Identification of hydrothermal alteration and mineralization in the Sancha magmatic Cu-Ni-Au sulfide deposit, NW China: Implications for timing and genesis of mineralization

Abstract

The Sancha Cu-Ni-Au deposit in NW China is a unique example of a sulfide deposit associated with magmatic Cu-Ni-(Au) and hydrothermal Au-(Cu) mineralization. Notable hydrothermal alteration and mineralization have been identified by geology, geochronology, mineralogy, and geochemistry, but they had no remarkable effect on the distribution of Ni and Co, although minor Au and Cu were mobilized, accumulated, and enriched within nearby ultramafic dikes and country rocks. The magmatic Ni-Cu-(Au) mineralization (489 ± 3 Ma) consists principally of magmatic pentlandite, pyrrhotite and chalcopyrite in the ultramafic rocks, while Au-(Cu) in beresites is closely related to hydrothermal mineralization (417 ± 5 Ma). The hydrothermal fluid altered magmatic minerals and textures, although some original pyroxenes and Pn-Po assemblages can still be recognized, and it released small amounts of metal elements (e.g., Cu, Co, and Ni) from magmatic sulfides. This transformation is exemplified by metasomatism on primary magmatic Po-Pn-Ccp to form a hydrothermal Py-Mlr-Ccp assemblage; replacement of olivine/pyroxene pseudomorphs by hydrothermal magnesite; varied concentrations of major elements; enrichments in W, Bi, Pb and Mo; and the liberation of Au, Cu, and Ni from altered ultramafic ores. Therefore, we suggest that Ni and Co predominantly originated from altered ultramafic ores and that strata with black shales in the mining area might have been one of the sources of As, S, and Au for hydrothermal mineralization. However, Au and Cu seem to have been more mobile and might have been redistributed by hydrothermal fluids in larger amounts and at a larger scale than Ni. In summary, Sancha is a magmatic Cu-Ni-Au deposit with hydrothermal Au-(Cu) mineralization that resulted in Au accumulation and Cu enrichment but slight changes in Ni or Co.