Nature and evolution of the ore-forming fluids in the Shazhou volcanic-related hydrothermal uranium deposit, Xiangshan ore field, SE China

The Xiangshan uranium (U) ore field in South-east China has been recognized as the largest volcanic-related hydrothermal U deposit in China since its discovery in the 1950s. The U mineralization at Xiangshan is mostly hosted in felsic volcanic and sub-volcanic rocks and occurs mainly in veins, which are controlled by high-angle normal faults in association with haematite, quartz, fluorite, apatite, and pyrite. Likewise, diverse hydrothermal alterations such as haematitization, silicification, fluoritization, and illitization have been identified adjacent to the ore-bearing faults and outward for tens to hundreds of metres into the volcanic rocks. Situated to the west in the Xiangshan ore field, the Shazhou deposit is the second-largest U deposit in the Xiangshan volcanic basin. Based on geological field evidence and petrographic observations, the ore-forming fluids in the Shazhou U deposit can be classified and separated into three stages (early, main, and late): (a) fluids associated with the early mineralization stage are characterized by the oxidizing feature with U-O hydroxide complexes as the dominant species; (b) fluids from the main mineralization stage were reducing fluids with black-purple fluorite and sulphide complexes of U-Ti-O; and (c) fluids from the late-stage were weakly oxidizing fluids with light purple fluorite and light red calcite complexes of U-O as the dominant species in the fluid. Fluid inclusions in alteration minerals from the main and late mineralization stage recorded homogenization temperatures (Th) of 280-340 degrees C and 200-260 degrees C, with salinities of 5-17 and 6-10% NaCleqv, respectively. Ore-forming fluids contain variable amounts of CO2, O-2, and H-2, in which the volatile content in the main mineralization stage is higher than that in the late stage. The temperature and salinity of the ore-forming fluids were fluctuating and gradually decreased as the geologic history of the deposit progressed. The delta DW-SMOW values calculated for the ore-forming fluids range from -97.4 to -65.1 parts per thousand. The delta O-18 values of syn-ore quartz occur mainly between 5.9 and 15.7 parts per thousand; calculated delta O-18(W-SMOW) values are between -1.5 and +8.3 parts per thousand. The delta S-34(CDT) values of syn-ore pyrite and galena range from 9.7 to 19.7 parts per thousand. The ore-forming fluids in the Shazhou U deposit are derived from different sources with a complex geological evolution history. The ore-forming fluids from the early mineralization stage with high O-2 contents may have been primarily derived from post-volcanic hydrothermal activity. Subsequently, fluids from the main mineralization stage (with lower O-2 and high H-2 contents) might have originated from the deep-sourced fluids, where sulphur was carried from the metamorphic basement. Finally, ore-forming fluids from the late mineralization stage could have been produced by mixing meteoric hydrothermal fluids. Large-scale fluid immiscibility and boiling occurred during the main mineralization stage, followed by fluid mixing during the late mineralization stage. Boiling and mixing of fluids were the two dominant mechanisms for the deposition of U in the Shazhou deposit.