Mineral chemistry of the Xiasai Ag–Pb–Zn deposit in the central Yidun Terrane, SW China: Insight into Ni–Ag–Bi mineralization and formation conditions

Abstract

Sulfides and sulfosalts are major ore minerals for most of Ag–Pb–Zn deposits and can be used to study enrichment and distribution of ore-forming elements and formation conditions. The Xiasai Ag–Pb–Zn deposit (Zn 0.141 Mt @ 4.30%, Pb 0.132 Mt @ 4.03%, Ag 1028 t @ 337.8 g/t, Cu 10058 t @ 0.56%, and Sn 20000 t @ 0.75%) is a hydrothermal vein-type deposit in the central Yindun Terrane, SW China. It consists of proximal quartz-cassiterite veins and distal Ag–Pb–Zn sulfide veins to a Cretaceous monzogranite. Three successive mineralization sub-stages are recognized for Ag–Pb–Zn veins, i.e., arsenopyrite + coarse-grained pyrite (Py1) + pyrrhotite, sphalerite + chalcopyrite + fine-grained pyrite (Py2), and silver-minerals + native bismuth + galena.Nickel-minerals, such as breithauptite and ullmannite were identified as inclusions hosted within pyrrhotite using Energy Dispersive X-ray Spectroscopy (EDS) and Electron probe microscopy (EPM) analysis and mapping. Trace amounts of invisible silver have been detected in chalcopyrite and galena, whereas a significant amount of silver is incorporated in pyrargyrite, freibergite, miargyrite, and electrum as intimate intergrowths with galena or inclusions within galena. A high content of invisible bismuth is detected within galena (0.21–16.95 wt%), as well as numerous grains of native bismuth occur as inclusions in galena. Based on the Bi concentrations determined and the modal composition a large-sized bismuth deposit would be estimated with about 23,800 t of Bi resource. The presence of nickel-minerals and native bismuth, together with Zn/Cd ratios (58–151) of sphalerite, indicate that the ore-forming materials are derived from the Xiasai monzogranite.Based on mineral assemblages and chemical features for sulfides and sulfosalts, the formation of Ag–Pb–Zn ores was triggered by a decrease in temperature, fs2, fO2, and sulfidation. The early episode of arsenopyrite, Py1, pyrrhotite, and nickel-minerals formed at temperature of 320–470°C, lower fs2 and decreasing sulfidation conditions from low alkaline fluids. In contrast, the late episode of silver-minerals, native bismuth, and galena deposited at 160–270 °C with a lower fs2 state and neutral or acidic pH. However, a slight increase of fs2, fO2, and sulfidation (intermediate sulfidation) occurred during precipitation of Zn–Cu.