Mobilization of ore-forming metals during post-magmatic hydrothermal overprinting of the Huangshandong Ni–Cu sulfide deposit, Eastern Tianshan, NW China

Abstract

The Huangshandong Ni–Cu sulfide deposit is the largest magmatic sulfide deposit in the Huangshan–Jing’erquan mafic–ultramafic belt in the Eastern Tianshan; it was overprinted by post-magmatic hydrothermal fluids, and metamorphosed and deformed as a result of regional ductile shearing. Ore-forming elements were remobilized as a result of post-magmatic hydrothermal fluid flow, but the extent of this remobilization is still unclear. In this contribution, detailed mineralogical–textural characteristics of silicates and base-metal sulfides in different types of ores in the Huangshandong Ni–Cu sulfide deposit were integrated with spatially resolved trace-element compositions of olivine, serpentine, pyrrhotite, pentlandite, and chalcopyrite to identify the metals that were mobilized, the scale and quantity of metal remobilization, and the phases into which the mobilized metals were incorporated. Based on varying degrees of serpentine and talc alteration, three different types of ores are identified – weakly serpentinized magmatic ores, which are most representative of the primary magmatic mineralization, strongly serpentinized and deformed ores, and talc-altered ores. The cracks consisting of serpentine and magnetite in weakly serpentinized magmatic ores caused by the expansion of olivine during serpentinization; this, serpentine is enriched in ore-forming metals (e.g., Ni, Co, Cu) relative to olivine. The expansion cracks extend from olivine to sulfides also enriched in ore-forming metals, suggesting that some metals were mobilized from olivine during serpentinization. Based on the higher Ni content of secondary pyrrhotite compared to primary pyrrhotite, it is suggested that base-metal sulfides in the primary, weakly serpentinized ores were dissolved and reprecipitated in the strongly serpentinized and deformed ores from metal-rich fluids that interacted with olivine.