Hydrothermal evolution and ore precipitation of the No. 2 porphyry Cu–Au deposit in the Xiongcun district, Tibet: Evidence from cathodoluminescence, fluid inclusions, and isotopes

Abstract

The No. 2 porphyry Cu–Au deposit in the Xiongcun district forms part of the Gangdese porphyry copper belt of Tibet. This study clarifies the fluid evolution and ore precipitation mechanisms of the magmatic-hydrothermal system that formed the deposit by combining the results of fluid inclusion petrography, microthermometry, scanning electron microscope-cathodoluminescence imaging, and stable isotope analyses of multiple quartz-dominated veins. Four distinct sets of quartz-dominated veins and eight generations of quartz (Q1–Q8) within the veins were identified, namely from early to late, early barren quartz veins (V1) comprised of equigranular CL-bright Q1 quartz, quartz–pyrite–chalcopyrite ± magnetite veins (V2) containing early equigranular CL-bright Q1 quartz and later CL-gray or CL-dark Q2–3 (Q2 + Q3) quartz grains intergrown with Cu–Fe sulfides, quartz–molybdenite ± pyrite ± chalcopyrite veins (V3) comprised of quartz grains (Q4–Q6) characterized by subhedral comb-like textures with well-developed oscillatory growth zoning, and late quartz veins (V4) containing CL-gray Q7 quartz with euhedral growth zones and later CL-dark Q8 quartz. Fluid inclusions in these veins can be classified into six types: B40, B40H, B80, B15H, B15H+, and B15. The B40 and B40H inclusions; B80 inclusions; and B15, B15H, and B15H+ inclusions contain vapor bubbles occupying 30–60 vol%, >70 vol%, and 5–25 vol%, respectively; in addition, B15H, B15H+, and B40H inclusions contain transparent daughter minerals (halite, and in some cases, sylvite). Raman spectra showed that CO2 is only present in B40 and B80 inclusions. In the Q1 quartz in V1 and V2 veins, the dominant type of inclusions is B40, which have homogenization temperatures (Th) of 278 °C–389 °C (peaking at 310 °C–330 °C) and salinities of 2.0–12.8 wt% NaCl equiv. In Q2–3 quartz in V2 veins, all inclusion types were observed. These were trapped at temperatures of approximately 325 °C–340 °C and had salinities of 1.8–69.7 wt% NaCl equiv. In V3 veins, only B15H and B15 inclusions were observed, which had Th values of 171 °C–320 °C (peaking at 240 °C–260 °C) and 173 °C–317 °C (peaking at 210 °C–230 °C) and salinities of 28.7–33.9 wt% NaCl equiv and 5.6–23.1 wt% NaCl equiv, respectively. In V4 veins, only B15 inclusions were observed. Primary B15 inclusions in V4 veins yielded Th values of 166 °C–229 °C (peaking at 170 °C–200 °C) and salinities of 5.1–17.1 wt% NaCl equiv. The 3He/4He and 40Ar/36Ar ratios of the fluid inclusions exhibited the ranges of 0.08–0.84 Ra and 451.3–1567.8, respectively, and the δ18Ofluid and δDfluid values varied from −3.2‰ to 5.8‰ and −92.8‰ to −80.3‰, respectively. By integrating all results from the fluid inclusion, cathodoluminescence, and isotopic analyses, we conclude that the initial ore-forming fluids of the No. 2 deposit were low-salinity, CO2-rich single-phase fluids of magmatic origin. Subsequently, fluid immiscibility developed in the initial ore-forming fluids, generating hypersaline liquid and low-salinity vapor phases and leading to the separation of a CO2 phase plus chalcopyrite precipitation from the fluids. As meteoric water was injected into the hydrothermal system, the ore-forming fluids gradually evolved to become meteoric water-dominated, low temperature, low-salinity, and CO2-poor; in addition, fluid-cooling due to the meteoric water input resulted in molybdenite precipitation.