Episodic magmatism contributes to sub-seafloor copper mineralization: Insights from textures and geochemistry of zoned pyrite in the Ashele VMS deposit

Abstract Prolonged and episodic magmatic-hydrothermal processes have been proposed to be important in forming large-tonnage Cu volcanogenic massive sulfide (VMS) deposits but are difficult to document and remain poorly understood in ancient VMS systems. In this study, we combine textural evidence and in situ analysis of sulfur isotopic composition with trace elements of pyrite from the well-preserved, large-tonnage Ashele VMS deposit (Central Asian Orogenic Belt, NW China) to elucidate the origin of the ore-forming materials and controlling factors of VMS deposits. The distribution of hydrothermal alteration and mineralization allows the Ashele deposit to be divided into five zones: massive sulfide, quartz-pyrite, chlorite-chalcopyrite-quartz-pyrite, quartz-chlorite-sericite-pyrite, and quartz-sericite-pyrite zones. Cu mineralization is mainly hosed in the massive sulfide and chlorite-chalcopyrite-quartz-pyrite zones. Three texturally and compositionally distinct types of pyrite from the Cu mineralization zones have been recognized. The first type of growth zones with high Cu, As, and volatile elements (e.g., Hg and Tl) concentrations and negative δ34S values (−7.83‰ to −0.35‰) are observed in pyrite grains from the massive sulfide zone most likely formed from the input of magmatic volatiles degassed from the underlying magmatic systems. The second type of Cu-As-rich growth zones in pyrite grains from the chlorite-chalcopyrite-pyrite zone is not enriched in volatile elements and has positive δ34S values (2.59–6.56‰), which are interpreted to have precipitated during fluid boiling. The third type of growth zone, without Cu-As enrichment but having positive δ34S values (0.30–9.76‰) in pyrite grains from both mineralization zones, was probably formed without boiling or magmatic volatile input. Our results suggest that multi-stage magmatic degassing or fluid boiling accompanied by Cu precipitation recorded in individual pyrite grains can reveal episodic magmatic-hydrothermal activity, which is an essential factor in forming large-tonnage VMS deposits.