Tectonic-diffusion estimates of global mineral resources: extending the method: granitic tin deposits
In this study, we have used tectonic-diffusion model calculations to estimate resources of bedrock-hosted granitic tin deposits of Phanerozoic age. These deposits range from proximal skarns, greisens and pegmatites in or near plutonic granites, to distal veins and disseminations, many of which are associated with subvolcanic rhyolites. Tin is also found in residual and placer deposits, some of which are associated with bedrock deposits. Most of the bedrock deposits formed at temperatures of 200–500 °C from fluids with a wide range of salinities and CO2 contents. Limited information suggests that they formed over a relatively continuous range of depths from a few hundred metres to as much as 6 km, with an average of about 2 km. Information was obtained on 547 deposits, including 435 with measured or estimated ages and 301 with estimated tin contents, almost all of which are bedrock rather than residual-placer deposits. The bedrock deposits have an average size of 58 000 t, and the largest five deposits, including one in Bolivia and four in China, account for 46% of the total tin resources in all deposits. Tectonic-diffusion model calculations indicate that approximately 14 520 granitic tin deposits remain in the crust and that they contain 8.4×108 t of tin. If about 50% of the deposits above depths of 1 km can be discovered and mined, current tin production of about 260 000 t annually can continue for about another 200 years.