Deciphering the Volcanic-Intrusive Evolution of the Giant Porphyry Tin Deposit at Yanbei, South China, by CA-TIMS + ICP-MS U-Pb Geochronology and Trace Element Geochemistry of Zircon

Abstract

The 141–134 Ma Yanbei porphyry tin deposit is the largest of its kind in South China and has been the focus of many studies. The tin orebodies are distributed along the contact zone between a volcanic sequence and intrusive granite porphyries. In the volcanic rocks, the ore is vein type, whereas in the porphyries it is disseminated. The precise temporal evolution of the magmatism remains obscure because of significant overlap in the ages of the volcanic and plutonic rocks. In this work, we investigate the magmatic history of the deposit by means of zircon high-precision chemical ablation-thermal ionization mass spectrometry and inductively coupled plasma-mass spectrometry U-Pb dating combined with zircon Hf isotope and trace element analyses. Zircon dating reveals that five volcanic units were generated over a short time interval of ca. 0.8 m.y. from 141.3 ± 0.2 to 140.5 ± 0.2 Ma. The zircon εHft values range from −13.9 to −9.1 with two-stage model ages ranging from 2.1 to 1.8 Ga, indicating that the magmas were derived from a relatively homogeneous Proterozoic basement. A hiatus in magmatic activity of at least 1.7 ± 0.2 m.y. preceded the initial crystallization of the granite porphyry, which evolved over a time interval of ca. 4.4 m.y. and included diverse batches of magma emplaced from 138.8 ± 0.2 to 134.4 ± 0.6 Ma. Significantly, the εHft values of the granite porphyry changed over time from −9.9 to −0.6, indicating an increasing incorporation of depleted mantle-derived material or a more juvenile crustal component. Zircon grains in the granite porphyry have higher contents of the incompatible elements Th, U, Nb, Ta, and Li and lower values of Eu/Eu* compared with the volcanic rocks. This indicates that the intrusive rocks experienced a higher degree of crystal fractionation that ultimately resulted in Sn enrichment in the magma reservoir. We postulate that the earlier volcanic rocks, although not the actual source of tin, acted as an impermeable cap, preventing the escape of later ore-bearing hydrothermal fluids.