The Origin of Ni-Cu-PGE Sulfide Mineralization in the Margin of the Zhubu Mafic-Ultramafic Intrusion in the Emeishan Large Igneous Province, Southwestern China

The Zhubu magmatic Ni-Cu-PGE sulfide deposit is hosted in a mafic-ultramafic intrusion which is part of the Emeishan large igneous province in southwest China. The Zhubu intrusion is composed of a layered sequence (750 X 400 X 600 m) with subhorizontal modal layering and a subvertical marginal zone of <40 m across. The marginal zone is composed of Iherzolite and olivine websterite with minor gabbroic rocks. The layered sequence is composed of Iherzolite, websterite, gabbro, and gabbrodiorite from the base to the top. The Zhubu intrusion can be explained by two stages of formation, an early conduit stage for the marginal zone and a late in situ differentiation stage for the layered sequence. Most important Ni-Cu-PGE mineralization in the intrusion occurs as disseminated sulfides within the marginal zone. Olivine crystals from the marginal zone contain 81 to 84 mol % Fo and 1,600 to 1,900 ppm Ni. The rims of zoned olivine phenocrysts in the Emeishan picrites have similar Fo contents but significantly higher Ni contents (2,300-2,600 ppm). The olivine data indicate that the parental magma of the Zhubu ultramafic rocks is similar to the transporting magma of the Emeishan picrites in MgO/FeO ratios but depleted in Ni due to sulfide segregation before olivine crystallization. The initial concentrations of PGE in the Zhubu magma, estimated from bulk sulfide compositions, are 7 ppb Pd, 9.3 ppb Pt, and 0.8 ppb Ir, similar to the values in the Emeishan picrites. Like the Emeishan picrites, the Zhubu intrusive rocks are characterized by light REE enrichments. Negative Nb anomalies relative to Th and Ta, which are rare in the Emeishan picrites, are present in the Zhubu samples. The (Sr-87/Sr-86)(i) and epsilon(Nd) values of the Zhubu intrusive rocks vary from 0.709591 to 0.710692 and from -2 to -3, respectively. The trace element and isotope compositions indicate that the Zhubu magma was contaminated by crustal materials, supporting the interpretation that sulfide saturation in the magma was triggered by crustal contamination. The area where the lower part of the conduit may have been brought up by faulting should be the focus of future exploration at Zhubu.

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