Genesis of the Maikhura tungsten-tin skarn deposit, Tajik Tien Shan: Insights from petrology, mineralogy, and fluid inclusion study

The Maikhura W-Sn skarn deposit is situated in the Gissar Segment of the Southern Tien Shan. It associates with a Late Carboniferous-Early Permian composite granitoid pluton, which includes earlier ilmenite-series, transitional metaluminous to peraluminous, I-type granodiorite, and later magnetite-series, biotite to biotite-tourmaline granite/leucogranite. The oxidized granitic intrusions exhibit A-type granite affinity and define a geochemically and genetically distinct group of W-Sn deposits. The deposit incorporates both pyroxene- and garnet-dominant prograde and retrograde skarns with scheelite and locally magnetite, overprinted by highly reduced (pyrrhotite-stable) post-skarn mineral assemblages of hydrosilicate and phyllic alteration stages, containing scheelite, cassiterite and sulfides. The fluid inclusions data indicate the involvement of carbonic-free aqueous, moderately-saline (9.9–16 wt% NaCl-eq.), high-pressure (2.1 ± 0.5 kbar), hot fluid, which was sourced from crystallizing magma and formed prograde calcic skarn. This fluid evolved into a sodic-chloride, slightly less saline (8.6–11 wt% NaCl-eq.), lower-pressure (1.7 ± 0.5 kbar), cooler fluids toward the retrograde skarn stage; this stage was characterized by deposition of scheelite in association with magnetite. The hydrosilicate (propylitic) alteration was formed from a low salinity, Ca-enriched, homogenous aqueous-carbonic (methane-rich) fluid at lower temperatures (∼350–400 °C), with precipitation of scheelite and cassiterite, particularly in phlogopite-chlorite-oligoclase-quartz assemblage. Corresponding pH decrease explains the trend toward overlapping fields of insoluble cassiterite and scheelite. The phyllic alteration assemblages were formed from a boiling aqueous-carbonic (CO2-dominated), low-salinity fluid at temperatures of ∼310–250 °C and pressure of 1.85 ± 0.1 kbar. The alteration evolving from chlorite- through muscovite- to albite-dominant paragenesis indicates a trend toward higher pH (less acidic) conditions and was accompanied by precipitation of scheelite and cassiterite, with further deposition of sulfides. A homogeneity of δ34S values suggests sulfur isotope homogenization in a magmatic chamber, whereas their proximity to the meteorite standard supports a magmatic source of sulfur. The depletion in heavy sulfur isotope of sulfides from hydrosilicate (propylitic) alteration (δ34S = +1.8 to +0.7‰) to sulfides from phyllic alteration (δ34S = −0.1 to −0.8‰) is consistent with the evolution from ilmenite-series to magnetite-series magmatic source.