Geology, mineralization, and fluid inclusion study of the Kuru-Tegerek Au-Cu-Mo skarn deposit in the Middle Tien Shan, Kyrgyzstan

Abstract

The Kuru-Tegerek Cu-Au-Mo deposit is situated in a system of Late Carboniferous subduction-related magmatic arcs of the Middle Tien Shan, which together constitute a metallogenic belt of Cu-Au-Mo (±W) porphyry, with local skarns, deposits. The deposit is related to magnetite-series gabbro-diorite to tonalite intrusion. It contains prograde magnesian and calcic skarns with abundant magnetite, associated with gabbro-diorite, and retrograde skarn with Cu mineralization, formed after intrusion of tonalite. Subsequent propylitic alteration introduced abundant chalcopyrite and pyrrhotite, and native Au culminating in zones overprinting magnetite and garnet skarn. Later quartz-muscovite-carbonate veins, formed after intrusion of late mafic quartz monzogabbro dikes, contain chalcopyrite, pyrite, arsenopyrite and other sulfides and sulfosalts, tellurides, and native Au. The earliest retrograde skarn garnet contains gaseous low-salinity (1.7–3.4 wt.% NaCl eq.) fluid inclusions homogenizing at 460–500 °C into vapor, indicating that the early fluid released from crystallizing magma was a low-density vapor. It was followed by more saline (4.0–5.0 wt.% NaCl eq.), high-temperature (400–440 °C) aqueous fluid, as fluid release from the magma progressed. Boiling of this fluid at temperatures of 420 to 370 °C and a pressure of ~350–300 bar produced a low-salinity (0.6–1.2 wt.% NaCl eq.), essentially gaseous, and high-salinity (from ~39 to ~31 wt.% NaCl eq.) brine, with possible metal (including Cu) partitioning into both gaseous and aqueous-saline phases. Boiling was coeval with sulfide deposition in the retrograde skarn. The latest episode of the retrograde skarn stage included direct separation of saline (~40–42 wt.% NaCl eq.) fluid from crystallizing magma. The separation of saline (~40 to ~14 wt.% NaCl eq.) fluids from a crystallizing magmatic melt continued during the propylitic stage, when fluid cooling from ~370 to 320 °C, together with decreasing fO2, caused Cu and especially Au precipitation. A new influx of possibly magma-derived, low-salinity (4.5–6.7 wt.% NaCl eq.) aqueous, and then NaCl-CO2-H2O fluids, corresponds to the phyllic (quartz-muscovite-carbonate-sulfide) stage. These fluids may have a deeper source, associated with the late mafic quartz monzogabbro dikes. Fluid cooling (from ~340 to 255 °C) and boiling of the NaCl-CO2-H2O fluid, together with increased fS2, increased the Au endowment.