Journal of African Earth Sciences, Vol. 6, No. 3, pp. 281-286, 1987 0731-7247/87$3.00 + 0.

00
Printed in Great Britain (~ 1987Pergamon Journals Ltd.

Geochemical prospecting for gold in the area north of Isanlu,

Nigeria
IBRAHIM GARBA

Exploration Division, Nigerian Mining Corporation, Jos, Nigeria

(Received for publication 10 July 1986)

Abstract--A combined heavy mineral and stream sediment survey was carried out in the area north of Isanlu to
delineate potential areas for a detailed search for gold deposits. The geology of the area is typical of the basement
of SW Nigeria with gneisses, schists, amphibolites and granites. Mineralization comprises gold/sulphide-bearing
quartz veins and amphibolites.
Data obtained from the stream sediment survey were subjected to statistical interpretation and it was found
that the log-normal pattern appears to be the one most applicable to the element distributions. No indicator for
gold has been identified, but the significant correlation that exists between Pb/Cu and Pb/Zn was taken to
distinguish vein mineralization from disseminated mineralization in amphibolites.
The heavy mineral survey, which was intended to simply check for gold occurrences in the field, also proved
useful in detecting other anomalies later confirmed by the stream sediment survey. It was found to be a better
exploration tool for gold at the reconnaissance stage. The overall high level of gold in the drainage indicates a
metallogenic area favourable for gold. The survey succeeded in delineating possible gold targets worthy of
follow-up.

INTRODUCTION HEAVY MINERAL SURVEY

Tins work was initiated by the Nigerian Mining Corpora- The heavy mineral concentrates obtained by panning
tion as part of a programme whose objective was to were intended for field-checking the occurrence of gold
locate gold deposits in parts of the Nigerian schist belts. in the drainage channels as the stream sediment survey
The area covered by the work is part of the Isanlu progressed. This quickly helped to delineate areas of
schist belt, situated between latitudes 8°15'N-8°30'N high gold values which might indicate alluvial placers or
and longitudes 5°38'E-8°00'E, in Oyi L.G.A., Kwara vein mineralization. The presence of visible alluvial gold
State, Nigeria. in panned concentrate has led to the discovery of a vast
The aim of the work was to quickly delineate potential number of the world's important gold deposits (Ant-
areas for gold that can be followed up and also to find weiler and Campbell 1982).
exploration guides for future work. The choice of sam- The method of sampling heavy mineral concentrates
pling procedures was designed to give reliable results consists of collecting two headpan-volumes (approxi-
quickly and cheaply. The sampling of the stream sedi- mately 0.016m 3) of sediment to represent one sample
ment and heavy mineral concentrates was carried out from a site considered favourable for heavy mineral
concurrently in the drainage system of the area. accumulation. The material was deslimed by panning
and screened through 11 and 18 mesh sieves. These
Climate, topography and geology fractions were further panned to about 10-20 g which
were later analysed by amalgamation in a field labora-
Climate conditions are typically tropical. Rainfall in tory.
Egbe, the nearest weather station, has a mean of The concentrates were moistened with water and
1845 mm per annum characterized by two peaks in July some mercury added, in test tubes. The mercury formed
and September and is concentrated between May and amalgam with any gold present in the samples. The
October (Jeje et al. 1982). amalgam was separated and dissolved in 1M HNO3 and
The area has a moderate relief with rolling terrain the gold precipitated. The gold grains thus obtained
with scattered occurrences of round topped hills. High were washed with water, dried and weighed on a Mettler
hill massifs of granites are restricted to the southern and balance. The weight of the gold (in grams) was converted
north central part of the area. Valleys of major streams and expressed as g m 3 of sample material. The values
and rivers are broad and often contain alluvial terraces. were plotted on the base map (Fig. 1).
Geologically the area consists of metavolcanic- One sample (2 headpan-volumes) = 16 1 (0.016 m3).
sedimentary rocks on a gneissic basement, both of which If, for example, there was 0.05 g of gold in the sample,
are intruded by granites. Gold mineralization consists of the values would be expressed as:
primary gold-sulphide in quartz veins, primary gold-
sulphide disseminated in amphibolites and secondary 0.05 x 62.5 = 3.125 g m 3
eluvial and alluvial gold deposits. (0.016 x 62.5 = 1 m 3)
281
AES 6/3 - C
282 IBRAHIMGARBA
5*40 IE 45' 50' 5* 55' E

25'

To Isanlu Mokut.u
Fig. 1. Gold values (g/m 3) in heavy mineral concentrates.

Only a few of the values are shown in Fig. 1, but selected and also Fe and Mn because of their role in
several hundred values were actually obtained from the adsorption and the creation of false anomalies.
exercise. The determination of Au, Ag, Cu, Pb, Zn, Fe and Mn
was done by AAS using the PYE UNICAM SP 9 model
in the N.M.C. laboratory in Jos. All samples were hot
extracted (Hx) using aqua regia. Routine checks were
STREAM SEDIMENT SURVEY carried out by duplicate analysis of every tenth sample.
Working standards were frequently analysed to deter-
Conventional geochemical sampling procedures are mine repeatability.
generally inadequate in the context of the use of
geochemistry in regional reconnaissance for gold Statistical treatment of analytical data
deposits. An appreciation of basic problem of sampling
related to the nature of gold and its distribution in The data obtained from the analysis of the stream
geologic media is needed (Harris 1982). sediments were subjected to simple statistical analysis
The sampling of the stream sediments was biased and interpretation. Most of the statistical analyses were
towards material with heavy mineral concentration, in done with the help of an electronic computer using the
most cases collected from the same location as the SPSS version.
material for panning. The basic statistical parameters obtained are shown in
Approximately 1-2 kg of sediments were collected as Table 1. The distribution pattern of gold was studied
one sample at a site. These samples, packed in polythene graphically using histograms of arithmetic and logarith-
bags, were sent to N.M.C.'s laboratory in Jos. Several mic values and cumulative frequency distribution (CFD)
hundred of such samples were collected, but due to high curves on a probability scale using the Lepeltier (1969)
costs of geochemical analysis only 160 samples were method. The main purpose of the CFD curves was to
selected for analysis. These samples were selected from check further if they fitted a log-normal distribution
two areas A and B (Fig. 1) considered to be mineralized (Lepeltier 1969), and if they did to estimate graphically
and barren with respect to primary gold veins. Area B is some basic parameters such as background, threshold
the control area, but both areas contain widespread and population groups. Figures 2 and 3 show the graphi-
occurrences of amphibolites. Of the 160 samples, 119 cal distribution patterns of the elements.
were selected from A and 41 from B. Again due to the Pearson correlation analysis was done to observe
need to minimize costs, only a few elements could be interelement relationships in A, B and the two areas
chosen for determination in the stream sediment sam- combined together. The correlation matrices are shown
ples. The gold mineralization in the area is known to be in Table 2. The correlation analysis was done using
gold-sulphide type, hence Au, Ag, Cu, Pb and Zn were log-transformed values of the elements.
 Geochemical prospecting for gold in the area north of Isanlu, Nigeria 283

[] ,o- T
30

20

% %
I0 I0 --

o
0 0.6 L2 1.8 2.4 3 3.6 -I.5 -0,9 -0.3 0.3 0.9 1.5

3o i
ppm Au Log p p m A u

4O

,° I
30

20 20
e/o %

I0 °

0
0.6 I. 2 I. 8 2.4 3 3.6 -I.5 -0.9 -0.3 0.3 0.9

ppm Au Log p p m A u

40 --

30 30

20 20 --
%
°/o

I0 iO --

0
o 0.6 i.e L8 2.4 3 3.6 -L5 -o.9 -0.3 03 0.9 1.5

pprn A u Log ppm Au

Fig. 2. Histograms of frequency distribution of gold in A, B and A + B.

Table 1. Some statistical parameters for the seven elements studied in A, B and A + B

Au Ag Cu Pb Zn Fe Mn

A (ll9Samples)
Mean 1.7 2 24 14 45 9333 736
Median 0.5 2 20 4 34 8263 502
Mode 0.3 3 20 1 36 6528 500
Standard deviation 0.5 1.5 31 17 44 6135 1215
Minimum value 0 0 4 1 3 726 80
Maximum value 37.5 5 332 64 270 28,000 12,500

B (41 Samples)
Mean 1.0 3 20 17 56 7873 604
Median 0.6 2 18 2 39 6462 497
Mode 0.5 2 20 1 22 3188 200
Standard deviation 1.5 3 12 10 76 5546 621
Minimum value 0.03 0 4 1 7 764 130
Maximum value 8.6 16 54 56 392 18,528 3800

A + B(160Samples)
Mean 1.6 2 23 15 48 8959 702
Median 0.5 2 20 3.5 35.5 7723 502
Mode 0.3 3 20 1 22 6528 500
Standard deviation 4.1 2 28 18 54 6007 1094
Minimum value 0 0 4 1 3 726 80
Maximum value 37.5 16 332 64 392 28,000 12,500
284 IBRAHIM G A R B A

A (I 19 samples] B [41 sampLes] A+ B ( 160 samples]

99.9

95.5

95
90
~',.. ~
e

5o -
-

IO
5
~.~
0.5--

0.1-

0.01 J I I J I i I I I I I I I J ,
QI Q2 0.5 I 2 5 PO Q2 Q5 I 2 5 I0 Q2 Q5 I 2 5 I0
ppm

Fig. 3. Cumulative frequency distribution ( C F D ) curves of gold in A , B and A + B.

Interpretation threshold values using the Lepeltier (1969) method.

Only the curves for Au distribution are shown here.
The log-normal pattern appears to be most applicable
to the results of the survey. In addition to the Au curves Background. A straight line on the CFD curves
shown (Figs. 2 and 3), Cu, Zn and Mn also show fairly denotes a single population log-normally distributed.
distinct log-normal patterns. There seem to be two The background (b) is estimated graphically by inter-
population groups of Pb. Fe is partly negatively skewed. section of the distribution line with the 50% ordinate.
The CFD curves have defined better the log-normal The values are rounded off. In the case of a perfect
patterns of the elements. Ag shows a confused, ill- log-normal curve the background thus estimated corre-
defined pattern which may be due to analytical problems sponds to the mode and the median and can be taken to
connected with the detection of Ag. The CFD curves be the geometric mean of the set of values (Lepeltier
(Fig. 3) have been used to estimate background and 1969). In this study, perfect curves have not been

Table 2. Correlation matrices showing Pearson correlation coefficients b e t w e e n the seven elements studied
inA, BandA + B

Au Ag Cu Pb Zn Fe Mn

1.000 0.110 -0.173 -0.025 0.045 0.109 0.547 Au

1.000 0.051 0.238 0.085 0.092 0.168 Ag
1.000 0.305 0.212 -0.002 0.056 Cu
1.000 0.529 -0.209 0.060 Pb
A 1.000 -0.092 -0.029 Zn
1.000 0.566 Fe
1.000 Mn

Au Ag Cu Pb Zn Fe Mn

1.000 0.217 0.096 0.167 -0.001 -0.173 0.064 Au

1.000 0.157 -0.067 -0.148 -0.212 0.027 Ag
1.000 0.212 0.394 0.260 0.366 Cu
1.000 0.283 -0.414 0.157 Pb
B 1.000 0.024 0.008 Zn
1.000 0.191 Fe
1.000 Mn

Au Ag Cu Pb Zn Fe Mn

1.000 0.130 -0.102 0.025 0.290 0.062 0.049 Au

1.000 0.069 0.135 0.023 0.022 0.134 Ag
1.000 0.278 0.252 0.047 0.106 Cu
1.000 0.459 -0.243 -0.076 Pb
A + B 1.000 -0.072 -0.022 Zn
1.000 0.521 Fe
1.000 Mn
 Geochemical prospecting for gold in the area north of Isanlu, Nigeria 285

5"40' 5"45' 5° 50' fl*30'

8*50' I J
i
ExpLanation
i /
/ • > 2ppm Au(high)
I to 2ppm Au (medium}
o Otolppm(Low)
I-q--~ Ouortz vein
-~ Medium-qrainedgranite
[-a--] ~phiboLi'tes

8"25' [ - ~ Schists
[ - ~ Gneisses
Streoms
--- GeoLogicaLboundary
~ Areas with "Anornotous"
GoLd voLuesnot related
to Vein mineraLization

ScaLe l: 150,000
8"20'

t5' 5* 50'
To TsanLu
Fig. 4. Gold distribution in the stream sediments.

achieved and thus the background values estimated are method. This method is therefore not useful even though
only close to the mode and the median. values may be normally or log-normally distributed.
In practical work such as this (that finds mineralization
Threshold. In the case of symmetrical distribution with the least possible efforts, i.e. funds), the pragmatic
(either normal or log-normal), 95% of the individual approach of the use of the concept of threshold is often
values fall between b + 2S and b - 2S; that is to say preferred to the highly scientific and mathematical
only 2½% of the population exceed, the upper limit methods. As such one should adjust the threshold to
b - 2S (Lepeltier 1969). This upper limit is convention- arrive at workable anomaly clusters (Borsch 1985).
ally taken to be the threshold level (t) above which the
values are considered as anomalous (Lepeltier 1969, Geochemical maps
Sinclair 1974, Rose et al. 1979). Graphically, threshold
levels can be estimated directly from the curves (CFD) Another method for selecting threshold is to plot the
as the intersection of the distribution line with the 2½% data (values) on a map and contour or otherwise separ-
ordinate. The values obtained are of course rounded off. ate high from low values (Rose et al. 1979). A cluster of
The background and threshold values estimated for high values has a low probability of occurring by chance
Au in the three cases are as follows. and may reflect a mineralized area. Low, medium and
high values of the elements are therefore plotted on the
Background (b) Thresholds (t) map and only that of Au is shown here (Fig. 4). It is
interesting to observe from the map that many clusters
A 0.6 6 of high and medium values can be directly related to
B 0.5 5
A+B 0.6 6 known mineralized bodies. Thus similar others may
indicate unknown mineralized bodies, or are at least
targets for a follow-up survey.
The threshold values are found to be very high con-
sidering the range of the data. This is, however, not
unusual since only 2½% of the populations will fall above G E O C H E M I C A L CHARACTERISTICS OF GOLD
the threshold. It has been observed that in the case of Au
only five values in A and one in B constitute anomalies. The most noticeable feature of Au is its general high
In an area such as that being investigated, where level of concentration in the stream sediments. This may
mineralization is very widespread and relatively few be attributed to two reasons, the first being the sampling
samples were analysed, it is apparent that values of biased towards heavy mineral concentration. The sec-
significance to the location of mineral occurrences will ond and probably the most important is that both areas
be more frequent than the ideal 2½% assumed in this (A and B) are geologically very favourable for Au,
286 IBRAHIM GARBA

which is believed to be dispersed through amphibolites scientific explanation but has not affected the practical
(Elueze 1982) in addition to the mineralized quartz purpose of the survey.
veins. It has been shown from this survey that either stream
The threshold value 6.5 p p m Au in A is higher than sediment survey or heavy mineral survey can be used
that of B (5 p p m Au). This is interpreted to indicate effectively for Au at the reconnaissance stage of explor-
more (additional) sources of A u in A than B in the form ation to outline favourable areas for follow-up. But,
of Au-bearing quartz veins. The extent of Au dispersion since the heavy mineral survey is cheaper, quicker and
train in stream sediments (as a heavy mineral) has been gives results in the field it is a m o r e effective exploration
shown to extend for m a n y kilometres downstream tool for reconnaissance searches for A u deposits such as
(Boyle 1979). Thus some of the Au values in the drainage the one carried out by this work.
can be related to suitable conditions for alluvial concen-
tration. Some areas, m a r k e d I, I I , . . . V in Fig. 4, which
show consistent high Au and other values may be CONCLUSION
associated with mineralized bodies.
Significant correlation has not been found between The geochemical distribution characteristics of gold
Au and any of the other elements. This may mean that studies have been useful in the application of geochemi-
only Au is carried as discrete grains in the train, while cal methods for prospecting for gold deposits in the
other elements move partially in solution in the medium Isanlu area.
sampled. The significant correlation that exists between A r e a B, hitherto considered barren to gold, has been
Pb/Cu and Pb/Zn, apart from indicating the presence of found to perhaps lack only primary vein mineralization.
a sulphide mineralization, may also distinguish area A This has shown that gold is dispersed through amphibo-
from B in having vein mineralization, since the lack of lite in both A and B, but with additional sources of gold
such correlation is observed in B. Therefore B might be in A from the quartz veins.
considered ' b a r r e n ' to vein mineralization. No indicator for gold has been identified from the
The heavy mineral survey was restricted to A u as a elements studied. Gold is probably the best indicator to
heavy mineral. This method of survey has been useful in itself. Gold in panned concentrate has been found to be
detecting the anomalies confirmed later by the stream a more reliable indicator of mineralized areas in the
sediment survey. Almost all mineralized and other A b s a r o k Mountains of Wyoming and Montana, U.S.A.
favourable areas for Au shown by the stream sediment (Antweiller and Campbell 1982). It is therefore thought
survey have been picked up by the heavy mineral survey. that the heavy mineral survey is a better exploration tool
However, this is not unexpected as the media sampled for gold deposits at the reconnaissance stage.
are favourable for the mechanical accumulation of A u in The overall high level of gold in the drainage system
the drainage. north of Isanlu is indicative of a metallogenic area
There may be differences in the relative values ob- favourable for gold. Favourable areas for gold have
tained from the heavy mineral survey and stream sedi- been outlined.
ment survey, but the practical application (particularly
the anomaly pattern) is essentially similar. For example Acknowledgements--My appreciation is extended to the Nigeria Min-
ing Corporation for support of the work and permission to publish it,
the heavy mineral survey shows concentration levels and also to Michael Woakes of the Geology Department, A.B.U. for
below and above 0.25 g A u / m 3 of gravel (equivalent of his useful contributions. I thank Dr O. M. Ojo for reading the text and
0.1 p p m Au). It may be questionable that the stream making useful suggestions.
sediments which usually show lower gold concentration
than the heavy mineral concentrates have a mean value
of 1.6 p p m Au (equivalent of 3.2 of A u / m 3 sediments); REFERENCES
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