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Magmatic Fluids in the Breccia-Hosted Epithermal Au-Ag Deposit of Roia Montan, Romania

Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland

Christoph A. Heinrich

Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland, and Faculty of Mathematics and Natural Sciences, University of Zürich, Switzerland

Stephen Leary

RSG Global, 1162 Hay Street, West Perth, Western Australia 6005

Gary O’Connor

Gabriel Resources Ltd, Suite 1501, 110 Yonge Street, Toronto, ON, Canada M5C 1T4

Clin G. Tma

Babe-Bolyai University, 1 Koglniceanu Street, 400084 Cluj-Napoca, Romania

Torsten Vennemann

Institute of Mineralogy and Geochemistry, University of Lausanne, BFSH-2, 1015 Lausanne, Switzerland

Thomas Ullrich

Pacific Center for Isotopic and Geochemical Research, University of British Columbia, Vancouver, BC, Canada

Corresponding author: e-mail, heinrich{at}erdw.ethz.ch

The breccia-hosted epithermal Au-Ag deposit of Roia Montan is located 7 km northeast of Abrud, in the northern part of the South Apuseni Mountains, Romania. Estimated total reserves of 214.91 million metric tons (Mt) of ore at 1.46 g/t Au and 6.9 g/t Ag (10.1 Moz of Au and 47.6 Moz of Ag) make Roia Montan one of the largest gold deposits in Europe. At this location, Miocene calc-alkaline magmatic and hydrothermal activity was associated with local extensional tectonics within a strike-slip regime related to the indentation of the Adriatic microplate into the European plate during the Carpathian orogenesis. The host rocks of the magmatic complex consist of pre-Mesozoic metamorphosed continental crust covered by Cretaceous turbiditic sediment (flysch). Magmatic activity at Roia Montan and its surroundings occurred in several pulses and lasted about 7 m.y.

Roia Montan is a breccia-hosted epithermal system related to strong phreatomagmatic activity due to the shallow emplacement of the Montana dacite. The Montana dacite intruded Miocene volcaniclastic material (volcaniclastic breccias) and crops out at Cetate and Cârnic Hills. Current mining is focused primarily on the Cetate open pit, which was mapped in detail, leading to the recognition of three distinct breccia bodies: the dacite breccia with a dominantly hydrothermal matrix, the gray polymict breccia with a greater proportion of sand-sized matrix support, and the black polymict breccia, which reached to the surface, contains carbonized tree trunks and has a dominantly barren clastic matrix. The hydrothermal alteration is pervasive. Adularia alteration with a phyllic overprint is ubiquitous; silicification and argillic alteration occur locally. Mineralization consists of quartz, adularia, carbonates (commonly Mn-rich), pyrite, Fe-poor sphalerite, galena, chalcopyrite, tetrahedrite, and native gold and occurs as disseminations, as well as in veins and filling vugs within the Montana dacite and the different breccias. The age of mineralization (12.85 ± 0.07 Ma) was determined by ⁴⁰Ar-³⁹Ar dating on hydrothermal adularia crystals from vugs in the dacite breccia in the Cetate open pit.

Microthermometric measurements of fluid inclusions in quartz phenocrysts from the Montana dacite revealed two fluid types that are absent from the hydrothermal breccia and must have been trapped at depth prior to dacite dome emplacement: brine inclusions (32–55 wt % NaCl equiv, homogenizing at T_h > 460°C) and intermediate density fluids (4.9–15.6 wt % NaCl equiv, T_h between 345°–430°C). Secondary aqueous fluid inclusion assemblages in the phenocrysts have salinities of 0.2 to 2.2 wt percent NaCl equiv and T_h of 200° to 280°C. Fluid inclusion assemblages in hydrothermal quartz from breccias and veins have salinities of 0.2 to 3.4 wt percent NaCl equiv and T_h from 200° to 270°C. The oxygen isotope composition of several zones of an ore-related epithermal quartz crystal indicate a very constant ¹⁸O of 4.5 to 5.0 per mil for the mineralizing fluid, despite significant salinity and temperature variation over time. Following microthermometry, selected fluid inclusion assemblages were analyzed by laser ablation-inductively coupled-plasma mass spectrometry (LA-ICPMS). Despite systematic differences in salinity between phenocryst-hosted fluids trapped at depth and fluids from quartz in the epithermal breccias, all fluids have overlapping major and trace cation ratios, including identical Na/K/Rb/Sr/Cs/Ba. Consistent with the constant near-magmatic oxygen isotope composition of the hydrothermal fluids, these data strongly indicate a common magmatic component of these chemically conservative solutes in all fluids. Cu, Pb, Zn, and Mn show variations in concentration relative to the relatively non-reactive alkalis, reflecting the precipitation of sulfide minerals together with Au in the epithermal breccia, and possibly of Cu in an inferred subjacent porphyry environment. The magmatic-hydrothermal processes responsible for epithermal Au-Ag mineralization at Roia Montan are, however, not directly related to the formation of the spatially associated porphyry Cu-Au deposit of Roia Poieni, which occurred about 3 m.y. later.

This article has been cited by other articles:

				C. A. Heinrich Fluid-Fluid Interactions in Magmatic-Hydrothermal Ore Formation Reviews in Mineralogy and Geochemistry, July 1, 2007; 65(1): 363 - 387. [Full Text] [PDF]