Genesis of tin-dominant polymetallic deposits in the Dachang district, South China: Insights from cassiterite U-Pb ages and trace element compositions

The Dachang tin-dominant polymetallic district in western South China, hosts a total reserve of 110 Mt ore @ 1% Sn, 3-5% by-products of Zn, Pb, Sb, In and Cu, and 100 to 300 g/t Ag. Economic tin orebodies in the district mainly developed in Devonian limestone, black shale and siliceous rocks, as massive/stratiform and vein/ stockwork mineralization. LA-ICP-MS U-Pb dating on seven cassiterite samples from the Tongkeng-Changpo and Gaofeng Sn-Zn-Pb deposits, and the Dafulou, Huile and Kangma Sn-Zn deposits, yields weighted mean Pb-206/U-238 ages ranging from 90.3 +/- 1.8Ma to95.4 +/- 4.9 Ma. These dates are significantly younger than the Devonian ore-hosted strata, but agree well with the previously published zircon U-Pb ages of 96.6-93.9 Ma for the Longxianggai biotite granite within analytical uncertainties, showing a close temporal link between the granitic magmatism and tin mineralization. Cassiterite grains from various ore types in the Dachang district show clear growth zonations as reflected from their cathodoluminescence images. LA-ICP-MS trace element analyses of cassiterite grains indicate that they have relatively high Ti, Fe and W but low Nb and Ta contents, which corresponds to the clear luminescencing. Chemical variations of cassiterite from different ore types, i.e. cassiterite-sulfide ores and cassiterite-bearing quartz veins, are indistinguishable, except for more enriched Fe in cassiterite from cassiterite-quartz veins. Combined with Cretaceous U-Pb ages, elevated Fe and W contents of cassiterite suggest tin-dominant polymetallic deposits in the Dachang district were formed in a granite-related magmatic-hydrothermal system controlled by the Longxianggai biotite granite, precluding their syngenetic origin