Primary Sn-rich titianite in the Qitianling granite, Hunan Province, southern China: An important type of tin-bearing mineral and its implications for tin exploration

The Qitianling calc-alkaline granite in Hunan Province (South China) has attracted much attention since the discovery of the Furong super-large tin deposit. The present study provides new mineralogical data to address their implications for exploration of tin deposits. In the Taoxiwo granite in the southeastern part of the Qitianling granite body, Sn-rich titanite was identified as an important type of Sn-bearing mineral. The titanite commonly occurs with biotite as euhedral crystals, exhibiting typical envelope-like shape and sector-zoning texture. These indicate that the titanite most likely crystallized in the magmatic stage. Electron-microprobe analyses show that the titanite is enriched in tin up to 1.12 wt% SnO₂, with an average of 0.43 wt%. With the crystallization of the granite, primary minerals undertook hydrothermal alteration by magma-derived fluids. Subsequently, in the hydrothermal stage, the primary Sn-bearing titanite was altered (at least partially), but still preserved its typical envelope-shaped outline. Micro-scale cassiterite is a representative product of such alteration; other secondary minerals include fluorite, ilmenite, Sn-bearing rutile and quartz. Therefore, the titanite, commonly present in the calcalkaline granites, can be treated as an important Sn-carrying mineral in the Qitianling granite, reflecting the primary magmatic environment with tin enrichment. The hydrothermal alteration of the primary titanite and subsequent crystallization of cassiterite recorded a process of leaching and accumulation of tin in magmatic-hydrothermal evolution of the Sn-bearing granite. Thus, this titanite has important implications for tin exploration. Supported by National Natural Science Foundation of China (Grant Nos. 40730423 and 40221301) and Project of China Geological Survey (Grant No. 1212010632100)