A genetic correlation between the granitic rocks and the Issia columbite-tantalite alluvial deposits, center-western Ivory-Coast, Africa.


Jacques Naho, Koffi Pothin, Jacques Carignan, Robert Yobou, Huan-Zang Lu, Jayanta Guha,

The only known production of columbite-tantalite in Ivory-Coast is the alluvium mining of the Issia deposit located in the center-west part of the country. Although a direct link to the granitic rocks were suspected, no attempt had been made to understand the genesis of the primary mineralization with respect to the evolution of the granites and associated pegmatites. These granites, which intrude the lower Proterozoic metasedimentary rocks, comprise the two mica oriented granite (Lobo), the oriented muscovite granite (Bitapia) and the non-oriented porphyritic muscovite granite (Issia s.s.). The results from this study indicate that Nb-Ta-Be-Li bearing pegmatites are related to the Issia s. s. granite. Field observations and bulk composition analysis of the granites show that: (1) the three major granites of the area are S-type and peraluminous in composition, (2) they were formed during late to post Eburnean orogeny, and (3) the Issia s.s. granite was the last to be emplaced. The pegmatites linked to the Issia s.s. granite have been classified into four types: A - Barren pegmatites; B - Be pegmatites; C- zoned Be-Nb-Ta pegmatites; and D - zoned Be-Nb-Ta-Li pegmatites. The zoned pegmatites can further be classified into: I - Aplite zone; II - Graphic zone; III - albite, microcline, beryl and columbite-tantalite zone; IV - Massive microcline zone; and V - Quartz core zone. These pegmatites have an external metasomatic alteration zone. The mineralization of Ta, Nb, and Be is related to zone III of Types C and D pegmatites, and Li is associated with greisens wich can be seen anywhere in the Type D pegmatites. The field and laboratory data also indicate that the Issia s.s. granite is the parental rock of rare element pegmatites of the area. The mineralised pegmatites are distributed only around the Issia s.s. granite and the REE patterns of the granite and those of the rare element pegmatites are very similar. The bulk composition analysis show a genetic link between Issia s.s. granite and the rare element pegmatites. It is suggested that the pegmatites were formed during the transport and fractional crystallisation of the magma, the rare elements remained in the residual liquid until the conditions favoured their precipitation. The establishment of this genetic link open up possibilities for new exploration strategies.
Mots Clés: Pegmatites, Columbite-tantalite, Peraluminous, Ivory-Coast, Granitoid, S-type, Granite, Issia