Genesis of the Jigongcun Re-rich quartz vein-type Mo deposit, southern Tibet: Constraints from mineralogy, fluid inclusions, geochronology, H–O–S isotopes, and in situ trace element compositions of molybdenite

Abstract

The Jigongcun Re-rich quartz vein-type Mo deposit is located in the Gangdese porphyry Cu belt, southern Tibet, China. The stratiform and lenticular orebodies are hosted within the quartz veins and are controlled by the NW-/EW-trending faults. In this study, we have identified four hydrothermal stages, namely the (1) pre-ore stage (stage Ⅰ) characterized by the development of thick Mo-free quartz veins; (2) stage Ⅱ, mainly forming thick quartz veins with Re-rich massive molybdenite (Mo1-type; mean = 1811 ppm) and bladed molybdenite (Mo2-type; mean = 1312 ppm); (3) stage III, mainly forming quartz veins with Re-barren bladed molybdenite (Mo3-type) and abundant galena and pyrite; (4) the post-ore stage (stage Ⅳ) characterized by formation of thick ore-barren quartz–calcite veins. Both electron probe micro-analysis and in situ laser ablation–inductively coupled plasma–mass spectrometry analysis of different types of molybdenite show that high Re concentrations are due to isovalent Re substitution for Mo in molybdenite.Fluid evolution during the four stages of mineralization was constrained by a detailed fluid inclusion study. Fluid inclusions during the formation of the early pre-ore stage indicate relatively high-temperature (311–337 °C), high-pressure (1300–1600 bars), low-salinity (0.2–2.6 wt% NaCl-equiv.), and CO2-rich hydrothermal fluids. Fluid immiscibility assemblages in CO2-rich inclusions coexisting with aqueous inclusions occur in quartz–K-feldspar veins of stage Ⅱ, and likely record phase separation at a temperature of 262–300 °C and pressure of 900–1100 bars, with the precipitation of Re-rich molybdenite. The subsequent fluid mixing occurs at a temperature of 220–271 °C, with the precipitation of Re-barren molybdenite at stage III. H–O–S isotope compositions show that sulfur and ore-forming fluids are derived from a deep-seated igneous source. Re-Os dating of molybdenite samples yielded an isochron age of 22.0 ± 0.4 Ma, indicating that the Jigongcun deposit formed in the Early Miocene, which is earlier than most of the porphyry Cu deposits in the Gangdese porphyry Cu belt. Combined our new geochemical and isotopic results with previous regional investigations, we suggest that the Re-rich Mo mineralization of Jigongcun deposit may be related to the Early Miocene northward subduction of the Indian continental plate.