Geology, igneous geochemistry, mineralization, and fluid inclusion characteristics of the late Palaeozoic North Katpar W-Mo(-Cu-Bi) skarn deposit, central Kazakhstan

The North (Northern) Katpar W-Mo skarn deposit in the central Kazakhstan contains resources of 90 Kt WO3 in altered skarn (averaging 0.23% WO3 and 0.04% Mo). It is associated with a cluster of magmatic intrusions comprising early Carboniferous monzonite-quartz monzonite-granodiorite, late Carboniferous granite (to leucogranite ?), and late Carboniferous-early Permian leucogranite, with their overall evolution from subduction-related to post-collisional intrusions. In contrast to most W-Mo deposits in central Kazakhstan, which correspond to greisen and/or stockwork types, the North Katpar deposit comprises large skarn zones. It has signatures of an intermediate-redox W-Mo skarn deposit, although a greater content of garnet (vs. pyroxene) in prograde and early retrograde skarns, together with andradite-rich composition of garnets and high Mo content in scheelite, indicate a higher fO2. Late retrograde skarn comprises a paragenesis of wollastonite-quartz-fluorite (with Mn-Fe pyroxene and almandine-spessartine-rich garnet), and the subsequent phyllic (carbonate-phyllic) alteration assemblages also comprise fluorite, together with W-Mo and Cu-Bi-Pb-Zn-sulphide mineralization. Two alternative genetic models are considered, namely (i) that involving a gradual (single-series) magmatic-hydrothermal evolution corresponding to the respective continuing (direct) evolution from magnetite-series to transitional magnetite-ilmenite-series intrusions, or (ii) that suggesting a hydrothermal fluid supply from unrelated magmatic intrusions. The fluid inclusion data indicate the involvement of an aqueous, moderately-saline (15–19 wt.% NaCl-eq.), Mg-Na-chloride and then higher-salinity (~26 wt.% NaCl-eq.) Ca-Na-chloride, high-temperature single-phase fluids, possibly exsolved from a cooling magmatic intrusion, during the prograde and then early retrograde skarn formation. No boiling occurred, probably due to the significant (>5 km) depth of magmatic crystallization and related hydrothermal mineral deposition. Then, a distinct geochemical shift occurred, possibly corresponding to the intrusion of leucogranite, with high-F, low-salinity (<5 wt.% NaCl-eq.), dominantly aqueous-fluorine fluids forming the late retrograde skarn and subsequent phyllic (carbonate-phyllic) alteration assemblages. Further evolution towards aqueous-carbonic fluids could be associated with the intrusion of the late mafic dikes.