Problems of PGE metallogenesis related to mafic–ultramafic complexes in North Xinjiang, China

Abstract

Platinum group element (PGE) mineralization is mostly related to mafic–ultramafic complexes of the dominant magmatic deposit type. Mineralization of this type mainly relies on two conditions: the abundance of PGE in magma, and the presence of mechanisms favorable to PGE enrichment and separation from sulfur-saturated magma during magmatic evolution. Mafic–ultramafic complexes are widely developed in North Xinjiang, including (1) the large-scale copper-nickel deposits of Kelatongke, Huangshan, Huangshan East, and Tulargen, (2) numerous small to medium-sized copper-nickel deposits such as Xiangshan, Tudun, Hulu, Baishiquan, and (3) the Xiangshan West and Weiya medium-sized V–Ti magnetite deposits. However, mafic–ultramafic complexes in North Xinjiang rarely form PGE deposits. Therefore, questions about PGE metallogenesis in North Xinjiang are discussed in this paper from the standpoint of the ore-forming mechanism of PGE deposits and the characteristics of the North Xinjiang magma sources. The rock types of the post-collisional mafic–ultramafic complexes in North Xinjiang are of a ferrous rock series formed by fractional crystallization, a rock type generally favorable for hosting PGE. For the Cu-Ni sulfide deposits of North Xingjiang, the assimilation of crustal material causes sulfide liquation during processes of magmatic evolution and mineralization. This can be shown petrogeochemically, including the characteristics of Sr, Nd, Pb, O, Os, and S isotopes, which indicate a magmatic ore-forming mechanism for the Xingjiang PGE deposits. The principal reason for weak PGE mineralization in North Xinjiang may be ascribed to a widely-developed depleted mantle source (with positive ɛ Nd values) that underlies it. The North Xinjiang lithosphere is not of typical Precambrian cratonal type, but has more affinities with modified oceanic lithosphere or immature continental lithosphere that would constitute PGE-poor original magma sources unfavorable to PGE mineralization.