Petrography and geochemistry of metavolcanic rock and related Intrusives and their alterations West of Butre, Ghana

Abstract

The metavolcanic rock and their related intrusives at Butre area in the Birimian Supergroup, southwest Ghana were studied to ascertain the mineralogical compositions, textures and metamorphism. The study showsthat the host rocks comprise of basalt and andesite whereas the intrusives are gabbroic diorite, diorite and granodiorite. The rocks are comagmatic emplaced in a subduction zone environment on a differentiation trend from tholeiitic basalt through gabbroic diorite, granodiorite, andesite and diorite. Intrusion of gabbroic diorite might have introduced arsenopyrite in the basalt whiles the diorite and granodiorite was accompanied by carbonate and pyrite. Amphibolite facies metamorphism was associated with garnet, epidote and magnetite whiles greenschist facies metamorphism was linked with epidote, chlorite, sericite, carbonate, pyrhotite and haematite. In general, all rock types are LREE depleted (La/Sm chondrite normalised ratios of basalt = 0.8 to 2.5; andesite = 2.03; diorite = 1.95 to 3.1; gabbroic diorite = 0.9 to 2.05; granodiorite = 1.66) with negative Eu and Ce anomalies. On the Alteration Index plot, the rocks show a general carbonatisation with sericite and chlorite alteration.