Neoproterozoic Nb-Ta-W-Sn bearing tourmaline leucogranite in the western part of Jiangnan Orogen: Implications for episodic mineralization in South China

Abstract

There are four episodes (Neoproterozoic, early Paleozoic, early and late Mesozoic) of Sn-W-Nb-Ta mineralization in South China, accounting for more than 50% W and 20% Sn reserves of the world and hosting considerable Nb and Ta resources. The genesis for such large-scale mineralization that may have seen multiple phases of recycling of ore elements is not well unterstood. The study of the earliest mineralized granites in South China, however, may be helpful to better understand the mechanism of mineralization. We present detailed mineralogical, geochemical, and isotopic data from two typical Neoproterozoic, mineralized intrusions, i.e., the Fanjingshan and Yuanbaoshan tourmaline leucogranites. There is a characteritstic mineral succession of diverse magmatic (columbite group minerals [CGMs], Nb-Ta-bearing cassiterite) and hydrothermal (wolframite, wolframoixiolite, Nb-Ta-poor cassiterite) Sn-W-Nb-Ta oxides. Hydrothermal cassiterite and W-bearing oxides (wolframite, wolframoixiolite) in the Yuanbaoshan intrusion were deposited separately by two types, i.e., Sn-rich and Nb-W-rich fluids. In-situ U-Pb dating of zircon, columbite and wolframoixiolite yields ages of 830 ± 5, 832 ± 5 and 821 ± 5 Ma, respectively. Whole-rock εNd(t) values of −2.5 to −6.4 correspond to crustal Nd model ages of 1.6–1.9 Ga. Paleoproterozoic model ages (~1.6–2.2 Ga) for most episodes of W-Nb-Ta mineralization in South China indicate that (i) Paleoproterozoic or older sediments may have been involved in the formation of W-Nb-Ta bearing granites and (ii) contributions of juvenile material was not important for these W-Nb-Ta mineralization.Neoproterozoic peraluminous granites along the Jiangnan Orogen formed in post-collision environments, but only those in the western part show Sn (-W-Nb-Ta) mineralization. The mineralized Neoproterozoic granites of the western part have experienced more extensive fractional crystallization (lower Nb/Ta, Zr/Hf ratios and more depleted in Fe, Mg, Ca, Sr, Ba, Eu, Ti) and are richer in volatiles (H2O, B) than the barren peraluminous intrusions of the eastern part. As similarly evolved Mesozoic metaluminous granites of the Nanling area have considerably more important tin mineralization than the mineralized Neoproterozoic granites, fractionation is not the only factor controlling mineralization. Instead, other factors, such as protolith and conditions of partial melting may account for the more important Mesozoic tin mineralization.