Mineralogy and He–Ar isotopic compositions of pyrites in the Paishanlou Au deposit on the northern margin of North China Craton

Abstract

The Paishanlou Au deposit is an altered‐rock type with the magmatic‐hydrothermal type deposit on the northern margin of the North China Craton and hosts a resource of ~40 tonnes Au. The deposit is hosted in the Neoarchean metamorphic rocks of the Jianping Group and is controlled mainly by ENE–NW‐trending ductile shear zones. The orebodies are stratoid, and mineralization styles are divided into auriferous altered felsic mylonite and altered biotite plagioclase mylonite. The sulphide minerals are dominated by pyrite that is recognized as pyrite 1, pyrite 2, and pyrite 3 based on crystalline form and structure, and pyrite 2 is the main auriferous mineral. Three types of pyrite were selected for electron microprobe analysis (EPMA), and the results show S deficiency, indicating that the pyrites are formed in reducing conditions. The pyrites have a trend from being Fe deficient to being Fe enriched, suggesting that the crystallization rate increases with the decrease of impurity composition from the early to the late stage. The Co/Ni ratios and Co–Ni–As diagram reflect that the Paishanlou Au deposit is associated with magmatic‐hydrothermal and is affected by various hydrothermal. He–Ar isotopic analyses of auriferous pyrite 2 indicate that the ore‐forming fluids are mixtures of mantle‐derived and modified air‐saturated water (MASW). 3He/4He ratios range from 0.21 to 1.51 Ra (Ra = 1.39 × 10−6 for air), corresponding to a 2.18%–19.06% mantle He contribution. The formation of mixed ore‐forming fluids was related to magmatism associated with craton destruction during the Early Cretaceous. Also, we attempt to use the thermoelectric characteristics of pyrite and a contour chart of Au grades to indicate potential mineralization prospecting areas.