Dating multiple fractionation trends in pegmatites – Implications for the genesis of a large lithium pegmatite province in West-Africa

Abstract

The race for lithium supply for the green energy transition has brought Li pegmatites, the primary source of Li in the world, to the forefront. Their origins and mechanisms of formation are still debated, and geochemical tools to distinguish granite-related from anatectic pegmatites are still needed. In the current study, we combine two classical approaches, i.e., alkali fractionation in micas coupled to Nb-Ta-oxide geochronology, to date multiple fractionation trends in pegmatites from a large lithium pegmatite province in Ghana. We investigated distinct fractionation trends in the different pegmatites and we timeframed these trends by dating columbite-group minerals associated with primary magmatic micas. Distinct Rayleigh-type Rb/Cs fractionation paths were modeled using different starting Rb and Cs melt compositions that can be attained by direct partial melting of the meta-sedimentary country rocks. Columbite-group mineral dating reveals that each of these fractionation paths occurred at different ages spanning ca. 140 Myr. Moreover, our data shows that the oldest pegmatites correspond to poorly fractionated, Li-rich, Nb-Cs-Ta-Sn-poor pegmatites, whereas the youngest are the most fractionated, Cs-Ta-rich pegmatites (up to levels of the major Lithium-Cesium-Tantalum pegmatites worldwide). The proposed model of formation for this large Li-bearing province involves at least three distinct stages of partial melting and melt extraction at ca. 2170, 2080 and 2030 Ma. The produced melt batches evolved independently through fractional crystallization, the spodumene-bearing pegmatites being the least fractionated, and the “true LCT” pegmatites the most fractionated.