The impact of early sulfur saturation and calc-crustal contamination on ore-forming process in the Posan mafic–ultramafic complex: Derived from the shallow depleted mantle, Beishan region, NW China

• In this paper, we have a detailed work, both field and office, for the newly founded ultramafic intrusion named Posan intrusion in Beishan region.
• We understand the petrography clearly of Posan intrusion through the geological section controlled by drill holes.
• We propose firstly the large scale of calc-crustal contamination on mafic–ultramafic rocks in Beishan region may make them poorly Ni–Cu mineralized.
• However, the Posan intrusion shows the potential of PGE mineralization because of the low Ni contents in olivine in every petrography.
• We also think the mafic–ultramafic in Beishan region come from the shallow depleted mantle without garnet, and they may have genetic connection with the early Permian mantle plume in Tarim block.

The Posan ultramafic intrusion is one of the ~280 Ma Pobei mafic–ultramafic complexes located in the Beishan rift, on the northeastern margin of the Tarim Craton. Given that three finished drill holes reveal less economic mineralization, it is necessary to understand the ore potential of this little ultramafic intrusion. Detailed fieldwork shows that it is comprised of five stages of magma events. Among them, stages I, II and III are the mafic bodies dominated by gabbro, gabbronorite and olivine gabbro, and stages IV and V are the ultramafic bodies characterized by layered intrusions. Theoretically, the Posan ultramafic intrusion meets the first requirement of ore mineralization because of its high MgO contents (14.76%) of the parental magma. Moreover, the low Ni content (less than 1900 ppm) in olivine and the paragenesis of sulfides and spinel demonstrate that the Posan intrusion experienced early sulfur saturation before or during the fractional crystallization of spinel and olivine. This part of the sulfide, which may contain Ni–Cu or PGE mineralization, deserves to be the focus of future work. In stage IV, the continuous decrease of Ni contents in olivine with the fractional crystallization, the calc-crustal contamination (irregular distributed plagioclase, high Th/Yb ratios, Nb and Zr depletion and K, Rb, Ba, U, Th and Pb enrichment) instead of sulfur-rich crustal contamination, and the high level of oxygen fugacity (+2.1 < fO2QFM < +3.0) result in no obvious disseminated sulfides created in this stage. A clinopyroxene TiO2-Alz diagram confirms that the Beishan mafic–ultramafic complexes were formed in a rift-related circumstance rather than the arc-related cumulus. Low TiO2/Yb and Nb/Yb ratios reflect that the Beishan complexes were derived from the shallow depleted mantle without garnet. Considering the temporal and spatial relationship of the Tarim large igneous province and the large numbers of mafic–ultramafic intrusions in the Beishan region, it is reasonable to speculate that the Beishan mafic–ultramafic complexes, including the Posan intrusion (275.8 ± 2.7 Ma), have a genetic affiliation with, but were not directly formed by, the early Permian Tarim Plume.