LITHIUM PEGMATITES IN THE HIMALAYAN CREST, 1: STRUCTURAL ENVIRONMENT, GEOCHEMISTRY, AND AGE

Lithium is a key strategic metal with important reserves in pegmatites. Here, we present field data, geochemistry, and geochronology for recently discovered lithium-rich pegmatites in the uppermost Greater Himalayan Sequence (GHS), southern China. These data characterize the formation of these rocks and form a foundation for a new model (presented separately) for creating and segregating these unusual melts from associated leucogranites.

Two main Li-rich pegmatite localities have been discovered ~50 km NW of Mount Qomolangma (Everest) at Pusi La and Ra Chu. Pegmatites at both localities intrude the normal-sense South Tibetan Detachment System (STDS) shear zone. At Ra Chu, Li-pegmatites occur in pervasively sheared Yellow Band Marble, <10 m below limestones of the Tethyan Himalayan Sequence and the brittle STDS (Qomolangma detachment). At Pusi La, Li-pegmatites additionally intrude ~500 m below the brittle STDS within sheared Neoproterozoic GHS schists and gneisses. Abundant sheared granite and leucogranite sills intrude the GHS from ~1 km below to <10 m below the Li-rich pegmatites.

Columbite and cassiterite U-Pb ages in the Li-rich pegmatites at Pusi La range from 23 to 25 ±1 Ma (2σ), comparable to intrusion ages of ~24 Ma for sheared granites and leucogranites in the uppermost GHS. Many trace elements are highly enriched in the Li-rich pegmatites (Li up to 13,000 ppm; Be and Sn up to 350 ppm; Rb up to 1700 ppm; Ta up to 80 ppm), while others are highly depleted (Ba <50 ppm; LREE < 5 ppm). Across proximal biotite and two-mica granites, and tourmaline and muscovite leucogranites, concentrations of these elements change only gradually (typically less than a factor of 2) but then shift abruptly for pegmatites (typically a factor of 10), creating compositional gaps. Spodumene and hydrothermal petalite are abundant in the pegmatites, but absent from other felsic bodies.

We interpret Li-rich pegmatites to be derived via fractionation from age-equivalent, parent, proximal granites and leucogranites. Intrusion into relatively cold rocks of the uppermost GHS and shearing within the STDS likely contributed to formation, segregation, and migration of Li-rich melts (see companion presentation). Similar characteristics along the c. 2200 km-long STDS suggest major lithium reserves may be widespread in the Himalaya.