Magmatic Fluids in the Breccia-Hosted Epithermal Au-Ag Deposit of Rosia Montana, Romania

• DOI: 10.2113/gsecongeo.101.5.923
• Publication Date: 2006-12-22T16:43:03Z
• Authors: S. Wallier; R. Rey; K. Kouzmanov; T. Pettke; C. A. Heinrich; S. Leary; G. O'Connor; C. G. Tamas; T. Vennemann; T. Ullrich
• Keywords: Breccia; Dacite

The breccia-hosted epithermal Au-Ag deposit of Roşia Montană is located 7 km northeast Abrud, in the northern part South Apuseni Mountains, Romania.Estimated total reserves 214.91 Mt ore at 1.46 g/t Au and 6.9 Ag (10.1 Moz 47.6 Ag) make one largest gold deposits Europe.At this location, Miocene calc-alkaline magmatic hydrothermal activity was associated with local extensional tectonics within a strike-slip regime related to indentation Adriatic microplate into European plate during Carpathian orogenesis.The host rocks complex consist pre-Mesozoic metamorphosed continental crust covered by Cretaceous turbiditic sediment (flysch).Magmatic its surroundings occurred several pulses lasted about m.y.Roşia system strong phreatomagmatic due shallow emplacement Montana dacite.The dacite intruded volcaniclastic material (volcaniclastic breccias) crops out Cetate Cârnic Hills.Current mining focused primarily on open pit, which mapped detail, leading recognition three distinct breccia bodies: dominantly matrix, gray polymict greater proportion sand-sized matrix support, black breccia, reached surface, contains carbonized tree trunks has barren clastic matrix.The alteration pervasive.Adularia phyllic overprint ubiquitous; silicification argillic occur locally.Mineralization consists quartz, adularia, carbonates (commonly Mn-rich), pyrite, Fe-poor sphalerite, galena, chalcopyrite, tetrahedrite, native occurs as disseminations, well veins filling vugs different breccias.The age mineralization (12.85 ± 0.07 Ma) determined 40 Ar-39 Ar dating adularia crystals from pit.Microthermometric measurements fluid inclusions quartz phenocrysts revealed two types that are absent must have been trapped depth prior dome emplacement: brine (32 -55 wt% NaCl eq., homogenizing Th > 460°C) intermediate density fluids (4.9 -15.6 between 345° 430°C).Secondary aqueous inclusion assemblages salinities 0.2 -2.2 eq. 200° -280°C.Fluid breccias 3.4 270°C.The oxygen isotopic composition zones an ore-related crystal indicate very constant 18 O +4.5 +5.0 ‰ for mineralizing fluid, despite significant salinity temperature variation over time.Following microthermometry, selected were analyzed laser-ablation inductivelycoupled-plasma mass spectrometry (LA-ICPMS).Despite systematic differences phenocryst-hosted breccias, all overlapping major trace cation ratios, including identical Na : K Rb Sr Cs Ba.Consistent near-magmatic isotope fluids, these data strongly common component chemically conservative solutes fluids.Cu, Pb, Zn Mn show variations concentration relative relatively nonreactive alkalis, reflecting precipitation sulfide minerals together possibly Cu inferred subjacent porphyry environment.The magmatic-hydrothermal processes responsible are, however, not directly formation spatially Cu-Au Poieni, 3 m.y.later.