Metallogenesis of the Bawang Sn-Zn Polymetallic Deposit, Wuxu Ore Field, Guangxi, South China: U-Pb Dating and C-O-S-Pb Isotopic Constraints

The Bawang deposit, located in the west of Wuxu ore field, southern section of the Nandan-Hechi metallogenic belt, is a medium-sized tin-zinc polymetallic deposit. Its genesis has been a matter of debate because of lacking constraints from isotope geology. In this study, LA-MC-ICP-MS U-Pb dating of cassiterite and C-O-S-Pb isotope analyses of monominerals were used to investigate the mineralization age and source of the ore-forming minerals in the Bawang deposit. LA-ICP-MS U-Pb dating of cassiterite yielded ages of 93.1 +/- 4.8 Ma and 85.3 +/- 6.3 Ma, indicating that the mineralization occurred in the early Late Cretaceous. The delta C-13(PDB) and delta O-18(SMOW) values of calcites at the ore stage range between -0.41 parts per thousand and 0.44 parts per thousand (average = -11 parts per thousand) and between 13.8 parts per thousand and 15.40 parts per thousand (average = 14.59 parts per thousand), respectively. This shows that ore fluid sources were a mixture of those derived from magma and stratum. Pyrite and sphalerite have uniform delta S-34 values (-4.45 parts per thousand~-2.20 parts per thousand), indicating that sulfur is also derived from the mixing of magmatic hydrothermal and stratum fluids. The Pb isotopic composition of sulfide (Pb-206/Pb-204 = 18.4055~18.7625, Pb-207/Pb-204 = 15.6745~15.7209, Pb-208/Pb-204 = 38.6232~39.0370) is consistent with the granite of the same age, indicating that ore-forming metals are mainly derived from magmatic hydrothermal solution. The Bawang deposit is a hydrothermal vein-type deposit in the external contact zone of Late Cretaceous granite, controlled by tectonic fractures, and formed by the interaction of magmatic hydrothermal fluid and carbonate rock. There may be large skarn-type ore bodies in the deep part. The results of this study provide insights into the research and exploration of similar deposits in Nandan-Hechi metallogenic belt and in the Youjiang Basin.