In situ trace element compositions and U[sbnd]Pb ages of cassiterite from tin-polymetallic deposits in the Dachang district, Guangxi, China: Implications for ore genesis and exploration

Abstract

The Dachang tin-polymetallic district in southwest China is one of the largest tin ore fields in the world containing approximately 1.5 Mt. Sn, 6.8 Mt. Zn, 1.8 Mt. Pb, 1.4 Mt. Sb, 0.4 Mt. Cu and other critical metals. Both cassiterite-sulfide and ZnCu skarn mineralization occur as stratiforms, veins, and stockworks and are hosted in the Mid-Upper Devonian carbonate-rich sediments adjacent to the underlying Late Cretaceous Longxianggai granite pluton and/or the granite/diorite porphyry dikes. The Tongkeng and Huile deposits are typical stratiform- and vein-type tin-polymetallic deposits in the district. Previous studies suggest a genetic link between these tin-polymetallic deposits and the prolonged Longxianggai granite which was dated to have emplaced for multiple stages within ca. 91–97 Ma. However, existing constraints on the timing of tin mineralization display a board range at 90–96 Ma, whereas the accuracy and precision of these ages are less satisfactory. These ages are broadly overlapping with both the prolonged Longxianggai granite and the porphyry dikes (91 Ma), thus gives rise to question on their genetic association, which is a question of importance for ore exploration in the district.In this study, a large number of measurements of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace elements and UPb dating on cassiterites from the Tongkeng and Huile deposits was performed and their mineralization ages with hitherto the best accuracy and precision were obtained. LA-ICP-MS trace element analyses show that cassiterites from the Tongkeng and Huile deposits contain concentrations of Fe, W, Nb, and Ta characteristic of granitic-related skarn-type tin mineralization. UPb dating on cassiterites from 391 spots analyses yielded well-constrained ages of 91.13 ± 0.61 Ma to 89.46 ± 0.47 Ma for the Tongkeng deposit and 89.0 ± 1.3 Ma for the Huile deposit. These ages are apparently younger than the dominated equigranular phase of the Longxianggai pluton, but they agree well with the emplacement ages of the porphyritic granite represented either by the latest phase of the Longxianggai granite or the granite porphyry dikes (ca. 91 Ma). This suggests that the tin-polymetallic deposits in the Dachang district were most likely genetically associated with later, higher evolved granitic phases in the district. In this regard, the dike-hosting fault systems were most likely served as fluid conduits and their surrounding host rocks are favorable targets for ore exploration in the Dachang district.