Pyrite trace element geochemistry of mafic granulite xenoliths from Xikeer: implications for the source of Cu in the sediment-hosted mineralization in the northwestern Tarim Basin (Northwest China)

The Xikeer mafic granulite xenoliths are hosted by a Cenozoic basanite sill (∼20 Ma) in the northwest Tarim Basin, northwest China. Sulfides, identified in these xenoliths consist mainly of pyrite. Two groups of pyrite (types A and B) can be distinguished based on petrography and trace element geochemistry, as determined by laser-ablation ICP-MS analysis. Type A pyrite is subhedral and in general has lower trace element abundance than type B pyrite. Type B pyrite is fractured with ragged anhedral morphology and has extremely high Cu, Tl, Ni, and Co abundance. The low Co and Ni contents combined with a positive correlation between Cu, Cr, and Pb all indicate that the type A pyrite may have formed from a high-temperature magmatic hydrothermal fluid. In contrast, the Cu-, Tl-, Ni-, and Co-rich type B pyrite was probably formed in a relatively low-temperature basinal environment. The Cu mineralization at Xikeer can be explained when Cu, Tl, Ni, and Co were leached from the intruded basanites and subsequently enriched in circulating basin brines. Such enrichment of Cu in basin brines could provide an important Cu source for the Cu mineralization in the Xikeer district and other Cenozoic sub-basins in the Tarim Basin.