Geochemical characteristic and B isotopic compositions of tourmaline from the Chaqiabeishan Li-rich pegmatite in the Quanji Massif, NW China

Abstract

The Chaqiabeishan Li-rich pegmatite deposit is located in Tianjun County, Qinghai Province. Due to the complex magmatic evolution, there are still numerous controversies surrounding the diagenetic and metallogenic mechanisms of pegmatites. In this study, we investigated tourmalines from pegmatites in Chaqiabeishan by means of analyzing the in situ major element compositions, trace element contents, and boron isotopes. The collected samples can be classified into two types: pegmatitic tourmaline (Tur 1) and fine tourmaline (Tur 2). Based on the observation of geological characteristics, specimens, and microphotographs, the tourmalines have a basically paragenetic relationship with the predominant lithium-bearing mineral (spodumene), tourmalines and spodumenes would crystallize at a late magmatic stage of the pegmatitic sequence. Both tourmalines have similar geochemical characteristics, and Tur 1 crystallized earlier than Tur 2. The major elements show that both tourmalines belong to the alkali group, schorl series, and Li-poor pegmatite. Additionally, they have ∑(Mg + Fe) values less than 3 apfu, and redundant Al (0.04–0.89 apfu, average of 0.61) in the Y-site occupancy. The trace elements show that both tourmalines are characterized by enrichment in LREE relative to the MREE, with a slight negative Eu anomaly, indicating that they are of magmatic origin. The high Li/Sr ratios ranging from 38.56 to 6292.28 (average of 701.01 ppm) further support the magmatic origin. The δ11B values of all tourmalines range from −13.86 to −12.46 ‰, with an average of −13.14 ‰, indicating that the boron in the pegmatites was derived from S-type granitic magma or Dakendaban Group mica schist basement. The δ11B values of Tur 1 (average of −13.13 ‰), the rim of Tur 1 (average of −12.95 ‰), the core of Tur 1 (average of −13.25 ‰), and Tur 2 (average of −13.19 ‰) show a slight variation, indicating that the tourmalines have a consistent boron source and basically no foreign B-rich fluids involved. In conclusion, the petrological and geochemical characteristics suggest that the pegmatites in Caqiabeishan are likely the direct products of anatexis, and the tourmalines and spodumenes were formed in secondary faults at the late magmatic stage.