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

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, fs(2), fO(2), and sulfidation. The early episode of arsenopyrite, Py1, pyrrhotite, and nickel-minerals formed at temperature of 320-470 degrees C, lower fs(2) and decreasing sulfidation conditions from low alkaline fluids. In contrast, the late episode of silver-minerals, native bismuth, and galena deposited at 160-270 degrees C with a lower fs 2 state and neutral or acidic pH. However, a slight increase of fs(2), fO(2), and sulfidation (intermediate sulfidation) occurred during precipitation of Zn-Cu.