Post-collisional Sb and Au mineralization related to the South Tibetan detachment system, Himalayan orogen

Abstract

More than 50 Sb–Au deposits and occurrences have recently been found in an E–W-striking structural–thermal dome zone, related to the South Tibetan detachment system (STDS) in the Himalayan orogen. They are mainly distributed around the thermal domes intruded by mid-Miocene leucogranite bodies, and show a metallic zoning varying from Au, Sb–Au, to Sb mineralization from the domes outwards. At least three mineralization styles are recognized, i.e., Sb-, Sb–Au-, and Au-styles of deposits, all hosted in the Mesozoic, Tethyan passive continental-margin sequences. The Shalagang-style Sb deposits, controlled by N–S-striking normal faults and E–W-striking bedding faults, are usually composed of quartz–stibnite veins with open-space filling structure. Low homogenization temperatures (135–367 °C) and low salinities (0.5–12.6 wt.% NaCl equiv.), as well as highly variable calculated δ 18O H2O values (− 11.5–12.3‰) and very low δD values (− 140 to − 166‰), suggest an ore-forming fluid dominated by geothermal or meteoric water. The Mazhala-style Sb–Au deposits were controlled by small-scale bedding faults and their intersections with N–S-striking normal faults. They consist of gold-bearing quartz–stibnite veins with minor disseminated ores, and are associated with silicification, sericitization, chloritization and carbonatization. CO 2-rich fluid inclusions in Au-bearing gangue quartz, calculated δ 18O H2O values (5.4–11.5‰) and measured δD values (− 72 to − 119‰) for precipitation of gangue quartz suggest a mixed ore-forming fluid containing both meteoric and magmatic waters. Langkazi-style Au deposits, controlled by detachment faults and normal faults along the margin of the thermal dome, consist of Au-bearing quartz veins and disseminated ores with intensive silicification, chloritization and sericitization. Gangue quartz from Au-bearing quartz veins yielded a limited range of δ 18O H2O values (1.8–8.2‰) and δD values (− 52 to − 83‰), suggesting an ore-forming fluid dominated by magmatic water. The genetic links among these deposits, the granite-intruded thermal domes and ore-controlled fault systems suggest a mid-Miocene metallogenic epoch. The spatial zonation in metallic associations, and the variation in compositions and sources of ore-forming fluids indicate that these Sb and Au deposits resulted from a post-collisional dome-centered geothermal system, driven by mid-Miocene granite magmas related to the STDS. Rb–Sr and S isotopic data indicate that the scavenging of the convective geothermal fluids through Mesozoic permeable strata play an extremely significant role in concentrating metallic Sb and minor Au, and forming Sb, Sb–Au, and Au deposits in southern Tibet.