Baimazhai, Yunnan Province, China: A Hydrothermally Modified Magmatic Nickel-Copper-PGE Sulfide Deposit

Abstract

The Baimazhai Ni-Cu-PGE (platinum-group-elements) sulfide deposit, southeast Yunnan Province, is hosted in mafic-ultramafic intrusions of the Permian Emeishan large igneous province, with which it is temporally and genetically related. The typical orthomagmatic sulfide ores of the Baimazhai deposit locally exhibit peculiar textural features, and are intimately associated with hydrothermal minerals such as biotite, amphibole, and chlorite. This suggests that magmatic sulfide ores were subjected to hydrothermal alteration and subsequent redistribution, resulting in enrichment in Cu, Pd, and Au. Whole-rock 40Ar-39Ar age data yielded plateau ages of about 160-170 Ma, at odds with the established Permian age of the host intrusions and the Emeishan large igneous province. We interpret these younger ages as due to thermal resetting during post-Permian tectonothermal events such as the Jurassic Cretaceous Yanshanian orogeny, and propose a model in which tectonic movements and hydrothermal fluids modified the pre-existing magmatic sulfides. Given the high degree of overprinting, we suggest two possible scenarios: (1) sulfide disseminations that surround the massive magmatic ores are the result of deformation and hydrothermal alteration; and (2) both magmatic massive and disseminated sulfides were produced initially, in which case the scale and metasomatic remobilization would have been smaller, but still detectable.