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Stream sediment geochemistry of Guberunde Horst, “Gongola basin”, Upper Benue Trough, Nigeria
Journal of African Earth Sciences, Vol. 7, No. 1, pp. 91-101, 1988
0731-7247/88 $3.00 + 0.00 Pergamon Journals Ltd.
Printed in Great Britain
Stream sediment geochemistry of Guberunde Horst, "Gongola Basin", Upper Benue Trough, Nigeria O. M. OJo Exploration Division, Nigerian Mining Corporation, Jos, Nigeria
(Received for publication 8 December 1986) Abstract--The crystalline basement complex of Guberunde Horst is mainly composed of granitic rocks, into which acid volcanics of the Burashika group of rocks were intruded during the Jurassic. The Horst itself was produced in the Lower Albian by uplift of the basement between two series of NE-SW striking border faults. Most of the fractures which pre-dated the uplift are filled with rhyolite and aplite of the Burashika group. A total of 374 stream sediment samples were collected over an area of 700 km 2. Each sample was analysed for Cu, Pb, Zn, Co, Ni, Fe, Mg, Mn and Ca; in addition, the pH was determined in each. Condescriptive and various multivariate statistical methods were employed in the interpretation of the resultant geochemical data. The various association of elements exhibits a four-factor model as follows: (1) Mn-Fe versus Ni: oxidizing environment (2) Ca-pH: environmental association (3) Ni-Co-Pb: lithological effect (4) Cu-Zn: basemetal mineralization association.
INTRODUCTION THE GUBERUNDEHorst, a product of Lower Cretaceous block faulting, is situated in the centre of the Gongola Basin (Upper Benue Trough) of Nigeria (Fig. 1). It is bounded in the northeast by Biu basalts and in the southwest by the Gongola River and flanked on other sides by Cretaceous sediments (Bima Formation). The structural features of the middle Gongola Basin suggest an aulacogen in the northern arm of Upper Benue (Ojo 1982). An environment which has been subjected to block faulting, uplifting and affected by granitization and volcanism suggested that regional stream sediment sampling of the study area would be an effective method of searching for base metals. No report of base metal mineralization had ever been made in the area. Consequently reconnaissance stream sediment sampling was conducted on a density of 1 sample per 2 km 2 to locate any possible mineral occurrences in the area. Three hundred and seventy four samples were collected and analysed by atomic absorption spectrophotometer for Cu, Pb, Zn, Ni, Co, Fe, Mg, Mn, and Ca; the pH of each sample was also determined. The study area lies within the tropics, where deep weathering prevails, and the stream sediment is a product of the bed rocks as well as physio-chemical changes in the weathered products. The base metal content of the stream sediment, therefore, usually reflects not only the sought mineral but also various other features that are not directly related to mineralization. To obviate these features and to make a meaningful interpretation of the stream sediment analytical data, various statistical techniques were employed; principal among these were correlation and factor analysis. Geochemical anomalies of Cu, Zn and, to less extent, Pb occur on the northern flank of Guberunde Horst. These anomalies are, in the main, associated with frac-
turing and volcanism which predated the uplift of the Horst (Ojo 1983).
GEOLOGICAL SETTING The Guberunde Horst is underlain by crystalline basement rocks, which were, before Cretaceous times, subjected to metamorphism, granitization and tectonism. The basement complex rocks in this area can be described under the following groups (Fig. 2). Migmatites are seen in scattered relics in the area; they have been affected by very high grade metamorphism of the hypersthene-pyroclase-granulite facies (750°C and 2 Kb pH20). There are, however, places where the migmatites show retrograde metamorphism-probably within the temperature range of 500-600°C. The gneissic complex was sheared and foliated by regional tectonics and metamorphosed in the amphibolite facies. Biotite and biotite-muscovite-granite gneisses that make up this group are closely associated with granites with which they often have gradational contacts. The granitic complex is composed of mafic and intermediate plutonic rocks (diorite and granodiorite), finegrained granites and syntectonic granites (porphyritic and equigranular). Although the mafic and intermediate plutonic rocks form only few and small outcrops in the entire area, they contribute in no small measure to the distribution of trace elements. At contact zones with granites, the mafic and intermediate plutonic rocks are affected by granitization which produces hybrid rocks such as adamellite and quartz diorite. Syntectonic granites are the most widespread of the basement complex rocks in the area. Their equigranular member can be texturally referred to as intermediate between migmatites and porphyritic granites. Their structure may be complex; they are in places massive, 91
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VOLCANICS I
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SEDIMENTS
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BASEMENT SCALE
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Fig. 1. M a p of U p p e r B e n u e Trough showing project area.
but they may also be foliated or form a complex mixture of both. The principal minerals of the basement rocks are: microcline, which in places, forms more than 50% of the mineral constituents; plagioclase (about 20%), quartz (not less than a fifth of the mineral composition); and micas (which may form up to a tenth). Intrusive bodies such as felsites and rhyolites are seen in fracture zones in the area. These rocks play a significant role in the contribution of trace metals. Pre-Turonian fracturing of the Horst is mostly in three directions. The most significant of these is NE-SW faulting that produced the Horst with its adjacent Bryel and Zange grabens to the north and south respectively. The north-south and east-west fractures systems are mostly filled with rhyolites; these form traps for migrating fluids and thus develop favourable environments for concentration of trace elements.
SAMPLING AND ANALYTICAL METHODS Stream sediments were collected in the study area at a density of one sample per 2 km 2 (Fig. 3). This coverage was considered reasonable for such a regional survey as it would give adequate reflection of anomalous areas of up to 4-5 km 2 in extent (Smith et al. 1976). The samples were air dried, disaggregated and sieved to minus 80 mesh for analysis. Copper, lead, zinc, cobalt, manganese, nickel, calcium and iron were determined by atomic absorption spectrophotometry after hot leaching with HNO3, HC104 and HC1. Lanthanum was used as a universal buffer serving as a releasing agent, ionization suppressor and to eliminate other less explicable interelement effects. The pH of stream sediments was determined in a 1 : 5 sediment-distilled water slurry by means of a standardized pH meter. Readings were taken after 30 sec and re-standardiza-
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FRACTURES GEOLOGICAL BOUNOARIES STRIKE AND DIP OF STRATA STRIKE AND DIP DIRECTION OF FOLIATION MAJOR ROADS MAJOR STREAMS AND RIVERS TOWNS AND VILLAGES
Fig. 2. Geological map of Guberunde Horst.
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Stream sediment geochemistry of Guberunde Horst, Upper Benue Trough, Nigeria
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Table 1. Statisticalparameters Element Cu Ni Co Pb Zn Mn Fe Ca Mg pH
N
Range
Mean (x)
St. Dev.
Variance
Skewness
x + 2s
373 360 355 354 372 309 361 356 362 281
1-1005 1-79 1-50 1-110 1-780 58-1984 4-192 3-60 3-15.6 44-8.2
27.4 15.1 6.3 9.1 61.6 597.3 4.8 1.0 7.8 6.0
72.0 13.5 8.3 12.8 55.2 280.1 2.5 0.7 4.5 0.6
5189.5 182.6 67.9 162.9 3047.2 78472.8 6.1 0.4 20,4 0.3
10.9 1.6 3.3 2.9 6.9 1.3 1.5 4.7 -0.1 0.2
171.4 42.1 22.9 34.7 172.0 1157.4 9.8 2.4 16.8 7.2
tion of the pH meter was done after a batch of ten measurements•
DATA ANALYSIS Analytical results of the stream sediments of the Guberunde Horst are presented in Table 1. All the economic elements (Cu, Pb, Zn, Ni and Co) are positively skewed, that is, they all cluster more to the left of the mean with anomalies to the right• In other words, a greater percentage of the data falls within the background and threshold values whereas the anomalous values are few and scattered. All the economic elements are log-normally distributed and also unimodal except lead which forms a bimodal distribution• Threshold values (Table 1) are estimated with the assumption that the mean plus two standard deviations mark the greatest upper band of threshold values, and that the upper 2.5%
of the values are anomalous (Garret and Nichol 1967, Sinclair 1976). The anomalies constitute therefore, the samples with analytical values higher than the mean plus twice standard deviation. The distribution of economic trace metals is displayed in Fig. 4. Copper and zinc have a close affinity in the location of their anomalous values near the northern border of the Horst. This area abounds in numerous fractures and rhyolite-filled veins. Lead is more closely associated with cobalt and nickel than with copper and zinc. While nickel has a weak but positive correlation with copper, lead and zinc, cobalt has a positive and stronger correlation with copper and zinc (Table 2). It is noted that cobalt is more strongly correlated with copper and zinc in volcano-sedimentary areas at the contact of the Horst and Cretaceous sediments east of Wuyo. The values and statistical distribution of Mn, Fe, Ca and Mg are directly related to the lithology of the underlying rocks and not to any economic mineralization. In intra-
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Table 2. Correlation coefficients of Guberunde Horst stream sediments
Cu Ni Co Pb Zn Mn Fe Ca Mg pH
Cu
Ni
Co
Pb
Zn
Mn
Fe
Ca
Mg
pH
1.0000
0.0163 1.0000
0.0285 0.0794 1.0000
-0.0885 -0.0054 -0.0984 1.0000
0.0869 0.0336 0.0839 0.1564 1.0000
-0.0431 -0.2041 -0.1312 0.0207 0.0829 1.0000
0.0083 -0.2570 -0.1431 -0.1476 0.1240 0.5752 1.0000
-0.0594 -0.1329 -0.1102 0.3491 0.0647 0.2879 0.2187 1.0000
0.0279 -(/.0168 0.0124 -0.5081 -0.1751 (/.2761 (t.2893 -0.0874 1.0000
0.0416 0.0424 -0.0808 -0.0236 0.0759 0.0510 0.0733 0.2731 0.1053 1.0000
continental basins, such as the study area, however, hydrous Fe and Mn oxides form very efficient traps for trace metals such as Co, Ni, Pb and Cu (Lecomte and Sondag 1980). In such environments, and particularly in oxidized environments, such as the middle Gongola basin that hosts the Guberunde Horst, oxides and hydrous oxides of Fe and Mn are the principal factors which control fixation of the trace metals. The strong and positive correlation that exists between Fe and Mn can, therefore, be attributed to a favourable environmental condition that aids Cu-Zn mineralization. The R-mode correlation (Table 2) shows a weak but positive correlation between the economic metals. Co and Zn are indicated as pathfinder elements for Cu. Similarly Ni is an indicator element for Pb. There are positive and significant correlations between the various lithology elements. Regression analysis of the elements were computed, and their statistics are summarized in Table 3. As observed in the correlation matrix (Table 2) the elements have weak correlation, as is often the case in regional stream sediments surveys of areas without large disseminated mineralization. This is equally reflected in the regression analysis. The regressions with correlation coefficient of at least 0.3 are plotted and are shown in Figs. 5-7, but these only reflect the lithology of the area. Though the correlation coefficient of Ni-Co (0.08) is low
Table 3. Scattergram of elements----statisticaldata
Cu/Ni Cu/Co Cu/Zn Ni/Co Ni/Zn Ni/Mn Ni/Fe Co/Fe Pb/Zn Pb/Ca Zn/Fe Mn/Fe Mn/Mg Mn/Ca Fe/Mg Fe/Ca
Correlation coefficient
Standard error of estimate
Intercept (A)
Slope (B)
0.02 0.03 0.09 0.08 0.03 -0.20 -0.26 -0.14 0.16 0.35 0.12 0.58 0.28 0.29 0.29 0.22
73.4 73.9 72.1 13.5 13.5 14.2 13.2 8.3 12.7 11.7 55.3 229.3 270.3 271.4 2.4 2.4
26.8 26.6 20.5 14.4 14.6 21.6 22.2 9.0 6.9 1.1 47.9 287.1 420.2 335.0 3.6 4.0
0.89 0.25 0.11 0.13 0.01 -0.01 - 1.44 -0.49 0.04 7.76 2.8 63.3 20.0 291.0 0.16 0.82
it does, however, show the bi-group distribution (Fig. 8) which influences the Cu-Zn distribution in the Guberunde Horst. R-mode factor analysis has been extensively used in recent years by geochemists to evaluate multivariate geochemical data. This is done by grouping the elements into associations influenced by the same factor (Nichol et al. 1966, 1969, Garret and Nichol 1967, Woodsworth 1971, Closs and Nichol 1975, Ukpong and Olade 1979, Elueze 1977, Olade et al. 1979, Zeegers 1978, Imeokparia 1980, Lecomte and Sondag 1980, Ojo 1982). Rmode factor analysis is used in this study to interpret the structure within the variance-covariance matrix of the analytical data (Davis 1973). Hence, element concentrations are grouped according to similarity in their behaviour in the study area. The factor analysis, therefore, determines the minimum integral factor responsible for the variance in the analytical data. The inter-element relationships for Cu, Ni, Pb, Zn, Co, Mn, Fe, Ca, Mg and pH values in stream sediments from the Guberunde Horst are summarized in a factor analysis matrix (Table 4). This has been rotated according to the varimax criterion. The four-factor model of the data are transformed as shown in the matrix of Table 5. Factor scores (Table 6) are calculated in order to appreciate the implication which geographic and mineralogic influences may have had on the data associations. The matrix factor scores computed reflects, in part, the covariance structure of the original variables as well as the structure of the factors.
Table 4. Varimax rotated factor matrix of Guberunde Horst
Cu Ni Co Pb Zn Mn Fe Ca Mg pH
Factor I
Factor 2
Factor 3
Factor 4
Communality
-0.08213 -0.50779 -0.07322 -0.02575 0.25454 0.81321 0.83816 0.44680 0.39042 -0.10169
0.04365 0.21259 -0.36167 -0.03937 -0.03547 0.07299 0.11041 0.60020 0.44552 0.80282
-0.23966 0.47684 0.42910 0.77118 0.38420 0.04849 -0.10189 -0.14925 0.30311 -0.02986
0.70331 0.13132 0.22442 -0.14482 0.71156 0.01772 0.13079 -0.10855 -0.24513 0.14195
0.56074 0,54767 0.37066 0.61791 0.71998 0.66930 0.74219 0.62564 0.50288 0.71433
Eigenvalue as % for 4-factor model 38.1% 22.4% 20.1%
19.3%
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Stream sediment geochemistry of Guberunde Horst, Upper Benue Trough, Nigeria
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It is usually desirable to plot the factor scores on a map as they give the combination of the effect of all the factors affecting an observation as a single number. This procedure was, however, not successful in this study as the resultant map is too complex for easy interpretation. The factor score coefficients are, nonetheless, applied in the interpretation of the data. Figure 9 represents the plots of principal components scores of variables with various combinations of factors.
RESULTS
The results of R-mode factor analysis show that the data fall into a four-factor model, and the orthogonal varimax solution yields the following metal associations. Factor 1, Mn-Fe versus Ni: This constitutes 38.1% of the variability of this model; lithologically it is in an oxidizing environment. The negative correlation of Ni with Mn and Fe is due to the paucity of basic and ultrabasic rocks in the entire area.
Table 5. Transformation matrix
Factorl Factor2 Factor3 Factor4
Factorl
Factor2
Factor3
Factor4
0.87594 0.27110 -0.18764 -0.35216
0.43095 -0.09197 0.55554 0.70512
-0.20031 0.73137 0.60000 -0.25490
-0.08293 0.61898 -0.54422 0.56019
Factor 2, Ca-pH: This factor accounts for 22.4% of variance in the model; and represents an environmental association reflected by strong loading of 0.80 for pH. Factor 3, Ni-Co-Pb: This explains 20.1% of the variability of this model. It is a lithological factor which has tendencies towards mineralization. The association suggests the formation of Cu-Zn mineralization, particularly in the volcanogenic areas in a NE-SW trend on the northern flank of the Horst (Fig. 4). Factor 4, Cu-Zn: The association of Cu-Zn, though accounting for only 19.3% of the variability of this model, signifies Cu-Zn mineralization in the Horst. Both Cu and Zn have strong loading of at least 0.70 while no other element has loading of up to 0.25. Pb has a negative loading in this factor and this explains the probable lack of lead in the economic mineralization of this area.
Table 6. Factor score coefficients
Cu Ni Co Pb Zn Mn Fe Ca Mg pH
Factor i
Factor 2
Factor 3
Factor 4
-0.05740 -0.26996 0.04142 0.03308 0.06345 0.41488 0.41487 0.13198 0.15019 -0.17468
0.06056 0.25902 -0.24342 -0.00555 -0.03018 -0.06557 -0.04210 0.37834 0.28256 0.63452
-0.21989 0.33509 0.29881 0.58996 0.26883 0.08208 -0.03513 -0.05925 0.27788 -0.01390
0.57075 0,08387 0.14458 -0.15468 0.54607 0.01963 0.13561 -0.13561 -0.19250 0.21861
100
O . M . OJo INTERPRETATION
Copper-zinc In general terms, most of the anomalous values of copper and zinc lie on the northern limit of the Guberunde Horst and conform with the NE-SW trending border faults. The distribution of copper is controlled by structural features and lithology. Copper has a higher threshold value in the areas bordering regional fractures and faults whereas low background values of copper are recorded in non-fractured areas or in areas with short and localized fractures. At the contact zone of the Horst with Cretaceous sediments, more anomalies are recorded on the basement side of the contact than in the sediments. This shows that the origin of enhanced copper in the area is the basement rocks of the Horst rather than the basal volcanogenic sediments. In other words, the source of the copper is attributed to weathering of uplifted continental masses in a riftogenic basin (Olade 1980). Such copper anomalies have been considered as syngenetic elsewhere in Africa such as at Cuanza in Angola, the Gabon Basin and in Morocco (Caia 1976). The underlying geology plays a major role in the distribution of copper anomalies. The concentration of high values of copper on the northern flank of Guberunde Horst coincides with the distribution of mafic and intermediate plutonic rocks in that area. These rocks are rich in femic minerals including some biotite; they are consequently, favourable host rocks for copper mineralization. As a result of weathering, copper is released from the various copper-bearing minerals in the country mafic and intermediate plutonic rocks. Some of the copper passes into soil where it co-precipitates with or adsorbed by clay minerals and organic matter (Boyle et al. 1966). It then passes directly through underground water into streams where it is co-precipitated in the sediments. Zinc has a close affinity with copper in its distribution in the stream sediments of the Guberunde Horst. Similar to copper, most of the anomalous values of zinc are related to the northern border faults of the Horst, but the metal is more closely related to the volcanics of the southwestern corner of the Horst. In such environments, zinc is concentrated in the extrusive zones of the volcanics where it may form massive strata bound deposits (Garret 1974). As a result of oxidation and weathering, zinc, like copper, is liberated from its sulphide combinations in zinc-bearing minerals in the country rocks as a soluble sulphate under acid to near neutral conditions. Its divalent ion is then adsorbed and co-precipitated with, or adsorbed to, sediment particles (Boyle et al. 1966). Nickel-cobalt-lead The association of Ni-Co-Pb partly reflects lithology and mineralization. The low nickel content in the stream sediment of the study area may be due to the paucity of
basic and ultrabasic rocks in the Horst area. The bulk of nickel in the basement complex areas is incorporated in silicate minerals, the most important of which are olivine and hypersthene while augite, amphibole and biotite are of less importance. However, in granites such as are present in the study area, almost all the nickel is contained in the biotite, and in such environments which have no ultrabasic bodies, economic nickel mineralization is rare. Although cobalt does not occur in significant anomalies in the area, relatively high values are found in the volcanic and intensely fractured southwestern corner of the area. The high cobalt values associated with volcanics suggest that this element may have been incorporated in the intrusives as a result of contact metasomatic process between intrusive volcanics and the host granites. Cobalt may, therefore, be used to indicate epigenetic stockwork copper-zinc mineralization such as may occur in the southwest corner of the Horst. Both cobalt and nickel have a medium to high density in the area east of Wuyo where chalcopyrite and pyrite are observed in the rhyolite filled veins. The low values recorded in the analysis for lead are not surprising as stream sediments in non-mineralized areas are generally low in lead, its relative high mobility notwithstanding. Lead has a weak but positive correlation with zinc and no correlation with copper. In fact, the few lead anomalies are not associated with high values of copper and zinc. Lead can, therefore, be only an indicator element for copper-zinc mineralization in the area. Manganese-iron-calcium-magnesium The association of Mn-Fe-Ca-Mg is a lithological factor reflecting the geology of the underlying rocks. In this survey, Mn has a positive correlation with Fe, Ca and Mg; Fe has a strong and positive correlation with Mn and Fe and a weak correlation with pH. The best linear regression is that of Mn-Fe. In intracontinental basin such as occurs in the study area, hydrous oxides of Fe and Mn are said to be very efficient traps for trace metals such as Cu, Pb, Zn (Lecomte and Sondag 1980). In such environments oxides and hydrous oxides of Fe and Mn are the principal factors which control fixation of various trace metals.
DISCUSSION The various interpretation methods adopted have together pointed to a probable Cu-Zn mineralization in a few localities along the northern flank of the Guberunde Horst. Noticeable indicator elements include Pb, Co and Ni. The area east of Wuyo contains significant anomalies of copper and zinc. This area is infested by criss-cross rhyolite-filled dykes. The rhyolite may have acted as trap for ore-bearing fluids migrating from the surrounding basement rocks. In fact, disseminated chalcopyrite and pyrite have been identified in the rhyolite of this area.
Stream sediment geochemistry of Guberunde Horst, Upper Benue Trough, Nigeria This area bears radiometric anomalies with spotty secondary uranium minerals such as uranophane found in a few places in rhyolite filled veins. Besides, the basal conglomerates at the contact between Cretaceous sediments and basement rocks have higher than normal radioactivity. Heavy mineral sampling revealed presence of cassiterite. A similar relationship of cassiterite and uranium has been found in Yukon (Canada) and in Mexico (Boyle 1986, personal communication). It is not unusual for gold to be associated with uranium in such environment. In the southwestern corner of the Horst there are numerous volcanic outcrops and also a network of fractures. The basement exhibits contact hybrid metamorphic rocks in the form of adamellite. The slightly low anomalies of copper and zinc in this area may be accounted for by the presence of volcanic sequences. In normal metallogenic sequences, rocks hosting volcanogenic massive Cu-Zn sulphide ore may have only a slightly higher zinc content than unmineralized volcanic sequences. In such an environment, veins and veinlets of hybrid rocks such as adamellite and rhyolite usually facilitate the concentration of pyrite, chalcopyrite, pyrrhotite and sphalerite. This is the situation to be expected in the southwestern corner of the Guberunde Horst.
REFERENCES Boyle, R. W., Tupper, W. M., Lynch, J., Friedrich, G., Ziauddin, M., Shafiqullah, M., Carter, M. and Bygrave, K. 1966. Geochemistry of Pb, Zn, Cu, As, Sb, Mo, Sn, W, Ag, Ni, Co, Cr, Ba and M in the waters and streams sediments of the Bathurst-Jaquet River District, New Brunswick. GSC Paper 65-42. Caia, J. 1976. Paleogeographical and sedimentological controls of copper, lead, zinc mineralizations in the Lower Cretaceous sandstones of Africa. Econ. Geol. 71,409-422. Closs, L. G. and Nichol, L. 1975. The role of factor and regression analysis in the interpretation of geochemical reconnaissance data. Can. J. Earth Sci. 12, 1316-1330.
101
Davis, J. C. 1973. Statistics and Data Analysis in Geology. John Wiley, New York. Elueze, A. A. 1977. Geology and geochemical studies in the Ilesha schist belt in relation to gold mineralization. Unpubl. M. Phil. Thesis, Ibadan University, Ibadan. Garret, R. G. 1974. Copper and zinc in Proterozoic acid volcanies as a guide to exploration in the Bear Province. Geochem. Explor. Vancouver IGES, 371-387. Garret, R. G. and Nichol, I. 1967. Regional geochemical reconnaissance in eastern Sierra Leone. IMM Trans. B. 76, B97-Blll. Imeokparia, E. G. 1980. Ore-bearing potential of granitic rocks from the Jos-Bukuru Complex, northern Nigeria. Chem. Geol. 28(1-2), 69-77. Lecomte, P. and Sondag, G. 1980. Regional geochemical reconnaissance in the Belgian Ardennes. Secondary dispersion patterns in stream sediments. Mineraliurn Deposita 15, 47--60. Nichol, I. 1969. The role of some statistical and mathematical methods in the interpretation of regional geochemical data. Econ. Geol. 64, 204-220. Nichol, I., Garrett, R. G. and Webb, J. S. 1967. Automatic data plotting and mathematical and statistical interpretation of geochemical data. GSC Paper 66-54, pp. 195-210. Ojo, O. M. 1980. Prospects of finding uranium deposits in Nigeria. J. Min. Geol. 17, 1-12. Ojo, O. M. 1982. Geology and stream sediment Geochemical Survey of Middle Gongola Basin (Upper Benue Trough) of Nigeria. Unpubl. Ph.D. Thesis, Ibadan University, Ibadan. Ojo, O. M. 1983. Pre-Turonian structural control of basemetal mineralization in Middle Gongola Basin. Nig. J. Min. Geol. Vol. 21 (1 & 2), pp. 13-17. Olade, M. A. 1980. Plate tectonics and me tallogeny of intracontinental rifts and aulacogens in Africa--a review. Proceedings of the 5th Qua'd IAGOD Syrup., pp. 91-111. Olade, M. A., Van de Kraats, A. H. and Ukpong, E. E. 1979. Effects of environmental parameters on metal dispersion patterns in stream sediments from the lead-zinc belt, Benue Trough, Nigeria using factor analysis. Geol. en Mijnbouw 58,341-351. Sinclair, A. J, 1976. Applications of probability graphs in mineral exploration. A. E. G. Spec. Vol. 4. Smith, A. Y., Cameron, J. and Barretto, P. M. 1976. Uranium geochemical prospecting in Austria. LA. E.A. Sym. Vienna, MarchApril 1976, pp. 657-670. Ukpong, E. E. and Olade, M. A. 1979. Geochemical surveys for lead-zinc mineralization, southern Benue Trough, Nigeria. IMM Trans. B. 88 81-93. Woodsworth, G. J. 1971. A geochemical drainage survey and its implication for metallogenesis, Central Coast Mountains, British Columbia. EG 66(8), 1104-1120. Zeegers, H. 1978. Regional geochemical prospecting in equatorial areas, an example in French Guiana. Geochem. Explor. Denver IGES, 209-225.
0731-7247/88 $3.00 + 0.00 Pergamon Journals Ltd.
Printed in Great Britain
Stream sediment geochemistry of Guberunde Horst, "Gongola Basin", Upper Benue Trough, Nigeria O. M. OJo Exploration Division, Nigerian Mining Corporation, Jos, Nigeria
(Received for publication 8 December 1986) Abstract--The crystalline basement complex of Guberunde Horst is mainly composed of granitic rocks, into which acid volcanics of the Burashika group of rocks were intruded during the Jurassic. The Horst itself was produced in the Lower Albian by uplift of the basement between two series of NE-SW striking border faults. Most of the fractures which pre-dated the uplift are filled with rhyolite and aplite of the Burashika group. A total of 374 stream sediment samples were collected over an area of 700 km 2. Each sample was analysed for Cu, Pb, Zn, Co, Ni, Fe, Mg, Mn and Ca; in addition, the pH was determined in each. Condescriptive and various multivariate statistical methods were employed in the interpretation of the resultant geochemical data. The various association of elements exhibits a four-factor model as follows: (1) Mn-Fe versus Ni: oxidizing environment (2) Ca-pH: environmental association (3) Ni-Co-Pb: lithological effect (4) Cu-Zn: basemetal mineralization association.
INTRODUCTION THE GUBERUNDEHorst, a product of Lower Cretaceous block faulting, is situated in the centre of the Gongola Basin (Upper Benue Trough) of Nigeria (Fig. 1). It is bounded in the northeast by Biu basalts and in the southwest by the Gongola River and flanked on other sides by Cretaceous sediments (Bima Formation). The structural features of the middle Gongola Basin suggest an aulacogen in the northern arm of Upper Benue (Ojo 1982). An environment which has been subjected to block faulting, uplifting and affected by granitization and volcanism suggested that regional stream sediment sampling of the study area would be an effective method of searching for base metals. No report of base metal mineralization had ever been made in the area. Consequently reconnaissance stream sediment sampling was conducted on a density of 1 sample per 2 km 2 to locate any possible mineral occurrences in the area. Three hundred and seventy four samples were collected and analysed by atomic absorption spectrophotometer for Cu, Pb, Zn, Ni, Co, Fe, Mg, Mn, and Ca; the pH of each sample was also determined. The study area lies within the tropics, where deep weathering prevails, and the stream sediment is a product of the bed rocks as well as physio-chemical changes in the weathered products. The base metal content of the stream sediment, therefore, usually reflects not only the sought mineral but also various other features that are not directly related to mineralization. To obviate these features and to make a meaningful interpretation of the stream sediment analytical data, various statistical techniques were employed; principal among these were correlation and factor analysis. Geochemical anomalies of Cu, Zn and, to less extent, Pb occur on the northern flank of Guberunde Horst. These anomalies are, in the main, associated with frac-
turing and volcanism which predated the uplift of the Horst (Ojo 1983).
GEOLOGICAL SETTING The Guberunde Horst is underlain by crystalline basement rocks, which were, before Cretaceous times, subjected to metamorphism, granitization and tectonism. The basement complex rocks in this area can be described under the following groups (Fig. 2). Migmatites are seen in scattered relics in the area; they have been affected by very high grade metamorphism of the hypersthene-pyroclase-granulite facies (750°C and 2 Kb pH20). There are, however, places where the migmatites show retrograde metamorphism-probably within the temperature range of 500-600°C. The gneissic complex was sheared and foliated by regional tectonics and metamorphosed in the amphibolite facies. Biotite and biotite-muscovite-granite gneisses that make up this group are closely associated with granites with which they often have gradational contacts. The granitic complex is composed of mafic and intermediate plutonic rocks (diorite and granodiorite), finegrained granites and syntectonic granites (porphyritic and equigranular). Although the mafic and intermediate plutonic rocks form only few and small outcrops in the entire area, they contribute in no small measure to the distribution of trace elements. At contact zones with granites, the mafic and intermediate plutonic rocks are affected by granitization which produces hybrid rocks such as adamellite and quartz diorite. Syntectonic granites are the most widespread of the basement complex rocks in the area. Their equigranular member can be texturally referred to as intermediate between migmatites and porphyritic granites. Their structure may be complex; they are in places massive, 91
92
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but they may also be foliated or form a complex mixture of both. The principal minerals of the basement rocks are: microcline, which in places, forms more than 50% of the mineral constituents; plagioclase (about 20%), quartz (not less than a fifth of the mineral composition); and micas (which may form up to a tenth). Intrusive bodies such as felsites and rhyolites are seen in fracture zones in the area. These rocks play a significant role in the contribution of trace metals. Pre-Turonian fracturing of the Horst is mostly in three directions. The most significant of these is NE-SW faulting that produced the Horst with its adjacent Bryel and Zange grabens to the north and south respectively. The north-south and east-west fractures systems are mostly filled with rhyolites; these form traps for migrating fluids and thus develop favourable environments for concentration of trace elements.
SAMPLING AND ANALYTICAL METHODS Stream sediments were collected in the study area at a density of one sample per 2 km 2 (Fig. 3). This coverage was considered reasonable for such a regional survey as it would give adequate reflection of anomalous areas of up to 4-5 km 2 in extent (Smith et al. 1976). The samples were air dried, disaggregated and sieved to minus 80 mesh for analysis. Copper, lead, zinc, cobalt, manganese, nickel, calcium and iron were determined by atomic absorption spectrophotometry after hot leaching with HNO3, HC104 and HC1. Lanthanum was used as a universal buffer serving as a releasing agent, ionization suppressor and to eliminate other less explicable interelement effects. The pH of stream sediments was determined in a 1 : 5 sediment-distilled water slurry by means of a standardized pH meter. Readings were taken after 30 sec and re-standardiza-
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Stream sediment geochemistry of Guberunde Horst, Upper Benue Trough, Nigeria
95
Table 1. Statisticalparameters Element Cu Ni Co Pb Zn Mn Fe Ca Mg pH
N
Range
Mean (x)
St. Dev.
Variance
Skewness
x + 2s
373 360 355 354 372 309 361 356 362 281
1-1005 1-79 1-50 1-110 1-780 58-1984 4-192 3-60 3-15.6 44-8.2
27.4 15.1 6.3 9.1 61.6 597.3 4.8 1.0 7.8 6.0
72.0 13.5 8.3 12.8 55.2 280.1 2.5 0.7 4.5 0.6
5189.5 182.6 67.9 162.9 3047.2 78472.8 6.1 0.4 20,4 0.3
10.9 1.6 3.3 2.9 6.9 1.3 1.5 4.7 -0.1 0.2
171.4 42.1 22.9 34.7 172.0 1157.4 9.8 2.4 16.8 7.2
tion of the pH meter was done after a batch of ten measurements•
DATA ANALYSIS Analytical results of the stream sediments of the Guberunde Horst are presented in Table 1. All the economic elements (Cu, Pb, Zn, Ni and Co) are positively skewed, that is, they all cluster more to the left of the mean with anomalies to the right• In other words, a greater percentage of the data falls within the background and threshold values whereas the anomalous values are few and scattered. All the economic elements are log-normally distributed and also unimodal except lead which forms a bimodal distribution• Threshold values (Table 1) are estimated with the assumption that the mean plus two standard deviations mark the greatest upper band of threshold values, and that the upper 2.5%
of the values are anomalous (Garret and Nichol 1967, Sinclair 1976). The anomalies constitute therefore, the samples with analytical values higher than the mean plus twice standard deviation. The distribution of economic trace metals is displayed in Fig. 4. Copper and zinc have a close affinity in the location of their anomalous values near the northern border of the Horst. This area abounds in numerous fractures and rhyolite-filled veins. Lead is more closely associated with cobalt and nickel than with copper and zinc. While nickel has a weak but positive correlation with copper, lead and zinc, cobalt has a positive and stronger correlation with copper and zinc (Table 2). It is noted that cobalt is more strongly correlated with copper and zinc in volcano-sedimentary areas at the contact of the Horst and Cretaceous sediments east of Wuyo. The values and statistical distribution of Mn, Fe, Ca and Mg are directly related to the lithology of the underlying rocks and not to any economic mineralization. In intra-
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Table 2. Correlation coefficients of Guberunde Horst stream sediments
Cu Ni Co Pb Zn Mn Fe Ca Mg pH
Cu
Ni
Co
Pb
Zn
Mn
Fe
Ca
Mg
pH
1.0000
0.0163 1.0000
0.0285 0.0794 1.0000
-0.0885 -0.0054 -0.0984 1.0000
0.0869 0.0336 0.0839 0.1564 1.0000
-0.0431 -0.2041 -0.1312 0.0207 0.0829 1.0000
0.0083 -0.2570 -0.1431 -0.1476 0.1240 0.5752 1.0000
-0.0594 -0.1329 -0.1102 0.3491 0.0647 0.2879 0.2187 1.0000
0.0279 -(/.0168 0.0124 -0.5081 -0.1751 (/.2761 (t.2893 -0.0874 1.0000
0.0416 0.0424 -0.0808 -0.0236 0.0759 0.0510 0.0733 0.2731 0.1053 1.0000
continental basins, such as the study area, however, hydrous Fe and Mn oxides form very efficient traps for trace metals such as Co, Ni, Pb and Cu (Lecomte and Sondag 1980). In such environments, and particularly in oxidized environments, such as the middle Gongola basin that hosts the Guberunde Horst, oxides and hydrous oxides of Fe and Mn are the principal factors which control fixation of the trace metals. The strong and positive correlation that exists between Fe and Mn can, therefore, be attributed to a favourable environmental condition that aids Cu-Zn mineralization. The R-mode correlation (Table 2) shows a weak but positive correlation between the economic metals. Co and Zn are indicated as pathfinder elements for Cu. Similarly Ni is an indicator element for Pb. There are positive and significant correlations between the various lithology elements. Regression analysis of the elements were computed, and their statistics are summarized in Table 3. As observed in the correlation matrix (Table 2) the elements have weak correlation, as is often the case in regional stream sediments surveys of areas without large disseminated mineralization. This is equally reflected in the regression analysis. The regressions with correlation coefficient of at least 0.3 are plotted and are shown in Figs. 5-7, but these only reflect the lithology of the area. Though the correlation coefficient of Ni-Co (0.08) is low
Table 3. Scattergram of elements----statisticaldata
Cu/Ni Cu/Co Cu/Zn Ni/Co Ni/Zn Ni/Mn Ni/Fe Co/Fe Pb/Zn Pb/Ca Zn/Fe Mn/Fe Mn/Mg Mn/Ca Fe/Mg Fe/Ca
Correlation coefficient
Standard error of estimate
Intercept (A)
Slope (B)
0.02 0.03 0.09 0.08 0.03 -0.20 -0.26 -0.14 0.16 0.35 0.12 0.58 0.28 0.29 0.29 0.22
73.4 73.9 72.1 13.5 13.5 14.2 13.2 8.3 12.7 11.7 55.3 229.3 270.3 271.4 2.4 2.4
26.8 26.6 20.5 14.4 14.6 21.6 22.2 9.0 6.9 1.1 47.9 287.1 420.2 335.0 3.6 4.0
0.89 0.25 0.11 0.13 0.01 -0.01 - 1.44 -0.49 0.04 7.76 2.8 63.3 20.0 291.0 0.16 0.82
it does, however, show the bi-group distribution (Fig. 8) which influences the Cu-Zn distribution in the Guberunde Horst. R-mode factor analysis has been extensively used in recent years by geochemists to evaluate multivariate geochemical data. This is done by grouping the elements into associations influenced by the same factor (Nichol et al. 1966, 1969, Garret and Nichol 1967, Woodsworth 1971, Closs and Nichol 1975, Ukpong and Olade 1979, Elueze 1977, Olade et al. 1979, Zeegers 1978, Imeokparia 1980, Lecomte and Sondag 1980, Ojo 1982). Rmode factor analysis is used in this study to interpret the structure within the variance-covariance matrix of the analytical data (Davis 1973). Hence, element concentrations are grouped according to similarity in their behaviour in the study area. The factor analysis, therefore, determines the minimum integral factor responsible for the variance in the analytical data. The inter-element relationships for Cu, Ni, Pb, Zn, Co, Mn, Fe, Ca, Mg and pH values in stream sediments from the Guberunde Horst are summarized in a factor analysis matrix (Table 4). This has been rotated according to the varimax criterion. The four-factor model of the data are transformed as shown in the matrix of Table 5. Factor scores (Table 6) are calculated in order to appreciate the implication which geographic and mineralogic influences may have had on the data associations. The matrix factor scores computed reflects, in part, the covariance structure of the original variables as well as the structure of the factors.
Table 4. Varimax rotated factor matrix of Guberunde Horst
Cu Ni Co Pb Zn Mn Fe Ca Mg pH
Factor I
Factor 2
Factor 3
Factor 4
Communality
-0.08213 -0.50779 -0.07322 -0.02575 0.25454 0.81321 0.83816 0.44680 0.39042 -0.10169
0.04365 0.21259 -0.36167 -0.03937 -0.03547 0.07299 0.11041 0.60020 0.44552 0.80282
-0.23966 0.47684 0.42910 0.77118 0.38420 0.04849 -0.10189 -0.14925 0.30311 -0.02986
0.70331 0.13132 0.22442 -0.14482 0.71156 0.01772 0.13079 -0.10855 -0.24513 0.14195
0.56074 0,54767 0.37066 0.61791 0.71998 0.66930 0.74219 0.62564 0.50288 0.71433
Eigenvalue as % for 4-factor model 38.1% 22.4% 20.1%
19.3%
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Stream sediment geochemistry of Guberunde Horst, Upper Benue Trough, Nigeria
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Fig. 9. Representation of variables in the space of factors 1--4 in Guberunde Horst.
It is usually desirable to plot the factor scores on a map as they give the combination of the effect of all the factors affecting an observation as a single number. This procedure was, however, not successful in this study as the resultant map is too complex for easy interpretation. The factor score coefficients are, nonetheless, applied in the interpretation of the data. Figure 9 represents the plots of principal components scores of variables with various combinations of factors.
RESULTS
The results of R-mode factor analysis show that the data fall into a four-factor model, and the orthogonal varimax solution yields the following metal associations. Factor 1, Mn-Fe versus Ni: This constitutes 38.1% of the variability of this model; lithologically it is in an oxidizing environment. The negative correlation of Ni with Mn and Fe is due to the paucity of basic and ultrabasic rocks in the entire area.
Table 5. Transformation matrix
Factorl Factor2 Factor3 Factor4
Factorl
Factor2
Factor3
Factor4
0.87594 0.27110 -0.18764 -0.35216
0.43095 -0.09197 0.55554 0.70512
-0.20031 0.73137 0.60000 -0.25490
-0.08293 0.61898 -0.54422 0.56019
Factor 2, Ca-pH: This factor accounts for 22.4% of variance in the model; and represents an environmental association reflected by strong loading of 0.80 for pH. Factor 3, Ni-Co-Pb: This explains 20.1% of the variability of this model. It is a lithological factor which has tendencies towards mineralization. The association suggests the formation of Cu-Zn mineralization, particularly in the volcanogenic areas in a NE-SW trend on the northern flank of the Horst (Fig. 4). Factor 4, Cu-Zn: The association of Cu-Zn, though accounting for only 19.3% of the variability of this model, signifies Cu-Zn mineralization in the Horst. Both Cu and Zn have strong loading of at least 0.70 while no other element has loading of up to 0.25. Pb has a negative loading in this factor and this explains the probable lack of lead in the economic mineralization of this area.
Table 6. Factor score coefficients
Cu Ni Co Pb Zn Mn Fe Ca Mg pH
Factor i
Factor 2
Factor 3
Factor 4
-0.05740 -0.26996 0.04142 0.03308 0.06345 0.41488 0.41487 0.13198 0.15019 -0.17468
0.06056 0.25902 -0.24342 -0.00555 -0.03018 -0.06557 -0.04210 0.37834 0.28256 0.63452
-0.21989 0.33509 0.29881 0.58996 0.26883 0.08208 -0.03513 -0.05925 0.27788 -0.01390
0.57075 0,08387 0.14458 -0.15468 0.54607 0.01963 0.13561 -0.13561 -0.19250 0.21861
100
O . M . OJo INTERPRETATION
Copper-zinc In general terms, most of the anomalous values of copper and zinc lie on the northern limit of the Guberunde Horst and conform with the NE-SW trending border faults. The distribution of copper is controlled by structural features and lithology. Copper has a higher threshold value in the areas bordering regional fractures and faults whereas low background values of copper are recorded in non-fractured areas or in areas with short and localized fractures. At the contact zone of the Horst with Cretaceous sediments, more anomalies are recorded on the basement side of the contact than in the sediments. This shows that the origin of enhanced copper in the area is the basement rocks of the Horst rather than the basal volcanogenic sediments. In other words, the source of the copper is attributed to weathering of uplifted continental masses in a riftogenic basin (Olade 1980). Such copper anomalies have been considered as syngenetic elsewhere in Africa such as at Cuanza in Angola, the Gabon Basin and in Morocco (Caia 1976). The underlying geology plays a major role in the distribution of copper anomalies. The concentration of high values of copper on the northern flank of Guberunde Horst coincides with the distribution of mafic and intermediate plutonic rocks in that area. These rocks are rich in femic minerals including some biotite; they are consequently, favourable host rocks for copper mineralization. As a result of weathering, copper is released from the various copper-bearing minerals in the country mafic and intermediate plutonic rocks. Some of the copper passes into soil where it co-precipitates with or adsorbed by clay minerals and organic matter (Boyle et al. 1966). It then passes directly through underground water into streams where it is co-precipitated in the sediments. Zinc has a close affinity with copper in its distribution in the stream sediments of the Guberunde Horst. Similar to copper, most of the anomalous values of zinc are related to the northern border faults of the Horst, but the metal is more closely related to the volcanics of the southwestern corner of the Horst. In such environments, zinc is concentrated in the extrusive zones of the volcanics where it may form massive strata bound deposits (Garret 1974). As a result of oxidation and weathering, zinc, like copper, is liberated from its sulphide combinations in zinc-bearing minerals in the country rocks as a soluble sulphate under acid to near neutral conditions. Its divalent ion is then adsorbed and co-precipitated with, or adsorbed to, sediment particles (Boyle et al. 1966). Nickel-cobalt-lead The association of Ni-Co-Pb partly reflects lithology and mineralization. The low nickel content in the stream sediment of the study area may be due to the paucity of
basic and ultrabasic rocks in the Horst area. The bulk of nickel in the basement complex areas is incorporated in silicate minerals, the most important of which are olivine and hypersthene while augite, amphibole and biotite are of less importance. However, in granites such as are present in the study area, almost all the nickel is contained in the biotite, and in such environments which have no ultrabasic bodies, economic nickel mineralization is rare. Although cobalt does not occur in significant anomalies in the area, relatively high values are found in the volcanic and intensely fractured southwestern corner of the area. The high cobalt values associated with volcanics suggest that this element may have been incorporated in the intrusives as a result of contact metasomatic process between intrusive volcanics and the host granites. Cobalt may, therefore, be used to indicate epigenetic stockwork copper-zinc mineralization such as may occur in the southwest corner of the Horst. Both cobalt and nickel have a medium to high density in the area east of Wuyo where chalcopyrite and pyrite are observed in the rhyolite filled veins. The low values recorded in the analysis for lead are not surprising as stream sediments in non-mineralized areas are generally low in lead, its relative high mobility notwithstanding. Lead has a weak but positive correlation with zinc and no correlation with copper. In fact, the few lead anomalies are not associated with high values of copper and zinc. Lead can, therefore, be only an indicator element for copper-zinc mineralization in the area. Manganese-iron-calcium-magnesium The association of Mn-Fe-Ca-Mg is a lithological factor reflecting the geology of the underlying rocks. In this survey, Mn has a positive correlation with Fe, Ca and Mg; Fe has a strong and positive correlation with Mn and Fe and a weak correlation with pH. The best linear regression is that of Mn-Fe. In intracontinental basin such as occurs in the study area, hydrous oxides of Fe and Mn are said to be very efficient traps for trace metals such as Cu, Pb, Zn (Lecomte and Sondag 1980). In such environments oxides and hydrous oxides of Fe and Mn are the principal factors which control fixation of various trace metals.
DISCUSSION The various interpretation methods adopted have together pointed to a probable Cu-Zn mineralization in a few localities along the northern flank of the Guberunde Horst. Noticeable indicator elements include Pb, Co and Ni. The area east of Wuyo contains significant anomalies of copper and zinc. This area is infested by criss-cross rhyolite-filled dykes. The rhyolite may have acted as trap for ore-bearing fluids migrating from the surrounding basement rocks. In fact, disseminated chalcopyrite and pyrite have been identified in the rhyolite of this area.
Stream sediment geochemistry of Guberunde Horst, Upper Benue Trough, Nigeria This area bears radiometric anomalies with spotty secondary uranium minerals such as uranophane found in a few places in rhyolite filled veins. Besides, the basal conglomerates at the contact between Cretaceous sediments and basement rocks have higher than normal radioactivity. Heavy mineral sampling revealed presence of cassiterite. A similar relationship of cassiterite and uranium has been found in Yukon (Canada) and in Mexico (Boyle 1986, personal communication). It is not unusual for gold to be associated with uranium in such environment. In the southwestern corner of the Horst there are numerous volcanic outcrops and also a network of fractures. The basement exhibits contact hybrid metamorphic rocks in the form of adamellite. The slightly low anomalies of copper and zinc in this area may be accounted for by the presence of volcanic sequences. In normal metallogenic sequences, rocks hosting volcanogenic massive Cu-Zn sulphide ore may have only a slightly higher zinc content than unmineralized volcanic sequences. In such an environment, veins and veinlets of hybrid rocks such as adamellite and rhyolite usually facilitate the concentration of pyrite, chalcopyrite, pyrrhotite and sphalerite. This is the situation to be expected in the southwestern corner of the Guberunde Horst.
REFERENCES Boyle, R. W., Tupper, W. M., Lynch, J., Friedrich, G., Ziauddin, M., Shafiqullah, M., Carter, M. and Bygrave, K. 1966. Geochemistry of Pb, Zn, Cu, As, Sb, Mo, Sn, W, Ag, Ni, Co, Cr, Ba and M in the waters and streams sediments of the Bathurst-Jaquet River District, New Brunswick. GSC Paper 65-42. Caia, J. 1976. Paleogeographical and sedimentological controls of copper, lead, zinc mineralizations in the Lower Cretaceous sandstones of Africa. Econ. Geol. 71,409-422. Closs, L. G. and Nichol, L. 1975. The role of factor and regression analysis in the interpretation of geochemical reconnaissance data. Can. J. Earth Sci. 12, 1316-1330.
101
Davis, J. C. 1973. Statistics and Data Analysis in Geology. John Wiley, New York. Elueze, A. A. 1977. Geology and geochemical studies in the Ilesha schist belt in relation to gold mineralization. Unpubl. M. Phil. Thesis, Ibadan University, Ibadan. Garret, R. G. 1974. Copper and zinc in Proterozoic acid volcanies as a guide to exploration in the Bear Province. Geochem. Explor. Vancouver IGES, 371-387. Garret, R. G. and Nichol, I. 1967. Regional geochemical reconnaissance in eastern Sierra Leone. IMM Trans. B. 76, B97-Blll. Imeokparia, E. G. 1980. Ore-bearing potential of granitic rocks from the Jos-Bukuru Complex, northern Nigeria. Chem. Geol. 28(1-2), 69-77. Lecomte, P. and Sondag, G. 1980. Regional geochemical reconnaissance in the Belgian Ardennes. Secondary dispersion patterns in stream sediments. Mineraliurn Deposita 15, 47--60. Nichol, I. 1969. The role of some statistical and mathematical methods in the interpretation of regional geochemical data. Econ. Geol. 64, 204-220. Nichol, I., Garrett, R. G. and Webb, J. S. 1967. Automatic data plotting and mathematical and statistical interpretation of geochemical data. GSC Paper 66-54, pp. 195-210. Ojo, O. M. 1980. Prospects of finding uranium deposits in Nigeria. J. Min. Geol. 17, 1-12. Ojo, O. M. 1982. Geology and stream sediment Geochemical Survey of Middle Gongola Basin (Upper Benue Trough) of Nigeria. Unpubl. Ph.D. Thesis, Ibadan University, Ibadan. Ojo, O. M. 1983. Pre-Turonian structural control of basemetal mineralization in Middle Gongola Basin. Nig. J. Min. Geol. Vol. 21 (1 & 2), pp. 13-17. Olade, M. A. 1980. Plate tectonics and me tallogeny of intracontinental rifts and aulacogens in Africa--a review. Proceedings of the 5th Qua'd IAGOD Syrup., pp. 91-111. Olade, M. A., Van de Kraats, A. H. and Ukpong, E. E. 1979. Effects of environmental parameters on metal dispersion patterns in stream sediments from the lead-zinc belt, Benue Trough, Nigeria using factor analysis. Geol. en Mijnbouw 58,341-351. Sinclair, A. J, 1976. Applications of probability graphs in mineral exploration. A. E. G. Spec. Vol. 4. Smith, A. Y., Cameron, J. and Barretto, P. M. 1976. Uranium geochemical prospecting in Austria. LA. E.A. Sym. Vienna, MarchApril 1976, pp. 657-670. Ukpong, E. E. and Olade, M. A. 1979. Geochemical surveys for lead-zinc mineralization, southern Benue Trough, Nigeria. IMM Trans. B. 88 81-93. Woodsworth, G. J. 1971. A geochemical drainage survey and its implication for metallogenesis, Central Coast Mountains, British Columbia. EG 66(8), 1104-1120. Zeegers, H. 1978. Regional geochemical prospecting in equatorial areas, an example in French Guiana. Geochem. Explor. Denver IGES, 209-225.