Country-rock contamination of magmas associated with the Baogutu porphyry Cu deposit, Xinjiang, China

Abstract

The Baogutu porphyry Cu deposit, located in the West Junggar Region, Xinjiang, China, has the reduced mineralization features. Baogutu is associated with a Late Carboniferous intrusive complex that was emplaced into Lower Carboniferous volcano-sedimentary sequences. The complex consists of a main-stage diorite stock and late-stage diorite porphyry dikes. The former hosts the most of the Cu mineralization. The main-stage stock has a wide range of composition, from minor gabbro through dominant diorite to tonalite porphyry. These are all inhomogeneous over a few meters, with no distinct boundary between the gabbro, diorite and tonalite porphyry. Some felsic microgranular enclaves and heterogeneous phenocrysts occur in these rocks: the latter is a single mineral with a range of compositions. The elemental and Sr–Nd compositions of the tonalite porphyry have a transitional character from the gabbro and diorite to the country rocks. These findings suggest possible country-rock contamination. Mineral composition data suggest that the primary magma at Baogutu is oxidized I-type magma. The gabbro and diorite have high positive εNd(t) (+3.3 to +6.0) and low initial 87Sr/87Sr ratios (0.7035–0.7038), and are characterized by moderately fractionated rare earth element (REE) patterns, pronounced Nb depletion and weak negative Eu anomalies. These observations suggest that the Baogutu magmas were derived from partial melting of a metasomatized mantle wedge and underwent significant country-rock contamination after emplacement. The contamination process involved the felsic components and reduced materials (e.g. Carbonaceous sediment), which caused the chemical variation of the Baogutu rocks and the reduced mineralization features in the Baogutu deposit. Thus, contamination played an important role in the formation of the host rocks and associated mineralization at Baogutu.