Petrography and Geochemistry of some Granitoids associated with Gold Mineralisation at Mpohor area in southeastern Ashanti Belt of the Birimian, Ghana

Abstract

Mpohor area in Western Region of Ghana is located within the Birimian at the southeastern corridor of Ashanti belt. Diamond drilling in the area which encloses Father Brown concession intersected steeply dipping igneous rocks on which petrographic and whole rock XRF analyses were conducted to describe the rock types according to mineralogy, texture, alteration and mode of mineralisation. The study showed that gabbro and diorite are weakly deformed, partially altered and of greenschist facies metamorphism. These rocks intruded into belt granitoids which accompanied metavolcanic rocks (well exposed at a coastal region to the southeast). Subsequent tonalite and granodiorite intrusives into sheared areas of gabbro are deformed with foliation being parallel to the major foliation in the host rock. This ductile deformation was accompanied by brittle deformation and infiltrated by hydrothermal fluids leading to silicification, carbonatisation, sericitisation, pyritisation of second and third generations and gold mineralisation. On the contrary, chloritisation with fine chalcopyrite together with pyrite of first generation characterise host gabbro and diorite. Hence deformed pyrite porphyroblast of second generation is found in tonalite and partly in contact with magnetite and also associated with gold. The ore zone (Au grade between 1.78 to 48.80 g/t) in tonalite is characterised with lower TiO₂ content (<0.73 wt %) in addition to Fe/Mg ratio above 2.4 and Zr values less than 100 ppm which may suggest that mineralised fluid was depleted of Ti and Zr and Mg/Fe ratio above 2.4. These probably show that hydrothermal fluid which has the potential for high sulphide-Au mineralisation accompanied more mafic host rocks.

Keywords: Petrography, Geochemistry, Granitoids, Gold Mineralisation, Southeastern Ashanti belt