Geology, mineralization, stable isotope, and fluid inclusion characteristics of the Vostok-2 reduced W-Cu skarn and Au-W-Bi-As stockwork deposit, Sikhote-Alin, Russia

Abstract

The large (>180KtWO3 and at least 10–15 t Au) Vostok-2 deposit is situated in a metallogenic belt of W, Sn-W, Au, and Au-W deposits formed in late to post-collisional tectonic environment after cessation of active subduction. The deposit is related to an ilmenite-series high-K calc-alkaline plutonic suite that, by its petrologic signatures, is transitional between those at W-dominant and Au-dominant reduced intrusion-related deposits. Consistently, besides large W-Cu skarns of the reduced type, the deposit incorporates quartz stockworks with significant Au-W-Bi mineralization also formed in a reduced environment. The hydrothermal stages include prograde and retrograde, essentially pyroxene skarns, hydrosilicate (amphibole, chlorite, quartz) alteration, and phyllic (quartz, sericite, albite, apatite, and carbonate) alteration assemblages. These assemblages contain abundant scheelite associated with pyrrhotite, chalcopyrite and, at the phyllic stage, also with Bi minerals, As-Bi-Sb-Te-Pb-Zn sulfides and sulfosalts, as well as Au mineralization. The fluid evolution included hot, high-pressure (420–460°C, 1.1–1.2kbar), low-salinity (5.4–6.0wt% NaCl-equiv.) aqueous fluids at the retrograde skarn stage, followed by lower temperature cyclic releases of high-carbonic, low salinity to non-carbonic moderate-salinity aqueous fluids. At the hydrosilicate stage, a high-carbonic, CH4-dominated, hot (350–380°C) low salinity fluid was followed by cooler (300–350°C) non-carbonic moderate-salinity (5.7–14.9wt% NaCl-equiv.) fluid. At the phyllic stage, a high-carbonic, CO2-dominated, moderately-hot (330–355°C, 0.9kbar) low salinity fluid was followed by cooler (230–265°C) non-carbonic moderate-salinity (6.6–12.0wt% NaCl-equiv.) fluid. A homogenized magmatic source of water (δ18OH2O=+8.3 to +8.7‰), and a sedimentary source of sulfur (δ34S=−6.9 to −6.2‰) and carbon (δ13Cfluid=−20.1 to −14.9‰) at the hydrosilicate stage are suggested. A magmatic source of water (δ18O=+8.6 to +9.2‰) and a sedimentary source of sulfur (δ34S=−9.3 to −4.1‰) but a magmatic (mantle- to crustal-derived) source of carbon (δ13Cfluid=−6.9 to −5.2‰) are envisaged for fluids that formed the early mineral assemblage of the phyllic stage. Then, the role of sedimentary carbon again increased toward the intermediate (δ13Cfluid=−16.4 to −14.5‰) and late (δ13Cfluid=−16.3 to −14.7‰) phyllic mineral assemblages. The magmatic differentiation was responsible for the fluid enrichment in W, whereas Au and Bi could also have been sourced from mafic magma. The decreasing temperatures, together with elevated Ca content in non-boiling fluids, promoted scheelite deposition at the early hydrothermal stages. The most intense scheelite deposition at the phyllic stage was caused by CO2 removal due to boiling of CO2-rich fluids; further cooling of non-boiling fluids favoured joint deposition of scheelite, Bi and Au.