Compositional characteristics of mineralised and unmineralised gneisses and schist around the Abansuoso area, southwestern Ghana

Abstract

Gold-bearing granitoid deposits have recently been discovered in the Birimian of Ghana but their mode of formation and ore genesis remain enigmatic. This study presents petrographic, and geochemical characteristics of mineralised and unmineralised (gold grade >0.05 and <0.05 g/t respectively) granitoids (now gneisses) and schists (metasedimentary) to evaluate their petrogenesis/provenance, and relationship to gold mineralisation in the Abansuoso area. The unmineralised rocks comprise biotite-and hornblende-biotite gneisses, sericite-quartz, carbonate-sericite, and biotite-quartz schist. The mineralised varieties are biotite-, muscovite gneiss, iron-carbonate-sericite, carbonate-sericite-quartz, chloritecarbonate, and biotite-carbonate schist. The mineralised and unmineralised gneisses are both metaluminous and peraluminous. Both mineralised and unmineraised gneiss and schist show Nb-Ta trough, depleted LILE and enriched HFSE although widespread overall trace element concentrations for the mineralised rocks on UCC-normalised multi-element diagram, suggestive of their formation in an arc setting. This suggests coeval granitic plutonism and sedimentation with subduction-accretion during the Eburnean orogeny, hence, mineralisation may be orogenic-type.

Application of a hybrid BWM-TOPSIS approach for mineral potential mapping

“…It was observed that, regions within the metavolcanic and the volcaniclastic sediment dominated terrane, with intense lineation (due to the existence of fracture and shear zones along the north-central portion of the area) were delineated to be of moderate to high potential. This corroborates with literature as gold occurrence in Abansuoso Area is mostly observed along strongly sheared zones [ 3 ]. The MPM developed was further discretised into three classes using the natural breaking classification method into low, moderate and high prospectivity classes as shown in Fig.…”

“…The mineral influencing factors derived from the radiometric data generally characterise the radioelement (K, eTh and eU) concentration distribution over a region of interest [ 8 ]. The radiometric ratio maps (K/eTh, eU/eTh) particularly are undoubtedly significant in outlining mineralised hydrothermal zones as well as alteration zones that are relevant towards gold mineralisation occurrence within the Abansuoso Area and its related geological terranes [ 3 , 12 ]. Thus, four radiometric-sourced layers comprising K ( Fig.…”

“…With a brittle component, the shear zone is ductile-sinistral. Several phases of metamorphism and deformation have caused the sheared biotite gneiss of intermediate composition and the variable composition schist to host the mineralisation [ 3 ]. Akin to other gold deposit of Ghana, arsenopyrite and, in some samples, a minor amount of pyrite constitutes the majority of the main ore minerals; both are found in the mineralised zones.…”

“…Sericite alteration associated with fine-grained dolomite and pyrite distinguishes the principal mineralised zones. It has been determined that the preliminary alteration sequence are chlorite-carbonate-hematite-sericite-quartz/pyrite with minor amount of albite [ 3 ]. Fracture planes and traces of hematite alteration transition into sericite alteration within brecciated fragments of mildly weathered schist define the mineralised zone.…”

“…The greater percentage of Ghana's mineral resources is gold, from which the country obtained its previous name i.e., Gold Coast. Gold occurrences, as well as mining operations in Ghana, are predominantly concentrated within the Paleoproterozoic Birimian greenstone belts, approximately 10 km–15 km wide, separated by deep sedimentary basins [ [1] , [2] , [3] ]. It is noteworthy that, gold occurrence in Ghana is largely structurally controlled within the Paleoproterozoic Birimian greenstone belts [ 4 ].…”