Feldspar traces mineralization processes in the Qongjiagang giant lithium ore district, Himalaya, Tibet

Abstract

Recently, a large spodumene pegmatite deposit was discovered in the Higher Himalayan orogenic zone near Qongjiagang. The granites exposed on about 1500 vertical meters of the outcrop (with elevation varying from 4077 m to 5533 m) show an evolution from two-mica granite to tourmaline muscovite granite and tourmaline albite granite. Various types of pegmatites are dispersed throughout the granite, creating an opportunity to examine the evolutionary relationship between these rock types and their potential as ore deposits rich in lithium. This study collected feldspar samples from various granites and pegmatites along a vertical cross-section, and analyzed their structures using various techniques including Raman spectroscopy, and transmission electron microscopy. Elemental and isotopic compositions were determined using electron microprobe and laser ablation inductively coupled plasma mass spectrometry, providing valuable mineralogical information on the formation of widespread lithium deposits in the Qongjiagang area. The observed variations in feldspar composition suggest that changes in granite lithology and ore-bearing potential of pegmatites with increasing altitude result from upward fractional crystallization of magma formed by partial melting of Himalayan crystalline basement. This process is likely driven by decreasing temperature, resulting in a significant decrease in concentrations of alkaline earth metals (e.g., Ca, Sr, Ba), and gradual enrichment of trace elements including Rb, Cs, and Li. As a result, pegmatites attain a melt environment similar to that of the adjacent granite at similar elevations. Structural features of feldspar suggest that the decrease in Li concentration in K-feldspar and the formation of lithium-rich zonation of pegmatites are a result of simultaneous growth of spodumene and blocky orthoclase during the magmatic stage, as well as deuteric fluid extraction by syntectonic processes during the transition to the stable stage of microcline feldspar.