Exploring volcanic-intrusive connections and chemical differentiation of high silica magmas in the Early Cretaceous Yanbei caldera complex hosting a giant tin deposit, Southeast China

Abstract

Study of the origin and chemical differentiation of silicic magmas can provide important insights into crustal evolution and rare metal metallogeny. Tin mineralization always tends to form in relatively reduced, highly fractionated granite systems. Recognition of distinctions between fertile and barren magmas is of enormous benefit to mineral exploration. The Yanbei caldera complex (YCC), is a typical inland volcanic-intrusive complex that also hosts a giant porphyry-type tin ore deposit in Southeast China. To study the origin and chemical differentiation of the YCC and constrain its relationship with tin mineralization, a comprehensive petrological, whole-rock major and trace element geochemical data, along with zircon UPb ages, trace element and Hf isotopic data of the Yanbei caldera complex (YCC) and the Xiaoji granite (XG) near the YCC are carried out. LA-ICP-MS zircon UPb dating indicates that the generation and evolution of the YCC and the XG took place in a short time (4 M.y.) of between 142.4 and 138.4 Ma. Distinct zircon Hf isotopic compositions of the volcanic and intrusive units from the YCC suggest that they were derived from different magma sources. The volcanic rocks and the XG have consistent and low zircon Hf isotopic compositions (εHf(t) = −14.9 ~ −9.0), implying that they were almost exclusively derived from melting of Paleoproterozoic crustal rocks. But the magma source of the intrusive units (the granite porphyry, GP and the biotite granite, BG) (εHf(t) = −6.0 ~ −0.8) contains a significant mantle-derived component input. The whole-rock and zircon compositions suggest that the compositionally zoned volcano of the YCC can be interpreted in terms of the “crystal mush model”. The rhyolite has the geochemical characteristics of highly evolved magmas which undergo crystal fractionation, while the dacite displays a complementary geochemical signature implying that it represents the residual crystal mush after extraction of the rhyolitic melts. The XG displays similar chronological, compositional, and isotopic features to the rhyolite, suggesting that it is the intrusive equivalent of the rhyolite. The intrusive units (GP and BG) of the YCC are highly-evolved granites with elevated tin contents, responsible for tin mineralization. By comparison of fertile and barren systems in the YCC, we suggest that simple crystal fractionation for the high-evolved/extracted granitic melt does not necessarily lead to tin mineralization. A tin-bearing and volatile-rich melt originated under high-temperature partial melting of crustal source induced by underplating/input of a hot mantle magma is an essential precondition for some magmatic-hydrothermal tin mineralization systems.