Lithium and Nd isotopic constraints on the origin of Li-poor pegmatite with implications for Li mineralization

Abstract

Although volumetrically minor in the upper continental crust, pegmatites host lots of rare-metal mineralization (e.g., Li, Be). Pegmatites and associated mineralization were generally thought to originate from extreme differentiation of a parent granite system. We propose an alternative model to interpret the genesis of pegmatite and factors controlling Li mineralization based on combined Nd and Li isotopic studies on the Li-poor pegmatites (barren) from the Qinghe pegmatite field of the Altai orogen (NW China). The Qinghe pegmatites show significantly varied Nd isotopic compositions (εNd(t) = −4.3 ~ −1.7), which overlap the surrounding schists (−6.5 ~ −1.5), but lower than the granites (−2 ~ +2.9) of the orogen. These data, together with the low REE (3.3–41 ppm) and absence of biotite in the pegmatites, suggest they were not derived from extreme differentiation of a parent granite as traditionally thought, but from low degrees of partial melting of the schists involving muscovite dehydration melting under amphibolite facies conditions. The pegmatites are characterized by heavy Li (δ7Li = 4.1–14.5‰) and low Li abundance (3.6–50 ppm), which contrast to that of local schist (δ7Li = +0.9 ~ +3.0‰, Li = 24–123 ppm) and granite (δ7Li = +0.9 ~ +3.0‰, Li = 24–70 ppm), and also to that of global Li-rich pegmatites (δ7Li = −1.0 ~ +10‰, Li >500 ppm). Modeling study indicates that Li-poor pegmatites always have heavy Li signature compared with Li-rich pegmatites due to the abundance of biotite in the source. The formation of Li-rich pegmatite with Li mineralization is attributable to the distinct source characteristics with abundant fluxing components like Li, Na, CO32−, and HCO3− and sufficient alumina-rich clay phases, and the fluxing components probably originated from evaporate interlayers in the source.