Scholarly Journal of Scientific Research and Essay (SJSRE) Vol. 3(8), pp. 100-111, September 2014Available online at http://www.scholarly-journals.com/SJSRE

ISSN 2315-6163 © 2014 Scholarly-Journals

Full Length Research Paper

Geochemical control of mineralization in Egbe-Isanlutantalite–gold fields, South-West (SW) Nigeria

Paulinus N. Nnabo1*, Silas S. Dada2 and Abiye O. Solomon3.

1Department of Geology and Exploration Geophysics, Ebonyi State University, Abakaliki. Nigeria2Department of Geology and Mineral Sciences, Kwara State University, Malete, Kwara State. Nigeria

3Department of Geology and Mining, University of Jos, Jos. Nigeria

Accepted 15 September, 2014

Gold, tantalite and associated mineralization in Isanlu-Egbe area, SW Nigeria is the product of injectionof Pan-African late phase quartz-rich (pegmatitic-pneumatolytic) material into the Archaean/EarlyProterozoic gneisses, metavolcanics and metasediments (amphibolite, talc schist, amphibole schist,quartz-mica schist). Mineralization includes gold, tantalite, tin, talc, tourmaline, beryl, sulphides andother precious stones. The presence of talc schist has reinforced the notion of metavolcanics as sourceof sulphides and associated gold and other ores. Geochemical studies show that the concentration ofgold in rocks and soils from the area is consistently low except in few areas with high values andrelated to occurrences in N-S quartz veins. The values of Au, Ag, As and Cu form one population whichconstitute the background. Significant correlation that exists between Cu/Pb/Zn and Ag/As indicates thepresence of sulphide mineralizations. Detailed field mapping reveals three deformation episodes thatled to the generation of three major structural trends: E-W, NE-SW and NW-SE and near vertical N-S. Thelatter and more prominent (N-S) shears served as conduits and pathway of the ore-bearing fluids withinwhich quartz stringers, veins and pegmatitic dykes were emplaced. Only such N-S trends appearfavourable in hosting any significant mineralization at depth and should be directly explored becausethe oxidized zones where eluvials were worked for years will not only lead to spurious geochemicalanomalies but have been so intensely leached and cannot be expected to lead to any targets at depth.

Key words: Geochemistry, Metavolcanics, Mineralization, Quartz veins, Pan-African Granite, Nigerian schistbelt.

INTRODUCTION

Falconer in 1911 reported the presence of gold in manyparts of Nigeria. Gold has since been found to bewidespread in quartz veins, stream channels and schistespecially in the area west of longitude 8E. Some lodeswere located along normal faults and in shatter belts.Exploration targets have been the Precambrian rocks inthe schist belts (Tattam, 1935, Tattam, 1936). Intensiveproduction of gold took place in many of these belts asearly as 1914. The areas include Niger province (Tattam,1936), Egbe-Isanlu, Ilesha, Bussa, Maru, Anka, etc.Exploration and mining were unsystematic andconcentrated mostly in alluvials along river channels, aswell as primary lodes and veins. Panning methods were

*Corresponding author. E-mail: nnabop@yahoo.com.

mainly used.The objective of this study is to define and identify the

structures related to gold mineralization. The areacovered by this study is part of Isanlu-Egbe belt, situatedbetween latitudes 825’00’’N to 830’N and longitudes544’00’’E to 546’30’’E, in south-western Nigeria (Figure1). It started with stream sediment survey carried out indrainage systems of the area followed by detailedgeological mapping, trenching and pitting, and soilsurvey.

GEOLOGICAL SETTING

The geology of Isanlu-Egbe area has been described(Bafor, 1988, Garba, 1987, Garba, 1988, Akande et al.,1988 and Annor et al., 1996). Geologically, the areaconsists of metavolcanic-sedimentary rocks overlying a

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Figure 1. Location of the Nigerian schist belt on the margin of the Pan African Province: 1- Zungeru-Bemin Gwari,2. - Kushaka, 3- Karau-Karau, 4-Kazuare, 5-Wonaka, 6- Maru, 7-Anka, 8- Zuru, 9-Iseyin-Oyin River, 10-Ilesha, 11-Igara, 12-Egbe-Isanlu.

gneissic basement, both of which are intruded by Pan-African Granites (Jacobson and Webb 1946, Matheis andCaen-Vachette, 1988 and Rahaman, 1989) (Figure 2).Migmatite-Gneiss Complex forms the major rock unit inthe Egbe-Isanlu area and covers >50% of the totalregion. A typical assemblage of the rocks in Egbe area

consists of metagabbros, metabasaltic tuffs,metamorphosed clastic sediments and calc-silicate rocksprobably of carbonate-rich affinities (Bafor, 1988.). Themetasedimentary rocks belong to the Egbe-Isanlu-Kabbaschist belt, one of the many NNE-trending belts of low- tomedium-grade (mainly greenschist to low amphibolite

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Figure 2. Geological map of part of Egbe-Isanlu schist belt

facies) Annor et al., 1996, supracrustal rocks that arebelieved to have been deposited as Proterozoic cover onolder basement rocks. The Pan-African granitic suite,varying from coarse-grained to porphyritic types, occursaround Egbe and Isanlu areas, forming the prominenttopographic features of the region. Gold mineralizationconsists of primary gold-sulphide in quartz veins, primary

gold-sulphide disseminations in amphibolites andsecondary alluvial and eluvial deposits (Dada et al.,2003).

All the major rock units have numerous pegmatitesdykes and quartz veins. In the Pan-African Granites,there is marked concentration of the dykes around theircontacts with the host rocks. The general field

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Table 1a. Channel samples from Kachakacha part of study area.

Sample No. Au (ppb) Ag (ppm) As (ppm) Cu (ppm) Pb (ppm) Zn (ppm)C/KK/056 0.01 <0.05 <0.05 6 13 66C/KK/057 0.01 <0.05 <0.05 13 12 91C/KK/058 0.01 <0.05 <0.05 10 13 75C/KK/059 <0.01 <0.05 <0.05 56 10 65C/KK/060 0.02 <0.05 <0.05 13 11 65C/KK/061 0.01 <0.05 <0.05 29 16 72C/KK/062 <0.01 <0.05 <0.05 28 15 80C/KK/063 0.01 <0.05 <0.05 37 13 80C/KK/064 <0.01 <0.05 <0.05 6 11 117C/KK/065 <0.01 <0.05 <0.05 17 15 74C/KK/066 <0.01 <0.05 <0.05 13 15 135C/KK/067 <0.01 <0.05 <0.05 4 16 143C/KK/068 <0.01 <0.05 <0.05 16 11 77C/KK/069 <0.01 <0.05 <0.05 6 11 108C/KK/070 <0.01 <0.05 <0.05 4 13 97C/KK/071 <0.01 <0.05 <0.05 14 14 66C/KK/072 <0.01 <0.05 <0.05 28 16 69C/KK/073 <0.01 <0.05 <0.05 4 17 93C/KK/074 <0.01 <0.05 <0.05 22 16 84C/KK/075 <0.01 <0.05 <0.05 35 18 99C/KK/076 0.01 <0.05 <0.05 27 14 86C/KK/077 0.01 <0.05 <0.05 2 8 63C/KK/078 <0.01 <0.05 <0.05 60 14 93C/KK/079 <0.01 <0.05 <0.05 27 15 90C/KK/080 <0.01 <0.05 <0.05 38 14 93C/KK/081 <0.01 <0.05 <0.05 44 20 78C/KK/082 0.03 <0.05 <0.05 73 11 70C/KK/083 <0.01 <0.05 <0.05 17 10 75C/KK/084 <0.01 <0.05 <0.05 5 12 43C/KK/085 <0.01 <0.05 <0.05 8 12 65C/KK/086 <0.01 <0.05 <0.05 41 12 94C/KK/087 <0.01 <0.05 <0.05 35 12 100C/KK/088 0.01 <0.05 <0.05 12 12 63C/KK/089 <0.01 <0.05 <0.05 10 12 125C/KK/090 <0.01 <0.05 <0.05 10 16 69C/KK/091 0.07 <0.05 <0.05 8 13 56C/KK/092 <0.01 <0.05 <0.05 41 12 91C/KK/093 <0.01 <0.05 <0.05 5 11 65C/KK/094 <0.01 <0.05 <0.05 12 12 68C/KK/095 <0.01 <0.05 <0.05 17 13 91C/KK/096 <0.01 <0.05 <0.05 10 12 81C/KK/097 <0.01 <0.05 <0.05 4 11 72C/KK/098 0.01 <0.05 <0.05 5 12 93C/KK/099 <0.01 <0.05 <0.05 16 13 88C/KK/100 <0.01 <0.05 <0.05 21 15 99DG-MM-3014 <0.01 <0.05 20 68 28 83DG-MM-3015 <0.01 <0.05 <0.05 54 28 75

associations give indication of a genetic relationshipbetween the pegmatites, including the tin-bearingvarieties, and the granites. The egmatitic facies aremineralized especially in Egbe and to the south of Isanluwhere they are currently being exploited for both metallicand non-metallic minerals such as cassiterite, columbite,tantalite, tourmaline and beryl (Jacobson and Webb1946). In areas where the dykes cut through amphiboliteand talcose rocks such as Kachakacha and Okolom, theyare commonly gold- bearing.

FIELD METHODS

Detailed geological and geochemical studies were carriedout over this area which has already been covered on areconnaissance scale (Boyle, 1983 and Dada, 1983). Themapping was concentrated in areas of better potentialsas determined from the results of significant mineralzones present in selected stream channels and outcrops.In Okolom North area, the geological mapping wasconducted in order to further investigate the following

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Table 1b. Channel samples from Dogondaji part of study area.

Sample No Au (ppm). Sample No Au (ppm) Sample No. Au (ppm)C/DD/001 <0.01 C/DD/021 <0.01 C/DD/041 0.01C/DD/002 <0.01 C/DD/022 <0.01 C/DD/042 0.01C/DD/003 <0.01 C/DD/023 <0.01 C/DD/043 0.01C/DD/004 <0.01 C/DD/024 <0.01 C/DD/044 0.01C/DD/005 <0.01 C/DD/025 <0.01 C/DD/045 0.01C/DD/006 <0.01 C/DD/026 <0.01 C/DD/046 0.01C/DD/007 <0.01 C/DD/027 <0.01 C/DD/047 <0.01C/DD/008 <0.01 C/DD/028 <0.01 C/DD/048 0.01C/DD/009 <0.01 C/DD/029 <0.01 C/DD/049 <0.01C/DD/010 <0.01 C/DD/030 <0.01 C/DD/050 <0.01C/DD/011 <0.01 C/DD/031 <0.01 C/DD/051 <0.01C/DD/012 <0.01 C/DD/032 0.05 C/DD/052 0.01C/DD/013 <0.01 C/DD/033 <0.01 C/DD/053 <0.01C/DD/014 <0.01 C/DD/034 <0.01 C/DD/054 <0.01C/DD/015 <0.01 C/DD/035 <0.01 C/DD/055 <0.01C/DD/016 <0.01 C/DD/036 <0.01 DG-MM-301 <0.01C/DD/017 <0.01 C/DD/037 <0.01 DG-MM-302 0.01C/DD/018 <0.01 C/DD/038 <0.01 DG-MM-303 0.01C/DD/019 <0.01 C/DD/039 <0.01 DG-MM-313 0.04C/DD/020 <0.01 C/DD/040 0.01 DG-MM-316 0.02

Table 1c. Channel samples from Okolom part of the study area.

Sample No. Au (ppb) Ag (ppm) As (ppm) Cu (ppm) Pb (ppm) Zn (ppm)C/UOK/01 0.02 <0.05 <0.05 7 45 145C/UOK/02 0.01 <0.05 <0.05 32 40 144C/UOK/03 0.01 <0.05 <0.05 10 38 119C/UOK/04 0.01 <0.05 <0.05 15 33 142C/UOK/05 0.02 <0.05 <0.05 6 80 153C/UOK/06 0.01 <0.05 24 7 25 278C/UOK/07 0.01 <0.05 <0.05 21 32 106C/UOK/08 0.01 <0.05 <0.05 23 31 116C/UOK/09 0.01 <0.05 <0.05 15 25 107C/UOK/10 <0.01 <0.05 <0.05 13 32 101C/UOK/11 0.01 <0.05 <0.05 15 26 105C/UOK/12 0.01 <0.05 <0.05 53 20 111C/UOK/13 0.01 <0.05 <0.05 49 39 157C/UOK/14 <0.01 <0.05 <0.05 74 31 124C/UOK/15 0.01 <0.05 <0.05 136 48 151C/UOK/16 <0.01 <0.05 <0.05 21 49 179C/UOK/17 0.01 <0.05 <0.05 96 46 152C/UOK/18 0.01 <0.05 <0.05 76 27 125C/UOK/19 0.01 <0.05 <0.05 40 26 124C/UOK/20 0.01 <0.05 <0.05 47 41 118

features: panned concentrate of gold anomalies, andpresence of talc schist along strike. The mapping wascarried out across the northern end of a worked vein,covering a distance of 4 km along its strike extension.The mapping was aimed at locating all outcrops anddefining areas of quartz float, combined with prospectingwhich entailed collection of samples representative of thevarious types of mineralization and alterations. Channeland grab samples representative for all the types ofmineralization were also collected with locations of

measurement corresponding to sample locations (SeeFigure 2).

GEOCHEMISTRY

Rock, soil and channel samples were taken from zonessuspected to be mineralized with gold based on results ofan earlier stream-sediment orientation survey. Tropicalsoils are known to host significant gold mineralization insome parts of West Africa (Bowel et al., 1996). Gold,

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Table 1d. Rock samples from the study area.

Sample No Au (ppb). Sample No Au (ppm)R/KK/003 <10 R/UOK/004 <10R/KK/010 <10 R/UOK/006 360R/KK/012 10 R/UOK/007 <10R/KK/015 <10 R/UOK/008 10R/KK/016 <10 R/UOK/009 10R/KK/017 <10 R/UOK/010 10R/KK/019 <10 R/UOK/011 <10R/KK/020 <10 R/UOK/014 <10R/KK/021 <10 R/UOK/025 <10R/KK/022 10 R/DD/001 <10R/KK/023 <10 R/DD/003 <10R/KK/024 <10 R/DD/004 10R/KK/025B <10 R/DD/006 <10R/KK/025C <10 R/DD/007 <10R/KK/026 <10 R/DD/008 <10R/KK/027 <10 R/DD/009 <10R/KK/028 <10 R/DD/010 <10R/UOK/001 <10 R/DD/011 <10R/UOK/002 <10 R/DD/022 <10R/UOK/003 20 R/DD/028 100

Table 1e. Soil samples from Okolom part of study area.

Sample No Au (ppb). Sample No Au (ppm)S/UOK/001A 30 S/UOK/021A 70S/UOK/002A 70 S/UOK/022A <10S/UOK/003A 1130 S/UOK/023A <10S/UOK/004A 10 S/UOK/024A <10S/UOK/005A 10 S/UOK/025A <10S/UOK/006A <10 S/UOK/026A 10S/UOK/007A <10 S/UOK/027A <10S/UOK/008A <10 S/UOK/028A 50S/UOK/009A 10 S/UOK/029A 30S/UOK/010A <10 S/UOK/030A <10S/UOK/011A <10 S/UOK/031A 30S/UOK/012A 980 S/UOK/032A 70S/UOK/013A 30 S/UOK/033A 20S/UOK/014A 80 S/UOK/034A 10S/UOK/015A 10 S/UOK/035A <10S/UOK/016A 10 S/UOK/036A <10S/UOK/017A 660 S/UOK/037A <10S/UOK/018A 70 S/UOK/038A <10S/UOK/019A 20 S/UOK/039A 60S/UOK/020A 40 S/UOK/040A 10

Table 2: Statistical parameters of the elements. (2a). Au inchannel samples from Dogondaji (in ppm).

Mean 0.012Median 0.011Mode <0.01Standard deviation 0.012Minimum value <0.01Maximum value 0.05

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Table 2b. Channel samples from Katchakatcha (values in ppm).

Au Ag As Cu Pb ZnMean 0.01 <0.5 <0.5 20.2 13.2 83.9Median 0.009 <0.5 <0.5 12.1 12.6 74.45Mode <0.01 <0.5 <0.5 4,5,10 12 65,93Standard deviation 0.009 0 0 16.65 2.33 20.28Minimum value <0.01 <0.5 <0.5 2 8 43Maximum value 0.07 <0.5 <0.5 73 20 143

Table 2c. Channel samples from Okolom North (values in ppm).

Au Ag As Cu Pb ZnMean 0.01 <0.5 1.675 37.75 36.7 137.85Median 0.01 <0.5 1.087 26.37 30.85 130.92Mode 0.01 <0.5 <0.5 15 25,26,31,32 124Standard deviation 0.002 0 5.25 34,64 13,27 39.28Minimum value <0.01 <0.5 <0.5 6 20 101Maximum value 0.02 <0.5 24 136 80 278

Table 2d. Au in rock samples from the Study Area (in ppm).

Mean 0.02Median 0.014Mode <0.01Standard deviation 0.058Minimum value <0.01Maximum value 0.38

Table 2e. Au in soil samples from Okolom North (in ppm).

Mean 0.09Median 0.05Mode <0.01Standard deviation 0.25Minimum value <0.01Maximum value 1.13

Table 3a: Au in Dogondaji channels.

Range of values No. of samples Cum. No Cum.% Log.ppm<0.01 45 55 100 <- 2.0

0.01-0.02 7 10 18.2 - 2.00.02-0.03 1 3 5.5 - 1.70.03-0.04 1 2 3.6 - 1.50.04-0.05 0 1 1.8 - 1.40.05-0.06 1 1 1.8 - 1.3

major and minor elements were analysed for (Table 1).The analytical data are presented in Tables 2 and 3.Table 2 presents the basic statistical parameters obtain-

-ned for the elements studied in the area.The distribution of the elements was studied using

cumulative frequency curves on a probability scale. The

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Table 3b: Au in Katchakatcha channels.

Range of values No. of samples Cum. No Cum.% Log.ppm<0.01 33 45 100 <- 2.0

0.01-0.02 9 12 26.7 - 2.00.02-0.03 1 3 6.7 - 1.70.03-0.04 1 2 4.4 - 1.50.04-0.05 0 1 2.2 - 1.40.05-0.06 0 1 2.2 - 1.30.06-0.07 0 1 2.2 - 1.20.07-0.08 1 1 2.2 - 1.1

Table 3c: Au in Okolom Channels.

Range of values No. of samples Cum. No Cum.% Log.ppm<0.01 3 20 100 <- 2.0

0.01-0.02 16 17 85 - 2.00.02-0.03 1 3 5 - 1.7

Table 3d: Au in Okolom Soils

Range of values No. of samples Cum. No Cum.% Log.ppm<0.01 15 40 100 <- 2.0

0.01-0.05 16 25 62.5 - 2.00.05-0.10 6 9 22.5 - 1.30.10-1.00 2 3 7.5 - 1.0

>1.00 1 1 2.5 0

curves for Au, Pb, Cu and Zn are shown in Figures3 (a – e). The concentration pattern for Au is similar tothose of Pb and Zn, an indication that Pb and Zn could bepathfinders for Au.Background. The background (b) is estimatedgraphically by inspection of the distribution line with 50%ordinate. For a perfect log-normal curve, the backgroundthus estimated corresponds to the mode and median andcan be taken to be the geometric mean of the values. Inthis work, perfect curves were not obtained and thus thebackground values estimated are only close to the modeand median.Threshold. For a symmetrical distribution, 95% of theindividual values fall between b + 2s and b – 2s, that is,only 2½% of the population exceed the upper limit, b –2s. This upper limit is taken normally to be the threshold(t) above which the values are considered anomalous.Graphically, the threshold corresponds to the intersectionof the distribution line with the 2½% ordinate. The valuesobtained are of course rounded off. The background andthreshold values estimated for Au in the cases studiedare as follows:

Background (b) Threshold (t)

Dogondaji channels <0.01 0.035

Kachakacha channels <0.01 0.035Okolom North channels 0.014 0.02Okolom North soils 0.017 1.00

From the data above, it can be observed that only onevalue in Dogondaji, Kachakacha and Okolom North soilconstitute anomalies, while there is none for OkolomNorth channels. In this area that was investigated wheremineralization may be widespread and relatively fewsamples collected and analysed, it is apparent thatlocations of significant occurrence will be more frequentthan the ideal 2½% assumed in this case. It is possible toadjust the threshold to arrive at workable clusters.

Relations of base metals and major elements to Aumineralization

In all the samples the concentration of gold is consistentlylow (<0.01 ppm) except in few areas where Au values areoccasionally ‘high’ (up to 1.13 ppm at most). These highvalues are related to occurrences in quartz veins, quartz-mica schist and disseminations in amphibolites. Theassociation of Au with Ag and As is consistently below0.5 ppm, below the detection limit. This value is commonto all the channels except in Okolom North where a valueof 24 ppm was recorded in amphibolites. This trend

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Figure 3. Cumulative frequency distribution curve of metals fromKachukacha channel sample, (a) Lead and Copper(b) Zinc (c) Gold.

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Fig. 3 Contd.. Cumulative frequency distribution curve of metals from OkolomNorth channel sample, (d) Zinc (e) Lead and Copper (f) Gold.

signifies that the Ag and As have only one populationwhich constitutes the background values. In Kachakacha

area, Cu, Pb and Zn values depict only one populationwith background values of 16.22 ppm (Cu) and 71 ppm

(Zn). The determined threshold values from probabilitycurves (not shown) are 71 ppm and 141 ppm for Cu andZn respectively. For Cu and Zn only one value each isabove the threshold concentration and therefore nosignificant anomaly. The concentration of Pb inKachakacha area is between 8 and 20 ppm, and alsoconstitutes the background values.

For Okolom North, the values of Cu, Pb and Zn formone population group with background values of 27, 29.5and 141 ppm respectively. The threshold was notreached. For both Okolom North and Kachakacha areas,the background values are as follows:

Cu Pb ZnBackground 16.22 21 95.5 ppmThreshold -- 47 166 ppm

The ‘anomalous’ values for Pb and Zn are 3 and 2respectively and which may not be considered to besignificant anomalies. Somewhat correlation has beenfound between Au and two of the other elements (Pb andZn) analysed. This means that Pb and Zn may be goodindicators for the presence of Au. During dispersion, goldmay be carried in discrete grains in the dispersion train,while the other elements may be moved in solution in themedium. The significant correlation that exists betweenCu/Pb/Zn and Ag/As indicates the presence of sulphidemineralizations.

MINERALIZATIONS

The Okolom-Dogondaji veins are hosted by bandedgneiss, talc schists and amphibolites (metavolcanics)within nearly N-S trending shear structures of the Isanlu-Egbe schist belt of SW Nigeria. This is one of theoccurrences of primary gold, with associated tantalite andcassiterite within the Precambrian to lower Palaeozoicschist belts of western Nigeria (Matheis, 1987). Primarygold also occurs in quartz-mica and quartz-sulphideveins, and in alteration zones surrounding the veins,particularly where veins intrude the host rocks. The Isanlugold-quartz mineralization comprises quartz, gold andsulphide hosted by quartz veins, lenses, stringers andother silicified bodies usually confined to the shatterzones brought about by fracturing and/or shearing(Garba, 1988). Syngenetic gold-sulphide mineralization inthe form of disseminations also occurs in theamphibolites (Garba, 1988). Secondary gold enrichmentoccurs in the alluvials of streams draining the areas. Goldfound in tin concentrates is derived from quartz veins ingneiss-schist suite, and highly probable that the goldmineralization is also connected with the Pan-AfricanGranites (Dada et al., 2003, Wright, 1970). These rocksthus have potentials for gold and other metals as they aretypically altered with strong development of sericite,chlorite, tourmaline, zircon and hematite. They are

Scholarly J. Sci. Res. and Essay 110

silicified particularly adjacent to the quartz veins andcommonly contains pyrite, galena, sphalerite,chalcopyrite, arsenopyrite, marcasite and argentite(Akande et al., 1988, Boyle, 1983, Dada, 1983). Inaddition to quartz, these represent 2-3% of the veinconstituents. In places quartz veins are remarkablyfractured with muscovite and tourmaline very common.

The prominent development of quartz, tantalite,sericite, chlorite and tourmaline in the wall rocks adjacentto veins is an indication that gold and sulphidemineralizations are accompanied and controlled by suchprocesses as mylonitization, sericitization andtourmalinization. This further suggests that tectono-metamorphic processes controlled the origin of Isanlu-Egbe primary gold mineralization. This involved theheating up and pushing the mineralizing fluid through thebasic volcanics and sediments under tectono-metamorphic conditions. Field evidence suggests thatvein mineralizations in the area are distributed withinshear zones traversing biotite gneiss, amphibolite, schist,and at sheared contacts of the Pan-African porphyriticgranite (Dada et al., 2003). Available data supportmetamorphism and shearing as the major processesinvolved in the gold mobilization and propagation in theschist belts. The area bears imprints of polyphaseddeformation (Annor et al., 1996). This process is ascribedto the magmatism of the Pan-African tectonic regimedated about 536 17 Ma (Matheis, 1987).

Available data (Garba, 1987, Annor et al., 1996, Dada,1983) point to two main stages of gold mineralization inthe area. The first was possibly synvolcanic and wasmarked was marked by early disseminations inamphibolites. The second episode was syntectonic andepigenetic in nature and utilizing the vein systems. Thelack of lithological preference for gold-bearing veins in theNigerian schist belts suggest that vein formationaccompanied the deformation of the Nigerian BasementComplex.

The vein trends are varied but the latter and moreprominent N-S shear fractures served as conduits, whichthe gold-bearing quartz stringers, veins and pegmatitedykes, were emplaced. The mineralizing hydrothermalsolutions have a tendency to migrate toward openfractures and shears to eventually form economic veinand fissure deposits of Au, Ag, Sn, base metals andassociated minerals. Only such N-S trends appearfavourable in hosting any significant mineralization.

DISCUSSION AND CONCLUSIONS

Geochemical studies show consistent low concentrationof Au and other elements analysed for except in fewareas where high values occur in quartz veins. Apart fromthe Okolom main vein, the quartz vein deposits so farencountered are mainly small structures. This seems tobe in conformity with the geochemical data from the area

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north of Isanlu. Egbe, as an old mining district in SWNigeria, is well known for its high tantalite output fromamphibolites and pegmatite’s. The trend of theassociation of Au with the other elements signifies thatthey form one population which constitutes thebackground. Only one value of Au in Dogondaji,Kachakacha and Okolom North soil constitute anomalies.However, somewhat correlation found between Au, andPb and Zn means that Pb and Zn may be good indicatorsfor gold deposits. The significant correlation that existsbetween Cu/Pb/Zn and Ag/As indicates the presence ofsulphide mineralization. In the area investigated,mineralization may be widespread and with the limitationsof geochemical analyses, it is apparent that locations ofsignificant occurrences will be more frequent than theideal 2½% assumed in this case.

REFERENCES

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Annor, A. E., Olobaniyi, S.B. and Mucke, A. (1996). A note on thegeology of the Isanlu area, in the Egbe-Isanlu schist belt, SW Nigeria.Jour. Min. Geol. 32(1): 47-51.

Bafor, B.E. (1988). Some geochemical considerations in the evolution ofthe Nigerian Basement Complex in the Egbe area of southwesternNigeria. Prec. Geol. Nigeria. Geol. Surv. Nigeria Publ., pp. 277-288.

Bowel, R.J., Afreh, E.O., Laffoley, N.d’A., Hanseen, E., Abe, S., Yao,R.K. and Pohl, D. (1996). Geochemical exploration for gold in tropicalsoils – four contrasting case studies from West Africa. Trans. Instn.Min. Metall. (Sect. B: Appl. Earth Sci.), 105: B12-B33.

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