In-situ sulfur isotope and trace element analysis of pyrite from the Xiwang uranium ore deposit in South China: Implication for ore genesis

Abstract

The Xiwang uranium ore deposit is one large-scale granite-hosted U deposit in the Xiazhuang U orefield in South China. Uranium orebodies occur as thin veins along the fracture zones within the Maofeng granitic pluton. Pyrite is the main sulfide mineral in the ores. Textural observation suggested that formation of hydrothermal pyrite can be divided into three stages: pre-mineralization stage pyrite (Py I), mineralization stage pyrite (Py II) accompanying with U-minerals, post-mineralization stage pyrite (Py III). In this study, in-situ sulfur isotopic compositions and trace element contents of pyrite from different stages were analyzed by laser ablation (MC-) ICP-MS methods. The δ34S values of Py I vary from −28.5‰ to −12.0‰ with an average value of −18.4‰, Py II from −15.9‰ to −10.2‰ with an average value of −12.5‰ and Py III from −22.3‰ to −14.8‰ with an average value of −19.4‰. The variations of sulfur isotopic compositions of pyrites were caused by the changes of oxygen fugacity of hydrothermal fluids. The hydrothermal fluids at the pre-mineralization stage had relatively higher oxygen fugacity (ƒO2 > 10−34). The decreasing of oxygen fugacity (to 10−36–10−35) at the mineralization stage might promote the precipitation of uraninite. The mineralization was terminated as the increasing of oxygen fugacity of hydrothermal fluids back to 10−35. Pyrite from the Xiwang deposit generally contains high U contents (up to ten thousand ppm) and very low Th contents (mostly <0.1 ppm). U and W contents in pyrite show good positive linear relationship, indicating that both U and W came from the hosting granite. Py II shows higher Mo and V contents than Py I, also suggesting more reduced environment at the U mineralization stage. Such high oxygen fugacity for initial hydrothermal fluids supports that the ore-forming fluids should be originated from meteoric water. The circulating meteoric water can leach U and other metals from the hosting granite to form U-rich hydrothermal fluids. In fracture zones and under reduced conditions, U6+ in the fluids can be reduced to precipitate uraninite (UO2). Thus, U-rich granites, circuiting meteoric water and reduced agents are pre-requisites for the formation of the granite-hosted U deposits in South China.