Chemical and boron isotopic compositions of tourmalines from the Lianyunshan Nb-Ta pegmatite in northeastern Hunan, China: Insights into fluid and metallogenic sources

Abstract

Although tourmaline is common in pegmatite-related Nb-Ta deposits and metasedimentary cover rocks in the Lianyunshan Complex of northeastern Hunan Province, South China, its fluid sources and genetic relationship to the host pegmatite remain unresolved. In this contribution, we chose the less-studied tourmaline minerals and analyzed their chemical (major, trace, and rare earth elements) and boron isotopic compositions to shed light on the process of pegmatite-related Nb-Ta mineralization. Lianyunshan tourmalines are both metasedimentary-hosted (Tourmalines I and II) and pegmatite-hosted (Tourmaline III), belong to the alkali group, have mainly schorlitic compositions, and contain dominantly hydroxyl anionic species. Element substitution is controlled by the activities MgFe-1, alkali-deficiency vector R1Al[Na(FeMg)]-1, AlO[R(OH)]-1, and □Al(NaR)-1. Lianyunshan tourmalines define a trend consistent with foitite to magnesio-foitite compositions. Lianyunshan tourmalines are poor in Fe3+, with a Fe3+/Fe2+ ratio of zero in all samples. This feature, as well as their non-alignment with the FeAl-1 vector, suggests precipitation from a reduced fluid. Fluid salinity may have been moderate to low given the low Na contents of all samples (0.60–0.66 apfu). Tourmaline I is rich in Mg and Ti, but the contents of these elements decrease in Tourmalines II and III. Fluid fractionation may have played a role in the decreasing Mg and Ti contents with corresponding enrichment in the contents of Fe, Na and X□ as Tourmalines II and III precipitated. Unusual enrichment of Li is found in the pegmatite-hosted Tourmaline III (mean 97 ppm) relative to Tourmalines I and II (means ∼ 31–33 ppm), and of Sn in Tourmaline II (mean 291 ppm) relative to Tourmalines I and III (means ∼ 3–10 ppm). δ11B values are similar for Tourmalines I (–14.21 to –12.09 ‰), II (–15.07 to –13.71 ‰), and III (–14.48 to –13.55 ‰), being consistent with the δ11B of typical S-type granites (∼−16 to −8 ‰) such as the S-type muscovite monzogranite host of the Lianyunshan tourmalines. We conclude that the Lianyunshan tourmalines precipitated from an evolved melt that formed muscovite monzogranite.

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