Geochemical Dispersion of Elements and Their Correlation with Gold in the Regolith at the Tetteh Prospect of the Chirano Gold Mines in the Sefwi Belt of the Birimian, Southwestern Ghana

The processes controlling gold (Au) mineralization in Chirano, a gold prospect in south-western Ghana have been investigated using a detailed geochemical dataset of 45 variables from 197 sampling locations. The data were subjected to advanced statistical analyses and bivariate graphical interpretations in the light of the underlying lithology. Four processes of significance have been identified from the multivariate analysis in terms of the geochemistry of the soil in the area. The most important process accounts for almost half of the total variance in the dataset and also correlates strongly with Au. Although Arsenic (As) registers the highest loading under this factor, Aluminium (Al), Silver (Ag), Manganese (Mn), Copper (Cu), and Iron (Fe) record higher correlations and could serve as better pathfinders than As in the study area. This study also shows that there is a second, less prominent process contributing to the availability of Au in the area. This process ranks third amongst the processes of significance in terms of the geochemistry, and correlates negatively with As, suggesting that the latter is not suitable as a pathfinder for Au in the traditional sense. However, since this third process is weaker compared to the first, the implication of this interpretation may be limited to only some locations in the study area. In addition to the revelations from the multivariate statistics, the regolith in the study area has evolved over the years and therefore the changes in regolith should be factored into the exploration protocols. The pre-existing preserved surfaces were found to be masked predominantly by transported materials consisting of ferruginized and exotic sediments. Within the surficial regolith and especially in the Fe-oxyhydroxide rich horizons, Au concentrations increase to significant values that are higher than those of the bedrock or saprolite thereby showing the significant Au re-distribution within the regolith. This study also highlights the apparent Au depletion in the mottled clay zones and in the saprolite and the enrichment of the metal in lateritic residuum and the topsoil. The high Au expressions in the upper parts of the regolith generally are transported whereas the residual soil anomalies have low Au concentrations and do not merit further exploration follow up. Keywords: Regolith, Gold mineralization, Pathfinder elements, Chirano mine, Sefwi belt, southwestern Ghana

Geochemical Characteristics of Granitoids (Ho Gneiss) from the Pan – African Dahomeyide Belt, Southeastern, Ghana: Implications for Petrogenesis and Tectonic Setting

The Pan-African Dahomeyide belt exposed in Southeastern, Ghana, consist of granitoid gneisses locally known as Ho gneiss. These rocks are thought to be part of the West African Craton (WAC) which was reworked during the Pan-African Orogeny, and may be time equivalent with the Kara gneisses. Petrographical and whole rock geochemical analyses have been used to evaluate the characteristics, petrogenesis and mode of emplacement of the granitoids. The new data reasonably suggest that the Ho gneiss consist mainly of biotite augen gneisses of both mafic and felsic rock suites. Geochemically, these rocks show tonalitic to quartz monzonite but mainly granodiorites affinities. They are metaluminous to weakly peraluminous, I-type, magnesian to ferroan and calcic to alkali calcic. With these varying features, the granitoids mimic mantle derived magma source which mixed or mingled with crustally derived melt. The rocks display varying REEs and trace elements patterns but, their LREEs and LILE enrichment with  the noticeable enrichment in Rb, Ba, K and especially Pb and negative Ti, Ta, Nb observed among majority of the samples are typical signature of ‘‘arc rocks’’ or continental crustal materials. Their incompatible trace element ratios, such as Th/U (1.07 – 13.87), K/Rb (272 – 574.47), Th/Yb (0.79 – 15.09), Ta/Yb (0.25 -0.64 ), Ce/Pb (mainly 1.62 – 7.88 ) and high Ba/Nb (19.55 – 314.17, with TA = 1565.00), are similar to those of the continental crust. The rocks are characterised by subduction related Sr/Y content (< 100), except sample MA8 that shows high concentration of Sr/Y (227.71). The magnesian affinity reflects relatively hydrous, oxidizing source which is consistent with origins that are broadly subduction related. Their high – K nature also points to an important petrogenetic role of remelting and differentiation of arc – accretionary complex crust. These geochemical signatures are likely to be related to metasomatism of the sub – continental lithospheric mantle via crustal recycling. In conclusion, the granitoids may have from melting of igneous source in a subduction related environment. Keywords: Pan-African belt, Dahomeyide, Southeastern Ghana, Ho gneisses, Geochemistry, petrogenesi

Pathfinder elements and indicator minerals of Au from the Kubi Gold ore deposits in Ghana

The Au mineralization in the Kubi Gold Mining Area in the Birimian of Ghana is associated with garnet (about 85 vol.%), magnetite, pyrrhotite, arsenopyrite, and sulfide minerals, as well as quartz with gold and calcite. These minerals and the included elements can act as indicator minerals or pathfinder elements. For the present work, we collected samples from drill holes at different depths, from the alluvial zone (0–45 m) to the ore zone (75–100 m). The distributions of minerals and elements in the rocks that act as indicator minerals and pathfinder elements in the concession area were investigated along the drill hole cross sections. X-ray diffraction shows that the samples contain garnet, pyrite, periclase, and quartz as the main indicator minerals. By energy-dispersive X-ray spectroscopy, Fe, Mg, Al, S, O, Mn, Na, Cu, Si, and K are identified as corresponding pathfinder elements. The results indicate that the Au mineralization in the Kubi Mine area correlates mostly with the occurrence of garnet, pyrite, goethite, and kaolinite in the host rocks, which show towards the surface increasingly hematitic and limonitic alteration in form of Fe(oxy-)hydroxides