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

Abstract

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 206Pb/238U ages ranging from 90.3 ± 1.8 Ma to 95.4 ± 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.