Micro-textures, in-situ trace elemental and sulfur isotopic analyses for pyrite and pyrrhotite from the Xiasai Ag-Pb-Zn-Sn deposit, central Yidun Terrane (SW China): Implication for ore formation

Xiasai is the largest magmatic-hydrothermal Ag-Pb-Zn-Sn deposit in the central Yidun Terrane (SW China). Two generations of pyrite are associated with Ag-Pb-Zn mineralization. The coarse-grained pyrite (PyI) occurs together with arsenopyrite and pyrrhotite, and the fine-grained pyrite (PyII) is associated with sphalerite and chalcopyrite. This study documents LA-ICPMS mapping, trace element and LA-MC-ICPMS sulfur isotopic analyses of the two types of pyrite and pyrrhotite, to further constrain the formation of Ag-Pb-Zn veins. Micro-textures reveal that pyrite and pyrrhotite have a homogeneous distribution of trace elements at the individual grain scale, however, some micro-inclusions containing trace elements, including Co, Ni, Cu, Pb, Zn, Ag, Bi, and As, are identified. Pyrite and pyrrhotite are enriched in Co, Ni, Cu, Pb, Zn, Ag, Bi, As, Se, and Sb. However, PyII has higher Cu, Pb, Zn, Ag, and Bi, but lower Co, Ni, As, Sb, and Te concentrations than PyI. The variations of Co and Ni concentrations, as well as the Co/Ni ratios in pyrite and pyrrhotite demonstrate a link with the distance to orerelated granite. PyI has in-situ delta 34S values of -6.8 to -3.4 %o (average of -5.9 %o), and PyII exhibits values ranging from -6.7 to -5.4 %o (average of -6.0 %o). Pyrrhotite has delta 34S values ranging from -6.9 to -5.5 %o. The in-situ sulfur isotopic compositions demonstrate a magmatic source for ore-forming materials, which were derived from the Early Cretaceous monzogranite. Trace elements and sulfur isotopic compositions of pyrite and pyrrhotite indicate that the formation of the Ag-Pb-Zn veins at Xiasai is due to a decrease of temperature, salinities, fS2 and fO2 of the hydrothermal fluids.