The Hongshan deposit is one of the largest Cu-polymetallic deposits in the Zhongdian area, southwest China. Two types of Cu–Mo ores, mainly developed in the skarns, have been recognized in the Hongshan deposit, i.e., massive or layered skarn and vein-type, with the former being dominant. The highly andraditic composition of garnet (Adr100
) and diopsidic composition of pyroxene (Di90
) indicate the layered skarn ores are of magmatic-hydrothermal origin that formed under oxidized conditions. Sm–Nd dating of garnet yield a well-constrained isochron age of 76.48 ± 7.29 Ma (MSWD = 1.2) for the layered skarn ores. This age was consistent with the Re–Os age for the pyrrhotite from the layered skarn ores, and thereby indicated that the layered skarn mineralization was formed in the Late Cretaceous, rather than in the Triassic as was previously thought. The coincidence of the geochronology from the layered skarn ores and vein-type mineralization further indicated that both ores were the result of a single genetic event, rather than multiple events. The recognition of the Late Cretaceous post-collisional porphyry–skarn Cu–Mo–W belt in the Zhongdian area exhibited a promising prospecting potential.
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