Kunche bölgesindeki (NW Ghana) lateritlerin mekansal dağılımı ve eser element jeokimyası: Gana'nın kuzey batısındaki altın arama hedefleri için önemli çıkarımlar
Gold Exploration in Tropical Landscapes
NOT: Araştırmacılar, Azumah Resources Limited'e (Gana) bu araştırma süresince verdiği destek için teşekkür etmektedirler. Gana Üniversitesi'nde Doç. Dr. Emmanuel ARHIN'e faydalı yorumları için özel olarak teşekkür etmektedirler. İlk yazar, Türkiye'de doktor araştırma görevlisi olarak verdiği sürekli destek için TÜBİTAK, BIDEB 2215 Uluslararası Öğrenciler için Lisansüstü Burs Programı'na teşekkürlerini belirtmektedir.
Tropikal Laterit
Regolitinde Altın Dağılımı
Tropikal bölgelerde, temel kaya derin bir şekilde aşınmıştır ve kalın bir laterit regolitine sahiptir.
Jeolojik zaman
boyunca tropikal iklimlere maruz kalmış altın taşıyan kuvars damarları, stok
işleri ve kayma bölgeleri, üst laterit regolitinde karakteristik mantar biçimli
bir altın dağılım deseni oluşturacaktır.
Fiziksel ve kimyasal işlemler süperjen altın zenginleşmesine ve yüksek saflıkta altın külçelerinin oluşumuna neden olur. Süperjen altın, kalan lateritte birikir ve erozyon, aşağı akışta bazı altın yıkanmalarını sağlar.
El yapımı ve
küçük ölçekli altın madencileri (ASGM) hem alüvyonlu hem de süperjen altını
hedefler.
ASGM'nin çakıl ve
tortuları işlemek için suya ihtiyacı olduğundan, genellikle dereler ve vadiler
boyunca altın kazmaya başlarlar ve geride bir dizi su dolu havuz ve beyaz
kuyruk yelpazesi bırakırlar. Bunlar alüvyonlu altın işletmeleridir.
ASGM derelerden
ayrılıp topografik yüksekliklere geçtiğinde, bir altın dağılım halesine rastlar
ve birincil bir altın kaynağına yaklaşırlar. Madencilik tarzları değişir ve süpergen
altın külçeleri ve ince altın tozu için kırmızı laterit toprağı hasadı yapmaya
başlarlar. Bunlar laterit altın işletmeleridir.
Birincil bir
altın sistemi ortaya çıkarmadıkları sürece laterit, su tablasındaki beyaz, kil
açısından zengin benekli bölgeye kadar sıyrılır. Bu durumda, su pompalarını
çalıştırırlar ve yumuşak saprolitin içine doğru damarları veya kayma bölgesini
takip ederek 10 ila 15 metre daha kazmaya başlarlar, ta ki taze kayaya ulaşana
kadar. Sert kayayı işleyemiyorlarsa, devam ederler ve zamanla derin çukur suyla
dolar.
Bir keşif jeoloğunun birincil altın sistemini vektörlemesi için alüvyonlu ve laterit altın işletmeleri arasındaki farkı bilmesi kritik öneme sahiptir.
Sonuç
Tropikal nemli
iklimlerde, ASGM laterit işletmelerindeki suyla dolu çukurlar birincil altın
sisteminin yerini işaretleyebilir. Bunları alüvyonlu su çukurlarından
ayırmaya dikkat edin.
ORİJİNAL MAKALE
Journal of African Earth Sciences
Volume 158, October 2019, 103519
Spatial distribution and trace element geochemistry of laterites in Kunche area: Implication for gold exploration targets in NW, Ghana
Emmanuel Daanoba Sunkari a, Michael Appiah-Twum b, Abdurrahman Lermi
Introduction
The term laterite is often used to describe indurated level (duricrust) present in some tropical regolith profiles (e.g., Arhin et al., 2015; Ilyas et al., 2016; Anand et al., 2019). Laterites are abundant with a widespread global distribution in many deeply weathered environments (Anand and Paine, 2002). Laterite materials and their mode of formation differ with some laterites forming in-place as residual products of weathering and others forming from a transported source (da Costa et al., 2016). Despite their different origins, both types of duricrusts have a similar orientation of chemistry. They are both poor in lessivable cations and mainly composed of Al/Fe oxides and more rare Al-rich clays. This is the direct result of their particular climatic conditions of formation, as they form in landscapes suffering a succession of extensive dry periods and short wet conditions (Aleva, 1994). Laterites are mainly formed as a result of secondary in situ cementation of unconsolidated materials overlying the parent bedrock (Taylor and Eggleton, 2001). Their formation generally depends on hydrology, topography and the primary rock. The formation of in situ duricrust is generally linked to the in situ accumulation of Fe/Al (Beauvais and Colin, 1993). It may result in iron oxide pisolith accumulation in the mottled zone and induration in the top of the profile (Nahon and Tardy, 1992). Recently, Chardon et al. (2018) opined that in West Africa, detrital duricrusts are often found covering lateritic pediments, which serve as obstacles to mineral exploration. The distribution of duricrusts on the landscape mainly depends on the past erosion, which translates into the present day by the local topography. However, laterite formation can also be the result of cementation of residual weathered materials as well as sediments from varied sources, hence their classification as residual and detrital laterites on the basis of origin (Anand et al., 2001). The lateritic pediments described by Chardon et al. (2018) have similar geomorphological characteristics with depositional environments in the Wa-Lawra greenstone Birimian belt in Ghana (Arhin et al., 2015). These environments characteristic of northern Ghana have gently inclined slopes of transportation and erosion that truncate the regolith and form at the confluence of eroding slopes and areas of sediment deposition (Arhin, 2013). Indurated and loose transported sediments are also common in the depositional environments of the Wa-Lawra greenstone Birimian belt as it was observed in the lateritic pediments area described by Chardon et al. (2018). During the period of secondary re-cementation in both in situ and detrital duricrusts, some elements behave as immobile elements due to changes in physico-chemical conditions causing unusual economical trace element enrichment such as detrital gold coatings on lithic units of clastic materials. The unusual enrichment of gold in such materials is because gold has an affinity for Fe-oxide, therefore, during the process of lateritization, gold grains may be coated to Fe-oxide (e.g., Freyssinet et al., 2005; Anand and Butt, 2010; Butt, 2016). Anand et al. (2001) documented erratic gold distributions in laterite capped areas in Australia. Butt and Bristow (2013) also documented auriferous laterite prevalence in the entire West African sub-region. Recently, Anand et al. (2019) reported auriferous ferricrete in the Yilgarn Craton of Western Australia and pointed out that the ferricrete gold deposits formed downslope of saprolite hills and are from proximal sources. However, the re-cemented unconsolidated units of different provenance identified in these studies appear erratic and thus, their elemental concentrations may not give a true representation of the underlying mineralization.
In such context, Anand (2001) and Cornelius et al. (2001) used several terminologies in describing iron coated lateritic materials (ferruginous materials). The authors mentioned that ferruginous duricrusts are regolith materials cemented by Fe regardless of their source. They are characterized by lateritic residuum (lateritic duricrust), ferricrete, and lateritic gravels (ferruginous lag) or a combination of all. The lateritic duricrust represents the ferruginous zone and occupies low rises, crests and mesas (Butt and Zeegers, 1992). As explained earlier, it forms as a result of ferruginization and residual accumulation of Fe and Al oxides as well as silica in the residual regolith. Depending on the formation conditions and materials, the lateritic duricrusts may have a uniform composition, which reflects the host rock lithology (Eggleton, 2001). Their chemical composition may still be similar due to the leaching/accumulation of geochemical elements from the host rocks. Ferricretes develop over/at the expense of sediments and have no distinguishable proximal bedrock source and thus, are sometimes considered to be of detrital origin (Anand et al., 2019). They may be reworked materials that originate from detrital sources with diverse jumbled materials and hence, could be termed detrital duricrusts (Cornelius et al., 2001). Accordingly, detrital duricrusts are formed as a result of cementation of sediments by Fe oxide impregnation with characteristics controlled by the nature of the matrix and cementing material (Anand, 1998). The matrix can be detrital clasts, sands and clays whereas the cementing material can be Fe-oxides, kaolinite, gibbsite, goethite and hematite. Ferruginous lag represents relics of Fe-rich regolith materials and varies depending on the degree of erosion and type of topography (Cornelius et al., 2001). It is generally composed of fragments of mottled saprolite, fragments of lateritic duricrust on hill crests, pisoliths and cutans on backslopes, and intercalations of polymictic materials in depositional (mixed) environments (Anand, 2001; Cornelius et al., 2001). In all, ferruginous duricrusts can be in situ or detrital in origin; when they come from in situ sources, they are simply referred to as lateritic duricrusts but if from detrital sources, they are generally known as detrital ferruginous duricrusts.
In the Wa-Lawra greenstone Birimian belt of Ghana, Arhin and Nude (2009) documented evidence of erratic gold distribution in the laterite dominated area. Identifying pathfinder elements of gold in such a complex regolith terrain is very difficult (Nude et al., 2012, 2014; Butt, 2016). Therefore, many workers have recognized laterites as the appropriate geochemical sample media for identifying pathfinder elements of gold in such terrains (Arhin, 2013; da Costa et al., 2016). The use of laterite trace element geochemistry in mineral exploration was initiated by Mazzucchelli and James (1966) in Western Australia and has since become a useful tool for successful gold exploration in the world (e.g., Anand et al., 2001; da Costa et al., 2016). By determining the behavior of trace elements in laterites within complex regolith environments, it is possible to completely understand secondary geochemical dispersion mechanisms useful for geochemical exploration programs (Arhin, 2013). Trace element associations in laterites can also point to where samples can be obtained and which type of sample to collect. Also, multi-element geochemistry stands as one of the best tools for delineating anomalies related to primary mineralization in tropical terrains covered by laterites and has been used by many researchers (e.g., Taylor and Thornber, 1992; Colin et al., 1993; Araújo, 1994; Marker et al., 1994; da Costa and Araújo, 1996). Since mineralization can be dispersed or concealed, laterally enhanced or diluted, delineating the actual anomaly using multi-element geochemistry is therefore worthwhile in geochemical exploration.
Kunche area in NW Ghana lies within the Wa-Lawra greenstone Birimian belt that is mainly composed of volcaniclastics, metasediments and some intrusive granitoids. The area is reported to have potential for gold mineralization associated with the volcaniclastics (Waller et al., 2012) but several attempts in harnessing and exploring for this valuable resource have been abortive over the years. Many exploration companies have since either abandoned their concessions or temporary halted their exploration activities in search for better ways of optimizing exploration. The exploration methods that were employed by the earlier exploration companies were adopted from those used in southern Ghana though the climates are different in both regions. The Kunche area is characterized by a savannah climate and such landscape is widely known to have abundant laterites and deeply weathered profiles (Butt and Zeegers, 1992). The regolith profiles of savannah areas are covered by very thick and consolidated ferruginous duricrust contrary to rainforest landscapes (Freyssinet, 1993). These variations in the landscape are usually due to modifications enhanced by climate, biological activities, topography, laterization, and thus, strictly require a different approach to interpreting element mobility and different exploration procedures. Southern Ghana has a homogenous regolith profile (Arhin, 2013) and gold (Au) is liberated from ore bodies in this area by chemical dissolution (Bowell, 1992). Au released by this process involves cyanide, fulvate, hydroxyl, and thiosulphate complexing and is re-precipitated via in-situ processes (Arhin, 2013). Accordingly, the mineralogy of Au in the regolith becomes complex. Also, the Au mineralogy in the regolith materials in southern Ghana is principally controlled by the prevailing physico-chemical processes during lateritic pedogenesis that led to in-situ and supergene Au mineralization (Arhin, 2013). Therefore, dispersion and mobilization mechanisms of elements in the savannah regions of Ghana like the Kunche area are different from the rainforest dominated southern parts. However, these geochemical processes are poorly known in the Kunche area since the mineralization is concealed by lateritic materials. This study seeks to identify the laterite types in Kunche area and to understand the nature of the underlying parent rocks using the trace element concentrations of the laterites complemented by multivariate geostatistical analysis. This information is used to determine areas of unusual element enrichment, the primary geological environment, and of the possible existence of base and precious metals mineralization, as there are few outcrops of primary rocks in the area. The study also aims at unravelling the distribution pattern of the pathfinder elements of gold and their geochemical associations, something not well understood in lateritic crusts.
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Section snippets
Geological setting and gold mineralization
Kunche area is located in NW Ghana and forms part of the Wa-Lawra greenstone Birimian belt (Fig. 1a). Undifferentiated pyroclastic rocks, volcaniclastics, granitoids, metavolcanics, and metasedimentary rocks of the Birimian system (Kesse, 1985; Hirdes et al., 1992; Arhin and Nude, 2009; Nude et al., 2012; Sunkari and Zango, 2018) dominate the Wa-Lawra greenstone Birimian belt. However, in the Kunche area, the predominant lithologies are volcaniclastics, metasediments and some intrusive
Data acquisition
A total of sixty-seven (67) laterite samples were collected from a series of plateaus and hills in Kunche, NW Ghana in close proximity to a gold prospect (Fig. 1b). The laterite samples were collected from in-situ materials on the surface of the landscape, old pits, and from 30 cm diameter-holes dug in the plateaus and hills by means of a digging hoe. During the sampling, the first 20 cm was discarded since a large proportion of the area is covered by soils influenced in some places by organic
Laterite types
Observations from old pits and surface materials in the laterite-capped Kunche area show that overlying the bedrock are pisoliths, quartz pebbles with ferricrete, clay units, and some rock fragments (Fig. 3). The pisoliths contain Fe-oxyhydroxides and the rock fragments encountered in some places on the surface are signatures of the up-shooting of the primary rocks mainly mafic volcaniclastics.
Formation of the laterites
The information gathered from the laterite mapping in the field showed that the study area is largely composed of detrital ferruginous duricrusts (detrital laterites) and lateritic duricrusts (residual laterites) (Fig. 4a). The pisoliths and quartz pebbles associated with these type of laterites are interpreted as transported detrital fragments overlying the regolith whilst the clay units are interpreted as remnants of residual laterites/duricrusts. However, the pisoliths found in the mottled
Conclusions
The following conclusions are drawn from the current study;
- ✓The studied laterites range from detrital laterites (detrital ferruginous duricrust) to residual laterites (lateritic duricrust) and are hosted in volcaniclastic rocks.
- ✓Geostatistical analysis indicates three element associations; (1) Fe, Pb, S, Co, Cr; (2) Ni, Y, Rb, Sr, Zn; (3) Ca, Cu, Mn, Ti, Zr, As, Au, Ag, which may be due to three factors; mainly the underlying volcanoclastic host rocks and sulphide-bearing deposits, mixed sources,
Acknowledgements
Azumah Resources Limited, Ghana is acknowledged for the support during the period of this research. Special thanks goes to Assoc. Prof. Emmanuel ARHIN at the University for Development Studies, Ghana for his useful comments. The first author thanks TÜBİTAK, BIDEB 2215 Graduate Scholarship Program for International Students for the continuous support as a doctoral research fellow in Turkey. The second author also acknowledges the support from the Graduate Scholarship Program for International.
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