Petrogenesis of Neoproterozoic rare metal granite-pegmatite suite in Jiangnan Orogen and its implications for rare metal mineralization of peraluminous rock in South China

Abstract

Recent studies have demonstrated the Mesozoic explosive metallogeny of rare metals in China; however, Precambrian rare metal mineralization event is rarely reported or studied. The Jiangnan Orogen (JO) is a well-preserved Neoproterozoic orogenic belt in South China, and although it abounds with a huge amount of Neoproterozoic granitoids, lacks the equivalent amount granitic pegmatite. In this study, the geology, mineralogy, whole-rock composition, zircon U-Pb age and Hf isotope compositions of the Fanjingshan granite and surrounding rare metal pegmatites, and bulk composition of sedimentary wall rocks of the Taojinhe Formation in Guizhou province, western Jiangnan Orogen (JO), were studied to reveal the petrogenesis of the granite and pegmatite and the source relationship of the pegmatite with the Paleozoic and Mesozoic rare metal ore deposits in South China. The Fanjingshan granite and pegmatite both show Sn-Ta-Nb mineralization and have consistent zircon U-Pb ages of ca. 830 Ma, indicating the existence of the Neoproterozoic rare metal granite-pegmatite suite in the JO. The geology, mineralogy, whole-rock composition and zircon U-Pb age and Hf isotope study results demonstrate that the Fanjingshan rare metal pegmatite belongs to the Li-P-B-Sn-Ta-Nb granitic pegmatite in the Chemical composition-Mineral assemblage-Structural geology (CMS) classification (Dill, 2016), and has close spatial–temporal and source-differentiation connections with the Fanjingshan granite. The petrography and whole-rock composition studies revealed that the Taojinhe formation was formed by the deposition of mature and immature source materials from acidic igneous rocks in a continental arc setting. Comparison of the Hf isotope compositions of the Fangjingshan granite-pegmatite suite with those of the Fangjinshan Group and the high Li, Sn, Rb, W, Ta and Nb background concentrations in the Taojinhe Formation support the conclusion that the Taojinhe Formation was a contributing source to the granite-pegmatite suite. A comparative study of the Fanjingshan granite with synchronous S-type granitoids revealed that the granitoids in the eastern and western JO have distinct sources and degrees of differentiation, indicating the limited ore-forming potential of the Neoproterozoic granitoids in the eastern JO. The Fanjingshan rare-metal granite-pegmatite suite has consistent Hf isotope model ages of 1.6–2.3 Ga with the Paleozoic and Mesozoic Nb-Ta ± Sn-W ± Be-Li ore deposits in South China, indicating a common origin from the Paleoproterozoic crust and the source inheritance of the western JO from the Cathaysia Block.