Shear zone-controlled post-magmatic ore formation in the Huangshandong Ni–Cu sulfide deposit, NW China

Abstract

The Huangshandong Ni–Cu sulfide deposit, located in the Kangguer–Jing’erquan ductile shear zone, belongs to the Jueluotage island arc belt in Eastern Tianshan of the Central Asian Orogenic Belt. The Huangshandong mafic–ultramafic pluton, which has a fusiform shape, is believed to have intruded strongly foliated Late Carboniferous volcanic rocks during the development of the regional ductile shear zone. Some orebodies in the deposit show intense ductile deformation. Among them, the No. 17 orebody is entirely controlled and hosted by the ductile shear zone. Host rocks and ores from this orebody show intense deformation and alteration, and could be called ore mylonites. Furthermore, numerous anastomosing veinlets can be observed in copper-rich carbonate-sulfide veins in the ductile shear zone. Geological field investigation and ore petrography studies indicate that sulfide ores at Huangshandong are composed of three types: undeformed magmatic ores, deformed magmatic ores, and hydrothermal overprinted ores. Magmatic ores with net fabrics remain unaltered and undeformed, and sulfide minerals in them show only brittle fractures. Silicate and sulfide minerals in deformed magmatic ores are characterized by intense deformation fabrics. Examples for such fabrics are kink bands, wavy extinction of uralite, domino structure of phlogopite, and elongated grains of pyrrhotite and pentlandite. Hydrothermal overprinted ores are characterized by strong deformation, flattened grains, and recrystallization of pyrrhotite; moreover, annealing of pyrrhotite is frequently observed. Pyrrhotite, pentlandite, and chalcopyrite in hydrothermal overprinted ores have high Ni and Co concentrations in pyrrhotite, a higher Ni concentration in pentlandite, and high Ag and Cd concentrations, respectively. We attribute Ni and Co in fluid-overprinted ores mainly to the alteration of olivine, and the dissolution of primary chalcopyrite is the likely source of copper in chalcopyrite-rich hydrothermal overprinted ores. During the intruding and cooling of the Huangshandong pluton, intense regional ductile shearing caused the ductile deformation of sulfide ores in the deposit, and subsequently, ore mylonites were formed. Gangue minerals in the deformed orebodies were crushed to fine-grained minerals and altered to serpentine, chlorite, and uralite. During these processes, fluid and ore elements were released and reacted with deformed rocks and ores, forming anastomosing veinlets on hydrothermal overprinted ores.