USGS Professional Paper 1361 - State of Arizona Department of ...

18 GEOLOGY AND GOLD MINERALIZATION OF THE GOLD BASIN·LOST BASIN MINING DISTRICTS, ARIZONAof the study area and 6.8 km south of 1'emple Baryields a K-Ar age of 10.9±0.6 Ma. This age is the same(11.3±0.3, 11.1±0.5, and 10.6± 1.1 Ma), within the analyticaluncertainty, as that of the Fortification BasaltMember of the Muddy Creek Formation as reported byAnderson and others (1972). These ages were subsequentlydiscarded and a new set ofages of 4.9,5.8, and 5.9 Mawere adopted for the age of the Fortification BasaltMember of the Muddy Creek Formation (Lucchitta, 1979).The locality south of Temple Bar is at least 30 km eastof Fortification Hill and 45 and 50 km east·southeast ofthe collection sites of the other basalt samples, so thesesamples are probably not from the same lava flow. Theconcordant ages on basalts indicate that basaltic volcanismwas widespread and from a number of vents in theregion from 11 Ma to 5-6 Ma.All of the silicic and intermediate volcanic rocks in theGold Basin-Lost Basin area (those correlated with thePatsy Mine Volcanics or Mount Davis Volcanics) are infault contact with surrounding older rocks. This low-anglefault is apparently some type of regional slide surface andnot a thrust fault. Other areas in southern Arizona, southeasternCalifornia, western Utah, and eastern Nevada inwhich the terrane is characterized by large slide blocksofTertiary rocks have almost universally been related tometamorphic core complexes. Gravity tectonics caused bydoming of the core complex is the driving force that causesmovement of these large slide blocks. In the Gold Basin­Lost Basin area, sliding postdates the rhyolite unit datedat 15.4±0.2 Ma. A second period of sliding is recordedby low-angle fault surfaces between Tertiary conglomerateof the Muddy Creek Formation that is dated about10.9 Ma and older rocks including Proterozoic gneiss andthe Late Cretaceous two-mica monzogranite (see fig. 2).This second period ofsliding is probably not much youngerthan about 10.9 Ma because the Hualapai LimestoneMember of the Muddy Creek Formation, a thick carbonateunit in the region about 8 Ma (Blair, 1978), is somewhatyounger (Lucchitta, 1979), is largely undeformed, and isnot in low-angle fault contact with older rocks.PETROCHEMISTRY OF CRYSTALLINE ROCKS ANDTHEIR RELATION TO MINERALIZATIONMetamorphic rocks in the Gold Basin-Lost Basin miningdistricts have been derived from igneous and sedimentaryprotoliths that were regionally metamorphosed to as highas upper-amphibolite-facies (Miyashiro, 1973) assemblagesand complexly deformed syntectonically during the olderEarly Proterozoic Mazatzal orogeny (see Wilson, 1939,1962). This orogeny occurred 1,650 to 1,750 Ma (Silver,1967). The Mazatzal orogeny thus correlates temporallywith the upper half of the Hudsonian orogeny in the CanadianShield, with final metamorphism in the Front Range,Colorado, and with the pre-Belt basement, Montana(King, 1969). Early Proterozoic metamorphic rocks inArizona broadly compose three tectonic belts, of whichthe northwesternmost one, in the general region of theGold Basin-Lost Basin mining districts, is mostly gneissderived largely from an epiclastie protolith (Anderson andGuilbert, 1979); it also includes schist in the Vishnu Complexof Brown and others (1979) derived from shale andgraywacke protoliths and some thick sequences ofgneissand amphibolite derived from mafic and, very locally,ultramafic protoliths. Metamorphic rocks in the districtsalso include metaquartzite, thin lenses of marble, calcsilicategneiss, banded iron formation, and metachert. Theprotolith(s) of these metamorphic rocks is difficult to correlateconvincingly with specific well-studied sequencesof Proterozoic rocks in central Arizona (see Donnelly andHahn, 1981, for descriptions of these sequences). TheEarly Proterozoic Bagdad Belt of Donnelly and Hahn(1981) includes relatively thick sequences of coarse- tofine-grained metamorphosed wackes ncar the top of theoverall metamorphic pile there. Some of these metamorphosedwackes may be correlative with quartzofeldspathicgneiss at Gold Basin-Lost Basin.The gneiss exposed in the Gold Basin-Lost Basin miningdistricts contains highly deformed and lithologically complexsequences that commonly grade or change abruptlyinto one another across short distances. Although aquartz-plagioclase (oligoclase or andesine) or quartzofeldspathicgneiss is probably the predominant rock typeoverall of the gneiss unit, quartzofeldspathic gneiss is complexlyinterlayered with other lithologies in many outcrops(fig. M). In addition, moderately thick, layered sequencesof predominantly quartzofe1dspathic gneiss may gradeinto sequences consisting mostly ofamphibolite by subtleyet marked changes in the relative proportions ofquartzo.feldspathic gneiss and amphibolite along strike. Some ofthe amphibolite in the districts will be shown, in the followingsection, to have an igneous protolith. In some sequenceswithin the gneiss unit, rather abrupt transitionsare also present across conformable contacts, perpendicularto lithologic layering between predominantlyquartzofe1dspathic gneiss and mixed zones of amphiboliteand associated gneissic. pegmatoid leucogranite dikes andthin sills (fig. 58). However, most ofthesc contacts couldnot be laterally extended sufficiently to be shown at ascale of 1:48,000, the scale of the map published by Blacet(1975). Further, many outcrops show an overall bandedaspect resulting from complex and close interlaminationand interlayering of quartzofeldspathic gneiss and amphibolite.In fact, many outcrops in the gneiss unit containabundant 0.3- to 5.Q-an, highly planar and continuousbands of amphibolite, some bands reaching thicknessesas great as 40 cm. Many such outcrops commonly showa pulling apart or boudinage of the amphibolite layers. In