Remote sensing satellite-based structural/alteration mapping for gold exploration in the Ketté goldfield, Eastern Cameroon
Introduction
In the tropics, prospecting for new mineral deposits is a cumbersome and expensive undertaking due to the challenging environmental conditions. Here we present a case of the challenges involved and techniques to overcome these to locate gold mineralizations in the tropical Ketté area. The Ketté is located in a subtropical zone within the Eastern Cameroon gold district (Fuh, 1990; Suh et al., 2006; Takodjou Wambo et al., 2016, 2018; 2020; Tata et al., 2018; Vishiti et al., 2019; Ngatcha et al., 2019) in the Pan-African North Equatorial fold belt (PANEFB; Nzenti et al., 1994). The area hosts both primary and alluvial gold mineralizations (Ngassam Mbianya, 2018). Primary mineralization is found in gold-sulphide bearing quartz veins and weathered rocks (Suh et al., 2006; Tata et al., 2018; Ngatcha et al., 2019; Vishiti et al., 2019; Takodjou Wambo et al., 2020), covered under a thick lateritic layer and controlled by shear zones. In-situ mineral prospecting studies are impeded by the scarcity of data on gold mineralizations and the difficulty to create structural lithological maps because of the heavy vegetation, thick lateritic profile and the sporadic distribution of the outcrops.
To overcome the challenges involved in locating gold mineralizations in hostile terrain remote sensing techniques were recommended (Sonbul et al., 2016; Sheikhrahimi et al., 2019; Adiri et al., 2020; Pour et al., 2021a, b). These techniques have become widespread in geology over the past decades due to the increasing number of observation satellites, satellite imagery and improvements of their spatial and spectral resolutions. Lineament and hydrothermal alteration mapping are among the most important applications of remote sensing in the fields of structural geology for the investigation of mesoscale phenomena (Pluijm and Marshak, 1997) and mineral exploration. The study of Lineaments through optical remote sensing constitutes an important decision-making tool for structural geology studies, and provides the foundation for prospecting, exploration and mining (Javhar et al., 2019).
Several authors have demonstrated the effectiveness of remote sensing in tropical regions for structural geology studies and mineral exploration (Hung et al., 2005; Ramli et al., 2009; Hashim et al., 2013; Metang et al., 2014; Pour and Hashim, 2014a, 2015a,b; Takodjou Wambo et al., 2016). The most commonly used lineament extraction techniques include manual and automatic methods (Ibrahim and Mutua, 2014; Hashim et al., 2013; Gannouni and Gabtni, 2015; Sedrette and Rebaï, 2016; Han et al., 2018; Javhar et al., 2019; Skakni et al., 2020; Beygi et al., 2020; Moradpour et al., 2020). The use of multispectral and hyperspectral remote sensing images has also shown great success in lithological mapping as well as hydrothermal alteration mapping in the tropical to subtropical domain (Pour and Hashim, 2014b; Kumar et al., 2020; Traore et al., 2020; Takodjou Wambo et al., 2020; Andongma et al., 2020), the arid to semiarid domain (Gad and Kusky., 2006; Pena and Abdelsalam., 2006; Zhang et al., 2007; Amer et al., 2010; Zoheir and Emam, 2013; Kumar et al., 2015; Hammam et al., 2018; Ge et al., 2018; Noori et al., 2019; Bolouki et al., 2020), as well as in the Arctic and Antarctic domains (Haselwimmer et al., 2010; Pour et al., 2018a,b, 2019a,b,c,d, 2021a,b).
The most commonly used processing methods involve spectral enhancement methods such as the Band Ratio (BR), Band Combination (BCs), Principal Component Analysis (PCA), Independent Component Analysis (ICA), Minimum Noise Fraction (MNF), Decorrelation Stretch and supervised automatic classification methods such as the Spectral Angle Mapper (SAM), the Support Vector Machine (SVM), Random Forest (RF), Matched filter, Maximum Likelihood classifier, Linear Discriminant Analysis (LDA), Artificial Neural Networks (ANN), and K-Nearest Neighbors (K-NN) (Sultan et al., 1986; Gad and Kusky, 2006; Haselwimmer et al., 2010; Yu et al., 2012; Abedi et al., 2012; Ge et al., 2018; Hammam et al., 2018; Kuhn et al., 2019; Kumar et al., 2020; Rezaei et al., 2020; Shirmard et al., 2020; Sekandari et al., 2020a,b; Pour et al., 2021a,b).
The formation of hydrothermal deposits involves four key elements: a metal source, a fluid source, a circulation engine and a precipitation site and mechanism (Arndt and Ganino, 2010). Brittle and ductile deformation structures usually contained in shear zones are good sites for precipitation and concentration. Hydrothermal deposits generally show a spatial association with faults, fractures and ductile-fragile shear zones at different scales (Austin and Blenkisop, 2009; Zoheir and Emam, 2013; Meshkani et al., 2013; Zoheir et al., 2019a,b). These structures increase the porosity and permeability of rocks, which then act as channels for hydrothermal fluid circulation. Lineaments are the expression of brittle and ductile geological deformation structures of deep origin (Fossen, 2010) and the distribution of hydrothermal gold mineralization is generally controlled by deformation zones. Several authors have demonstrated a close relationship between gold occurrences and the distribution of lineaments (Bonham-Carter, 1985; Al-Mokredi et al., 2007; Meshkani et al., 2013; Yousefi et al., 2018; Pour and Hashim, 2016).
The aims of this study is: (i) to provide some insight on the deformation history and the structures controlling the distribution of gold mineralization in the Ketté gold field, (ii) to map the different hydrothermal alterations of the Precambrian Ketté basement and (iii) to determine the most suitable zones for primary gold mineralization through remote sensing and GIS methods using Landsat-7 ETM+ and Landsat-8 OLI satellite data and field survey data.
Section snippets
Geological setting
The Pan-African-North Equatorial Fold Belt (PANEFB) is the main Precambrian unit in Cameroon. It comprises three distinct geodynamic domains: the northern, central and southern domains (Nzenti et al., 1988, 1994; Ngnotué et al., 2000). The Ketté area belongs to the central domain, which is an intermediate domain located between the northern and southern part of the PANEFB (Fig. 1). This domain is marked by multiple regionally significant strike-slip faults and contains abundant granitoids with
Data acquisition
The different data used in this work are: (1) Landsat-8 OLI level 1T and Landsat-7 ETM + satellite images; (2) thematic maps including the Batouri 1/200 000 topographic sheets and the Batouri-E. 1/500 000 geological map (Gazel and Gerad, 1954); and (3) field data. The main characteristics of the Landsat images used are summarized in Table 1. These images were acquired from the USGS online site and correspond to the zone 33 of the Universal Transverse Mercator UTM projection in the WGS 84
Alteration mapping
The PCA transformation was applied to Landsat-8 OLI and Landsat-7 ETM + data. The results are presented in the form of eigenvector matrices (Table 3, Table 4). PC8-1 (Landsat-8) and PC1-7 (Landsat-7) contain 78.12% and 79.86%, respectively, of all variance of the datasets. According to Loughlin (1991), this is mainly due to the fact that these PCs receive an overall scene brightness (albedo) which is responsible for the correlation between the different bands. Other PCs highlight particular
Structural significance of lineaments and structural control of gold mineralization
Fieldwork data were collated to demonstrate the structural significance of the various trends of the obtained lineaments. According to Ngassam Mbianya (2018), the Ketté basement rocks were affected by a ductile-to-brittle deformation classified into four main deformation phases.
The first deformation D1 involves ductile tectonics characterized by a S1 foliation structure (Fig. 13A) and strongly transposed by D2 and D3 deformation phases. It is also marked by P1 folds and B1 boudins. The D2
Conclusion
This investigation highlights the utility of using Landsat-8 OLI and Landsat-7 ETM + images combined with field data for lineaments mapping and hydrothermal alteration mapping for mineral exploration in the Ketté area. Lineament mapping both automatic and semi-automatic extraction methods enabled to identify predominantly NE-SW to ENE-WSW, WNW-ESE, NW-SE, N–S and NNE-SSW structure trends in the study area. These lineaments are associated with ductile and brittle structures of a deep origin
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The data presented here form a part of the primary author's Ph.D. thesis supervised by T. Ngnotué at the University of Dschang, Cameroon. We gratefully acknowledge several anonymous reviewers for their critical and constructive comments of the manuscript.
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