Tourmaline geochemistry as genetic evolution and metallogenic potential indicators of pegmatites: a case study from the Tugeman Be-Li deposit, Middle Altun

Tourmaline is a common borosilicate mineral in granite and pegmatite, and it is also an important carrier to record the physicochemical evolution information of magmas. However, the genetic relationship between pegmatite-type rare metal mineralization potential and tourmaline-bearing pegmatites is not clear, and the tourmaline geochemical indication of the mineralization type of rare metals needs to be supplemented. In the middle Altun region, extensive tourmaline-bearing pegmatite dykes are prevalent within and surrounding the Tugeman complex. In this paper, the petrology and geochemical characteristics of tourmaline within the pegmatite of the Tugeman Be-Li deposit have been comprehensively analyzed. The research delves into the genetic evolution and metallogenic potential of the pegmatite, emphasizing the indicative role of tourmaline geochemistry in discerning mineralization types in the Tugeman Be-Li deposit. Major and trace element analysis of tourmaline reveals varying contents of FeO (9.37-17.10 wt%), Al₂O₃ (28.64-35.22% wt%), and MgO (0.13-2.47 wt%) in the Tugeman Be-Li pegmatite. Li, Zn, and Sn contents range from 204 ppm to 1590 ppm, 1143 ppm to 4089 ppm, and 74 ppm to 396 ppm, respectively. The tourmaline analyzed in this study is magmatic origin and belongs to schorl series, exhibits substitution relationships, including (Na, Fe²⁺)(^X□, Al)_-1, (Fe²⁺, Fe³⁺)(Mg, Al)_-1, (Al, ^X□)(R²⁺, Na)_-1, (Na, Mg)(^X□, Al)_-1 and ^X□NaR²⁺_-1. The trace element data of tourmaline show that lithium has four enrichment stages in different mineral crystallization phases. Notably, the tendency of tourmaline in the Tugeman Be-Li pegmatite to evolve towards elbaite implies a magmatic evolution sequence from low to high, transitioning through microcline-albite pegmatite, muscovite-microcline-albite pegmatite, albite pegmatite, and microcline pegmatite. Furthermore, we also propose that the Sc content in schorl can serve as an indicator of pegmatite mineralization type and the Tugeman Be-Li pegmatite has evolved to a beryllium-rich stage and has certain lithium enrichment potential.