Geochemistry of Volcano-sedimentary and Plutonic Formations of the Agbaou Gold Deposit, Ivory Coast

Abstract

Located in the south-west part of the Fettekro greenstone belt, Agbaou gold deposit is marked by three major lithological units: (i) a volcano-plutonic unit composed of basaltic to andesitic lavas, amphibolites, chlorite-schists and sills of microdiorite and microgabbro; (ii) a volcano-sedimentary unit containing pyroclastic lavas (basaltic and dacitic) and sediments (shale and grauwacke); (iii) the late felsic dikes (rhyolite and rhyodacite) probably contemporary with the formation of granitoids form the third unit. These host rocks are mostly intensely deformed and altered. Alteration phenomena were revealed by the high values in fire loss, the decreasing of silica contents, the sometimes high values of alkaline for rocks also basic, the constant depletion in LREE and LILE. The Eu and Nb negative anomalies reveal a crustal contamination of magmatic series. Basaltic lavas are volcanic arc tholeiites nearing N-MORB type; they are associated to a magmatogenesis of ocean floor. Their magmatic source would probably be spinel lherzolitic type. Andesites have a calc-alkaline composition and seem far link to active subduction margin. Geodynamics context would be that of an area where transcurrent faults of lithospheric extension generate heat corridors able of generating by fusion the andesitic calc-alkaline magma. This context would probably be the one that prevailed during the establishment of the gold mineralization. Pyroclastic rocks of dacitic composition as acid lavas (rhyolite and rhyodacite) have also evolved in this same geotectonic context. Plutonic rocks are located in arc-volcanic granites field, while metasediment are linked to active continental margin field.