Geology, mineralization, and fluid inclusion characteristics of the Meliksu reduced tungsten skarn deposit, Alai Tien Shan, Kyrgyzstan: Insights into conditions of formation and regional links to gold mineralization

Abstract

The Meliksu W skarn deposit (∼10Kt WO3, with average grade of 6.6% WO3) in the Alai Segment of the Southern Tien Shan is related to an Early Permian granitoid pluton that comprises ilmenite-series to ilmenite-titanite-bearing, medium-K, metaluminous to peraluminous I-type granitoid (monzonite/diorite-tonalite-granodiorite to granite) rocks. There is a regional-scale continuum from these intrusions to transitional weakly reduced to weakly oxidized metaluminous I-type granitoid suites accompanied by reduced intrusion-related Au deposits. Magmatic evolution also included coeval high-K to shoshonitic suites, and younger Early Permian, post-collisional to within-plate, alkaline (from monzogabbro to nepheline syenite) and carbonatite intrusions, indicative of an extensional tectonic regime. Consistent with this evolution toward an extensional environment, reduced W skarns and associated reduced intrusion-related Au mineralization in the region may evolve toward Sb-Hg (+As, Au, W) mineralization exhibiting the Carlin-type affinity.The deposit bears signatures of reduced W skarn deposits, with dominant pyroxene (hedenbergite-rich) prograde and retrograde skarns. Scheelite was mostly deposited during the propylitic (amphibole-chlorite-calcite-quartz, with abundant pyrrhotite and minor chalcopyrite) and phyllic (quartz-sericite-albite-Fe-carbonate, with sulfides, Bi, Ag and Au minerals) alteration stages. Silver mineralization is particularly abundant, that, together with very high W grades, low Au grades, intense calcic-sodic and sodic alteration, and coarse-grained sericite (muscovite)-quartz aggregate, possibly reveal a root-level of reduced W skarn deposits. Fluid inclusions data indicate formation of retrograde (mainly pyroxene-quartz-amphibole) skarn from aqueous, dominantly magnesian-sodic-chloride, moderately-saline (16–18 wt% NaCl-eq.), high pressure (1.9 ± 0.7 kbar), moderately-hot (∼400–500 °C) fluid. Propylitic alteration assemblages were formed from slightly less saline (11–13 wt% NaCl-eq.), high-pressure (1.8 ± 0.5 kbars), lower temperature (∼370–450 °C), Ca-enriched fluid. Phyllic alteration assemblages were formed from high-pressure (2.2 ± 0.1 kbars) boiling, methane-rich carbonic-aqueous, cooling (from 350 to 335 °C to 275–265 °C) fluid. High fluid pressures at different hydrothermal stages are consistent with a deep level of mineralization at a lithostatic depth of ∼9.0 km. A strong enrichment in heavy sulfur isotope (δ34Sfluid varies from +6.5 ± 0.4‰ to +8.0 ± 0.3‰) can reflect sulfur sourcing from sedimentary rocks enriched in seawater sulfate, with sulfur isotope homogenization in a magmatic chamber.