Petrogenetic relationships between Paleoproterozoic granitoids and rare-element pegmatites from Dibilo (Liptako, West Niger, West African Craton)

Abstract

The rare-element pegmatites of Dibilo form intrusions in a tonalitic gneiss of the Paleoproterozoic Téra-Ayorou pluton about 4 km from the contact zone with the Diagorou-Darbani greenstone belt, in the Nigerien Liptako (NE portion of the Man Ridge of the West African Craton). The general objective of the present study is to determine the petrogenetic relationships between the granitoids and the Dibilo pegmatites through a multidisciplinary approach combining petrology, geochemistry, structural geology and metallogeny. Two generations of granitoids have been distinguished at Dibilo. A first generation of metaluminous to weakly peraluminous granitoids (tonalitic gneiss, metadiorite and two-mica granitic gneiss) constitute a calc-alkaline series. Trace element geochemical signatures suggest that these granitoids were emplaced in an island arc and subduction zone context. A second generation of granitoids (leucogranites), encompassing two-mica, muscovite and two-mica-garnet granites, are peraluminous and calc-alkaline. These leucogranites are fertile, i.e., rich in incompatible elements, with Li concentrations attaining 250 ppm in the richest. A model of partial melting shows that they could derive from the partial melting of Li-rich metadiorite, which is itself highly enriched in lithium (150 ppm). One sample of migmatite has Li contents of 300 ppm, which evidences the fertile nature of the tonalitic-dioritic-granitic gneiss complex.

Three types of pegmatite have been identified at Dibilo: sterile pegmatites (type I), peraluminous and poorly evolved; Nb-Ta pegmatites (type II-a) and Nb-Ta, Mo pegmatites (type II-b), moderately evolved, peraluminous, respectively mineralized with columbite-group minerals (type II-a) and columbite-group minerals and Mo-bearing sulfide (type II-b); and highly evolved, peraluminous Li and Nb-Ta pegmatites (type III) mineralized with spodumene and columbite-group minerals. The type I barren pegmatites share similarities with the leucogranites and could also originate from the partial melting of the dioritic gneiss. The highly evolved type II-a, type II-b and type III pegmatites correspond to the residual liquids resulting from the fractional crystallization of leucogranites. Therefore, we propose that the Dibilo rare-element pegmatites derive from both fractional crystallization and partial melting processes within the tonalitic-dioritic-granitic complex.