Textures, trace elements, sulfur, lead and He-Ar isotope analyses of pyrite: Implications for ore-forming processes and the origin of the Shenjiayao gold deposit, southern margin of the North China Craton

Abstract

The Shenjiayao gold deposit (8.50 t Au, average grade of 2.21 g/t), hosted in a Precambrian metamorphic terrane, is located in the Xiaoshan region on the southern margin of the North China Craton. The gold mineralization is characterized by vein ores comprising native gold and gold-bearing pyrite. Paragenesis of the Shenjiayao deposit can be divided into four stages. Stage I is pyrite – milky quartz veins; Stage II consists of grey quartz – pyrite veins; Stage III is characterized by quartz – polymetallic sulfide veins; and Stage IV minor pyrite – carbonate – quartz veins. Native gold and electrum grains are mainly distributed in the fractures of quartz, pyrite, chalcopyrite, and other sulfides. In this study, three types of pyrite were distinguished based on textural and geochemical results. Py1 has silicate inclusions and mainly occurs as euhedral–subhedral grains formed in the Stage I. Py2 within quartz veins from Stage II shows oscillatory zoning and can be distinguished into two subtypes by their distinct textures (Py2a and Py2b). Py3 coexisting with native gold, chalcopyrite, sphalerite, and galena is characterized by sulfide and silicate inclusions and is divided into two subtypes (Py3a and Py3b). The porosity and heterogeneous textures of Au-bearing pyrite indicate a process of dissolution and reprecipitation. The replacement was likely the result of dissolution–reprecipitation. However, Py1 has very low lattice-bound gold contents, indicating that obtaining high-grade gold orebodies through remobilization of lattice-bound gold is unreasonable. We suggest that the deposition of abundant native gold was induced by a new volume of Au-rich ore-forming fluids and triggered by extensive fluid phase separation during Stage II and Stage III. The δ34S values for the sulfides are similar, ranging from +0.9 ‰ to +5.8 ‰, and the lead isotope compositions of Stage II and Stage III sulfides are consistent. Fluid inclusions extracted from gold-bearing pyrite have elevated 3He/4He ratios (0.18–1.13 Ra) and 40Ar/36Ar (329.5–848.5), indicating the presence of mantle-derived fluids in the ore system.