The reduction process contributes to the generation of subduction-related Langmuri Cu-Ni sulfide deposits in the East Kunlun Orogen, Northern Tibetan Plateau

Abstract

Ore-genesis of the subduction-related Cu-Ni sulfide deposits has not been well studied. This article focuses on the Langmuri Cu-Ni sulfide deposit in the East Kunlun Orogen (EKO) by studying the symbiotic ore-bearing olivine pyroxenite and ore-free gabbro. Twenty-seven Cu-Ni sulfide orebodies were defined in these ultramafic and mafic rocks in past years with Ni average grades up to 1.5 % and Cu average grades up to 0.18 %. The ore mineralization comprises sulfides (pyrite, chalcopyrite, pentlandite, and pyrrhotite) and oxides (chromite and magnetite). Zircon U-Pb dating shows that the Langmuri mafic-ultramafic rocks have similar emplacement ages (449.9 ± 3.9 Ma for the olivine pyroxenite, 450.8 ± 1.8 Ma for the gabbro), and they were generated during the northward subduction of the Proto-Tethys Ocean. These rocks were sourced from partial melting of a continuously enriched lithosphere mantle resulting from interaction with slab-derived fluids. The ore-bearing olivine pyroxenite had a basaltic parental magma. It underwent accumulation of olivine and minor chromite and crustal contamination during magmatic evolution, while the ore-free mafic rocks underwent fractional crystallization of olivine and chromite and the crustal contamination was insignificant. The oxidation state of the ore-bearing olivine pyroxenite at QFM-1 to QFM is lower than the values for normal arc magma and the ore-free gabbro, which should be associated with the crustal contamination. Based on these clues, we inferred that fractional crystallization of olivine and minor chromite occurred in a deep magmatic chamber, and these minerals accumulated locally to form olivine-rich ultramafic rocks. The magma that underwent fractional crystallization of these minerals is the parental magma for the ore-free gabbro, while the magma with accumulation of these minerals is heavier and transported more slowly, which makes it undergo significant crustal contamination during magmatic rising. The crustal contamination reduced the magma, resulting in the sulfur saturation in magma and the generation of Cu-Ni sulfide orebodies in the Late Ordovician subduction setting at EKO.