Petrography and geochemistry of Akyem Mine gold deposit, Ashanti belt-Ghana

Abstract

Petrographic and geochemical investigations on rocks that host the Akyem Mine gold deposit situated in the north eastern flank of the Ashanti Belt, Ghana were used to determine the lithologies and textures within the host rocks and ore zone. The work also studied the styles of alteration and mineralisation and their relationship. The rocks generally are fine to medium grained, weakly to strongly foliated with mineral compositions dominantly made up of plagioclase, amphibole, chlorite and quartz with moderate sericite, epidote, dolomite, calcite. Ore minerals such as pyrite, pyrrhotite, magnetite, arsenopyrite, gold and chalcopyrite occur in the deposit. Appropriate rock names could be amphibole-chlorite schists and quartz-chlorite schist. The primary minerals are strongly altered leaving relict bytownite, therefore the rocks are subjected to geochemical classification. Geochemical data in wt % SiO2 (39-62.5, average 54.19), Al2O3 (9.44-19.4), Na2O (0.51- 8.64), total FeO (4.37-14.2), CaO (0.48-10.6), MnO (0.07-0.22), MgO (1.13-6.31), and TiO2 (0.39-1.33). The values were used in discrimination diagrams to deduce protoliths of andesite to sub-alkaline basalts. The altered rocks in the Akyem Mine deposit exhibit varying intensity of silicification, sulphidation, sericitisation, carbonatisation and albitisation, so, were categorised based on the extent of alteration. Hence, weakly altered rocks contain alteration package of dolomite-calcite-chlorite-sericite with gold up to 0.98 g/t. Moderate to highly altered rocks are characterised by silica-albite-calcite-epidote with gold grade >0.98 g/t. Two generations of gold, pyrite and arsenopyrite occur respectively. Both generations of gold are more closely associated with fine anhedral pyrite 2 and arsenopyrite 1 and 2 respectively. Gold 1 is fine grained and may also occur along sheared quartz veins whilst Au 2 is medium grained and usually overprint these veins.