Texture and geochemistry of pyrite from the Jinya, Nakuang and Gaolong gold deposits in the Youjiang Basin: implications for basin-scale gold mineralization

To understand the controls of basin paleogeographic evolution on sediment-hosted gold metallogenesis, a study was undertaken of pyrite texture and geochemistry of the Jinya, Nakuang, and Gaolong gold deposits in the Youjiang Basin, SW China. Syn-sedimentary/diagenetic pyrite and three generations of hydrothermal pyrite (As-Au-poor core, anomalously As-rich but Au-poor mantle, and As-Au-rich rim) are defined in the three gold deposits. The syn-sedimentary/diagenetic pyrites deposited in the basin facies at Jinya and slope facies around platforms at Gaolong have δ 34 S values of –3.4 ‰ to + 7.3 ‰ and + 14.8 ‰ to + 16.1 ‰, respectively. Compared to As-Au-poor pyrite with abundant fine-grained galena inclusions and high-radiogenic Pb isotopic ratios, the As-Au-rich pyrite shows a ~ 2–3 ‰ increase in average δ 34 S values (Jinya: –5.9 ‰; Nakuang: –7.6 ‰; Gaolong: + 12.2 ‰) towards those of syn-sedimentary/diagenetic pyrite and a decrease in average Pb isotopic ratios. These phenomena can be explained by the interaction of ore fluids with the pyritic sedimentary host rocks. A regional feature of the Youjiang Basin is that most of the gold deposits share similar pyrite textures and trace element compositions. However, the δ 34 S values of auriferous pyrite range from –9 ‰ to –5 ‰, –5 ‰ to + 5 ‰, and + 5 ‰ to + 19 ‰ in these gold deposits with host rocks deposited in the basin facies, open platforms, and slope facies around platforms, respectively. Thus, it is demonstrated that the sulfur isotope compositions of auriferous pyrite were mostly determined by the syn-sedimentary/diagenetic pyrite formed under different sedimentary facies.