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 (~ 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 ~ 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 ~ 40 Ma after the emplacement of the Issia granitic complex.

Issia 岩基在科特迪瓦中部延伸超过 100 公里，周围环绕着重要的铌钽矿砂矿床，与深度蚀变的锂-铯-钽伟晶岩有关。考虑到其中一个花岗岩与钶钽铁矿化之间的显着年龄差异（~40 Ma），花岗岩杂岩与在钶钽铁矿起源的矿化锂-铯-钽伟晶岩之间的成因联系并不简单。为了限制花岗岩侵入体和钶钽铁矿砂矿之间的联系，我们完成了对 18 个花岗岩露头的岩石学和地球化学研究。一些花岗岩被伟晶岩岩脉横切，但花岗岩及其侵入的伟晶岩岩脉均未表现出原生 Nb-Ta 矿化。我们的结果揭示了三个花岗岩系列（G1、G2、G3）。以黑云母为主的 G1 花岗岩分馏最少，呈金属铝质至过铝质，微量元素组成接近平均上大陆地壳。G2 和 G3 花岗岩富含白云母，过铝质，中间磷含量高达 0.5 wt% P₂ O ₅. G2 和 G3 具有相似的成分，但 G2 中的 Cs 和 Ta 较低，Th、Hf 和 Zr 较高。G2 和 G3 被强烈分馏，高达 22 ppm Be、57 ppm Cs、505 ppm Rb、16 ppm Sn、11 ppm Ta，并且 Nb/Ta 比 < 1。G3 花岗岩及其侵入的伟晶岩岩脉含有丰富的电气石，石榴石和富含 Nb-Ta 的钛铁矿。我们得出结论，G2 和 G3 花岗岩组通过分级结晶在遗传上联系在一起。三个假设可以解释富含 Li-Cs-Ta 的液体的形成：（1）一个分级结晶模型，其中 G2-G3-伟晶岩系列通过变质沉积物熔融产生的深熔熔体的分级结晶演化，（2）所有花岗岩和伟晶岩都是通过直接深熔形成的深熔模型，并且它们的强分异程度反映了富集的变沉积物源，（3）花岗岩成因，其中最分馏的 G3 花岗岩在花岗岩冷却后产生了富含 Li-Cs-Ta 的液体~ 40 Myrs。花岗岩熔融的年代学结果和构造证据倾向于支持第三种模型，这表明地壳再加热发生在 Issia 花岗岩杂岩就位后约 40 Ma。