The superlarge Tyrnyauz skarn W-Mo and stockwork Mo(-W) to Au(-Mo, W, Bi, Te) deposit in the Northern Caucasus, Russia: Geology, geochemistry, mineralization, and fluid inclusion characteristics

Abstract

The Tyrnyauz deposit (>850 Kt WO3, >220 Kt Mo, ~60 t Au, with additional ~140 t Au at a satellite deposit) is the largest W skarn deposit in Russia and the FSU. It comprises large mineralized (molydoscheelite) skarn zones of the intermediate-redox type, followed by overprinting and much wider systems of post-skarn quartz-garnet-pyroxene, quartz-garnet, quartz-wollastonite, quartz-feldspar and quartz veins with molybdenite and minor scheelite. Younger quartz-molybdenite (±scheelite) stockworks in zones of propylitic (quartz-amphibole-chlorite) alteration are typically more distal, and so are the complex W-Au-Bi-As-Te-Sb quartz-sulfide stockworks occurring in zones of phyllic (carbonate-phyllic) alteration. The mineralization appears to be associated with at least two separate igneous suites, possibly a Late Paleozoic low-K “trondhjemite” plutonic suite (quartz diorite, tonalite-granodiorite, and plagiogranite) followed by the formation of mineralized skarns, and a Neogene moderate- to high-K, plutonic to subvolcanic suite (leucocratic granite, biotite granite, rhyolite to rhyolite-dacite, vitroandesite, to trachyandesite-trachybasalt). The Neogene intrusions exhibit a reverse (toward less silicic rocks) overall differentiation trend, cut skarns, and alternate with the post-skarn hydrothermal stages.Prograde calcic skarn was formed from a carbonic-free aqueous, low-salinity (~3 wt% NaCleq.), high-pressure (~1.3 kbar), hot fluid, which could be sourced from crystallizing magma. As the magma degassing progressed, a Na-K-Ca-chloride, high salinity (~50 wt% NaCleq.), but still high-pressure (~1.4 kbar) fluid formed mineralized (with molybdoscheelite) retrograde skarn. After the Neogene leucocratic granite emplacement, the molybdenite (±scheelite) stockworks were formed from a boiling aqueous-carbonic fluid at decreasing (from 420 to 390 °C to 370–350 °C) temperatures and much lower (~0.5 kbar) pressures. A short (?) episode of the late quartz-pyroxene-fluorite-calcite veins with scheelite occurred after the rhyolite emplacement, and a corresponding influx of boiling high-temperature (600–650 °C), Na-K-Ca-chloride, high-salinity (>50 wt% NaCleq.), methane-rich fluid interrupted the generally low- to moderate-salinity carbonic-aqueous fluid supply. A moderate- to low-salinity (22 to 4–9 wt% NaCleq.), aqueous Na-K-chloride fluids formed the late Cu-Zn sulfide to W-Au-Bi-As-Te-Sb mineralization. The superposition of different fertile igneous suites and related mineralization styles may have been the key reason for the giant metal endowment.