Petrogenetic relationships between peraluminous granites and Li-Cs-Ta rich pegmatites in south Issia zone (Central-West of Cote d'Ivoire): Petrography, Mineralogy, Geochemistry and zircon U-Pb Geochronology

The Issia batholith extends over 100 km in central Ivory Coast and is surrounded by important placer deposits of columbite-tantalite associated with deeply altered lithium-cesium-tantalum pegmatites. The genetic link between the granitic complex and the mineralized lithium-cesium-tantalum pegmatites at the origin of the coltan placers is not straightforward, considering the significant age difference (similar to 40 Ma) between one of the granites and the coltan mineralization. In order to constrain the link between the granitic intrusions and the coltan placers, we have completed a petrological and geochemical study on eighteen granitic outcrops. Some granites are crosscut by pegmatite dykes, but neither the granites nor their intruding pegmatites dykes exibit primary Nb-Ta mineralization. Our results reveal three granite series (G1, G2, G3). The biotite-dominant G1 granites are the least fractionated, metaluminous to peraluminous, with trace element compositions close to average upper continental crust. The G2 and G3 granites are muscovite-rich, peraluminous with intermediate phosphorus contents up to 0.5 wt% P2O5. G2 and G3 have similar compositions except for lower Cs and Ta and higher Th, Hf and Zr in G2. G2 and G3 are strongly fractionated with up to 22 ppm Be, 57 ppm Cs, 505 ppm Rb, 16 ppm Sn, 11 ppm Ta, and a Nb/Ta ratio < 1. The G3 granite and its intruding pegmatite dykes contain abundant tourmaline, garnet and Nb-Ta-rich ilmenite. We conclude that the G2 and G3 granite groups are genetically linked through fractional crystallization. Three hypotheses can explain the formation of the Li-Cs-Ta-rich liquids: (1) A fractional crystallization model where the G2-G3-pegmatite series evolves by fractional crystallization of an anatectic melt produced by melting of metasediments, (2) an anatectic model where all granites and pegmatites were formed by direct anatexis, and their strong differentiation degree would reflect an enriched metasedimentary source, (3) a granitic origin where melting of the most fractionated G3 granite produced the Li-Cs-Ta-rich liquids similar to 40 Myrs after the granite cooling. Geochronological results and textural evidence of granite melting tend to favor the third model, which indicates that crust re-heating occurred similar to 40 Ma after the emplacement of the Issia granitic complex.