Epithermal gold mineralization in Cretaceous volcanic belt, SE China: Insight from the Shangshangang deposit

Abstract

Low sulfidation (LS) epithermal gold deposits within the Southeastern China Fold Belt (SCFB) are generally associated with Early Cretaceous volcanism (91–110 Ma). The Shangshangang (SSG) gold deposit (3 t Au @ 2.7 g/t) is proposed to be a LS ore system hosted by Early Cretaceous volcanic-subvolcanic rocks in the Dongkeng Basin (DVB), northern SCFB. Early Cretaceous felsic volcanic-subvolcanic rocks within the DVB possess high-K calc-alkaline and metaluminous to weakly peraluminous signatures. They show rare trace element (REE) and trace element patterns resembling those of Early Cretaceous A-type granites and rhyolites in the SCFB with significantly Eu negative anomalies (Eu/Eu* = 0.1–0.3). These volcanic-subvolcanic rocks are potentially formed by mixing of melts from crust and Early Cretaceous enriched mantle under an extensional tectonic setting. This is indicated by their negative εHf(t) values (−8.8 to −5.2), young two-stage Hf model ages (TDM2 = 1.32–1.48 Ga), and variable Sr-Nd isotopic compositions.The epithermal event at SSG is represented by three mineralization stages characterized by four distinct quartz vein types. These are: (1) greyish green crustiform comb quartz veins (type A) and smoky grey fine-grained (<0.5 mm) quartz veins (type B) in the early stage, (2) ivory medium- to coarse-grained (>1 mm) quartz veins (type C) in the middle stage, and (3) late white calcite-rich quartz veins (type D). Quartz veins are typically accompanied by hydrothermal alteration with zoning (kaolinite-illite-chlorite zone, adularia-chlorite-quartz zone, and adularia-sericite-quartz zone). Gold mineralization is identified in the early stage and free gold exists as micrometer-sized inclusions of native gold within pyrite. Pb–Zn mineralization dominates the middle stage. Hydrothermal fluids within the ore system are of low salinity (<7 wt% NaCl) and gas-poor. The fluid temperatures decreases from 224 − 316 °C for Au mineralization to 190–261 °C for Pb-Zn mineralization. No indication for fluid boiling is found. The H–O isotopes indicate mixing of magmatic and meteoric fluids and a larger portion of meteoric water at a later stage. The δ34S values of pyrite related to Au mineralization are 1.1–2.0‰ and of sphalerite and galena related to Pb-Zn mineralization range from −1.8 to 2.5‰. The Pb isotopic compositions of sulfides from the early and middle stages are relatively homogeneous and similar with those of subvolcanic rocks within the DVB. Deposit geology, microthermometric result, and isotopic data support a LS epithermal origin for the SSG gold deposit, and therefore suggest a co-existing relationship between low and intermediate sulfidation epithermal gold mineralization within the DVB.