Genesis of the oxidized Sn ores in the Gejiu district, Yunnan Province, SW China
The Gejiu Sn ore district, one of the largest Sn producing area (300Mt @1 wt. % Sn) in the world, is located in the western margin of the Cathaysia Block, SW China. There are five types of Sn ores in the Gejiu district, including greisen, skarn, veined tourmaline, massive sulfide and oxidized. Among these types of Sn ores, the genesis of the oxidized Sn ores is most controversial, such as magmatic hydrothermal origin and sedimentary exhalative (SEDEX) or hot water deposition (Red Sea type). This paper reports U-Pb ages, fluid inclusions, trace elements and H-O isotopes of cassiterite from the oxidized Sn ores, aiming to solve this issue. LA-MC-ICPMS cassiterite U-Pb dating yielded an age of 86.7±2.8Ma, which is accordant with the formation ages (∼86 Ma) of granitic intrusions in the Gejiu district. Two types of fluid inclusions in cassiterite from the oxidized Sn ores were observed, one type is CO2-rich inclusions (Type 1) and the other type is aqueous inclusions (Type 2). The homogenization temperatures range from 220 to 366°C and the salinities range from 10.9 to 21 wt. % NaCl equiv. for the two types of fluid inclusions. Furthermore, fluid inclusions in cassiterite from the oxidized Sn ores located at 300m far away from granitic intrusions have higher temperatures (357-366°C) than those located at 500m (273-330°C).Hence, the ore-forming temperatures tend to decrease with the distance far away from the magmatic hydrothermal center. The vapor components for the two types of fluid inclusions are CO2 and H2O, respectively, and the liquid phase compositions of them are all H2O. The CO2 in the ore-forming fluids may source from marble by decarbonization. This indicates that the precipitation of cassiterite is resulted from fluid immiscible. The δ18O values of fluid inclusions in cassiterite range from +6.1 to +7.7‰, and the δD values range from -135 to -153‰, indicating a mixture of magmatic and meteoric waters. Trace elements show that cassiterite is rich in In, Rb and U, but Ba, Nb and Ti are depleted. In addition, the lower Sr (0.4-2.1ppm) and higher Sc (35.9-53.7ppm) contents reflect that the formation of cassiterite is related to F-rich intermediate-acid magma. Cassiterite has a relatively low total REE contents (2.7-9.2ppm) with negative Eu (δEu=0.14-0.76) and Ce (δCe=0.1-0.6) anomalies, indicating that the ore-forming environment is oxidized. This is consistent with the cassiterite precipitation conditions. Hence, all the evidence supports that the oxidized Sn ores are genetically associated with the Gejiu granitic magmatism.