Abstract
ABSTRACT Extensive lateritization and widespread sheet wash and alluvial deposits characterize the thick regolith in the savannah regions of northern Ghana. As often is the case in these areas, the presence of these cover materials mask geochemical gold (Au) response in soils during surficial gold exploration. Anomaly detection thus becomes very difficult perhaps due to gold grain encrustation during lateritization and anomaly dilution by sheet wash deposits. Termite mound samples collected from areas of thick regolith, transported overburden and laterite cap in gold bearing areas of northern Ghana which were analyzed for gold defined anomalous zones. Gold contents were determined from size fractions consisting of −125 μm, +125–250 μm, +250–500 μm and +500 μm. The gold contents show relatively insignificant changes in concentration and in repeat samples in the −125 μm and +125–250 μm size fractions, but there were significant differences when sub-samples were re-analysed in the coarser samples. Gold content repeatability was relatively better in the fine size fractions (−125 μm) and decreased in the coarser size fractions. The study showed that termite mounds can be used as a geochemical sample medium to support conventional soil surveys especially in areas under thick regolith and transported cover, and the −125 μm size fraction appears the most appropriate.
INTRODUCTION
Sampling of termitaria, or termite mounds, was carried out in parts of the savannah regions of northern Ghana, which is characterized by thick regolith and transported overburden (Arhin & Nude 2009). The presence of these cover materials mask geochemical gold response in soils during surficial gold (Au) exploration and thus anomaly detection becomes very difficult; perhaps due to gold grain encrustation during lateritization and anomaly dilution by sheet wash deposits (Butt 1992). Termite mounds have been used in search of economically important metals especially in regions where thick regolith materials mask bedrock mineralization (D'Orey 1975; Gleeson & Poulin 1989; Kebede 2004; Petts et al. 2009). According to these authors, mineralization overlying bedrock is carried along by termites and mixed with fine regolith materials used in building the termite mound. For this reason, during regional geochemical surveys in gold exploration in areas masked by thick regolith and transported cover, termite mounds have been used as a sampling medium to support conventional soil surveys.
Over time, the termitaria may collapse and subsequently become eroded, transported and redeposited in low-lying areas. Although these materials may be residual or transported in character (Butt & Zeegers 1992), mineralization expressed by sampling termite mounds represents site-specific or in-situ anomalies (D'Orey 1975; Kebede 2004), as the materials sampled are usually from the subsurface and generally unaffected by surficial processes. Studies by Gleeson & Poulin (1989), Kebede (2004) and many more confirm the direct relationship between the concentration of metals in termite mounds and their contents in subsurface horizons of the regolith. Sampling termite mounds is based on the assumption that the mound building activity results in an upward transfer of clay, silt, sand and fine metal grain particles to the surface, a process opposite to leaching, which often results in significant mobilization and dispersion of trace elements (Roquin et al. 1991; Chatupa & Direng 2000).
In the savannah regions of northern Ghana, termitaria occur as preserved or eroded relict mounds of various sizes and structures ubiquitously in the gently undulating terrain. We have determined the Au contents from size fractions consisting of −125 μm, +125–250 μm, +250–500 μm and +500 μm in termite mound samples collected in Au-bearing areas in the Wa east region, northern Ghana, where the sample medium has been used to support conventional soil surveys (Affam & Arhin 2004).
GEOLOGICAL SETTING, PHYSIOGRAPHY AND REGOLITH
Geology
The study area is within the Birimian gold-bearing belts of northern Ghana (Kesse 1985); the regional geological setting is shown in Figure 1. The Birimian terrain in Ghana comprises narrow sedimentary basins and linear volcanic belts intruded by several generations of granitoid batholiths. The regional geology has been described by several authors including Leube et al. (1990) and Hirdes et al. (1992). The rocks of the sedimentary basins include thick isoclinally folded and steeply dipping alternating phyllites, slates, sheared conglomerates, graywackes and argillaceous rocks with some tuffaceous schists and lava. The metavolcanic rocks are mainly metabasalts, metaandesites and pyroclastic rocks. The package is interbedded by a coarse clastic sedimentary sequence belonging to the Tarkwaian Group.
Regional geological map of northern Ghana showing the distribution of the lighological units. Insert shows the study area.
In Ghana the granitoid rocks that intrude the Birimian have been classified into two broad categories. These are a hornblende-rich variety that is associated with the volcanic rocks and known as the ‘Dixcove or belt’ type, and a mica-rich variety that is associated with the metasedimentary rocks and referred to as ‘Cape Coast or basin type’ granitoids (Leube et al. 1990; Taylor et al. 1992; Hirdes et al. 1992). Pink microcline-hornblende granitoids with biotite subgroup known as the ‘Bongo granites’ are exposed close to Bolgatanga area in Upper East region (Leube et al. 1990). The general foliation in the Birimian rocks trends N–NNE to S–SSW. Sheared and brecciated quartz veins are extensive in the Birimian; these features together with the extensive faults control the gold mineralization (Dzigbodi-Adjimah 1993).
Physiography
The physiographical setting of the study area generally is a gently undulating landform relict with elevations from 100–250 m above mean sea level (Meyertons 1976). The area is characterized by alluvial plains, colluvial plains, flood plains, and pediments, erosional plains, rolling hills, escarpments, isolated hills and ridges. The high relief landforms occur at the north-eastern and central parts of area, which are underlain by granitic batholiths. Areas underlain by metasedimentary units are generally associated with pediments, colluvial plains, and erosional plains. Preserved and eroded relict termite mounds, consisting of cathedral and mushroom types (Roquin et al. 1991; Petts et al. 2009), and of various structures and colourations are common over the generally flat and undulating terrain. The cathedral type mounds dominate: they have broad bases with multiple sides, and range from 2.5 m to c. 5 m in height, and relatively resistant to erosion. The mushroom types are easily susceptible to collapse. They are relatively short (up to 1.5 m in height), with slender trunks and broad tops that give it an ‘umbrella’ shape.
Regolith
The regolith profile of the study area has been described by Arhin & Nude (2009) as deep, with extensive ferruginous duricrust, that has been truncated in places. The lateritic profiles generally have a surface veneer of pisoliths, whereas sheet-wash deposits cover low lying areas. The upland areas are generally marked by talus that decreases in clast size down-slope and the sheet wash areas are characterised by thin layers of colluvium, which are interspersed with alluvial plain sediments.
The spatial distributions of the regolith materials consist of residual regolith, typically preserved at ridge tops and on pediments, and proximal transported materials at the base of ridges and at moderately elevated settings (Arhin & Nude 2009). Regolith materials are preserved in the landscape as colluvium soils and scree/talus, whereas duricrust generally occurs on topographic highs and is preserved with an equigranular groundmass. In addition, transported laterites are widespread in the area and occur in the low lying areas and sometimes near streams. Areas overlain by ferruginous saprolite are typically indurated by hardpan that is encrusted with calcium carbonate and iron oxides.
SAMPLING AND ANALYTICAL METHODS
Termitaria sampling
Composite samples were collected from the termitaria following their natural distribution over an area of 10 km2 with a regular density of 100 m × 50 m that corresponds to the soil sampling grid used in a previous geochemical survey during gold exploration in the area. The density of sampling was approximately six termite mounds over 1 km2. Representative samples from the termite mounds were chipped from around the circumference of the mound at the base and 0.5-m intervals upwards to the apex of the termitaria to make up 4–6 kg samples. The quantity of chipped samples collected was controlled by the size and height of the termitaria. For example, a termitarium which is 1.5 m high was sampled at four sections: at the base and at every 0.5 m. Composite samples were also collected from areas with termite mound clusters. The samples largely consist of detrital quartz, feldspars, fine pisolithic materials and clay mineral particles with negligible compositional differences from the fine to the coarser samples. To ensure that there is Au mineralization in the subsurface, the samples were collected from mounds in Au-bearing areas that are in close proximity to known illicit mining areas. In this regard mounds that were suspected to be affected by nearby mining activities, or eroded and redeposited, were discarded in this study.
Analytical methods
The samples collected were dried in the sun for about eight hours, after which they were disaggregated and sieved to remove very coarse materials, granules and organic materials. The samples were then separated into size fractions of −125 μm, +125–250 μm, +250–500 μm and +500 μm of c. 1 kg each. Each size fraction was analysed by conventional fire assay-atomic absorption spectrometry (FA-AAS) (Delaney & Fletcher 1999) at a commercial laboratory at Tarkwa, Ghana, operated by SGS Mineral Services. FA-AAS is generally accepted as dependable analytical method for Au (Juvonen & Kontas 1999). The method was also chosen for comparison purposes based on previous work carried out in northern Ghana by gold exploration companies (Griffis et al. 2002).
The analytical procedure has been described by Nude & Arhin (2009). It consists of two consecutive metallurgical separations: lead fire assay followed by determination of Au by AAS. Each size sample was pulverized to 90% passing 75 μm and a 50-g charge was dissolved in a molten flux and fused in a graphite furnace at 1100° C. The Pb button was removed by cupellation at 950° C. The resultant Au prill was digested with aqua regia mixture and the solution was analysed using a Varian 55B atomic absorption spectrometer with an air-acetylene burner, to yield a detection limit of 5 ppb. Gold standards were prepared from 1000-ppm stock solution in a 10% HCl matrix. Replicate analyses of standards and field-split duplicates were used to estimate analytical precision and relative errors according to the quality control procedures of SGS Mineral Service Laboratories.
RESULTS AND DISCUSSION
Distribution of gold contents in the different size fractions
The Au contents (ppb) in the various size fractions of the termite mounds are presented in Table 1. Gold contents in −125-μm sizes were compared with those of the +125–250 μm, +250–500 μm and +500 μm size fractions. The data indicate that the four size fractions appear to contain relatively significant Au concentrations defining the same anomalous zones (Fig. 2).
Gold values in ppb from FA-AAS analyses of the different size fractions from the termite mounds
Comparison of gold (Au) contents in the −125 μm size fractions with Au contents in the +125–250 μm, +250–500 μm and +500 μm size fractions.
The −125 μm and +125–250 μm size samples have similar Au geochemical patterns to the plots of the original data but with higher absolute Au contents in the smaller size (−125 μm) fraction (Fig. 3). Replicate analyses show insignificant variation in the −125 μm size samples but relatively pronounced variations were shown in the replicate analyses of the +125–250 μm size fraction. For example, in the −125 μm size fractions from termite mound number 31, the original concentration of Au of 464 ppb returned 451 ppb when the sample was re-analysed. However, the Au content in the +125–250 μm size fraction from the same termite mound which was originally 178 ppb returned only 50 ppb when the samples were re-analysed. Again, in termite mound number 40, the Au content in the −125 μm size fractions was 310 ppb in the original sample with 300 ppb in the repeat analysis; however, the +125–250 μm size fraction showed 290 ppb in the original analysis but only 59 ppb in the repeat analysis. In both cases, the repeat analyses of the −125 μm size fractions were still significant whereas the +125–250 μm size fraction decreased considerably. Hence, based on enhanced Au content and repeatability (Table 1), the −125 μm size fraction appears to be more dependable for Au than the +125–250 μm size fraction.
Comparison of gold (Au) contents in the −125 μm size fractions with Au contents in the +125–250 μm size fractions.
Similarly, in Figures 4 and 5, Au contents in the −125 μm size fraction are relatively more enhanced than in the +250–500 μm and the +500 μm size fractions, although they show similar patterns. However, the latter two size fractions have relatively poor precision (Table 1).
Comparison of gold (Au) contents in the −125 μm size fractions with Au contents in the +125–500 μm size fractions.
Comparison of gold (Au) contents in the −125 μm size fractions with Au contents in the +500 μm size fractions.
Importantly, the repeatability in analysis for Au is more variable with the coarser size fractions. Chatupa & Direng (2000) attribute the enhanced Au contents in the fine fractions in the sandveld regolith, Botswana, to the greater surface area to volume ratio of the particles. It is also possible that the poor reproducibility of analysis in the coarser size fractions shown in this study could be due to detrital Au grains or nugget effects. The implication is that using coarser size fractions as sample media could be problematic, as false anomalies could erroneously be interpreted as residual.
From this study, it is evident that there is gold mobilization through the thick regolith and transported cover as a result of the termite mound building process. The results suggest that termite mound surveys can be used as a supplemental exploration tool to conventional soil surveys, especially in areas of extensive cover, and more appropriately when the finer −125 μm size fractions is used.
Quantitative analysis
The Au results were also subjected to a statistical t-test in order to compare the mean Au values of the different size fractions. The −125 μm size fraction was used as the standard measure and the Au content in that size fraction was compared with the mean values of the other size fractions at a confidence level of 95%. A comparative test was conducted with the Null Hypothesis set assuming no significant difference in means with any of the other three size fractions, namely, +125–250 μm, +250–500 μm and +500 μm size fractions. Other alternative hypotheses were also set and tested to aid in identifying the most appropriate sample size fraction of the termitaria for gold exploration.
In the alternative hypotheses; the mean of Au contents in the −125 μm size fraction was considered to be less than the means of Au in the +125–250 μm, +250–500 μm and +500 μm fractions. In addition, it assumed that the mean of Au in the −125 μm size fraction is not equal to the means of Au in the +125–250 μm, +250–500 μm and +500 μm size fractions. Nonetheless, the last alternative hypothesis testing assumed the mean of Au in the −125 μm fraction to be greater than the means of Au in the +125–250 μm, +250–500 μm and +500 μm fractions.
For the first t-test between the mean Au content in the −125 μm and +125–250 μm size fractions, the p-value was 0.021 against a 0.05 level of significance, which implies that the test was significant. This results in the rejection of the Null Hypothesis in favour of the alternative which implies that the mean Au content in the −125 μm fraction is greater than that of the +125–250 μm size fraction. The second t-test between the mean Au content in the −125 μm and +250–500 μm fractions resulted in p-value of 0.0006 against a 0.05 level of significance which shows that the test was significant. Again the Null Hypothesis was rejected in favour of the alternative, implying that the mean Au content in the −125 μm size fraction is greater than the mean Au in the +250–500 μm size fraction. The last t-test between the mean Au content in the −125 μm and +500 μm size fractions resulted in a p-value of 0.0297 against a 0.05 level of significance, which indicates that the test was significant. This hypothesis testing also favours the alternative testing but rejects the Null Hypothesis that the mean in the −125 μm size fraction is greater than the mean for Au in the +500 μm fraction.
The statistical analyses suggest that the mean Au content in the −125 μm size fraction is greater than that in any of the other three size fractions. This again shows that whereas termitaria sampling can be used to support soils during surficial geochemical surveys for gold exploration in northern Ghana, the fine size fractions are the most appropriate.
CONCLUSIONS
Conventional methods for gold exporation in regional soil surveys in northern Ghana have often resulted in erratic gold results (Griffis et al. 2002), probably due to the thick regolith, including transported overburden and laterite caps that mask subsurface mineralization. This study confirms the remobilization of gold during the construction of termitaria. A termite mound survey can therefore be useful as a supplementary tool to conventional soil surveys especially in areas with extensive cover. The materials, irrespective of the size fractions, have consistent gold contents. This strongly suggests that the relative enhanced gold values in the fine fractions are a result of dispersion during the building of the termite mounds rather than compositional differences in the termitaria. As termites can burrow to depths of the water table (D'Orey 1975), samples from termite mounds are residual and therefore more likely to be representative of bedrock and subsurface mineralization. For this reason, gold contents from the termitaria can often be more dependable than the surficial soils.
The results from this study show that the +125–250 μm, +250–500 μm and +500 μm size fractions lack sample repeatability during re-analyses for gold. However, the −125 μm size fraction has better precision for gold. Therefore, based on enhanced gold content and repeatability, the −125 μm size fraction is considered to be the most appropriate and practical size fraction for supporting soil surveys in gold target delineation during ‘greenfields’ exploration in areas of cover.
Acknowledgments
The research collaboration reported here was generously supported by Ghana Government research grants. We sincerely appreciate reviews by Steven Hill and Anna Petts, and the helpful comments by the journal editor, which led to improvements of the manuscript.
- © 2010 AAG/Geological Society of London
References
Use of termitaria in surficial geochemical surveys: evidence for >125-μm size fractions as the appropriate media for gold exploration in northern Ghana
