Controls on crystallization of cassiterite from the southern Hunan: Evidence from cathodoluminescence, trace elements and geochronology

Distinct types of tin polymetallic mineral deposits were formed in the southern Hunan as an important section of the Nanling non-ferrous and rare metallogenic belt. It is still not fully understood about the fluid source, composition and physicochemical state of various tin deposits. Five types of tin ores, including proximal skarn, distal skarn, greisen, chlorite vein and quartz vein from the Xianghualing, Furong and Hongqiling deposits, were taken as the cases to discuss controls on the crystallization process of cassiterite based on the analysis of microstructure, chronology and in-situ trace element.LA-ICP-MS cassiterite U-Pb dating of in the Hongqiling tungsten-tin deposit yielded an age of 153.7±2.4 Ma. The color change of CL images of different types of cassiterite from southern Hunan is mainly related to the relative content of Ti, Nb and Ta. The color of CL images is lighter when the content of Ti is high, while the color of CL images is darker when the content of Nb and Ta is high. The Zr/Hf ratio of cassiterite is interpreted as assimilation of wall rock and fractional degree of the ore-forming fluid. Cassiterites from skarn and chlorite vein have a higher Zr/Hf ratio than its ore-bearing granite, which is related to the strata. On the contrary, cassiterites from greisen and chlorite vein show a low Zr/Hf ratio indicating a highly fractionated fluid. The primary and secondary texture developed in cassiterites from chlorite veins and quartz veins demonstrates an opposite change of Fe, W, U content and Zr/Hf ratio, probably implying addition of meteoric water and plused magmatic fluid, respectively.