Sb-Bi Alloys and Ag-Cu-Pb-Sb-Bi Sulphosalts in the Jialong Cu-Sn Deposit in North Guangxi, South China

Abstract

Although native bismuth is a relatively common mineral, native antimony is less abundant, and Sb-Bi alloys are relatively rare phases in Nature. Sb-Bi alloys and Ag-Cu-Pb-Sb-Bi sulphosalts have been discovered in the Jialong vein-type Cu-Sn deposit in North Guangxi, South China. The Jialong deposit is hosted by schist within the contact zone of a Neoproterozoic granite. Four stages of ore formation are recognised, with the Sb-Bi alloy- and sulphosalt-bearing assemblage formed during the third stage. Sulphosalts include Pb-Bi-Ag sulphosalts (pavonite), Sb-Bi sulphosalts (tintinaite, terrywallaceite), and Sb sulphosalt (ullmanite, freibergite, bournonite). Grains of Sb-Bi alloy measure 2–20 μm in diameter, show rounded margins and occur together with galena along the edges or internal fissures of sulphosalts. The Sb-Bi alloys do not coexist with bismuthinite, BiS (an unnamed mineral), or with native bismuth. Two phases of Sb-Bi alloys are identified based on back-scattered electron image observations and electron microprobe analysis. The textural and thermodynamic relationships indicate that Phase I was formed before Phase II. Phase I contains high Sb (69.15–80.12 wt %) and lower Bi (18.01–27.85 wt %), while Phase II contains low Sb (0.89–25.24 wt %) and high Bi (72.95–98.89 wt %). Cooling in the range of 270–400 °C and decreasing sulphur fugacity promote precipitation of Sb-Bi alloys and sulphosalts during the late stage of incursion of Sb- and Bi-bearing magmatic hydrothermal fluids.