Geochemistry and geochronology of native copper mineralization related to the Emeishan flood basalts, Yunnan Province, China

摘要

Native copper mineralization in northeastern Yunnan is related to the Emeishan flood basalts, which have high Cu background concentrations. The deposits are located in the Ludian, Maoling and Yiche synclines between the flood basalts and the overlying Xuanwei Formation. Evidence from REE and HSFE concentrations, oxygen isotope compositions, bitumen reflectance, and mineral paragenesis indicates that the high-grade copper orebodies were deposited from moderate to low temperature hydrothermal solutions (420 to 100 deg C) in a reducing environment. ~(40)Ar/~(39) Ar spectra for the laumontites that coexist with native copper in the high-grade ores yield consistent plateau and isochron ages (226 to 228 Ma). These are inteipreted to represent the age of the first stage of hydrothermal mineralization, which took place at temperatures of 200 to350 deg C, concordant with that of Zn-Pb-Ag-Ge mineralization also occurring in the area. The heulandites yield a stable ~(40)Ar/~(39) Ar plateau age of 134.0 + - 1.7Ma, consistent with isochron ages indicated by U-Th-Pb isotopic systematics, indicating a second stage of native copper mineralization at lower temperatures (100 to 200 deg C) that took place during the Early Cretaceous.