Geology of the Duchess-Urandangi region,Mount Isa Inlier ...

Geology of the Duchess -Urandangi region ,Mount Is a Inlier ,Queensland

DEPARTMENT OF RESOURCES & ENERGYBUREAU OF MINERAL RESOURCES,AND GEOPHYSICSGEOLOGYBULLETIN 219Geology o f th e Duchess-Urandang i region ,Mount Is a Inlier , Queenslan dD. H. BLAKE, R. J. BULTITUDE*, P. J. T. DoNCHAKt,L. A. I. WYBORN, & I. G. HONE* formerl y BMR , no w Geologica l Surve y o f Queenslan df Geologica l Surve y o f Queenslan dAUSTRALIAN GOVERNMEN T PUBLISHIN G SERVIC ECANBERRA 198 4

DEPARTMENT O F RESOURCE S A N D ENERG YMINISTER: SENATO R TH E HON . PETE R WALSHSECRETARY: A . J . WOODSBUREAU O F MINERA L RESOURCES , GEOLOG Y AN D GEOPHYSIC SDIRECTOR: R . W . R . RUTLANDPublished for the Bureau of Mineral Resources, Geology and Geophysicsby the Australian Government Publishing Service© Commonwealt h o f Australi a 1984ISBN 0 64 4 0374 3ISSN 0084-708 9This Bulletin was edited by G. M. Bladon. Line drawings by A. Jaensch andG. A. Clarke.Cover: Disharmonic folding of bedded calc-silicate rocks in the Corella Formation 7 km westof Duchess (M2633/4)Printed by Graphic Services Pty Ltd, 516-518 Grand Junction Road, Northfield, SA 5085

CONTENTSThe authorship of each major section is indicated in parentheses; authors' initials are identified on the titlepage.ABSTRACT (DHB) . . . .INTRODUCTION (DHB & RJB)Climate . . . .Vegetation . . . .Topography and geomorphologyPrevious investigations .Definitions of termsAcknowledgementsGENERAL GEOLOGY (DHB) .Regional setting . . . .Tectonic frameworkStructures . . . . .Igneous intrusionsEffects of metamorphismPRECAMBRIAN OF THE WESTERN AREASulieman Gneiss .Kallala Quartzite .Saint Ronans MetamorphicsJayah Creek MetabasaltOroopo MetabasaltHaslingden GroupCarters Bore RhyoliteMcNamara GroupMount Isa Group?Sybella Granite .Minor intrusions .StructureFolds .Faults .Metamorphic effects(RJB)PRECAMBRIAN OF THE CENTRAL AREA (DHB, RJB, & PJTD)Plum Mountain GneissTewinga Group .Undivided Tewinga GroupLeichhardt VolcanicsMagna Lynn Metabasalt and Argylla FormationBottletree FormationHaslingden GroupMount Guide QuartziteEastern Creek VolcanicsMary Kathleen GroupBallara QuartziteCorella Formation and Corella Formation?Stanbroke SandstoneMakbat SandstoneCarters Bore Rhyolite? .Surprise Creek Formation?Mount Isa Group .Kalkadoon BatholithWonga Batholith .Mount Philp Breccia .Minor intrusions .StructureFoldsFaults .Metamorphic effectsPRECAMBRIAN OF THE EASTERN AREA (DHB & PJTD)Tewinga Group . . . . . . . .Argylla FormationiiiPage

Classification of mineral deposits and metallogenesis, by D. J. Perkin . . . 87Characteristics of Duchess-type stratabound deposits . . . . 88Characteristics of Pegmont-type stratiform deposits . . . . 88Discussion . . . . . . . . . . . . 88SUMMARY OF GEOLOGICAL HISTORY (DHB) 89Pre-1900 m.y.? 89About 1900 m.y.? 89About 1880-1810 m.y 89About 1810-1750 m.y 89About 1750-1700 m.y 90About 1700-1650 m.y 91About 1650-1350 m.y 91About 1350-1100 m.y 91About 1100 m.y. to present . . . . . . . . . 91REFERENCES 92TABLES1. Details of stratigraphy—Sulieman Gneiss, Kallala Quartzite, and Saint RonansMetamorphics, western area . . . . . . . . . .102. Details of stratigraphy—older cover sequence, western area . . . .123. Details of stratigraphy—younger cover sequence, western area . . . .134. Sybella Granite, western area . . . . . . . . . . 135. Details of stratigraphy—Plum Mountain Gneiss and Tewinga Group, central area 176. Details of stratigraphy—Bottletree Formation and Haslingden Group, central area 237. Details of stratigraphy—Stanbroke Sandstone, Makbat Sandstone, and Mary KathleenGroup, central area . . . . . . . . . . 248. Details of stratigraphy—Carters Bore Rhyolite?, Surprise Creek Formation?, andMount Isa Group, central area . . . . . . . . . . 279. Granites of the Kalkadoon Batholith, central area . . . . . .2810. Garden Creek Porphyry, Mount Philp Breccia, and granites of the Wonga Batholith,central area . . . . . . . . . . . . 2911. Details of stratigraphy—Tewinga Group, Malbon Group, and Double CrossingMetamorphics, eastern area . . . . . . . . . . 4012. Details of stratigraphy—Soldiers Cap Group, eastern area . . . . .4113. Details of stratigraphy—Mary Kathleen Group and Roxmere Quartzite?, easternarea . . . . . . . . . . . . . . . 4314. Granites of the Williams Batholith, eastern area . . . . . .5015. Representative chemical analyses of felsic volcanics from the Duchess-Urandangiregion . . . . . . . . . . . . . 5716. Representative chemical analyses of granites from the Duchess-Urandangi region 6017. Representative chemical analyses of mafic volcanics from the Duchess-Urandangiregion . . . . . . . . . . . . . 6318. Summary of density measurements (grouped by rock unit and rock type) . 6919. Summary of density measurements (grouped by rock type only) . . .7020. Summary of magnetic susceptibility and remanence measurements . . .7621. Airborne magnetic anomalies from rock units of the Duchess-Urandangi region . 8022. Recorded production data for mines in the Duchess-Urandangi region . . 85FIGURES1. Location map . . . . . . . . . . . . . viii2. Key to 1:100 000 and 1:250 000 geological maps 23. Geomorphological divisions . . . . . . . . . . . 34. Rocky ridge of calc-silicate rocks, Corella Formation . . . . . . 35. Low hill of sheared Leichhardt Volcanics . . . . . . . 46. Foliated felsic porphyry, Argylla Formation 57. Tectonic elements . . . . . . . . . . . . 68. Typical small-scale fold styles . . . . . . . . . . 79. Distribution of metamorphic facies . . . . . . . . . 810. Banded gneiss with boudinaged pegmatite veins, Sulieman Gneiss . . .1111. Inclusions of country rock (Jayah Creek Metabasalt) in Sybella Granite . .1412. Porphyritic granite cut by veins of microgranite, Sybella Granite . . . .1513. Migmatitic gneiss, undivided Tewinga Group . . . . . . . 1814. Interbedded gneisses, undivided Tewinga Group . . . . . • .1815. Ignimbrite, Leichhardt Volcanics . . . . . . • • .1916. Quartz-filled vesicles in mafic lava, Magna Lynn Metabasalt . . . .1917. Steeply dipping beds of mainly volcanic detritus, Magna Lynn Metabasalt . . 2018. Deformed volcanic conglomerate, Magna Lynn Metabasalt . . . .20v

19. Foliated felsic tuff with mafic inclusions, Magna Lynn Metabasalt . . .2020. Mafic spatter in felsic tuff, Magna Lynn Metabasalt and Argylla Formation . .2121. Laminated felsic tuff, Bottletree Formation . . . . . . 2122. Granite boulder in greywacke-conglomerate, Bottletree Formation . . . 2 223. Conglomerate in the Yappo Member, Mount Guide Quartzite . . . .2224. Thin-bedded calc-silicate rocks, Corella Formation . . . . . . 2525. Scapolite porphyroblasts in calc-silicate rock, Corella Formation . . . .2526. Tightly folded calc-silicate rocks, Corella Formation . . . . . . 2527. Transposed bedding, Corella Formation 2628. Porphyritic Kalkadoon Granite . . . . . . . . . . 2929. Granitic gneiss with feldspar augen, Birds Well Granite? . . . . .3130. Gently folded pegmatite vein cutting tightly folded calc-silicate rocks of theCorella Formation . . . . . . . . . . . .3131. Crenulated gneissic granite, Revenue Granite . . . . . . .3132. Mixture of dolerite, granite, and hybrid rocks, Mount Erie Igneous Complex . 3233. Aerial view of the Myubee Igneous Complex . . . . . . .3334. Inclusions of gabbro in granite, Myubee Igneous Complex . . . . .3435. Mount Philp Breccia 3536. Infrared airphoto of structures north of the Lady Fanny mine . . . .3637. Infrared airphoto of synclines southeast of Dajarra . . . . . .3738. Disharmonic folding, Corella Formation . . . . . . . .3839. Two generations of folding, Corella Formation 3840. Aligned minerals in fold hinge, Corella Formation . . . . . .3941. Zoned garnet porphyroblasts in skarn, Corella Formation . . . . .3942. Migmatitic gneiss, Soldiers Cap Group . . . . . . . . 4243. Small folds in Overhang Jaspilite . . . . . . . . . 4244. Infrared airphoto of folded metadolerite sills . . . . . . . 4445. Mica schist, Kuridala Formation . . . . . . . . . 4446. Infrared airphoto of the area around Kuridala . . . . . . . 4547. Pegmatite vein within the Kuridala Formation . . . . . . . 4648. Andalusite porphyroblasts in schist, Kuridala Formation . . . . .4649. Photomicrograph of andalusite porphyroblasts in carbonaceous slate, KuridalaFormation . . . . . . . . . . . . . . 4650. Photomicrograph of staurolite porphyroblasts in mica schist, KuridalaFormation . . . . . . . . . . . . . . 4651. Banded and brecciated calc-silicate rocks, Doherty Formation . . . .4752. Calc-silicate breccia, Doherty Formation 4753. Leucogranite cutting banded calc-silicate rocks of the Doherty Formation . . 4854. Convolute bedding of recumbent-fold style, Staveley Formation . . . .4855. Infrared airphoto of structures north of Kuridala . . . . . 4956. Aerial view of the Mount Cobalt Granite pluton 5257. Infrared airphoto of folds east of Malbon . . . . . . . . 5358. Infrared airphoto of structures south of Selwyn . . . . . . . 5459. Airphoto of folds and faults north of Kuridala . . . . . . . 5560. Infrared airphoto of structures in the northeast of the region . . . .5661. Abundances of selected major oxides and trace elements plotted against SiO o forthe felsic volcanics of the Duchess-Urandangi region . . . . . . 5862. Variation diagrams of selected major oxides and trace elements for granites ofthe Duchess-Urandangi region . . . . . . . . . .6163. Absolute frequency-distribution diagram for SiO o based on 412 analyses ofigneous rocks from the Duchess-Urandangi region . . . . . . 6264. Correlations of stratigraphic units, igneous events, and tectonism . . .6665. Mesas of the Gilbert River Formation overlying the Squirrel Hills Granite . . 6766. Airborne radiometric surveys and detailed metalliferous surveys . . . .6867. Density model for the Proterozoic rocks in the Duchess-Urandangi region . . 7068. Regional Bouguer anomaly field . . . . . . . . . .7169. Bouguer anomaly map of the Duchess-Urandangi region . . . . . 7 270. Bouguer anomaly profiles and simplified interpreted geological sections acrossDUCHESS 7371. Observed Bouguer gravity values and modelled Bouguer gravity profile along latitude21°42'S in the centre of the Duchess-Urandangi region . . . . 7472. Observed Bouguer gravity values and modelled Bouguer gravity profile along latitude21°42'S in the east of the Duchess-Urandangi regionr. . . . . 7 473. Total magnetic field contours . . . . . . . . . . 7774. Zones defined from characteristics of the total magnetic field . . . . .7875. Aeromagnetic profile along an east-west line in northern DUCHESS . . .7976. Total-count radiometric contours, DUCHESS . . . . . . .81vi

77. Ruins of the smelter at Kuridala in 1975 8378. Open cut of the Young Australia copper mine . . . . . . . 8479. Concordant band of quartz-hematite rock northwest of Selwyn . . . .88MAPGeology of the Duchess-Urandangi region (1:250 000 scale)ABSTRACTThe Duchess-Urandangi region covers the southern part of the Precambrian Mount IsaInlier and small parts of the early Palaeozoic Georgina and Mesozoic Eromanga Basins.The Precambrian is represented by sedimentary rocks and volcanics which have beentightly folded about mainly northerly trending axes, extensively faulted, intruded by numerousgranite plutons and mafic bodies, and regionally metamorphosed to the greenschist and amphibolitefacies. Basement rocks, older than 1850 m.y., are overlain unconformably by an oldercover sequence, between about 1810 and 1700 m.y. old, and a younger cover sequence, betweenabout 1680 and 1600 m.y. old. The cover sequences were laid down mainly in shallow water,and are many thousands of metres thick. They were last deformed on a major scale betweenabout 1600 and 1550 m.y. Two major north-trending fault zones, the Wonomo/MountAnnable/Mount Isa fault system in the west and the Pilgrim Fault Zone in the east, subdividethe region into three parts—the western, central, and eastern areas. Correlations across thesefault zones are generally uncertain.The igneous rocks are markedly bimodal; rocks of andesitic composition are rare. Fivedistinct geochemical suites of felsic volcanics are recognised; at least two of them are comagmaticwith granite. Of the four granite batholiths present, one, the oldest, has I-type characteristics,and the others are mainly uranium-rich A-types. Most mafic lavas and intrusivesresemble continental tholeiites in their geochemistry.A regional gravity high corresponding to the Mount Isa Inlier is characterised in theregion by elongate Bouguer anomalies trending north-south, parallel to geological trends; relativehighs are mainly over metasediments and mafic rocks, and lows are mainly over granites.Broad aeromagnetic zones correlate well with the known geology. Most mafic volcanics andintrusives and some metasediments, felsic volcanics, and granites are significantly magnetic.Regional radiometrics are strongly influenced by the presence or absence of uranium-richgranites.The Precambrian rocks are hosts to numerous, mainly small deposits of copper, gold,silver, lead, zinc, cobalt, tungsten, uranium, silica, and calcite, many of which have beenexploited. Most copper deposits are localised along shear zones, especially in carbonaceousslates, calc-silicate rocks, and metabasalt. Most of the gold has been obtained as a by-productof copper-mining, as at the Duchess mine, the most productive in the region, from which27 717 t Cu (average grade 12.3%) and 69 474 g Au were produced between 1906 and itsclosure in 1920. Silver ore has been obtained from two mines and cobalt from one. Severalstratiform lead-zinc-copper deposits, at present uneconomic, occur within the Kuridala Formationand Soldiers Cap Group; the best known deposit is that at the Pegmont prospect. Majordeposits of phosphate rock occur within the Palaeozoic Georgina Basin.The geological evidence suggests that the Mount Isa Inlier evolved during the Proterozoicin an intracratonic tectonic setting, rather than at a continental margin. Since about 1500 m.y.ago the inlier has been an essentially stable tectonic unit.vn

138° 140 ° 142 °DUCHESS 1 250 000 Sheet area = Main roadDUCHESS 1:100 000 Standard Sheet area === Secondary road| Duchess-Urandangi region " H I " Railway• HomesteadFig. 1 . Locatio n map .viii

The Duchess-Urandangi region, comprising theDuchess 1:250 000 Sheet area and the eastern part ofthe adjoining Urandangi 1:250 000 Sheet area, isbounded by latitudes 21 and 22°S, and longitudes 139and 141°E (Fig. 1). It includes the southern part ofthe Precambrian Mount Isa Inlier (except for somesmall outcrops which extend south into the Bouliaand Glenormiston 1:250 000 Sheet areas), part ofthe early Palaeozoic Georgina Basin (mainly the BurkeRiver Structural Belt), and a small part of the MesozoicEromanga Basin. The Mount Isa Inlier is formedof metamorphosed sedimentary, volcanic and intrusiverocks, and the two basins consist of unmetamorphosedsedimentary rocks.The Precambrian rocks in the region were mappedbetween 1975 and 1980 as part of a joint Bureau ofMineral Resources (BMR) and Geological Survey ofQueensland (GSQ) project to update the results of theBMR/GSQ reconnaissance survey of the 1950s(Noakes, Carter, & Opik, 1959; Carter, Brooks, &Walker, 1961; Carter & Opik, 1963). Colour aerialphotographs of about 1:25 000 scale were used formapping and airphoto-interpretation. More detailedresults of the mapping are reported in map commentariesfor the following 1:100 000-scale geologicalmaps (Fig. 2): ARDMORE* (Bultitude, 1982),DAJARRA (Blake & others, 1982a), DUCHESSREGION (Bultitude & others, 1982), SELWYNREGION (Blake & others, 1983a), and KURIDALAREGION (Donchak & others, 1983); and also in preliminarydata records (Blake & others, 1978, 1979;Mock, 1978; Bultitude & others, 1978; Noon, 1978,1979; Donchak & others, 1979; Bultitude, 1980). Newand revised Precambrian stratigraphic and intrusiverock units in the region are defined by Blake & others(1981a); their definitions are summarised by Blake& others (1981b).The main population centres in the region (Fig. 1)are the townships of Dajarra (population about 170 m1980), Duchess (population about 30 in 1980),Malbon (population about 20 in 1980), and TheMonument, which was built in the mid-1970s as acompany town to serve the nearby phosphate mine atPhosphate Hill. The Monument had a population ofabout 150 in 1977, but became largely deserted followingthe suspension of mining in 1978. The onlyother permanent settlements in the region are stationhomesteads.Access to and within the region is generally good(Fig. 1). The bitumen-sealed Diamantina DevelopmentalRoad from Mount Isa to Boulia crosses thewestern part of the region via Dajarra. Regularlymaintained formed gravel roads branch from thisroad north of Ardmore homestead to Urandangi, andsouth of Dajarra to The Monument. Similar gravelroads link Duchess to Mount Isa, to the sealed BarklyHighway near Cloncurry (via Malbon), and to Dajarra.The partly sealed Landsborough Highway crosses thenortheastern corner of the region, and the unsealedbut regularly maintained McKinlay to Boulia roadcrosses the southeastern part. Other unsealed roadsconnect the former township of Selwyn with Malbon(via the former township of Kuridala) to the north,Boulia to the southwest, and Hamilton Hotel to the* Names of 1:100 000-scale geological maps are printed incapital letters.INTRODUCTION1south. Duchess and Malbon are on the Townsvilleto Mount Isa railway; Dajarra is the terminus of abranch line from Duchess; and another branch lineconnects the Phosphate Hill mine near The Monumentwith the Townsville to Mount Isa railway east ofDuchess. Numerous vehicle tracks in variable statesof upkeep provide access to other parts of the region.Many of the roads and tracks become impassable afterheavy rain. The townships and most homesteads havelanding grounds suitable for light aircraft. The regionis served by the Royal Flying Doctor Service basedat Mount Isa.The main local industry is cattle-raising on unimprovednatural pastures. Mining has been importantin the past, but most of the many mainly small minesin the region are now abandoned. The Precambrianoutcrops are prospective for copper, gold, silver, lead,zinc, cobalt, uranium, and tungsten, and also for silicaflux and marble (limestone), which are used insmelting operations at Mount Isa. Major deposits ofrock phosphate occur within the Cambrian of theGeorgina Basin sedimentary succession. Severalmineral exploration companies are prospecting in theregion.Climate (based mainly on Slatyer, 1964; Bureau ofMeteorology, 1975, 1977)The region is semi-arid and has a tropical monsoonalclimate with a short wet 'summer' season characterisedby hot days, and a long dry mild 'winter', characterisedby southeast winds. At Cloncurry, to the north,average daily temperatures range from about 10°Cminimum and 24°C maximum during July, up to 24°Cminimum and 38°C maximum during November,December, and January. Diurnal variations in temperatureare marked, particularly in the 'winter'months, when night-time temperatures commonly fallbelow freezing-point. The average annual rainfall isabout 380 mm, and tends to decrease from northeastto southwest. However, rainfall is variable, rangingfrom more than 500 mm in some years to less than200 mm in others; droughts are not uncommon. Mostrain falls between November and March, but heavyrains have occasionally been recorded in the 'winter'months (for example, 91 mm of rain fell at Dajarrabetween 2 and 10 July 1978, including 66 mm on 10July). Relative humidity ranges from about 15 percentin 'winter' to 50 percent in 'summer'; annual evaporationgreatly exceeds rainfall.Vegetation (based mainly on Perry & Lazarides, 1964;Horton, 1976)Most of the region supports a vegetation of spinifex(Triodia spp.), kerosene grasses (Aristida spp.),a diverse array of shrubs (generally dominated byAcacia spp. and Cassia spp.), and scattered trees(mainly Eucalyptus spp.). Some of the more commontrees are snappy gum (E. brevifolia), western box(E. argillacea), bloodwood (E. terminalis), ghost gum(E. papuana), and silver-leaf box (E. pruinosa).Scattered, small dense stands of gidgee (Acacia georginaeand A. cambagei) are common in places. Dense,in places almost impenetrable, stands of 'turpentine'bush—mainly A. lysiphloia and A. chisholmii —arelocally extensive, especially on soils derived from calcareousmetasediments. Grasslands, in which Mitchell(Astrebla spp.) and Flinders (Iseilema spp.) grassespredominate, cover most of the extensive cracking

138°0O' 138°30' 139°17' 139°30 ' 1*0°00' I40°15 ' 140°30 'URANOANG11:250 00 0 Shee tOBANWwWDUCHESSP^smEGIONPiMALBONmmDUCHESS-URAr IDANGI REGIONbkiMi^///////MT MERLI N'WWDUCHESS 1:25 0 00 0 Shee tMT ANG f l ^yO^^yIOAIHEGION- S E L w f N ^ ^ ^ ^ ^ ' 'WYNREGIONH1°00'2I°00'22°O0'1:100 OOO preliminary-edition map1:100 OOO coloured first-edition mapj 1:250 OOO coloured special mapFig. 2 . Ke y t o 1:10 0 00 0 an d 1:25 0 000 geologica l maps .clay-soil plains formed on the lower Palaeozoic rocksof the Georgina Basin succession in the western andcentral parts of the region, and on Mesozoic rocks ofthe Eromanga Basin succession in the east.Topography and geomorphologyThe region has a maximum elevation of about 550 mabove sea level near Mount Guide in the northwest anda minimum of about 235 m on the alluvial plain of theHamilton River in the southeast. Local relief isgenerally less than 100 m. A low divide separatingthe catchment of the southerly flowing Georgina-Diamantina drainage system from the northerly flowingLeichhardt-Flinders drainage system trends northwestacross the region north of Duchess (Fig. 3); inthe southeast, the Selwyn Range—formed of plateaus,mesas, buttes, and rocky ridges—is part of this divide.The geomorphology of the terrain west of 139°30'Ehas been described by Stewart (1954) and that to theeast by Twidale (1964, 1966). The geomorphologicaldivisions generally applied to the region are those ofTwidale (1964, 1966) and these are shown in Figure3. The Isa Highlands and the Carpentaria and InlandPlains of Twidale correspond to the Erosional LandSurface and Depositional Land Surface subdivisions,respectively, of Stewart (1954).The Precambrian metasediments exposed in theregion are generally more resistant to erosion than theigneous rocks. Metamorphosed sandstones (mainlyquartz arenite) typically form prominent north-trendingstrike-ridges with pale airphoto tones, separatedby valleys developed in less resistant rocks. Vein quartzforms narrow steep-sided ridges, commonly along faultlines. Calc-silicate rocks, especially where they aremetamorphosed to amphibolite facies, give rise torugged hilly terrain that has moderately dark airphototones (Fig. 4).Mafic igneous rocks generally form flat to gentlyundulating terrain, and typically have smooth darkairphoto tones; hence they can be readily distinguishedfrom most other rock types on aerial photographs.Mafic dykes are mostly less resistant to erosion thanthe rocks they intrude, and tend to form depressions.Felsic volcanic rocks are less recessive, and mainlyform low hills and tors (Figs. 5, 6) which have mediumto pale airphoto tones.Granitic rocks—granite, granodiorite, tonalite,diorite, and quartzofeldspathic gneiss—are exposed assteep-sided hills and mesas (especially in the east)capped by much weathered granite, Iaterite, or flat-lyingMesozoic and Cambrian rocks (e.g., Fig. 65); as torsand boulder-covered hills; and as undulating terrainand plains largely covered by granitic soil and rubblebut commonly with some partly exposed large roundedboulders. Outcrop areas generally have pale airphototones, except where ironstained Iaterite cappings arepresent, and dendritic drainage patterns. Boulders ofnon-foliated granitic rocks and felsic volcanics aretypically spheroidal, whereas those of foliated rocks arcellipsoidal.The lower Palaeozoic rocks in the western andcentral parts of the region form broad treeless plainsexcept locally near the Pilgrim Fault Zone, wherethey are uplifted and tilted to form mesas and cucstas.Flat cappings of Mesozoic sedimentary rocks, and alsoof Iaterite and deeply weathered Precambrian bedrock,are widespread, but are most extensively preserved inthe Selwyn Range in the southeast. Broad alluvial plainsand gently undulating terrain with some low ridgesare present in the far east and far west of the region.The crests of many of the ridges are planated surfacesrepresenting remnants of a former peneplain(Carter & others, 1961; Twidale, 1964). Widespreadremnant cappings of Mesozoic rocks and weatheredPrecambrian rocks provide further evidence of planation,which probably took place during the late Proterozoic,late Palaeozoic to early Mesozoic, and earlyTertiary.Previous investigationsThe Duchess-Urandangi region was first mappedsystematically during the reconnaissance survey of the

16/F5d/50Drainage divide J ~ "j Isa HighlandsCarpentariaPlainsj | Inland PlainsFig. 3 . Geomorphologica l divisions .Mount Isa Inlier by joint BMR and GSQ field partiesin the 1950s. The results of this work have beenreported by Carter & others (1961), who gave anaccount of the geology of the whole Mount Isa Inlier,and by Noakes & others (1959) and Carter & Opik(1963), who described the geology of the Urandangiand Duchess 4-mile (1:253 440) Sheet areas respectively.The early history of the region and previousgeological work were reviewed in these publications;the early history of the Duchess area is also summarisedby Bultitude & others (1978). A detailedreport on the geology of an area near Selwyn waswritten by White (1957). Syvret (1966), Krosch &Sawers (1974), and Brooks (1977, 1979b) havedescribed several of the small mines, and miningcompany activity in the region has been summarisedby Noon (1976, 1977). Locsei (1977) and Stanton &Vaughan (1979) have given accounts of the Pegmontprospect southeast of Selwyn; descriptions of this andsome other prospects in the eastern part of the regionhave also been given by Nisbet & Joyce (1980), Orridge(1980), and Stanton (1982).The Late Proterozoic to Ordovician succession inthe Burke River Structural Belt (part of the GeorginaBasin) in the central part of the Duchess-Urandangiregion, and the Cambrian rocks of the GeorginaBasin succession in the west, have been described byde Keyser (1968, 1972, 1973), de Keyser & Cook(1972), and Shergold & Druce (1980). The discoveryof rock phosphate in Cambrian strata near The Monumentand at other localities in the region has beendescribed by, amongst others, Russell (1967), Thomson& Russell (1971), Russell & Trueman (1971), andCook & Shergold (1979). An account of the Mesozoicrocks of the Eromanga Basin, exposed mainly in thefar southeast, is given by Senior & others (1978).Definitions of termsMost of the terms used are defined in the AGIGlossary of Geology (2nd Edition; Bates & Jackson,1980). Sandstones are classified according to Pettijohn& others (1972); grainsize definitions are as follows:fine, 0.125 to 0.25 mm; medium, 0.25 to 0.5 mm;coarse, 0.5 to 1 mm. The term 'quartzite' is used in adual sense (Bates & Jackson, 1980) to describe aFig. 4 . Rock y dar k ridg e o f calc-silicat e rock s o f th e Corell aFormation, an d relativel y smoot h ridg e o f Overlande rGranite i n th e background , 3 9 k m north-northeas t o fDuchess (nea r G R UB8976) . (M2633/26A )3

Fig. 5 . Lo w hill y terrai n develope d o n sheare d felsi c rock s o f th e Leichhard t Volcanic s 2 8 k m northwes t o f Duches s(at G R UB623584) . (M2633/7) .hard but little-metamorphosed arenite which has beendiagenetically cemented by silica, and a siliceousgranoblastic rock produced by regional or contactmetamorphism; all gradations exist between the twotypes. Bedding thickness terms are: laminated, less than1 cm; thin-bedded, 1 to 50 cm; medium-bedded, 50cm to 2 m; thick-bedded, more than 2 m. The nomenclaturefor plutonic igneous rocks follows that ofStreckeisen & others (1973). Grainsizes for igneous,and also metamorphic rocks are: fine, less than 1 mm;medium, 1 to 5 mm; coarse, 5 mm to 3 cm; very coarse,pegmatitic, more than 3 cm. Terms describing metamorphicfacies are as defined by Turner & Verhoogen(1960). The term 'granofels' (Goldsmith, 1959) isused for medium to coarse granoblastic rocks whichdo not have a marked foliation or lineation; theserocks may be uniform in mineral composition, or theymay contain layers of different composition in whichnon-directional minerals predominate. The prefix 'meta'added to a rock name indicates that the rock nowhas a metamorphic fabric or mineralogy, or both, butits original nature is readily apparent.The term 'concordant' is used to describe contactsbetween units displaying parallelism of bedding orstructure, where a hiatus cannot be recognised butmay exist (Bates & Jackson, 1980).The term 'migmatite' is used to describe a composite(mixed) rock consisting of igneous or igneouslookingand metamorphic components which aregenerally distinguishable megascopically (Bates &Jackson, 1980).The term 'batholith' refers to a grouping of spatiallyassociated granitic intrusions which may or may notbe genetically related or similar in age.A cknowledgmentsWe wish to express our gratitude to the residentsof the region who provided hospitality and assistanceduring the course of the project. Special thanks are dueto D. Macdonald of Devoncourt station; Mr and MrsC. Kelly of Duchess; the McConachy family of Ashoverstation; L. Cozzi, formerly of the Revenue mine;P. Rons of the Answer mine; M. Dando of MountIsa; and the Bode family of Percol Plains station. Wealso wish to acknowledge the assistance provided byC. M. Mock (BMR), R. N. England (formerly ofBMR), and T. A. Noon (GSQ), who were membersof the 1975-76 mapping parties; by A. L. Jaques(BMR), who took part in the 1978 fieldwork; by CarpentariaExploration Company in supplying geologicalinformation; and by Queensland Phosphate Limited inallowing access to the facilities at The Monument.Throughout the course of the project the authors havegreatly benefited from many stimulating discussionsheld with G. M. Derrick, R. N. England, and W. B.Dallwitz, all formerly of BMR, both in the field andin Canberra.4

Fig. 6 . Exposure s o f foliate d felsi c porphyr y o f th e Argyll a Formatio n 3 7 k m nort h o f Duches s (a t G R UB805748) .(M2633/10)GENERAL GEOLOG YRegional settingThe Duchess-Urandangi region covers the southernmostexposed part of the Precambrian Mount IsaInlier (Geological Survey of Queensland, 1975),apart from a few outcrops in the Boulia and Glenormiston1:250 000 Sheet areas to the south. The Precambrianrocks belong to the 'Cloncurry Complex' ofCarter & others (1961), and consist of northerlytrending, tightly folded and extensively faulted sedimentsand volcanics which have been intruded bynumerous granite plutons and mafic bodies, andregionally metamorphosed to the greenschist andamphibolite facies. The inlier is flanked to the westby the early Palaeozoic Georgina Basin and to the eastby the Mesozoic Eromanga Basin, and is overlain inthe central part of the region by sedimentary rocksof the Burke River Structural Belt, an embayment ofthe Georgina Basin.Since the work of Carter & others (1961), theMount Isa Inlier has generally been considered toconsist of a narrow central basement belt (Kalkadoon-Leichhardt Block) flanked to the west and east bythick sequences of stratigraphically equivalent coverrocks, commonly known as the western and easternsuccessions. This is the model favoured, for example,by Plumb & Derrick (1975), Plumb & others (1980,1981), and Derrick & Wilson (1981b). However,Blake (1980, 1981b) has criticised some aspects ofthis model and made the following conclusions: (1)the oldest cover rocks in the west, mainly those of theHaslingden Group, were deposited to the west of alarge and broad, rather than narrow, landmass composedof crystalline rocks which are now exposed inthe central and possibly eastern parts of the MountIsa Inlier; (2) these cover rocks predate those at thebase of the eastern succession; (3) the basement of previousworkers contains volcanic units which are youngerthan part of the western succession; and (4) the mostwidespread unit previously recognised in the easternsuccession, the Corella Formation, includes rocks thatare probably older as well as younger than the HaslingdenGroup.The Precambrian succession in the southern partof the Mount Isa Inlier has been dated by U-Pbzircon geochronology of felsic volcanics and intrusives,mainly from north of the Duchess-Urandangi region(Page, 1978, 1983a, b; Wyborn & Page, 1983)*. Itis characterised by the occurrence of similar rock* Unless state d otherwise , cite d isotopi c age s hav e bee ndetermined b y R . W . Page , BMR , usin g th e U-P b zirco nmethod. Almos t al l age s o f rock s determine d usin g Rb-S rand K-A r technique s ar e to o young , an d ar e presume d t odate metamorphi c an d othe r events .5

25 IFig. 7 . Tectoni c elements .types—such as mafic and felsic volcanics, quartzarenite, calc-silicate rocks, and black slate—at severaldifferent stratigraphic levels and geographic locations,and by major breaks in the stratigraphy markedmore commonly by disconformities than by angularunconformities.A major unconformity separates metamorphic andigneous basement rocks older than about 1850 m.y.from the overlying Proterozoic cover rocks. The 'oldercover' includes metasedimentary and metavolcanicsequences ranging in age from about 1810 to about1700 m.y. The 'younger cover' comprises sequencesbetween about 1680 and 1600 m.y. old. The oldergranites in the region predate the cover sequences,and were probably intruded before 1850 m.y.; theyounger granites range in age from about 1740 toperhaps about 1500 m.y.Two major northerly trending fault zones, theWonomo/Mount Annable/Mount Isa fault system inthe west and the Pilgrim Fault Zone in the east, dividethe Duchess-Urandangi region into three parts: thewestern, central, and eastern areas (Fig. 7). Correlationsbetween these areas are generally uncertain,and few Precambrian units have been mapped in morethan one of the areas. Each area includes unitsassigned to the basement, to the older and youngercover sequences, and to younger granite; however,older granite, represented by the Kalkadoon Batholith,has been recognised only in the central area.Tectonic frameworkAccording to Plumb & others (1980), there arefour principal Precambrian tectonic elements in theDuchess-Urandangi region: the Leichhardt RiverFault Zone, Kalkadoon-Leichhardt Block, MalbonBlock, and Mary Kathleen Fold Belt. The LeichhardtRiver Fault Zone, which corresponds to the LeichhardtRiver Fault Trough of Derrick (1980), contains theolder and younger cover sequences in the western areaand in the western part (the Dajarra Belt of thisBulletin) of the central area (see Fig. 7, and thestructural and tectonic sketch on the accompanyingFig. 8 . Typica l small-scal e fol d style s i n th e Duchess-Urandang i region .(a) Fold s i n calc-silicat e rock s o f th e Corell a an d Dohert y Formations . I n highl y calcareou s layers , tigh t t o isoclina l fold sare develope d wit h littl e o r n o associate d axial-plan e foliation . I n les s calcareou s an d therefor e mor e competen t layers ,relatively ope n fold s ar e develope d wit h a n axial-plan e foliatio n denne d b y alignmen t o f mafi c minerals .(b) Secon d o r third-generatio n fold s i n schis t interbedde d wit h meta-arenite , a s i n th e Kuridal a Formatio n an d part s o fthe Soldier s Ca p Group . Relativel y incompeten t schist s hav e well-develope d crenulation s an d a n associate d cleavage ,whereas th e mor e competen t meta-arenit e bed s ar e commonl y no t cleaved .(c) Fold s i n part s o f th e Overhan g Jaspilite : tightl y folde d jaspilit e ban d enclose d i n massive , cleave d phyllite .(d) First-generatio n structure s i n gneis s o f th e Soldier s Ca p Group . Th e first-generatio n foliatio n i s define d b y micaceou sand felsi c band s an d boudinage d pegmatite , aplite , an d quart z veins . Simila r feature s occu r i n othe r gneissi c units .(e) Second-generatio n fold s i n th e Plu m Mountai n Gneiss . Si , th e first-generation foliation , i s define d b y alignmen t o fmineral aggregate s an d feldspa r augen . Second-generatio n crenulations , wit h S - a s axia l plane , hav e stretched-ou t auge n o nfold limbs .(f) Structure s i n th e Bush y Par k Gneiss . Th e mai n foliatio n (Si ) i s define d b y alignmen t o f minera l grain s an d feldspa raugen. Som e auge n consis t o f relic t feldspa r phenocryst s wit h deformatio n trail s paralle l t o Si . Quart z an d aplit e vein swhich ar e folded , an d quart z vein s whic h ar e boudinaged , probabl y predat e th e mai n deformation . Th e interfingerin ggneiss-metabasite contac t ma y resul t fro m intens e deformation , an d th e origina l contac t ma y hav e bee n almos t a t right -angles t o th e presen t foliation .6

map sheet). The Kalkadoon-Leichhardt Block, formedlargely of basement rocks, occupies the central partof the central area. The Malbon Block contains theolder cover sequence in the western part of theeastern area. The Mary Kathleen Fold Belt liesmainly in the eastern area, where it corresponds to theStaveley Belt and Squirrel Hills Belt of this Bulletin,formed of younger and older cover sequences respectively,but it also includes tightly folded older coverand possible basement rocks which form the DuchessBelt in the eastern part of the central area (Fig. 7).StructuresThe style, orientation, and degree of development offolds in the Precambrian rocks vary according to rocktypes, presence and frequency of bedding planes andprevious structures such as cleavage, position relativeto zones of particularly intense deformation, andproximity to granite plutons and major faults. Thesemainly local factors, together with a general lack ofoverprinting criteria and a paucity of detailed structuralmeasurements, make it difficult to correlatedeformational events between different areas of theregion, and hence to decipher the overall structuralhistory. Some typical small-scale fold styles are illustratedin Figure 8. Major folds show a comparablerange in style.The earliest recognisable structural feature in mostrocks is a subvertical, northerly trending cleavage/schistosity/foliation which in some cases can be relatedto tight to isoclinal major and minor folds. However,in the more metamorphosed rocks, none of the majorfolds identified can be related to the earliest andmost prominent foliation present. Prevailing northerlytrends may reflect the superimposition of similarlyoriented (co-axial) folds of different ages, or the reorientationof early structures and lithological boundariesby later intense folding about north-southaxes.The major faults and fault zones—the northerlytrending and steeply dipping Wonomo, Mount Annable,and Cloncurry Faults, the Rufus, Mount Isa, andPilgrim Fault Zones, and the northeasterly trendingFountain Range Fault—probably represent long-actingdeep-seated crustal discontinuities along which bothvertical and horizontal movements have taken place.Many faults mark lithologic boundaries, and someoccur in the axial zones of major folds, especiallysynclines. Most faults shown on the Duchess-Urandangiregion map are either at least partly syntectonicor post-tectonic, but faulting undoubtedly took placeduring sedimentation and volcanism, and may accountfor some abrupt changes in thickness of rock units.Some faults, including those of the Pilgrim and RufusFault Zones, displace Cambrian strata, so were activein Phanerozoic as well as Precambrian times. Shearzones many metres wide are associated with severalfaults; in these zones, schistose, mylonitic. or brecciatedrocks predominate, but recognisable remnantsof the pre-existing rocks are generally also present.Fig. 9 . Distributio n o f metamorphi c facies .8

Igneous intrusionsMafic intrusions of several different ages occurthroughout the region. Most may be related to maficvolcanic units. Northerly trending metadolerite andamphibolite dykes, commonly concentrated in swarms,predominate, but sills and irregular pods are commonin some units. There are also some unmetamorphoseddolerite dykes which cut across the regional northerlytrends. Some mafic bodies have been intruded intogranite to form net-veined complexes (Blake, 1981a).Many mafic intrusions, especially the larger ones, havemassive interiors, but their margins are generally schistose,probably because of slippage during deformation.Metadolerite ranges from fine-grained to gabbroic andin a few places shows compositional zoning. Somerelict igneous features, such as ophitic textures andplagioclase phenocrysts, are commonly preserved inmetadolerite, but not in amphibolite, which is athoroughly recrystallised metamorphic rock.Felsic dykes are much less common than mafic dykesbut occur in most parts of the region. All show somerecrystallisation effects.The numerous granite plutons in the region formparts of four large batholiths—from west to east, theSybella, Kalkadoon, Wonga, and Williams Batholiths.Most of the granitic plutons appear to have been intrudedpassively, causing little disturbance to beddingand pre-existing cleavage trends in surroundingcountry rocks.Effects of metamorphismThe Precambrian rocks have been affected byregional metamorphism of low-pressure/high-temperaturetype (e.g., Jaques & others, 1982), which presumablyaccompanied the main deformation events.Mineral assemblages range from lower greenschist toupper amphibolite facies. No poly-metamorphic progrademineral assemblages have been recognised, butretrograde assemblages marking relatively late lowgrademetamorphic events are common. The distributionof prograde greenschist-facies and amphibolitefaciesrocks is shown in Figure 9, which is based onthin-section studies—including microprobe mineralanalyses—of mafic, calc-silicate, and pelitic rocks.In greenschist-facies rocks, primary igneous andsedimentary textures are commonly well preserved.Typical metamorphic mineral assemblages are quartz ±alkali feldspar ± chlorite ± greenish brown biotite ±sericite/muscovite ± tremolite-actinolite ± epidote infelsic igneous rocks and non-calcareous sedimentaryrocks; tremolite/actinolite + albite + calcite or dolomitein calcareous rocks; and actinolite + albite +epidote in mafic igneous rocks.Lower amphibolite-facies rocks are generally morerecrystallised, and most primary igneous and sedimentarytextures are either poorly preserved or obliterated.Typical metamorphic assemblages are quartz+ feldspar (albite and/or microcline) ± red-brownbiotite ± muscovite ± garnet in felsic igneous rocks;the same minerals ± andalusite ± staurolite in noncalcareousmetasediments; green hornblende (commonlychlorine-bearing) + sodic plagioclase + clinopyroxene(generally salitic) ± garnet ± scapolite ±quartz ± K-feldspar in calc-silicate rocks; and greenhornblende + oligoclase/andesine ± clinopyroxene ±garnet in amphibolite.Upper amphibolite-facies rocks are distinguished bythe presence of sillimanite in gneiss and schist; andraditegarnet and calcic plagioclase in calc-silicate rocks;and clinopyroxene ± andradite garnet in amphibolite.Contact metamorphic effects are apparent locallywithin 100 m or so of some granitic plutons, and alsowithin a few metres of some mafic intrusions. In themetamorphic aureoles, arenites and felsic volcanics arerecrystallised to quartzitic rocks in which tourmalineneedles are commonly present, and pelitic rocks mayform hornfels containing adalusite, cordierite, andfibrolitic sillimanite. Skarns are commonly developedin. calcareous units. Alkali feldspar (K-feldspar oralbite) and hematite metasomatism, resulting in theformation of 'red rock', is extensive in places,especially within the Duchess Belt of the central area.The metasomatic effects may extend more than 100 mfrom igneous intrusions.PRECAMBRIAN O F TH E WESTER N ARE AThe western area is bounded by the Wonomo/Mount Annable/Mount Isa fault system to the east andthe Cambrian Georgina Basin sedimentary successionto the west (Fig. 7), and is part of the LeichhardtRiver Fault Zone of Plumb & others (1980). Many ofthe Precambrian stratigraphic units in this area appearto be similar to formations (mainly assigned to theHaslingden Group) exposed west of the Mount IsaFault in MOUNT ISA, to the north (Hill & others,1975; BMR, 1978a). However, they are referred tonew formations (Blake & others, 1981a,b) because(i) their stratigraphic relations (particularly in thesouth) are uncertain; (ii) their isotopic ages have notbeen established; (iii) they cannot be correlatedunequivocally with units of the central area; and (iv)not all of them may be equivalent to the HaslingdenGroup.In the south the western area consists of twonortherly trending belts—one to the east and the otherto the west of the north-northeast-trending RufusFault Zone (Fig. 7). This fault zone, which is part ofthe Mount Remarkable fault system of Derrick &others (1980), truncates the Wonomo Fault (which isthought to be the southerly extension of the MountIsa Fault Zone), so that only the western belt is presentin the north.In the eastern belt the oldest units are probablythe Sulieman Gneiss and the Kallala Quartzite, and themost extensive unit is the Jayah Creek Metabasalt,which includes numerous metasedimentary lenses—thethickest being the Timothy Creek Sandstone Member.The relative ages of these three formations are uncertainbecause they have concordant contacts andgenerally lack facing evidence in contact zones. Metaareniteand possibly carbonaceous metasiltstone andshale preserved in the keel of a small syncline in thesouth, and probably in faulted contact with JayahCreek Metabasalt, is tentatively assigned to the MountIsa Group.In the western belt, west of the Rufus Fault Zone,the oldest rocks exposed are the Saint Ronans Metamorphics.This formation appears to be similar lithologicallyto the Yaringa Metamorphics exposed to thenorth, west of Mount Isa; Carter & others (1961)regarded the Yaringa Metamorphics as possibly theoldest unit exposed in the Mount Isa Inlier, and Page

& others (1982) have provisionally dated them as olderthan 2000 m.y. The Saint Ronans Metamorphics areoverlain to the south by the Oroopo Metabasalt, which—together with the Jayah Creek Metabasalt—may becorrelated with the Eastern Creek Volcanics in thecentral area. Rocks assigned to the Haslingden Group,Carters Bore Rhyolite, and McNamara Group cropout in the north.The Sulieman Gneiss, Kallala Quartzite, and SaintRonans Metamorphics (Table 1) are considered to bepart of the basement; the Jayah Creek Metabasalt,Oroopo Metabasalt, and Haslingden Group (Table 2)are assigned to the older cover sequence; and theCarters Bore Rhyolite and Mount Isa and McNamaraGroups (Table 3) belong to the younger coversequence.Sulieman GneissThe Sulieman Gneiss (Table 1) crops out in thesouthwest of the eastern belt. It is highly deformed,extensively recrystallised, and regionally metamorphosedto the amphibolite facies. Most of the mainrock type, quartzofeldspathic gneiss (Fig. 10), mayrepresent metamorphosed felsic volcanics and interlayeredvolcaniclastic sediments, though some augengneisses within the unit may be metamorphosed graniteand feldspar porphyry intrusives. The presence of somechlorite (replacing biotite or, less commonly, hornblende)and sericite (replacing plagioclase) indicatesthat the amphibolite-facies rocks have undergone alater greenschist-facies retrogressive metamorphism.About 24 km southwest of Dajarra the SuliemanGneiss appears to have a concordant contact withmetasediments assigned to the Jayah Creek Metabasalt.The presence of pods and lenses of coarse chloriteand veins of pegmatite in both units adjacent to thiscontact indicates that the contact may be tectonic.Original lithologies in the Sulieman Gneiss are notobvious, whereas those in the Jayah Creek Metabasaltare readily discernible in most places, and the twounits are possibly separated by a major metamorphicunconformity.Two or more sets of pegmatite and granite veinscut the Sulieman Gneiss. The oldest, which are mainlyless than 20 cm thick and are generally roughly concordantwith, the foliation in the host rocks (Fig. 10),may have been produced by local partial meltingduring regional metamorphism and deformation. Theyare extensively boudinaged, generally have irregularcontacts, and in places show small-scale ptygmaticfolds; those of pegmatite commonly contain largefeldspar augen. Younger pegmatite veins are littledeformed and cut across the foliation; they are probablyrelated to the nearby Sybella Granite.The Sulieman Gneiss may correlate with the SaintRonans Metamorphics exposed west of the Rufus FaultZone, with the Yaringa Metamorphics in MOUNTISA (Carter & others, 1961; Hill & others, 1975),and with quartzofeldspathic gneiss of the undividedTewinga Group in the central area to the east. Anotherpossible correlative is the May Downs Gneiss inMOUNT ISA (Hill & others, 1975), which Derrick &others (1976b) considered to be a metamorphosedequivalent of the lower part of the Mount GuideQuartzite and therefore younger than the TewingaGroup.Kallala QuartziteThe Kallala Quartzite (Table 1) forms a narrownortherly trending belt in the far south. It containsTABLE 1. DETAILS OF STRATIGRAPHY—SULIEMAN GNEISS, KALLALA QUARTZITE AND SAINTRONANS METAMORPHICS, WESTERN AREAName of unit(reference to definition)max. thickness (m)Saint Ronan s Metamorphic s(Blake & others , 1981a )1000 +Kallala Quartzit e(Blake & others , 1981a )350 +Sulieman Gneis s(Blake & others , 1981a )1000 +LithologyFine-grained quart z + biotit e + muscovit e ± feld -spar ± andalusit e schist , quartzofeldspathi c gneiss ,extensively recrystallise d even-graine d t o porphyrini cfelsic metavolcanics , amphibolite , amygdaloida l meta -basalt, meta-arenite , quartzite , an d metasiltstone ;minor agglomerat e an d para-amphibolite ; som e small -scale cross-beddin g i n meta-arenit e i n SMedium t o coars e glass y quartzite , muscovit e quartzit eand feldspathic ? quartzite ; mino r amphibolit e an dhornblende-biotite schis t an d gneiss . Fe w sedimentar ystructures preserve dMedium-grained quartzofeldspathi c gneis s an d auge ngneiss, amphibolite , hornblend e schist , glass y quart -zite, muscovit e quartzite , an d feldspathic ? quartzite ;minor calc-silicat e rocks , para-amphibolite , pegmatite ,feldspar metaporphyry , an d amygdaloidal ? metabasal tGneiss: mediu m t o thin-banded ; consist s o f quart z +plagioclase + biotit e ± microclin e ± mino r mus -covite, hornblende , garnet , sphene , epidote/clino -zoisite, an d chlorite ; som e myrmekit e commonl ypresent. Foliatio n commonl y folde d an d crenulate dAmphibolite and hornblende schist: mediu m t o coars eaggregates o f plagioclas e (An„.n0) + brownis hgreen hornblend e + mino r sphene , quartz , epidote /clinozoisite, an d opaqu e oxid eCalc-silicate rocks: thinl y banded ; consis t o f plagio -clase (oligoclase-andesite ) + clinopyroxen e(salitic?) • + quart z ± garne t ± mino r sphene ,epidote/clinozoisite, an d apatit ePara-amphibolite: commonl y contain s som e biotit eand apatit eRelationshipsBase no t exposed ; overlai n apparentl yconcordantly bu t probabl y unconform -ably b y Oroop o Metabasalt , an d wit hangular unconformit y b y Cambria n sedi -ments o f Georgin a Basi n succession ;intruded b y Sybell a Granite , muscovit eand tourmaline-bearin g pegmatite , an damphibolitic metadolerit eConcordant an d apparentl y gradationa lcontact wit h Suliema n Gneiss ; concor -dant wit h rock s assigne d t o Jaya h Cree kMetabasalt; intrude d b y Sybell a Granit eBase no t exposed ; concordan t o r faulte dcontacts with , Jaya h Cree k Metabasalt ;appears t o grad e int o Kallal a Quartzit e(not know n whic h i s olde r unit) ;intruded b y Sybell a Granite , amphiboliti cmetadolerite, dolerite , an d pegmatit e10

and mafic volcanics are the predominant rock types inthe Saint Ronans Metamorphics (Table 1), the oldestunit exposed west of the Rufus Fault Zone. The felsicmetavolcanics contain variable amounts of epidote,little or no magnetite, abundant thin layers and lenticlesrich in metamorphic biotite, and quartz and feldsparphenocrysts which tend to be concentrated inthin layers alternating with phenocryst-poor or phenocryst-freebands. Interlayered schists locally containandalusite porphyroblasts, generally replaced by muscoviteor chlorite. Lenses of fine to medium-grainedamphibolite and amygdaloidal metabasalt consist ofgranular aggregates of mainly hornblende and relativelycalcic plagioclase (An>17). indicating amphibolitefaciesmetamorphism. The replacement of andalusiteporphyroblasts by white mica and chlorite, the partialreplacement of biotite by chlorite, and the presence ofbent mica Hakes in some of the schists indicate laterretrograde metamorphism and deformation.The Saint Ronans Metamorphics may be equivalentto the Sulieman Gneiss east of the Rufus Fault Zone,to part of the undivided Tewinga Group of the centralarea, and to the Yaringa Metamorphics in MOUNTISA. Other possible correlatives are the LeichhardtVolcanics and even the much younger Bottletree Formationof the central area.Fig. 10 . Bande d gneis s wit h boudinage d quartz-feldspa rpegmatite veins , Suliema n Gneis s 3 9 k m southwes t o fDajarra (a t G R UA199700) . M2633/11 )lenses of amphibolite and mafic schist and gneiss whichare thought to be mainly metamorphosed mafic dykesand sills; however, some of these lenses contain possibledeformed amygdalcs and include thin bands ofquartzite and epidotic quartzite, indicating that somemafic lava flows may be present. The formation hasbeen tightly folded about steeply dipping to vertical,north to northeast-trending axial planes. A fracturecleavage is well-developed in the hinge zones of thesefolds. The Kallala Quartzite may be equivalent toquartzite mapped as Mount Guide Quartzite andreported to overlie the May Downs Gneiss west of theMount Isa Fault in MOUNT ISA (Hill & others,1975).Saint Ronans MetamorphicsRegionally metamorphosed argillaceous and arenaceoussediments and extensively recrystallised felsicJayah Creek MetabasaltThe Jayah Creek Metabasalt (Table 2) crops outin a wide north-northwest-trending band in theeastern belt of the western area. Many of the individuallava flows in the formation, especially in the easternpart of the outcrop area, have well-preserved amygdaloidalzones and marginal breccias. Mafic rocks in thewest, adjacent to intrusions of Sybella Granite, aremore intensely foliated and more coarsely recrystallisedthan those to the east. Thin bands of relativelycoarse-grained non-foliated to schistose mafic rocks inthe sequence are probably dykes and sills.The mafic volcanics are interlayered with beddedsediments ranging in thickness from less than 1 m upto about 2000 m. These may represent shallow-marineor fluvial deposits. Argillaceous metasediments arecommon in the northwest, but elsewhere arenaceousmetasediments predominate. The thickest sedimentarylayer, the Timothy Creek Sandstone Member, forms aprominent north-northwest-trending range of closelyspaced planated strike ridges.The presence of cordierite in interlayered mctaargillitesand of co-existing calcic plagioclase (An>17)and hornblende in metabasalt indicate amphibolitefaciesregional metamorphism. Thermal contact-metamorphiceffects, such as hornfels, are generallyapparent only within 5 m or less of Sybella Graniteintrusions.The Jayah Creek Metabasalt may be equivalent tothe Eastern Creek Volcanics of the central area. TheTimothy Creek Sandstone Member is possibly a timeequivalent of the Lena Quartzite Member (Derrick &others, 1976b) of the Eastern Creek Volcanics.Oroopo MetabasaltThe Oroopo Metabasalt is exposed west of theRufus Fault Zone, mainly in the far southwest. It

consists of basaltic lava flows and interlayered sedimentaryrocks (Table 2) which have been regionallymetamorphosed to the upper greenschist or loweramphibolite facies. The mafic metavolcanics are lithologicallysimilar to those of the Jayah Creek Metabasaltto the east and the Eastern Creek Volcanics ofthe Dajarra Belt in the central area, which are possiblecorrelatives. The sedimentary rocks in the OroopoMetabasalt. and also in the Jayah Creek Metabasalt,are generally finer-grained than those in the EasternCreek Volcanics of the Dajarra Belt, an indication thatthe sedimentary source may have been to the east. Asequence of meta-arenite, calcareous meta-arenite, andlimestone exposed in the centre of a partly faultboundedstructural basin at about GR UA1274 istentatively mapped as Oroopo Metabasalt, but maybelong to a younger unit.Haslingden GroupRocks assigned to the Mount Guide Quartzite,Eastern Creek Volcanics, and Judenan beds of theHaslingden Group (Table 2) crop out in the northernpart of the western area, west of the Mount Annableand Mount Isa Faults. They are generally more highlydeformed and more extensively recrystallised thanthose of the Haslingden Group rocks of the centralarea: they have been regionally metamorphosed mainlyto the amphibolite facies and are commonly foliated,and facing evidence is generally lacking. Schist andgneissic migmatitic rocks mapped as Eastern CreekVolcanics adjacent to the Sybella Granite near GRUB3140 have a crenulated foliation. The contactaureole of this granite pluton also contains quartzofeldspathiccordierite-garnet-biotite hornfels—somewith partly retrogressed sillimanite—and granoblasticquartz-bearing amphibolite which are also mapped asEastern Creek Volcanics.The arenaceous and argillaceous metasedimentswhich make up the Judenan beds may be equivalentto the upper part of the lithologically similar MyallySubgroup (Derrick & others, 1976b) exposed east ofthe Mount Isa Fault to the' north, as suggested byMock (1978). However, the stratigraphy of theJudenan beds is obscured by faulting, tight folding,and intrusions of Sybella Granite, and part or all ofthe unit may represent interbeds within the EasternCreek Volcanics, rather than an overlying sequence.Carters Bore RhyoliteThe Carters Bore Rhyolite (Table 3) crops out onthe limbs of a syncline in the far northwest. A samplefrom the formation to the north has been isotopicallydated at 1678 ± 1 m.y. (Page, 1978).McNamara GroupThe McNamara Group (Table 3) is preserved in thecentre of the small syncline in the far northwest, whereit has a maximum thickness of about 400 m. The twoformations present, Torpedo Creek Quartzite andGunpowder Creek Formation, extend northwards intoMOUNT ISA, where the Torpedo Creek Quartzite hasbeen mapped as basal Gunpowder Creek Formation(Hill & others, 1975). The McNamara Group sedimentsare thought to have been deposited in a shallowmarineenvironment. The Torpedo Creek Quartzite iscorrelated with the Warrina Park Quartzite of theTABLE 2. DETAILS OF STRATIGRAPHY—OLDER COVER SEQUENCE, WESTERN AREA»-> o0£o „-Z sa o3-81/5Name of unit(reference to definition)max. thickness (m)Judenan bed s(Carter & others , 1961 ;Derrick & others , 1976b )1000?Eastern Cree k Volcanic s(Carter & others , 1961 ;Derrick & others , 1976b )1000+?Mount Guid e Quartzit e(Carter & others , 1961 ;Derrick & others , 1976b )1000+?Jayah Cree k Metabasal t(Blake & others , 1981a )15 000?Timothy Cree kSandstone Membe r2000Oroopo Metabasal t(Blake & others . 1981a )1000 +LithologySchistose an d cleave d sericiti c an d feldspathi c meta -arenite, quartzit e (commonl y ferruginous) , quartz -mica schist , an d spatte d gre y mic a schist ; mino rgneiss. Primar y texture s largel y obliterate d b yfaulting, tigh t folding , an d granit e intrusio nInterlayered amphibolite , meta-arenite , quartzite ,and schistos e t o gneissi c metasediments , som e o fwhich contai n cordierit e and/o r sillimanite ; mino rmigmatitePebbly, quartzose , feldspathic , an d sericiti c meta -arenites, an d quartzite . Primar y texture s poorl ypreserved. Commonl y schistos eSlightly t o highl y foliated , variabl y epidotic ,amphibolitic metabasalt ; subordinat e quart z +biotite ± muscovit e ± feldspa r ± cordierit e schis tand gneiss , quartzite , pebbl y meta-arenite , musco -vite ± quart z schist ; mino r siliceou s an d micaceou smetasiltstone, para-amphibolite , greywacke , meta -greywacke, recrystallise d limestone , impur e lime -stone, an d calcareou s meta-arenite . Cross-beddin gand rippl e mark s commo n i n E an d fa r SW , bu tlargely obliterate d b y well-develope d cleavag e i nW. Smal l isoclina l fold s an d crenulation s i ngneissic rock s adjacen t t o Suliema n Gneis s S W o fDajarraMeta-arenite; mino r metasiltstone , quartzite , meta -basalt, an d thi n conglomerat e lense sPartly epidoti c amygdaloida l metabasalt ; epidoti cquartzite; quartzose , feldspathic . an d calcareou smeta-arenites an d metasiltstones ; recrystallise dlimestone; dolomite ; meta-arkose ; som e conglo -meratic an d gritt y meta-arenit e lenses ; mino rchlorite schist . Rippl e mark s an d cross-beddin gcommon i n mela-arenite sRelationshipsThought t o b e conformabl e o n Easter nCreek Volcanics ; overlai n unconform -ably b y Carter s Bor e Rhyolite ; intrude dby Sybell a Granit eThought t o b e overlai n conformabl y b yJudenan bed s an d unconformabl y b yCarters Bor e Rhyolit e an d Torped oCreek Quartzite ; intrude d b y Sybell aGranite an d amphiboliti c metadolerit eContacts wit h othe r formation s poorl yexposedConcordant o r faulte d contact s wit hSulieman Gneiss ; concordan t wit hKallala Quartzite ; intrude d b y Sybell aGranite, metadolerite , an d mino r quart zporphyry, quartz-feldspa r porphyry , an dnon-porphyritic rhyolit e dyke sConformable laye r withi n Jaya h Cree kMetabasaltOverlies Sain t Ronan s Metamorphic sapparently concordantl y an d probabl yunconformably; overlai n unconformabl yby Cambria n an d Mesozoi c strata ;intruded b y metadolerit e an d rar e peg -matite (Sybell a Granite? )12

Mount Isa Group, and the Gunpowder Creek Formationis equated with the Moondarra Siltstone andBreakaway Shale (e.g., Plumb & others, 1980). Thegroup has been affected by only low-grade regionalmetamorphism.Mount Isa Group?A poorly exposed sequence of quartzitic and schistosemetasediments preserved in a small and at leastpartly fault-bounded syncline in the south, at aboutGR UA3483, has been tentatively assigned to theundivided Mount Isa Group (Table 3). The sequenceappears to be much less metamorphosed and deformedthan the adjacent Jayah Creek Metabasalt, and hencemay be separated from this formation by a metamorphicunconformity.Sybella GraniteThe Sybella Granite (Carter & others, 1961) includesall granitic rocks exposed in the western area.The several discrete plutons present, none of whichhave been isotopically dated, are part of a major batholith,the Sybella Batholith, which extends into Sheetareas to the north and south of the region. Granitesof the batholith in MOUNT ISA have been dated atbetween 1680 and 1670 m.y. (Page, 1981a).The Sybella Granite includes several different typesof granite (Table 4). The oldest are intensely foliatedto gneissic types which form small plutons withgenerally concordant intrusive contacts (e.g., 10-15km east of Ardmore homestead). They are cut inplaces by mafic dykes which have been regionallymetamorphosed to the amphibolite facies, and are alsointruded by bodies of less foliated to non-foliated andpossibly significantly younger biotite granite, leucogranite,and pegmatite.The narrow elongate pluton of foliated SybellaGranite intruding migmatitic and amphibolitic rocksmapped as Eastern Creek Volcanics near GR UB3140has a core of medium-grained biotite granite containingaugen of K-feldspar, and an outer zone ofrecrystallised crenulated microgranite; aplite dykes,pegmatite veins, and small stocks of foliated leucograniteintrude this granite and adjacent country rocks,and hybrid rocks ranging from quartz-rich granite tomafic hornblende granodiorite have developed locallyalong intrusive contacts (Mock, 1978).The main granite type, biotite granite, forms severallarge plutons. It generally contains sparse to abundantfeldspar phenocrysts up to about 8 cm long, and xenolithsof biotite-rich mafic rock and foliated to gneissicgranodiorite; large inclusions of country rock arecommon in the marginal zones of plutons (Fig. 11).The granite is made up of about equal amounts ofquartz, microcline, and plagioclase, 10-15 percentbiotite, and commonly minor hornblende and muscovite;purple fiuorite is present in places. Myrmekite isalmost ubiquitous, and rapakivi textures are locallycommon. The granite is cut by veins and pods of biotitemicrogranite, pegmatitic granite, pegmatite, andaplite (Fig. 12). The pegmatites commonly containtourmaline, muscovite, and, in the far north, beryl.In general the main granite plutons have sharpintrusive contacts, commonly irregular in detail, whichcut across, without obviously deflecting, the beddingand foliation of the country rocks. Hornfels in adjacentTABLE 3. DETAILS OF STRATIGRAPHY—YOUNGER COVER SEQUENCE, WESTERN AREA— —hhName of unit(reference to definition)max. thickness (m)MOUNT IS A GROUP ?(Derrick & others , 1976c )170Gunpowder Creek Formation(Carter & others , 1961 ;Hutton & others , 1981 )250Torpedo Cree k Quartzit e(Hutton & others , 1981 )150Carters Bor e Rhyolit e(Derrick & others , 1978 )

ocks is generally restricted to within 5 m of intrusivecontacts. Tourmaline, probably resulting from boronmetasomatism, is common in nearby schistose metasediments.In shear zones adjacent to the Rufus FaultZone the biotite granite has been converted to myloniticrock consisting mainly of small quartz and feldsparaugen and muscovite flakes in a fine-grained quartzofeldspathicgroundmass.Small bodies of leucocratic muscovite and biotite—muscovite granite intrude the Saint Ronans Metamorphicswest of the Rufus Fault Zone, and similar leucocraticgranite is exposed about 13 km southeast ofArdmore homestead, where it appears to be a marginalphase of a pluton formed mainly of biotite granite.The leucocratic granite contains pegmatitic patches,scattered white feldspar megacrysts up to about 15 cmlong, and mafic xenoliths, and it is cut by veins oftourmaline-bearing muscovite pegmatite.The field evidence indicates that the massive toweakly foliated granites were probably emplaced bystoping after at least some of the country rocks hadbeen folded and regionally metamorphosed. However,petrographic evidence shows that these granites havethemselves been regionally metamorphosed, probablyto the greenschist facies.The few amphibolitic metadolerite dykes whichcut the older foliated granites may have been intrudedbefore the emplacement of the main granite type. Theyare presumed to be older than a northeast-trendingdyke of apparently unmetamorphosed dolerite whichcuts porphyritic biotite granite west of Dajarra (atGR UB260010).Minor intrusionsDykes and pod-like intrusions of foliated to massivemetadolerite and amphibolite of probably several differentages are widespread in the western area. Theyrange from less than 1 m to more than 1000 m thick,and consist mainly of plagioclase more calcic thanAn,7and hornblende, indicating that they have beenregionally metamorphosed to the amphibolite facies.Fig. II . Inclusion s o f countr y roc k (Jaya h Cree k Meta -basalt) i n porphyriti c granit e a t th e margi n o f a Sybell aGranite pluto n 1 9 k m west-northwes t o f Dajarr a (a t G RUB290070). (M2633/12 )Many of the thicker bodies have fine-grained, extensivelyrecrystallised. foliated margins and coarser, nonfoliatedcentres in which primary igneous textures arepreserved. Some of the dykes contain feldspar phenocrysts,and those cutting the Jayah Creek Metabasalt inthe eastern part of the area commonly have amygdaloidalzones and show compositional banding.A few of the mafic intrusives do not appear to bemetamorphosed. These intrusions, which are probablythe youngest Precambrian rocks in the western area,generally consist of plagioclase (partly sericitised),pyroxene, opaque oxide, and minor primary hornblende,biotite, and interstitial and commonly granophyricquartz and K-feldspar. They may be relatedto the Lakeview Dolerite, which Derrick & others(1977b, 1978) described in MARY KATHLEEN.A few felsic dykes have been found. They aregenerally formed of either quartz ± feldspar porphyryor non-porphyritic rhyolite.StructureFoldsThe Sulieman Gneiss and much of the KallalaQuartzite in the eastern belt display a well-developedfoliation (Fig. 10). However, no large-scale folds havebeen identified in the Sulieman Gneiss, though tightfolds with mainly north to northeast-trending axialplanes are present in the Kallala Quartzite. These foldsmay be synchronous with similarly oriented folds inthe Haslingden and Mount Isa Groups to the northand east, and are thought to postdate the foliationformingevent.The Jayah Creek Metabasalt generally appears toform an ordered succession with only minor internalfolding. However, near plutons of Sybella Granite, theformation is more deformed, has a northerly trendingfoliation which obliterates bedding, and shows somesmall-scale north-plunging isoclinal folds and crenulations.The Saint Ronans Metamorphics in the south of thewestern belt have a well-developed foliation, but nofolds related to this foliation have been identified. Thefoliation is folded around a large south-southeastplunginganticline truncated in the east by the RufusFault Zone, and largely concealed beneath Cambrianand superficial deposits to the west. To the northeast, aprominent synform separates this fold from a similarlyoriented but smaller antiform cored by a SybellaGranite pluton, indicating a possible link between foldingand granite emplacementEquivalent folds in the overlying Oroopo Metabasalt,and the earliest recognised in the formation, are opensouth-plunging structures. The Oroopo Metabasalt wassubsequently extensively deformed along the RufusFault Zone, where bands of kinked and crenulatedchlorite and quartz-muscovite schist have beendeveloped.North to northwest-plunging open to tight foldswith wavelengths of up to 1 km characterise theHaslingden, Mount Isa, and McNamara Groups.Farther north, some folding of the Haslingden Groupevidently occurred before the Mount Isa andMcNamara Groups were deposited (Derrick & others,1980), but no such early folding has been recognisedin the western area. Drag-folds are common adjacentto faults, especially in the Mount Isa Group (cf.Wilson, 1973).14

Fig. 12 . Porphyriti c granit e cu t b y vein s o f microgranite , Sybell a Granit e 2 0 k m wes t o f Dajarr a (a t G R UB017260) ,(M2633/8)FaultsThe oldest faults recognised in the western area arenortherly trending fractures, not shown on the accompanyingmap, developed along lithologic boundariesparallel to bedding and foliation trends. Some of theseare marked by metadolerite dykes and, in the SaintRonans Metamorphics, possibly by quartz and pegmatiteveins. They are displaced by a conjugate setof more-or-less vertical northeasterly and northwesterlytrending faults which postdate the latest folding andhave apparent lateral displacements of up to 1 km;those trending northeast are mainly dextral strike-slip,and those trending northwest are mainly sinistral strikeslip.Movements along the conjugate set of faults indicatean overall east-west compression.The largest faults are the north-northeasterly trendingMount Annable Fault and those of the Rufusand Mount Isa Fault Zones, and the north to northwesterlytrending Wonomo Fault, which is truncatedto the northwest by the Rufus Fault Zone (Fig. 7).The Rufus Fault Zone and Mount Annable Fault maybe part of the right-lateral Mount Remarkable faultsystem (Derrick & others, 1980). No displacementshave been measured along these major and probablylong-acting faults, but the difficulty in directly correlatingany units across them implies movements ofmany kilometres. The preservation of Cambrian sedimentsin the Ardmore Outlier (de Keyser & Cook,1972), essentially a graben within the Rufus FaultZone, indicates that some movement along this faultzone took place after the deposition of the PhanerozoicGeorgina Basin succession. Cambrian sedimentsare also displaced by some faults of the conjugate setin the far southwest.Metamorphic effectsCalc-silicate rocks and amphibolite within the SuliemanGneiss have amphibolite-facies mineral assemblages,characterised by co-existing clinopyroxene andoligoclase/andesine, and hornblende and andesine/labradorite, respectively. Quartzofeldspathic gneissesin this unit commonly have triple-point junctionsbetween grains, and locally display myrmekitic intergrowthsof quartz and plagioclase. The SuliemanGneiss also contains thin pegmatite veins that mayhave been produced by partial melting during metamorphism.The adjacent Kallala Quartzite is extensively recrystallised,and contains metabasite lenses with mineralassemblages indicating amphibolite-facies metamorphism.The Jayah Creek Metabasalt to the east hasalso been regionally metamorphosed to the amphibolitefacies.Gneissic phases of the Sybella Granite are cut byrare amphibolite-facies metadolerite dykes, implying asimilar grade for these phases, but the metamorphicgrade of the younger and generally less recrystallised15

and deformed main phase of biotite granite mapped asSybella Granite may be lower. Although most SybellaGranite plutons do not have marked contact aureoles,the metamorphic grade of the Jayah Creek Metabasaltgenerally increases towards the granite, indicating thatthere may be a genetic relationship between the graniteand the regional metamorphism.In the Saint Ronans Metamorphics in the southwestthe presence of extensively recrystallised felsicand mafic metavolcanics, metasediments with a welldevelopedschistosity which commonly obliteratesbedding, meta-argillites with andalusite porphyroblasts,and metabasites with coexisting hornblende and calcicplagioclase (An> ] 7) indicates amphibolite-faciesregional metamorphism. Mineral assemblages in theOroopo Metabasalt to the north and south indicatethe upper greenschist to lower amphibolite facies.The Haslingden Group in the north consists mainly,if not entirely, of amphibolite-facies rocks. Metabasiteswithin the Eastern Creek Volcanics contain hornblendeand relatively calcic plagioclase, and cordierite andsillimanite are developed in some associated metaargillites.Cordierite and garnet in meta-argillites adjacentto intrusions of Sybella Granite are attributedto contact metamorphism.In the far northwest, the McNamara Group andprobably the Carters Bore Rhyolite, and also rockstentatively assigned to the Mount Isa Group, havebeen regionally metamorphosed to only the greenschistfacies.PRECAMBRIAN O F TH E CENTRA L ARE AThe central area, bounded to the west by theWonomo/Mount Annable/Mount Isa fault systemand to the east by the Pilgrim Fault Zone, is madeup of three main structural elements (Fig. 7): theKalkadoon-Leichhardt Block, formed mainly of basementrocks; the Duchess Belt, to the east, whichconsists of basement and older cover rocks—includingthe Corella Formation—and granites of the WongaBatholith; and the Dajarra Belt, to the west, whichcomprises older and younger cover sequences. Thehighly deformed western part of the Duchess Beltis the southerly continuation of the Wonga Belt ofDerrick (1980) and Derrick & Wilson (1981a,b).Details of stratigraphic and intrusive units in thecentral area are listed in Tables 5-10.The oldest rocks exposed are considered to begneisses and schists mapped as Plum Mountain Gneissin the Duchess Belt and undivided Tewinga Group inthe Kalkadoon-Leichhardt Block; these rocks—togetherwith the Leichhardt Volcanics of the Tewinga Group,the Kalkadoon Granite, and possibly the CorellaFormation (a unit of mainly calcareous metasedimentsforming most of the Duchess Belt)—make up thebasement of the central area. The older cover comprisesthe Bottletree Formation and Haslingden Groupof the Dajarra Belt; the Magna Lynn Metabasalt andArgylla Formation of the Tewinga Group, which cropout within the Kalkadoon-Leichhardt Block and alongthe western margin of the Duchess Belt; and theBallara Quartzite, Corella Formation?, StanbrokeSandstone, and Makbat Sandstone, which are confinedto the Kalkadoon-Leichhardt Block. Units ofthe younger cover in the central area—Carters BoreRhyolite?, Surprise Creek Formation?, and Mount IsaGroup—crop out only in the Dajarra Belt. Granitesof the Wonga Batholith, which are thought to postdatethe basement and older cover units, have been emplacedwithin both the Kalkadoon-Leichhardt Block andDuchess Belt, and bodies of Mount Philp Breccia cutthe Corella Formation of the Duchess Belt.The Proterozoic units are intruded by metadoleriteand amphibolite, which mainly form dykes withnortherly trends, and also by some felsic dyke-likebodies and a few younger dykes of non-metamorphoseddolerite. One felsic minor intrusion which intrudesthe Mount Guide Quartzite of the Haslingden Groupin the Dajarra Belt has been formally named theGarden Creek Porphyry.Plum Mountain GneissThe Plum Mountain Gneiss (Table 5) is a complexunit exposed in the western part of the DuchessBelt south of Duchess. It appears to consist mainlyof felsic volcanics, subordinate interlayered partlyvolcaniclastic sedimentary rocks, and abundant concordantto discordant granitic intrusives, all of which havebeen regionally metamorphosed to amphibolite-faciesgneissic rocks; it is also taken to include metasedimentsthat Blake & others (1982a) mapped as possibleCorella Formation 13 km east-southeast of Stanbrokehomestead. Most of the non-intrusive gneissic rocksmay be correlatives of the undivided Tewinga Groupto the west, and some may be stratigraphic equivalentsof the Corella Formation to the east. They are olderthan the Leichhardt Volcanics as they are intrudedby the One Tree Granite, which is overlain by theLeichhardt Volcanics. Contacts with adjacent Precambrianunits are generally concealed.Tewinga GroupAs defined by Derrick & others (1976a), theTewinga Group (Table 5) includes the oldest rocksexposed in the Kalkadoon-Leichhardt Block, formspart of the crystalline basement on which cover rocksof the western and eastern successions (mainly theHaslingden, Malbon, and Mary Kathleen Groups)were deposited, and comprises three formations: theLeichhardt Metamorphics (the oldest). Magna LynnMetabasalt, and Argylla Formation (the youngest).In the Duchess-Urandangi region the LeichhardtMetamorphics of these and earlier workers aremapped partly as Leichhardt Volcanics, a formation ofreadily recognisable felsic volcanic rocks, and partlyas undivided Tewinga Group (i.e., not subdivided intoformations), which consists mainly of gneissic andschistose metamorphic rocks rather than volcanicrocks. A major unconformity representing about 80m.y. separates the Leichhardt Volcanics from the overlyingMagna Lynn Metabasalt and Argylla Formation.These two younger formations may be younger, ratherthan older, than most of the Haslingden Group, andhence may be part of the older cover sequence, ratherthan part of the basement (Blake. 1980). Consequently,in a strict sense the term 'Tewinga Group', as originallydefined, is probably not a valid stratigraphic unit.However, it has been referred to in numerous recentpublications (e.g., Plumb & others, 1980, 1981; Blake,16

TABLE 5 . DETAILS OF STRATIGRAPHY—PLUM MOUNTAIN GNEISS AND TEWINGAGROUP, CENTRAL AREAName of unit(reference to definition)max. thickness (m)Argylla Formatio n(Carter & others , 1961 ;Derrick & others , 1976a )2250Magna Lvn n Metabasal t(Derrick & others , 1976a )1300Leichhardt Volcanic s(Blake & others , 1981a )1000 +Undivided1000 +Plum Mountai n Gneis s(Blake & others . 1981a )1000 +LithologyReddish brow n t o grey , massiv e t o intensel yfoliated, mainl y porphyriti c felsi c volcanics , in -cluding ignimbrite s showin g fragmenta l textures ,lavas showin g contorte d flow-banding, an d possibl ysubvolcanic intrusions ; mino r agglomerate , bedde dfelsic an d mafi c tuff , metabasalt , an d metasedi -ments—sericitic t o quartzos e meta-arenit e (com -monly showin g cross-beddin g an d rippl e mark soutlined b y lamina e ric h i n heav y minerals) ,conglomerate, meta-arkosc , mic a schist , an d meta -siltstone; cordierite-anthophyllit e roc k expose d atGR UB78766 6Felsic porphyry: generall y magnetic ; contain sdihedral t o anhedra l phenocryst s o f micro -cline. an d commonl y sericiti c plagioclas e an dquartz, i n fine-grained recrystallise d quartzo -feldspathic groundmas s containing biotit e ± gree nhornblende ± muscovit e ± sphen e ± granule sand porphyroblast s o f magnetit e + apatit e ± epi -dote ± chlorite ; myrmekit e i s develope d i n som esamples. Phenocryst s commonl y recrystallise d t ogranular aggregate s i n amphibolite-facie s meta -porphyryMassive t o amygdaloida l metabasal t consistin gmainly o f gree n hornblend e an d plagioclase , brec -ciated metabasal t (flow-margi n breccia) , mafi cschist wit h biotit e and/o r chlorit e and/o r amphi -bole; mino r quartzose , epidotic . an d feldspathi cmeta-arenites an d quartzite s locall y showin g cross -bedding, felsi c porphyry , metasiltstone . commonl ydeformed conglomerate . an d thin-bande d t olaminated para-amphibolit e an d biotitc-ric h tuf -faceous? metasediment sBulf, pink , maroon , an d greenis h t o bluis h gre yquartz-feldspar porphyr y (mainl y ignimbrite , bu talso som e flow-banded lav a flows) ; mino r feldspa rporphyry, altere d basalt , bedde d tuff , quartzos e t ovolcaniclastic arenite , an d conglomerate ; little -recrystallised t o locall y foliate d an d schistos eFelsic porphyry: generall y non-magnetic ; contain ssparse t o abundan t phenocryst s les s tha n 5 m macross o f plagioclas e + microclin c + quartz ,and streak y aggregate s o f opaqu e mineral s +biotite + chlorit e + epidot e + sphen e +muscovite. se t i n a fine t o ver y fine-grainedgroundmass o f quart z + alkal i feldspa r ±biotite, sericite , chlorite , an d epidote ; frag -mental an d eutaxiti c texture s common . Pheno -crysts recrystallise d t o granula r aggregate s i nplacesPink t o grey , massiv e t o banded , fine t o coarse -grained gneis s an d auge n gneiss , migmatiti c gneis sand schis t showin g streak y an d contorte d compo -sitional bandin g an d concordan t an d discordan tveins an d patche s o f apliti c t o pegmaliti c granite :minor gneissi c felsi c agglomerate , metarhyolit e lava ,massive t o schistos e quartzite . bande d calc-silicat erocks, conglomerate , an d massiv e t o schistos emetabasalt; foliatio n locall y crenulate dFelsic rocks: non-magneti c t o intensel y magnetic ;consist o f quart z + microclin e + plagioclas e+ biotit e ± hornblende , muscovite . epidote .garnet, sphene . allanite . apatite , chlorite , fluorite.opaque minerals . tourmaline , an d zircon ;myrmekite commo nMetabasalt: consist s o f plagioclas e + hornblend e+ quart z + epidot e ± biotite , garnet , sphene ,opaque minerals , an d chlorit ePink l o grey , massiv e t o banded , fine t o coarse -grained gneis s an d auge n gneiss ; concordan t t ocross-cutting, slightl y t o intensel y foliated , even -grained t o porphyriti c granit e containin g biotit e ±hornblende ± garnet , leuco-granite , aplite . an dpegmatite; bande d micaceous , feldspathic , an darkosic meta-arenite , mic a schist , bande d calc -silicate rocks , para-amphibolite , an d schistos e meta -basalt; foliatio n crenulate d i n place sRelationshipsConformable o n Magn a Lyn n Meta -basalt; overlai n concordantl y an d pos -sibly conformabl y b y Ballar a Quartzit eand unconformabl y b y Stanbrok e Sand -stone; concordan t contac t wit h Corell aFormation. Intrude d b y Bird s Wel l an dMairindi Cree k Granites , Bush y Par kGneiss, an d probabl y Bowler s Hol eGranite, an d b y metadolerit e an d som efelsic dyke sUnconformable o n Leichhard t Vol -canics; overlai n conformabl y b y Argyll aFormation an d unconformabl y b y Stan -broke Sandstone ; concordan t contac twith Corell a Formation . Intrude d b yBowlers Hole , Mairind i Creek , an d Bird sWell Granites , Bush y Par k Gneiss , an dmafic an d felsi c dyke sOverlies On e Tre e Granit e an d possibl yunnamed monzonit e (ma p uni t E?g,„) ;may b e unconformabl e o n undivide dTewinga Group ; overlai n unconformabl yby Magn a Lyn n Metabasalt , Stanbrok eSandstone, an d probabl y Makba t Sand -stone. Intrude d b y Will s Creek , Woo -nigan. Bowler s Hole , an d Bird s Wel lGranites, probabl y b y Kalkadoo nGranite, an d b y mafi c an d felsi c dyke sOverlain unconformabl y b y Bottletre eFormation, b y Moun t Guid e Quartzit e(Yappo Member) , probabl y b y Stan -broke Sandstone , an d possibl y b yLeichhardt Volcanics . Intrude d b yKalkadoon, Will s Creek , an d probabl yOne Tre e Granites , b y granit e mappe das possibl e Wooniga n Granit e (WS W o fDuchess), an d b y mafi c an d felsi c dyke sConcordant contac t wit h Corell a Forma -tion t o E . Intrude d b y On e Tree , Sain tMungo. an d Bird s Weil ? Granites . Bush yPark Gneiss , an d Metadolerit e dykes .17

lent of the Leichhardt Volcanics; this metadacite isalso chemically similar to the Leichhardt Volcanics(Bultitude & Wyborn, 1982). Similarities in chemistryindicate that other undivided Tewinga Group rocksmay be equivalent to the Bottletree Formation (Bultitude& Wyborn, 1982).Fig. 13 . Migmatiti c gneis s o f th e undivide d Tewing a Grou pat Yarrama n Bore , 3 3 k m northwes t o f Duches s (a t G RUB558578). (G B 2339 )1980), and is currently being used by Derrick and coworkersnorth of the Duchess-Urandangi region, soits usage is considered justified on the grounds of continuityand general acceptance.Undivided Tewinga GroupThe undivided Tewinga Group crops out in the westof the Kalkadoon-Leichhardt Block. It consists ofmainly gneissic rocks, some of which arc migmatitic(Fig. 13), and is considered to represent a complexsequence of felsic and minor mafic volcanic rocks,interlayered sedimentary rocks which are largelyvolcaniclastic but include some quartz arenite, andprobably some intrusions of felsic porphyry andgranite, all regionally metamorphosed to the amphibolitefacies. Because of structural complexities, relativelyhigh metamorphic grade, and a lack of facingevidence and useful marker beds, no stratigraphicsuccession has been determined within the sequence.Many of the gneisses are broadly banded fine tomedium-grained quartzofeldspathic rocks containingwispy mafic inclusions, and are possible meta-ignimbrites(Fig. 14). Some are massive, show vaguely preservedcontorted flow-banding, and probably representmetarhyolite lava. Gneissic coarse fragmental rocks,possibly representing agglomerate, are also present.The gneisses commonly contain megacrysts and augenof feldspar, which may be relict phenocrysts, andstreaky clots of quartz.The quartzofeldspathic gneisses of the undividedTewinga Group may be correlatives of similar gneissesin the Plum Mountain Gneiss to the east and in theSulieman Gneiss of the western area. They aregenerally in faulted contact with the adjacent littlemetamorphosedfelsic porphyries of the LeichhardtVolcanics, and were thought by Blake (1980) to beprobably a much older unit separated from the LeichhardtVolcanics by a period of tectonism and regionalmetamorphism. However, extensively recrystallisedmetadacite from the undivided Tewinga Group west ofBushy Park homestead has yielded an imprecise zirconage of between 1870 and 1850 m.y. (Page, 1983a),and so may be either an extrusive or intrusive equiva­Leichhardt VolcanicsThis formation is equivalent to most of the LeichhardtMetamorphics mapped in Sheet areas to thenorth (and also to the informally named Standishvolcanics of Blake & others, 1978; Bultitude & others,1978; and Blake, 1980). It is termed the LeichhardtVolcanics in the Duchess-Urandangi region becauseit consists predominantly of readily recognisable felsicvolcanic rocks, not metamorphic rocks. To designatesuch rocks as metamorphics is not only misleading—in that almost all other Precambrian stratigraphic unitsin the region are as metamorphosed or more so—butalso conceals the essentially volcanic nature of theformation. Gneissic rocks previously mapped in thecentral area as Leichhardt Metamorphics (Carter &Opik, 1963) are now assigned to the undividedTewinga Group.The Leichhardt Volcanics crop out in a series ofnortherly trending belts in the central and easternparts of the Kalkadoon-Leichhardt Block. The formationconsists mainly of massive porphyritic felsic ignimbrite(Fig. 15), although it also includes some flowbandedfelsic lava and rare bedded volcaniclasticsedimentary rocks, tuff, and basaltic lava. A basalconglomerate containing clasts of granite is presentlocally in the south, where the formation overlies theFig. 14 . Interbande d felsi c fine-graine d gneis s an d auge ngneiss (metamorphose d bedde d felsi c tuffs? ) o f th e un -divided Tewing a Grou p 2 5 k m southeas t o f Dajarr a (a t G RUA555794). (M2633/13 )18

One Tree Granite. This granite is cut by dykes ofporphyry which are pctrographically and chemicallysimilar to the overlying Leichhardt Volcanics.Contacts of the Leichhardt Volcanics with theundivided Tewinga Group arc either not exposed orare marked by faults. However, across contacts thedegree of recrystallisation changes abruptly from littlealteredporphyry (probably mainly greenschist facies)to quartzofeldspathic gneiss (amphibolite facies). Thisindicates that the porphyry of the Leichhardt Volcanicsmay be younger than the undivided Tewinga Grouprocks, and may postdate a major regional metamorphicevent affecting the gneissic rocks (Blake, 1981b,1982).The Leichhardt Volcanics are probably intrudedby the Kalkadoon Granite, although relationships atsome exposed contacts are equivocal. Similarities inchemistry and isotopic age indicate that these two unitsare comagmatic (e.g., Bultitude & Wyborn, 1982;Wyborn & Page, 1983).No fold structures have been mapped within thebelts of Leichhardt Volcanics because of the predominanceof massive (unbedded) porphyries and alack of marker units. However, in several of the beltsthe formation is overlain by either the Magna LynnMetabasalt, Stanbroke Sandstone, or Makbat Sandstonepreserved in synclines or half-synclincs (one half'removed' by faulting), and the belts are commonlybounded by probably older rocks (mainly undividedTewinga Group). Hence the Leichhardt Volcanics mayFig. 16 . Quartz-fille d vesicle s i n mafi c lav a o f th e Magn aLynn Metabasal t 3 7 k m nort h o f Duches s (a t G RUB807739). (M2633/14 )be largely confined to major synclinal structures andto downfaulted blocks.Two samples of felsic porphyry from the LeichhardtVolcanics, one from southeast of Dajarra and theother from north of Duchess, have been isotopicallydated at 1875 +] j m.y. (Page, 1983a), indicatingthat the formation is similar in age to the LeichhardtMetamorphics—dated at 1865 ± 3 m.y. (Page, 1978)—north of the region.Fig. 15 . Felsi c ignimbrit e o f th e Leichhard t Volcanic s 1 6km west-northwes t o f Duches s (a t G R UB689470) .(GB 2325 )Magna Lynn Metabasalt and Argylla FormationThe Magna Lynn Metabasalt and conformablyoverlying Argylla Formation crop out in the northernpart of the Kalkadoon-Leichhardt Block and in theadjoining part of the Duchess Belt. They have beenregionally metamorphosed to mainly the amphibolitefacies, and are most intensely deformed and recrystallisedadjacent to the Bowlers Hole Granite in theKalkadoon-Leichhardt Block and adjacent to theCorella Formation in the Duchess Belt, where theMagna Lynn Metabasalt is intruded by the Bushy ParkGneiss. The Argylla Formation north of the area hasbeen dated at 1777 ± 7 m.y. (Page, 1978) and 1783± 5 m.y. (Page, 1983a).The Magna Lynn Metabasalt consists mainly ofmafic lavas (Fig. 16), but also includes some intercalatedmetasediment (Figs. 17, 18) and minor felsicmetavolcanics of Argylla type (Fig. 19). The ArgyllaFormation consists mainly of felsic metavolcanics19

Fig. 17 . Steepl y dippin g bed s forme d mainl y o f mafi c volcani c detritus , Magn a Lyn n Metabasal t nea r th e Lad y Fann ymine, 1 4 k m north-northwes t o f Duches s (a t G R UB748506) . (G B 2364 )Fig. 18 . Deforme d conglomerat e containin g mafi c volcani c ^^^"t - •^"^•^^^'"^•^••^^ —

which are generally strongly magnetic (in contrastto those of the Leichhardt Volcanics), but also includessome metasediments, especially in the upper part of theunit. Most of the felsic rocks probably represent ignimbrites,but flow-banded lava, bedded tuff and coarserfragmental rocks, and possible subvolcanic intrusivebodies are also present. At GR UB735495, the Argyllasequence includes a metabasalt lava as well as at leastone flow-banded felsic lava. Some interfingering and' 'Jjf- — • » ^ ^ ^ f lFig. 20 . A resul t o f contemporaneou s felsi c an d mafi cvolcanism—mafic lav a spatte r i n felsi c tuf f a t th e contac tbetween th e Magn a Lyn n Metabasal t an d Argyll a Forma -tion 3 9 k m nort h o f Duches s (a t G R UB769766) .(M2231/29)mixing of felsic and mafic metavolcanics commonlyoccurs at the base of the Argylla Formation and top ofthe Magna Lynn Metabasalt (Fig. 20), indicating thatthe two formations are similar in age.Bottletree FormationWe consider that the Bottletree Formation (Table6) is the oldest unit of the western succession andhence is part of the older cover sequence. However,Derrick & Wilson (1981b) and Derrick (1982) haveregarded it as part of the basement. The formationcrops out in the Dajarra Belt, and unconformably overliesthe undivided Tewinga Group and KalkadoonGranite along the western margin of the Kalkadoon-Leichhardt Block. Main rock types are felsic lava,ignimbrite, bedded tuff (Fig. 21), and greywacke-typesedimentary rocks, including conglomerate, but somebasaltic lavas and other sedimentary rocks are alsopresent; all of the rocks have been regionally metamorphosedto the amphibolite facies. The sedimentsare thought to have been deposited as fans in a partlyfluvial and partly shallow-marine environment flankinga landmass to the east.The conglomerates in the formation contain clastsderived from penccontemporaneous volcanics andfrom basement rocks, including granite, to the east(Fig. 22). Clasts of felsic volcanics generally predominate;many of them are intensely flattened, evenwhere other clasts are little deformed, and may originallyhave been pumice.The formation ranges in thickness from 0 to about3000 m. This variation is attributed partly to depositionon a surface having a considerable topographicrelief, and partly to localised accumulations of volcanicrocks and fanglomerate-type sediments.The upper boundary of the formation is placed at21

Fig. 22 . Granit e boulde r i n steepl y dippin g greywacke-conglomerat e o f th e Bottletre e Formatio n 3 0 k m west-northwes tof Duches s (a t G R UB525459) . (M2230/3 )the top of the uppermost readily recognisable volcaniclayer. In the overlying Yappo Member of the MountGuide Quartzite, thin beds of probable felsic tuff, greywacke-typesedimentary rocks identical with those ofthe Bottletree Formation, and bedding parallel to thatin the Bottletree Formation are taken to indicate thatthe two units are conformable (e.g., Blake, 1980,1981b); indeed Bultitude & others (1977) originallymapped the two units as a single unit, informallynamed the Rifle Creek beds. However, Derrick &Wilson (1981b) have suggested that the contactbetween the two units is an unconformity.Metamorphosed dacitic volcanics of the BottletreeFormation from west of Bushy Park homestead andfrom 25 km to the north (in MARY KATHLEEN)have been isotopically dated at 1808 m.y. and1790 ± v £ m.y. respectively (Page, 1983a). In the surveyof MARY KATHLEEN, the formation was mappedas Argylla Formation (Derrick & others, 1977b); itscorrelation with the 1780-m.y.-old Argylla Formationcropping out to the east has continued to be favouredby Derrick & Wilson (1981b), but not by Blake (1980,1981b), who considers that the Bottletree Formationis probably the older unit.Haslingden GroupThe Haslingden Group (Table 6) crops out in theDajarra Belt, where it is represented by the MountGuide Quartzite, which includes the Yappo Member,and the Eastern Creek Volcanics. These units, whichare part of the older cover sequence, have beenregionally metamorphosed to the greenschist facies andFig. 23 . Conglomerat e i n th e Yapp o Membe r o f th e Moun tGuide Quartzit e 2 0 k m north-northwes t o f Dajarr a (a t G RUB427204). (M2633/16 )22

more extensively to the amphibolite facies, andcommonly schistose.Mount Guide QuartziteThis formation is made up of a named lowermember, the Yappo Member, and an unnamed uppermember. The Yappo Member consists mainly of greywacke-typemetasediments, commonly conglomeratic(Fig. 23), identical with many of the metasediments inthe underlying Bottletree Formation; it similarlyrepresents outwash fans deposited on the flanks of anelevated landmass to the east. It is equivalent to mostof the lower Mount Guide Quartizite in MARYKATHLEEN, to the north (Derrick & others, 1976b,1977b), and grades upwards into quartzose metaareniteof the unnamed upper member, which isthought to have been deposited in a shallow-waternearshore environment. The upper member formsprominent hills and ridges, including Mount Guidein the north.The Mount Guide Quartzite is intruded byaremaficdykes, many of which may be related to the basalticlavas of the overlying Eastern Creek Volcanics,although most—according to Glikson & others (1976)—postdate not only the Haslingden Group but alsothe Mount Isa Group.Eastern Creek VolcanicsThis formation comprises a thick sequence of metabasaltlava flows which generally form undulatingterrain, and interlayered metasediments—mainly ridgeformingmeta-arenites but including locally abundantconglomerate beds. No pillow lavas have beenrecorded, and the sequence appears to have been laiddown on a shallow-marine shelf or fluvial plain whichsubsided at about the same rate as the rocks accumulatedupon it. Traces of copper minerals are commonlypresent in the metabasalt lavas. Conglomerates in thesequence contain clasts of quartzite, non-magneticfelsic porphyry probably derived from the LeichhardtVolcanics to the east, and penecontemporaneous metabasalt.TABLE 6 . DETAIL S O F STRATIGRAPHY—BOTTLETREE FORMATIO N AN D HASLINGDE N GROUP ,CENTRAL ARE AName of unit(reference to definition)max. thickness (m)Eastern Cree k Volcanic s(Carter & others , 1961;Derrick & others , 1976b )2000 +Mount Guid e Quartzit e(Carter & others , 1961;Derrick & others , 1976b )Unnamed uppe rmember2000Yappo Membe r(Blake & others ,2750Bottletree Formatio n(Blake & others , 1981a )30001981a)LithologyMassive t o schistos e metabasal t whic h i s commonl yamygdaloidal, brecciate d metabasal t (flow-margi nbreccia), interlayere d metasediments—mediu m t othin-bedded quartzose , feldspathic , an d sericiti cmeta-arenites commonl y showin g cross-bedding ,conglomerate, metasiltstone , recrystallise d lime -stone, para-amphibolit e (bedde d basalti c tuff )Metabasalt: amphibolite-facie s assemblage s typi -cally consis t o f gree n hornblende , oligoclase , andopaque mineral s ± actinolite , albite , epidote ,sphene, biotite , quartz , an d calcite ; greenschist -facies assemblage s typicall y consis t o f albite ,actinolite, quartz , titanomagnetite , epidote ,chlorite, an d K-feldspa rThick t o thin-bedded , massiv e t o schistose ,medium-grained quartzos e t o micaceou s meta -arenite; sericitic , feldspathic , an d lithi c meta -arenites; metagreywack e an d metasiltston e nea rbase; mino r pebbl y beds . Cross-beddin g commo nthroughout, an d rippl e mark s commo n i n uppe rpartPebbly an d gritt y metagreywack e an d metagrey -wacke-conglomerate; mino r meta-arkose , sericitic ,feldspathic, an d quartzos e meta-arenites , sericit eschist, muscovite-biotit e schist , metasiltstone , epi -dotic quartzite , metabasalt , an d pin k t o gre yrecrystallised felsi c tuff? . Cross-beddin g presen t i nplacesConglomerate: clast s o f felsi c volcanic s (includ -ing flattened possibl e pumice) , quartzite , vei nquartz, graniti c t o dioriti c plutoni c rocks , mic aschist, amphibolite , an d phyllit e i n a generall yabundant matri x containin g biotit e ± muscovit e± epidot e ± chlorite ; clast s commonl y mode -rately t o highl y deforme dInterlayered pin k t o gre y rhyoliti c t o daciti c meta -volcanics (pyroclastic s an d lav a flows), an d meta -morphosed greywack e an d greywacke-conglomrat e(identical wit h tha t i n overlyin g Yapp o Member) ;subordinate metabasalt , mainl y nea r bas e an dquartzite, epidoti c quartzite , an d thinl y bande dpara-amphibolite (probabl y mafi c tuff )Felsic metavolcanics: commonl y magnetic , massiv eto foliated ; lava s sho w contorte d flow banding ,quartz-filled vesicles , an d recrystallise d spheru -lites, an d contai n spars e t o abundan t pheno -recrystalised quart z ± biotit e ± hornblend e i nrecrystallised quartzofeldspathi c groundmas scrysts o f feldspa r (mainl y plagioclase ) ~ ±commonly ric h i n biotit e an d magnetit e an dlocally containin g metamorphi c garne tMetabasalt: schistos e t o massive , commonl y amyg -daloidal; consist s mainl y o f gree n hornblend eand oligoclas eRelationshipsOverlies Moun t Guid e Quartzit e wit happarent conformity ; overlai n concor -dantly b y Surpris e Cree k Formation ?and unconformabl y (mainl y discon -formably) b y Warrin a Par k Quartzit eof Moun t Is a Grou pOverlies Yapp o Membe r conformabl yand Kalkadoo n Granit e an d undivide dTewinga Grou p unconformably ; overlai nconcordantly b y Easter n Cree k Vol -canics. Intrude d b y Garde n Cree k Por -phyry an d b y mafi c an d mino r felsi cdykesUnconformable o n Kalkadoo n Granit eand undivide d Tewing a Group ; con -formable o n Bottletre e Formation .Intruded b y mafi c an d som e felsi cdykesUnconformable o n Kalkadoo n Granit eand undivide d Tewing a Group ; over -lain conformabl y b y Moun t Guid eQuartzite (mainl y Yapp o Member) .Intruded b y mafi c dyke s23

The Eastern Creek Volcanics may be similar in ageto the Magna Lynn Metabasalt exposed farther eastin the central area, and to the Jayah Creek Metabasaltand Oroopo Metabasalt of the western area. Theformation has been correlated with the Marraba Volcanics,which overlie the Argylla Formation in theeastern area (e.g., Carter & others, 1961; Plumb &Derrick, 1975; Plumb & others, 1980), but Blake(1980) considers it to be older than both theseformations.Mary Kathleen GroupIn the central area the Mary Kathleen GroupDerrick & others (1977a) is represented by the BallaraQuartzite, Corella Formation, and Corella Formation?(Table 7). These units crop out in theofLeichhardt Block and the Duchess Belt. As rocksKalkadoon-assigned by previous workers to the Corella Formationhere and in MARY KATHLEEN to the northmay be of two or more different ages, as suggestedby Blake (1980, 1981b, 1982), those that cannot becorrelated with reasonable confidence to the type sectionof the Corella Formation in MARRABA areshown as Corella Formation? on the Duchess-Urandangiregion map.Ballara QuartziteThe Ballara Quartzite is exposed in the northernpart of the central area, where it delineates a northplungingsyncline truncated to the north by the FountainRange Fault. It probably represents nearshoreshallow-shelf sediments (Derrick & others, 1977b),and may be partly a correlative of the Stanbroke andMakbat Sandstones, also of the central area, and theTABLE 7 . DETAIL S O F STRATIGRAPHY—STANBROK E SANDSTONE , MAKBA T SANDSTONE ,KATHLEEN GROUP , CENTRA L ARE AAND MARYa*D-Oaw1-1x

Mitakoodi Quartzite of the eastern area. It has beenregionally metamorphosed to the amphibolite facies.the calc-silicate rocks generally represents bedding,but transposed bedding (metamorphic layering) isapparent locally (Fig. 27).The scapolite in the metasediments is thought tohave formed during regional metamorphism of halitebearingbeds (Ramsay & Davidson, 1970), rather thanresulting from regional soda-metasomatism accompaniedby the introduction of chlorine, as suggestedThe nature of the contact between the BallaraQuartzite and underlying Argylla Formation has beendiscussed at some length by Blake (1980, 1981b) andDerrick & Wilson (1981b). Derrick & Wilson maintainthat this contact is a major unconformity representingthe period during which the sediments of the Haslingdenand Malbon Groups were deposited to the west andeast, respectively. Blake, on the other hand, suggeststhat the contact does not mark a significant timebreak, and that in places the Ballara Quartzite mayinterfinger with felsic volcanics of the Argylla Formation.Corella Formation and Corella Formation?The Corella Formation, a unit consisting of beddedcalc-silicate rocks (Fig. 24) which are commonlyscapolitic (Fig. 25), other metasediments, and interlayeredfelsic and mafic metavolcanics, is exposed in abroad band up to 11 km wide in the Duchess Belt,along the eastern margin of the central area. It isbounded to the east by the Pilgrim Fault Zone, and tothe west by intensely deformed metamorphic rocksassigned to the Magna Lynn Metabasalt, Argylla Formation,Bushy Park Gneiss, and Plum MountainGneiss, and by the Saint Mungo Granite. Probablyintrusive felsic gneiss assigned to the Bushy ParkGneiss is interbanded with calc-silicate rocks of theCorella Formation northwest of Mayfield homestead(Bultitude & others, 1982).The calc-silicate rocks are considered to representmetamorphosed impure calcareous and dolomiticsediments deposited in a shallow-marine and locallyevaporitic environment. The formation is tightly foldedabout generally steeply dipping to vertical axial planes(Fig. 26), and mineral assemblages indicate regionalmetamorphism to the amphibolite facies. Banding inFig. 25 . Densel y packe d scapolit e porphyroblast s i n calc -silicate rock , Corell a Formatio n 3 6 k m nort h o f Duches s(at G R UB843738) . (M2633/17 )Fig. 24 . Thin-bedde d calc-silicat e rock s o f th e Corell a For -mation 1 1 k m nort h o f Duches s (a t G R UB790485) .(M2443/6)Fig. 26 . Tightl y folde d calc-silicat e rock s o f th e Corell aFormation 1 1 k m nort h o f Duches s (a t G R r 11790485) .(M2443/29)

quite separate stratigraphic units was suggested byBlake (1980) and has since been discussed by Derrick& Wilson (1981b), who refute the possibility, and byBlake (1981b, 1982). The Corella Formation (= 'old'Corella Formation of Blake, 1980) differs from theCorella Formation? exposed to the west (= 'young'Corella Formation of Blake, 1980) in lithology, thickness,stratigraphic relationships, and perhaps geologicalhistory (Blake, 1982): (1) it contains abundant felsicand mafic volcanics; (2) north of the Duchess-Urandangiregion it is reported to be at least 2000 m thickand may be 4000 m or more thick, whereas the 'young'Corella Formation has a maximum thickness of about1260 m; (3) its stratigraphic relationships are uncertain,as it cannot be shown, beyond reasonable doubt,to overlie the Ballara Quartzite or to underlie anequivalent of the Deighton Quartzite (which overliesthe 'young' Corella Formation in MARY KATH­LEEN); and (4) it appears to have been metamorphosedand deformed before it was intruded bygranite plutons and rhyolite dykes 1740-1720 m.y.ago (e.g., Derrick, 1980; Plumb & others, 1980; Bultitude& others, 1982), whereas the 'young' CorellaFormation was not metamorphosed or significantlydeformed until after the Mount Albert Group (whichincludes the Deighton Quartzite) and probably also the1670-m.y.-old Mount Isa Group had been deposited.The lithologic and thickness differences may be dueto lateral facies changes, as stated by Derrick & Wilson(1981b), or may indicate that the two units are notcorrelatives, the interpretation favoured by Blake.Fig. 27 . Transpose d beddin g i n calc-silicat e rock s o f th eCorella Formatio n 1 5 k m nort h o f Duches s (a t G RUB835535). (M2633/34A )by Edwards & Baker (1954). Scapolite present inassociated metamorphosed igneous rocks presumablyformed during the same regional metamorphic event,when these rocks recrystallised—probably in the presenceof chlorine-rich fluids derived from adjacentsediments (Ramsay & Davidson, 1970).Rocks mapped as Corella Formation? crop outnorthwest of Duchess, mainly in the centre of thenorth-plunging syncline truncated by the FountainRange Fault. Here grey scapolitic and amphiboliticcalc-silicate rocks, representing calcareous siltstone andarenite and impure limestone regionally metamorphosedto the amphibolite facies, lie conformably onthe Ballara Quartzite. The unit is more extensivelyexposed in MARY KATHLEEN to the north, whereit has been mapped as Corella Formation (Derrick &others, 1977b), but its correlation with the main beltof the Corella Formation to the east, which containsthe type section of the formation (Carter & others,1961), is doubted by Blake (sec below).The possibility that the Corella Formation in thecentral area, and in MARY KATHLEEN andMARRABA to the north, may represent two or moreStanbroke SandstoneThis formation, which consists mainly of ridgeformingarenites (Table 7), crops out within the Kalkadoon-LeichhardtBlock in narrow northerly trendingsynclinal belts partly bounded by faults. Thearenites probably represent shallow-marine sediments.They generally appear to be little metamorphosed,except near faults, where they are locally sheared,brecciated, and silicified. Possible correlatives are theMakbat Sandstone in the south and the Ballara Quartzite,Corella Formation?, and Surprise Creek Formation?in the north. Like the Ballara Quartzite theStanbroke Sandstone differs from the Makbat Sandstoneand Surprise Creek Formation? (but not from theSurprise Creek Formation of Derrick & others, 1980,north of the Duchess-Urandangi region) in generallycontaining calcareous and/or dolomitic beds, but unlikethe Ballara Quartzite it mostly lies directly on theLeichhardt Volcanics rather than on the ArgyllaFormation.Makbat SandstoneThe Makbat Sandstone (Table 7) forms lowranges in the southern part of the Kalkadoon-LeichhardtBlock, cropping out in the troughs of relativelyopen north-northwesterly trending synclines. Like itspossible correlative, the Stanbroke Sandstone, the formationwas probably deposited in a shallow-marineenvironment. It appears to be little-metamorphosed,and generally overlies little-metamorphosed felsic porphyriesof the Leichhardt Volcanics.Carters Bore Rhyolite?Interbeddcd porphyritic rhyolitic tuff and quartziteexposed in the far south of the Dajarra Belt aremapped as Carters Bore Rhyolite? (Table 8) and26

hence as part of the younger cover sequence. Thestratigraphic relationships of these rocks are uncertain.Surprise Creek Formation?Clastic sedimentary rocks mapped as Surprise CreekFormation? (Table 8) crop out in the north, on thewestern side of the Kalkadoon-Leichhardt Block. Theyform prominent ridges which continue north intoMARY KATHLEEN, where they were mapped asSurprise Creek beds by Derrick & others (1977b). Theunit, which has been regionally metamorphosed onlyto the greenschist facies, may be partly or entirelyequivalent to either the Surprise Creek Formation orthe Quilalar Formation of Derrick & others (1980).Other possible correlatives are the Myally Subgroup(Derrick & others, 1976b), which forms the upperpart of the Haslingden Group north of the Duchess-Urandangi region, the Ballara Quartzite and StanbrokeSandstone to the east, and the Mount Isa Group tothe west.Mount Isa GroupRocks assigned to the Mount Isa Group (Table 8)are preserved in partly fault-bounded synclinal beltsin the Dajarra Belt, where they have been tightly foldedand regionally metamorphosed to the greenschistfacies. Four constituent formations, each consisting ofprobably shallow-water sedimentary rocks, have beendistinguished. These are, from oldest to youngest, theWarrina Park Quartzite, the Moondarra Siltstone,which is host to several small copper mines andprospects, the Breakaway Shale, which forms lowrubbly ridges with a moderately sparse vegetation, andthe Native Bee Siltstone?, which may alternatively bepart of the Moondarra Siltstone. Non-faulted contactsbetween the Warrina Park Quartzite and underlyingHaslingden Group rocks (Eastern Creek Volcanics)are mainly concordant, but are locally discordant;this discordance appears to be due to tilting,rather than folding, before the Mount Isa Group wasdeposited. The group has been isotopically dated northof the region at about 1670 m.y. (Page, 1981b).Kalkadoon BatholithThe Kalkadoon Batholith is made up of the OneTree, Kalkadoon, Wills Creek, and Woonigan Granitesand a small body of unnamed porphyritic monzonite(Table 9), all of which were mapped as KalkadoonGranite by Carter & others (1961) and Carter & Opik(1963). It probably consists of numerous plutons,and may have been emplaced over several millionyears.The One Tree Granite forms a large intrusion ofmainly foliated biotite granite in the southeastern partof the Kalkadoon-Leichhardt Block. It is exposed aslow hills and tors, as scattered spheroidal boulders onflat to gently undulating terrain, and as small mesascapped by weathered granite and Iaterite. Ten kilometressouth-southeast of Stanbroke homestead thegranite is overlain by felsic porphyries of the LeichhardtVolcanics: a thin bed of conglomerate locallypresent] underlying the volcanics contains fragments ofthe granite, and felsic dykes similar in chemical compositionto the volcanics intrude the granite in thegeneral vicinity.The Kalkadoon Granite forms a large compositebody extending along the western side of the Kalkadoon-LeichhardtBlock from south of the Duchess-TABLE 8 . DETAILS OF STRATIGRAPHY—CARTERS BORE RHYOLITE?, SURPRISE CREEK FORMATION?,AND MOUNT ISA GROUP, CENTRAL AREAMoa>DOO

Urandangi region northwards for more than 250 km.In the central area if consists mainly of massive tointensely foliated, commonly porphyritic (Fig. 28)biotite granite and granodiorite, which are exposedas tors, low hills, and undulating terrain with scatteredspheroidal boulders.Samples of the Kalkadoon Granite from the Cloncurry1:250 000 Sheet area, to the north, have beendated at 1862 +| 7 m.y. (Page, 1978), and a similarage (1856 i; 1^ m.y.) has been obtained on a sampleof the granite from 24 km south-southeast of Dajarra(Wyborn & Page, 1983). These are the oldest graniteages so far determined by the U-Pb zircon methodin the Mount Isa Inlier. The Kalkadoon Granite intrudesthe undivided Tewinga Group and probably theLeichhardt Volcanics (with which it is consideredto be comagmatic), but it predates the Magna LynnMetabasalt and Argylla Formation of the TewingaGroup. It is also older than the Bottletree Formationand Haslingden Group, which overlie it to the west.Migmatitic complexes are developed where gneissicrocks of the undivided Tewinga Group are intricatelyveined by the Kalkadoon Granite. Small net-veinedcomplexes (Blake, 1981a) are present at a few localitieswhere mafic dykes appear to have intersectedpartly molten granite. Recrystallisation and foliationeffects and cross-cutting metamorphosed dolerite dykesshow that the granite has been regionally metamorphosedto the greenschist facies and, more extensively,to the amphibolite facies; Rb—Sr isotopic data indicateTABLE 9. GRANITES OF THE KALKADOON BATHOLITH, CENTRAL AREAName of unit(reference to definition)LithologyRelationshipsWills Cree k Granit e(Blake & others , 1981a )Woonigan Granit e(Blake & others , 1981a )Kalkadoon Granit e(Carter & others , 1961 )Pink t o buff , massiv e (non-foliated) , mediu m t ocoarse-grained, even-graine d t o slightl y porphyritic ,generally leucocrati c biotit e granite ; mino r aplit ewhich i s pyriti c i n places, an d rar e porphyriti c micro -graniteGranite: generall y deepl y weathere d an d friable ;consists o f straine d quartz , microcline/orthoclas eand generall y subordinat e plagioclase , les s tha n 5percent biotite , an d accessor y ana . secondar yallanite, apatite , chlorit e (mainl y afte r biotite) ,epidote/clinozoisite, muscovite , sericite , sphene , an dzircon; n o myrmekit e recorde dPale pink , massiv e (non-foliated) , coars e t o fine -grained, even-graine d t o slightl y porphyriti c leuco -cratic biotit e granit e whic h contain s spars e smal lmafic xenolith s and , in places, inclusion s o f Leichhard tVolcanics an d Kalkadoo n Granite ; mino r aplit e an dporphyritic microgranit eGranite: Generall y deepl y weathere d an d friable ;petrographically simila r t o Will s Cree k Granit ePink t o pal e grey , massiv e t o intensivel y foliated ,medium t o coarse-graine d biotit e granit e an d grano -diorite, commonl y containin g feldspa r phenocryst s u pto 5 c m long , smal l mafi c xenoliths , an d large rinclusions o f countr y rocks ; mino r tonalite , leuco -granite, microgranite , porphyriti c granophyre , diorite ,aplite, an d pegmatit eGranitic rocks: consis t o f straine d an d recrystallise dquartz; oligoclase/andesin e showin g alteratio n t osericite/muscovite an d epidote/clinozoisite ; micro -cline; 5-1 5 percen t biotit e whic h generall y form sfine-grained aggregates , som e pseudomorphin g horn -blende and/o r pyroxene , an d i s commonl y partl yaltered t o chlorit e an d epidote ; an d accessor yopaque minerals , apatite , zirco n ± hornblende ,monazite, muscovite , sphene , an d metamic tminerals (mainl y allanite) ; myrmekit e i s commonl ypresentInclusions: schist , quartzite , meta-arkose , amphibolite ,diorite, and , especiall y nea r contact s wit h un -divided Tewing a Grou p an d Leichhard t Volcanics ,recrystallised felsi c volcanics . Som e metadolerit einclusions hav e rounded , pillow-lik e shape s an dmay for m th e mafi c componen t o f smal l net-veine dcomplexesUnnamed monzonit e Generally massiv e (non-foliated ) monzonite , consist -ing o f pin k tabula r phenocryst s o f microclin e an dscattered smal l xenolith s i n a gre y medium-graine dgranitic groundmas s o f microclin e an d plagioclas e i nabout equa l amounts , abou t 1 5 percen t chloritise dbiotite, an d accessor y apatite , zircon , an d opaqu emineralsOne Tre e Granit e(Blake & others , 1981a )Pale pin k t o pal e grey , massiv e intensel y foliated ,medium t o coarse-graine d biotit e granite , an d dar kgrey, slightl y foliate d t o gneissic , finer-grained biotit egranite; mino r microgranite , aplite , an d pegmatit eGranites: Commonl y contai n feldspa r phenocryst s an dscattered smal l xenoliths ; consis t o f abou t equa lamounts o f plagioclase , microcline , an d straine dquartz, u p t o abou t 3 0 percen t biotite , an d aces -sory an d secondar y allanite , apatite , chlorite ,epidote, opaqu e minerals , sericite , sphene , an dzirconIntrudes undivide d Tewing a Group ,Leichhardt Volcanics , an d probably On eTree an d Kalkadoo n Granites ; cu t b ymafic dykes . Probabl y overlai n b y Stan -broke Sandston e an d Makbat Sandston eIntrudes Leichhard t Volcanics , un -divided Tewing a Group , an d Kalkadoo nGranite; cu t b y dyke s o f metadolerit eand porphyriti c granophyre . Granit emapped a s Wooniga n Granite ? 1 1 k mWSW o f Duches s intrude s undivide dTewinga Grou pIntrudes an d locall y form s migmatiti ccomplexes wit h undivide d Tewing aGroup; probabl y intrude s Leichhard tVolcanics; intrude d b y Wooniga nGranite, Bird s Wel l Granite , an dprobably Will s Cree k Granite , b yinnumerable mafi c dykes , an d b y som efelsic dykes . Overlai n b y Bottletre eFormation, Moun t Guid e Quartzit e(mainly Yapp o Member) , an d Surpris eCreek Formation ?May b e overlai n b y Leichhard t Vol -canics an d intrude d b y dyke s o f felsi cporphyryIntrudes Plu m Mountai n Gneis s an dprobably undivide d Tewing a Group ;inferred t o b e intrude d b y Will s Cree kGranite an d Bird s Wel l Granite? . Over -lain b y Leichhard t Volcanics , an d cu tby felsi c dyke s relate d t o thes e vol -canics; als o cu t by mafi c dyke s28

Fig. 28 . Porphyriti c Kalkadoo n Granit e 3 0 k m wes t o fDuchess (a t G R UB5334S4) . (M2231/3 )that this metamorphism took place no earlier than 1640m.y. ago (Wyborn & Page, 1983).The Woonigan Granite forms mainly poorly exposedplutons spatially associated with the Kalkadoon Granitein tne northern half of the area. It consists mostlyof non-foliated leucocratic biotite granite which maybe correlated with the Wills Creek Granite to thesouth and with leucocratic granite mapped as part ofthe Kalkadoon Granite (part of unit Bgko ) in MARYKATHLEEN, to the north (Derrick & others, 1977b).Biotite granite mapped as possible Woonigan Granitewest and southwest of Duchess may be a more maficvariant.Several separate bodies of Wills Creek Granite,forming mainly low hills and undulating terrain, arepresent in the southern half of the central area. Thegranite is readily distinguished from nearby KalkadoonGranite and One Tree Granite by its non-foliated,even-grained leucocratic character. It intrudes theLeichhardt Volcanics, which are baked and recrystallisedto hornfels adjacent to it. ^Pb/^Pb data onzircon from the granite indicate a minimum age of1830 m.y., which is similar to the minimum agesof zircon fractions from the Kalkadoon Granite(Wyborn & Page, 1983).A small body of unnamed porphyritic monzonite isexposed 20 km northeast of Dajarra. On the westernside of the body, rubbly, rather than hornfelsic porphyryof the Leichhardt Volcanics appears to gradelaterally into rubbly (regolithic?) monzonite, and atleast one dyke-like body of felsic porphyry occurswithin the monzonite, indicating that the monzonitemay predate the Leichhardt Volcanics.Wonga BatholithThe Wonga Batholith, which lies mainly to theeast of the Kalkadoon-Leichhardt Block, is made upof several discrete intrusions, most of which are clearlyyounger than most granites of the Kalkadoon Batho-TABLE 10. GARDEN CREEK PORPHYRY, MOUNT PHILP BRECCIA, AND GRANITES OF THE WONGABATHOLITH, CENTRAL AREAName of unit Lithology Relationships(reference to definition)Garden Cree k Porphyr y Massiv e t o locall y sheare d an d quartz-veine d por - Intrude s Moun t Guid e Quartzit e(Blake & others , 1981a ) phyriti c microgranit e an d mino r metadolerit e (a tmargins)Microgranite: contain s abundan t phenocryst s o f ande -sine an d microcline , som e mor e tha n 1 c m across ,and smalle r rounde d phenocryst s o f straine d an drecrystallised quart z i n a pin k t o gre y fine-grainedgroundmass o f quart z + microclin e + sodi cplagioclase + biotit e + opaqu e mineral s (includ -ing magnetite ) + accessor y apatite , muscovite , an dzircon, an d secondar y chlorit e an d epidote/clino -zoisite; als o contain s biotite-ric h xenoliths , som ewith smal l feldspa r euhedr aMetadolerite: consist s mainl y o f gree n hornblend e an dvariably altere d plagioclas eMount Phil p Brecci a Brecci a o f angula r t o rounde d fragment s (derive d Intrude s Corell a Formatio n an d meta -(Blake & others , 1981a ) fro m adjacen t Corell a Formation ) enclose d i n a pin k dolerite ; ma y b e intrude d b y dolerit eto re d matri x containin g amphibol e phenocryst s an dmiarolitic cavitie s i n a groundmas s forme d mainl y o falbite lath sWONGA BATHOLIT HBirds Wel l Granit e(Blake & others , 1981a )Massive t o intensel y foliate d an d gneissic , slightl y t orichly porphyritic , mediu m t o coarse-graine d biotit egranite; mino r quartzofeldspathi c gneis s (locall ygarnetiferous), auge n gneiss , hornblende-biotit egranite, aplite , an d microgranite ; rar e pegmatite ;scattered mafi c xenoliths ; inclusion s o f gneissi cgranodiorite, granite , an d calc-silicat e rock s presen tin place sGranitic rocks: consis t mainl y o f quart z + microclin e+ plagioclas e largel y altere d t o sericit e + biotit e± dar k blue-gree n hornblende ; accessor y an dsecondary mineral s includ e allanite . chlorite , epidote ,muscovite, opaqu e minerals , sphene , zircon , and , i nplaces, abundan t scapolite ; myrmekit e i s commonl ypresentIntrudes Leichhard t Volcanics , Magn aLynn Metabasalt , Argyll a Formation ,Kalkadoon Granite , an d perhap s Corell aFormation (possibl e sourc e o f calc-sili -cate inclusions) . Bird s Wel l Granite ? i nS intrude s Plu m Mountai n Gneiss , pro -bably On e Tre e Granite , an d possibl ySaint Mung o Granite . Cu t b y meta -dolerite dyke s29

Name of unit(reference to definition)Bowlers Hol e Granil e(Blake & others , 1981a )Bushy Par k Gneis s(Blake & others , 1981a )Mairindi Cree k Granit e(Blake & others , 1981a )Overlander Granit e(Blake & others , 1981a )Revenue Granit e(Blake & others , 1981a )Mount Eri e Igneou sComplex(Blake & others , 1981a )Myubee Igneou s Comple x(Blake & others , 1981a )Saint Mung o Granit e(Blake & others , 1981a )TABLE 10 —Lithology(Continued).Foliated t o gneissic , slightl y porphyritic , medium -grained biotite-hornblend e granite ; mino r gneissi cbiotite granite , microgranite , aplite , pegmatite , an ddark contaminated ? granite ; inclusion s o f recrystallise dmafic rocks , probabl e felsi c metavolcanics , an dcoarse-grained granit e commo nGranite: show s reductio n i n grainsiz e toward s intru -sive contacts ; consist s mainl y o f granula r aggregate sof quart z + plagioclas e (largel y altere d t o mus -covite/sericite) + microclin e + dar k blue-gree nhornblende + generall y subordinat e biotite ;accessory an d secondar y mineral s recorde d ar eallanite, calcite , chlorite , epidote , muscovite/seri -cite, opaqu e minerals , sphene , an d zircon ; myrme -kite i s uncommo nMedium t o coarse-graine d hornblende-biotit e quartzo -feldspathic gneis s commonl y containin g auge n (de -formed phenocrysts) , slightl y t o intensel y foliate d(gneissic) porphyriti c biotite-hornblend e granite ;minor biotit e graniti c gneiss , fine-grained leucogneiss ,medium t o coarse-graine d leucogranite , aplite , an dtourmaline-bearing pegmatite . Gneisse s ar e locall yscapolitic. Myrmekit e an d inclusion s o f amphibolite ,quartzite, an d felsi c metavolcanics ? ar e commo nMedium t o coarse-graine d foliate d biotit e granit e an dminor aplite . Foliatio n locall y crenulate dGranite: contain s scattere d phenocryst s o f microcline .quartz, an d rar e plagioclas e i n groundmas s o fquartz + plagioclas e + microclin e + biotit e +accessory muscovite , blue-gree n hornblende , opaqu eminerals, allanite , an d fluoriteMassive t o foliated , leucocratic , mediu m t o coarse -grained, even-graine d t o slightl y porphyriti c horn -blende-biotite granite ; mino r pegmatit e an d aplit eSimilar t o Overlande r Granite . Contain s spars e mafi cxenoliths an d larg e inclusion s o f calc-silicat e rocks .Crenulated gneissi c granit e presen t i n place sGranite, mctadolerite/gabbro , dioriti c hybri d rocks ,and inclusion s o f calc-silicat e rocks : mino r pegmatite ,aplite, an d veinlet s o f prehnite . Granit e an d meta -dolerite for m net-veine d complexe sGranite: leucocratic , mediu m t o fine-grained, locall yslightly porphyritic , generall y foliate d an d recrystal -lised; consist s o f quart z -f - orthoclas e o r microclin e+ sodi c plagioclas e + mino r biotit e an d gree nhornblende, an d accessor y an d secondar y allanite ,apatite, calcite , chlorite , epidote . opaqu e minerals ,scapolite, sericite/muscoviie , sphene , an d zircon ;myrmekite i s rar eMetadolerite/ gabbro: dar k grey , mediu m t o finegrained,ophiti c t o granulitic , generall y no t foliated ,locally contain s plagioclas e phenocrysts . Ophiti cvarieties consis t mainl y o f plagioclas e lath s +clinopyroxene i t orthopyroxen e ± olivin e ±secondary amphibol e ± primar y biotite ; granuliti cvarieties consis t mainl y o f plagioclas e + hornblend e+ biotit e ± clinopyroxen e remnant s ± scapolite ;accessory mineral s includ e apatite , partl y micro -graphic quart z quart z an d alkal i feldspar , an dopaque mineral sGranite, gabbro ; mino r diorite , aplite , pegmatite .Granite an d gabbr o locall y for m net-veine d complexe sGranite: leucocrati c hornblende-biotit e granit e simila rto tha t o f Overlande r Granite : i n place s contain sinclusions o f coarse-graine d granit e an d calc-silicat erocksGabbro: dilferentiate d fro m olivine-ric h norite .through pyroxen e gabbro , hornblend e gabbro , an dhornblende leucogabbro , t o pegmatoida l horn -blende diorit eSlightly t o intensel y foliated , mediu m t o coarsegranitegrained, recrystallise d hornblende-biotit ecrowded wit h microclin e megacryst s u p t o 3 c macross; mino r foliate d porphyriti c biolit e granite,aplite, an d pegmatite . Xenoliths , som e wit h microdinemegacrysts , ar e commo nGranile: consist s mainl y o f quartz , microcline , partl yaltered plagioclase , an d 10-1 5 percen t biotit e an dgenerally subordinat e dar k blue-gree n hornblende ;myrmekite i s common ; accessor y an d secondar yminerals recorde d ar e apatite , calcite , chlorite ,epidote, fluorite, metamic t mineral s (includin gallanite). opaqu e mineral s (includin g magnetite) ,scapolite, sphene . an d zircon ; man y o f microclin emegacrysts ar e augen-lik eRelationshipsIntrudes Leichhard t Volcanics , Magn aLynn Metabasalt , an d probabl y Argyll aFormation; cu t b y schistos e metadolerit edykesIntrudes Magn a Lyn n Metabasal t an dCorella Formatio nIntrudes Magn a Lyn n Metabasal t an dArgylla Formation ; cu t b y metadolerit edykesIntrudesdolerite;IntrudesdoleriteCorella Formatio n an d meta -cut b y dolerit e dyk eCorella Formatio n an d meta -Intrudes Corell a Formatio nIntrudes Corell a Formation ; cu t b ydolerite dyk eIntrudes Plu m Mountai n Gneis s an dCorella Formation ; intrude d b y mafi cdykes an d possibl y b y Bird s Wel lGranite?30

lith. These intrusions are generally elongated parallelto regional northerly trends. In the Duchess-Urandangiregion the Wonga Batholith comprises the Birds Well,Bowlers Hole, Mairindi Creek, Overlander, Revenue,and Saint Mungo Granites, the Bushy Park Gneiss,and the Mount Erie and Myubee Igneous Complexes(Table 10). To the north, in MARY KATHLEENand MARRABA, it includes granites, mapped asWonga Granite (Carter & others. 1961) and BurstallGranite (Derrick & others, 1978), which have beenisotopically dated at 1740 to 1670 m.y. (Page, 1978.1980, 1981a, 1983b).Most of the Wonga Batholith granites are pale pink,weakly foliated to gneissic, and extensively recrystallised,and all have been regionally metamorphosed tothe greenschist or amphibolite facies.The Birds Well Granite forms a pluton with anoutcrop area of roughly 70 km- about 12 km west ofDuchess. Similar granite, mapped as possible BirdsWell Granite, crops out to the south, 30 km east ofDajarra (Fig. 29). Both outcrops were previouslyFig. 30 . Metamorphose d pegmatit e vein , gentl y folded , cut -ting tightl y folde d calc-silicat e rock s o f th e Corell a Forma -tion 1 9 k m nort h o f Duches s (a t G R UB819S69) .(M2443/10)Fig. 29 . Graniti c gneis s wit h feldspa r augen , Bird s Wel lGranite? 2 9 k m eas t o f Dajarr a (a t G R UB845050) .(M2443/18)mapped as Kalkadoon Granite. The Bowlers HoleGranite, in the northeast, which forms an oval pluton11 km long from north to south and 3.5 km wide, andthe Mairindi Creek Granite, a. small pluton 8 to 12km northwest of Duchess, were also previously mappedas Kalkadoon Granite; however, like the Birds WellGranite, they intrude units younger than the KalkadoonGranite. The Bushy Park Gneiss is exposed as a seriesof intensely deformed intrusive pods and lenses alongthe western margin of the Duchess Belt in the northeastof the central area; associated veins of aplite andpegmatite and the gneissic foliation locally show tightto isoclinal minor folds.The Overlander Granite and Revenue Granite formelongate plutons, previously mapped as Wonga Granite,within the Duchess Belt north of Duchess. They intrudethe Corella Formation, which is converted to skarn inplaces adjacent to the plutons, and are taken to includeswarms of tourmaline-bearing pegmatite dykes andveins (Fig. 30). Some of these dykes cut the granite,but others appear to merge into granite at plutonmargins. The Revenue Granite includes crenulatedgneissic granite (Fig. 31) considered by Joplin &Walker (1961), but by neither RJB nor PJTD, torepresent metasomatised (granitised) calc-silicate rocks.The Mount Erie Igneous Complex crops out overabout 30 km- in the vicinity of Duchess. It consistsmainly of leucocratic granite and dolerite/gabbrowhich are intimately mixed locally with dioritic hybridrocks (Fig. 32) to form net-veined complexes (Blake,1981a). In these the dolerite/gabbro is present asangular to rounded pillow-like inclusions enclosed inand veined by granitic rock; it may be either similarin age to or younger than the granite, and is inferredto represent mafic magma intruded into either graniticmagma or melted granitic rock. Inclusions of calcsilicaterocks derived from the adjacent Corella Formationare concentrated near the margins of the complex.Although igneous textures and mineralogy arepreserved in places, and the dolerite/gabbro componentis commonly not deformed, several features suggestthat probably all components of the Mount ErieIgneous Complex have been metamorphosed to theFig. 31 . Crenulate d gneissi c granit e a t th e margi n o f th eRevenue Granit e 9 k m nort h o f Duches s (a t G RUB795465). (GB1370 )31

The Saint Mungo Granite, previously mapped a spart of the Kalkadoon Granite, crops out over about90 km 2 in the southeast part of the central area. Themain rock type, porphyritic foliated granite containingboth biotite and hornblende, is similar to much ofthe Bushy Park Gneiss, a probable correlative.Fig. 32 . Net-veine d comple x o f th e Moun t Eri e Igneou sComplex: a mixtur e o f dolerit e (rounde d dar k gre y inclu -sions), granit e (pal e grey) , an d heterogeneou s hybri ddioritic rock s (mediu m grey ) 5 k m sout h o f Duches s (a tGR UB823325) . Fiel d o f vie w i s abou t 1 m across .(M2231/35)amphibolite facies: the presence of symplectic texturesin gabbro; mineral assemblages and granulitic texturesin much of the dolerite/gabbro; and the recrystallisedand foliated nature of the granite.The Myubee Igneous Complex forms a circularbody about 2 km in diameter (Fig. 33) 16 km north ofDuchess. It consists of an outer zone of granite partlysurrounding a core of gabbro. The granite containssparse angular inclusions of coarser-grained granite.The gabbro is cut by granitic veins, and is locally convertedto amphibolite adjacent to granite. It also formsa net-veined complex with the granite (Fig. 34),similar to that of the Mount Erie Igneous Complex;hence it was probably emplaced either at the same timeas the granite or later. The gabbro may be related tothe dolerite and gabbro of the Mount Erie Igneous Complex,and also to the Lunch Creek Gabbro in M ARRABA(Derrick, 1980), which in places forms net-veinedcomplexes with the Burstall Granite (Blake, 1981a).Although the gabbro generally appears to be an unalteredigneous rock, coronas formed of fine-grainedmagnetite and orthopyroxene? between olivine andplagioclase grains and the presence of symplectic intergrowthsof hornblende and spinel indicate regionalmetamorphism to the amphibolite facies, as in theMount Erie Igneous Complex.The granitic components of the Mount Erie andMyubee Igneous Complexes are similar in compositionto, and may be comagmatic with, the Overlander andRevenue Granites, and also with the Burstall Graniteto the north, in MARRABA, which has been isotopicallydated at 1720-1740 m.y. (Page, 1980, 1981a,1983b). All these granitic units may have been emplacedafter the surrounding Corella Formation hadfirst been tightly folded and regionally metamorphosedto the amphibolite facies (Derrick, 1980; Blake,1981b).Mount Philp BrecciaThe Mount Philp Breccia intrudes the Corella Formationin the northern half of the Duchess Belt. Twomain bodies are known: one in the far north whichextends into MARY KATHLEEN and was previouslymapped as Mount Philp Agglomerate (Carter & others,1961; Carter & Opik, 1963; Derrick & others, 1977a,b); and the other 7 km southeast of Duchess.The Mount Philp Breccia (Table 10) consists ofdisoriented fragments, ranging in shape from angularto rounded and in size from less than one centimetreto several tens of metres, dispersed in a pink to redmatrix (Fig. 35). The fragments are formed of bandedcalc-silicate rocks, albitite, metabasalt, amphibolite,quartzite. schist, quartz-feldspar pegmatite, and possiblymetadolerite, all of which can be matched withrocks in the adjacent Corella Formation. The matrix ofthe breccia has an igneous-type texture, and consistsmostly of stubby phenocrysts of amphibole (mainlytremolite) enclosed in a groundmass of interlockingalbite laths in which there are many small and irregularmiarolitic cavities lined with inward-growingalbite crystals. A detailed description of the MountPhilp Breccia and a discussion of its origin are givenby Blake & others (1982b).Minor intrusionsDykes and pod-like intrusions of metadolerite andamphibolite of several different ages occur throughoutthe central area, and at some localities metamorphosedmafic dykes form 10 percent or more of the bedrock.They generally consist mainly of green hornblendeand variably sericitised oligoclase/andesine, typical ofthe amphibolite facies, and commonly also contain epidote,chlorite, and biotite. Some of the intrusionscontain relict clinopyroxene, and those intruding theCorella Formation in the east are typically rich inscapolite. Textures range from relict ophitic (relictigneous) to schistose or granulitic (completelyrecrystallised).Non-metamorphosed dolerite dykes are relativelyrare. They have ophitic textures, and generally consistof plagioclase (partly sericitised), clinopyroxene,opaque oxides, minor primary hornblende and biotite,and interstitial and commonly granophyric quartz andalkali feldspar. These dykes may be related to theLakeview Dolerite in MARRABA and MARY KATH­LEEN, to the north, which has an Rb-Sr age ofabout 1116 m.y. (Derrick & others, 1978; Page,1983b).Felsic dykes, though common in parts of the centralarea, are much less numerous than mafic dykes. Theyare generally formed of grey or pink fine-grainedquartzofeldspathic rock containing phenocrysts offeldspar and quartz. The largest felsic minor intrusionis the Garden Creek Porphyry (Table 10), which formsa discontinuous subvertical sheet up to 250 m wideintruding the Mount Guide Quartzite more or less concordantlyon the east side of the Dajarra Belt. It mayrepresent a composite intrusion, as the porphyry isalmost invariably flanked by metadolerite. It was prc-32

Fig. 33 . Aeria l view , fro m th e south , o f th e circula r Myube e Igneou s Comple x 1 6 k m north-northwes t o f Duchess .Gabbro i n th e centra l par t o f th e comple x i s encircle d b y granit e an d ridge-forming granite-veine d calc-silicat e rock sof th e Corell a Formation . (M2443/31 )viously mapped as Kalkadoon Granite (Carter & others,1961; Carter & Opik. 1963), which is now known topredate the Mount Guide Quartzite.StructureFoldsThe first major folding event within the Kalkadoon-Leichhardt Block (and probably the whole region)is thought by Blake (1980, 1981b) to have affectedmuch of the undivided Tewinga Group, and to havepredated the Leichhardt Volcanics; alternatively, itmay have been associated with the emplacement of theKalkadoon Granite, although this is now consideredunlikely. A gneissic foliation and some small-scalefolds in undivided Tewinga Group rocks and also inthe Plum Mountain Gneiss (Fig. 8e) to the cast mayhave been developed during this event.A later deformation resulted in the Magna LynnMetabasalt and older units, and the Argylla Formation.Ballara Quartzite, Corella Formation?, StanbrokeSandstone, Makbat Sandstone, and Surprise CreekFormation? being moderately to tightly folded (Figs.36, 37). The trends of these folds are subparallel tothe trends of the postulated earlier structures. One ofthe later folds is the large north-plunging synclinewhich is truncated in the north by the Fountain RangeFault, and in which rocks mapped as Corella Formation?are preserved (Fig. 36). This syncline is similarin style and probably age to synclines in MARYKATHLEEN, to the north (Derrick & others, 1977b).This folding appears to have been the first to affectthe Magna Lynn Metabasalt to Corella Formation?sequence, and was possibly also the first to affect theLeichhardt Volcanics (Blake, 1981b, 1982). Otherfolds of probably similar age include the synclines andhalf-synclines in which the Stanbroke and MakbatSandstones (Fig. 37) and Surprise Creek Formation?are preserved. In the south, east of Dajarra, largefolds possibly related to this later folding have foldedearlier structures in the undivided Tewinga Group(Blake & others, 1982a). In the north, near GRUB5953, small-scale crenulations in foliated undividedTewinga Group rocks may also be related to the laterfolding.The dominant folding event in the Dajarra Belt producedmajor open to tight, mainly north-plungingstructures with steeply dipping to vertical northerlytrending axial planes. The Eastern Creek Volcanics ofthe Haslingden Group, for example, are largely preservedin partly fault-bounded synclines, the faultsparalleling the axial planes of the folds. These foldsappear to be the earliest that affected the Dajarra Belt,and may be related to the folds affecting the MagnaLynn Metabasalt and younger units in the Kalkadoon-Leichhardt Block. Angular discordance of up to 30°between the Haslingden and Mount Isa Groups locally33

Fig. 34 . Net-veine d comple x o f th e Myube e Igneou s Complex : predominantl y angula r fragment s o f gabbr o enclose d i ngranite. (M2633/19 )evident west of Dajarra appears to have resulted fromtilting, rather than tight folding, before the MountIsa Group was deposited. In the far south a geneticrelationship between faulting and folding is indicatedby an increasing number of parasitic folds (in theEastern Creek Volcanics and overlying Warrina ParkQuartzite) westwards, towards the Wonomo Fault, onthe western side of a broad north-plunging anticline(near GR UA4176).The western margin of the Duchess Belt is markedby a highly deformed zone a few kilometres widerepresented by interlayered augen gneiss, graniticgneiss, schistose metasediments and metavolcanics,foliated granite, and amphibolite. No major folds havebeen recognised in this zone (the southern continuationof the Wonga Belt), probably because of widespreadtransposition, but small-scale tight to isoclinalfolds are common in cross-cutting veins of quartz andaplite; the axial planes of the folds are parallel to themain gneissic foliation (Fig. 8f). The small foldsindicate that some, if not all, the deformation in thiszone took place after granite emplacement (cf.Holcombe & Fraser, 1979; Holcombe, 1981).To the east, in the Corella Formation, the mainlycalc-silicate rocks have generally been deformed plasticallyto produce spectacular small and medium-scalefolds which are locally disharmonic (Figs. 26, 38, 39,front cover). The earliest folds are tight to isoclinal,and commonly have high amplitude-to-wavelengthratios, shallow plunges, and subvertical axial planes.Axial-plane foliations are generally restricted to hingeareas, and range from a fracture cleavage to an alignmentof mafic mineral grains and porphyroblasts(Fig. 40). Tectonic transposition and metamorphicdifferentiation—pseudobedding—are developed in someof the most deformed rocks (Fig. 27).Later open to tight folds, coaxial with the earlierfolds, are sporadically developed in calc-silicate rocks—for example, in a small area about 20 km north ofDuchess (Fig. 39). Early minor folds have beenreoriented around the hinge of one fold here, a northplungingsynform 2 to 3 km wide. This synform is ina zone of open folds and basin-and-dome structures,which may represent interference fold patterns, lyingto the west of the Revenue and Overlander Graniteplutons. The later folding event in the Corella Formationmay represent an initial ductile response of thecalc-silicate rocks to compressive forces which subsequentlyformed a northeast-trending dextral strikeslipfault system, or it may represent large-scale dragfoldingrelated to movements along the Pilgrim FaultZone.Late open folds occur in the Corella Formationadjacent to the Revenue Granite. These appear to havebeen formed during the emplacement of this granite.Associated foliation and lineation styles are indistinguishablefrom those of the earlier folding event. Theage of the late folds relative to the late folds farthernorth is unknown.Rocks of the Duchess belt south of the Plum MountainFault generally show evidence of only one majordeformation event. The Plum Mountain Gneiss, SaintMungo Granite, and Corella Formation here are highlydeformed and possess a prominent subvertical,northerly to northwesterly trending foliation, butwhether this is the result of one or more major tectonicevents is not clear. Minor folds related to the foliationarc rare, and no related major folds are evident.34

major northeast-trending faults, where some verticalmovement is also commonly evident. A good exampleis the Fountain Range Fault, which juxtaposes asynclinal sequence of Ballara Quartzitc-Corella Formation?rocks to the south against older Argylla Formationrocks to the north (Fig. 36); the lateral dextralmovement along this fault was estimated by Derrick& others (1977b) to be about 25 km.A series of north-trending faults predate the conjugateset. Many of these earlier faults mark boundariesbetween rocks of contrasting competencies, andare thought to have resulted from differential beddingplaneslip during folding. Other faults related to foldinginclude north-trending axial-plane shears, alongwhich the horizontal and vertical movements haveseparated or removed adjacent fold limbs to form'half-synclines'. The north-trending Dajarra Fault inthe south of the Kalkadoon-Leichhardt Block mayhave a considerable displacement, as in places itseparates amphibolite-facies rocks to the west fromgreenschist-facies rocks to the east.Fig. 35 . Top: Fragment s o f quartzit e (white , amphibolit e(dark grey) , an d othe r rock s type s i n Moun t Phil p Brecci a38 k m north-northeas t o f Duches s (a t G R UB907756) .Bottom: Larg e fragmenta l amphibol c crysta l i n Moun tPhilp Brecci a 3 8 k m north-northeas t o f Duches s (a t G RUB907756). (M2633/20 , 21 )Younger folds have been recognised in the Plum MountainGneiss immediately south of the Plum MountainFault, whose movement has resulted in the developmentof large open drag-folds.FaultsThe most prominent faults in the central area forma northeast and northwest-trending conjugate strikeslipset which postdates the latest major folding event.These faults may result from east-west compressivestresses, as suggested by Carter & other (1961). Thenortheast-trending faults commonly have dextral displacementsand are best developed in the DuchessBelt, whereas the northwest-trending faults have sinistralmovements and are best developed in the DajarraBelt. The greatest horizontal displacements occur alongMetamorphic effectsIn the central area, as in the western and easternareas, the Precambrian rocks are metamorphosed tothe greenschist and amphibolite facies (Fig. 9). Thehighest-grade rocks occur in the northeast and probablythe south of the Kalkadoon-Leichhardt Block and inthe Duchess Belt. The lowest-grade rocks are probablythose of the younger cover in the Dajarra Belt, butrelatively low-grade rocks are also present in theKalkadoon-Leichhardt Block, alongside much highergraderocks.The felsic gneisses which predominate in the PlumMountain Gneiss of the Duchess Belt and undividedTewinga Group of the Kalkadoon-Leichhardt Blockgenerally contain no diagnostic metamorphic mineralassemblages. However, several features indicate thatthese units have been regionally metamorphosed tothe amphibolite facies: the common presence of myrmekite;the extensive obliteration of primary igneousand sedimentary textures owing to extensive recrystallisation;the local occurrence of leucocratic veinsand migmatitic patches resulting from partial melting;and the presence of hornblende and relatively calcicplagioclase in associated metabasites.The main granites of the Kalkadoon Batholith—theKalkadoon Granite and One Tree Granite—have alsobeen metamorphosed mainly to the amphibolite facies.Metamorphism in these units is indicated by thebreakdown of primary biotite and in places hornblendeinto fine-grained biotite aggregates, the straining andcommon recrystallisation of quartz and locally feldspar,and the development of secondary chlorite, epidote,and white mica. Most cross-cutting mafic dykes haveamphibolite-facies mineral assemblages. In contrastthe Wills Creek and Woonigan Granites appear to bemetamorphosed only to the greenschist facies. Nometamorphic aureoles have been found at intrusivecontacts of the Kalkadoon Granite, but some contacteffects occur in the Leichhardt Volcanics adjacent tothe Wills Creek Granite: the volcanics are mottled andpartly recrystallised, and some contain small tourmalineneedles.The felsic volcanics of the Leichhardt Volcanicsgenerally have well-preserved primary igneous textures,show no marked recrystallisation effects, and containonly small amounts of metamorphic biotite, white mica,35

Fig. 36 . Infrare d airphot o o f th e are a nort h o f th e Lad y Fann y min e an d centre d abou t 2 3 k m northwes t o f Duchess ,where a majo r north-plungin g synclin e i s truncate d b y th e northeast-trendin g Fountai n Rang e Fault . Recessiv e calc -silicate rock s o f th e Corell a Formation ? i n th e cor e o f th e synclin e overli e hill-formin g meta-arenite s o f th e Ballar aQuartzite an d Argyll a Formatio n (outline d b y dashe d lines) . Th e upstandin g terrai n nort h o f th e faul t consist s mainl y o fKalkadoon Granit e an d Leichhard t Volcanics , bu t als o include s strike-ridge s o f Stanbrok e Sandstone . Northerl y trendin gBushy Par k Gneiss , Argyll a Formation , Magn a Lyn n Metabasalt , an d Corell a Formatio n o f th e Duches s Bel t ar e ex -posed alon g th e easter n margi n o f th e photo .16/F54/54sphene, and epidote. These features have been takento indicate greenschist-facies metamorphism, but otherfeatures show that in places the Leichhardt Volcanicsare more intensely metamorphosed: more pronouncedrecrystallisation effects; development of a metamorphicfoliation; presence of cross-cutting amphibolite-faciesmetadolerite dykes; and, in the north, amphibolitefaciesmineral assemblages in the overlying MagnaLynn Metabasalt.Greenschist-facies metabasalts with well-preservedprimary textures are rare within the Magna LynnMetabasalt, and most parts of this unit, and also ofthe overlying Argylla Formation, have been metamorphosedto the amphibolite facies. Metabasites in thesetwo formations generally have granoblastic to intenselyfoliated fabrics and contain co-existing hornblende andrelatively calcic plagioclase. The highest-grade metabasitesare adjacent to the Bowlers Hole, MairindiCreek, and Birds Well Granites. Like the rocks theyintrude, these granites are extensively recrystallised andcommonly intensely foliated. Cordierite-anthophylliteassemblages in rocks of the Ballara Quartzite, andclinopyroxene and hornblende in overlying calc-silicaterocks of the Corella Formation?, show that thesetwo units have also been regionally metamorphosedto the amphibolite facies.The Surprise Creek Formation? and the Stanbrokeand Makbat Sandstones are generally not extensivelyrecrystallised and have well-preserved primary sedimentarytextures (except near faults). They are inferredto be regionally metamorphosed mainly to thegreenschist facies.The Duchess Belt consists almost entirely of amphibolite-faciesrocks. Felsic and mafic metavolcanics,36

16/F54/55Fig. 37 . Infrare d airphot o o f th e are a centre d abou t 3 7 k m southeas t o f Dajarra , showin g northwest-trendin g syncline sof ridge-formin g Makba t Sandston e ( A an d B ) mainl y overlyin g th e Leichhard t Volcanics . Th e On e Tre e Granit e i n th eeast (aroun d C ) ha s a pronounce d north-trendin g structura l grai n whic h appear s t o predat e th e synclines .metasediments, and gneissic granites in the highlydeformed western zone are extensively recrystallisedand tectonically transposed. Cordierite-anthophylliteassemblages have been recorded in metasediments atone locality, and metabasites have hornblende-calcicplagioclase assemblages. The amount of deformationindicates that pressures may have been higher in thisbelt than in most other parts of the central areaduring the main metamorphism.Metasediments in the Corella Formation typicallyhave granoblastic textures, and contain K-feldspar,scapolite, hornblende (rather than trcmolite/actinolite),clinopyroxene (mainly salitic), epidote, quartz, plagioclase,sphene, and less commonly muscovite, garnet,biotite, zircon, apatite, and opaque oxides. Sillimaniteand cordierite have been recorded locally. Hornblendecrystals are commonly aligned parallel to the axialplanes of the oldest folds identified, but in places arealso aligned parallel to the axial planes of youngerfolds. Metabasites within the Corella Formation havetypical amphibolite-facies mineral assemblages. Thepresence of wollastonite, vesuvianite, and garnet incalc-silicate rocks adjacent to the Myubee IgneousComplex indicates hornblende-hornfels facies contactmetamorphism. Contact metasomatic effects arecommon in the Corella Formation alongside graniteintrusions, and banded to massive garnet-rich andclinopyroxene-rich skarns (Fig. 41) and alkali feldsparrichmetasomatic rocks persist for more than 100 mfrom some granite margins.The Haslingden Group of the Dajarra Belt comprisesboth greenschist and amphibolite-facies rocks.The sedimentary rocks of the Mount Isa Group containsmall amounts of fine-grained metamorphic biotite,muscovite, and chlorite, and have probably beenregionally metamorphosed to mainly the lower greenschistfacies.37

PRECAMBRIAN O F TH E EASTER N ARE AThree main tectonic elements are distinguished in theeastern area (Fig. 7); from west to east these arethe Malbon Block of Plumb & others (1980), and thenewly named Staveley and Squirrel Hills Belts, whichform part of the Mary Kathleen Fold Belt of Plumb& others (1980).The Proterozoic formations in the western part ofthe area lie in the core and on the flanks of a majoranticlinal structure, the Malbon Block, which continuesnorthwards into MARRABA where it forms theBulonga and Duck Creek Anticlines (Derrick, 1980).The sequence here is reasonably well established,although it is partly concealed beneath Cambrian andyounger sediments. Felsic metavolcanics and associatedmetasediments of the Argylla Formation in the coreof the anticlinal structure are overlain to the east andwest by the Malbon Group, comprising the basalticMarraba Volcanics and the Mitakoodi Quartzite, andthe Overhang Jaspilite of the overlying Mary KathleenGroup. The Overhang Jaspilite on the eastern sideof the anticlinal structure crops out mainly in thenorth, and to the south it appears to pass along strikeinto the Answer Slate.Farther east the stratigraphy is less clear. TheAnswer Slate is in contact eastwards with the DoubleCrossing Metamorphics, a probably older formation,and with metasediments of the Staveley Belt—theStaveley Formation and Agate Downs Siltstone—which are probably younger. In the south, the AnswerSlate grades eastwards into generally coarser metasedimentsof the Kuridala Formation, which in turngrades eastwards into coarser and more feldspathicmetasediments and interlayered metabasalt of theSoldiers Cap Group. These last two units, togetherwith the Doherty Formation, form the Squirrel HillsBelt. In the north, the Overhang Jaspilite is overlainto the east, possibly unconformably, by units of theStaveley Belt sequence—the Staveley Formation,Marimo Slate, and Roxmere Quartzite?. This sequenceis faulted to the east against the probably olderDoherty Formation, an extensive unit of banded andbrecciated calc-silicate rocks previously assigned to theCorella Formation. Near Kuridala and to the south,the Staveley Formation is inferred to overlie theFig. 38 . Disharmoni c foldin g o f bedde d calc-silicat e rock sin th e Corell a Formatio n 7 k m wes t o f Duches s (a t G RU15840447). (M2633/24A )Fig. 39 . Thinl y bedde d calc-silicat e rock s showin g small -scale earl y isoclina l fold s refolde d b y late r mor e ope nfolds whic h hav e a n associate d axial-plan e cleavage —Corella Formatio n 1 9 k m nort h o f Duches s (a t G RUB806570). (M2633/22 )38

The Proterozoic succession is intruded by numerousmafic dykes and sills and by granites of the WilliamsBatholith. The youngest intrusions are non-metamorphoseddolerite dykes trending cast-west, roughly atright-angles to the trends of most of the older rocks.Tewinga GroupArgylla FormationOf the three formations making up the TewingaGroup of Derrick & others (1976a), only the youngest,the Argylla Formation, is exposed in the eastern area.It crops out in the core of the Malbon Block, andconsists mainly of felsic volcanics, which include ignimbriticdeposits that were probably deposited subaerially,and quartzose to feldspathic arenites (Table 11). Itsthickness is uncertain, as it may be tightly folded inplaces, and its base is not exposed; however, it is atleast 2000 m thick in MARRABA, to the north(Derrick. 1980). Sedimentary rocks predominate inthe upper part of the formation, and arc much morevoluminous generally than in the Argylla Formation ofthe central area. The formation is little-deformed ormetamorphosed in the far north, but is regionallymetamorphosed to amphibolite-facies gneissic andquartzitic rocks in southwestern and southeasternexposures. A felsic volcanic sample from the formationin MARRABA has been isotopically dated at 1766m.y., statistically indistinguishable from ages of about1780 m.y. obtained for the Argylla Formation in thecentral area, and a more highly metamorphosed samplefrom GR VB251096 is inferred to be of a similar age(Page, 1983a).Fig. 40 . Mafi c mineral s aligne d paralle l t o th e axia l plan ein th e hing e o f a smal l fol d i n calc-silicat e rock s o f th eCorella Formatio n 7 k m north-northeas t o f Duches s (a tGR UB842444) . (M2633/23 )Malbon GroupThe Malbon Group comprises two formations, thebasaltic Marraba Volcanics and the conformably over-Kuridala Formation, perhaps unconformably. TheDoherty Formation may overlie the Soldiers CapGroup either conformably or disconformably, butmost contacts between these two units are marked byfaults.The favoured stratigraphic interpretation for theeastern area is that the Argylla Formation, MalbonGroup, Overhang Jaspilite, and Answer Slate form aconformable sequence (representing the Malbon Block)which correlates in part with the Kuridala Formation,Soldiers Cap Group, and Doherty Formation (SquirrelHills Belt) to the east. All these units are interpretedto be part of the older cover sequence, and are inferredto be overlain unconformably (mainly disconformably)by the younger cover sequence, comprising theStaveley Formation, Agate Downs Siltstone, MarimoSlate, and Roxmerc Quartzite? (Staveley Belt).Alternatively, the entire succession in the western halfof the area, from the Argylla Formation to the RoxmercQuartzite?, may be essentially conformable (e.g.,Derrick, 1980). Also, as suggested by Carter & others(1961) and other workers, the Soldiers Cap Groupmay be separated from the calc-silicate rocks nowmapped as Doherty Formation by an unconformityrepresenting a considerable time break. The stratigraphicposition of the Double Crossing Metamorphicsis uncertain. Detailed descriptions of the stratigraphicunits are listed in Tables 11-13.Fig. 41 . Zone d garne t porphyroblast s i n skarn , Corell aFormation adjacen t t o th e Myube e Igneou s Comple x 1 9 k mnorth o f Duches s (nea r G R UB790540) . (M2204/4 )39

lying Mitakoodi Quartzite (Table 11). Sediments inthese two formations were probably deposited inshallow water, and interlayered volcanics may havebeen erupted subaerially. Like the Argylla Formation,the Malbon Group has been regionally metamorphosedto the amphibolite facies, except in the far north, wherethe regional metamorphism only reached the greenschistfacies.Marraba VolcanicsThis formation has been subdivided by Derrick &others (1976d) into three members, two of whichhave been recognised in northern outcrops (thoughnot distinguished on the Duchess-Urandangi regionmap): the (lower) Cone Creek Metabasalt Member,which consists mainly of basaltic lava, and the (upper)Timberoo Member, which consists mainly of metasiltstonebut locally includes some mafic and felsicvolcanics (Donchak & others, 1983). To the south, theformation consists of interlayered metasediments andbasaltic metavolcanics. Metasediments become morearenaceous upwards, and the formation has a gradationalcontact with the overlying Mitakoodi Quartzite.Disseminated sulphides, mainly pyrite and chalcopyrite,and malachite stains are widespread, andseveral small copper mines and prospects occur withinthe formation.Mitakoodi QuartziteThis formation consists of ridge-forming metaarenitesand minor mainly silty interbeds, some ofwhich are calcareous, and some interlayered mafic andfelsic metavolcanics. It is tightly folded in the north.Cross-bedding and graded bedding are well developedin northern outcrops, but are generally difficult torecognise to the south because of recrystallisationeffects resulting from more intense metamorphism. TheWakefield Metabasalt Member of Derrick & others(1976d) has been recognised in northeastern outcrops(Donchak & others, 1983), though not distinguishedon the Duchess-Urandangi region map, and thin metabasaltlayers occur elsewhere within the MitakoodiQuartzite. Felsic porphyry of Argylla-type, probablyextrusive, is interlayered with sedimentary rocks of theMitakoodi Quartzite both east and west of the BurkeRiver Structural Belt; in the east the porphyry is associatedwith volcaniclastic breccia and a possibly laharicconglomerate.The Mitakoodi Quartzite becomes generally finegrainedtowards its top, and grades upwards intoconformably overlying Overhang Jaspilite and, in thesouth, Answer Slate. The presence of some bandedjaspilite in the upper part of the formation in thesoutheast indicates possible equivalence to part of theOverhang Jaspilite to the north.TABLE 11 . DETAIL S O F STRATIGRAPHY—TEWING A GROUP , MALBO N GROUP , AN D DOUBL E CROSSIN GMETAMORPHICS, EASTER N ARE AName of unit(reference to definition)max. thickness (m)Double Crossin gMetamorphics(Blake & others , 1981a )1000 + ?Mitakoodi Quartzit e(Carter & others , 1961 )2000 +Marraba Volcanic s(Carter & others , 1961 )~3000Argylla Formatio n(Carter & others , 1961 ;Derrick & others , 1976a )2000 +LithologyMicaceous an d feldspathi c gneis s an d schist ,migmatitic gneis s wit h leucosomes , auge n gneiss ,amphibolite, quartzite . meta-arkosc , bande d quartz -tourmaline an d quart/.-hematit e rock ; concordan tand discordan t vein s o f quartz-feldspa r pegmatit e(containing tourmalin e and/o r muscovite) , leuco -granite, an d quart zGneiss and schist: consis t o f biotit e ± muscovit e+ quart z + microclin e and/o r plagioclas ePinkish brown , mainl y fine-grained feldspathi cand quartzos e meta-arenite ; mino r calcareou smeta-arenite, metagreywacke , micaceou s metasilt -stone, phyllite , schist , metabasalt , quartz-feldspa rporphyry, conglomerate , jaspilite , scapoliti c calc -silicate rocks , pyriti c tuff? , an d quartz-hematit erock. Meta-arenite s commonl y sho w cross-beddin gand grade d bedding , especiall y i n NDark greenis h gre y metabasal t an d possibl e meta -andesite, gre y metasiltstone , locall y cross-bedde dmeta-arenite, an d biotit e schist ; mino r felsi c meta -volcanics, conglomerate , an d calcareou s metasilt -stone an d meta-arenit eMetabasalt: mainl y massiv e i n fa r N an d schistos eto S ; locall y porphyritic ; contain s trace s o fmalachite, pyrite , chalcopyrite , rar e azurite ;commonly contain s amygdale s wit h chlorite ,epidote, an d calcit e i n N ; represente d b y biotit eschist an d oligoclase-gree n hornblend e roc k i n SMassive (i n N ) t o intensel y foliate d an d recrystal -lised (i n S ) felsi c volcanics , clasti c sedimentar yrocks, an d mino r mafi c volcanics ; regionall y meta -morphosed t o greenschis t facie s i n N an d amphi -bolite facie s i n SFelsic volcanics: generall y magnetic , pinkis h brow nto gre y ignimbrite , tuff , flow-banded lava ; con -tain phenocryst s o f sodi c plagioclas e and/o rmicrocline an d commonl y quart z i n fine-graine dgroundmass o f quart z + alkal i feldspa r +minor biotit e ± dar k blue-gree n hornblend e(in S ) ± opaqu e mineral s (mainl y magnetite )Sedimentary rocks: commonl y cross-bedde dquartzose an d feldspathi c arenite s an d meta -arenites; mino r interbedde d siltstone , phyllite ,and schis tRelationshipsIntruded b y Gi n Cree k Granite ; concor -dant an d probabl y partl y faulte d con -tacts wit h Answe r Slat e an d Stavele yFormationConformable o n Marrab a Volcanics ;concordant (conformable? ) o n Argyll aFormation; overlai n conformabl y b yOverhang Jaspilit e an d Answe r Slate .Intruded b y Wimber u Granit e an d meta -doleriteOverlies Argyll a Formatio n wit h apparen tconformity; overlai n conformabl y b yMitakoodi Quartzite . Intrude d b y Wim -beru Granit e an d metadolerit eOverlain concordantl y b y Marrab aVolcanics an d Mitakood i Quartzite .Intruded b y Wimber u Granit e an dmetadolerite40

Double Crossing MetamorphicsThis somewhat enigmatic formation crops out overan area of about 30 km 2on the eastern margin of theMalbon Block 20 km southwest of Selwyn. It consistspredominantly of gneiss and schist which appear to bemainly quartzofeldspathic metasediments, but alsoincludes augen gneiss and amphibolite which possiblyrepresent felsic and mafic metavolcanics (Table 11).The formation has been tightly to isoclinally folded,regionally metamorphosed to the amphibolite facies,and partly granitised, and appears to be more deformedand metamorphosed than adjacent rocks mapped asAnswer Slate and Staveley Formation, which arepelitic rather than quartzofeldspathic. It may representpre-Argylla Formation basement rocks or lateralequivalents of the Argylla Formation and MalbonGroup rocks in the core of an anticline or horst;alternatively, it may represent rocks brought up fromdeeper in the crust during the emplacement of thespatially associated Gin Creek Granite, which inplaces contains abundant large inclusions of rocksidentical with those of the Double Crossing Metamorphics.The only sample chemically analysed, afelsic gneiss from near Gin Creek Bore, is similar inchemistry to felsic volcanics of the Argylla Formation(Bultitude & Wyborn, 1982).Soldiers Cap GroupThe Soldiers Cap Group (the Soldiers Cap Formationof Carter & others, 1961; Carter & Opik, 1963)crops out extensively in the east—in the Squirrel HillsBelt, mainly east of the Cloncurry Fault—and is probablyseveral thousands of metres thick (Table 12).It is less resistant than the adjacent Doherty Formation,and forms mainly low hills and ridges and undulatingterrain. Rocks of the group are generally tightly foldedand steeply dipping, and have been regionally metamorphosedto a high grade. The group was divided intothree formations by Derrick & others (1976e): fromoldest to youngest, the Llewellyn Creek Formation,Mount Noma Quartzite, and Toole Creek Volcanics.These formations have been recognised only in thefar north; elsewhere in the eastern area the SoldiersCap Group is not divided into formations.The main rock types exposed are mica schist,quartzofeldspathic gneiss, meta-arenite, and amphibolite;some banded iron formation is also present.Many of the gneissic rocks are migmatitic (Fig. 42).Amphibolite thought to be metabasalt lava forms concordantlayers, commonly has heterogeneous and garnetiferouszones and patches which may representoriginally scoriaceous and amygdaloidal flow margins,and is locally interlayered with thinly banded paraamphibolite.No undoubted intrusive bodies of amphibolitein the region have been found to contain garnet.Felsic volcanics are much less widespread thanmafic volcanics in the Soldiers Cap Group, but metarhyoliteis present in the north near the WilliamsRiver, and possible felsic metavolcanics representedby garnetiferous quartzofeldspathic gneiss petrographicallysimilar to the Potosi Gneiss of the Broken Hillarea, New South Wales, are exposed in the east, nearGR VB9723 (Donchak & others, 1983).Banded iron formation forms finely laminated beds,averaging about 1 m thick, which commonly showtight folding. In places the beds are hosts to low-gradecopper, lead, and zinc deposits of possibly exhalativetype. They may be confined to the same stratigraphicTABLE 12. DETAILS OF STRATIGRAPHY—SOLDIERS CAP GROUP, EASTERN AREAName of unit Lithology Relationships(reference to definition)max. thickness (m)Toole Cree k Volcanic s(Derrick & others , 1976c )1000 +Mount Nom a Quartzit e(Derrick & others . 1976c )1000 +Llewellyn Cree k Formatio n(Derrick & others , 1976c )1000 +Amphibolitic metabasalt , mic a schist , carbonaceou smetasiltstone, an d quartzit eFeldspathic meta-arenite , quartzite , an d mic aschist commonl y wit h garne t and/o r andalusite ;minor conglomerate , metagreywacke , an d amphi -bolitic metabasal tMica schist , commonl y wit h garne t an d andalusite ;minor phyljit e an d amphiboliti c metabasal tConformable o nMount Nom aQuartziteConformable o nLlewellyn Cree kFormation; i ncontact wit h(faulted against? )Doherty Formatio nbrecciaBase no t expose dIntruded b ySaxby Granit eand amphiboliti cmetadolerite sills ."Pass laterall yinto undivide dSoldiers Ca pGroupL'ndivided1000-Mica schist , gneiss , migmatiti c gneis s wit h leuco -somes, meta-arenit e (includin g quartzite) , amphi -bolitic metabasalt ; mino r finely bande d para -amphibolite, calc-silicat e rocks , chert , bande d iro nformation, metarhyolite . quart z + feldspa r ±tourmaline ± muscovit e pegmatit eSchist, gneiss, and meta-arenite: pal e t o dark ;medium t o coarse-grained ; consis t mainl y o fquartz + plagioclas e ± microclin e ± biotit e ±muscovite ± sillimanit e ± porphyroblast s o fgarnet an d andalusit eAmphibolitic metabasalt and para-amphibolite:schistose t o gneissic ; consis t mainl y o f plagio -clase (An>„ ) + gree n hornblend e + sphen e+ opaque s ± quart z ± garne t ± clinopy -roxene ± coppe r mineral s (mainl y malachite )Calc-silicate rocks: banded , massive , an d brec -ciated; simila r i n mineralog y t o thos e o fDoherty Formatio n excep t mor e commonl yscapoliticBanded iron formation: dar k brow n t o black ; ver yfine t o medium-grained ; typicall y consis t o fquartz + feldspa r + hematit e + magnetit e± gahnit e ± apatit eBase no t exposed ; concordan t an d pos -sibly interfingerin g contact s wit hDoherty Formation ; appear s t o grad elaterally t o W i n S int o Kuridal a For -mation; intrude d b y Cowie , Maramun -gee, Moun t Angelay , Saxby , an d Squirre lHills Granites , unname d granite , pegma -tite, metadolerite , an d dolerit e41

greenschist and amphibolite facies, and they were presumablyconsidered by Derrick & others to form anessentially conformable sequence. However, the OverhangJaspilite, Answer Slate, Kuridala Formation,and Doherty Formation may form an older sequenceseparated by a major unconformity from an overlyingmuch younger sequence comprising the Staveley Formation,Agate Downs Siltstone, and Marimo Slate ofthe Mary Kathleen Group, and also the RoxmereQuartzite?Isotopic age data indicate that rocks of the MaryKathleen Group probably range in age from pre-1740m.y. to about 1600 m.y. (Page, 1983a,b): the OverhangJaspilite is younger than the underlying ArgyllaFormation (1780 m.y.) but is considered to be olderthan the Burstall Granite (1720-1740 m.y.) exposedin MARRABA (Derrick, 1980); metarhyolite in theDoherty Formation has been dated at about 1720 m.y.;in MARRABA, rocks possibly equivalent to theStaveley Formation/Marimo Slate sequence in theeastern area, but mapped as Corella Formation, includefelsic volcanics 1600 m.y. old.Fig. 42 . Irregula r mino r fold s i n migmati c gneis s o f th eSoldiers Ca p Grou p 4 5 k m southeas t o f Kuridal a (a t G RVB873271). (M2304/33 )level as banded iron formation in the Kuridala Formationat the Pegmont prospect.The stratigraphic relationship between the SoldiersCap Group and the Doherty Formation is uncertain.In the eastern area no evidence has been found tosupport the view—for example, of Carter & others(1961) and Glikson (1972)—that a major angularunconformity separates the Soldiers Cap Group fromthe Doherty Formation (= Corella Formation ofearlier workers), and there is no clear indication asto which is the older unit. At exposed contacts, schistoserocks of the Soldiers Cap Group invariably lieadjacent to calc-silicate breccia of the Doherty Formation,but bedding orientation in the two units isgenerally concordant, indicating a possible conformablerelationship. This is supported by the presence nearsome contacts of interbedded schists and Doherty-typecalc-silicate rocks within the Soldiers Cap Group. Ourfavoured interpretation is that the Soldiers Cap Groupis overlain conformably by, and locally interfingerslaterally with, the Doherty Formation, and that thecalc-silicate breccia at the contact was formed whenthe two units were tightly folded.Mary Kathleen GroupThe Mary Kathleen Group, as defined by Derrick &others (1977a), includes the following formationsexposed in the eastern area: the Overhang Jaspiliteand Answer Slate of the Malbon Block, the KuridalaFormation and Doherty Formation (= Corella Formationof previous workers) of the Squirrel Hills Belt,and the Staveley Formation, Agate Downs Siltstone(formerly a member of the Staveley Formation), andMarimo Slate of the Staveley Belt (Table 13). Theseformations consist mainly of fine-grained sedimentaryrocks, especially carbonates, siltstones, and shales,which have been regionally metamorphosed to theOverhang JaspiliteThis formation crops out mainly in the northerncentral part of the area, where it is tightly folded on amajor scale, and in the northwest. It consists mainlyof thin-bedded fine-grained calcareous and chertysedimentary rocks (Fig. 43), including some jaspilite.In northeastern exposures, and especially inMARRABA to the north (Derrick, 1980), the uppermostpart of the formation comprises iron-rich andmanganese-rich brccciated beds (part of the ChumvaleBreccia in MARRABA). The breccias may bepart of a fossil regolith representing a major erosionalFig. 43 . Smal l fold s i n thinl y bedde d cher t o f th e Overhan gJaspilite 2 8 k m north-northeas t o f Duches s (a t G RUB915647). (G B 2297 )42

TABLE 13. DETAILS OF STRATIGRAPHY—MARY KATHLEEN GROUP AND ROXMERE QUARTZITE?,EASTERN AREAName of unit(reference to definition)max. thickness (m)Roxmere Quartzite ?(Carter & others , 1961 )Marimo Slat e(Carter & others , 1961 )1000 +Toby Bart y Sandston eMember(Derrick & others , 1977a )1000?Agate Down s Siltston e(Blake & others , 1981a )500 +Staveley Formatio n(Carter, 1959 ; Carte r &others, 1961 ; Blak e & others ,1981a)2000 +Doherty Formatio n(Blake & others , 198 k1000 +Kuridala Formatio n(Carter, 1959 ; Carte r &others, 1961 )1000 +Answer Slat e(Carter, 1959 ; Carte r &others, 1961 ; Blak e &others, 1981a )1000?Overhang Jaspilit e(Derrick & others , 1977a )1000?LithologyBrown, fine t o medium-graine d feldspathi c arenit eand pink , fine-grained quartzite ; mino r gre y cal -careous siltstone , micaceou s siltstone , an d sedi -mentary breccia . Rippl e mark s an d cross-beddin gcommon; grade d beddin g an d convolut e beddin gpresent i n place sGrey t o black , variabl y carbonaceou s slat e an dsiltstone; subordinat e interbedde d arenite , chert ,cherty siltstone , breccia , an d impur e limestone .Some arenit e show s small-scal e cross-beddin gWhite t o grey , fine t o medium-grained , generall ypoorly sorted , feldspathi c arenit e an d quartzit ePale gre y t o brown , chert y an d slat y metasiltstone ,phyllite, an d fine-grained meta-arenit e an d quart -zite; mino r breccia ; som e calcareou s an d fer -ruginous beds , mainl y i n lowe r part . Rar e cross -bedding an d rippl e mark sInterbedded arenite , siltstone , an d phyllite/shale /mudstone whic h ar e variabl y calcareous , ferru -ginous, feldspathic , siliceous , an d micaceous ;breccia; mino r pyriti c siltstone , impur e limestone ,marble, bande d calc-silicat e rocks , mic a schist ,conglomerate, bande d quart z + hematit e +magnetite rock , an d altere d basal t consistin g mainl yof epidote , actinolite , an d albite , wit h garne t i nsome amygdale s (S W o f Selwyn) . Arenit e an dsiltstone commonl y sho w convolute/recumben tbedding, cross-bedding , grade d bedding , an dripple mark s (partl y outline d b y laminat e ric h i nheavy minerals) . Halit e cast s i n place sThin-banded t o laminate d calc-silicat e granofel sand massiv e calc-silicat e breccia ; mino r massiv ecalc-silicate granofels , marble , mic a schist , car -bonaceous blac k slate , chert , variabl y calcareou sand feldspathi c quartzite , metarhyolite , para -amphibolite, metabasalt , an d bande d quartz-tour -maline roc kCalc-silicate rocks: pink , grey , an d green ; consis tof albit e ± microclin e ± quart z ± calcit e ±hornblende ± actinolit e ± saliti c clinopyroxen e± epidot e ± sphen e ± opaqu e mineral s ±garnet (rare ) ± scapolit e (rare )Marble: consist s o f calcit e ± scapolit e ±tremoliteBlack slate: andalusit e needle s i n places ; locall ypyritic; hos t t o C u an d M o deposit sMetarhyolite: pal e pin k t o grey ; contain s smal lphenocrysts o f albite , microcline , an d quart z i nfine-grained recrystallise d quartzofeldspathi cgroundmassPale t o dar k gre y o r ironstained , mediu m t o thin -bedded mic a schist , schistos e metagreywacke , meta -arkose, quartzite , an d graphiti c slat e an d meta -siltstone; als o phyllit e an d mino r thin-bande d t olaminated calc-silicat e granofels , chert , chert yquartz-albite rock , metarhyolite , felsi c tuff? ,banded iro n formation , bande d quart z + hema -tite + magnetit e rock , para-amphibolite , an dmetabasalt?Schists: mediu m t o fine-grained; crenulate d i nplaces; commonl y contai n garne t porphyro -blasts u p t o 5 m m acros s an d andalusit e poikilo -blasls u p t o 5 c m long ; staurolit e porphyro -blasts i n place sGraphitic rocks: commonl y contai n 'matchstick 'porphyroblasts o f andalusit e an d anhedra l poi -kiloblasts o f muscovit e (pseudomorphs? )Calc-silicate rocks: commonl y contai n clinopy -roxene, les s commonl y scapolit eBull t o brow n o r dar k grey , intensel y cleave d an dlocally crenulate d metasiltstone , slate , an d phyl -lite; mino r interbedde d feldspathi c quartzite ,quartzite, metagreywacke , chert , chert y quartz -albite rock , impur e limestone , mic a schist , an dbanded quartz-hematit e rock . Carbonaceous ,pyritic, an d calcareou s bed s common . Scapolit epresent i n places . Widesprea d quartz-veinin gThin-bedded ferruginou s an d calcareou s siltston ewith band s o f re d jaspilite , buf f t o brow n arenit eand quartzite , gre y t o brow n cher t an d phyllite ,impure scapoliti c limestone , an d hematit e +magnetite roc kRelationshipsOverlies Stavele y Formatio n wit happarent conformit yPartly overlie s an d ma y interfinge r wit hStaveley Formation . Intrude d b y smal ldolerite bod yPartly faulte d concordan t contact s wit hStaveley Formatio n an d Marim o Slat eFaulted agains t Answe r Slate ; overlie sStaveley Formatio n wit h apparen tconformityInferred t o overli e wit h possibl e uncon -formity Doubl e Crossin g Metamorphics ,Answer Slate , Overhan g Jaspilite , Kuri -dala Formation , an d Dohert y Forma -tion; overlai n wit h apparen t conformit yby Agat e Down s Siltstone , Marim oSlate, Roxmer e Quartzite? , an d possibl yToby Bart y Sandston e Member . Intrude dby Gi n Creek , Wimberu , an d Squirre lHills Granites , unname d granite , meta -dolerite, an d feldspa r porphyr yProbably overlie s Soldier s Ca p Group ,either conformabl y o r disconformably ;faulted agains t Stavele y Formation ,Kuridala Formation , an d Marim o Slate .Intruded b y Wimberu , Saxby , Moun tAngelay, Squirre l Hills , Cowie , an dBlackeye Granites , unname d granite ,metadolerite, an d dolerit eAppears t o merg e laterall y wit h Answe rSlate J a W an d Soldier s Ca p Grou p t oE; faulte d agains t Dohert y Formation ;inferred t o b e overlain , possibl y uncon -formably, b y Stavele y Formation .Intruded b y Gi n Creek , Squirre l Hills ,Mount Cobalt , Moun t Dore , an dYellow Waterhol e Granites , unname dgranite, metadolerite , an d dolerit eConformable o n Mitakood i Quartzit eand Overhan g Jaspilite ; may b e unconfor -mable o n Doubl e Crossin g Metamor -phics; appear s t o merg e laterall y wit hKuridala Formation ; faulte d agains tAgate Down s Siltstone ; inferre d t o b eoverlain, possibl y unconformably , b yStaveley Formation . Intrude d b yWimberu an d Gi n Cree k Granite s an dmetadoleriteConformable o n Mitakood i Quartzite ;overlain conformabl y b y Answe r Slat eand possibl y unconformabl y b y Stavele yFormation. Intrude d b y Wimber uGranite

16/F54/56Fig. 44 . Infrare d airphot o o f th e are a centre d abou t 2 0 k m west-southwes t o f Selwyn , showin g metadolerit e sill s (dar kgrey) withi n th e Mitakood i Quartzite , Answe r Slate , an d Stavele y Formation . Th e sill s i n th e circle d area s outlin etight t o isoclina l first-generatio n fold s wit h axia l plane s paralle l t o th e prevailin g first-generatio n cleavag e i n th e adjacen tAnswer Slate .unconformity, or they may be a tectonic feature(Derrick & others, 1977a; Derrick, 1980). The OverhangJaspilite was probably deposited in a stableshallow-marine or lagoonal environment (Derrick,1980), possibly on a shelf or platform close to a lowlyinglandmass.A nswer SlateThe Answer Slate forms a northerly trending beltof low strike ridges and undulating terrain 72 km longand up to 6 km wide on the east side of the MalbonBlock. Its contacts with the Staveley Formation andAgate Downs Siltstone to the east are at least partlyfaulted. In the south the Answer Slate appears tograde laterally eastwards into the Kuridala Formation,and it may represent the distal facies equivalents ofturbidites and other sediments of the Kuridala Formationdeposited in a quiet, moderately deep marineenvironment. The presence of pyritic and carbonaceousbeds may be attributed to local reducing conditions.Fig. 45 . Crenulate d Hne-graine d mic a schis t i n th e Kuridal aFormation 1 2 k m southeas t o f Selwy n (a t G R VB596065) .(M2303/32)44

The Answer Slate is tightly folded, and several Kuridala Formationmajor folds outlined by metadolerite sills are clearly The Kuridala Formation crops out as hills, strikevisible on aerial photographs (Fig. 44). Many of these ridges, and undulating terrain in the western centralsills are multiple intrusions which include thin meta- and southern parts of the Squirrel Hills Belt. Thesedimentary screens. Mineral assemblages in the meta- highest ridges are formed of quartzite and bandeddolerites are typical of regional metamorphism of the quartz-hematite rock. The main rock types of the foruppergreenschist and lower amphibolite facies. Quartz mation—schistose greywacke, siltstone, and shaleveins are common throughout the formation, and many (Fig. 45)—may be moderately deep-water turbiditesoutcrop areas are partly covered by quartz rubble. derived from a landmass to the east. The formationSeveral copper lodes are known within the forma- has been tightly folded, probably at least twice, andtion, the largest being at the Answer mine.many large, medium, and small-scale tight to isoclinalThe Answer Slate is a possible correlative of both folds have been identified. Several major fold structheOverhang Jaspilite and Kuridala Formation. We tures are outlined by metadolerite bodies, as at Kuritentativelyconsider it to be older than the Marimo dala, where a series of sills intruded parallel to theSlate exposed to the north, unlike earlier workers (e.g., schistosity has been folded to form a tightly appressed,Carter & others, 1961; Noon, 1978) who have sug- northerly trending structural basin (Fig. 46). Thegested that they are correlatives.formation is intruded by a number of granite plutons,Fig. 46 . Infrare d airphot o o f th e are a aroun d Kuridala , snowin g a north-trending , probabl y second-generation , tigh tstructural basi n (synform) , outline d b y dar k gre y metadolerit e sills , withi n th e Kuridal a Formation . Th e dominan tlayering i n th e Kuridal a Formatio n i n th e synfor m i s a first-generation schistosit y whic h i s generall y paralle l t obedding an d th e sills . A wea k crenulatio n cleavag e presen t i n th e hing e area s i s paralle l t o th e axia l plan e o f th esynform. Small-scal e first-generation foldin g i s inferre d wher e beddin g i s a t a hig h angl e t o th e schistosity . Folde dmetadolerite t o th e west , a t localit y A , ma y defin e a first-generation fold . I n th e northeast , third-generatio n bo x fold sat localit y B an d southeast-plungin g ope n fold s a t localit y C ma y b e relate d t o movemen t alon g th e nearb y Straigh tEight Faul t (labelle d I)) . Northwest-plungin g second-generatio n fold s ar e eviden t i n th e Kuridal a Formatio n a t E , i nthe south . A post-tectoni c granit e body , mappe d a s Squirre l Hill s Granite , form s subdue d terrain i n th e east , nea r localit y F .4516/F64/57

Fig. 47 . Tourmaline-bearin g quartz-feldspa r pegmatit evein withi n th e Kuridal a Formatio n 6. 5 k m sout h o fSelwyn (a t G R VB482128) . Th e pegmatit e ma y b e relate dto th e nearb y Moun t Dor e Granit e o f th e William s Batho -lith. (M2304/25 )by a few tourmaline-bearing pegmatite veins (Fig.47), and, in the vicinity of Pegmont prospect in thesouth, by irregular veins of muscovite granite.Because of the structural complexities, a lack ofmarker beds, and a paucity of sedimentary structuresgiving facing directions, no stratigraphic sequenceshave been recognised within the formation. Also,neither the top nor base of the formation has beenFig. 49 . Photomicrograp h o f matchstick-typ c andalusit eporphyroblasts i n carbonaceou s slat e o f th e Kuridal aFormation a t th e Moun t Elliot t min e nea r Selwy n (a t G RVB443179). (M2442/25 )unequivocally determined. Hence the thickness of theKuridala Formation, though probably in the order ofseveral thousand metres, is unknown.Mineral assemblages indicate regional metamorphismto the amphibolite facies and some possible retrogressionto the greenschist facies. Porphyroblastic garnet,andalusite, and staurolite developed in the metasediments(Figs. 48-50) either during or before the mainschistosity-forming event. Hornblcndc-hornfcls faciesrocks—consisting of fibrolitic sillimanite, andalusite,cordierite, muscovite, biotite, quartz, microcline, andsodic plagioclase—occur in metamorphic aureoles upFig. 48 . Stubb y andalusit e porphyroblast s i n mic a schis t o fthe Kuridal a Formatio n 1 3 k m southeas t o f Selwy n (a tGR VB605064) . (M2303/33 )Fig. 50 . Photomicrograp h o f euhcdra l porphyroblast s o fstaurolite (poikilitic ) an d garne t (left ) i n mic a schis t o fthe Kuridal a Formatio n wes t o f th e Moun t Cobal t min eand 2 3 k m sout h o f Selwy n (a t G R VA477969) .(M2442/32)46

to 100 m wide in places adjacent to granite intrusions;however, at most granite contacts the rocks of theKuridala Formation show little evidence of contactmetamorphism.Doherty FormationThe Doherty Formation crops out in a broad belt,up to 21 km wide, extending from the southeasternpart of the Squirrel Hills Belt northwards for over100 km into the Cloncurry 1:250 000 Sheet area. Itconsists predominantly of thinly banded calc-silicategranofels and massive calc-silicate breccia (Figs. 51,52) which—being more resistant to erosion than mostadjacent rocks—form rugged hills and ridges. Bandingin the metasediments generally represents bedding.No thicknesses of the formation have been determinedbecause of medium to large-scale open to tight folding,faulting, a general lack of both marker beds andfacing evidence, and uncertain stratigraphic relationships.However, a maximum thickness of severalthousand metres is likely.Massive metarhyolite isotopically dated at 1720 ±7 m.y. (Page, 1983a) occurs within the formation inthe east (Blake & others, 1983a). It is considered (byDHB) to be extrusive because breccia, possible agglomerate,and regularly banded rhyolitic rock which mayrepresent contemporaneous bedded tuff occur near itsmargins. Metabasalt and para-amphibolite are presentin the north of the Doherty Formation outcrop.Calc-silicate breccia making up much of theDoherty Formation forms irregular bands and lenses,some of which are hundreds of metres thick. Thebreccia consists of angular to rounded fragmentsranging from a few centimetres to tens of metres longenclosed in an abundant to sparse matrix of generallysimilar overall composition. Some breccia bodies areoligomictic, consisting of essentially one type of calc-silicate rock; others are polymictic, and may containfragments of jasper, chert, and amphibolite, in additionto various calc-silicate rocks. Although the breccia ismainly massive and chaotic, some is banded and possiblyof sedimentary origin (Fig. 51). Most of thebreccia, though, is considered to be tectonic, and maybe related to postdepositional folding, faulting, andigneous intrusion (e.g., Glikson, 1972).The thinly banded calc-silicate rocks representmedium to fine-grained, possibly shallow-marine,impure calcareous sediments which may include sometuffaceous material derived from local contemporaneousrhyolitic and basaltic volcanism. Subsequenttectonism resulted in the formation being folded,regionally metamorphosed to the amphibolite facies,and partly brecciated. Black carbonaceous and locallypyritic slate present in places was probably depositedin a quiet reducing environment; it is host to copperand minor molybdenum and silver deposits (e.g., atMount Arthur mine).The stratigraphic relationship between the DohertyFormation and the Soldiers Cap Group is uncertainbecause most contacts between the two units are probablyfaulted, and the Doherty Formation at thecontacts is invariably represented by calc-silicatebreccia. However, where bedded rocks are present inboth units close to their contacts, bedding is concordant,indicating conformity or disconformity, and aFig. 51 , Bande d an d brecciate d calc-silicat e rock s o f th eDoherty Formatio n 4 3 k m east-southeas t o f Selwy n (a tGR VB880025) . Thi s brecci a appear s t o b e sedimentar y o rdiagenetic, rathe r tha n bein g relate d t o late r tectonism .(M2304/6)Fig. 52 . Tectoni c calc-silicat e brecci a i n th e Dohert y For -mation 3 8 k m eas t o f Selwy n (a t G R VB870188) .(M2304/22)47

possible interfingering relationship is apparent fromthe presence of some Soldiers Cap-type schist withinthe Doherty Formation and some Doherty-type calcsilicaterocks within the Soldiers Cap Group. Previouslythe Doherty Formation (= Corella Formation ofearlier workers) was considered to be unconformableon the Soldiers Cap Group (e.g., Derrick & others,1977a), as calc-silicate breccia of the formation wasthought to form flat-lying outliers overlying steeplydipping Soldiers Cap Group rocks near Cloncurry(Carter & others, 1961; Glikson, 1972). However, atleast some of these so-called outliers have been foundto be dyke and pod-like bodies which were probablyinjected during tectonism (Blake & Derrick, 1980;Wilson, 1979).Contacts between the Doherty Formation andgranite are generally clear-cut (Fig. 53), but in places,especially in the north, calc-silicate rocks appear tograde laterally into contaminated clinopyroxene-bearinggranite.The Doherty Formation has been defined as a newformation (Blake & others, 1981a; see also Blake,1982) for several reasons: it is geographicallyseparated from the outcrop area of the Corella Formationtype section in MARRABA; it is in contactwith units which are not obvious correlatives of thoseadjacent to the Corella Formation exposed to thewest; and it includes metarhyolite (considered to beextrusive) dated at 1720 — 7 m.y., whereas the CorellaFormation in the outcrop area of its type sectionis intruded by granite dated at 1720-1740 m.y., andhence is significantly older than the metarhyolite.and soil-covered plains in a belt up to 16 km wideand more than 75 km long, extending northwards fromtne southern central part of the eastern area. It consistsmostly of well-bedded to locally brecciated sandy,silty, and clayey sedimentary rocks (Table 13). Thesehave been tightly folded and regionally metamorphosedmainly to the lower greenschist facies. Uppergreenschist to lower amphibolite-facies rocks arepresent locally, mainly near granite plutons; theyinclude mica schist, some of which contains stauroliteporphyroblasts, and calc-silicate rocks containing hornblendeand/or clinopyroxene. Rock types and sedimentarystructures (e.g., Fig. 54) indicate probableshallow-water deposition. Some contemporaneoustectonism is indicated by the presence of basaltic lavaat GR VB455140, and possibly by convolute to recumbentbedding structures and the widespread occurrenceof breccia. The thickness of the formation is uncertain.Contacts between the Staveley Formation and adjacentunits are either concordant or faulted, or both.In the north of the Staveley Belt, and farther north inMARRABA, rocks of the Staveley Formation and itscorrelatives overlie breccia consisting almost entirelyof chert fragments enclosed in a siliceous and ferruginousmatrix; if this breccia, which is mapped as partof the Overhang Jaspilite (and also, in MARRABA,as Chumvale Breccia), is regolithic, as favoured byBlake (in Blake & others, 1983a; Donchak & others,Staveley FormationThe Staveley Formation, which includes some outcropspreviously mapped as Marimo Slate and CorellaFormation, is the most extensive unit in the StaveleyBelt. It forms undulating terrain, low hills and ridges,Fig. 53 . Cross-cuttin g contac t o f leucogranite , possibl yrelated t o th e Blackey e Granite , an d bande d calc-silicat erocks o f th e Dohert y Formatio n 5 0 k m southwes t o fSelwyn (a t G R VA9I4937) . (M2304/16 )Fig. 54 . Convolut e beddin g o f recumbent-fol d typ e i n inter -bedded arenit e an d siltston e o f th e Stavele y Formatio n 6 k msouthwest o f Selwy n (a t G R VB451146) . (M2304/26 )48

1983), a low-angle unconformity is implied betweenthe Staveley Formation and underlying rocks.A possible unconformable contact between theStaveley Formation and the Kuridala Formation isexposed near GR VB457138; here little-metamorphosedrocks of the Staveley Formation appear tooverlie finely crenulated schistose rocks of the KuridalaFormation which are cut by ptygmatically foldedquartz veinlcts, and conglomeratic beds in the StaveleyFormation nearby contain schistose clasts possiblyderived from the Kuridala Formation. The crenulatedcleavage in the Kuridala Formation rocks heredeveloped cither during a tectonic event predating thedeposition of the Staveley Formation, or during laterfaulting. Another possible unconformable contactoccurs 7 km north of Kuridala, where small masses ofStaveley Formation breccia appear to be discordant onsteeply dipping Kuridala Formation schists (Donchak& others, 1983). To the south the Kuridala Formationforms two large second-generation synforms partlybounded by the Staveley Formation, but which is theolder unit here is not clear. Several interpretations arepossible from the available evidence: (1) the StaveleyFormation may overlie the Kuridala Formation unconformably(the interpretation favoured at present) orconformably; (2) the two formations may be lateralequivalents; (3) the Staveley Formation may be theolder unit; or (4) there may be two quite separateunits mapped as Kuridala Formation, one older and theother younger than the Staveley Formation.In the north the Staveley Formation is faultedagainst the Doherty Formation. These two formationsare possible correlatives, as both contain bedded andbrecciated calcareous rocks. However, the Staveley16/F54/58Fig. 55 . Infrare d airphot o o f th e are a centre d abou t 2 5 k m north-northeas t o f Kuridala . Th e Dohert y Formatio n her e(I'kd) display s tight t o isoclina l first-generation fold s (e.g. , a tigh t synclina l basi n outline d b y a metadolerit e sil lat A ) whic h i n place s ar e refolde d b y ope n north-plungin g second-generatio n folds , a s a t B) . Bot h foldin g episode swere associate d wit h regiona l metamorphis m an d recrystallisation . Unit s t o th e wes t o f th e Dohert y Formation , wes t o fthe postfoldin g Marti n Cree k an d Bi g Mic k Faults , ar e o f lowe r metamorphi c grade . Som e second-generatio n foldin gis eviden t locall y i n th e lower-grad e rocks , a s a t C i n th e northwest , wher e a north-plungin g second-generatio n fol dhas refolde d th e first-generation cleavag e develope d i n th e Marim o Slate . Th e Roxmer e Quartzite ? (E ? pr) form s a nisolated tilte d bloc k i n th e wes t (D ) an d a southeas t plungin g antiform , whic h i s a n overturne d syncline , i n th enorth (E) .49

Formation is a predominantly clastic unit, unlike theDoherty Formation, and is generally of much lowermetamorphic grade. Consequently, it is consideredto be the younger unit, and it may postdate an episodeof deformation and regional metamorphism affectingthe Doherty Formation.The Staveley Formation is overlain, apparently conformably,by the Agate Downs Siltstone and MarimoSlate, and probably includes lateral equivalents of thesetwo units. It is also overlain with apparent conformityby the Roxmere Quartzite?Agate Downs SiltstoneThis formation forms a north-trending belt 25 kmlong and up to 2.5 km wide in the central part of thearea, on the west side of the Staveley Belt. It consistsmainly of ridge-forming metasiltstone. fine-grainedmeta-arenite, and quartzite. together with less resistantphyllite, and appears to have been regionally metamorphosedonly to the greenschist facies. Cross-beddingand ripple marks near the base of the AgateDowns Siltstone in the south indicate that the formationoverlies the Staveley Formation and is preservedin a complex isoclinal syncline. The closure of themajor fold is outlined in the south by a band of thinbeddedfine-grained quartzite and cherty metasiltstone.The formation may be a facies equivalent of part ofthe Marimo Slate and the upper part of the StaveleyFormation exposed to the northeast. Its top is notexposed.Marimo SlateThe Marimo Slate forms low hills and ridges in thenorth of the Staveley Belt. Following Derrick & others(1977a), we have included in it ridge-forming feldspathicarenite and quartzite mapped as the Toby BartySandstone Member, but the relationship of this memberto the remainder of the formation is uncertain becauseof faulting. The Marimo Slate consists mainly of carbonaceousslate and siltstone. which are cupriferous inplaces, and appears to be regionally metamorphosedto about the middle of the greenschist facies. The carbonaceousbeds are commonly pitted, possibly owingto the leaching out of small pyrite cubes. Most of thenon-carbonaceous rocks previously included within theformation (Carter & Opik, 1963; Noon. 1978) arenow placed in the Staveley Formation, which is probablya facies equivalent of part of the Marimo Slate.Open to tight large-scale folds and minor folds withaxial planes parallel to the slaty cleavage are evidentin places. The thickness of the Marimo Slate may rangebetween 1000 and 2000 m; the uncertainty is becausethe formation shows widespread folding, and its topis not exposed.TABLE 14. GRANITES OF THE WILLIAMS BATHOLITH, EASTERN AREAName of unit(reference to definition)LithologyRelationshipsFoliated granitesBlackeve Granit e(Blake & others , 1981a )Cowie Granit e(Blake & others, 1981a )Maramungee Granit e(Blake & others , 1981a )Non-foliated granitesWimberu Granit e(Carter & others. 1961)Gin Cree k Granit e(Blake & others , 1981a )Foliated leucocrati c granodiorit e an d mino r pegmatit e Intrudes Dohert y Formatio nGenerally foliate d biotitc-bearin g leucocrati c granite ,granodiorite, an d tonalite ; pegmatit eGenerally foliate d biotite-bearin g leucocrati c granite ,granodiorite, an d tonalite ; pegmatit eCommonly porphyriti c granit e an d granodiorit econtaining biotit e and/o r hornblend e ± clinopy -roxene; subordinat e finer-graine d non-porphyriti cbiotite granite ; mino r aplit e an d pegmatit eIntrudes Soldier s Ca p Grou p an dDoherty Formation ; intrude d b y Squirre lHills Granit eIntrudes Soldier s Ca p Group ; cu t b ydolerite dyk eIntrudes Argyll a Formation , Marrab aVolcanics, Mitakood i Quartzite , Over -hang Jaspilite , Answe r Slate , Dohert yFormation, Stavele y Formation , an dmetadoleritePartly porphyriti c biotit e granite ; subordinat e slightl y Intrude s Doubl e Crossin g Metamorphics ,foliated fin e t o coars e leucogranit e containin g mus - Answe r Slate , Stavele y Formation ,covite an d tourmaline , an d intensel y foliate d biotit e Kuridal a Formation , an d metadolerit egranite wit h abundan t inclusion s mostl y o f gneis s an dschist derive d fro m th e Doubl e Crossin g Metamor -phics; mino r biotit e microgranite , aplite , an dgreisenMount Angela v Granit e(Blake & others , 1981a )Mount Cobal t Granit e(Blake & others , 1981a )Mount Dor e Granit e(Blake & others , 1981a )Saxby Granit e(Blake & others , 1981a )Squirrel Hill s Granit e(Blake & others , 1981a )Yellow \ \ .Hi rin. Ii Granit e(Blake & others , 1981a )Partly porphyriti c granil e containin g biotit e and/o r Intrude s Soldier s Ca p Grou p an dhornblende and/o r clinopyroxene ; mino r leucogranite , Dohert y Formation ; cu t b y dolerit eporphyritic microgranite , contaminate d gre y granite , dyke saplite, an d pegmatite ; xenolith s commo n nea r margin sof granit eBiotite granit e an d mino r aplit e Intrudes Kuridal a Formatio n an d meta -doleriteBiotite an d hornblende-biotit e granite , porphyriti c i nplaces; mino r microgranite , aplite , pegmatite , an dgreisenBiotite an d hornblende-bearin g granite ; mino r leuco -granite, xenolithi c diorite , monzonite , granodiorite ,aplite, an d pegmatit eCommonly porphyriti c granit e containin g hornblend eand/or biotit e an d locall y clinopyroxene ; mino r aplite ,porphyritic microgranite , monzonite , an d granodiorite ;rare pegmatit ePartly porphyriti c biotit e an d hornblende-biotit egranite: mino r aplit eUndivided granile Foliated leucocrati c tonalit e an d pegmatite ; biotit egranite: mino r hornblende-biotit e tonalit eIntrudes Kuridal a Formatio n an d meta -doleriteIntrudes Soldier s Ca p Group , Dohert yFormation, an d metadolerite ; cu t b ydolerite dyke sIntrudes Soldier s Ca p Group , an dKuridala, Doherty , an d Stavele y Forma -tions, Cowi e Granite , an d metadolerite ;cut b y dolerit e dyke sIntrudes Kuridal a Formatio nIntrudes Soldier s Ca p Group , Kuridal aFormation, an d Dohert y Formatio n50

The Marimo Slate is similar in lithology to theAnswer Slate, a possible correlative. However, theMarimo Slate is thought more likely to be a youngerunit which, like the Staveley Formation, was depositedin mainly shallow water. The Toby Barty SandstoneMember may be a thick lens within the Marimo/Staveley sequence, or a younger unit, possibly equivalentto the Roxmere Quartzite?Roxmere Quartzite?Ridge-forming arenites mapped as Roxmere Quartzite?(Table 13) crop out within the Staveley Belt inthe northern central part of the area, forming a partlyfault-bounded block about 20 km north of Kuridalaand a southeasterly trending ridge to the northeast(Fig. 55). The southern outcrop was mapped as RoxmereQuartzite by Carter & others (1961) and Carter& Opik (1963), but both it and the northern outcropare geographically separated from, and cannot bedirectly correlated with, the outcrop area containingthe type section of the formation near Cloncurry. Thenorthern outcrop is a southeasterly plunging antiformwhich is an overturned syncline bounded by brecciaof the Staveley Formation. This antiform may be partof a large recumbent fold or nappe structure.The arenites making up the unit are generally similarin lithology to those in the nearby Staveley Formationand Marimo Slate, and are considered to be part ofthe same, mainly shallow-water, sequence. They appearto have been regionally metamorphosed to about thelower greenschist facies.Williams BatholithThe Proterozoic stratigraphic succession in theeastern area is intruded by a series of granitic plutonswhich collectively form the Williams Batholith (Table14). Except for the most westerly intrusion, formed ofWimberu Granite, the plutons were previously mappedas Williams Granite (Carter & others, 1961; Carter &Opik, 1963).In the field the granites of the Williams Batholithcan be divided into two main types. One type isrecrystallised and foliated, leucocratic (pale pinkish),texturally heterogeneous (at any one exposure), andgenerally non-porphyritic. It forms small plutons inthe east which have generally concordant and in placeslit-par-lit intrusive contacts. The foliation in thesegranites is concordant with the foliation/mctamorphiclayering in adjacent country rocks. In some contactzones, migmatitic complexes are developed. The othertype is mostly non-foliated, darker pink, and texturallyhomogeneous, and commonly contains plagioclase andmicrocline phenocrysts 1-5 cm across. It forms mainlylarge plutons and groups of plutons which have crosscuttingintrusive contacts. The foliated, heterogeneousgranites may be syntcctonic, and are inferred to beolder than the non-foliated, homogeneous granites,which arc post-tectonic. These younger granites wereprobably emplaccd between 1500 and 1550 m.y. ago(Blake & others, 1983b).Foliated granitesNortherly trending intrusions of fine-grained to pegmatitic,foliated granitic rocks cropping out along theeastern margin of the Williams Batholith are mappedas Blackcye, Cowie, and Maramungee Granites andunnamed granite. The intrusions are similar in compositionand probably age. They typically contain upto 5 percent primary ferromagnesian minerals (amphibole± biotite ± clinopyroxene), are commonly xenolithic,and have been regionally metamorphosed to thegreenschist facies.The Blackcye Granite forms a small but well-definedpluton 1.6 km long intruding the Doherty Formation inthe southeast. The Cowie Granite forms a pluton 11.5km long and up to 3 km wide 3 km to the west;it is intruded by non-foliated porphyritic micrograniteprobably related to the adjacent Squirrel Hills Granite.The Maramungee Granite forms a pluton 5 km long12 km north of the Blackcye Granite; biotite fromthe Maramungee Granite has given a K-Ar age of1400 m.y. (Richards & others, 1963, sample GA 134),but this is probably a cooling age rather than theage of granite emplacement.Non-foliated granitesThe voluminous non-foliated granites of the WilliamsBatholith, represented by the Wimberu, Gin Creek,Mount Angelay, Mount Cobalt, Mount Dore, Saxby,Squirrel Hills, and Yellow Waterhole Granites andsome unnamed granite, are probably closely related toone another both magmatically and in age. They aremainly medium to coarse-grained, and typically consistof strained quartz, about equal amounts of slightlyzoned oligoclase and commonly perthitic microcline,5 to 15 percent of biotite and/or pale to dark greenhornblende and/or pale green clinopyroxene, up to 5percent magnetite and sphene, and accessory andsecondary allanite, apatite, calcite, chlorite (mostlyafter biotite), epidote, fluorite, muscovite, sericite(mostly after plagioclase), sulphide minerals, andzircon. Micrographic quartz and microcline and myrmekiticquartz and plagioclase occur in some samplesexamined microscopically. Xenoliths and pegmatitesare generally rare, but veins and patches of aplite arepresent at most exposures. Thin veins of hematiteoccur locally within the Mount Dore and SquirrelHills Granites.In detail the bodies of the non-foliated granitesgenerally have sharp, commonly irregular intrusivecontacts which cut across without obviously deflectingthe bedding and foliation of the country rocks. However,some granite contacts, especially in the north,are broadly concordant with adjacent stratigraphicunits. At many localities the country rocks appear toshow no contact metamorphic effects, but metamorphicaureoles up to 100 m wide, consisting of hornfels, arepresent in places.The field evidence indicates that the non-foliatedgranites were emplaced, probably by stoping, after thecountry rocks had been folded and regionally metamorphosed.However, petrographic evidence shows thatmost, if not all, these granites have been affected by alow-grade metamorphic event. Though termed 'nonfoliated',the granites are sheared and foliated in places,mainly near faults.The Wimberu Granite forms a large composite body,up to about 42 km across, which was emplaced withinthe major anticlinal structure in the northwestern partof the area, and it also forms some small intrusions,possibly connected to the main body at depth, to theeast. The main intrusion contains several large pendantsof country rocks and is partly concealed beneathCambrian sediments of the Burke River StructuralBelt. Two isotopic ages are available for the granite:a K-Ar biotite age of 1498 m.y. (Richards & others,51

Fig. 56 . Aeria l view , fro m th e northwest , o f th e smal l pluto n o f Moun t Cobal t Granit e intrudin g th e Kuridal aFormation 1 9 k m sout h o f Selwyn . (M2303/26 )1963); and an Rb-Sr total-rock age of 1530 m.y.(Richards, 1966) derived from a decay constant forRb 8 7 of 1.39 X 10-Wjr 1 , or 1498 m.y. according tothe current recommended decay constant for Rb 8 7of1.42 X 10-iiy-i.The Gin Creek Granite crops out on the southwesternside of the Williams Batholith, forming anortherly trending composite body 24 km long and upto 6 km wide and also a few much smaller intrusionsnearby. Unlike other named granites of the WilliamsBatholith, it includes both non-foliated and foliatedtypes of granite.The Mount Angelay Granite forms a north-northwesterlytrending intrusion that crops out over an areaof about 200 km 2in the northeast on the western sideof the Cloncurry Fault. It includes some heterogeneouspale to dark grey granitic hybrid rocks. Contaminatedgranite present in places adjacent to theDoherty Formation contains scapolite.The Mount Cobalt Granite is exposed as a prominentstock about 1 km across (Fig. 56) at GR VB5100. Thestock is surrounded by a metamorphic aureole about100 m wide in which rocks of the Kuridala Formationand metadolerite have been metamorphosed to hornfels.The Mount Dore Granite forms a northeasterlytrending pluton 14 km long and 7 km wide intrudingthe Kuridala Formation between Selwyn and theMount Cobalt Granite. It has been dated by Rb-Sr at1509 ± 22 m.y. (Nisbet & others, 1983).The Saxby Granite forms closely spaced, irregularlyshaped plutons up to 10 km across in the northeast,mainly east of the Cloncurry Fault. It includes a complexbody of xenolithic diorite, monzonite, and granodioritenear the headwaters of the Williams River.The Squirrel Hills Granite forms a large compositebody about 100 km long and up to 25 km wide trendingnorth-northwest in the eastern half of the easternarea. Porphyritic microgranite associated with thisgranite intrudes the foliated Cowie Granite in thesoutheast.The Yellow Waterhole Granite forms an east-trendingintrusion 19 km long and up to 2.5 km wide in thesouthern central part of the eastern area. The intrusioncuts the surrounding northerly trending KuridalaFormation, which in a few places is metamorphosedto hornfels adjacent to the granite. A K-Ar biotite ageof 1458 m.y. (Richards & others, 1963, sample GA172) is regarded as a minimum age for the emplacementof this granite and probably also for the othernon-foliated granites of the Williams Batholith.Minor intrusionsSills, dykes, and pod-like bodies of metadolerite and,in the east, amphibolite are widespread in the easternarea. They intrude most Proterozoic stratigraphicunits, and range from less than a metre to severalhundreds of metres in thickness. Many of the largerbodies are multiple sills in which some individual intrusionsare separated from one another by thin screensof country rock.The metadolerite and amphibolite intrusions predatethe last major folding and metamorphic event affectingthe area, and most are probably older than the granitesof the Williams Batholith. Some, such as the sills atKuridala (see below), postdate one folding event butwere folded during a subsequent event. A small net-52

veined complex of granite and metadolerite at BooramaWaterhole (GR VB818316—shown incorrectly on theaccompanying map as Boomerang Waterhole) indicatesthat at this locality the metadolerite is either similar inage or younger than the associated granite (Blake,1981a).Non-metamorphosed dolerite forms a series ofwidely spaced east-trending dykes up to about 50 mthick cutting granite and older rocks. The doleriteconsists of ophitic pyroxene (augite ± hypersthene) +calcic plagioclase laths (commonly much altered) ±possible pseudomorphs after olivine + opaqueminerals + apatite — biotite — interstitial and partlymicrographic quartz and alkali feldspar. These dykespostdate the emplacement of granite and the last metamorphicevent to affect the area. They are the youngestPrecambrian rocks exposed in this part of the MountIsa Inlier, and may be correlated with the LakeviewDolerite to the north, which has been dated by theRb-Sr whole-rock method at 1116 ± 12 m.y.(Derrick & others, 1978; Page, 1983b).StructureFoldingIn the western part of the eastern area, the ArgyllaFormation to Overhang Jaspilite sequence has beenfolded into a large anticlinal structure about 50 kmacross. This structure, which corresponds to theMalbon Block, continues northwards into MARRABAas the northeasterly plunging Bulonga and DuckCreek Anticlines and intervening syncline (Derrick,1980). Thin chert and jasper bands in the OverhangJaspilite locally show related small-scale folds (Figs.8c, 43). The eastern limb of the anticlinal structurein the north, north of the Malbon-Kuridala road, hasbeen refolded to produce a series of large northeastplunging,tight to isoclinal folds (Fig. 57): these differin style, orientation, and possibly age from other foldsin the region.Immediately to the east of the major anticlinalstructure there is a complex folded and faulted zonemade up of the Answer Slate of the Malbon Blockand the Staveley Formation, Agate Downs Siltstone,Marimo Slate, and Roxmere Quartzite? of the StaveleyBelt. The Staveley Formation and Marimo Slate in thefar north have been folded into north-plunging structures.The northernmost outcrop of the RoxmereQuartzite? is a southeasterly plunging antiform representingan overturned syncline (Fig. 55), possibly partof a nappe structure, surrounded by Staveley Formationbreccias. This structure, as well as the partlyfault-bounded block of Roxmere Quartzite? a fewkilometres to the south, may have resulted from majorovcrthrusting, shortly after lithification, along a decollemcntzone formed of breccia.To the south, the Answer Slate, Staveley Formation,and Agate Downs Siltstone show tight to isoclinal foldswith mainly gentle plunges. Many of the folds in theAnswer Slate, an intensely cleaved unit, and also somein the Staveley Formation, are outlined by metadoleritesills (Fig. 44). The Agate Downs Siltstone defines alarge isoclinal north-plunging syncline complicated bytranscurrent and strike-slip faults.The Kuridala Formation to the east, part of theSquirrel Hills Belt, is a sequence of complexly foldedmetapclites and meta-arenites. The metapelites have awell-developed schistosity which in places has beenfolded about open to tight major antiforms and synforms.Folds related to the early schistosity appear tobe rare, but some examples of early major and minorfolds have been found (Fig. 58), and larger folds maybe inferred where bedding is clearly at a high angleto the schistosity; early open anticlines and tight synclineswith wavelengths of up to several hundred metreshave been described by Donchak (in Blake & others,1983a) from a locality 9 km southeast of Mount Dore.A good example of a later fold is the tightly appressednorth-trending 20-km-long structural basin outlined bydolerite sills near Kuridala (Donchak & others, 1983;Fig. 46). Small subsequent folds here may be relatedto movements along the Straight Eight Fault to theeast. Near Mount Cobalt mine, southwest of Mount16/F54/59Fig. 57 . Infrare d airphot o o f th e are a centre d abou t 1 0km eas t o f Malbon , showin g tigh t t o isoclina l northeast -plunging large-scal e fold s i n ridge-formin g Mitakood iQuartzite o n th e easter n sid e o f th e Malbo n Block . Thes efolds appea r t o di e ou t t o th e west , i n th e underlyin gMarraba Volcanic s an d Argyll a Formation , an d d o no tpersist abov e th e overlyin g Overhan g Jaspilit e t o th e east .They appea r t o fol d pre-existin g cleavage s associate d wit hthe Duc k Cree k Anticline , an d henc e probabl y postdat ethis structure . N o fold s o f thi s styl e an d orientatio n hav ebeen foun d elsewher e i n th e region .53

Dore, Nisbet & others (1983) have recognised twogenerations of tight to isoclinal folds and a later phaseof more open folds.To the east, tight to isoclinal early folds in theDoherty Formation have wavelengths ranging fromseveral metres to several kilometres, steeply dippingaxial planes, and moderate to steep plunges. Thesefolds have been found only in thin-banded calc-silicaterocks in which an associated axial-plane cleavageis rarely evident. They are commonly refolded aboutsteeply dipping axial planes (Fig. 59) to form northand south-plunging structures, including some basinand-domeinterference folds.The easternmost Precambrian unit, the Soldiers CapGroup, is also multiply deformed. A well-developedearly schistosity and metamorphic layering generallytrend north, dip steeply, and are commonly parallelto boudinaged pegmatite and quartz veins. Compositionalbanding in the gneisses probably resultsfrom transposition of original bedding, locallyenhanced by metamorphic differentiation. Recognisablebedding is generally parallel to the schistosity, exceptwhere shallowly plunging isoclinal and intrafolial rootlessminor folds are present (Fig. 42). Large earlyfolds related to the schistosity have been recognisedonly in the vicinity of the Fairmile prospect, where anisoclinal fold is outlined by a bed of banded ironformation. Later major tight folds, generally plungingnorth, and also some basin-and-dome-type folds occursporadically in the eastern part of the Soldiers CapGroup (Fig. 60). Axial planes are mainly verticaland locally have an associated crenulation cleavagewhich in a few areas is defined by mica-rich bands thatobliterate the earlier schistosity. A southeast-plungingyounger antiform occurs in the far north, betweenSnake and Sandy Creeks (Donchak & others, 1983).FaultingIn the eastern area the main faults, which includethe Cloncurry, Straight Eight, Big Mick, Martin Creek,Mr > ocz

and Happy Valley Faults, trend between northwestand northeast. The latest movements along these faultspostdate folding, and some postdate the emplacementof the Williams Batholith. Some smaller faults, such asthose in the Mitakoodi Quartzite east of Malbon, arealigned parallel to the axial planes of folds and wereprobably formed when ductile folding could no longerdissipate applied stresses. As in other parts of theregion, many lithologic boundaries are marked by faultsresulting from bedding-plane slip during deformation.Metamorphic effectsThe highest-grade regionally metamorphosed rocksof the eastern area are exposed in the southwest andfar east (Fig. 9), as described by Jaques & others(1982).Fig. 59 . Airphot o o f a n are a centre d 2 2 k m nort h o f Kuridala , showin g well-bedde d calc-silicat e rock s o f th e Dohert yFormation i n th e south , th e northeast-trendin g Mic k Cree k Fault , an d north-trendin g ridge s o f westerl y dippin g Roxmer eQuartzite? i n th e nort h an d northwest . Th e bedde d calc-silicat e rock s sho w tw o generation s o f folding— a south-south -west-plunging lirst-gcncratio n fol d (singl e arrowhead ) an d north-northeas t plungin g second-generatio n fold s (doubl e arrow -heads). Th e hill y terrain t o th e eas t an d northeas t i s forme d o f calc-silicat e breccia .55

In the north, west of the Doherty Formation outcrop,delicate primary structures and textures in sedimentaryand igneous rocks are commonly well preserved,suggesting regional metamorphism to no morethan the greenschist facies. However, rims of bluegreenhornblende around cores of actinolite in somebasaltic rocks of the Marraba Volcanics, and intenselyrecrystallised felsic volcanics at some exposures of theArgylla Formation, indicate that the lower amphibolitefacies was reached in places.To the south the Argylla Formation, MarrabaVolcanics, and probably also the Mitakoodi Quartziteand Answer Slate consist predominantly of amphibolitefaciesrocks, whereas the Staveley Formation and AgateDowns Siltstone to the east appear to have been metamorphosedmainly to the greenschist facies.The Kuridala and Doherty Formations, east of theStaveley Formation outcrop, are formed of amphibolite-faciesrocks. Pelitic schists and meta-arenitesof the Kuridala Formation typically consist of quartz-I- biotite + muscovite ± alkali feldspar + porphyroblastsof andalusite, garnet, and less commonly staurolite,and associated metabasites contain hornblendeand plagioclase more calcic than Ani;; hornblendehornfels containing andalusite, cordierite, and fibroliticsillimanite is present in metamorphic aureoles upto 100 m wide adjacent to some non-foliated graniteintrusions of the Williams Batholith. Typical assemblagesin the calc-silicate rocks of the Doherty Formationare calcite + sodic plagioclase (mostly albite) +microcline + quartz ± hornblende ± clinopyroxene(salitic) ± actinolite + epidote + sphene ± scapolite(rare); in the north the Martin Creek and BigMick Faults separate these amphibolite-facies rocksfrom greenschist-facies arenites and breccias of theStaveley Formation to the west.Fig. 60 . Infrare d airphot o o f Soldier s Ca p Grou p an d Saxb y Granit e outcrop s centre d abou t 3 5 k m east-northeas t o fKuridala, i n th e northeas t o f th e region . Th e dominan t trend s i n th e metasediment s represen t bedding , whic h i s generall yparallel t o th e first-generation foliation . Tigh t majo r second-generatio n fold s occu r i n th e circle d areas . A pluto n o f post -tectonic Saxb y Granit e occupie s th e genera l depressio n i n th e north . Mesa s i n th e wes t ar e cappe d b y flat-lyin g Mesozoi csediments o f th e Gilber t Rive r Formation .16/F54/6256

In the Soldiers Cap Group the metamorphic gradegenerally increases eastwards (Jaques & others, 1982),and in places the rocks probably reach the upperamphibolite facies. Partial melting is indicated byabundant veins and pods of leucogranite and pegmatiteand the local development of migmatite (Fig. 42).Mineral assemblages include quartz + sodic plagioclase+ microcline + biotite + muscovite ± garnet± andalusite ± sillimanite in schist and gneiss, greenhornblende + oligoclase/andesine in amphibolite, andamphibole 4- clinopyroxene + albite ± garnet ±scapolite in calc-silicate rocks.The amphibolite-facies regional metamorphism maybe related to the earliest major folding event recognisedin the eastern area. The sporadic development ofchlorite in the amphibolite-facies rocks may be relatedto a later, less intense, tectonic evpnt .GEOCHEMISTRY O F IGNEOU S ROCK SThe following account is mainly based on chemicalanalyses of 412 samples collected between 1975 and1980. The samples were analysed for major elementsat The Australian Mineral Development Laboratories(AMDEL), Adelaide, and for trace elements at BMR,mainly by X-ray spectrometry, though abundancesof Co, Cr, Cu, Li, Zn, Ni, and V were determinedby atomic absorption spectroscopy. Representativechemical analyses are listed in Tables 15-17, andvariation diagrams for selected elements in felsicvolcanics and granites are shown in Figures 61 and62. Wyborn (in preparation) has compiled a completelist of analyses and sample localities, and separatepublications discuss in more detail the geochemistry ofTABLE 15. REPRESENTATIVE CHEMICAL ANALYSES OF FELSIC VOLCANICS FROM THEDUCHESS-URANDANGI REGION(from Bultitude & Wyborn, 1982)No. o f sample s orsample numbers*SiO,TiO,A1 20 3FeOMnOMgOCaONa 20K 20p 2O 5BaRbSrPbThUZrNbYLaCeLeichhardtVolcanicsArgyllaFormationBottletreeFormationCorellaFormationCarters Bore Rhyolite34 13 19 15 7653- 7653- 7653-X s X s X s X s 1143A 1143B 1143C72.36 2.59 69.03 2.52 70.00 3.60 67.40 4.24 76.10 74.60 73.200.23 0.11 0.62 0.20 0.64 0.15 0.48 0.24 0.47 0.45 0.4413.84 0.86 12.82 0.38 11.93 2.97 12.88 0.67 11.20 10.70 11.101.19 0.50 3.04 1.24 2.71 1.34 3.23 2.13 0.49 3.75 3.881.18 0.86 2.05 0.81 3.19 0.80 1.64 0.95 0.24 0.35 0.350.03 0.02 0.04 0.03 0.06 0.02 0.05 0.08 0.01 0.01 0.010.45 0.31 0.56 0.36 0.45 0.26 0.61 0.28 1.02 0.13 0.131.60 0.64 1.68 1.14 2.43 0.98 1.74 1.13 0.24 0.09 0.092.90 0.58 2.55 0.95 2.10 0.81 1.67 1.00 0.20 0.10 0.164.84 0.96 5.52 1.70 4.76 1.60 7.71 1.58 8.82 9.20 9.440.06 0.03 0.15 0.09 0.14 0.06 0.11 0.06 0.15 0.06 0.07817 318 856 503 1483 523 1194 332 210 200 170197 53 190 53 175 50 241 64 100 130 140152 60 89 58 180 74 40 28 24 14 1326 23 7 5 18 10 3 3 5 3

igneous rocks in the region: volcanic rocks (Bultitude dolerite samples collected before 1975 (Ellis && Wyborn, 1982), granites of the Kalkadoon Batho- Wyborn, in press). Papers discussing the geochemistrylith (Wyborn & Page, 1983), and dolerite and meta- of the other batholiths in the region are planned.1I + \ •A" + \70 7 5 8 0Si0 2(%)WESTERN ARE A— Field of Leichhardt £ x Carter s Bor e Rhyolit eMetamorphics + Suliema n Gneis s• — Field of Argylla FormationCENTRAL AREAT Corell a Formatio n+ Argyll a Formatio n• Bottletre e Formatio nA Leichhard t Volcanic sO Tewing a Grou p (undivided)® Plu m Mountain Gneis sEASTERN ARE AEl Dohert y Formatio n• Mitakood i Quartzit eH Doubl e Crossin gMetamorphics• Soldier s Ca p Group16/F54/27Fig. 61 . Abundance s o f selecte d majo r oxide s an d trac e element s plotte d agains t Si O • fo r th e felsi c volcanic s o f th eDuchess-Urandangi region . Als o show n ar e th e field s outline d b y th e Leichhard t Metamorphic s (5 3 analyses ) an dArgylla Formatio n (7 9 analyses ) fro m nort h o f th e regio n usin g dat a fro m Kossite r & Ferguso n (1980 ) an d Wybor n(in preparation) .58

Almost all the analysed rocks have been regionallymetamorphosed to either the greenschist or amphibolitefacies, and many are extensively recrystallised. However,as the various suites show generally consistenttrends for most elements on variation diagrams, themajority of analysed samples are thought to be close totheir original composition, rather than being extensivelymodified by metamorphism.Silica values for the igneous rocks show a welldefinedbimodal distribution (Fig. 63), and igneousrocks of intermediate composition are rare, as in otherparts of the Mount Isa Inlier.Felsic volcanicsBultitude & Wyborn (1982) recognised five distinctivegeochemical suites of felsic volcanics in theDuchess-Urandangi region: the Leichhardt suite,characterised by high Sr and low Zr, Nb, and Yabundances (relative to the other suites); the Argyllasuite, which has low Sr and high Zr and Nb; theBottletree suite, which has high Ba, Sr, Zr, and Nb;the Duchess-Corella suite, characterised by low Sr andZr and high Nb and Y; and the Carters Bore suite,which has high K 20 and low AI2O3, Na 20, CaO, Pb,Sr, and Ba. Some of these differences are illustratedin Figure 61. Each suite is thought to be derived froma chemically unique source.Western areaThe two analysed samples of felsic metavolcanicsfrom the Sulieman Gneiss may belong to the Leichhardtsuite, although they are anomalously high in Nb.No analyses are available for felsic volcanic rocks fromthe Saint Ronans Metamorphics. Three samples of themuch younger Carters Bore Rhyolite have beenanalysed; these have the high K2O contents characteristicof the Carters Bore suite, and their T1O2 andMgO contents are notably high (Table 15) for rockswith high silica values (73-76 wt%).Central areaThe 34 analysed felsic volcanic rocks of the LeichhardtVolcanics show the typical features of the Leichhardtsuite: low Y, Zr, Nb, Ti02, and total Fe contents,relatively low K2O, Th, and U abundances, andhigh AI2O3, Pb, Sr, Na20, and CaO contents. They arechemically and also isotopically identical with those ofthe Leichhardt Metamorphics north of the Duchess-Urandangi region (Wilson, 1978), and with thegranites of the Kalkadoon Batholith (Wyborn & Page,1983), their comagmatic intrusive equivalents.The felsic volcanics of the Argylla Formation (13analyses), like those to the north (Wilson, 1978;Rossiter & Ferguson, 1980), are distinguished by highZr, Nb, Y, Ti0 2, and total Fe contents, relatively highK2O, Th, and U contents, and low CaO, Na 20, AI2O3,Pb, Sr, and Zn contents, which are characteristic of theArgylla suite.Felsic volcanics of the Bottletree Formation (19analyses) belong to the Bottletree suite. They resemblethe felsic volcanics of the Argylla Formation, excepttheir CaO, Sr, and Pb contents are more similar tothose of the Leichhardt suite.Felsic volcanic rocks in the Corella Formation of theDuchess Belt (15 analyses) comprise the Duchess-Corella suite. They have high Nb and Y contents, likethose of the Argylla and Bottletree Formations, butlow Zr, Ti02, and total Fe, as in the Leichhardt Volcanics.Sr, Pb, CaO, and AI2O3 values are low, andTh and U contents are high. Very low Na20 andexceptionally high K2O may be alteration effects.Among the felsic volcanic rocks from the undividedTewinga Group (6 analyses), one sample, which hasalso been isotopically dated (Page, 1983a), is indistinguishablein chemistry and age from the LeichhardtVolcanics; the other 5 samples resemble those of theBottletree suite. Two analysed samples from the PlumMountain Gneiss, both probably intrusive rather thanextrusive, appear to belong to the Leichhardt suite.Eastern areaThe only analysed sample from the Double CrossingMetamorphics, a felsic gneiss thought to be a metamorphosedvolcanic rock, is enriched in incompatibleelements, like the Argylla suite. The two felsic volcanicsamples analysed from the Mitakoodi Quartzite aretypical Argylla suite rocks. Two samples of rhyolitefrom the Doherty Formation have been analysed: one,isotopically dated at about 1720 m.y. (Page, 1983a),is enriched in incompatible elements, whereas the other,from a separate but nearby body, is depleted in theseelements. One sample of a possible felsic metavolcanicrock from the Soldiers Cap Group, a quartzofeldspathicgneiss, has low Sr, Y, Zr, and AI2O3, and highTi02, total Fe, and K2O contents. This rock and theDoherty Formation rhyolites do not fit into any of thefive main suites. No analyses of felsic volcanic rocksfrom the Kuridala Formation are available.GranitesParts of four major granitic batholiths are exposedin the Duchess-Urandangi region. From west to eastthese are the Sybella, Kalkadoon, Wonga, and WilliamsBatholiths. Each batholith is made up of severalplutons, most of which have been foliated and/orrecrystallised to some extent owing to postemplacementdeformation and metamorphism, and each has itsown particular chemical characteristics, some of whichare illustrated in Figure 62.The Sybella Batholith consists of northerly trendingelongate plutons and some associated porphyry dykes,and is confined to the western area; it has no undoubtedextrusive equivalents. Most of the plutonsare intensely foliated and have been metamorphosedto at least the lower amphibolite facies. The main rocktype is a biotite-bearing granite characterised by largeK-feldspar augen and commonly containing deep bluegreenhornblende and purple fluorite. Similar graniteforms most of the plutons of Sybella Granite to thenorth of the Duchess-Urandangi region, in MOUNTISA and KENNEDY GAP, where some of the granitehas been dated at about 1670 m.y. (Page, 1981a). Themain granite type has very high levels of incompatibleelements, and has higher T1O2, P2O5, La, Ce, and Pband lower AI2O3 and Sr contents than most othergranites of the Mount Isa Inlier. The batholith alsoincludes one or more plutons of a petrographically andchemically distinct granitic type, here termed theKahko type, which crops out mainly between Kahkoand Jayah Bore Creeks. The Kahko type consists ofbiotite-rich granodiorite and subordinate granite. Comparedwith the main granite of the Sybella Batholith,it is mostly finer-grained, richer in plagioclase phenocrysts,and at similar silica values has less amphibole.It also has lower Ti02, Zr, Y, and La, and higherAI2O3 and Sr contents; in these chemical features it59

TABLE 16 . REPRESENTATIV E CHEMICA L ANALYSE S O F GRANITE S FRO M TH E DUCHESS-URANDANG I REGIO NKalkadoonSybella B atholith B atholith W onga B atholith W illiams B atholithBowlers Hole Bushy Park Overlander Maramungee Wimberu SaxbyTotal Main type Kahko type type type type type type typenumber of 48 13 50 3 9 19 4 25 6samplesanalysed ~x s ~x s X s ~x s X s ~x s ~x s X s ~x sSiO, 71.12 3J4 70.38 4.40 68.80 4.56 69.13 0.21 70.81 1.45 72.86 4.93 70.05 5.24 71.79 3.86 69.28 6.58Ti0 2 0.50 0.32 0.44 0.26 0.44 0.22 0.51 0.04 0.44 0.08 0.23 0.15 0.32 0.29 0.35 0.24 0.54 0.46Al 2O rt13.15 0.54 14.06 1.66 14.76 1.59 12.93 0.29 13.27 0.46 13.32 1.66 14.73 1.39 13.71 0.72 13.42 0.75Fe,O 1.22 0.78 1.02 0.65 0.76 0.37 1.86 0.82 0.94 0.18 1.04 0.53 1.94 1.90 1.33 1.31 1.86 1.50tFeO 2.31 1.35 2.36 1.19 2.81 1.35 3.65 0.43 2.82 0.67 1.26 1.36 1.39 1.52 1.07 0.67 2.79 2.26Tot. F e 3.41 1.62 3.28 1.57 3.50 1.50 5.32 0.33 3.66 0.74 2.20 1.42 3.13 3.21 2.26 1.40 4.47 3.27MnO 0.05 0.02 0.05 0.02 0.05 0.02 0.09 0.01 0.04 0.01 0.04 0.03 0.05 0.05 0.03 0.01 0.06 0.04MgO 0.61 0.41 0.77 0.60 0.98 0.61 0.27 0.09 0.56 0.07 0.29 0.23 0.66 0.36 0,67 0.47 0.82 0.82CaO 1.92 1.03 2.14 1.06 2.74 1.35 2.12 0.36 1.50 0.39 1.05 0.60 2.31 1.86 1.66 1.01 1.91 1.59Na 20 2.69 0.57 2.79 0.70 2.88 0.80 2.65 0.28 2.60 0.29 3.80 1.10 4.45 1.10 3.87 0.98 3.67 0.32K 20 5.11 1.30 4.75 1.71 4.17 1.13 5.32 0.23 5.54 0.45 4.97 1.37 2.68 1.40 4.17 1.22 3.97 0.84P aO B0.13 0.10 0.14 0.09 0.11 0.07 0.09 0.01 0.11 0.04 0.06 0.05 0.10 0.09 0.13 0.11 0.19 0.14Ba 656 393 711 275 793 426 880 72 861 250 369 388 343 192 761 488 465 384Li 16 9(27)* 21 12 13 9(35) 14 (1) 22 9(7) 5 6(12) 6 4(2) 7 4(19) 9 3Rb 263 103 248 104 172 57 299 18 261 30 236 95 100 71 176 67(24) 268 100Sr 120 85 157 82 230 136 90 14 103 12 42 34 232 289 134 73 101 139Pb 32 12 27 15 27 14 22 15 19 7 14 12 26 24 11 8 13 5Th 53 29 45 32 24 8 31 3 32 3 54 27 32 34 51 31 41 23U 10 7 6 4 4 3 5 9 5 1 12 6 7 8 10 5 15 12Zr 354 136 247 100 218 87 557 112 255 52 239 167 157 47 206 90 242 162Nb 31 9 21 7 11 4 35 6 14 4 30 17 9 3 24 7 33 12Y 59 18 40 19 24 10 85 6 46 10 61 37 20 15 35 13 40 16La 116 51 112 68 62 23 84 5 75 12 71 46 37 10 68 36 42 22Ce 201 81 188 110 110 39 167 6 131 23 122 75 56 5 114 54 80 35Nd 52 32(4) n.a. 49 15(35) 75 (1) 61 12(7) 62 36(12) 28 16(2) 42 16(18) 39 18Sc 4 (4) n.a. 7 5(35) 6 (1) 7 11(7) 4 11(12) 11 10(2) 5 5(18) 11 17V 25 24 26 26 35 23 19 12 21 12 22 40 14 13 28 31 45 55Cr 9 3(4) n.a. 17 12(35) 6 (1) 11 2(7) 8 4(12)13 5 9 3(23) 10 6Co 5 (4) n.a. 9 5(28)

ea.S 4 0120900403•02040065 7 0Si0 2 (%)+657 07 58 0Si0 2 (% )WESTERN ARE ASybella Batholit• Mai n phas eO Kahk o typ ehCENTRAL ARE A+ Kalkadoo n Batholit hWonga Batholit h# Bowler s Hol e typ e• Bush y Par k typ e£ Overlande r tvn eEASTERN ARE AWilliams BatholitA Maramunge e typ e• Saxb y typ e• Wimber u typ eIWNMhFig. 62 . Variatio n diagram s o f selecte d majo r oxide s an d trac e element s fo r granite s o f th e Duchess-Urandang i region .61

more closely resembles the granites of the KalkadoonBatholith to the east than the main granite type of theSybella Batholith. However, it has higher Th, U, andNb contents than the Kalkadoon-type granites. TheKahko type may include the granodiorite of Kalkadoontype that Joplin & Walker (1961) reported northwestof Sulieman Bore.The Kalkadoon Batholith forms part of the Kalkadoon-LeichhardtBlock of the central area. It comprisesthe Kalkadoon Granite, which has been datedat 1865 +U m.y. (Wyborn & Page, 1983), and theOne Tree, Wills Creek, and Woonigan Granites. Likethe comagmatic Leichhardt Volcanics, the granites ofthe Kalkadoon Batholith have higher AI2O 3 and Srcontents and lower levels of incompatible elements,especially Nb, TiO^, Zr, and Th, than most otherMount Isa granites. Different plutons of the KalkadoonBatholith show minor variations in chemistry; forexample, the Kalkadoon Granite in central DAJARRAhas lower TiO^ contents, and the One Tree Granitehas lower Sr and Al-O.i and higher Fe and TiO^ contents,than typical Kalkadoon Granite. However, thesevariations are much less than those between theKalkadoon Batholith and the other batholiths. Analysedsamples of the Wills Creek and Woonigan Graniteshave more than 74 percent silica and cannot be readilydistinguished from other high-silica granites of theregion.The Wonga Batholith comprises a series of deformedand metamorphosed elongate plutons in the easternpart of the central area. Three main petrographic andchemical types have been distinguished, all of whichare enriched in incompatible elements. One type, thatforming the Bowlers Hole and Mairindi CreekGranites, is characterised petrographically by darkblue-green amphibole and only minor amounts ofplagioclase, and chemically by distinctively low MgO.Pl'O.-,, and AbOa contents. This Bowlers Hole typemay be comagmatic with the 1780-m.y.-old felsicvolcanics of the Argylla Formation. A second type,the Bushy Park type, is made up largely of gneissescontaining K-feldspar augen. It includes the BushyPark Gneiss, Birdswell Granite, and Saint MungoGranite, which form intensely foliated plutons intrudingthe Magna Lynn Metabasalt, Argylla Formation,70 , -and Corella Formation. This granite type commonlycontains blue-green hornblende and also some scapolite.It is generally coarser-grained than the other granitetypes of the Wonga Batholith, and has lower Nb andgenerally lower Th and U contents, although valuesfor these elements are higher than in granites of theKalkadoon Batholith. The third compositional typewithin the Wonga Batholith forms the Overlander andRevenue Granites and the granitic components ofthe Mount Erie and Myubee Igneous Complexes, allof which intrude the Corella Formation of the DuchessBelt. This Overlander type may be comagmatic withthe Burstall Granite in MARRABA, which has beendated at 1720-1740 m.y. (Page, 1983b). It containsless plagioclase than the Bushy Park type, andchemically has unusually high Th, U, Nb, and Ycontents. Of the three types of granite, those of theBushy Park type are chemically the most similar to thefelsic volcanics of the Duchess-Corella suite.The Williams Batholith, which is confined to theeastern area, is petrographically and chemically morediverse than the other batholiths in the Duchess-Urandangi region. Three broad types of granite—theMaramungee, Wimberu, and Saxby types—can be distinguished,all of which have higher Na^O and lowerK-O, C u, Zn, and Pb contents than most other granitesof the Mount Isa Inlier. The Maramungee type formsthe Maramungee, Cowie, and Blackcye Granites, andnumerous small plutons, including some mapped asSaxby Granite, which intrude the Soldiers Cap Groupand Doherty Formation; it is probably either syntectonicor pretcctonic. It is mostly fine-grained, non-porphyritic,and extensively recrystallised, and has lowerTiOj, PjO.-,, Zr, Nb, Ce, and V contents than the othertwo types and is also relatively low in Cr. The Wimberutype includes the Wimberu, Mount Cobalt, Mount Dore,and Yellow Waterhole Granites, part of the Gin CreekGranite, and perhaps most of the Squirrel Hills andMount Angelay Granites. The granitic rocks of thistype are generally massive, and were emplaced afterthe country rocks had been tightly folded andregionally metamorphosed. They typically containyellow-brown biotite, some pale green clinopyroxeneand/or blue-green amphibole, abundant dark redbrownsphene, and magnetite; muscovite is present insome of the more fractionated phases. Chemical characteristicsare high MgO, ?>Or„ Ba, Rb, Sr, Th. U. Zr,Nb, Y. La. and Ce contents, and low Cu, Pb, and Zncontents. The Saxby type forms the large plutonsmapped as Saxby Granite and also part of the GinCreek Granite and possibly parts of the Squirrel Hillsand Mount Angelay Granites. These mainly massiveand post-tectonic plutons are characterised by thepresence of dark blue-green hornblende, brown biotite,and muscovite, and by relatively low AljO:i. CaO. Ba,and Sr, and high Fe contents. One pluton of this type,that centred 15 km west of Maronan homestead, hasdistinctly high contents of Th (>70 ppm) and U (25-30 ppm). There are insufficient data available to determinewhether any of the three granite types werecomagmatic with the rhyolites of the Doherty Formation.S1O2 (%)I6/F54/29Fig. 63 . Absolut e frequency-distributio n diagra m fo r SiO obased o n 41 2 analyse s o f igneou s rock s fro m th e Duchess -Urandangi region . Dat a fro m Wybor n (i n preparation) .Mafic volcanicsMafic volcanic rocks are present in the SuliemanGneiss (1 analysis), Saint Ronans Metamorphics (1analysis). Jayah Creek Metabasalt (3 analyses),Oroopo Metabasalt (2 analyses), and Eastern Creek2

Volcanics (no analyses) of the western area; in theundivided Tewinga Group (1 analysis), LeichhardtVolcanics (no analyses), Magna Lynn Metabasalt (3analyses), Argylla Formation (no analyses), BottletreeFormation (2 analyses), Eastern Creek Volcanics(5 analyses), and Corella Formation (1 analysis) ofthe central area; and in the Marraba Volcanics (1analysis), Soldiers Cap Group (3 analyses), KuridalaFormation (1 analysis), Doherty Formation (2analyses), and Argylla Formation, Double CrossingMetamorphics, Mitakoodi Quartzite, and StaveleyFormation (no analyses) in the eastern area. Theanalysed samples (Table 17) are chemically similarto those of the Pickwick Metabasalt Member of theEastern Creek Volcanics described by Derrick & others(1976b) and Glikson & Derrick (1978). As shown byBultitude & Wyborn (1982), they are characterised bylow and uniform contents of most incompatible elements(e.g., Ti, P, Zr, Y), and are similar to continentaltholeiites of the Karroo province of southernAfrica (Le Roex & Reid, 1978) and the AntrimPlateau Volcanics of northern Australia (Bultitude,1976). The eastward progression from rocks of continentaltholeiitic affinities near Mount Isa to rocks ofocean-floor tholeiitic affinities near Cloncurry, asrecorded by Glikson & others (1976) and Glikson &Derrick (1978), is not apparent in the Duchess-Urandangi region. All mafic volcanics are magnetic,although those of the Soldiers Cap Group are onlyweakly so.DoleritesSeveral generations of metamorphosed and one ormore of unmetamorphosed dolerite dykes are presentin the region.TABLE 17 . REPRESENTATIV E CHEMICA L ANALYSE S O F MAFI C VOLCANIC S FRO M TH EDUCHESS-URANDANGI REGIO N(from Bultitud e & Wyborn , 1982 )EasternUndividedCreek Bottletree Tewinga Magna Lynn Corella OroopoVolcanics Formation Group Metabasalt Formation MetabasaltNo. o f sample s o r _ 5 7653- 7653- 7753- 7753- 7753- 7753 - 7753- 7853- 7953-sample numbers i x s 5071 5100 0836 0452 2310 5077 C 0148 3020 2376 ASiOo 50.49 3.0 7 50.20 50.90 49.70 49.60 53.50 49.9 0 49.60 54.50 51.00Tio; 1.30 0.4 9 1.86 1.21 1.97 1.79 1.29 1.4 0 1.83 0.87 0.92Al>6 14.47 0.8 1 15.80 15.40 13.30 14.60 14.00 15.1 0 12.80 13.20 14.403Feo0 4.99 0.7 9 6.12 3.14 4.65 3.11 4.07 3.8 1 5.52 2.51 4.933FeO 6.71 2.1 0 6.78 8.55 10.30 10.30 6.69 8.0 8 9.88 7.67 6.18MnO 0.19 0.0 1 0.19 0.20 0.24 0.23 0.29 0.3 5 0.18 0.17 0.14MgO 6.62 1.9 5 2.62 5.92 5.70 6.04 6.20 7.0 4 4.40 6.61 6.14CaO 9.51 2.2 9 11.70 10.80 8.73 10.00 5.99 7.9 7 8.06 10.70 9.06KoO 1.04 0.5 2 0.70 0.33 0.83 0.60 2.21 1.5 2 1.18 0.22 1.48NaoO 2.70 0.4 4 1.35 1.65 1.65 1.94 2.46 3.1 2 3.68 1.51 2.00P 2O 50.13 0.0 7 0.53 0.20 0.19 0.20 0.18 0.2 0 0.23 0.10 0.16Ba 239 10 9 380 170 310 140 520 33 0 210 55 350Rb 30 2 3 26 3 22 11 65 7 0 24 2 32Sr 181 5 5 390 260 130 130 180 16 0 120 120 230Pb 37 3 3 380 160 38 280 140 7 5 60 65 5Zr 120 6 3 280 170 140 150 160 18 0 120 70 130Nb 6 4 22 14 14 16 18 1 8 14 8 8Y 26 1 1 34 26 28 30 32 3 2 26 17 26La 38 2 5 40 35 15 50 30 3 0 30 30 20Ce 42 1 9 180 55 30 40 80 7 0 90 20 30TABLE 1 7continuedSaintRonansMarrabaJayah Creek Meta­ Sulieman Soldiers Cap Vol­ Kuridala DohertyMetabasalt morphics Gneiss Group canics Formation Formation7853- 7853- 7853- 7953- 7953- 7853- 7853- 7853- 7853- 7853- 7853- 7853-2405D 2408A 3035 2034 2477 0166A 4666 4365 4346A 0821 0065F 0072C52.80 52.40 56.70 51.40 49.60 48.30 48.80 49.30 51.10 50.80 49.20 50.101.02 1.21 1.25 0.92 1.56 2.85 1.62 1.43 2.03 1.42 2.23 1.5914.30 14.30 13.90 14.90 15.10 12.90 13.70 13.90 12.80 13.70 12.10 12.503.84 6.43 4.36 3.61 6.65 0.48 5.26 0.34 5.95 4.73 6.01 7.55'6.67 5.29 7.33 7.43 5.39 14.10 10.00 11.20 9.37 7.14 9.02 6.650.20 0.17 0.14 0.18 0.19 0.49 0.21 0.24 0.21 0.47 0.15 0.126.10 4.60 5.65 6.49 6.80 5.85 5.40 6.50 5.14 5.87 4.96 5.209.48 9.61 3.95 9.55 10.20 9.10 8.25 11.20 7.83 8.67 8.78 8.801.32 0.54 0.85 0.65 0.36 1.72 0.61 0.73 0.54 0.49 0.81 1.121.70 3.34 2.86 2.20 1.88 1.03 3.77 2.66 2.20 4.12 3.89 4.900.14 0.17 0.14 0.13 0.19 0.30 ' 0.1 7 0.15 0.19 0.12 0.19 0.18460 360 460 240 170 200 140 100 150 780 410 15034 17 48 22 10 85 16 20 15 7 26 24130 230 110 190 190 70 60 150 110 300 120 95120 60 36 150 13 5

In the western area, metamorphosed dolerites intrudethe basement and older cover rocks, and a fewalso intrude granite of the Sybella Batholith.At least four groups of metadolerite intrusions occurin the central area. One group forms a swarm of northtrendingsheets intruding the older cover rocks of theDajarra Belt. Another forms a conjugate swarm, trendingnorth-northwest and north-northeast, intruding theundivided Tewinga Group, Leichhardt Volcanics, andgranites of the Kalkadoon Batholith; the dykes of thisgroup are much less magnetic than other doleriteintrusions in the region, and do not give rise toanomalies on aeromagnetic maps. A third group formsrelatively large doleritic to gabbroic bodies intrudingthe Corella Formation, and is associated with granitein net-veined complexes of the Mount Erie and MyubeeIgneous Complexes; this group may be related to theLunch Creek Gabbro associated with the BurstallGranite to the north, in MARRABA, which has beendated at about 1740 m.y. (Page, 1983b). The fourthgroup forms elongate bodies mainly intruding theArgylla Formation and Corella Formation.In the eastern area, metadolerite forms folded sillsand northerly trending dykes and pod-like bodies.These intrude the Mary Kathleen Group and olderrocks, but predate most of the Williams Batholith.Unmetamorphosed dolerite dykes are rare in thewestern and central areas, but are relatively commonin the eastern area, where they form several widelyspaced easterly trending dykes.All analysed samples of dolerite, plotted on theconventional total alkalies-silica diagram, fall in thesubalkaline field. The majority are hypersthene-normative,and on an AFM diagram show the tholeiitic trendof iron-enrichment with increasing differentiation. Mostdolerites have

Fig. 65 . Mesa s forme d o f flat-lying Mesozoi c Gilber t Rive r Formatio n overlyin g th e Squirre l Hill s Granit e o f th eWilliams Batholit h eas t o f Selwyn . (M2303/34 )SEDIMENTARY ROCK S OVERLYIN G TH E MOUN T IS A INLIE RLate Proterozoic and early Palaeozoic rocks of theGeorgina BasinSedimentary rocks of the Georgina Basin successioncrop out in the Burke River Structural Belt, mapped in1967 by de Keyser and others (de Keyser, 1968), andin the far west, where they were mapped in 1969 byde Keyser & Cook (1972). There are also some smallscattered outcrops, remnants of a formerly moreextensive cover, between the two main outcrop areas.The succession is mostly flat-lying, and rests with amarked angular unconformity on the rocks of theMount Isa Inlier. It contains several phosphatedeposits, mostly in the Middle Cambrian Beetle CreekFormation; the main phosphate deposit is near TheMonument, 45 km south of Duchess (Russell & Trueman,1971).The Georgina Basin succession in the region hasbeen described recently by Shergold & Druce (1980).The oldest unit is the upper Adelaidean Little BurkeTillite exposed 9 km east-northeast of Duchess. It isup to 34 m thick and consists of tillite capped bydolomite. The younger units of the succession range inage from late Early Cambrian or early Middle Cambrianto Early Ordovician They consist of clastic, calcareous,dolomitic, and cherty sedimentary rocks, whichare mainly shallow marine, and may reach a maximumthickness of about 1000 m. The succession containsseveral disconformities and diastcms. Further detailsare given on the Duchess-Urandangi region map, inDUCHESS REGION, DAJARRA, ARDMORE, andSELWYN REGION map commentaries (Bultitude &others, 1982; Blake & others, 1982; Bultitude, 1982;Blake & others, 1983). in the paper by Shergold & Druce(1980), and in works referred to in these publications.Details of macrofossil localities in the Burke RiverStructural Belt are given by Carter & Opik (1963),de Keyser (1968), and Shergold (1980); lithologiclogs of stratigraphic holes penetrating the Cambrianunits are summarised by de Keyser (1968), Shergold& Walter (1979), and Simpson (1980).Mesozoic rocks of the Eromanga BasinA flat-lying cover of sediments may have beendeposited over the whole region during the Mesozoic,but only remnants are now preserved, generally ascappings on summit surfaces (Fig. 65). The mostextensive remnants are in the east, where they aremainly mapped as the Gilbert River Formation —apartly fluvial and partly shallow-marine clastic unit ofLate Jurassic to Early Cretaceous age; plant fossilshave been recorded from this unit about 6.5 km northof Selwyn (Carter & Opik, 1963). Probable equivalentsto the west are mapped as unnamed Mesozoic rocks.These Mesozoic sediments are generally muchweathered, and so are the immediately underlyingPrecambrian rocks; all this weathering may postdatethe Mesozoic sedimentation. Three other Mesozoicformations are exposed in the southeast: the CretaceousIValhtmbilla Formation, Toolebnc Formation,and Allartt Mudstone, which represent shallow-marine,67

partly calcareous sediments. These units, and theGilbert River Formation, are part of the EromangaBasin succession (Senior & others, 1978); the GilbertRiver Formation also forms part of the CarpentariaBasin succession (Smart & others, 1980).CainozoicFour Cainozoic units have been distinguished onthe Duchess-Urandangi region map. Leached bedrockconsists of bleached to ironstained, highly weatheredrock which has pre-weathering textures and structurespartly preserved. It forms cappings up to 10 m thick onsummit surfaces, especially on granite, and is part ofa deep-weathering profile which is at least partlyCainozoic and may be Tertiary. Laterite and lateriticrubble also form cappings, and probably represent partof the same weathering profile as the leached bedrock;they show no pre-weathering textures or structures.Limestone and minor chalcedony of probable Tertiaryage form low mounds in broad drainage depressions.They may represent modified lacustrine sediments(Hutton & Dixon, 1981) or chemical deposits resultingfrom the evaporation of groundwater (Goudie, 1972).Alluvial, colluvial, and residual unconsolidated sediments,in places over 15 m thick, overlie older rockson gently undulating plains, broad alluvial fans, andriver flood plains, and also occur as a thin patchycover on many areas mapped as bedrock. They areprobably mainly Quaternary.This account is based on the results of regionalgravity, airborne magnetic and radiometric, anddetailed metalliferous ground surveys carried out byBMR in the Duchess-Urandangi region.The regional gravity was determined between 1957and 1966 from surveys in which stations on a gridspacing of about 11 km (plus some fill-in stations)gave an average density of one station every 90 km 2 .Maps showing contoured Bouguer anomalies at1:250 000 scale (Urandangi) and 1:253 440 scale(Duchess) are available through the AustralianGovernment Printer Copy Service.REGIONAL GEOPHYSIC SWells & others (1966) described the results andpresented profiles of alternate lines of an aeromagneticsurvey of the Georgina Basin, including the Urandangi1:250 000 Sheet area, in which east-west lines werespaced 3.2 km apart 600 m above mean sea level;BMR (1966) published a contour map of the magneticfield at a scale of 1:250 000. An airborne magnetometerand gamma-ray spectrometer survey was carriedout over the Duchess 1:250 000 Sheet area in 1975along east-west flight lines 1.5 km apart 150 m aboveground level; the data are presented at 1:250 000scale as (1) contours and stacked profiles of themagnetic and total-count radiometric results, (2)stacked profiles of the potassium, uranium, and thoriumchannels, and (3) stacked profiles of the ratios uranium:thorium, uranium:potassium, and thorium:potassium(BMR, 1978b), and at 1:100 000 scale as contourmaps of the magnetic field (BMR, 1978c)—all ofwhich can be obtained from the Australian GovernmentPrinter Copy Service. Airborne scintillographsurveys made in 1958—60 in the west along east-westflight lines 300-500 m apart 60 m above ground levelare reported on by Gardener (1961) and Mulder(1961a,b). In 1969, a detailed airborne gamma-rayspectrometer survey was made along lines 320 mapart 80 m above ground level over phosphate depositsin the Burke River Structural Belt (Waller & others,1971). Areas covered by airborne radiometric surveysare shown in Figure 66.Several detailed metalliferous ground surveys madein the 1950s were reported on by Horvath (1952,1955), Horvath & Tate (1954), and Howard (1960).Earlier ground geophysical investigations were conductedby the Aerial, Geological and GeophysicalSurvey of Northern Australia southwest of Duchess(Rayner & Nye, 1936). Figure 66 shows locations ofdetailed metalliferous surveys.Survey Instrumenarea>oooQQQCScintillographScintillographScintillographSpectrometerSpectrometerLocation ReferencAIRBORNE RADIOMETRI C SURVEYtFlying Lin eheight spacin g(a.g.l.; (mi60m60m60m80m150 m400300-500320-4803201500SReferenceGardener 119611Mulder(1961a>Mulder(1961bjWaller & others (1971)BMRDETAILED METALLIFEROU S SURVEY® Rayner & Nye (1936)eHorvath (1952)X Horvath it Tate (1954); Horvath (1955)S(1978b—cjc=i Howard (1960) 16/F54/6522°SFig. 66 . Area s covere d b y airborn e radiometri c survey s an ddetailed metalliferou s surveys .68

Rock densityAlthough insufficient samples were measured to adequatelyrepresent the densities of all rock units exposedin the region, Tables 18 and 19 provide the bestavailable guide for estimating bulk rock densities (formore details see Hone & others, in preparation). Themeasured densities are consistent with those of rocksfrom other parts of the Mount Isa Inlier. The resultsare summarised diagrammatically in Figure 67.Porosity measurements show that weathered rocksin the Mount Isa Inlier have porosities of up to 27percent, whereas fresh drillhole samples have porositiesbetween 0 and 2.6 percent, and fresh surfacerocks have recorded porosities between 0 and 2.0percent. The rock densities in Tables 18 and 19 arebased on assumed porosities of 2 percent or less forsedimentary rocks, and 0.5 percent or less for igneousrocks.TABLE 18. SUMMARY OF DENSITY MEASUREMENTS (GROUPED BY ROCK UNIT AND ROCK TYPE)Lithology No. Wet density (t/m 3 )Rock unit samples Range Mean SDMcNAMARAGROUPGunpowder Cree k Formation * sandstone 3 2.62-2.69 2.66 0.04dolomitic siltston e 15 2.65-3.03 2.77 0.09shale 3 2.61-2.66 2.63 0.02MOUNT IS A GROUPMoondarra Siltstone * siltstone 1 2.72MARY KATHLEE N GROU PMarimo Slate * siltstone, shale , slate , cher t 8 2.30-2.72 2.55 0.15Staveley Formatio n basalt 1 2.94Kuridala Formatio n banded iro n formatio n 2 3.13-3.20 3.17Overhang Jaspilite * shale, jaspilit e 9 2.77-3.46 2.93 0.22Corella Formatio n wes t o fWonga Belt *in Wong a Belt *shale, quartzit emarblegranofelsmetaconglomerate, metavolcanic smarble1312212.63-2.82 2.702.792.712.752.76calc-silicate rock , amphi -bolite, granofel s 5 2.77-3.08 2.92 0.14schist 1 2.69skarn 2 3.48-3.63 3.56east o f Wong a Belt * shale, siltstone , quartzit e 6 2.61-2.69 2.64 0.03metasiltstone 3 2.71-2.80 2.75 0.05limestone, calc-silicat e 25 2.64-3.06 2.81 0.12Ballara Quartzit e conglomerate 1 2.90MALBON GROU PMitakoodi Quartzite * meta-arenite 1 2.63siltstone, shal e 3 2.79-2.83 2.81 0.02basalt 2 2.84-2.87 2.86Marraba Volcanics * mafic schist , metabasal t 2 2.78-2.84 2.82SOLDIERS CAP GROU PUndividedamphibolitegarnetiferous gneis s213.00-3.10 3.052.78Toole Cree k Volcanics * dolerite, metabasalt , amphibolit e 5 2.80-3.11 3.03 0.13quartzite 3 2.67-3.02 2.89 0.19Mount Nom a Quartzite * metasediments 10 2.63-2.78 2.68 0.05rhyolite 2 2.57-2.60 2.59metadolerite 1 3.05banded iro n formatio n 3 3.14-3.73 3.48 0.30HASLINGDEN GROU PEastern Cree k Volcanics * basalt 6 2.87-3.07 2.99 0.07chlorite-albite-quartz roc k 2 2.70-2.77 2.74Yappo Membe r (Moun tGuide Quartzit e conglomerate 1 2.78Bottletree Formatio n metadacite 1 2.65TEWINGA GROU PArgylla Formatio nfelsic metavolcanic samphibolite212.682.95Leichhardt Volcanic s ignimbrite 2 2.64quartz-feldspar porphyr y 2 2.63-2.69 2.66quartz-feldspar porphyr y dyk e 1 2.66Undividedgneiss9 2.58-2.72 2.64 0.05metagreywacke conglomerate ,meta-agglomerate 3 2.72-2.79 2.75 0.04WONGA BATHOLIT HMount Eri e Igneou s Comple x dolerite, metadolerit e 2 2.98-3.04 3.01WILLIAMS BATHOLIT HWimberu Granit e 4 2.64-2.67 2.65 0.01Gin Cree k Granit e granite1 2.58Unnamed granit e1 2.65Mount Dor e Granit e 1 2.64KALKADOON BATHOLIT HWills Cree k Granit e 1 2.65One Tre e Granit e granite 5 2.66-2.73 2.69 0.03Woonigan Granit e 1 2.60Kalkadoon Granit e 10 2.67-2.80 2.74 0.04Dolerite, gabbro , metadolerit edykes 5 2.96-3.03 3.00 0.03Mount Phil p Brecci a 1 2.62Note tha t densitie s o f sedimentar y roc k sample s wit h porositie s o f ove r 2 percen t hav e bee n correcte d t o densitie s fo r porositie s o f 2 percent ,and densitie s o f igneou s roc k sample s wit h porositie s o f ove r 0 5 percen t hav e bee n correcte d t o densitie s fo r porositie s o f 0. 5 percent .* Sample s fro m nort h o f th e Duchess-Urandang i region .0.0569

TABLE 19. SUMMARY OF DENSITY MEASUREMENTS (GROUPED BY ROCK TYPE ONLY)Numberof Wet density (t/m')Rock type samples Range Mean SDskarn 2 3.48-3.63 3.56banded iro n formatio n 2 3.13-3.20 3.16mafic igneous rock s 15 2.84-3.10 2.98 0.08granofels 3 2.77-2.87 2.82 0.05conglomerate, metaconglomerat e 5 2.75-2.90 2.79 0.06metavolcanics 1 2.75calc-silicate rock s 3 2.66-2.79 2.73 0.07meta-agglomerate 1 2.72mafic schis t 1 2.71granite 24 2.64-2.80 2.69 0.05gneiss 12 2.58-2.78 2.66 0.06felsic volcanic s 4 2.63-2.68 2.65 0.02breccia 1 2.62Basement rocks, mainly felsic gneiss and granite,are generally of low to medium density (

139°30141-00'H 24-00 'Possible lineamentsLimit of outcrop of Mt Isa InlierBouguer anomaly values > 200 pm/s 2Bouguer anomaly values < -200 pm/s'0 pm/s contour lineContour interval 50 pm/s 2Duchess-Urandangiregion— — Cloncurry Regional Gravity High(after FraserS others 1977)Fig. 68 . Regiona l Bougue r anomal y field.71

139°00'to22°00'Bouguer anomaly values > 100 pm/sLetters refer to features andprofiles mentioned in text.Bouguer anomaly values < - 100 pm/s 2 Bouguer reduction density: 2.67 t/m 350kmOpm/scontour lineContour interval 20pm/s 2 Fig. 69 . Bougue r anomal y ma p o f th e Duchess-Urandang i region .

Belt, are considered to be due to underlying Proterozoicrocks of Corella Formation type. A small area of highBouguer values in the south, along longitude 139°30'E,is mainly over the Eastern Creek Volcanics and JayahCreek Metabasalt, and is attributed to the basalticcomponent of these formations.Medium Bouguer anomaly gravity fields (between-100 and 100 yu.rn.s- 2 ) in the east are considered to becaused by the exposed rocks of the Soldiers CapGroup and Staveley Formation. Those over the BurkeRiver Structural Belt are interpreted as being causedby underlying Proterozoic older and younger coverrocks.The sources of most low Bouguer anomaly gravityvalues (less than -100 fj.rn.s-'-) are considered to begranites of the Williams and Sybella Batholiths.Georgina an d Eromanga Basi n cove r rock s+ + +f + + + Younger granit e* * *J Mar y Kathlee n Group ,Malbon Group , an d Argylla Formatio n.•k „ Dohert y Formatio nSoldiers Ca p GroupHaslingden an d Mount Is a Group s|j Basemen t 30 k m_JCorella FormationFig. 70 . Bougue r anomal y profile s an d simplifie d interprete d geologica l section s alon g latitude s 2 1 4 5 S an d 2 2 S ,DUCHESS.73

HO°OS'JPalaeozoicsedimentary rock sArgylla Formatio n o rMary Kathlee n Grou pObservedvalueProfile calculatedfrom model10 k m_JFig. 71 . Observe d Bougue r gravit y value s an d modelle d Bougue r gravit y profil e alon g latitud e 2 1 4 2 S betwee n point sC an d D (se e Fig . 6 9 fo r location) .10--25-E140°13'AlluviumF141°00'Cambriansedimentaryrocks2.76AnswerSlate2.74 2.67Williams BatholithMesozoic2.76Soldiers CapGroupintruded b ysome granit eDouble Crossin gMetamorphicsDohertFormationObservedvalueProfile calculatedfrom modelFig. 72 . Observe d Bougue r gravit y value s an d modelle d Bougue r gravit y profil e alon g latitud e 21°42' S betwee n point sE an d F (se e Fig . 6 9 fo r location) .74

The wide spacing of the gravity stations in theregion precludes definition of the gravity field overunits of limited areal extent, such as the OverlanderGranite and other small granite bodies of the WongaBatholith. Local anomalies of over 100 Am.s -2amplitudesuch as some of those reported by Smith (1966a,b, 1968) from the Dobbyn, Cloncurry, and Mount Isa1:250 000 Sheet areas would not be detected by theregional survey.Quantitative interpretation of major features of thegravity fieldThree major features of the Bouguer gravity map ofthe region and surrounds are the long gravity highlabelled UV and the elongate gravity lows labelled WXand YZ in Figures 68 and 69. The western flank ofthe gravity high, UV, lies partly over the Kalkadoon-Leichhardt Block, which—being granitic—would beexpected to generate a gravity low. The gravity low,WX, is located in the southwest in an area of sparseoutcrop. The other gravity low, YZ, which is associatedwith the Williams Batholith, has a gentler gravitygradient on its western flank than on its eastern flank.To try to explain the sources of these gravity features,a quantitative interpretation was carried out alonglatitude 21°42'S (lines CD arid EF in Fig. 69). Theobserved gravity values and the profiles calculated frominterpreted generalised densities along lines CD andEF are shown in Figures 71 and 72.The gravity high, UV, peaks over the Corella Formation(Fig. 71), and its western flank is over the Kalkadoon-LeichhardtBlock. The measured average densityof Corella Formation rocks, around 2.8 t/m 3 , is consistentwith the modelling calculation, which shows theCorella Formation as the main cause of the anomaly.Yet the average density of granite and gneiss of theKalkadoon-Leichhardt Block (^2.74 t/m 3 ) is toolow to account for the western flank of the anomaly.Measurements on samples of Kalkadoon and OneTree Granites indicate that the density of granite andgneiss under the western flank could be about 2.72t/m 3 —too low by 0.03 t/m 3to account for the feature.However, if—for example—10 percent of thevolume of the rock corresponding to the western flankof the anomaly were to comprise mafic dykes (averagedensity of 3.0 t/m 3 ) and the other 90 percent granite(density 2.72 t/m 3 ), the bulk average density would beabout 2.75 t/m 3 . This speculation may be close to thetruth, as the 1:100 000-scale geological maps of thesouthern part of the Mount Isa Inlier show that maficdykes form a significant component of the Kalkadoon-Leichhardt Block; being readily eroded, and hence oftennot exposed, such dykes may be more voluminous thanmapped. In contrast, the presence of gravity lows overthe Williams and Sybella Batholiths is consistent withtheir composition of low-density granite intruded byonly sparse mafic dykes.The northern part of an elongate gravity low in thesouth along longitude 139°45'E (WX, modelled inFig. 71) occurs partly over outcrops of the WillsCreek Granite. This granite (or another granite),intruded by few mafic dykes, is suggested to be thesource for most of this gravity low.Modelling of the gravity field over the WilliamsBatholith along line EF (Fig. 72), in order to interpretthe cause of anomaly YZ, indicates that the batholithhas a complex western boundary. The gravity field canbe accounted for by supposing that metasedimentsform a wedge in the granite, and that the granitecontact dips westwards at a low angle.Magnetic properties of Proterozoic rocksThe results of laboratory magnetic susceptibility andremanence measurements made on surface samples ofrocks collected during the 1975-1980 geologicalmapping program, and on surface and drillhole samplesfrom north of the region, are summarised in Table 20.The rocks measured exhibit a wide range of susceptibilities,and the mean value is high—more than 0.01(SI). The strengths of measured remanences, and theKoenigsberger ratio (Q, the ratio of the effect ofremanence to that of susceptibility, calculated for anambient field strength of 50 000 nT), also have a widerange. The measurments show that many of the Proterozoicrocks are magnetic. The most magneticsamples (based on a combination of susceptibility andremanence) were basalts from the Eastern CreekVolcanics and a sample of banded iron formation fromthe Kuridala Formation (Table 20). Other very magneticsamples came from the Corella Formation, OverhangJaspilite, and Mount Noma Quartzite (bandediron formation). Of the granite samples measured, allbut one (the only sample of Gin Creek Granite) fromthe Williams Batholith were magnetic, whereas all thosefrom the western part of the region were non-magnetic.Regional magnetic characteristicsThe total magnetic field over the Duchess-Urandangiregion (Fig. 73) was compiled from the resultsof two surveys. A change in style of the anomaliesacross longitude 139°30'E is due to the use of differentsurvey parameters and contouring methods. The surveyin the Duchess 1:250 000 Sheet area was flownat a line spacing of 1.5 km and an altitude of 150 mabove ground level, whereas for the Urandangi1:250 000 Sheet area the line spacing was 3.2 km andthe flying height 600 m above mean sea level—thatis, 180 m above ground level in the northeast and400 m above ground level in the south. The resultsfrom Duchess were contoured by computer, whereasthose from Urandangi were hand-contoured.The total magnetic field strength is about 51 500nT, the inclination is —52°, and the declination is6°E (Finlayson, 1973). Narrow sources with an eastweststrike, or small sources with a roughly equidimensionalhorizontal cross-section, produce an inducedmagnetic anomaly with a high to the north and a lowto the south of the source. The strength of the inducedmagnetic field across a north-south striking verticalsource is symmetrical.The region can be divided into seven broad magneticareas (Fig. 73) which correlate well with gross geologicaldivisions. These areas are divided into zones(Fig. 74) on the basis of changes in anomaly trends,anomaly shapes, magnetic field level, and degree ofmagnetic disturbance. The zones outline areas of consistentmagnetic properties, and can be used to assistmapping and structural interpretation.In the westernmost area, magnetic anomalies areelongate and generally trend northwest, parallel toregional gravity trends. They are interpreted as arisingmainly from mafic rocks in the Oroopo Metabasalt,Sulieman Gneiss, and Saint Ronans Metamorphics,and from mafic intrusions. The magnetic anomalies inthe far west indicate places where mafic rocks underlie75

oOSTABLE 20. SUMMARY OF MAGNETIC SUSCEPTIBILITY AND REMANENCE MEASUREMENTSUnit LithologySusceptibility (SIxlO-*) Remanence (mA/m) Koenigsberger ratioN Range Median Mean N Range Median Mean N Range Median MeanMcNAMARA GROUP *Gunpowder Cree k Formatio ndolomite 4 5-1634 22 421 4 0.1-290 1.0 73 4 0.01-0.45 0.11 0.17sandstone, siltstone , dolomiti c17 0-38 13 19 17 0.1-10 0.7 1.2 16 0.02-0.66 0.08 0.13siltstone, shal eMOUNT IS A GROUP *Moondarra Siltston e siltstone 1 0 1 1MARY KATHLEE N GROU PStaveley Formatio n basalt 1 678 1 100 10.37Kuridala Formatio n banded iro n formatio n 2 88-1005 547 2 12400-102700 57550 2 257-354306Overhang Jaspilite " shale, jaspilit e 181 100-32794 106 2011 9 0.2-1700 50 410 9 0.01-6.68 0.20 1.15Corella Formatio nwest o f Wong a Belt *quartzite, shale , marble , granofel s 218 0-1767 75 104 15 0.05-340 10 43 15 0.002-17.0 0.56 2.03amphibolite, calc-silicat e roc k 11 0-151 88 262 11 0.1-1300 3 203 10 0.002-87 0.24 9.70cast o f Wong a Belt "metavolcanics, granofel scalc-silicate rock , siltstone , limestone , 126 0-13210 76 356 11 0.1-560 10 122 10 0.08-8.30 0.45 1.75Ballara Quartzit emetasediments, amphibolite ,granofels, shal econglomerate 1 12692 1 2750 1 0.54MALBON GROU PMitakoodi Quartzit eshale 40 30-5958 3309 3052 2 20 2 0.001-0.003 0.002basalt 2 4700-5200 4450Marraba Volcanics " mafic schist , basal t 2 251-1200 726 1 1050 1 10.5SOLDIERS CA P GROU PundividedToole Cree k Volcanics "Mount Nom a Quartzite "HASLINGDEN GROU PEastern Cree k Volcanic sYappo Membe r (Moun tGuide Quartzite )Bottletree Formatio nTEWINGA GROU PArgylla Formatio nLeichhardt Volcanic sundividedamphibolitegarnet gneis samphibolite, metabasalt , quartzite ,doleritebanded iro n formatio nsedimentary rocks , rhyolite , meta -doleritebasaltchloritc-albitc-quartz roc kconglomeratemetadacitegneiss, amphibolit eporphyry, mcta-ignimbrit emeta-ignimbriteconglomerategneissWONGA BATHOLIT HMount Eri c Igneou sdolerite, metadolerit eComplexWILLIAMS BATHOLIT HWimberu Granit eGin Cree k Granit eunnamed granit egraniteMount Dor e Granit eKALKADOON BATHOLIT HWills Cree k Granit eOne Tre e Granil egraniteKalkadoon Granit emafic intrusive sMount Phil p Brecci aN Numbe r o f sample s measured .* Include s som e sample s fro m nort h o f th e Duchess-Urandang i region .2 75-94 85 2 1 2 0.031 18850 1 2400 1 0.328 6-580 112 166 1 2 1 0.023 210-22000 15000 1240313 2-3600 78 4055 792-10681 6723 6436 6 2500-390000 48735 132261 6 0.72-173 16.66 59.802 36-261 149 2 30-300 165 2 2.07-2.90 2.491 50 1 10 1 0.501 5027 1 580 1 0.293 50-2199 189 813 3 15-6000 60 2025 3 0.75-6.86 0.80 2.803 38-1583 201 607 2-1700 460 721 3 0.12-5.75 2.70 2.861 2890 1 20500 1 17.832 2890-6660 4775 2 160-420 290 2 0.14-0.160.159 0-182 25 48 9 0.3-2950 7 337 8 0.03-65.56 0.92 10.452 31-75 53 2 1-40 21 2 0.08-1.33 0.714 113-4147 201 1166 4 20-4300 48 1104 4 0.33-2.61 0.66 1.061 0 1 0.21 5027 1 1400 1 0.701 1005 1 40 1 0.106 0-38 22 21 6 0.1-15 1.3 3.8 5 0.01-1.2 0.1 0.315 25-31 25 27 5 0.2-9 3 3.8 5 0.02-0.72 0.24 0.336 13-38 28 26 6 0.4-2 0.5 0.7 6 0.03-0.40 0.05 0.105 50-4926 316 1840 5 20-12000 860 3215 5 0.44-31.6 1.75 8.091 50 1 7100 1 355

0 5 0 kmG H Magnetic profile shown in figure 75 i iM N Feature mentioned in textContour interval 10 nTFig. 73 . Tota l magneti c field contours , Duchess-Urandang i region .

1390 5 0 kmI I16/F54/73Fig. 74 . Zone s denne d fro m characteristic s o f th e tota l magneti c field.

the essentially non-magnetic sediments of the GeorginaBasin.Immediately to the east, a magnetic area characterisedby many short-wavelength high-amplitude complexmagnetic anomalies mainly overlies the DajarraBelt. The anomalies form long narrow zones whichhave north-northeast to north-northwest trends. Mostof the anomalies arise from basalts of the EasternCreek Volcanics and Jayah Creek Metabasalt, andfrom mafic intrusions. Zones of low magnetic intensityoccur over Mount Guide Quartzite and Mount IsaGroup rocks.The area of generally quieter and lower magneticfield farther east mainly overlies the Kalkadoon-LeichhardtBlock. Although many mafic intrusives aremapped in this block, most do not produce noticeablemagnetic anomalies in the regional contours. A profilebetween G and H in northern DUCHESS REGION(Fig. 73) reveals magnetic anomalies that are of muchsmaller amplitude over mafic intrusives in the Kalkadoon-LeichhardtBlock than over nearby Eastern CreekVolcanics, Magna Lynn Metabasalt, and Argylla Formation(Fig. 75).Many high-amplitude short-wavelength magneticIV v VV VV V Vv- "> - > •Surprise Cree k Formation ?Argylla Formatio nMagna Lyn n Metabasal tEastern Cree k Volcanic sMafic intrusiveMafic intrusion mappedwithin 100m of profileShear zoneFault•st—r—x x Kalkadoo n Granit eLeichhardt Volcanic sFig. 75 . Aeromagneti c profil e alon g lin e G-I I i n norther n DUCHES S (se e Fig . 6 9 fo r locatio n o f line) .79

anomalies occur over the Duchess Belt and the westernpart of the Malbon Block. They result from magneticrocks in the Corella Formation, Argylla Formation,and Wimberu Granite.The magnetic background over the Burke RiverStructural Belt is higher than that over the Kalkadoon-Leichhardt Block. The magnetic anomalies are smooth,and arise from sources in the Proterozoic basementunderlying the Georgina Basin sediments. The northwesterlytrending dashed line (MN) in the area correspondingto the Burke River Structural Belt (Fig. 73)marks a break in magnetic characteristics: southwestof this line the anomalies are broader and of loweramplitude than those to the northeast, indicating deepersources to the southwest. This line is thought to representa fault along which the basement to the south hasbeen displaced downwards several hundred metres.East of the Burke River Structural Belt, as to thewest, the magnetic field is dominated by intense shortwavelengthanomalies. The sources are granites of theWilliams Batholith, and magnetic units in the Tewinga,Malbon, Mary Kathleen, and Soldiers Cap Groups.High amplitudes of anomalies over the Doherty Formationoutcrop, and lower amplitudes to the east,reflect the more magnetic nature of the Doherty Formationrelative to the Soldiers Cap Group.Stratigraphic locations of magnetic sourcesStratigraphic locations of magnetic anomaly sourcesare summarised in Table 21, which lists the typicalmaximum anomaly recorded in the airborne data. Insome rock units, more than one distinct source isapparent. Where multiple sources are evident, becauseof either multiple layering of sources or repetition ofsources by faulting or folding, only the most intensesource has been identified.Few high-amplitude magnetic anomalies occur overbasement rocks of the Plum Mountain Gneiss, Kalkadoonand One Tree Granites, Leichhardt Volcanics,and undivided Tewinga Group. Some anomaliesappear to be caused by mafic intrusions, often alongfaults, and some anomalies over the KalkadoonGranite may be caused by ingested rocks. However,most of the mafic rocks intruding the basement unitsproduce only low-amplitude anomalies, unlike many ofthose elsewhere in the region.In the older and younger cover sequences most maficvolcanics and intrusives are magnetic, and so are manyfelsic lavas and some metasediments. The quartzhematitebodies in the Kuridala and Staveley Formationsand parts of the Corella Formation are especiallymagnetic.Of the granitic rocks, only those of the WilliamsTABLE 21. AIRBORNE MAGNETIC ANOMALIES FROM ROCK UNITS OF THEREGIONRock unit Anomaly amplitude (nT)MetadoleriteDolerite dyk eWILLIAMS BATHOLIT HWimberu Granit eGin Cree k Granit eMount Dor e Granit eYellow Waterhol e Granit eSquirrel Hill s Granit eMount Angela y Granit eSaxby Granit eMaramungee Granit eUnnamed granit eWONGA BATHOLIT HBirds Well Granit eSaint Mung o Granit eGarden Cree k Porphyr yMOUNT IS A GROU PMARY KATHLEE N GROU PMarimo Slat eStaveley Formatio nDoherty Formatio nKuridala Formatio nCorella Formatio nOverhang Jaspilit eSOLDIERS CA P GROU PToole Cree k Volcanic sMount Noma Quartzit eundividedMALBON GROU PMitakoodi Quartzit eMarraba Volcanic sDouble Crossin g Metamorphic sHASLINGDEN GROU PEastern Cree k Volcanic sBottletree Formatio nOroopo Metabasal tJayah Creek Metabasal tTEWINGA GROU PArgylla Formatio nMagna Lyn n Metabasal tLeichhardt Volcanic sundividedKALKADOON BATHOLIT HWoonigan Granit eWills Creek Granit eKalkadoon Granit eOne Tree Granit ePlum Mountai n Gneis sDUCHESS-URANDANGI500, -300 (wester n area) ; 500 , 20 (centra l area) ; 2000 (easter narea)400100(a), 400(b )0100(a), 300(b )100(a),300(b)400(a),600(b)200(a), 600(b )300(a),600(b)0000100 (mafi c margins )0505800 (quartz-hematit e body )1600500 (schist) , 200 (graphiti c slate), 540 0 (quartz-hematit e body )3800, 100 0 (felsi c volcanics) , 10 0 (metabasalt), 30 0 (calc-silicat erock)800100100040 (metabasalt) , 200 , 400, 90 01200 (uppe r part) , 200 (lowe r part )100 (mafi c schist) . 1400 , 100 01200 (quartz-hematit e body? )1000 (wester n area) , 140 0 (centra l area )600 (metabasalt? )500-200,10002100 (easter n area) , 400 (centra l area )2400?10040 (quartzite) , 600? (rare )(a) short-wavelengt h anomalies ; (b ) long-wavelengt h anomalies .NOTE: Amplitude s o f 0 indicat e n o anomalie s attributabl e t o th e roc k unit . Negativ e amplitude s indicat e reversel y magnetise d sources .0000080

Batholith arc characteristically magnetic. Thesegranites generate a distinctive pattern of complexanomalies in which randomly oriented short-wavelengthanomalies with amplitudes of 100 nT to 400 nT aresuperimposed on longer-wavelength anomalies withamplitudes of 300 to 600 nT. Other granites are nonmagneticor only weakly magnetic.The Phanerozoic rocks in the region do not appearto contain magnetic sources.Regional radiometric characteristicsTotal-count and spectral data are available for theDuchess 1:250 000 Sheet area, but not for the Urandangi1:250 000 Sheet area. Detailed or quantitativeinterpretation of BMR spectrometer data is plaguedby uncertainties owing to the small detector crystalused and the lack of quantitative calibration data. Thesmall detector results in low count rates, so thestatistical accuracy of data collected over small areasis poor, but the data collected are useful in regionalreconnaissance where large areas of similar rock typeare sampled. Over much of the Duchess-Urandangiregion, however, there are marked changes in rock typeover distances too short to give meaningful results. Theregional interpretation presented here is therefore, bynecessity, broad.The Duchess 1:250 000 Sheet area can be dividedinto five broad zones of radioactivity. From west toeast these are classed as of low, moderate, low, high,and low activity (Fig. 76). The zones correlate broadlywith structural groupings, and their shapes and radioactivitycharacteristics are strongly influenced by thepresence or absence of granite and its radioactivity.Not surprisingly, the granites with the highest uraniumcontents (see section GEOCHEMISTRY OFIGNEOUS ROCKS) generate the highest count rates.In the westernmost low zone the total count rate isless than 75 cps. The zone lies mainly over theDajarra Belt, in which the main rock units belong tothe Haslingden Group and, in the south, the MountIsa Group. Haslingden Group rocks generate very lowcount rates except in the extreme northwest, wherepart of the Mount Guide Quartzite generates countrates of over 100 cps.In the moderate zone, which covers most of theKalkadoon-Leichhardt Block, the Duchess Belt, andthe western part of the Malbon Block, the total countrate is mainly in the range 50-150 cps. Higher countrates occur over the Overlander (up to 350 cps),Revenue (up to 200 cps), and Bowlers Hole (over175 cps) Granites, part of the Wimberu Granite (upto 250 cps), parts of the Argylla Formation, and alsothe phosphate deposits at Phosphate Hill, DAJARRA,where counts of up to 175 cps were recorded. TheBirds Well and Mairindi Creek Granites are not quiteas radioactive, and the Saint Mungo Granite, BushyPark Gneiss, and granites of the Kalkadoon Batholith—which yield total-count recordings usually less than125 cps—are even less so.In the central low zone the count rate is mostlyFig. 76 . Total-coun t radiometri c contours , DUCHESS .81

in the range 25-75 cps. The zone corresponds largelyto the area occupied by Palaeozoic sedimentary rocksof the Burke River Structural Belt, but it also coverssmall parts of the Corella Formation in the west.The high zone to the east is dominated by highcount rates: values of over 300 cps have been recordedover non-foliated granites of the Williams Batholith.The Mary Kathleen, Malbon, and Soldiers Cap Groupsin this zone generate mainly moderate count rates.In the eastern low zone, which in the south joinswith the central low zone, the count rate is usuallyin the range 25-75 cps. Higher count rates occurover watercourses, such as those of the Fullarton River,Sandy Creek, and Bustard Creek, which drain granitesof the Williams Batholith. Most of the zone is coveredwith Cainozoic sediments, although there are someoutcrops of Soldiers Cap Group, Kuridala Formation,and Phanerozoic sedimentary rocks.Spectral resultsIn the Duchess 1:250 000 Sheet area, profiles areavailable for thorium and stripped uranium and potassiumchannels, and for thorium:potassium, uranium:potassium, and uranium:thorium. In addition, spectralresults in the form of contours are available from adetailed airborne survey covering 640 km 2 in thesouth of the Sheet area (Waller & others, 1971). Thedata show that the high total count rates over theWilliams Batholith and the Overlander and RevenueGranites are accompanied by high count rates in eachof the thorium, uranium, and potassium channels,suggesting that these granites are enriched in all radioactiveelements. Granites with a lower total countrate appear to have a lower enrichment. Exceptionallylarge thorium-channel anomalies (well over 50 cps),accompanied by high thorium:potassium and uranium:potassium ratios, occur over parts of the WilliamsBatholith where weathered granite and, in places,Mesozoic sedimentary rocks form mesa cappings, andmay be attributed to surface concentration of thoriumand uranium during lateritic weathering processes.Uranium-channel anomalies, often unaccompaniedby anomalies in other channels, occur over someCambrian outcrops, mainly the Beetle Creek Formation,and may overlap onto adjacent formations. Somealso occur over parts of the Staveley Formation,Marimo Slate, Overhang Jaspilite, and Doherty Formation.Uranium:thorium ratios typically appear to beinversely proportional to the total count rate, exceptover the Cambrian Beetle Creek Formation, where theyare high and the total count rate is moderate to high.Mining historyMINERALThe Duchess-Urandangi region contains what werethe most productive mines of the Cloncurry Gold andMineral Field during the early part of this century.Significant numbers of miners were first attracted tothe region in the 1860s, following the discovery ofrich alluvial gold deposits at Top Camp on the CloncurryRiver east of Malbon. However, within a decademost of the gold reserves were exhausted. The prospectorsthen extended their searches farther south andwest, and by 1900 most of the major copper depositsin the region had been discovered. In 1906, largescalemining operations were initiated by two companieswhich were soon to dominate the field—MountElliott Ltd and Hampden-Cloncurry Copper MinesLtd. The latter's operations were centred on theHampden group of mines and its associated servicetownship of Kuridala, whereas the former's operationswere concentrated at the Mount Elliott mine, 28 kmto the south, near Selwyn.In 1906, Hampden-Cloncurry Copper Mines Ltdacquired the Duchess copper mine, which had beendiscovered in 1897 by Jack Kennedy, son of the pioneerpastoralist Alexander Kennedy. This mine was tobecome the richest producer in the Duchess-Urandangiregion. By 1912, Hampden-Cloncurry had also gainedcontrol of the rich Trekelano copper mine and thesmaller Mount Mascotte and Answer copper mines.However, its rival, Mount Elliott Ltd, achieved a businesscoup in 1912 by purchasing the richest mine ofthe Hampden group, the Hampden Consols. Withinthe region at that time, only one copper mine withany significant output, Saint Mungo, remained in independenthands. This mine produced over 6700 t ofore averaging 23 percent Cu between 1910 and 1918(Table 22).From 1911, ore obtained from the mines operatedby the two major companies was processed at theirrespective smelters at Kuridala (Fig. 77) and MountRESOURCES82Eiliott, both now in ruins. The blister copper wastransported to Cloncurry and then to Townsville viathe newly completed railway. Despite industrialdisputes, inefficient smelter designs, breakdowns, andthe rapid depletion of the high-grade oxidised orein most of the operating mines (ore grades fell fromabout 12% Cu in the oxidised zones mined in 1910to about 4% in the primary zones mined in 1918),the copper smelting operations on the field remainedprofitable, and the smelters were the most importantproducers of blister copper in Australia during WorldWar I.However, in 1918 continued labour troubles, breakdowns,and fires in the Hampden Consols mine, andthe exhaustion of the high-grade oxidised ore, togetherwith a drastic postwar drop in world copper prices,forced the closure of the Duchess, Hampden, MountElliott, Mount Mascotte, and Answer mines. After abrief rally in 1919, copper prices fell once more in1920, and the low prices then persisted for more thana decade. As a result, by 1920, the Hampden, MountElliott, and Duchess mines had closed, after each hadproduced more than 200 000 t of copper ore (Table22), and mining activities in the Cloncurry Gold andMineral Field had almost ceased. Only the Trekelanomine continued to be worked, and it closed in 1922.The Mount Mascotte mine was reopened in 1927, butclosed again in 1928. In that year, Trekelano mine wassold to an independent syndicate, and was workedprofitably until labour problems forced its closure in1943.During the first half of this century the only majornon-cupriferous mining operation in the region tookplace at the Mount Cobalt mine, 50 km south ofKuridala. Here cobalt ore was mined between 1920and 1934, and hand-picked or milled on the site forshipment. The mine closed because of decreasing oregrades and prolonged drought.During the last 20 years, intermittent small-scale

Fig. 77 . Ruin s o f th e smelte r fo r th e Hampde n grou p o f coppe r mine s a t Kuridala . Photograp h take n i n 1975 . (G B 2906) .operations have taken place at the Hampden, Trekelano,Answer, and Mount Mascotte mines, and opencutmining has been carried out at the Mount Hopemine (north of Duchess) and the Young Australiamine (southwest of Kuridala). The Mount Hope mineproduced over 300 000 tonnes of cupriferous silicaflux and about 10 000 tonnes of copper ore between1967 and 1973, and more than 170 000 tonnes of lowgrade(2.2%) copper ore was extracted from theYoung Australia mine (Fig. 78) in 1967. Since theclosure of these two mines, only intermittent smallscalegouger activity has been recorded in the region,except for the open-cut mining of phosphate ore inCambrian rocks at Phosphate Hill, south of Duchess,between 1975 and 1978.Recorded production data for many of the minesin the region (Table 22) have been obtained fromMines Department Annual Reports, QueenslandGovernment Mining Journals, Carter & others (1961),Krosch & Sawers (1974), Brooks (1977), and Noon(1978). Descriptions of the most productive mines arecontained in map commentaries (Blake & others, 1982a,1983a; Bultitude & others, 1982; Bultitude, 1982;Donchak & others, 1983) and in BMR Records (e.g.,Bultitude & others, 1978; Mock, 1978).Ore distribution, controls, and associationsCopperCopper has been by far the most abundant andeconomically significant base-metal resource in theregion. Copper ore has been won from a great numberof widely scattered and mostly small mines whichgenerally extended no deeper than the zone ofsecondary enrichment.Some deposits are broadly stratabound, but no syngeneicstratiform types are known. Almost all of thedeposits are localised along faults, shear zones, ortension cracks. These sites probably acted as channelsfor circulating metal-rich solutions moving in responseto deformational stresses and/or thermal gradientsassociated with igneous intrusions. Dolerite intrusions,in particular, are very commonly associated with theore deposits. The intrusions may have either contributeddirectly to the metal content of the deposits, orset up convection cells involving metal-concentratingmeteoric waters. The highest metal concentrations aregenerally developed in, or derived from, rocks whichprobably had high primary base-metal contents. Particularlyfavourable rock types are carbonaceous blackshales (deposited in euxinic environments), calc-silicaterocks, and metabasalt.83

In the eastern area, minor copper deposits arecommon in the Marraba Volcanics, especially whereshear zones cut mafic volcanics. Examples includethose at the Quark, Deadlock, Hugarty, and Just-in-Time mines. In the north of the central area, at theLady Fanny and Mount Hope mines, the copperorebodies are located in fissure veins in faulted amphiboliteand felsic and mafic volcanics of the MagnaLynn Metabasalt and Argylla Formation. At the SaintMungo mine, south of Duchess, the copper ore is atfaulted contacts of Saint Mungo Granite and amphibolitedykes.South of Kuridala, ridge-forming banded quartzhematitebodies within the Mitakoodi Quartzite,Answer Slate, Staveley Formation, and KuridalaFormation, and in screens of country rock within intrusionsof metadolerite and Gin Creek Granite (Blake& others, 1983a), have anomalously high copper andgold contents. Some of these bodies, which are thoughtto be epigenetic (Blake & others, 1983a), have economicpotential.In the western part of the region, minor copperdeposits are known in metabasalt of the Eastern CreekVolcanics, in shale and siltstone of the Mount IsaGroup, and in some metadolerite intrusions. The mainmines here are located near major fault zones.Fig. 78 . Ope n cu t o f th e Youn g Australi a coppe r mine , 5km southwes t o f Kuridal a (a t G R VB383403) . Th e lod ewas situate d i n a steepl y dipping , north-trending , kaolinise dfault zone , an d containe d malachite , chalcocite , cuprite ,tenorite, an d hematit e (Doncha k & others , 1983) . Photo -graph take n i n 1980 . (M2491/3A )The mineral deposits at the Mount Elliott, Tip Top,Labour Victory, Occidental, and Mount Dore mines,and at the Hampden group of mines, are in carbonaceousblack slate of the Kuridala Formation. TheAnswer and Young Australia deposits occur in faultedcarbonaceous rocks of the Answer Slate. Black shaleor slate units within predominantly calc-silicatesequences are almost invariably mineralised. In theDoherty Formation, for example, copper and minorsilver minerals at the Two Bobs mine, 13 km north ofKuridala, occur in fracture zones in quartzite adjacentto carbonaceous and sulphide-bearing shales of the'Bipygo Basin', and copper ore containing minormolybdenum occurs in black shale at the Mount Arthurmine, northeast of Kuridala. Minor copper depositsare present in faulted lenses of black shale within theCorella Formation at the Pelican leases about 33 kmnorth-northeast of Duchess (near GR UB920690).Calc-silicate rocks of the Corella Formation arehosts to copper deposits at several mines. For example,in the north, at the Overlander group of mines, faultcontrolledand possibly stratabound copper depositsoccur in calc-silicate rocks adjacent to felsic metavolcanics.The Duchess mine, the most productive inthe region, is located on a shear zone cutting thinbandedcalc-silicate rocks, and metadolerite andgranite of the Mount Erie Igneous Complex. Similaramounts of copper and gold ore were recovered fromthe Trekelano mine, to the south, where a shear zonecuts schistose amphibolite and calc-silicate granofels.Smaller copper deposits in the Corella Formation arelocated in fissure veins along faults, shears, andtension cracks at, for example, the Revenue andMount Mascotte mines.GoldThe two main mines worked primarily for gold arethe Last Call and Top Camp mines in the north ofthe eastern area. The Last Call mine produced a fewhundred grams of gold between 1880 and 1894 fromsmall lenses of black shale and impure limestone incalcareous breccias of the Staveley Formation. TopCamp was a more prolific producer during the latterpart of the last century, but no production figures areavailable; here alluvial gold was recovered from valleyscutting through hills of calcareous siltstone, phyllite.and quartzite of the Overhang Jaspilite.SilverSilver ore has been produced mainly from O'BriensSoak, a small but very rich fissure vein deposit withcalcite gangue in undivided Tewinga Group gneisswest of Bushy Park homestead in the west of thecentral area, and from the Silver Phantom mine in theeastern area, where the ore is present in a pendantof Mitakoodi Quartzite in the Wimberu Granite.Lead, zincStratiform lead, zinc, and copper deposits areknown within the Kuridala Formation and SoldiersCap Group. At the best known prospect, Pegmont,lead, zinc, and subordinate copper are contained inbanded iron formation within the Kuridala Formation.Accounts of this deposit have been given by Locsei(1977), Stanton & Vaughan (1979), Orridge (1980),Stanton (1982), and Taylor & Scott (1982). Almostidentical deposits are present in similar banded ironformation within the Soldiers Cap Group at the CowieMaramungee, and Dingo prospects (Nisbet & Joyce,1980), and the Black Rock and Fairmile prospects.These deposits are considered to be essentially syngeneic.The banded iron formation at all the prospectsis possibly at the same stratigraphic level, andmay be an exhalative deposit related to contemporaneousvolcanism, perhaps associated with mafic and/orfelsic volcanics in the Soldiers Cap Group and KuridalaFormation.8 I

TABLE 22. RECORDE D PRODUCTIO N DAT A FO R MINE S I N TH E DUCHESS-URANDANG I REGIO N"ppt" refer s t o productio n o f coppe r cemen t fro m leachin g > operationsMine \ Grid Ore Copper Gold Silver 1958-80reference t t g g productionAnswer VB340035 10826 1023 11090 1967-76, 78 , 7 925.44(ppt) 1974-76Ardath UB786520 58.91 1.49 1979Bald Hill s UB241234 244.97 27.43 1961. 63-6 5Beauty UB782610 63.91 1.73 224 1969Beleium VB432120 359.68 90.12Burke an d Will s UB732492 685.02 88.29 367 1968Chum Victori a UB735523 74.23 3.84 9 1976-77Deadlock VB268746 202.04 34.85 1965,73Duchess UB818374 204865 25155 76141 61833Edgarda (Crown ) UB784503 312.45 20.23 1968, 69 , 73-7 9Empress UB801347 5.56 0.2 1971Green Valle y VB375609 65.54 2.34 1969-70Hampden Grou p (include s VB487465 214852 15114 376096 173619 1973-77,79, 8 0Hampden, Hampde n 8.4(ppt) 5.1 1974-76Consols, Hampden Queen )Hampden Wes t VB438479 43.7 8H.B. UB904253 257.26 11.07 1968, 6 9Horse Cree k VB299326 125.89 40.38 1967,68Hugarty VB228671 143.85 8.69 1969, 70 , 7 2Just-in-Time VB244686 604.55 133.20 196Labour Victor y VB445281 1367.66 260.79 606 1959-62Lady Barbar a UB829341 94.59 4.27 1968-70Lady Fanny (includin g Lad y UB737493 4673.06 460.93 1676 1966, 67,69,70,73,7 4Fanny North ) 3.99(ppt) 1.23 1974Lady Mari a UB736458 168.46 10.97 (also limeston e 1966-67Last Cal l VB526702 130 445Little Bi t (include s Bi g Bit ) UA762868 1654.55 154.83 1961-65, 8 0Little Monastr y 7UB811036 43.69 2.13 1971Lotta Copp a VB517496 192.03 11.78 1967(hand-picked)Malbon VB238683 520.22 110.24 563McPhail VB349585 18.29 3.45 31Mount Agat e VB305352 87.90 3.36 1970,71, 7 3Mouth Arthu r (Lanha m VB600534 11.68 1.12 1964Shaft)Mount Birni e UB872097 13.21 1.63 22Mount Cobal t VA474960 3286.92 (778.69 t cobalt )(hand-pickedore an d con -centrates)Mount Devoncour t VB358437 432.73 32.51 1966-68(Kangaroo)Mount Dor e VB472043 16.26 5.99Mount Ellio t VB485178 268201 24862 1054698Mount Hope Grou p UB765582 11962 657.8 109 1959-61,66-70, 72 , 7 4(includes Mount Hope ; 309175 5874 1967-73Mount Hope North , South ,(silica flux)and West; Greens Creek ;Binna Burra ; The Stubbie )Mount Kalkadoo n VB699541 214.69 40.13Mount McCab e VB467702 4.06 1.221960,62-66, 68-6 9Mount Mascott e UB812574 5154.93 889.55 7037 1967,68,70Myubee UB728479 25.20 1.02 1970,71Nil Desperandu m (include s UB728461 1790.03 81.48 9 1967,71,73,75Nil Desperandu m Nos . 2and 3 )O'Briens Soa k UB546523 8.84 314891Occidental VB446258 13.02 0.49 1969Overlander (include s UB862722 913.23 44.91 1966-68, 7 0Overlander Nos. 1 and 4 )Oxford UB802306 170.09 17.58 1966, 6 8Pindora Grou p (include s UB956706 1039.95 77.59 174 1969-74Pindora Nos. 1 to 5 ) 1822.12 (silic a flux) 1971,72Pokara VB281620 53.55 2.95 16 1967,68QuarkVB238765 6.08 0.42 19701998.96 (silic a flux) 1977Rainbow Ridg e VB029465 8.03 0.51 1972Revenue Grou p (include s UB790452 4957.99 302.44 311 1964,67-77Revenue, Revenue Central ,Revenue Extended , Luck yRevenue)[Lucky Revenue ] [UB791454] [9.54] [1240] [1975]Saint Mung o UB809080 6701.87 1561.67Silver Phanto m VB377427 98.42 435387 1967, 77,7 8Sinking Su n VB388570 295.69 17.64 1968-73Southern Cros s VB380758 319.34 56.59 361 1968-70Squirrel Hil l?8.74 1.12 1968Straight Eigh t VB528485 439.85 22.56 1966-69Stuart 18 6 VA454917 1607.24 84.74 1968-70, 76 , 79-80Sweet Willia m VB428770 783.61 61.51 202 1968,71,75Tip To p VB437310 44.71 13.72Trekelano UB859238 187939 20472 423236 338560 1971True Blu e UA892930 209.92 36.58 31 1966, 69,7 1Trump VB382563 3.05 1.22Two Bob s VB480597 272.29 22.44 68 5089 1968-70, 72,74,7 6Vulcan VB529719 284.49 11.89 19Young Australi aVB383403 1182.46 135.66 1959-66173151.2 3775.6 1967I1UA )85

CobaltThe one major cobalt deposit in the region is atMount Cobalt mine, south of Kuridala. The lode here,situated along a shear zone at the contact between ametadolerite sill and metasediments of the KuridalaFormation, also contains minor copper and traces ofgold, silver, and tungsten (see below).Calcite, silicaSubstantial amounts of calcite and silica arerequired as fluxes in smelting operations by Mount IsaMines Ltd at Mount Isa. Large near-vertical lensesof coarsely crystalline calcite (marble) suitable forflux occur within the Corella Formation north andsouth of Duchess (Bultitude & others, 1982). Thelenses formed as fissure fillings, and are generallyoblique to bedding in the surrounding calcareousmetasediments. Significant producers include theRevenue, Lady Maria, Lucky Chance (GR UB730480),Semigem (GR UB775350), Atina (GR. UB813610),Lime King (GR UB80560O), Big Chance (GRUB845735), and Mount Morah (GR UB846607)mines.Cupriferous silica flux has been obtained from threedeposits in the region. These are at the Mount Hopemine, 20 km north of Duchess, where silicified faultscut mafic schist and amphibolite of the Magna LynnMetabasalt and felsic gneiss and granite of the BushyPark Gneiss; at the Pindora mine, to the northeast,where a silicified shear zone cuts schist and felsic volcanicsof the Argylla Formation; and at the Quarkmine, north of Malbon, where a silicified shear zonecuts metabasalt of the Marraba Volcanics.PhosphateExtensive deposits of sedimentary phosphorite(rock phosphate) are present in the Middle CambrianBeetle Creek Formation at Phosphate Hill (Russell &Trueman, 1971), in the Burke River Structural Belt.The ore is mainly pelletal, although some laminae ofcollophane mudstone are also present. Open-cut miningof the phosphorite began at Phosphate Hill in 1975, butwas suspended for economic reasons in mid-1978, bywhich time more than one million tonnes of marketablephosphate rock had been produced.Subeconomic phosphate deposits have also beenfound in the Beetle Creek Formation in the ArdmoreOutlier (GR UA2295) in the west (Thomson & Russell,1971; de Keyser & Cook, 1972).Notes on selected minesDuchess mine (Broadhurst, 1953; Carter & others,1961)The Duchess orebody was situated on a steepwesterly dipping shear zone at the northern end ofthe north-northeast-trending Juenburra Fault (Bultitude& others, 1982). The country rocks are graniteand dolerite of the Mount Erie Igneous Complex, andmica schist and calc-silicate rocks of the Corella Formation.The lode in the upper levels of the mineconsisted of bornite and subordinate chalcocite, malachite,and cuprite in a calcite gangue, but at deeperlevels chalcopyrite with quartz gangue was dominant.The mine was worked to a depth of 259 m, belowwhich the ore showed an abrupt decrease in grade.The main shaft extended to a depth of 326 m.Mount Elliott mine (Nye & Rayner, 1940; Sullivan,1953b; White, 1957; Carter & others, 1961)The ore at Mount Elliott mine occurred in a shearzone cutting andalusite-bearing graphitic slate of theKuridala Formation adjacent to a metadolerite sillwith dyke-like offshoots to the east. The shear zonetrends north-northwest and dips steeply to the east,subparallell to bedding. Four en-echelon orebodies wereworked. In the oxidised zone (down to about 75 m)the ore minerals were malachite, cuprite, and subordinatetenorite, azurite, chrysocolla, and nativecopper, whereas chalcopyrite, magnetite, pyrite, andpyrrhotite occurred in the primary zone. Calcite, diopside,scapolite, gypsum, apatite, sphene, and prehnitewere gangue minerals.Trekelano mine (Shepherd, 1953; Carter & others,1961)Trekelano mine is situated about 14 km south ofDuchess, where a steep westerly dipping shear zonecuts hornblende-biotite schist, amphibolitic granofels,red and grey banded calc-silicate rocks, and scapolitepyroxenegranofels of the Corella Formation, andaplitic granite of the Mount Erie Igneous Complex.The mine was worked to a depth of 263 m. The oreconsisted mainly of chalcopyrite and pyrite/marcasitein calcite gangue, but also contained some chalcocite,malachite, chrysocolla, and tenorite.Hampden group of mines (Broadhurst, 1936; Sullivan,1953a; Carter & others, 1961; Brooks, 1977)The orebodies at the Hampden group of minesoccur along a north-trending shear zone, the HampdenFault, which is up to 60 m wide and cuts slate of theKuridala Formation and metadolerite forming theelongate structural basin centred on Kuridala. Oreshoots are up to 12 m wide in the oxidised ore, butaverage less than 1 m wide in the primary zone. Theoxidised ore—consisting mainly of malachite, chrysocolla,tenorite, and chalcocite—is reported to extendto a depth of 107 m. The primary ore zone, containingmainly chalcopyrite, marcasite, and pyrite, was workedto a maximum depth of 183 m. Quartz and kaolinare gangue minerals.Recent small-scale open-cut operations, describedby Brooks (1977), have exposed veins of chalcocite,malachite, cuprite, and hematite in kaolinised slate.The main lode is reported to dip 75° to the east, anda secondary ore shoot exposed in the open cut dipsabout 25° to the east.Saint Mungo mine (Carter & others, 1961)The Saint Mungo orebody was situated in a narrowfissure vein at the contact between the Saint MungoGranite and an amphibolite dyke. The main oreshoots, as described by Carter & others (1961),occurred at the intersections of horizontal and verticalfaults or fracture planes. In the lower workings, one oreshoot was at right-angles to the northerly trend of themain deposit. The mine was worked to a depth of116 m, but the main shaft extended to a depth of137 m. The main ore mineral appears to have beenchalcopyrite.Mount Mascotte mine (Carter & others, 1961)The copper deposit at the Mount Mascotte mine,20 km north of Duchess, was a narrow fissure vein ina north-northeast-trending fault cutting fine-grainedquartz-biotite schist and amphibolite of the CorellaFormation. The oxidised ore consisted of malachite,chalcocite, and minor cuprite in a gangue of quartzand calcite. Primary sulphides included bornite andchalcopyrite. Ore shoots of massive sulphide 30 cmto 1 m thick were reported.86

Answer mine (White, 1957; Carter & others, 1961;Brooks, 1977, 1979b)The Answer lode is situated along a shear zoneparallel to bedding in black and grey carbonaceousslate and minor phyllite of the Answer Slate. Almostall the ore mined came from the oxidised zone, abovethe 30.5-m level, and consisted mainly of chalcocite,bornite, chalcopyrite, and pyrite in a quartz gangue.The highly pyritic primary sulphide lode was exposedat the 61-m level. Profitable small-scale copper leachinghas been practised since 1974, using scrap iron(to fix the copper) and the highly acidic mine water.Mount Cobalt mine (Reid, 1921; Rayner, 1938, 1953;Honman, 1941; Brooks, 1960, 1979a; Carter &others, 1961; Croxford, 1974; Nisbet & others, 1983).The Mount Cobalt lode occurs in a shear zone atthe contact between a metadolerite sill and steeplydipping schist, metagreywacke, and meta-arenite of theKuridala Formation. The lode is up to 9 m wide, andhas been worked over a length of 220 m and to adepth of 34 m. The cobalt ore consists of hypogenecobaltite and supergene erythrite and black oxide,together with minor galena, pyrite, chalcopyrite, sphalerite,and traces of gold, silver, and scheelite. Thegangue minerals include quartz, calcite, magnetite,siderite, and biotite. Small amounts of copper ore consistingof chalcopyrite, pyrite, and supergene chalcocite,covellite, cuprite, malachite, azurite, and chrysocollawere also produced. Indicated reserves from diamondand percussion drilling are 60 000 t of 1.25percent Co (Nisbet & others, 1983).Mineral potentialPerhaps the most obvious exploration targets in theregion are shales and slates, many of which—especially those that are carbonaceous and pyritic—were probably deposited in euxinic basins and henceoriginally may have been anomalously rich in basemetals,precious metals, uranium, and sulphur.One of the most productive cupriferous areas inthe region in the past has been the Corella Formationnorth and south of Duchess. This predominantly calcsilicatesequence has been intruded by numerous graniteand dolerite bodies which may have contributed copperby hydrothermal processes or may have promotedconvective movement of copper-scavenging sodiumand chloride-rich solutions. Numerous faults providegood structural traps for mineral-bearing solutions,and many contain cupriferous vein calcite and silica.The Corella Formation can therefore be consideredprospective for both small-scale epigenetic medium tohigh-grade and large-scale low-grade copper deposits,and also for calcite and silica flux. Favourable sitesfor possible syngenetic sulphide accumulation withinthe Corella Formation may be located near interlayeredfelsic metavolcanics, such as those recordednear the Overlander group of mines (Bultitude &others, 1982).In the east, similar calc-silicate rocks intruded bydolerite and granite occur in the Doherty Formation.However, three criteria imply that the mineral potentialof this formation is less significant: (1) the unithas a poor record as an ore producer; (2) few interlayeredvolcanic rocks have been found within it; and(3) much of the granite intruding it postdates themain deformation and regional metamorphism. Nevertheless,the formation does host low-grade coppermolybdenumdeposits in black shales interbedded withcalc-silicate rocks north and south of Mount Arthur,and can still be considered prospective, as explorationhas been discouraged by the rugged terrain.Other areas of economic interest in the east includethe northern part of the Staveley Formation, wherehalite casts indicate a shallow-water evaporitic depositionalenvironment in which intrastratal brines mayhave been active in concentrating and depositing copperminerals during sedimentation and diagenesis; blackslate of the Marimo Slate, in which traces of copperminerals have been found in shear zones; and metabasaltof the Marraba Volcanics, which remain prospectivefor small-scale copper deposits, particularlywhere cut by cross-faults.Within the Kuridala Formation and Soldiers CapGroup, several low-grade lead-zinc-copper depositsare associated with stratiform syngenetic banded ironformation, the best-known prospect being Pegmont(mainly lead-zinc). Prospecting for similar bodies hasbeen undertaken in recent years throughout most ofthe Soldiers Cap Group and Kuridala Formation outcropareas, and it is considered unlikely that moreprospects will be located in areas of reasonable outcrop.However, some potential still exists in the farsouth and east, where the sequences are eitherintensely weathered and leached or are concealedbeneath superficial sediments.Similar prospecting targets occur in a north-trendingzone west of Selwyn, where some essentially concordantridge-forming quartz-hematite bodies (Fig.79), mainly in the Kuridala Formation, are anomalouslyrich in copper and gold (Blake & others, 1983a);Donchak & others, 1983). These iron-rich bodiesappear to be epigenetic, perhaps formed by hydrothermalreplacement of readily altered grains andmatrix during the intrusion of nearby granite. Manyof them have been evaluated during recent exploration,but there is still scope for further exploration.Granites, quartzofeldspathic gneisses, and felsicvolcanic units within the region generally show littleevidence of mineralisation, and are considered prospectivefor base-metals only where they are cut bymafic dykes. The uranium-rich granites of the region,however, especially those of the Sybella and WongaBatholiths, which have been regionally metamorphosed,and adjacent country rocks, are potential hosts foreconomic uranium deposits (see section GEO­CHEMISTRY OF IGNEOUS ROCKS).The outcrop areas of Mount Isa Group sedimentaryrocks in the west of the region have considerablepotential for Mount Isa-type copper deposits, and alsosome potential for Mount Isa-type lead-zinc deposits,although no equivalents of the Urquhart Shale, thehost rock at Mount Isa mine, appear to be present.Classification of mineral deposits and metallogenesisby D . J . Perki nTwo main types of metalliferous mineral depositscan be recognised in the Duchess-Urandangi region.One of these is the Duchess-type copper-gold (±cobalt, tungsten, molybdenum, uranium, silver) stratabounddeposit into which most of the known economicdeposits fall. The other is the Pegmont-type lead-zincsilverstratiform deposit (Mount Isa shale-hosted style).However, tectonic, metamorphic, and metasomaticoverprinting has caused the original characteristics ofthe deposits to be blurred, and has led to a greatdiversity in the present form of these two main types.HI

Fig. 79 . Concordan t quartz-hematit e bod y withi n th e Kuridal a Formatio n formin g a wal l mor e tha n 2 m hig h o n aridge cres t 1 5 k m southwes t o f Kuridal a (a t G R VB405332) . (G B 2326 )Characteristics of Duchess-type stratabound deposits— disseminated ore minerals within a distinctivelithologic unit, commonly concentrated wherethere is enhanced shearing, faulting, and metasomatism— diagenetic through to late metamorphic mineralisation— generally high gold contents and appreciableamounts of cobalt, tungsten, molybdenum, silver,or uranium— associated iron oxide facies rocks uncommon— some evidence of hypersaline sedimentary environment:e.g., high potassium and/or sodium contents,presence of scapolite and/or barite, presenceof pseudomorphs after evaporite minerals.Characteristics of Pegmont-type stratiform deposits— banded to massive sulphides of lead, zinc, and ironin thinly bedded metasediments— syngenetic mineralisation— generally minor copper and only traces of gold— sulphides invariably associated with bedded ironoxide facies rocks which in the unoxidised zonecomprise mainly magnetite— evidence in host rocks of hypersaline and/oralkaline depositional environmentDiscussionThe Duchess-type stratabound deposits are consideredto have resulted from the leaching of copperfrom mafic igneous rocks into sulphur-bearing blackshale, dolomite, calc-silicate rock, or arenite at somestage between early diagenesis and the end of regionalmetamorphism. Basaltic volcanics were probably theultimate source for most of the copper and othermetals, although doleritic dykes, sills, and other intrusionsmay have contributed copper to some of thedeposits, such as those of the Hampden group atKuridala, through hydrothermal leaching; however,metamorphic convergence (Walker & others, 1960)obscures the identity of the protoliths of many amphibolitebodies close to the mineral deposits. Favourablezones for copper ore deposition are where reactiverocks such as carbonates are located near permeablezones connected with nearby mafic rocks; the permeablezones may be faults, breccia zones, or conglomeratebands. Most copper is thought to havebeen originally deposited during late diagenesis in asabkha or rift-shelf (shallow-lacustrine) environmentin an intracratonic tectonic setting. However, muchof the copper was remobilised and concentrated duringsubsequent tectonism and metamorphism.The Pegmont-type stratiform deposits, like theMount Isa lead-zinc ores, were probably originallyshale-hosted and arc of exhalative origin. Subsequentmetamorphism and metasomatism has effectivelymasked the original textures and mineralogy of thehost sedimentary rocks. The source of the metals is88

envisaged as lead-zinc-rich brines or exhalites derivedfrom hydrothermally leached felsic volcanics, ratherthan mafic volcanics. The chemical precipitation mayhave taken place in a fault-controlled local basin orrift.The Duchess-type deposits bear some resemblanceto portions of the Bushmanland copper deposits of theOkiep district in northwest Cape Province, SouthAfrica, described by Lombaard & Schreuder (1978).Both the Duchess-type and Okiep-type deposits maybe regarded as examples of metamorphosed, metasomatised,and remobilised Zambian-type copperdeposits. The Pegmont-type deposits, on the other hand,are similar to the Broken Hill lode in New SouthWales, and also to the Broken Hill lead-zinc-copperdeposit at Aggeneys, 120 km north of Okiep (Ryan,1982), to the nearby zinc deposit at Gamsberg, SouthAfrica (Rozendaal, 1978), and to the lead-zinc depositsof Ducktown, Tennessee (Nesbitt, 1982); I considerthat these deposits are metamorphosed and metasomatisedequivalents of shale-hosted stratiform lead-zinc(± copper) deposits.In accord with the correlation scheme favoured forthe Duchess-Urandangi region, the geological historyoutlined below generally follows the interpretation ofBlake (1980, 1981b, 1982), rather than that proposedby Carter & others (1961) and Derrick and his coworkers(e.g., Plumb & Derrick, 1975; Plumb & others,1980, 1981; Derrick, 1980; Derrick & Wilson, 1981b),for the evolution of the Mount Isa Inlier. The geochronologicalframework is based on U-Pb zircon andRb-Sr dating by Page (1978, 1980, 1981a, 1981b,1983a,b; Page & others, 1982). The main periods ofsedimentation, volcanism, igneous intrusion, regionalmetamorphism, and deformation are shown in Figure64. The tectonic setting throughout the Proterozoic isconsidered to be intracratonic or ensialic (Wyborn &Blake, 1982).Pre-1900 m.y.?Volcanism. Thick sequences of felsic volcanics, predominantlypyroclastic flows (ignimbrites) but includingsome lavas, and minor basaltic lavas wereextruded over much of the region. These volcanics,together with associated volcaniclastic and other sediments,are represented by the older basement rockunits—probably the Sulieman Gneiss and Saint RonansMetamorphics of the western area, the Plum MountainGneiss, much of the undivided Tewinga Group, andprobably part of the Corella Formation of the centralarea, and possibly the Double Crossing Metamorphicsof the eastern area. The abundance of ignimbriticdeposits in the undivided Tewinga Group indicatesthat much of the volcanism was subaerial.Sedimentation. Fluvial and shallow-marine sedimentsof mainly volcaniclastic quartzofeldspathic types butincluding some quartz-rich sands, such as those of theKallala Quartzite in the southwest, were depositedduring the period of volcanism. The calcareous andevaporitic sediments of the Corella Formation in theDuchess Belt of the central area may be of similar age.Intrusive activity. The emplacement of felsic andmafic dykes, and perhaps some granite comagmaticwith the volcanics, accompanied the volcanism.Palaeogeography. During the igneous activity mostof the region was probably a land area, but marineconditions may have prevailed locally, especially in theDuchess Belt of the central area.SUMMARY O F GEOLOGICA L HISTOR Y89Remarks. Some of the older basement rock unitsmay be Early Proterozoic—similar in age to theYaringa Metamorphics (west of Mount Isa), whichare probably between about 2000 and 2100 m.y. old(Page & others, 1982). Others, including parts of theSulieman Gneiss, undivided Tewinga Group, and PlumMountain Gneiss, may be similar in age to lithologicallysimilar upper Archaean rocks of the GawlerCraton, South Australia (e.g., Parker, 1980).About 1900 m.y.?Tectonism. During a major tectonic event some timebefore 1880 m.y., perhaps at about 1900 m.y., theolder basement was tightly folded and regionally metamorphosedto amphibolite facies gneissic, schistose, andgranulitic rocks.Intrusive activity. Some granitic intrusions, suchas the One Tree Granite of the Kalkadoon Batholith,may have been emplaced during or shortly after thefolding and metamorphism.Palaeogeography. During the tectonism the regionwas uplifted to form part of a mountainous land areathat may have extended well beyond the present confinesof the Mount Isa Inlier.About 1880-1810 m.y.Volcanism. Extensive felsic and minor mafic volcanicactivity, represented by the Leichhardt Volcanics,took place in the central area. The abundance of ignimbriticdeposits and paucity of associated volcaniclasticand other sedimentary rocks indicate subaerial eruptions.The volcanism probably commenced about 1875m.y. ago or a little earlier, and may have lasted for10 m.y. or more. The volcanism represented by theSaint Ronans Metamorphics of the western area ispossibly of a similar age.Intrusive activity. Granites of the Kalkadoon Batholith,comagmatic with the felsic volcanics, were emplacedin the central area before, during, and after theLeichhardt volcanism. Some mafic dykes wereemplaced at about the same time, and these formedsmall net-veined complexes where they intersectedpartly molten granite. Younger mafic dykes, formedof non-magnetic dolerite, intruded the Kalkadoon-Leichhardt Block after 1850 m.y. but before 1780 m.y.Deformation and metamorphism. Minor contactmetamorphism and local folding and faulting accompaniedgranite emplacement and volcanism. However,there does not appear to have been a major deformationaland regional metamorphic event at this time(Blake, 1981b). From about 1850 m.y., when felsicigneous activity probably ceased, to about 1810 m.y.the region seems to have been tectonically stable.Palaeogeography. Throughout the period the regionprobably remained a land area subjected to erosionalprocesses. By 1810 m.y. parts of the Kalkadoon Batholithhad been unroofed and large areas of metamorphosedbasement rocks were exposed.About 1810-1750 m.y.The older cover sequences were laid downthis period.during

Sedimentation. At about 1810 m.y., subsidence inthe Dajarra Belt of the central area and probably inthe western area resulted in sedimentation commencingin a linear depression corresponding to the LeichhardtRiver Fault Trough (Zone) of Glikson & others(1976), Plumb & others (1980, 1981), and Derrick(1982). The first sediments laid down, those of theBottletree Formation and the Yappo Member of theMount Guide Quartzite, were immature fluvial andshallow-marine deposits derived from contemporaneousvolcanics and from older rocks mainly to the east.They were succeeded by the more mature, probablyshallow-marine sediments of the upper part of theMount Guide Quartzite. Shallow-water sedimentationcontinued in the depression during the accumulation ofthe Eastern Creek Volcanics, Jayah Creek Metabasalt,Oroopo Metabasalt, and Judenan beds. To the east,penecontemporaneous fluvial sediments present withinthe Magna Lynn Metabasalt and Argylla Formationwere laid down on the Kalkadoon-Leichhardt Blockof the central area, which was part of an extensivelandmass at this time. Subsequent subsidence farthereast—after the deposition of the Haslingden Groupto the west—resulted in thick accumulations of mainlyshallow-water clastic and other sediments in the easternarea—those of the upper part of the Argylla Formation,and the Marraba Volcanics, Mitakoodi Quartzite,Overhang Jaspilite, Answer Slate, Kuridala Formation,and Soldiers Cap Group. During this subsidence theKalkadoon-Leichhardt Block also became a depositionalarea, accumulating shallow-marine clastic andcarbonate sediments represented by the Ballara Quartzite,Corella Formation?, and possibly the StanbrokeSandstone, Makbat Sandstone, and part or all of theSurprise Creek Formation?; these were laid down conformablyto locally unconformably on volcanics ofthe Argylla Formation and unconformably on olderrocks.Volcanism. Mafic and felsic volcanism, some ofwhich may have been subaqueous, accompanied theearliest sedimentation in the Dajarra Belt—that of theBottletree Formation. After a period of quiescence,mafic volcanism recommenced in the west withvoluminous outpourings of basaltic lavas—those of theEastern Creek Volcanics, which were laid down conformablyon sands (Mount Guide Quartzite) in thecentral part of the depression and overlapped eastwardsonto basement rocks. Basaltic lavas of probablysimilar age, those of the Jayah Creek Metabasalt andOroopo Metabasalt in the southwest, were laid downunconformably on basement rocks. At this time, onthe Kalkadoon-Leichhardt Block to the east, thebasaltic lavas of the Magna Lynn Metabasalt wereerupted, and were accompanied and succeeded by theeruption of the mainly felsic volcanics, especially pyroclasticflows, of the Argylla Formation. North of theDuchess-Urandangi region some Argylla-type felsicvolcanism continued on the Kalkadoon-LeichhardtBlock into Ballara Quartzite time (Blake, 1981b).In the eastern area an early phase of felsic volcanism,represented by the Argylla Formation, was followedby eruption of the basaltic lavas of the MarrabaVolcanics. Abundant interlayered sedimentary rocksindicate that both the felsic and mafic volcanism heremay have been largely subaqueous. Similar felsic andmafic volcanism persisted on a smaller scale duringthe deposition of the overlying Mitakoodi Quartzite.Farther east, contemporaneous or younger basalticand rhyolitic lavas and pyroclastics were erupted,probably under water, during the deposition of theKuridala Formation and Soldiers Cap Group sediments.These two units include exhalative-type bandediron formation formed by chemical sedimentationduring one or more of the volcanic episodes.Intrusive activity. Many mafic and some felsic dykeswere emplaced during this period, and some probablyvented to the surface, forming feeders for extrusiveflows.Palaeogeography. The depression in the west wasprobably bounded initially by a large hilly or mountainouslandmass to the east—the main source of atleast the early detritus—and a lower land area to thewest. Subsidence in the depression more or less keptpace with sedimentation, the depositional area wasgradually enlarged, especially to the west, and the landareas were eroded down to form broad plains withsome residual hills. Volcanic eruptions on the landareas—especially on the Kalkadoon-Leichhardt Block,which was part of the eastern landmass—resulted inextensive sheets of basaltic lavas and felsic pyroclasticflows filling topographic depressions, rather than theformation of prominent volcanic edifices. A relativerise in sea level and regional subsidence in the easttowards the end of Argylla Formation time causedmuch of the region to become a mainly shallow depositionalarea—perhaps part of a broad shelf. By about1750 m.y., sediments derived from land areas nearbyhad reached thicknesses of at least 10 000 m in partsof the western and eastern subsidence areas.Remarks. In this interpretation the Magna LynnMetabasalt and Argylla Formation postdate theMount Guide Quartzite; that is, they are not part ofthe basement underlying the Haslingden Group, asproposed by Derrick and his co-workers (e.g., Derrick& Wilson, 1981b); also, the basaltic lavas of the EasternCreek Volcanics are regarded as similar in age to thoseof the Magna Lynn Metabasalt and older than thoseof the Marraba Volcanics.About 1750-1700 m.y.Sedimentation. No sediments of this age are preservedin the western and central areas. In the easternarea, shallow to moderately deep-water sedimentation(Doherty Formation and perhaps the upper parts ofthe Overhang Jaspilite, Answer Slate, Kuridala Formation,and Soldiers Cap Group) may have continuedwithout major interruption until at least 1720 m.y.Volcanism. In the eastern area, minor mafic andfelsic volcanism accompanied the deposition of the _predominantly sedimentary Kuridala Formation,Soldiers Cap Group, and Doherty Formation. Rhyolitelavas in the Doherty Formation were erupted 1720± 7 m.y. ago (Page, 1983a).Intrusive activity. Isotopic dating of granites northof the region (Page, 1980, 1983b) indicates that mostof the Wonga Batholith plutons in the eastern partof the central area were probably emplaced between1740 and 1720 m.y. ago. The mafic component of theMount Erie Igneous Complex and Myubee IgneousComplex was emplaced either at this time or sometimelater. Some mafic and felsic dykes, comagmaticwith contemporaneous volcanics, probably intrudedthe eastern area.Tectonism. Granite emplacement in the central areawas probably accompanied by local folding and faulting,as well as by contact metamorphism, but no tight90

folding and regional metamorphism of region-wideextent are thought to have taken place during thisperiod (Blake, 1981b).Palaeogeography. The eastern area was part of adepositional basin for all or most of the period. Theremainder of the region—except perhaps the easternpart of the central area, the loci for granite plutonsand a likely zone of uplift—may also have been belowsea level for much of the time.About 1700-1650 m.y.Volcanism and sedimentation. Minor felsic volcanism(Carters Bore Rhyolite) followed by deposition ofshallow-water sediments (Mount Isa Group,McNamara Group, and possibly part or all of theSurprise Creek Formation?) took place in the westernpart of the region between about 1680 and 1670m.y. ago (Page 1978, 1981b, 1983a). The sediments ofthe Staveley Formation, Agate Downs Siltstone,Marimo Slate, and Roxmere Quartzite? in the easternarea may have been deposited at this time or duringa later period.Intrusive activity. Most of the granitic plutonsmaking up the Sybella Batholith, and some spatiallyassociated mafic dykes, were probably emplaced about1670 m.y. ago (Page, 1981a), presumably shortlyafter the deposition of the Mount Isa Group. TheGarden Creek Porphyry intrusion in the western partof the central area may be of a similar age.Tectonism. A widespread deformational eventcausing tight to isoclinal folding and regional metamorphismto the greenschist and amphibolite faciesmay have taken place during this period, more or lesscoincident with the emplacement of the Sybella Batholith.However, the main folding and metamorphicevent evident in the region is thought to have takenplace between about 1610 and 1550 m.y.Palaeogeography. Between 1700 m.y. and about1680 m.y. the western part of the region was probablyemergent, and rocks overlying the Eastern CreekVolcanics and Judenan beds and their correlatives hadbeen removed by erosion by the time the Carters BoreRhyolite was erupted. Between about 1680 and 1670-m.y. a depositional basin developed once again in thewest while the remainder of the region may have beena low-lying land area. With the onset of graniteemplacement, region-wide uplift took place; thegreatest amount of uplift was possibly in the west.About 1650-1350 m.y.Sedimentation and volcanism. The shallow-watersediments of the Staveley Belt—Staveley Formation,Agate Downs Siltstone, Marimo Slate, and RoxmereQuartzite?—may have been deposited between about1650 and 1600 m.y. ago. Minor basaltic volcanismaccompanied the deposition of the Staveley Formation.Plutonism and tectonism. Some time after the depositionof the Staveley Belt sediments, between about1610 and 1550 m.y. ago, major metamorphic anddeformational events affected the entire region. Duringthese events the Staveley Belt sediments and olderrocks were tightly folded about mainly northerlytrending axes and regionally metamorphosed to thegreenschist and amphibolite facies, and some possiblenappe structures were formed; units folded before1880 m.y. were refolded. The foliated granites of theWilliams Batholith, and sills and other intrusions ofdolerite in the eastern area, including those nearKuridala, may have been emplaced at about this time.The tectonism was closely followed by the intrusionof the main plutons of the Williams Batholith in theeastern area and perhaps by the formation of theMount Philp Breccia bodies in the east of the centralarea. The granite emplacement may have beenaccompanied and/or followed by mild low-graderegional metamorphism, which resulted in a redistributionof rubidium and strontium isotopes (see Page,1978) and relatively young (1450-1350 m.y. old)K-Ar biotite ages (Richards & others, 1963).Palaeogeography. Between about 1650 and 1600m.y. part of the eastern area may have been submerged,forming a depositional basin. Subsequentmajor deformation and granite emplacement resultedin widespread uplift, and a period of erosion commencedwhich probably continued to the end of thePrecambrian.Remarks. Many of the ore deposits in the regionmay have formed during the tectonism of this period,when metals present in trace amounts in the rockswere mobilised, concentrated, and deposited infavourable sites.About 1350-1100 m.y.Sedimentation and volcanism. None have beenrecorded.Intrusive activity. A series of dolerite dykes cuttingacross regional trends was emplaced between about1200 and 1100 m.y. ago.Palaeogeography. The region probably remained aland area throughout the period, and was graduallyworn down to a peneplain.About 1100 m.y. to presentSedimentation and palaeogeography. After prolongederosion, tillite was deposited near Duchess duringone of the glacial epochs which affected northernAustralia in the Late Proterozoic. Shallow-marinesedimentation recommenced early in the Cambrianand continued, with several breaks marked by disconformities,into the Ordovician. The Cambrian andOrdovician sediments, which included calcareous andphosphatic types, may have blanketed the entire region,but are now largely confined to the Burke RiverStructural Belt in the centre of the region and to themain part of the Georgina Basin in the far west.After relative uplift and resulting erosion, duringwhich much of the lower Palaeozoic sedimentaryblanket was removed, the region was partly or entirelycovered by fluvial and shallow-marine sediments of theJurassic to Cretaceous Eromanga and CarpentariaBasins. Since the Mesozoic the region has been part ofa continental landmass. Planation and lateritisation ofthis landmass took place during the Tertiary. Thepresent, largely dissected landscape can be attributedto late Tertiary and Quaternary changes in climateand relative sea level.Tectonism. Some faulting took place during thePalaeozoic, mainly along reactivated Precambrianstructures, and in places within the Pilgrim Fault Zonethe Cambrian strata were steeply tilted. In general,though, the Cambrian and younger strata are flatlying,indicating that the region has remained essentiallystable tectonically throughout the Phanerozoic.91

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AGSO LIBRAR YANG0054507 *DATE DU E

AUSTRALIA 1:25 0 000 GEOLOGICAL SPECIA LGEOLOGY O F THE DUCHESS-URANDANG I REGIO N QUEENSLANDMT ISA 35 kmMT ISA 37 kmCLONCURRY42 kmCLONCURRY57 km1 39°00'21°00'|f/ « 3 20 '«9WONOMO FAUL TMOUNT IS AFAULT ZON EPILGRIMFAULTZONECzTsTISand, silt, gravel: alluvial, colluvial and residualLimestone: minor cbalcedonyiaterite: later/tie rubbleieacbed bedrock : mesa cappingsooNOFQ.C/3 = >cc O111 o c_lO nV) oOWILLIAMS BATHOLIT HPgm, Pgg , Pgid,Pgic, Pgiy, Pgis, Pgia, Pgix Mainly biotite and hornblendebiotitegranite, porphyritic and non-porphyriticEg Massive to foliated granitic rocksPgc,Pge, Pgq Mainly foliated leucocratic granodiorite,granite, and tonalitePiPinPibPimPiwPmwPmpPrPhrPhr?PgpMeta-arenite, quartzite, siltstone, shaleDolomitic siltstone, siltstone, shaleCarbonaceous?, pyritic shale and siltstoneDolomitic pyritic siltstone, limestone, dolomiteMeta-arenite, quartzite; minor conglomerate, metasiltstoneMetasiltstoneMeta-arenite, quartzite; minor conglomerateArenite; minor conglomerate, shale, siltstonePorphyritic rhyoliteRhyolitic tuffPorphyritic biotite microgranite; minor amphiboliteSYBELLA BATHOLIT HEdbPgsMainly biotite granite, commonly foliated, porphyriticto non-porphyriticIntrusive brecciaWONGA BATHOLIT HPgbr.Pgbo Medium to coarse leucocratic hornblende-biotite granitePbm GabbroPbrrtg Leucocratic graniteGgd Leucocratic granite, metadolerite, dioritic hybrid rocks, pegmatitePgz Foliated biotite granite; minor quartzofeldspathic gneiss, augen gneissPgo Foliated biotite-hornblende granitePgr Foliated biotite granitePgy Quartzofeldspathic gneiss, augen gneiss, foliated to gneissic granitePgn Foliated porphyritic hornblende-biotite granitePprPkgPkmPkyPksPkaPkrPkr sBkr fPkdPkd sPkcPkc?PkjPkbPxsPxmPoPo difj Po t3 Po nO Po lLU „cn Pn mrr Pn aPeaO Per nrrLUQ Pir n_iOPovPjvPjtArenite; minor siltstoneMetasiltstone, phyllite, quartziteMainly carbonaceous slate and siltstoneFeldspathic arenite and quartziteMainly arenite, siltstone and phyllite, commonly calcareousand ferruginousMetasiltstone, slate, phyllite, mica schistMainly schist, meta-arenite. slate, phylliteGraphitic slate, phyllite. metasiltstoneMainly meta-arkoseMainly banded and brecciated calc-silicate granofelsBlack slateScapolitic and amphibolitic calc-silicate rocks, mafic andfelsic metavolcanics, marble, schist, ironstoneScapolitic and amphibolitic ca/c-silicate rocksCalcareous siltstone, arenite. chert limestone, jaspilite.ironstoneMeta-areniteArenite, dolomite, limestone, siltstone. basal conglomerateArenite, siltstone, basal conglomerateGneiss, schist, quartzite; some banded iron formationsMainly amphibolite, commonly garnetiferousAmphibolite, mica schist metasiltstone, quartziteMainly meta-arenite, mica schist, amphiboliteMica schist, phyllite, amphiboliteMainly meta-areniteMetabasalt. metasiltstone. meta-arenitePorphyritic felsic metavolcanics. meta-arenite (especially in east}Metabasalt, meta-arenite, mafic schist conglomerateGneiss, schist, amphibolite, qoartziteMetabasalt, meta-arenite, metasdtstone, limestoneMetabasalt, amphibolite, schist quartzite, meta-areniteMeta-arenite; minor metasiltstone. quartzite, metabasaltCJoNoDCLUODCQ_22°00'3Q0 000 mr;Published b y the Burea u o f Minera l Resources , Geolog y an d Geophysics ,Department o f Nationa l Developmen t an d Energy. Prepare d i n collaboratio nwith th e Geological Surve y o f Queensland . Issue d unde r th e authorit yof the Minister fo r National Developmen t an d Energy. Bas e mapsupplied b y the Division o f Nationa l Mapping , topographi c informatio nshown i s correct t o 1979 for the Duchess ma p sheet an d 1980 fo rthe Urandang i ma p sheet(©Commonwealth o f Australi a 1983STRUCTURAL AN D TECTONI C SKETC HBOULIA 112 kmEROMANGA BASI NCretaceous sedimentary sequenceGEORGINA BASI NCambro-Ordovician sedimentary sequence5I —SCALE 1:25000 05 1 0 25 Kilometre sBLUE NUMBERE D LINE S AR E 1 0 000 METRE INTERVAL S O F TH E AUSTRALIA N MA P GRID , ZON E 54TRANSVERSE MERCATO R PROJECTIO NDIGBYPEAKS 76 km TOOLEBUC HOMESTEAD 25 km 50' j' B 0GRID ZON E DESIGNATIO N54K100 00 0 METR ESQUARE IDENTIFICATIO NTBUBTA U A'0 "0VBVAIGNORE th e SMALLE R figure s o fany gri d number , thes e ar e forfinding th e ful l c o ordinates Us eONLY th e LARGE R figure s o fthe gri d number ; example :3Q 0 00 0UNIVERSAL GRI D REFERENC ETO GIV E A STANDAR D REFERENC E O NTHIS SHEE T T O NEARES T 10 0 METRE SSAMPLE POINT : • Birds Well1 Rea d letter s identifyin g 10 0 000 metr esquare i n whic h th e poin t lies :2 Locat e firs t VERTICA L gri d lin e t oLEFT o f poin t an d rea d LARG E figure slabelling th e lin e eithe r i n th e to p o rbottom margin , o r o n the lin e itself :3 Estimat e tenth s fro m gri d lin e t o point :4 Locat e firs t HORIZONTA L gri d lin eBELOW poin t an d rea d LARG E figure slabelling th e lin e i n eithe r th e lef t o rright margin , o r o n th e lin e itsel f5 Estimat e tenth s fro m gri d lin e t o point:SAMPLE REFERENCE :If reportin g beyon d 18 ° in any direction,prefix Gri d Zon e Designation , as: 54KUB733 24 9TOOLEBUC HOMESTEAD 2! kmGeology 196 7 b y F . deKeyser, J.H . Shergold, C.G . Gatehouse,R. Thiem e BMR ; C.G . Murray GSQ(Burke Rive r Structura l Belt )1969 b y F . deKeyser, P.J . Cook BM R (Georgina Basin )1975-1980 b y R.J. Bultitude. D.H . Blake, CM. Mock ,A.L. Jaques , R.N . England, G.M . Derrick,R.M. Hill , K.R . Walker BMR ; P.J.T . Donchak ,T.A. Noon , I.H . Wilson GSQCompiled 198 0 by D.H. Blake, R.J . Bultitude, A.J . Retter, BMREdited 198 1 by G.W. D'Addari oDesign b y Cartography Sectio n BMRDrawn b y Mercury-Walc hPrinted b y Mercury-Walc hGeological boundaryAnticlineSynclineOverturned synclineAntiformSynformSaint Ronan sMetamorphicsPsPqKallalaQuartziter— ?—iPsgSuliemanGneissLUQo