In-situ boron isotopic and geochemical compositions of tourmaline from the Shangbao Nb–Ta bearing monzogranite, Nanling Range: Implication for magmatic-hydrothermal evolution of Nb and Ta

Abstract

The enrichment mechanism of tantalum and niobium are still in dispute due to complex textural features of Nb–Ta minerals. Tourmaline is an accurate passive geochemical monitor related to the W-Sn-Nb-Ta-Li mineralization, which can record Nb and Ta variations in melt/fluid. The newly discovered Shangbao Nb–Ta bearing monzogranite is located in Nanling Range, South China, which is characterized by tourmaline wide occurrence. In this paper, we systematically investigate the different types of tourmaline using in-situ major, trace elemental and boron isotope for studying the origin of tourmaline, boron source, and Nb–Ta enrichment process. Three types of tourmaline have been identified, namely, disseminated tourmaline (TD type) in monzogranite, nodular tourmaline (TN type), consisting of the yellow core (TNy type) and blue rim (TNb type), and veined tourmaline (TV type). Almost all tourmalines from the Shangbao Nb–Ta deposit exhibit very low Ca content, X-site occupancy and high ratio of Fe / (Fe + Mg), pertaining to the group of schorl and alkali. Trace elements primarily present the low concentrations of <10 ppm, while the contents of Sc, Li and Zn are higher.Based on the petrography and Al occupations in the Y-site of tourmaline, TD type tourmaline was precipitated straightly from the melt at the late magmatic stage, while TN type tourmaline crystallized during the magmatic-hydrothermal transition process, and TV type tourmaline was the origin of hydrothermal fluid. In-situ boron isotope analyses of TD and TN type tourmaline by MC-ICP-MS show the narrow δ11B values from −17.16‰ to −14.81‰, demonstrating the same boron source. TV type tourmaline shows the weak higher δ11B values of −14.48‰ to −13.46‰ likely resulted by Rayleigh fractionation. The concentration of Nb in TD and TV type tourmaline can reach up to 20 ppm, which is higher than that in TN type tourmaline. However, TN type tourmaline contains higher Ta content than other types, revealing magmatic-hydrothermal transitional stage is critical to Ta enrichment. Compared with tourmaline in other type deposits, tourmaline in Shangbao monzogranite contains high Nb and Ta contents, indicating that it is a suitable tool for exploration, and the tourmaline is available to targeting Nb and Ta mineralization.