Li-Be-Nb-Ta mineralogy of the Kuqu leucogranite and pegmatite in the Eastern Himalaya, Tibet, and its implication

The rare -element minerals record important information of rock -forming process and metallogenesis. Himalaya is a famous leucogranite belt. The Kuqu intrusion is located in the Tethys Himalayan sedimentary sequence in the Eastern Himalaya. This study investigated two -mica granite, muscovite granite, tourmaline granite and granitic pegmatites in the Kuqu intrusion. The granitic pegmatites refer to either the pegmatite rocks that are included in and related to granite or the pegmatite zones that make up a single pegmatite dyke. The rare -element minerals identified in the Kuqu intrusion include spodumene, cookeite, beryl, columbite-tantalite, Y-hatchettolite and microlite. They are distributed in quartz -feldspar-muscovite pegmatite, quartz-albite-muscovite pegmatite, blocky feldspar-aplite, block feldspar-tourmaline aplite, spodumene-blocky feldspar-aplite, pegmatite rock in muscovite granite, and tourmaline granite. The EMPA and LA-ICP-MS analytical results indicate that spodumenes show four occurrences which consists of coarse -grained subhedral to euhedral spodumene, fine-grained symplectitic reaction border composed of spodumene and quartz, middle to fine-grained mosaic structure of spodumene, K -feldspar, albite and muscovite, and coarse -grained spodumene filled by cookeite veins. These occurrences of spodumene reveal a complex and fluid -rich unstable crystallization environment. There are homogeneous and heterogeneous beryl crystals in the intrusion. The internal zonation patterns for heterogeneous beryls are alteration border, patches and complex irregular zoning in BSE images. The substitution mechanisms of beryl in the Kuqu intrusion are channel -octahedral, channel -tetrahedral and alkali cations in channel. The columbite-group mineral crystals are composed of primary part and alteration border or display irregular zoning pattern, some of which are altered by Y-hatchettolite and microlite. Compared with primary parts, the alteration border and irregular zoned crystals have higher Ta/ ( Nb + Ta) values and lower Mn/ ( Fe + Mn) values, indicating fractional crystallization, supersaturation caused by strong undercooling, and fluid activities. Based on the studies of rare -element minerals, the single pegmatite dyke ( quartz -feldspar-muscovite pegmatite), pegmatite rock in muscovite granite and tourmaline granite display differences on the degree of fractionation of magma, evolution processes and fluid activities and they might be magma products of different stages. The Rb and Zn contents of beryl, and Sc20 Si02 and Pb0 contents of columbite-group minerals show correlation with the existed indicators for evolution degree of magma, but more studies are needed to carry out for them to be possible indicators. Considering the spodumene-bearing pegmatite indentified in the intrusion, the high degree of magma evolution, multistage of magmatism, complex fluid activities and being in a high lithium abundance value area, the Kuqu intrusion is an important potential area for future lithium prospecting in the Eastern Himalaya.

Automated summary

The study investigated the rare-element minerals found in the Kuqu intrusion, including spodumene, cookeite, beryl, columbite-tantalite, Y-hatchettolite, and microlite. These minerals are distributed in various types of pegmatites within the intrusion, indicating a complex and fluid-rich unstable crystallization environment. The presence of these minerals suggests a potential for lithium prospecting in the Eastern Himalaya.