Geology, mineralization, and fluid inclusion characteristics of the Lermontovskoe reduced-type tungsten (±Cu, Au, Bi) skarn deposit, Sikhote-Alin, Russia

Abstract

The Lermontovskoe deposit (∼48Kt WO3; average 2.6% WO3, 0.24% Cu, 0.23g/t Au) is situated in a W-Sn-Au metallogenic belt that formed in a collisional tectonic environment. This tungsten skarn deposit has a W-Au-As-Bi-Te-Sb signature that suggests an affinity with reduced intrusion-related Au deposits. The deposit is associated with an intrusion that is part of the ilmenite-series, high-K peraluminous granitoid (granodiorite to granite) suite. These rocks formed via mantle magma-induced melting of crustal sources.The deposit comprises reduced-type, pyroxene-dominated prograde and retrograde skarns followed by hydrosilicate (amphibole-chlorite-pyrrhotite-scheelite-quartz) and phyllic (muscovite/sericite-carbonate-albite-quartz-scheelite-sulfide, with abundant apatite) alteration assemblages. Fluid inclusions from the skarn assemblages indicate high-temperature (>500°C), high-pressure (1400–1500bars) and high-salinity (53–60wt% NaCl-equiv.) magmatic-hydrothermal fluids. They were post-dated by high-carbonic, methane-dominate, low-salinity fluid at the hydrosilicate alteration stage. These fluids boiled at 360–380°C and 1300–1400bars. The subsequent phyllic alteration started again with a high-temperature (>450°C), high-pressure (1000–1100bars) and high-salinity (42–47wt% NaCl-equiv.) fluid, with further incursion of high-carbonic, methane-dominated, low-salinity fluid that boiled at 390–420°C and 1150–1200bars. The latest phyllic alteration included the lower-temperature (340–360°C), lower pressure (370–400bars) high-carbonic, methane-dominated (but with higher CO2 fraction), low-salinity fluid, and then the low-temperature (250–300°C) H2O-CO2-CH4-NaCl fluid, with both fluids boiled at the deposit level. The high-salinity aqueous fluids are interpreted to have come from crystallizing granitoid magma, whereas the reduced high-carbonic fluids probably came from a deeper mafic magma source. Both of these fluids potentially contributed to the W-Au-As-Bi-Te-Sb metal budget. Decreasing temperatures coupled with high aCa2+ and fluid boiling promoted scheelite deposition at all post-skarn hydrothermal stages.The deposit is characterized by limited downdip extent of mineralized zones and abundant coarse-grained muscovite-quartz (+apatite, scheelite) aggregates that formed at the phyllic alteration stage. Together with presence of high-temperature, high-pressure and high-salinity fluids directly exsolving from crystallizing magma, this suggests a root level of the mineralized magmatic-hydrothermal system of reduced W skarn deposits.