Laterite is a soil type rich in iron and aluminium, which forms mostly in tropical areas and is usually of rusty-red coloration as a result of high iron oxide content (Widdowson, 2009). Based on the source of the material from which they are formed, laterites can be categorized as lateritic duricrust (residual laterites) and transported (detrital) laterites, since some form by the in-situ cementation of weathered products while others form via the cementation of weathering products from different sources (Anand et al., 2001). Laterites result from intense and prolonged weathering of the underlying bedrock. That is, in climates of a tropical nature where there is severe in-situ weathering of rocks, these products of weathering that are rich in iron form (Widdowson, 2009). Thus, laterites can be said to be a relevant group of iron and aluminium rich products of weathering together with other products such as paleosols and ferricretes (Sunkari et al., 2019). Laterites have a vertical extent up to 20 m and a horizontal extent that is laterally vast, thereby forming blankets or layers of few to several thousands of square kilometers, which are relatively resistant to weathering (Widdowson, 2009).
Laterites are becoming a focus in tropical regions during mineral exploration programs, in many parts of the world because they serve as a suitable sampling medium for discovering hidden or concealed gold in laterite-capped tropical terranes. The advances in laterite geochemistry have resulted in critical approaches for the interpretation of anomalies in laterite samples (Anand, 2001). Since it was used by Mazzucchelli and James (1966) for geochemical prospecting within the enclave of Western Australia, trace element geochemistry utilizing laterites has proven to be an important tool (Anand et al., 2001). To comprehend the mechanisms of geochemical dispersion in mineral exploration, there is the need to ascertain how trace elements behave in lateritic materials, especially in terranes of deeply weathered regolith (Nude et al., 2012). According to Ghezelbash et al. (2019), statistical analysis and geostatistical methods can be used to delineate anomalous patterns related to mineralization. Rate (2018) also indicated that statistical tools are useful in determining patterns, groupings, and associations in geochemistry. This is possible because variables in geochemistry have a multivariate behavior as well as a regionalized nature (Ghezelbash et al., 2019).
Multivariate statistical methods can reveal geochemical signatures which can be linked to the hidden geology, weathering, and processes that have occurred or are occurring, and the associated mineralization within a given area after a sampling program (Sunkari et al., 2019). Nude et al. (2012) said that to show deviation from the normal distribution, various descriptive statistics are determined for the elements under investigation. These are the minimum, maximum, range, mean, median, and standard deviation. The deviation shown by these statistics is attributed to high values due to outliers (Nude et al., 2012). Moreover, when analyzing the dataset obtained to identify relationships between gold and the elements selected, correlation analysis, factor analysis with principal component analysis (PCA) method, and hierarchical cluster analysis (HCA) are used (Nude et al., 2012). Correlation measures the association between two variables while HCA calculates the similarity/dissimilarity between elements and clusters them together based on their similarities (Lermi and Sunkari, 2020). Factor Analysis extracts principal components from the dataset and presents observations in 2-3 dimensional charts, which show the degree of similarity between variables and make interpretations easier (Sunkari et al., 2020).
The Wa-Lawra greenstone belt, located in Ghana, belongs to the Birimian supra-crustal domain of the Paleoproterozoic age which has been folded, metamorphosed, and intruded by granitoids (Amponsah et al., 2015). Of all the greenstone belts in Ghana, this belt is the only N-S trending belt contrasting the approximately NE-SW trend of the rest (Arhin and Nude, 2009). The greenstone belts in Ghana are known to host several important gold mines (Nude et al., 2012). However, it is difficult to establish gold reserves within the Wa-Lawra Belt although it has identical lithologies, mineralization styles, and structures when compared to the other greenstone belts found in the southwestern parts of the country (Arhin et al., 2015). Arhin et al. (2015) attributed this difficulty to the regolith profile in such complex terranes since it is difficult to replicate successful gold exploration in areas of deeply weathered terranes with extensive cover of ferruginous materials and vast detrital sediments. Unaware of this fact, in northern Ghana, many exploration firms still carry out gold analysis on crushed and raw field samples (Sunkari et al., 2019).
The Wa-Lawra Belt shows great promise for commercially viable gold mineralization but several attempts in exploring and exploiting the resource have been largely unsuccessful over the years (Waller et al., 2012). Sunkari et al. (2019) indicated that most companies exploring for gold in the belt use methods drawn from those used in the southern parts of Ghana without consideration of the fact that there is a contrast between the climatic conditions prevailing in northern and southern parts of the country. Savannah climate exists in northern Ghana leading to a landscape that is a deeply weathered terrane with abundant laterites (Fig. 1). Sunkari et al. (2019) mentioned that there is a great thickness of cemented residual and transported laterites in savannah-dominated regions unlike the terranes existing in many rainforest areas. The landscape contrasts between the savannah regions and rainforest regions (Fig. 1) are a result of changes to differences in climate, the nature of biological activities in both regions, topography variations, and the mode of lateritization (Arhin, 2013). This implies that how elements are mobilized and/or dispersed by different geochemical processes in northern Ghana varies from that of southern Ghana and hence, different methods of mineral exploration, as well as different methods of interpretation of geochemical signature are required in the Wa-Lawra Belt (Sunkari et al., 2019). Hence, several researchers used statistical methods to identify pathfinder elements of gold in the Wa-Lawra Belt. Nude et al. (2012) identified indicator elements for Au to be Pb, Ag, As, Cu, Fe, and Mn, using soil samples from the Wa-Lawra Belt. Sunkari et al. (2019) also identified Pb, Cu, As, and Ag as indicator elements for Au using laterites from the Wa-Lawra Belt. These studies agree that laterites are the best geochemical sampling material in studies carried out purposefully to find the indicator elements of Au in deeply weathered regolith terranes and the most useful pathfinder elements of Au in the area are As, Ag, Cu, and Pb. In both cases, both residual and transported laterites were used for the analysis. The petrography of the laterites was also not studied. Generally, in-situ (residual) laterites will better represent their underlying formations than transported soils. Also, transported laterites may contain extraneous sources of element enrichment caused by pollution, which is unrelated to gold mineralization.
There is, therefore, a gap in the previous works done in the area. The gap this study seeks to fill is to determine the minerals associated with the laterites and the implications when only residual laterites are used for the analysis. This brings the following research questions to the forefront: Will the residual laterites be more accurate and effective in determining pathfinder elements of gold or will similar results be obtained as before? What are the rock-forming minerals and ore minerals associated with the formation of the residual laterites? What will be the implications for gold exploration in the area? This study is therefore aimed at conducting petrographic studies, multi-element geochemical analysis, and multivariate statistical analysis of residual laterites in the Wa-Lawra Belt. The implications for identifying pathfinder elements of gold in the area will also be elucidated.