Petrogenesis, Mineralization and Prospecting Information of Permian Mafic-Ultramafic Rocks, Beishan, Xinjiang

A series of Permian mafic-ultramafic complexes associated with large and medium-sized Ni deposits have been found in recent decades in the Beishan area, Xinjiang, NW China. Based on the new data obtained from the geological survey, a few controversial scientific issues, such as geochronology, the origin and nature of parental magma, tectonic setting, sulfide immiscibility mechanism, etc., are redefined. Two episodes of mafic magmatism in the Beishan area were recognized. i.e. the Early Permian and Middle Permian (261-266 Ma). The parental magma may have originated from high degree partial melting of the depleted mantle which suffered from metasomatism and modification by subduction-related fluid in relatively shallow depth. It is characterized by high temperature, high Mg content, oxidized and hydrous nature. The Beishan mafic-ultramafic rocks are of non-plume origin, as well as of non-arc origin. The tectonic setting seems more likely slab break-off and consequent upwelling from an asthenosphere mantle in post-orogenic setting. The Pobei sub-belt and the Hongshishan sub-belt have similar rock types and geochemical features, but different mineralization types, suggesting multi-stages and differences of sulfide segregation mechanism. Addition of external S from adjacent VMS Cu deposits triggers sulfide segregation prior to olivine crystallization. Injection of ore pulp to later norite gives rise to vein-type mineralization with high Ni tenor. Finally, some indicators are proposed for the future prospecting, for example, relatively low Ni content in olivine from ultramafic rocks, high Mg but low Ti contents, high m/s and m/f ratios, tholeiitic parental magma (>11.5% MgO), relatively high degree of partial melting (>10%) and high solidification index (>50). The area along the Hongshijing deep fault is of further prospecting potential.