Early pyrite and late telluride mineralization in vanadium-rich gold ore from the Oroya Shoot, Paringa South mine, Golden Mile, Kalgoorlie: 3. Ore mineralogy, Pb-Te (Au-Ag) melt inclusions, and stable isotope constraints on fluid sources

Abstract

The Oroya Shoot (>\u200962 t Au) in the giant Golden Mile deposit, Yilgarn Craton, Western Australia is controlled by reverse faults, which offset altered quartz diorite porphyry but are crosscut by a late-mineralization kersantite dyke (>\u20092642\u2009±\u20096\xa0Ma). Oroya stage 1 ore (12–14 g/t Au) is characterized by chert-like quartz, Fe-chlorite, siderite and pyrite (10–20\xa0vol%), minor arsenopyrite, chalcopyrite and sphalerite, and accessory pyrrhotite, gersdorffite, and melonite. Dendritic pyrite and arsenopyrite-chlorite thermometry indicate rapid precipitation from an H 2 S-dominant fluid of intermediate sulfidation and low oxidation state cooling from 450–400 to 340\xa0°C. Oroya stage 2 ore (120–600\xa0g/t Au) forms veins and breccia filled or cemented by quartz, muscovite (≤\u200913.2 wt% V 2 O 3 ), ankerite, chlorite, and tourmaline (≤\u200914.7 wt% V 2 O 3 ). The assemblage pyrite-arsenopyrite-chalcopyrite brackets telluride deposition. Bornite-chalcopyrite aggregates, sphalerite, and tetrahedrite are associated with native gold, montbrayite, altaite, calaverite, petzite, tellurantimony, coloradoite, and melonite. Myrmekitic altaite-tellurium\u2009±\u2009sylvanite and altaite-krennerite symplectites represent melt droplets deposited at >\u2009400\xa0°C and log f Te2 \u2009=\u2009−\u20094\xa0bar (400\xa0°C, 200\xa0MPa). Tellurium fugacity declined as the fluid cooled during the deposition of native gold and free telluride grains terminating in melonite replacement at 340–300\xa0°C. Pyrite-nolanite- and pyrite-magnetite-telluride assemblages suggest oxidation states up to 2 log units higher than during stage 1. After telluride deposition, the fluid evolved to a high sulfidation state (log f S2 \u2009=\u2009−\u20095\xa0bar at 300\xa0°C) and increased salinity indicated by digenite-covellite and Cl-bearing altaite. The average ore is enriched in lithophile (K, Rb, Cs), siderophile (Fe, V, Ni, W), and chalcophile elements (e.g., Te, Se, Cu, Zn, Pb) implicating local monzodiorite plutons as the fluid source. Stable isotope data from the Oroya kersantite and from Cu-Au skarn and monzodiorite-granodiorite stocks southeast of Kalgoorlie constrain the composition of the magmatic fluid to δ 13 C PDB \u2009=\u2009−\u20093.0 to −\u20092.2‰ and δ 18 O SMOW \u2009=\u20098.3 to 9.7‰. In the Golden Mile, the fluid oxygen isotope ratios of the gold ore bodies (8.2 to 9.8‰) are consistent with I-type magmatic water. The fluid carbon isotope ratios of all altered rocks (δ 13 C PDB \u2009=\u2009−\u20091.7 to −\u20090.5‰) are 13 C-enriched, perhaps due to the reduction of fluid CO 2 to CH 4 by interaction with ferrous greenstones close to the intrusion.