Geochemical and Petrogenetic Discrimination of Productive and Barren Pegmatites in the Kohistan Ladakh Island Arc (KLIA), Northern Pakistan

Abstract

Granitic pegmatites constitute major repositories of critical metals including Li, Be, Nb, Ta, Cs, and Sn, yet distinguishing fertile pegmatites from barren equivalents remains a fundamental challenge in complex orogenic and arc-related terranes. This study investigates the geochemical and petrogenetic characteristics of pegmatites within the Kohistan–Ladakh Island Arc (KLIA) of northern Pakistan, focusing on the Gilgit Formation and the Nyit-Bruk–Bisil district of the Shigar Valley. Integrated field observations, petrographic analysis, and whole-rock geochemical data obtained through X-ray fluorescence (XRF) and ICP-OES techniques are used to evaluate pegmatite fertility and magmatic evolution. Field relationships indicate structurally controlled emplacement of pegmatite bodies along shear zones, lithological contacts, and late-stage fractures within granitoid–metasedimentary host rocks. Petrographic evidence reveals internally zoned pegmatites with mineral assemblages dominated by quartz, feldspar, mica, and accessory rare-element phases. Major-element compositions indicate strongly peraluminous, highly fractionated melts, while trace-element data demonstrate significant enrichment of incompatible elements including Li, Rb, Cs, Nb, Ta, and Sn in spodumene-bearing pegmatites relative to barren granitoid-related bodies. Diagnostic elemental ratios such as Rb/Sr, K/Rb, Nb/Ta, and Cs/Rb effectively discriminate fertile pegmatites from barren populations, reflecting progressive magmatic differentiation and late-stage fluid–melt interaction. Bivariate trace-element relationships (e.g., Li–Rb and Ta–Be) further indicate advanced fractionation trends consistent with Li–Cs–Ta (LCT) pegmatite evolution. The geochemical signatures suggest that fertile pegmatites in the (KLIA) originated from highly evolved peraluminous melts derived from differentiated granitoid magmas and subsequently modified by volatile-rich magmatic fluids. These results demonstrate that pegmatite fertility within the (KLIA) is controlled by a combination of melt differentiation, structural focusing of residual melts, and proximity to evolved granitoid sources. The study establishes a robust geochemical exploration framework for identifying rare-metal pegmatites in arc-related tectonic settings and provides new insights into the metallogenic evolution of the Kohistan–Ladakh Island Arc.

Keywords: Granitic pegmatites; Rare metals; Geochemistry; Kohistan–Ladakh Island Arc; Gilgit Formation; Nyit-Bruk–Bisil;