Formation of the Lubei magmatic Ni–Cu deposit in a post-subduction setting in East Tianshan, North West China

Abstract

The Lubei Ni–Cu sulfide ore deposit with 7.8 Mt sulfide ores at an average grade of 0.88 wt% Ni and 0.5 wt% Cu, is associated with a mafic–ultramafic intrusion in the western part of the East Tianshan in Xinjiang Uygur Autonomous Region, NW China. The primary mineralization is hosted by ultramafic rocks which are part of a 2.1 km2 mafic–ultramafic intrusion located within the Kanggurtag fault zone. In this paper, we intend to probe the ore-formation processes for the Lubei Ni–Cu sulfide ore deposit and discuss key factors that control the ore formation processes in a post-subduction setting. The Lubei primary magmas were derived from 13 to 17 % partial melting of a hydrous mantle containing ∼0.29 wt% H2O estimated using the silicate-liquid model pMELTS. The Lubei sulfide ores are characterized by low platinum group elements (PGE) and high mantle-normalized Pd/Ir ratios, and disseminated sulfide ores have higher Cu, Ni and PGE tenors than massive and vein sulfide ores indicative of crystal fractionation of Fe-rich monosulfide solid solution. The PGE-depleted Lubei sulfides possibly resulted from early sulfide segregation from the Lubei primary magmas as indicated by high Cu/Pd ratios (up to 106), and the proportion of early sulfide losses was 0.02 wt% which has depleted the Lubei parental magmas and later fractionated sulfides in chalcophile elements. This depletion would been compensated at different levels by later sulfide–magma interaction when the parental magmas reached in situ sulfide saturation, with an R factor (mass ratio of silicate to sulfide liquids) of 100–1000. The formation of the observed Lubei disseminated sulfide ores was associated with 5–15 vol% magma crystallization with the main silicate minerals being spinel and olivine, as modeled using the silicate-liquid model MELTS. The δ34S values of the Lubei sulfides vary over a narrow range of −0.3 to +1.8‰, indicating a mantle origin for the sulfur. The key factor that controls the formation of magmatic Ni–Cu deposits in post-subduction settings is a metasomatized mantle accompanied by activation of translithospheric faults during post-collisional extension. This mechanism alternatively interprets the widespread magmatic Ni–Cu deposits in the southern Central Asian Orogenic Belt and implies translithospheric fault as a potential target for Ni–Cu exploration.