Tourmaline as an indicator for pegmatite evolution and exploration: A case study from the Chakabeishan deposit, northeastern Tibetan Plateau

Abstract

To assess the potential of tourmaline as an indicator for pegmatite evolution and exploration, we measured and compared the geochemical compositions in tourmalines of the barren, Be-rich and Li-rich pegmatites from the Chakabeishan (CKBS) deposit, eastern North Qaidam Tectonic Belt, northeastern Tibetan Plateau. Two types of tourmalines from the barren pegmatites are identified: optically homogeneous fine-grain tourmalines (H-Tur) and optically zoned coarse-grain tourmalines (OZ-Tur). Both H-Tur and OZ-Tur are of magmatic origin, showing high Na/(Na + Ca) (0.961–0.990), YAl (0.389–0.757 apfu) and low Mg/(Fe + Mg) (0.085–0.230). The core-to-rim δ11B increase within H-Tur and OZ-Tur, together with the enrichment of 11B from H-Tur (crystallized earlier) to OZ-Tur, indicates that fluids were unsaturated in the evolution of barren pegmatites. Compared to H-Tur and OZ-Tur, tourmalines from the Be- and Li-rich pegmatites show higher Nb/Sc (>3:4) and lower Mn/Zn (<1:1), Mn/Li (<3:1), and Sc/Li (<1:500), which are useful indirect criteria of distinction between the fertile and the barren pegmatites. The reliable tourmaline geochemical criteria for further distinguishing the Li-rich from the Be-rich pegmatites are Pb/Sr < 0.11, Nb/Sr < 0.15, and Pb/Co < 4. A compilation suggests that diagrams of (V + Sr)-(Li + Zn), Li-Zn/Sr, Sn/10Nb-Li-100Nb, and Zn/Sn-Li/10Sc-5Sc could discriminate tourmalines of granite-related pegmatite-type Li deposits from those of other deposits and rocks, showing implications for determining prospective areas for further Li-rich pegmatite exploration by using tourmalines recovered from surface sediments.