Rosia Poieni porphyry Cu-Au and Rosia Montana epithermal Au- Ag ...

• BULGARIAN ACADEMY OF SCIENCESGEOCHMISTRY, MINERALOGY AND PETROLOGY • 4 3 • S OFIA • 200 5Au-Ag-Te-Se depositsIGCP Project 486, 2005 Field Workshop, Kiten, Bulgaria, 14-19 September 2005Rosia Poieni porphyry Cu-Au and Rosia Montana epithermal Au-Ag deposits, Apuseni Mts., Romania: Timing of magmatism andrelated mineralisationKalin Kouzmanov 1* , Albrecht von Quadt 1 , Irena Peytcheva 1,2 ,Caroline Harris 1 , Christoph A. Heinrich 1 , Emilian Rou 3 , GaryO’Connor 41ETH Zürich, Institute of Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETHZentrum NO, 8092 Zürich, Switzerland; 2 Central Laboratory of Mineralogy and Crystallography, BulgarianAcademy of Sciences, 1113 Sofia, Bulgaria; 3 Geological Institute of Romania, Mineral Resources andEnvironmental Geology Department, 012271Bucharest, Romania; 4 Gabriel Resources Ltd, Suite 1501, 110Yonge Street, Toronto, ON, Canada M5C 1T4; *Present address: Institut des Sciences de la Terre, Universitéde Genève, 1205-Genève, Switzerland; E-mail: kalin.kouzmanov@terre.unige.chAbstract. The Rosia Poieni porphyry Cu-Au and Rosia Montana epithermal Au-Ag deposits are the largestoperating mines in the South Apuseni mineral district, Romania. Rosia Montana is a breccia-hosted low- tointermediate-sulfidation epithermal system, related to strong phreatomagmatic activity due to shallowemplacement of a dacitic dome structure. The Rosia Poieni deposit, situated about 4 km NE of RosiaMontana, is a porphyry copper system with a high-sulfidation epithermal overprint. This study provides thefirst high-precision U-Pb dating of Miocene magmatic and hydrothermal events in the South ApuseniMountains. Together with the existing 40 Ar- 39 Ar data, the new results show clearly that the two depositsbelong to two separate magmatic-hydrothermal systems, the porphyry copper system of Rosia Poieni beingabout 3 Ma younger than the neighbouring Rosia Montana epithermal deposit. The timing of the porphyrymineralization at Rosia Poieni has been bracketed by dating the main Poieni diorite intrusion (9.42 ± 0.14Ma) and a later intra-mineralization dike (9.16 ± 0.10 Ma), cutting a mineralized porphyry stockwork and cutby later porphyry veins. Whereas the magmatism at Rosia Montana is dated at 13.15-13.61 Ma by U-Pbdating on zircons and the epithermal mineralization there – at 12.71-12.85 Ma by 40 Ar- 39 Ar dating onhydrothermal adularia.Key words: porphyry, epithermal, U-Pb dating, Rosia Poieni, Rosia Montana, Apuseni Mts.IntroductionThe calc-alkaline Miocene magmatism in theSouth Apuseni Mountains was related totranstensional and rotational tectonics. Themagmatic activity was focused within NW-SEoriented extensional basins and developedmainly between 14.7 and 7.4 Ma (Rosu et al,2004). These structures host some of theEurope’s largest porphyry Cu-Au andepithermal Au-Ag deposits, both associatedwith shallow subvolcanic intrusions. One of theparticularities of the South Apuseni districtconsists of the common spatial associationbetween porphyry Cu-Au and epithermal113

deposits. Some recent studies (Alderton andFallick, 2000; Wallier et al, submitted)demonstrated that the mineralizing fluids inboth systems have a dominantly magmaticorigin, a direct genetic link between the largeporphyry and associated epithermal deposits inthe area is still controversial. With the presentstudy we tried to highlight the temporal andgenetic succession of magmatic and hydrothermalevents within the Rosia Montana-Bucium volcano-intrusive structure in thenorthern part of the district, hosting the largestoperating mines in the South Apuseni Mts –Rosia Poieni porphyry Cu-Au deposit andRosia Montana breccia-hosted epithermal Au-Ag deposit. Bucium-Tarnita is a porphyrystock cut by epithermal veins, about 3 kmsouth of the map area of Fig. 1.Geological setting and sampledescriptionFigure 1 summarizes the geology of the RosiaMontana – Rosia Poieni area. The RosiaMontana deposit is intimately related to adiatreme breccia complex associated with twodome-like dacitic bodies. The Rosia Poienideposit is hosted by the Poieni dioriteporphyritic stock, about 4 km NE of RosiaMontana. Rotunda type volcanic andesites andtheir pyroclastic derivates, cropping out in thearea between the two deposits and also east andsouth of Rosia Poieni, have a subhorizontalbasis and they cover the volcanoclastic brecciaand dacites from the Rosia Montana structure,as well as earlier folded Cretaceous sedimentaryrocks.Fig. 1. Geological map of the Rosia Montana-Rosia Poieni area, showing the proximity of the two deposits.Location of samples used for U-Pb dating is indicated as wellFor the purpose of the present study, wehave selected two samples from the RosiaMontana area and three samples from the RosiaPoieni porphyry deposit. Sample locations areshown on Fig. 1. Sample RM03SW19 comesfrom the Cetate open pit and is a representative114

sample of the Montana dacite, affected byquartz-sericite ± adularia alteration – the mainmagmatic rock, hosting the epithermal mineralization.Sample RM03SW29 is a clast ofquartz-bearing amphibole-biotite andesite fromthe volcanoclastic breccia, about 1 km south ofthe Cetate open pit, where a high-magneticanomaly has been mapped by Rosia MontanaGold Corporation. Sample RP04CH30 is arepresentative sample of the Poieni dioriteporphyry, cropping out in the western part ofthe Poieni open pit. Sample RP04KK08 isbiotite-bearing diorite porphyry from 662mdepth from drill core RPSP-002 (GabrielResources Ltd) in the central part of the RosiaPoieni open pit. Sample RP02RR07 is an intramineralizationfine-grained dark dike, cuttingthe Poieni diorite intrusion, as well as earlystage porphyry veins and cut by later quartzand quartz-pyrite-sericite veins.data. In Fig. 2, only the results for concordantzircons of all studied zircon crystals are shown.The concordant age is calculated using Isoplotprogram (Ludwig, 2001), with 95% confidence,decay-constant errors included.Figure 2a summarizes the results for theRosia Montana samples. The calculated agesare 13.61 ± 0.07 Ma and 13.15 ± 0.04 Ma forthe Montana dacite and the clast from thevolcanoclastic breccia, respectively. Thus themagmatic activity at Rosia Montana isconfined in the time interval 13.6-13.2 Ma.Geochronology and geochemistry of hostrocksConventional U-Pb single zircon dating hasbeen used to determine the crystallization agesof the studied magmatic rocks, as this methodcombines the relative resistance of zircons tohydrothermal overprint with the high precisionof the ID-TIMS (Isotope Dilution – ThermalIonization Mass Spectrometry) technique (fordetails, see von Quadt et al., 2002). Scanningelectron microscopy-cathodoluminescence(SEM-CL) images and laser ablation (LA) ICP-MS analyses of zircons help with theinterpretation of the zircon data and contributeto the understanding of the chemical evolutionof the magma source.The zircons of the five studied samplesreveal similar morphology and are usuallytransparent, orange or beige-rose-orangecolored. For the analyses the most elongatedprismatic morphology has been chosen with theaim of avoiding inheritance of older zircongenerations. A reason for this is given by theSEM-CL images of the zircons from the RosiaMontana samples showing inherited cores insome of the separated grains, which were alsoconfirmed by discordant older U-Pb isotopeFig. 2. U-Pb concordia diagrams for magmaticzircons: a) Rosia Montana area; b) Rosia Poieni areaResults from the Rosia Poieni porphyrydeposit (Fig. 2b) differ significantly from thosein Rosia Montana. The Poieni diorite porphyryhas an age of 9.42 ± 0.14 Ma; the biotitebearingdiorite from the deep drill core has anage of 9.23 ± 0.15 Ma, and the late intra-115

mineralization dike – an age of 9.16 ± 0.10 Ma.XRF major element and ICP-MS traceelement analyses of the studied samples, aswell as other representative samples frommagmatic rocks in the studied area, shownormal calc-alkaline character of magmas.They are more evolved at Rosia Montana,where they have dacitic composition, comparedto Rosia Poieni, where the compositions plotwithin the andesitic field. No adakitic-liketrend has been observed (Rosu et al., 2004).Discussion and conclusionsThe new data, in combination with the existing40 Ar- 39 Ar dating on hydrothermal adularia fromthe Rosia Montana deposit (Manske et al.,2004) and Wallier et al. (submitted), and K-Ardating on whole rocks (Rosu et al., 2004),allow reconstruction of the magmatic andhydrothermal history of the Rosia Montana-Bucium volcano-intrusive structure in thenorthern part of the district (Fig. 3).Fig. 3. Summary diagram for the timing of magmatism and mineralization in the northern sector of the SouthApuseni district, including the U-Pb data on magmatic zircons from this study, as well as published Ar-Ardating on hydrothermal adularia (Manske et al., 2004; Wallier et al., submitted) and K-Ar dating on wholerock (Rosu et al., 2004)Bucium-Tarnita, occupying the southernpart of the complex is a porphyry-coppersystem with high-sulfidation style epithermaloverprint, and according to K-Ar data of Rosuet al. (2004) is the oldest part of the volcanointrusivestructure (14.87-14.60 Ma). Themagmatic activity in the Rosia Montana areatook place between 13.61 and 13.15 Ma andwas immediately followed by the economichydrothermal mineralization (12.85-12.71 Ma;Fig. 3). The 40 Ar- 39 Ar amphibole age of 11.0 ±0.8 Ma of a clast from the high-magneticanomaly in Corna valley, south of the Cetateopen pit (Fig. 1), reported by Manske et al.(2004) has a suspiciously high analytical errorfor an 40 Ar- 39 Ar analysis and doesn’t help tounderstand the exact age relationship betweenthis high-magnetic breccia body and themineralization at Rosia Montana.The timing of magmatism and relatedporphyry mineralization at Rosia Poieni rangeswithin the narrow interval of 9.42-9.16 Ma.The timing of the porphyry mineralization hasbeen bracketed by dating the main Poienidiorite intrusion and a later intra-mineralizationdike. 40 Ar- 39 Ar dating of the high-sulfidationepithermal veining overprinting this porphyrysystem (Kouzmanov et al., 2004) is in progress.The similar K-Ar age for Rotunda andesite(9.30 Ma; Rosu et al., 2004) suggests a direct116

genetic link between the Poieni dioriteporphyry and the overlying volcanic rocks.Finally, the magmatism ended by theemplacement of small barren basaltic andesiteand andesite intrusions in the eastern part of thestructure at 7.8-7.4 Ma.This study provides the first highprecisionU-Pb dating of Miocene magmaticand hydrothermal events in the South ApuseniMountains. The results show clearly thatdespite their proximal location, and themagmatic character of the mineralizing fluidsin both, the Rosia Montana and Rosia Poienideposits belong to two separate magmatichydrothermalsystems. The porphyry coppersystem at Rosia Poieni is about 3 Ma youngerthan the neighbouring Rosia Montanaepithermal deposit, which is much longer thanthe thermal lifetime even of a very large uppercrustalmagma chamber.Acknowledgements. This study was financed by theSwiss National Science Foundation project 200020-100735 within the GEODE program. Themanagements of Gabriel Resources Ltd, RosiaMontana Gold Corporation and SC Cupru Min SA,Abrud are thanked for their logistic support in thefield.ReferencesAlderton, D.H.M., Fallick, A.E. 2000. The natureand genesis of gold-silver-tellurium mineralizationin the Metaliferi Mountains of westernRomania. Economic Geology, 95, 495-515.Kouzmanov, K., Wallier, S., Rey, R., Pettke, T.,Ivascanu, P., Heinrich, C. 2004. Fluid processesat the porphyry-to-epithermal transition: RosiaPoieni copper-gold deposit, Romania. In: J.Muhling, R. Goldfarb, N. Vielreicher, F.Bierlein, E. Stumpfl, D.I. Groves, S. Kenworth(Eds.), Predictive Mineral Discovery underCover. Proceedings, SEG 2004 Conference,Perth, Western Australia, 383-386.Ludwig, K.R. 2001. Isoplot/Ex - A GeochronologicalToolkit for Microsoft Excel. BerkeleyGeochronological Center Special Publication,No. 1a, p. 43.Manske, S., Ullrich, T., Reynolds, J., O'Connor,G.V. 2004. Vein sets and hydrothermalalteration in the Cetate - Carnic Area, RoiaMontan district, Romania. Romanian Journalof Mineral Deposits, 81, 122-125.Rou, E., Seghedi, I., Downes, H., Alderton, D.,Szakacs, A., Pecskay, Z., Panaiotu, C., Nedelcu,L. 2004. Extension-related Miocene calcalkalinemagmatism in the Apuseni Mountains,Romania: Origin of magmas. Schweiz. Mineral.Petrograph. Mitt., 84, 153-172.von Quadt, A., Peytcheva, I., Kamenov, B.K.,Fanger, L., Heinrich, C.A., Frank, M. 2002. TheElatsite porphyry copper deposit in thePanagyurishte ore district, Srednogorie zone,Bulgaria: U-Pb zircon geochronology andisotope-geochemical investigations of magmatismand ore genesis. In: D.J. Blundell, F.Neubauer, A. von Quadt (Eds.), The Timing andLocation of Major Ore Deposits in an EvolvingOrogen. Geological Society of London SpecialPublication, 204, 119-135.Wallier, S., Rey, R., Kouzmanov, K., Pettke, T.,Heinrich, C.A., Leary, S., O'Connor, G., Tamas,C., Vennemann, T., Ullrich, T., submitted.Magmatic fluids in the breccia-hostedepithermal Au-Ag deposit of Rosia Montana(Romania). Economic Geology.117