Magmatic-hydrothermal evolution at the Lyangar redox-intermediate tungsten-molybdenum skarn deposit, western Uzbekistan, Tien Shan: Insights from igneous petrology, hydrothermal alteration, and fluid inclusion study

Abstract

The Lyangar W-Mo skarn deposit is situated in a metallogenic belt of W-Mo, Cu-Mo, W-Au, and Au deposits and is related to an Early Permian multiphase (monzodiorite)-granodiorite-granite-leucogranite pluton that was emplaced in a tectonic domain with possible ancient continental crust. The latter, coupled with an inferred ascent of hot asthenospheric material, has defined an enrichment in Mo and fluorine, together with W and Au, to form a distinct metallogenic assemblage in a post-collisional environment. The parental moderately- to high-K, transitional metaluminous to peraluminous, weakly oxidized-weakly reduced, ilmenite-titanite-series granitoids are accompanied by hydrothermal alteration and mineralization that formed in redox-intermediate environment.The deposit incorporates prograde and retrograde pyroxene and garnet skarns composed of hedenbergitic pyroxene and grossular- to almandine+spessartine-rich garnet, respectively. Fluorite is present in the retrograde skarn together with scheelite and sulfides, the latter including most abundant pyrrhotite and molybdenite. Hydrosilicate (amphibole-epidote-chlorite-quartz, locally with albite, sulfides, and scheelite) assemblage has limited development. Phyllic (quartz-sericite-Fe-carbonate, locally with albite, fluorite, scheelite, and sulfides, also Bi and Au minerals) alteration assemblage partially overprints altered skarns and extends over much larger areas. Overall, the deposit represents a superposition of the early W-Mo mineralization related to retrograde skarn and hydrosilicate alteration, and the late Au-W-Cu-Bi mineralization in phyllic alteration assemblages.Prograde and retrograde skarns were formed from dominantly magnesian-sodic-chloride, moderately-saline (11.7–13.9 wt% to 8.6–10.5 wt% NaCl-eq.), high pressure (1500 to 1350 bars), hot aqueous fluids that were followed by moderately-saline (12.3–14.5 wt% NaCl-eq.), Ca-rich, lower pressure (600 bars) fluids during deposition of most scheelite. Hydrosilicate (propylitic) assemblages were formed from boiling fluids at the temperatures of 380–430 °C and pressures of 600±300 bars, whereas phyllic alteration assemblages - from boiling carbonic-aqueous, low-salinity fluids at the temperatures of 300–320 °C and pressures of ~1000 bars. δ18OH2O values range from +7.8 to +8.6‰ for retrograde skarn, and from +6.6 to +7.0‰ for phyllic alteration, whereas δ13CCO2- from −3.3 to −0.5‰ for retrograde skarn, and from −5.5 to −4.5‰ for phyllic alteration. δ34S increases from retrograde skarn (δ34S = +2.0 to +2.3‰) to phyllic alteration (δ34S = +4.3 to +4.6‰) stages.