The Qitianling granitic stock is a stanniferous granite located in Hunan Province, China. Recently, the Furong tin ore deposit was found in this stock, with a tin reserve of approximately 600,000 tons. The main rock-forming minerals including amphibole, biotite, plagioclase and ore minerals including cassiterite and rutile were analysed chemical compositions by electron microprobe. The biotite is Fe-rich annite, and has high Ti and Cl concentration. The biotite has high Fe3+/(Fe2++Fe3+) ratios and the oxygen fugacity calculated by biotite compositions is above Ni-NiO (NNO), and near the Fe2O3-Fe3O4 (MH). The amphiboles are ferropargasite and ferro-edenite hornblende. The pressure of the granite estimated by Al-in-hornblende barometer is 3.6 ± 0.9 kbar. An amphibole-plagioclase thermometry and a semiquantitative hornblende thermometer yield a forming temperature of 750∼820°C. The chlorite from the orebody has negligible K2O, Ti2O, F, and Cl, but shows similar Fe/(Fe+Mg) ratios with amphibole and biotite. Cassiterite is observed closely associated with rutile disseminated in chlorite alteration veins and envelopes. The ore-forming temperature is estimated to be 290∼405°C from chlorite geothermometry.
The Qitianling granite contains Sn-rich hornblende and biotite, and shows a high oxygen fugacity for the magma, which is slightly different from the common S-type stanniferous granites worldwide. Fractional crystallization of the magma and tin deposition directly from exsolved magmatic-hydrothermal fluids may not be the major mechanism for the tin mineralization in this deposit. Instead, we suggest that post-magmatic hydrothermal alteration of the granite may have released tin and other metals (e.g., Ti) from the Sn-bearing biotite and hornblende in the granite. Then cassiterite and rutile precipitated together with chlorite when the physical and chemical condition of the Sn- and Ti-rich fluids changed.