Geology, geochronology, mineral chemistry and geochemistry of the Hongnieshan mafic–ultramafic complex in the Beishan area, southern Central Asian orogenic Belt, NW China: Implications for petrogenesis and regional Ni mineralization

Abstract

In recent decades, new breakthroughs in Ni prospecting have been made in the Beishan area, southern Central Asian Orogenic Belt, NW China—more vein-type sulfide mineralization with high Ni tenor has been discovered. Here, novel discussions on the petrogenesis and sulfide immiscibility of the Permian mafic–ultramafic rock are presented based on petrological, geochronological, mineralogical, and Hf–S isotopic studies of the newly discovered Hongnieshan Ni occurrence. The complex is predominantly composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro and norite. The gabbro has a zircon age of 282.9 Ma and positive εHf(t) values (+5.5–+7.8). The complex may have originated from partial melting of the depleted mantle, which was metasomatized by subduction fluid in a post-orogenic extensional setting. The parental magma is characterized by high temperature (1396 ℃) and high Mg (picritic affinity, 14.4 wt% MgO) and Ti (2.3 wt%) contents, and an oxidized nature (FMQ + 0.59). The disseminated and vein-type sulfide mineralization are the results of multistage segregation. The first segregation was induced by the assimilation of Archean strata, as indicated by the presence of Δ33S of sulfide minerals (1.4‰ to 3.6‰), which is within the range of Archean crust (-2.5‰–+11‰) and is clearly beyond the range of post-Archean rocks (0 ± 0.1‰). The sulfide was removed at depth, leading to that the later segregated sulfide was poor in platinum group element. Vein-type high Ni tenor mineralization was ascribed to the second segregation before olivine crystallization and later injection of sulfide ore pulp. The third sulfide segregation was accompanied by fractional crystallization and gave rise to disseminated mineralization with a low Ni tenor. The δ34S in the Beishan Ni-Cu deposits ranges from-2.01 to 2.07‰, exhibiting a mantle-like variation and arguing against the incorporation of crustal S. Crustal assimilation, especially the addition of Si-rich components from country rocks is the main trigger factor. The Beishan area is still a favorable exploration target in future prospecting.