Geological, Mineralogical and Geochemical Study of the Aquamarine-Bearing Yamrang Pegmatite, Eastern Nepal with Implications for Exploration Targeting

Abstract

The Yamrang Pegmatite in the Ikhabu Pegmatite Field, Eastern Nepal is Nepal’s primary source of aquamarine. This paper reports detailed mineralogy and whole rock granite and pegmatite geochemistry, and major and trace element data for alkali feldspar and muscovite in order to classify the aquamarine-bearing Yamrang Pegmatite, elucidate beryl-saturation processes and evaluate potential geochemical exploration tools for beryl-pegmatites. Five internal mineralogical/textural zones were identified in the Yamrang Pegmatite; zone 1 (saccharoidal albite); zone 2 (blocky perthitic microcline); zone 3 (muscovite–microcline–quartz); zone 4 (beryl-quartz), and zone 5 (miarolitic cavities). Zones 1–4 represent the magmatic stage, while zone 5 formed during the hydrothermal stage of pegmatite genesis. Spectacular aquamarines are recovered from miarolitic zone 5, while beryl saturation is found in zones 3, 4, and 5. Based on beryllium (Be) content, Be partition among co-existing minerals at the magmatic stage is beryl > muscovite > tourmaline > alkali feldspar > quartz. In contrast, the sequence at the hydrothermal stage is beryl > muscovite > albite > tourmaline > quartz. The Be content in rock-forming minerals decreases from pegmatite margin to core, and tourmaline could have played a significant role in Be enrichment processes in the marginal pegmatite zone. High temperature, a low degree of fractionation, and the dominance of Be-compatible mineral phases such as muscovite, calcium-rich alkali feldspar and tourmaline resulted in beryl undersaturation in marginal zones. However, low temperature, high fractional crystallization, and low abundance of Be-compatible mineral phases resulted in beryl saturation in inner zones. The strongly peraluminous nature, low total REE content (<500 ppm), mineral assemblage of beryl, tourmaline, spessartine, columbite-tantalite, depletion of Ba, Nb, and enrichment of Pb, Rb, Cs in the primitive mantle normalized multi-element plots suggest that the beryl-bearing Yamrang Pegmatite corresponds to the LCT pegmatite family. Alkali feldspar with K/Rb values of 30–150, Rb ~3000 ppm, Cs >100 ppm, and muscovite, with K/Rb ranging 18–50, Rb ~6000 ppm, Cs > 500 ppm, and Ta > 65 ppm in inner zones (3–5), indicate that the Yamrang Pegmatite is an intermediate-fractionated, beryl-type rare-element (REL) pegmatite. It is probable that whole rock Be content of >10 ppm could be considered an exploration guide to beryl mineralization in the region.