Formation of Li-bearing pegmatites in the Arabian Shield: A case study from the Mount Ablah, Asir Terran, Saudi Arabia

There is currently no lithium mining activity in Saudi Arabia, although the Arabian Shield is rich in rocks suitable for economic concentrations of lithium, including pegmatites, greisen and rare metal granites, as well as ancient brines and salt deposits. The present work is the first to report lepidolite, the major lithium-bearing mineral, hosted by the Mount Ablah pegmatite, located north of the Ablah Graben, Asir Terrane. The pegmatitic rocks represent the dominant rock types in the Mount Ablah, where they are strongly brecciated along the margins and surrounded by alteration zone of sericitized rocks, and cross-cut by a number of quartz veins. They are also covered by a massive muscovite-fluorite body, here described as a greisen deposit. The studied pegmatites are medium- to coarse-grained and are of two types: lepidolite-bearing and lepidolite-absent pegmatites. The lepidolite-bearing pegmatite consists of K-feldspar, muscovite, quartz and lepidolite, whereas the lepidolite-absent pegmatite consists of K-feldspar and quartz. Fluorite, cassiterite, topaz, tourmaline and beryl are the main accessory minerals. The lepidolite forms crystals of anhedral to subhedral shapes and clusters and varies in colour from pink to purple. Geochemically, the Ablah pegmatite is characterized by low amounts of Mg, Ca, Ba and Sr, but high contents of alkaline elements, Sn (870–1205 ppm), Ta (34–61 ppm), Nb (351–469 ppm), Y (130–165 ppm), Rb (1916–2606 ppm), Cs (265–357 ppm), Li (4727–7865 ppm) and Be (309-597 ppm). These compositional features are typical of post-collisional, rare metal-bearing pegmatites of the mixed NYF-LCT family. All pegmatitic samples show positive Ta anomalies and distinct negative Eu anomalies (Eu/Eu* = 0.14–0.21). We suggest that the source magma of the pegmatitic rocks of Mount Ablah was mostly derived from the juvenile continental crust of the Arabian Shield through partial melting, probably during the evolution of the major Shear Zone of Umm Farwah. The anatectic crustal melts were subsequently subjected to a high degree of fractional crystallization and during the latter hydrothermal stage the exsolution of F-rich fluids transported some elements and locally increased their concentrations to economic grades.