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Overbank sediments as appropriate geochemical sample media in regional stream sediment surveys for gold exploration in the savannah regions of northern Ghana
Journal of Geochemical Exploration 103 (2009) 50–56
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Overbank sediments as appropriate geochemical sample media in regional stream sediment surveys for gold exploration in the savannah regions of northern Ghana Prosper M. Nude a,⁎, Emmanuel Arhin b a b
Department of Geology, University of Ghana, P.O. Box LG 58, Legon, Ghana Department of Earth and Environmental Sciences, University for Development Studies, P.O. Box 24, Navrongo Campus, Ghana
a r t i c l e
i n f o
Article history: Received 28 November 2008 Accepted 2 June 2009 Available online 6 July 2009 Keywords: Gold Geochemical survey Active stream sediment Overbank sediment Northern Ghana
a b s t r a c t Conventional stream sediment sampling in which sediments are taken from the active channels during reconnaissance regional geochemical surveys in gold exploration has over the years failed to delineate prospective target zones in northern Ghana, where the relict is flat. Whereas the technique has been successful in the south western Ghana, which is characterised by moderate to high relief, generally the savannah north is associated with low relief, commonly with flat topographies and generally decoupled stream channels. Geochemical comparison of active stream and overbank sediments in this study demonstrate that active stream channels may contain contaminated materials of recent origin, but overbank sediments, except for the uppermost horizons, represent alluvial regolith of earlier depositional cycles over time. Based on gold value repeatability, composite samples taken from the overbank sediment layers were relatively less erratic and are considered to be an appropriate geochemical medium in delineating potential regional gold targets for follow up. The results show that overbank sediment sampling can be used as a costeffective method to define broad anomalous zones; and the technique must be considered useful during reconnaissance geochemical surveys in the savannah regions. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Stream sediment geochemistry is based on the assumption that samples collected either from the active channel or the overbank or the bank of a stream, represent the products of weathering upstream from the sampling site. Conventional or traditional methods of collecting −125 µm fractions of active channel sediments as a quick method in defining prospective targets for follow up surveys for gold exploration, have been very successful in south western Ghana, and resulted in the discovery of many world class gold deposits. Some examples are Anglogold-Ashanti, Obuasi and Newmont Ghana Gold, Kenyasi and several others. However, gold exploration in the savannah regions of northern Ghana has been faced with erratic and nonrepeatable gold contents, both locally on the streambeds and along the length of streams. This serves as an impediment to successful application of stream sediment surveys as a reconnaissance tool in defining regional anomalous targets for gold exploration in the savannah regions of Ghana. In south western Ghana, the landform and regolith are generally homogeneous and less complex irrespective of the period in the year. The thick vegetative canopies in the southern Ghana reduce the effects
⁎ Corresponding author. Tel.: +233 24 411 6879. E-mail address: [email protected] (P.M. Nude). 0375-6742/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2009.06.005
of temperature and rainfall, the main agents for weathering, generally throughout the year. Modifications of the landscape that affect stream catchment areas resulting in drastic variations in concentrations of gold in wet and dry seasons are non-existing. In contrast, the landform characteristics and regolith situations in the savannah north are different. Whereas southern Ghana is characterised by moderate to high relief with distinct stream channels; the savannah north is associated with low relief commonly with flat topographies and generally decoupled stream channels. The notion that a stream sediment sample should represent equally all parts of the catchment basins upstream of the sample site is most likely to be obtained in first and second order streams where the valley slopes and the stream channels are closely linked or coupled (Fletcher, 1997). However, very few streams in the savannah regions of Ghana have these characteristics; the supply of sediments to stream channels in these areas appear to be complex as most of the streams are decoupled from the valleys by erosion and agricultural activities. Thus stream sediments from these areas are not representative of all parts of the catchments equally, thereby deviating from the situation as pertains in south western Ghana. In this study we have compared the results of gold contents in sediment samples from overbanks of streams from gold bearing areas in the savannah north of Ghana with samples from the active channels. The overbank samples have been found to be appropriate medium which are relatively less erratic and capable of delineating potential regional gold targets for follow up.
P.M. Nude, E. Arhin / Journal of Geochemical Exploration 103 (2009) 50–56
2. Geological setting and physiography 2.1. Geological setting The study area falls within the Birimian gold bearing belts of northern Ghana (Kesse, 1985) and the regional geology is shown in Fig. 1. The area is underlain by metavolcanic, pyroclastic and metasedimentary rocks. The metavolcanic rocks are of basaltic and gabbroic in compositions and most of them have been altered into various schist. The metasedimentary rocks consist of sandstones, siltstones, tuff, carbonaceous phyllites, tuffaceous phyllites, cherts and manganoferous rocks. The sandstones are fine to medium grained, grey, and with typical whitish tarnish from the surface; the siltstones in the area are less distinct in granularity; they are grey, but usually darker (Meschcherakov and Yakzhin, 1964). The schists are fine to medium grained and are usually dark grey or black, but greenish and reddish schists are also not uncommon. Quaternary deposits consist of alluvium, and are associated with shallow streams. Intruding the Birimian rocks are migmatitic bodies and porphyritic granitoids that have generally been classified into two broad categories. These are (a) hornblende-rich varieties that are closely associated with the volcanic rocks and known as the ‘Dixcove’ or ‘belt’ type; and (b) mica-rich varieties which tend to border the volcanic belt and are in the metasediment units, and referred to as ‘Cape Coast’ or ‘basin’ type granitoids (Leube et al., 1990; Taylor et al., 1992; Hirdes et al., 1992). The belt granitoids are small discordant to semi-discordant, late or post tectonic soda-rich hornblende-biotite granites or granodiorites which grade into quartz diorite and hornblende diorite. They are generally massive but in shear zones they are strongly foliated. The basin granitoids are large concordant and syntectonic batholitic granitoids, commonly banded and foliated. The basin granitoids are
51
two-mica potassic granitoids, containing both biotite and muscovite, with biotite dominating (Leube et al., 1990). The belt granitoids are non-foliated but the basin granitoids are strongly foliated to gneissic. These rocks are generally isoclinally folded, with dips usually greater than 50°. The general foliation in the rocks is N–NNE to S–SSW. Sheared and brecciated quartz veins are extensively developed in the south western part of the area. Extensive faults and fissure zones are found to be the most important structural features which control Birimian gold mineralization (Dzigbodi-Adjimah, 1993). 2.2. Physiography The study area, shown in Fig. 1, represents a typical physiographical setting of northern Ghana and described by Meyertons (1976) as being generally gently undulating relict with elevations from 100 to 250 m above mean sea level. Erosion by sheet wash and flashfloods has commonly exposed the bedrock in many upland localities. The weathering profile is deep and preserved, with extensive ferruginous duricrust, which are truncated in places. The lateritic profiles generally have surface veneer of pisoliths, whereas sheet-wash deposits cover low lying areas. The upland areas are generally marked by talus that decreases in fragment size down-slope and the sheet wash areas are characterised by thin layers of colluvium, which are interspersed with alluvial plains. 2.3. Regolith Northern Ghana is generally associated with low topographical terrains and thus most areas retain relicts of lateritic weathering profiles. Arhin and Nude (2009) described the characteristics of the regolith in the area. The spatial distributions of the regolith materials consist of residual regolith, commonly preserved at ridge tops and on
Fig. 1. Geological map of northern Ghana showing area of study.
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P.M. Nude, E. Arhin / Journal of Geochemical Exploration 103 (2009) 50–56
pediments, and proximal transported materials which are found at base of ridges and often at moderate elevated terrains. These materials are preserved on the landscape as colluvium soils and screes/talus. Duricrust usually occurs at the topographic highs and are preserved as equigranular groundmass. Ferricretes or transported laterites are widespread in the area and occur at the low lying areas and sometimes near streams. In addition to the above, certain areas have been overlain by ferruginous saprolite and usually appear hardpanized. Truncated areas exposing the saprolite are generally uncommon. 2.4. Drainage characteristics Based on the sediment characteristics, which are as a result of the nature of (a) the landform (b) underlying rocks, and (c) the vegetation types, four different types of sediments were identified from the 63 streams sampled. Silt-clay sediments from undisturbed watershed fill stream channels draining areas underlain by the metasedimentary rocks, whereas streams draining areas underlain by granitoids have sands of varying thickness in their channels. Streams at the low lying areas, and decoupled streams have their channels filled with sheet-wash sediments of varying weathered materials; and at sparsely vegetated and laterite-covered environments, the stream channels are generally filled with pisolith-rich sediments. These four sediment types have different porosities and permeabilities, and as such gold grains are likely to have different modes of occurrence in the various sample media (Butt and Zeegers, 1992). This heterogeneity of the stream sediments suggests possible dilution effects from the sheet wash and encrustation of gold during lateritization in some areas and enhancement of gold content in some localities. Fletcher (1990), Cohen et al. (2005) and many others have shown that geochemical gold patterns of stream sediment samples change due to agricultural land disturbance and that the changes vary from severe to minor depending on the rates of soil erosion and extent of sediment transport in relation to extent of the catchment areas. Therefore in northern Ghana where agricultural activities are concentrated along the waterways and floodplains of streams, soil erosion and contamination are enhanced, these are likely to affect the gold geochemical patterns through land disturbances and chemicals used. The implications are that the results obtained from stream surveys in northern Ghana may appear unrepresentative, more so if sample media are not differentiated based on their mode of formation. 3. Stream sediment sampling and gold analysis 3.1. Sampling Stream sediment samples, consisting of composite samples from three different sample media, were collected from 63 selected streams for gold analysis. The samples were taken from (a) the silt-clay fraction in the active stream channels, (b) dug-out slot of about 10 cm at the interface between the overlying sediments in the active channel and the preserved pre-existing surface or the paleo-surface of the stream, and (c) the whole thickness of the overbank sediment sequence below the surface sediments, in the exposed areas of both sides of the stream channel and within 10 m radius of the streams that constitute the flood plains. For comparison purposes the samples were consistently collected from approximately the same sample sites as practically as possible. 3.2. Gold analysis The samples from the three media were analysed for gold by conventional fire assay-atomic absorption spectrometry (FA-AAS) (Delaney and Fletcher, 1999) at a commercial laboratory at Tarkwa, Ghana, operated by SGS Mineral Services. FA-AAS is generally accepted as dependable
analytical method for gold (Juvonen and Kontas, 1999). The method was also chosen for comparison purposes based on previous works carried out both in southwestern and northern Ghana (e.g. Griffis et al., 2002). The analytical process involved two consecutive metallurgical separations that consist of lead fire assay followed by determination of gold by atomic absorption spectrometry. The samples were dried
Table 1 Fire assay analyses of gold contents (ppb) in the sediments. Sample points
Sample media
Position
UTM-E
UTM-N
Active channel
Dug-out slot
Overbank
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
593900 592900 595900 597200 589800 590100 586700 585400 584200 582700 578900 576800 576100 589900 589400 591400 589300 590800 597200 596600 594900 595300 591300 592400 591300 592400 587600 584900 582600 580400 580400 578400 579400 598400 575900 575200 576800 577100 579500 576200 576900 578900 582700 584300 585300 586700 588500 590300 589800 578100 577500 579100 580400 582000 583600 586400 592200 596400 602600 601400 607400 604000 608000
1114600 1116400 1117400 1114300 1117400 1120800 1117400 1118200 1117000 1118500 1117700 1117000 1116700 1117400 1113300 1113400 1113400 1110600 1114200 1111600 1110300 1109800 1109200 1105300 1109200 1105400 1107700 1106600 1108700 1106600 1104000 1105600 1108900 1105700 1105300 1106300 1107000 1107600 1108900 1116800 1117800 1117600 1118600 1116900 1118200 1117300 1118600 1117800 1117400 1122200 1123000 1125100 1123300 1124300 1125300 1124800 1124200 1124500 1122100 1119500 1118800 1114400 1115800
205 170 92 114 38 14 8 5 5 10 30 30 75 20 89 20 10 5 15 5 5 65 18 20 5 10 25 10 105 38 70 50 75 45 30 10 10 10 5 75 50 40 170 118 98 35 285 115 131 20 15 25 80 52 30 10 10 10 5 117 97 10 5
90 65 250 20 78 13 56 5 5 24 35 28 67 35 211 25 10 5 23 5 5 68 36 34 5 5 5 5 24 30 120 52 35 67 5 12 5 5 5 54 34 37 214 134 76 76 100 89 125 5 5 5 57 52 25 5 5 4 5 56 45 78 5
108 120 80 87 24 5 5 5 5 8 5 15 20 17 102 17 6 5 14 5 5 8 20 15 5 5 6 5 5 20 57 59 20 45 34 10 5 5 5 34 23 27 112 98 58 19 89 79 102 5 5 5 20 23 70 5 5 5 5 101 132 5 7
P.M. Nude, E. Arhin / Journal of Geochemical Exploration 103 (2009) 50–56
and pulverized to 90% passing 75 µm. A 50 g charge was dissolved in a molten flux and fused in a graphite furnace at 1100 °C. The obtained lead button was removed by cupellation at 950 °C. The resultant gold prill was digested with aqua regia mixture and the solution was analysed by Varian 55B atomic absorption spectrometer with airacetylene burner, and a lamp current of 4 mA at a detection limit set at b5 ppb. Gold standards were prepared from 1000 ppm stock solution with 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. Gold values of 50 ppb or more were considered as anomalous based on previous work in south west Ghana and in the savannah regions of northern Ghana (e.g. Griffis et al., 2002). 4. Results and discussion 4.1. Comparison of samples from active channel and overbank The gold contents (in ppb) in the various sample media are presented in Table 1, and the geochemical distribution and anomalous zones defined according to the gold assay values of the active channel sediments are shown in Fig. 2. For the purpose to compare the anomalous zones of the gold assay values in the overbank sediments, the results have been superimposed with the assays of the active channel samples and shown in Fig. 3. In the figures the two media appear to have similar anomalous patterns defining the same catchments areas. The only difference is that the active channel samples showed relatively higher gold values than the overbank samples (Fig. 4). In addition the overbank sediments failed to confirm the Guripie anomaly defined by the active channel sediments. The higher gold results in the active channel samples can be due to the presence of detrital gold, which was easily identified by panning some of the sediments. Whereas surficial overbank sediments and active channel sediments may be contaminated, overbank sediments taken at depth are commonly paleo-sediments that had been deposited as a
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result of re-concentration, and redistribution from the leaching and hydromorphic dispersion mechanisms over time. In contrast active stream sediments represent active material of recent origin temporarily suspended on the stream bed (Peh and Miko, 2001). From Fig. 4, the gold content repeatability between the sets of assay values from the two sample media were minimal for the overbank samples compared to those from the active channel. None of the field-split duplicates from the active stream sediment samples gave assay values close to the original sample results but a better precision was obtained for the overbank samples. 4.2. Comparison of samples from dug-out slots in active channels and overbank In Fig. 5, gold values obtained in sediments from the dug-out slots in the active channels delineated the same anomalous zones as the overbank sediments. The difference is that the anomalous area defined by the gold contents in the overbank sediments at Danyokura is larger than the dug-out slot. Again the anomalous zone east of Mangwe that was defined by gold contents in the sediments from the dug-out slot was not confirmed by the overbank sediment anomaly. In case of absolute contents, the gold values in samples taken from the dug-out slots in the active stream bed were found to be relatively higher than those from the overbank samples (Fig. 6). But the high peak values obtained from the dug-out samples are likely to have been taken from the clayey saprolite or the mottled plasmic zone of the regolith. Therefore assay values from such a sample represent an anomaly in the underlying bedrock and would appear uncertain in delineating anomalous catchments areas that are representative of the area. Alternatively, even if the dug-out sample from the stream bed contained transported paleo-sediment, the overall length of time spent in excavating such sediments affects the turnaround time of getting samples ready for assay. This implies that the overbank sediment sampling is more cost-effective than sampling from dugout active channels. It is therefore practical and appropriate to use
Fig. 2. Geochemical distribution of gold in the active stream sediments.
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P.M. Nude, E. Arhin / Journal of Geochemical Exploration 103 (2009) 50–56
Fig. 3. Anomalous sites of gold in the overbank sediments as coloured triangles on the gold distribution in the active stream sediments as the coloured background image. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
overbank sediment as sample medium over the dug-out samples. Sampling preparation of overbank medium was found to be easier than that of the dug-out slot since the latter was relatively wet. Obtaining fine clay-silt fraction from wet samples is often difficult and is usually associated with contaminations that influence gold responses. Additionally, nugget effect may not be a problem in the overbank sample as compared to the dug-out medium since the pure gold has specific gravity of about 19.2 g/t and may accordingly tend to settle on the interface of the supposed clayey saprolite and the transported infill sediments in the stream channel (Fletcher, 1990).
4.3. Comparison of samples from dug-out slots in the active channels and the active channels Gold contents in samples from the dug-out slots in the active channel and sediments from active channel all defined the same anomalous catchments areas with only a slight variation in the area of extent. The active channel sediments defined broader and weaker anomalous areas than samples from the dug-out slots which defined more discrete anomalies with higher gold concentrations in the sediments (Fig. 7). The anomaly defined near Guripie by the active channel sediment got decayed and only reproduced background gold values when samples were collected from the dug-out sediments in the active channel. This emphasizes the problem of repeatability of gold values in sediments from the active channel. However, some of the gold values in the sediments from the dugout slot are site specific anomalies from the saprolitic clay that may not represent the catchment areas marked out for follow up. Notwithstanding, the dug-out samples gave better precision and more repeatable gold assays compared with the active stream sediments. 4.4. The case for overbank sediment sampling
Fig. 4. Comparison of gold contents in the active stream and overbank sediments.
In Fig. 8, samples from the three media appear to delineate the same zones of prospectivity, but with differing gold contents (Fig. 9). The active channel sediments have broader anomalous areas with relatively higher gold contents (5-285 ppb) than the overbank sediments which have gold values in the range of 5–200 ppb. An important observation is that the overbank anomalous targets appear to be a part of the zones defined by the active channel sediments; the targets defined by the overbank sediments confined the most prospective areas defined by active channel sediments. Furthermore the intersectional points of the two media had correspondingly high gold contents, emphasizing the prospectivity of the defined zones by overbank sediments. Thus the overbank medium is capable of detecting anomalous targets and may be a
P.M. Nude, E. Arhin / Journal of Geochemical Exploration 103 (2009) 50–56
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Fig. 5. Anomalous zones of gold in the dug-out sediments as the coloured symbols on the gold distribution in the active stream sediments as the coloured background image. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
smarter technique over the active stream sediments which show problems of repeatability of gold values and false anomalies such as the one in Guripie and East Mangwe. In the active stream, sediments have been found to be loaded with abundant pisolithic and sheet-wash materials hence there was difficulties in obtaining silt-clay fractions desirable for gold assays. Therefore bulk sediments are needed to be processed to obtain the silt-clay fractions; this in itself could result in contamination and prolong the time of processing. Above all, the gold results obtained from these sediments are generally erratic with non-repeatability. The origins of the sampled sediments from the dug-out slots in the active channels are often uncertain, as these sediments could be part of the
paleo-surface and representing regional anomalies arising from ancient sediments or from the truncated regolith that could represent a single point anomaly. Thus, although the evidence of gold mineralization in the area is shown, interpreting such anomalies as regional will lead to false targets for follow up work.
Fig. 6. Comparison of gold contents in the overbank and dug-out sediments.
Fig. 7. Comparison of gold contents in the active stream and dug-out sediments.
5. Conclusions Stream sediment geochemical surveys provide a robust, costeffective method of defining prospective anomalous catchment areas for follow up work in gold exploration. However, these surveys work well wherever stream drainage systems are well developed. The streams in the savannah regions of northern Ghana are often
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Fig. 8. Anomalous zones of gold in the active stream as the coloured background image, overbank sediments as the coloured triangles and dug-out sediments gold distribution in the active stream sediments delineated as continuous dotted line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
decoupled, and the channels disturbed by agricultural activities, hence the characteristics of the sediments in the channels are different. Thus the use of regional stream sediment in gold exploration in northern Ghana based solely on the south western Ghana models may partly be responsible for the lack of success in delineating regional anomalies for follow up. The results from this study show that active stream channels may be loaded with contaminated materials of recent origin. Overbank sediments, except for the uppermost sections, represent alluvial regolith of earlier depositional cycles over time. Based on gold value repeatability, overbank sediments are considered as a cost-effective method that is useful in defining gold targets during geochemical surveys in the savannah regions.
Fig. 9. Comparison of gold contents in the active stream, dug-out slot, and overbank sediments.
Acknowledgements This research collaboration reported here was partly supported by SEMAFO (Gh) Ltd. Reviews by an anonymous journal reviewer and helpful comments by the journal editor are very much appreciated. References Arhin, E., Nude, P.M., 2009. Significance of regolith mapping and its implication for gold exploration in northern Ghana: a case study at Tinga and Kunche. Geochemistry: Exploration, Environment, Analysis 9, 63–69. doi:10.1144/1467-7873/08-189. Butt, C.R.M., Zeegers, H., 1992. Regolith exploration in tropical and subtropical terrains. Handbook of exploration geochemistry 4. Elsevier, Amsterdam, p. 607. Cohen, G.R., Dunlop, A.C., Rose, T., 2005. Contrasting dispersion patterns for gold in stream sediments at Timbara NSW, Australia. Journal of Exploration Geochemistry 85, 1–16. Delaney, T.A., Fletcher, W.K., 1999. Efficiency of cyanidation in gold exploration using soils. Journal of Geochemical Exploration 66, 229–239. Dzigbodi-Adjimah, K., 1993. Geology and geochemical patterns of the Birimian gold deposits, Ghana, West Africa. Journal of Geochemical Exploration 47, 305–320. Fletcher, W.K., 1990. Dispersion and behaviour of gold in stream sediments; B.C. ministry of energy, mines and petroleum resource, open file 1990–28. Fletcher, W.K., 1997. Stream sediment geochemistry in today's exploration world. In: Gubins, A.G. (Ed.), Proceedings of Exploration 97: Fourth Decennial International Conference on Mineral Exploration, pp. 249–260. Griffis, J., Barning, K., Agezo, F.L., Akosa, F., 2002. Gold Deposits of Ghana. Mineral Commission, Accra, Ghana. 432 p. Hirdes, W., Davis, D.W., Eisenlohr, B.N., 1992. Reassessment of Proterozoic granitoids ages in Ghana on the basis of U/Pb zircon and monazite dating. Precambrian Research 56, 89–96. Juvonen, R., Kontas, E., 1999. Comparison of three analytical methods in the determination of gold in six Finnish gold ore, including a study on sample preparation and sampling. Journal of Geochemical Exploration 65, 219–229. Kesse, G.O., 1985. The mineral and rock resources of Ghana. Balkema Publishers, p. 610. Leube, A., Hirdes, W., Mauer, R., Kesse, G.O.,1990. The Early Proterozoic Birimian Supergroup of Ghana and some aspects of its associated gold mineralization. Precambrian Research 46, 139–165. Meschcherakov, S. and Yakzhin, A., 1964. Report on the work carried out by revision estimation party at the Gold Coast Deposits of Dokrupe and Yoyo during 1962 – 1963. Ghana Geological Survey Department archival report, p. 111. Meyertons, E.T., 1976. Ghana study NRG/247. A metallogenic map of northwestern Ghana. Minerals Department, ESSO Eastern Inc, Canada, p. 109. Peh, Z., Miko, S., 2001. Geochemical comparison of stream and overbank sediments: a case study from Zumberak Region, Croatia. Geologia Croatia 54 (1), 119–130. Taylor, P.N., Moorbath, S., Leube, A., Hirdes, W., 1992. Early Proterozoic crustal evolution in the Birimian of Ghana: constraints from geochronology and isotope geology. Precambrian Research 56, 77–111.
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Overbank sediments as appropriate geochemical sample media in regional stream sediment surveys for gold exploration in the savannah regions of northern Ghana Prosper M. Nude a,⁎, Emmanuel Arhin b a b
Department of Geology, University of Ghana, P.O. Box LG 58, Legon, Ghana Department of Earth and Environmental Sciences, University for Development Studies, P.O. Box 24, Navrongo Campus, Ghana
a r t i c l e
i n f o
Article history: Received 28 November 2008 Accepted 2 June 2009 Available online 6 July 2009 Keywords: Gold Geochemical survey Active stream sediment Overbank sediment Northern Ghana
a b s t r a c t Conventional stream sediment sampling in which sediments are taken from the active channels during reconnaissance regional geochemical surveys in gold exploration has over the years failed to delineate prospective target zones in northern Ghana, where the relict is flat. Whereas the technique has been successful in the south western Ghana, which is characterised by moderate to high relief, generally the savannah north is associated with low relief, commonly with flat topographies and generally decoupled stream channels. Geochemical comparison of active stream and overbank sediments in this study demonstrate that active stream channels may contain contaminated materials of recent origin, but overbank sediments, except for the uppermost horizons, represent alluvial regolith of earlier depositional cycles over time. Based on gold value repeatability, composite samples taken from the overbank sediment layers were relatively less erratic and are considered to be an appropriate geochemical medium in delineating potential regional gold targets for follow up. The results show that overbank sediment sampling can be used as a costeffective method to define broad anomalous zones; and the technique must be considered useful during reconnaissance geochemical surveys in the savannah regions. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Stream sediment geochemistry is based on the assumption that samples collected either from the active channel or the overbank or the bank of a stream, represent the products of weathering upstream from the sampling site. Conventional or traditional methods of collecting −125 µm fractions of active channel sediments as a quick method in defining prospective targets for follow up surveys for gold exploration, have been very successful in south western Ghana, and resulted in the discovery of many world class gold deposits. Some examples are Anglogold-Ashanti, Obuasi and Newmont Ghana Gold, Kenyasi and several others. However, gold exploration in the savannah regions of northern Ghana has been faced with erratic and nonrepeatable gold contents, both locally on the streambeds and along the length of streams. This serves as an impediment to successful application of stream sediment surveys as a reconnaissance tool in defining regional anomalous targets for gold exploration in the savannah regions of Ghana. In south western Ghana, the landform and regolith are generally homogeneous and less complex irrespective of the period in the year. The thick vegetative canopies in the southern Ghana reduce the effects
⁎ Corresponding author. Tel.: +233 24 411 6879. E-mail address: [email protected] (P.M. Nude). 0375-6742/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2009.06.005
of temperature and rainfall, the main agents for weathering, generally throughout the year. Modifications of the landscape that affect stream catchment areas resulting in drastic variations in concentrations of gold in wet and dry seasons are non-existing. In contrast, the landform characteristics and regolith situations in the savannah north are different. Whereas southern Ghana is characterised by moderate to high relief with distinct stream channels; the savannah north is associated with low relief commonly with flat topographies and generally decoupled stream channels. The notion that a stream sediment sample should represent equally all parts of the catchment basins upstream of the sample site is most likely to be obtained in first and second order streams where the valley slopes and the stream channels are closely linked or coupled (Fletcher, 1997). However, very few streams in the savannah regions of Ghana have these characteristics; the supply of sediments to stream channels in these areas appear to be complex as most of the streams are decoupled from the valleys by erosion and agricultural activities. Thus stream sediments from these areas are not representative of all parts of the catchments equally, thereby deviating from the situation as pertains in south western Ghana. In this study we have compared the results of gold contents in sediment samples from overbanks of streams from gold bearing areas in the savannah north of Ghana with samples from the active channels. The overbank samples have been found to be appropriate medium which are relatively less erratic and capable of delineating potential regional gold targets for follow up.
P.M. Nude, E. Arhin / Journal of Geochemical Exploration 103 (2009) 50–56
2. Geological setting and physiography 2.1. Geological setting The study area falls within the Birimian gold bearing belts of northern Ghana (Kesse, 1985) and the regional geology is shown in Fig. 1. The area is underlain by metavolcanic, pyroclastic and metasedimentary rocks. The metavolcanic rocks are of basaltic and gabbroic in compositions and most of them have been altered into various schist. The metasedimentary rocks consist of sandstones, siltstones, tuff, carbonaceous phyllites, tuffaceous phyllites, cherts and manganoferous rocks. The sandstones are fine to medium grained, grey, and with typical whitish tarnish from the surface; the siltstones in the area are less distinct in granularity; they are grey, but usually darker (Meschcherakov and Yakzhin, 1964). The schists are fine to medium grained and are usually dark grey or black, but greenish and reddish schists are also not uncommon. Quaternary deposits consist of alluvium, and are associated with shallow streams. Intruding the Birimian rocks are migmatitic bodies and porphyritic granitoids that have generally been classified into two broad categories. These are (a) hornblende-rich varieties that are closely associated with the volcanic rocks and known as the ‘Dixcove’ or ‘belt’ type; and (b) mica-rich varieties which tend to border the volcanic belt and are in the metasediment units, and referred to as ‘Cape Coast’ or ‘basin’ type granitoids (Leube et al., 1990; Taylor et al., 1992; Hirdes et al., 1992). The belt granitoids are small discordant to semi-discordant, late or post tectonic soda-rich hornblende-biotite granites or granodiorites which grade into quartz diorite and hornblende diorite. They are generally massive but in shear zones they are strongly foliated. The basin granitoids are large concordant and syntectonic batholitic granitoids, commonly banded and foliated. The basin granitoids are
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two-mica potassic granitoids, containing both biotite and muscovite, with biotite dominating (Leube et al., 1990). The belt granitoids are non-foliated but the basin granitoids are strongly foliated to gneissic. These rocks are generally isoclinally folded, with dips usually greater than 50°. The general foliation in the rocks is N–NNE to S–SSW. Sheared and brecciated quartz veins are extensively developed in the south western part of the area. Extensive faults and fissure zones are found to be the most important structural features which control Birimian gold mineralization (Dzigbodi-Adjimah, 1993). 2.2. Physiography The study area, shown in Fig. 1, represents a typical physiographical setting of northern Ghana and described by Meyertons (1976) as being generally gently undulating relict with elevations from 100 to 250 m above mean sea level. Erosion by sheet wash and flashfloods has commonly exposed the bedrock in many upland localities. The weathering profile is deep and preserved, with extensive ferruginous duricrust, which are truncated in places. The lateritic profiles generally have surface veneer of pisoliths, whereas sheet-wash deposits cover low lying areas. The upland areas are generally marked by talus that decreases in fragment size down-slope and the sheet wash areas are characterised by thin layers of colluvium, which are interspersed with alluvial plains. 2.3. Regolith Northern Ghana is generally associated with low topographical terrains and thus most areas retain relicts of lateritic weathering profiles. Arhin and Nude (2009) described the characteristics of the regolith in the area. The spatial distributions of the regolith materials consist of residual regolith, commonly preserved at ridge tops and on
Fig. 1. Geological map of northern Ghana showing area of study.
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pediments, and proximal transported materials which are found at base of ridges and often at moderate elevated terrains. These materials are preserved on the landscape as colluvium soils and screes/talus. Duricrust usually occurs at the topographic highs and are preserved as equigranular groundmass. Ferricretes or transported laterites are widespread in the area and occur at the low lying areas and sometimes near streams. In addition to the above, certain areas have been overlain by ferruginous saprolite and usually appear hardpanized. Truncated areas exposing the saprolite are generally uncommon. 2.4. Drainage characteristics Based on the sediment characteristics, which are as a result of the nature of (a) the landform (b) underlying rocks, and (c) the vegetation types, four different types of sediments were identified from the 63 streams sampled. Silt-clay sediments from undisturbed watershed fill stream channels draining areas underlain by the metasedimentary rocks, whereas streams draining areas underlain by granitoids have sands of varying thickness in their channels. Streams at the low lying areas, and decoupled streams have their channels filled with sheet-wash sediments of varying weathered materials; and at sparsely vegetated and laterite-covered environments, the stream channels are generally filled with pisolith-rich sediments. These four sediment types have different porosities and permeabilities, and as such gold grains are likely to have different modes of occurrence in the various sample media (Butt and Zeegers, 1992). This heterogeneity of the stream sediments suggests possible dilution effects from the sheet wash and encrustation of gold during lateritization in some areas and enhancement of gold content in some localities. Fletcher (1990), Cohen et al. (2005) and many others have shown that geochemical gold patterns of stream sediment samples change due to agricultural land disturbance and that the changes vary from severe to minor depending on the rates of soil erosion and extent of sediment transport in relation to extent of the catchment areas. Therefore in northern Ghana where agricultural activities are concentrated along the waterways and floodplains of streams, soil erosion and contamination are enhanced, these are likely to affect the gold geochemical patterns through land disturbances and chemicals used. The implications are that the results obtained from stream surveys in northern Ghana may appear unrepresentative, more so if sample media are not differentiated based on their mode of formation. 3. Stream sediment sampling and gold analysis 3.1. Sampling Stream sediment samples, consisting of composite samples from three different sample media, were collected from 63 selected streams for gold analysis. The samples were taken from (a) the silt-clay fraction in the active stream channels, (b) dug-out slot of about 10 cm at the interface between the overlying sediments in the active channel and the preserved pre-existing surface or the paleo-surface of the stream, and (c) the whole thickness of the overbank sediment sequence below the surface sediments, in the exposed areas of both sides of the stream channel and within 10 m radius of the streams that constitute the flood plains. For comparison purposes the samples were consistently collected from approximately the same sample sites as practically as possible. 3.2. Gold analysis The samples from the three media were analysed for gold by conventional fire assay-atomic absorption spectrometry (FA-AAS) (Delaney and Fletcher, 1999) at a commercial laboratory at Tarkwa, Ghana, operated by SGS Mineral Services. FA-AAS is generally accepted as dependable
analytical method for gold (Juvonen and Kontas, 1999). The method was also chosen for comparison purposes based on previous works carried out both in southwestern and northern Ghana (e.g. Griffis et al., 2002). The analytical process involved two consecutive metallurgical separations that consist of lead fire assay followed by determination of gold by atomic absorption spectrometry. The samples were dried
Table 1 Fire assay analyses of gold contents (ppb) in the sediments. Sample points
Sample media
Position
UTM-E
UTM-N
Active channel
Dug-out slot
Overbank
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
593900 592900 595900 597200 589800 590100 586700 585400 584200 582700 578900 576800 576100 589900 589400 591400 589300 590800 597200 596600 594900 595300 591300 592400 591300 592400 587600 584900 582600 580400 580400 578400 579400 598400 575900 575200 576800 577100 579500 576200 576900 578900 582700 584300 585300 586700 588500 590300 589800 578100 577500 579100 580400 582000 583600 586400 592200 596400 602600 601400 607400 604000 608000
1114600 1116400 1117400 1114300 1117400 1120800 1117400 1118200 1117000 1118500 1117700 1117000 1116700 1117400 1113300 1113400 1113400 1110600 1114200 1111600 1110300 1109800 1109200 1105300 1109200 1105400 1107700 1106600 1108700 1106600 1104000 1105600 1108900 1105700 1105300 1106300 1107000 1107600 1108900 1116800 1117800 1117600 1118600 1116900 1118200 1117300 1118600 1117800 1117400 1122200 1123000 1125100 1123300 1124300 1125300 1124800 1124200 1124500 1122100 1119500 1118800 1114400 1115800
205 170 92 114 38 14 8 5 5 10 30 30 75 20 89 20 10 5 15 5 5 65 18 20 5 10 25 10 105 38 70 50 75 45 30 10 10 10 5 75 50 40 170 118 98 35 285 115 131 20 15 25 80 52 30 10 10 10 5 117 97 10 5
90 65 250 20 78 13 56 5 5 24 35 28 67 35 211 25 10 5 23 5 5 68 36 34 5 5 5 5 24 30 120 52 35 67 5 12 5 5 5 54 34 37 214 134 76 76 100 89 125 5 5 5 57 52 25 5 5 4 5 56 45 78 5
108 120 80 87 24 5 5 5 5 8 5 15 20 17 102 17 6 5 14 5 5 8 20 15 5 5 6 5 5 20 57 59 20 45 34 10 5 5 5 34 23 27 112 98 58 19 89 79 102 5 5 5 20 23 70 5 5 5 5 101 132 5 7
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and pulverized to 90% passing 75 µm. A 50 g charge was dissolved in a molten flux and fused in a graphite furnace at 1100 °C. The obtained lead button was removed by cupellation at 950 °C. The resultant gold prill was digested with aqua regia mixture and the solution was analysed by Varian 55B atomic absorption spectrometer with airacetylene burner, and a lamp current of 4 mA at a detection limit set at b5 ppb. Gold standards were prepared from 1000 ppm stock solution with 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. Gold values of 50 ppb or more were considered as anomalous based on previous work in south west Ghana and in the savannah regions of northern Ghana (e.g. Griffis et al., 2002). 4. Results and discussion 4.1. Comparison of samples from active channel and overbank The gold contents (in ppb) in the various sample media are presented in Table 1, and the geochemical distribution and anomalous zones defined according to the gold assay values of the active channel sediments are shown in Fig. 2. For the purpose to compare the anomalous zones of the gold assay values in the overbank sediments, the results have been superimposed with the assays of the active channel samples and shown in Fig. 3. In the figures the two media appear to have similar anomalous patterns defining the same catchments areas. The only difference is that the active channel samples showed relatively higher gold values than the overbank samples (Fig. 4). In addition the overbank sediments failed to confirm the Guripie anomaly defined by the active channel sediments. The higher gold results in the active channel samples can be due to the presence of detrital gold, which was easily identified by panning some of the sediments. Whereas surficial overbank sediments and active channel sediments may be contaminated, overbank sediments taken at depth are commonly paleo-sediments that had been deposited as a
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result of re-concentration, and redistribution from the leaching and hydromorphic dispersion mechanisms over time. In contrast active stream sediments represent active material of recent origin temporarily suspended on the stream bed (Peh and Miko, 2001). From Fig. 4, the gold content repeatability between the sets of assay values from the two sample media were minimal for the overbank samples compared to those from the active channel. None of the field-split duplicates from the active stream sediment samples gave assay values close to the original sample results but a better precision was obtained for the overbank samples. 4.2. Comparison of samples from dug-out slots in active channels and overbank In Fig. 5, gold values obtained in sediments from the dug-out slots in the active channels delineated the same anomalous zones as the overbank sediments. The difference is that the anomalous area defined by the gold contents in the overbank sediments at Danyokura is larger than the dug-out slot. Again the anomalous zone east of Mangwe that was defined by gold contents in the sediments from the dug-out slot was not confirmed by the overbank sediment anomaly. In case of absolute contents, the gold values in samples taken from the dug-out slots in the active stream bed were found to be relatively higher than those from the overbank samples (Fig. 6). But the high peak values obtained from the dug-out samples are likely to have been taken from the clayey saprolite or the mottled plasmic zone of the regolith. Therefore assay values from such a sample represent an anomaly in the underlying bedrock and would appear uncertain in delineating anomalous catchments areas that are representative of the area. Alternatively, even if the dug-out sample from the stream bed contained transported paleo-sediment, the overall length of time spent in excavating such sediments affects the turnaround time of getting samples ready for assay. This implies that the overbank sediment sampling is more cost-effective than sampling from dugout active channels. It is therefore practical and appropriate to use
Fig. 2. Geochemical distribution of gold in the active stream sediments.
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Fig. 3. Anomalous sites of gold in the overbank sediments as coloured triangles on the gold distribution in the active stream sediments as the coloured background image. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
overbank sediment as sample medium over the dug-out samples. Sampling preparation of overbank medium was found to be easier than that of the dug-out slot since the latter was relatively wet. Obtaining fine clay-silt fraction from wet samples is often difficult and is usually associated with contaminations that influence gold responses. Additionally, nugget effect may not be a problem in the overbank sample as compared to the dug-out medium since the pure gold has specific gravity of about 19.2 g/t and may accordingly tend to settle on the interface of the supposed clayey saprolite and the transported infill sediments in the stream channel (Fletcher, 1990).
4.3. Comparison of samples from dug-out slots in the active channels and the active channels Gold contents in samples from the dug-out slots in the active channel and sediments from active channel all defined the same anomalous catchments areas with only a slight variation in the area of extent. The active channel sediments defined broader and weaker anomalous areas than samples from the dug-out slots which defined more discrete anomalies with higher gold concentrations in the sediments (Fig. 7). The anomaly defined near Guripie by the active channel sediment got decayed and only reproduced background gold values when samples were collected from the dug-out sediments in the active channel. This emphasizes the problem of repeatability of gold values in sediments from the active channel. However, some of the gold values in the sediments from the dugout slot are site specific anomalies from the saprolitic clay that may not represent the catchment areas marked out for follow up. Notwithstanding, the dug-out samples gave better precision and more repeatable gold assays compared with the active stream sediments. 4.4. The case for overbank sediment sampling
Fig. 4. Comparison of gold contents in the active stream and overbank sediments.
In Fig. 8, samples from the three media appear to delineate the same zones of prospectivity, but with differing gold contents (Fig. 9). The active channel sediments have broader anomalous areas with relatively higher gold contents (5-285 ppb) than the overbank sediments which have gold values in the range of 5–200 ppb. An important observation is that the overbank anomalous targets appear to be a part of the zones defined by the active channel sediments; the targets defined by the overbank sediments confined the most prospective areas defined by active channel sediments. Furthermore the intersectional points of the two media had correspondingly high gold contents, emphasizing the prospectivity of the defined zones by overbank sediments. Thus the overbank medium is capable of detecting anomalous targets and may be a
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Fig. 5. Anomalous zones of gold in the dug-out sediments as the coloured symbols on the gold distribution in the active stream sediments as the coloured background image. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
smarter technique over the active stream sediments which show problems of repeatability of gold values and false anomalies such as the one in Guripie and East Mangwe. In the active stream, sediments have been found to be loaded with abundant pisolithic and sheet-wash materials hence there was difficulties in obtaining silt-clay fractions desirable for gold assays. Therefore bulk sediments are needed to be processed to obtain the silt-clay fractions; this in itself could result in contamination and prolong the time of processing. Above all, the gold results obtained from these sediments are generally erratic with non-repeatability. The origins of the sampled sediments from the dug-out slots in the active channels are often uncertain, as these sediments could be part of the
paleo-surface and representing regional anomalies arising from ancient sediments or from the truncated regolith that could represent a single point anomaly. Thus, although the evidence of gold mineralization in the area is shown, interpreting such anomalies as regional will lead to false targets for follow up work.
Fig. 6. Comparison of gold contents in the overbank and dug-out sediments.
Fig. 7. Comparison of gold contents in the active stream and dug-out sediments.
5. Conclusions Stream sediment geochemical surveys provide a robust, costeffective method of defining prospective anomalous catchment areas for follow up work in gold exploration. However, these surveys work well wherever stream drainage systems are well developed. The streams in the savannah regions of northern Ghana are often
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Fig. 8. Anomalous zones of gold in the active stream as the coloured background image, overbank sediments as the coloured triangles and dug-out sediments gold distribution in the active stream sediments delineated as continuous dotted line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
decoupled, and the channels disturbed by agricultural activities, hence the characteristics of the sediments in the channels are different. Thus the use of regional stream sediment in gold exploration in northern Ghana based solely on the south western Ghana models may partly be responsible for the lack of success in delineating regional anomalies for follow up. The results from this study show that active stream channels may be loaded with contaminated materials of recent origin. Overbank sediments, except for the uppermost sections, represent alluvial regolith of earlier depositional cycles over time. Based on gold value repeatability, overbank sediments are considered as a cost-effective method that is useful in defining gold targets during geochemical surveys in the savannah regions.
Fig. 9. Comparison of gold contents in the active stream, dug-out slot, and overbank sediments.
Acknowledgements This research collaboration reported here was partly supported by SEMAFO (Gh) Ltd. Reviews by an anonymous journal reviewer and helpful comments by the journal editor are very much appreciated. References Arhin, E., Nude, P.M., 2009. Significance of regolith mapping and its implication for gold exploration in northern Ghana: a case study at Tinga and Kunche. Geochemistry: Exploration, Environment, Analysis 9, 63–69. doi:10.1144/1467-7873/08-189. Butt, C.R.M., Zeegers, H., 1992. Regolith exploration in tropical and subtropical terrains. Handbook of exploration geochemistry 4. Elsevier, Amsterdam, p. 607. Cohen, G.R., Dunlop, A.C., Rose, T., 2005. Contrasting dispersion patterns for gold in stream sediments at Timbara NSW, Australia. Journal of Exploration Geochemistry 85, 1–16. Delaney, T.A., Fletcher, W.K., 1999. Efficiency of cyanidation in gold exploration using soils. Journal of Geochemical Exploration 66, 229–239. Dzigbodi-Adjimah, K., 1993. Geology and geochemical patterns of the Birimian gold deposits, Ghana, West Africa. Journal of Geochemical Exploration 47, 305–320. Fletcher, W.K., 1990. Dispersion and behaviour of gold in stream sediments; B.C. ministry of energy, mines and petroleum resource, open file 1990–28. Fletcher, W.K., 1997. Stream sediment geochemistry in today's exploration world. In: Gubins, A.G. (Ed.), Proceedings of Exploration 97: Fourth Decennial International Conference on Mineral Exploration, pp. 249–260. Griffis, J., Barning, K., Agezo, F.L., Akosa, F., 2002. Gold Deposits of Ghana. Mineral Commission, Accra, Ghana. 432 p. Hirdes, W., Davis, D.W., Eisenlohr, B.N., 1992. Reassessment of Proterozoic granitoids ages in Ghana on the basis of U/Pb zircon and monazite dating. Precambrian Research 56, 89–96. Juvonen, R., Kontas, E., 1999. Comparison of three analytical methods in the determination of gold in six Finnish gold ore, including a study on sample preparation and sampling. Journal of Geochemical Exploration 65, 219–229. Kesse, G.O., 1985. The mineral and rock resources of Ghana. Balkema Publishers, p. 610. Leube, A., Hirdes, W., Mauer, R., Kesse, G.O.,1990. The Early Proterozoic Birimian Supergroup of Ghana and some aspects of its associated gold mineralization. Precambrian Research 46, 139–165. Meschcherakov, S. and Yakzhin, A., 1964. Report on the work carried out by revision estimation party at the Gold Coast Deposits of Dokrupe and Yoyo during 1962 – 1963. Ghana Geological Survey Department archival report, p. 111. Meyertons, E.T., 1976. Ghana study NRG/247. A metallogenic map of northwestern Ghana. Minerals Department, ESSO Eastern Inc, Canada, p. 109. Peh, Z., Miko, S., 2001. Geochemical comparison of stream and overbank sediments: a case study from Zumberak Region, Croatia. Geologia Croatia 54 (1), 119–130. Taylor, P.N., Moorbath, S., Leube, A., Hirdes, W., 1992. Early Proterozoic crustal evolution in the Birimian of Ghana: constraints from geochronology and isotope geology. Precambrian Research 56, 77–111.