Sulfide chemistry and sulfur isotope characteristics of the Cenozoic volcanic-hosted Kuh-Pang copper deposit, Saveh county, northwestern central Iran

The several-hundred-m-thick Eocene-Oligocene volcanic units in the Urmia-Dokhtar magmatic arc in northwestern Central Iran host stratabound and fault-controlled copper mineralization. The Kuh-Pang deposit (2.8 Mt at 1.65 wt% Cu, 0.52 g/t Au, 34 g/t Ag) has vein-style copper mineralization, with primary Cu-sulfides of chalcopyrite, bornite, chalcocite and digenite, and supergene Cu-sulfides of chalcocite, covellite, malachite and azurite, in a tectonic hydrothermal breccia zone within rhyodacite and andesite flows. The mineralization is accompanied by a variety of alterations including silicic, carbonate, argillic and advanced argillic within a broad-scale propylitic halo. The main ore formation is related to hydrothermal breccias, and has a close association with silicic and argillic alterations. Sulfides formed during hydrothermal alteration, as indicated by: 1) the occurrence of disseminated sulfides in the groundmass of hydrothermally altered rocks, 2) the co-precipitation of sulfides and alteration minerals (e.g., kaolinite, alunite) in the cement of hydrothermal breccias, and 3) the occurrence of sulfides filling fissures and voids of hydrothermally altered rocks (vuggy quartz). The main mineralization stages include: (i) pre-ore stage with pyrite and arsenopyrite, (ii) main ore-stage with chalcopyrite, bornite, chalcocite I and digenite, and (iii) late-ore and supergene enrichment stage with chalcocite II and covellite. Electron microprobe analyses of pyrite show that As, Cu and Co are the most abundant minor elements. Back-scatter electron imaging of pyrite shows zoning features that closely reflect variations in the AO ratio. High Cu concentration in pyrite is probably related to the presence of sub-micrometer particles and aggregates of chalcopyrite or other Cu-bearing sulfide minerals. Silver is preferentially enriched in supergene chalcocite and covellite. The mineral assemblage indicates a trend from intermediate to high sulfidation state, and arsenopyrite geothermornetry defines temperatures for the pre-ore sulfides ranging from 250 to 370 degrees C. The sulfur isotope composition of pre-ore pyrite and arsenopyrite shows a delta S-34 range of 1 to 3 parts per thousand, indicating a magmatic sulfur-source. The copper sulfides have lower delta S-34 values (-9 to -3 parts per thousand), indicating that the ore-forming fluids gradually became SO42- -enriched and relatively oxidized during Cu mineralization. The tectono-magmatic setting, mineral assemblage and hydrothermal alteration, including advanced argillic and silicic zones, suggest that the Kuh-Pang copper deposit is a high sulfidation epithermal deposit in a volcanic arc terrain. (C) 2017 Elsevier B.V. All rights reserved.