Vol.3, No.8, 646-650 (2011) Natural Science
http://dx.doi.org/10.4236/ns.2011.38088
Major elements and lithostratigraphic study of the
contact rocks of the Togo and the Dahomeyan
formations in Ghana
Mawutorli Nyarku1*, Samuel Yao Ganyaglo1, Eric Tetteh Glover1, Yaw Serfor-Armah2
1National Nuclear Research Institute, Ghana Atomic Energy Commission, Accra, Ghana;
*Corresponding Author:
2Graduate School of Nuclear and Allied Sciences, University of Ghana, Accra, Ghana.
Received 4 April 2011; revised 26 April 2011; accepted 30 April 2011.
ABSTRACT
The thrust contact between the Togo and the
Dahomeyan formations in Ghana is a lithotec-
tonic boundary that exists between two major
Precambrian formations which are of impor-
tance to geologists owing to the fact that Pre-
cambrian rocks in Ghana host almost all eco-
nomic minerals and metals. The lithostratigra-
phy of the Togo-Dahomyan thrust contact rocks
from a borehole in Kwabenya near Accra (the
Capital of Ghana) has been studied and major
crustal chemical elements assayed using In-
strumental Neutron Activation Analysis (INAA)
techniques. The results have revealed elemental
compositions and the mineralogical make up of
the lithostratigraphic units of these contact
rocks and the general geology of the Togo and
the Dahomeyan formations. The profile shows a
thrust contact that exists between the Togo for-
mations which are underlying the Dhomeyan
formations. The Togo formations here are made
mainly of quartzite in fresh schist and the Da-
homeyan made of gneisses. In between these
two major geologic formations are the rocks of
the contact which are intercalation of quartzite
and schist. The rocks are felsic with an average
felsic index (F) of 85.74 and feldspar rich with
K-(orthoclase) feldspars dominating the Da-
homeyan rocks. Iron and titanium oxides are
depleted with depth from 52 m depth below
surface downward and potassium oxide was
enriched with depth from 42 m below surface
downward. Major mineral forming elements
such as aluminum and calcium had varied lev-
els in the Togo and in the Dahomeyan rocks.
Keywords: Dahomeyan; Togo; Contact Rocks;
Elemental; Concentrations; INAA; Ghana
1. INTRODUCTION
Thrust contact of the Togo and the Dahomeyan for-
mations underlie some suburbs of Accra such as Kwa-
benya and Achimota. The geology of the Togo and Da-
homeyan formations and their contact are well described
by Blay and Kesse [1,2]. Rocks of these two formations
and their thrust contact are important owing to the fact
that the Togo and the Dahomeyan formations are among
Precambrian rocks in Ghana which happened to host
majority of the economic minerals and metals. The li-
thologic successions of these two formations are known
to be complex. The Togo has been classified into three
stratigraphic divisions namely: Basic, Arenaceous and
Argillaceous groups while the Dahomeyan formations
have been classified into an order of Acidic Dahomeyan,
Alkalic gneiss, Basic intrusives and the Metabasics re-
spectively with the first in each case being the youngest
and at the top of the lithosuccession. The Togo forma-
tions are Upper Precambrian while the Dahomeyan for-
mations are Middle to Late Precambrian [3,4]. The Togo
formations are deformed thrusted supracrustal rocks and
trend northeast-southwest. The Dahomeyan formations
are belts with the same northeast-southwest trending east
of the Togo, they are highly metamorphosed and are
associated with much thermotectonic activities. The
Dahomeyan formations are found in the easternmost part
of Ghana. Mineral and for that matter elemental compo-
sition of these contact rocks have always been of interest
to geologists, as this has helped to establish origin, evo-
lution and the geology of the contact. Since quantity of
chemical elements in a mineral is what determines the
mineral type and the mineral name, elemental data on
rocks and minerals of the thrust contact of the Togo and
the Dahomeyan formations is needful for the proper

M. Nyarku et al. / Natural Science 3 (2011) 646-650
647647
characterization of the geology of this contact. In this
research, lithostratigraphic profile of rocks of the To-
go-Dahomeyan contact from a borehole drilled on the
site of the Ghana Atomic Energy Commission in Kwa-
benya (shown on the map of Figure 1), one of the ter-
ranes of the Togo-Dahomeyan contact was logged and
studied. The study was in two aspects-assay of major
crustal and mineral forming elements using Instrumental
Neutron Activation techniques, and the study of the
minerals profile of the bedrock which happens to be
within a site that has been earmarked for a radioactive
waste disposal facility. The logged cuttings of the bore-
hole were examined to reveal the lithostratigraphic pro-
file of the rocks and then their elemental composition
assayed. Results of this work have revealed mineralogi-
cal and elemental make up of lithostratigraphic units of
the borehole; and the general geology of the Togo-Da-
homeyan contact at the area. These results have been
used to characterize the site earmarked for the radioac-
tive waste disposal facility.
2. EXPERIMENTAL METHODS
2.1. Sample Logging and Preparation
A quantity of about 3 kg of cuttings and chips was
collected from the borehole into polyethylene packs after
every 3 m depth was drilled starting from the depth of
5.5 m which is the depth of unconsolidated lateritic soil.
Twenty two (22) samples were collected from the bore-
hole for up to the 72 m depth below surface. The sam-
ples were coded and pre-fixed with BF followed by the
depth in meters at which the sample was taken. For ex-
ample a sample with code BF12 is one taken from the
depth 12 m below the surface level in the borehole. Each
sample was examined to identify its rock composition
before being air-dried. After air-drying, each sample was
disaggregated and pulverized into powder using the
agate mortar (Vibratory Disc Mill RS100) to ensure ho-
mogeneity and also to form a composite sample. Two
replica representative samples that weighed 100 mg each
were then taken from every composite sample and
wrapped in thin polyethylene papers. Replica samples of
100 mg each were also prepared for certified rock refer-
ence materials (CRM’s) GBW07106 and GBW07107.
The samples and standards were packed into 7 mL plas-
tic vials and heat-sealed. For the purpose of results vali-
dation, single element gold standard solution (SRM 3121)
of concentration 10.00 ± 0.03 μg/g was pipetted (using
Eppendorf tip ejector pipette; Brinkmann Instruments,
Inc. Westbury, New York) into a clean 1.5 mL polyeth-
ylene vial and weighed. Weight of the empty vial was
zeroed (pre-weighed) in order to obtain the weight of the
Figure 1. Location map of sample collection and study area.
standard solution. Ground sucrose (from SIGMA-ALD-
RICH, Inc. 3050 Spruce Street St Louis MO 63103 US)
was added to the solution—in order to solidify it and
then allowed to dry at room temperature before being
heat-sealed and placed into a 7 mL vial for irradiation.
2.2. Irradiation and Counting of Samples
Samples and their standards were irradiated in order
for the various elements of interest to be activated and
subsequently assayed. The samples were divided into
two groups to enable all elements of interest to be de-
termined. One group was used for the determination of
medium and long lived nuclides and the other group for
the determination of short-lived nuclides. Irradiation of
samples and standards was done using the pneumatic
rabbit system of the Ghana Research Reactor-1
(GHAAR-1) operating at half-full power of 15 KW
(thermal) and neutron flux of 5.0 × 1011 n/cm2·s. Sam-
ples used for the determination of long and medium
lived nuclides were irradiated for 1 hour. After their ir-
radiation, the medium-lived samples were decayed for
between 2 to 4 days before counting while the long-lived
nuclides samples were decayed for between 4 to 8 weeks
before counting. Samples used to assay short-lived nu-

M. Nyarku et al. / Natural Science 3 (2011) 646-650 648
clides were irradiated for 10 seconds and counted imme-
diately after irradiation. The PC-interfaced N-type HPGe
(High Purity Germanium) gamma ray detector system
was used for detection, counting and gamma spectra
acquisition. The efficiency of the detector system was
25% relative to standard 2'' × 2'' NaI detector and it op-
erated in a bias voltage (of –3000 V) with a FWHM
(Full Width at Half Maximum) resolution of 1.8 keV for
60Co gamma ray energy of 1332 keV. Spectra intensities
of the samples and standards obtained by means of a
MCA (Multi-Channel Analyser) card (ORTEC, 2002)
that is coupled to the PC were used along with the cer-
tificate of the CRM’s used to calculate the concentra-
tions of both oxides and elements.
3. RESULTS AND DISCUSSIONS
Drilled cuttings collected from different depths of the
borehole examined to determine the strategraphic profile
of the rocks reveal a profile shown in Figure 2. Rocks of
the borehole were predominantly schist, gneiss, quartzite,
phyllite. The profile of the borehole confirms earlier
works that have been done with respect to the sequence
of rocks below and above the thrust contact that, rocks
of the Togo are underlain by Dahomeyan formations [6].
Elemental assay results are captured in two tables: Table
1 for major oxides and Table 2 for major elements. Ma-
jor oxides determined in this work were: Al2O3, CaO,
Fe2O3, K2O, Na2O and TiO2 (Table 1). The results show
that the concentration by weight of these oxides was in
the order: TiO2 > Al2O3 > Na2O > K2O > CaO > Fe2O3;
showing that, the rocks are Fe2O3, and CaO deficient.
Felsic index (F) is found by the ratio of (Na2O + K2O) X
100 and (CaO + Na2O + K2O) and mafic index (M) is
found by the ratio of (FeO + Fe2O3) X 100 and (MgO +
FeO + Fe2O3). Felsic index has been computed for these
rocks.
Table 1. Concentrations of oxides.
Sample Code Al2O3 (%wt) CaO (%wt) Fe2O3 (%wt) K2O (%wt) Na2O (%wt) TiO2 (ppm) Felsic Index
BF9 23.79 ± 0.10 1.47 ± 0.22 0.91 ± 0.09 2.43 ± 0.12 6.49 ± 0.03 9811 ± 1400 85.85
BF12 10.74 ± 0.09 1.59 ± 0.17 0.90 ± 0.08 3.34 ± 0.13 3.30 ± 0.02 10300 ± 1570 80.68
BF15 20.77 ± 0.08 3.11 ± 0.34 0.92 ± 0.10 4.51 ± 0.08 3.08 ± 0.01 21972 ± 1552 70.93
BF18 20.75 ± 0.04 2.70 ± 0.13 1.61 ± 0.12 1.76 ± 0.11 2.47 ± 0.01 9663 ± 1290 61.03
BF21 10.23 ± 0.04 0.61 ± 0.12 0.85 ± 0.09 2.81 ± 0.14 6.95 ± 0.02 2449 ± 886 94.11
BF24 13.80 ± 0.06 2.24 ± 0.26 1.60 ± 0.09 3.38 ± 0.12 3.09 ± 0.01 15387 ± 1574 74.28
BF27 10.10 ± 0.05 0.94 ± 0.17 0.77 ± 0.07 5.21 ± 0.11 4.87 ± 0.01 1926 ± 681 91.47
BF30 8.69 ± 0.05 0.56 ± 0.12 1.10 ± 0.09 1.91 ± 0.17 6.53 ± 0.02 2079 ± 703 93.77
BF33 18.20 ± 0.07 4.65 ± 0.29 0.97 ± 0.09 2.78 ± 0.09 2.97 ± 0.01 8780 ± 843 55.28
BF36 18.93 ± 0.06 3.15 ± 0.28 0.87 ± 0.09 3.03 ± 0.10 4.67 ± 0.01 13196 ± 1159 70.96
BF39 20.63 ± 0.05 1.72 ± 0.19 1.26 ± 0.09 3.79 ± 0.12 5.57 ± 0.01 14905 ± 1537 84.47
BF42 8.92 ± 0.04 1.12 ± 0.15 1.23 ± 0.09 3.57 ± 0.11 5.12 ± 0.01 7332 ± 1230 88.58
BF45 10.10 ± 0.05 0.53 ± 0.12 0.87 ± 0.09 5.39 ± 0.13 4.88 ± 0.01 4276 ± 548 95.09
BF48 10.59 ± 0.5 1.05 ± 0.17 1.10 ± 0.11 4.38 ± 0.11 2.52 ± 0.01 4262 ± 581 86.79
BF51 23.10 ± 0.06 0.50 ± 0.15 0.45 ± 0.05 3.26 ± 0.13 6.83 ± 0.02 1810 ± 647 95.27
BF54 12.61 ± 0.07 0.70 ± 0.16 0.65 ± 0.08 5.33 ± 0.14 6.38 ± 0.02 2721 ± 453 94.35
BF57 12.41 ± 0.05 0.56 ± 0.17 0.46 ± 0.07 6.14 ± 0.18 4.80 ± 0.20 1458 ± 495 95.13
BF60 3.10 ± 0.02 0.45 ± 0.10 0.36 ± 0.04 6.34 ± 0.02 4.55 ± 0.02 1040 ± 487 96.03
BF63 10.21 ± 0.04 0.60 ± 0.10 0.32 ± 0.05 5.98 ± 0.20 4.18 ± 0.02 <500> 94.42
BF66 1.14 ± 0.10 0.82 ± 0.18 0.26 ± 0.05 5.92 ± 0.13 4.21 ± 0.01 3040 ± 568 92.51
BF69 23.66 ± 0.01 1.00 ± 0.27 0.22 ± 0.02 6.93 ± 0.23 5.10 ± 0.02 3413 ± 674 92.32
BF72 6.27 ± 0.32 0.67 ± 0.17 0.21 ± 0.03 5.17 ± 0.13 3.65 ± 0.01 1134 ± 380 92.93

M. Nyarku et al. / Natural Science 3 (2011) 646-650
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Table 2. Elemental concentrations in ppm.
Sample Code Ba Co Cr Cs Cu La Mn Sc U V
BF9 <4.30>
2
1.21 ± 2.5519.22 ± 6.69 5.09 ± 1.64281 ± 8116 ± 12 2600 ± 30020.63 ± 0.68 <0.05> 123 ± 8
BF12 124 ± 44 17.34 ± 3.65<0.50> 3.74 ± 1.12183 ± 978.51 ± 10.321892 ± 16316.42 ± 0.64 <0.05> 108 ± 7
BF15 <4.30> 13.65 ± 4.25<0.50> 1.85 ± 0.15136 ± 587.34 ± 8.215965 ± 34017.50 ± 0.68 <0.05> 244 ± 7
BF18 114 ± 32 44.04 ± 4.69<0.50> 4.36 ± 1.68128 ± 948.75 ± 6.411438 ± 19044.66 ± 1.10 6.88 ± 1.2295 ± 6
BF21 162 ± 66 13.5 ± 2.91160 ± 15 <0.01> 288 ± 861.25 ± 8.631162 ± 19113.35 ± 0.58 4.39 ± 1.6435 ± 4
BF24 <4.30> 31.11 ± 3.31124 ± 12 <0.01> 128 ± 451.41 ± 6.832592 ± 18639.98 ± 0.63 <0.05> 92 ± 8
BF27 <4.30> 16.87 ± 3.70<0.50> 4.30 ± 0.88204 ± 6122 ± 10 1189 ± 20710.96 ± 0.49 <0.05> 33 ± 9
BF30 104 ± 24 13.81 ± 2.6499.63 ± 9.19 4.11 ± 1.37314 ± 10108 ± 10 936 ± 80 11.87 ± 0.53 <0.05> 20 ± 4
BF33 151 ± 60 10.80 ± 1.9896.56 ± 11.23 3.14 ± 0.89140 ± 642.46 ± 7.526015 ± 32112.91 ± 0.61 <0.05> 157 ± 9
BF36 <4.30> 5.88 ± 0.9995.49 ± 17.62 2.17 ± 0.79220 ± 797.33 ± 10.594748 ± 24214.90 ± 0.69 <0.05> 125 ± 8
BF39 <4.30> 19.05 ± 3.53<0.50> 10.48 ± 1.83221 ± 7109.73 ± 12.333783 ± 23718.89 ± 0.65 <0.05> 113 ± 8
BF42 100 ± 28 24.10 ± 3.10<0.50> 1.51 ± 0.27216 ± 6112.13 ± 12.341898 ± 17824.19 ± 0.72 2.33 ± 0.3368 ± 6
BF45 <4.30> 11.43 ± 2.14<0.50> 5.69 ± 1.26200 ± 7135 ± 12 1393 ± 18414.62 ± 0.57 4.04 ± 0.6336 ± 5
BF48 <4.30> 17.77 ± 2.88150 ± 50 5.25 ± 1.91120 ± 5130 ± 14 1811 ± 19213.76 ± 0.66 5.92 ± 0.5646 ± 5
BF51 <4.30> 11.10 ± 2.92254 ± 86 4.46 ± 1.72287 ± 975.93 ± 11.93910 ± 2275.41 ± 0.44 5.18 ± 0.4317 ± 5
BF54 <4.30> 10.54 ± 2.3137.36 ± 7.25 6.30 ± 1.34174 ± 7160 ± 12 1120 ± 2196.33 ± 0.44 <0.05> 23 ± 5
BF57 <4.30> <0.02> 59.42 ± 6.35 3.95 ± 1.17211 ± 12161 ± 11 800 ± 2132.69 ± 0.42 1.51 ± 0.58<10.00>
BF60 <4.30> 11.53 ± 3.3428.35 ± 0.85 1.10 ± 0.47215 ± 10138 ± 10 598 ± 1743.19 ± 0.30 6.59 ± 1.3810.87 ± 4.32
BF63 117 ± 41 2.56 ± 0.44<0.50> 4.42 ± 1.07200 ± 9132 ± 10 573 ± 1794.37 ± 0.36 3.59 ± 0.55<10.00>
BF66 <4.30> 5.17 ± 1.64<0.50> 3.19 ± 1.07180 ± 7123 ± 11 928 ± 2134.26 ± 0.32 9.28 ± 4.2512.45 ± 3.67
BF69 <4.30> 6.10 ± 2.43<0.50> 2.21 ± 0.24213 ± 10209 ± 11 1685 ± 2433.94 ± 0.33 8.50 ± 2.8014.76 ± 4.31
BF72 <4.30> 6.88 ± 2.3334.65 ± 5.93 2.10 ± 0.28156 ± 6112 ± 12 384 ± 98 3.77 ± 0.31 2.87 ± 0.42454 ± 1.30
Felsic index of each of the samples was computed and
this is captured on Table 1. The average felsic index (F)
calculated for all the sample is: 85.74. This implies that
the rocks of the contact are generally felsic and less ma-
fic. The comparatively high content of oxides of sodium
and aluminum and the felsic index indicate that the rocks
are feldspar rich [7]. The relatively high content of po-
tassium-aluminum oxides as compared to calciumalu-
minum oxides indicates that orthoclase (K-) feldspars
are the dominants feldspars. Iron and titanium oxides got
depleted with depths from the 52 m below surface
downward while potassium oxide is enriched from 42 m
depth below surface downward. Concentrations of the
other major oxides varied at different depths though not
in trend as could be seen from Table 1.
From Table 1 concentrations of K2O are compara-
tively higher in the Dahomeyan segment of the borehole
than in the Togo segment of the borehole indicating that
the Dahomeyan rocks are K-feldspar richer than the To-
go rocks, though; both rock types are generally feldspar
rich. This shows that the Dahomeyan formations are a
higher grade metamorphic rocks which were formed
under higher temperatures and pressures. The Togo rocks
on the other hand, have higher iron content as compared
to the Dahomeyan; and therefore abound in iron miner-
als such as biotite and hornblende which are predomi-
nant minerals in the Togo schist, quartzite and phyllite;
the main rock types of the Togo formations [8]. Biotite
schist crystallizes from a relative lower temperatures
therefore it can be established that the Togo formations
were formed under relatively lower temperatures there-
fore the two terranes—the Togo rocks and the Da-
homeyan rocks were formed under different geothermal
conditions with the Dahomeyan being formed under
higher geothermal gradient.
Elemental occurrences varied in the Togo rocks and

M. Nyarku et al. / Natural Science 3 (2011) 646-650 650
Figure 2. Lithostratigraphic profile of the borehole
as per examination of drilled cuttings.
the Dahomeyan rocks show the clear mineralogical dis-
tinction between the two types of geologic terranes [9].
For instance, uranium occurrence in the Dahomeyan was
within detectable limits in all the lithostratigraphic levels
studied in exception of BF54 as against the Togo rocks
were uranium content was below detection limit in many
of the lithostratigraphic units. The Togo rocks had high-
er scandium concentrations than the Dahomeyan rocks.
This however, is attributable to the fact that though
scandium is widely dispersed in most minerals of the
crust, aluminum substitution with scandium could give
rise to elevated scandium concentrations in rocks/min-
erals with high aluminum content. The thrust contact is
well depicted in the values of elemental concentrations
(see Table 1). The contact occurred at 48 m below sur-
face level up to around 51 m.
4. CONCLUSIONS
This work has revealed the elemental and mineralogi-
cal compositions of rocks of the various lithostratigrphic
units of the famous Togo-Dahomeyan contact at Kwae-
benya near Accra. The results have been useful in the
prediction of origin and evolution of the Togo-Dahome-
yan contact. The rocks are felsic having average felsic
index (F) of 85.74. They are feldspar rich with K- (or-
thoclase) feldspars dominating. Iron and titanium oxides
are depleted with depths from 52 m below surface level
downward in the borehole, and potassium oxide is en-
riched from 42 m depth below surface level downward
in the borehole. The Togo and the Dahomeyan rocks had
different concentrations of the major elements and ox-
ides assayed as shown on Table 1.
5. ACKNOWLEDGEMENTS
The authors acknowledge the sponsor of the drilled borehole: The
Ghana Atomic Energy Commission and the contributions of staff of
the Ghana Research Reactor-1 (GHARR-1) centre in sample irradia-
tion and counting.
REFERENCES
[1] Blay, P.K. (1982) Geology of 1/4 Field Sheets 184, 185
and 187. Ghana Geological Survey Bulletin, 45.
[2] Kesse, G.O. (1985) The mineral and rock resources of
Ghana, Balkema, Rotterdam, 32-41.
[3] Wright, J.B. (1985) Geology and mineral resources of
West Africa. Science, 58-59.
[4] Mani, R. (1978) The geology of the Dahomeyan of Gha-
na. Geology of Ghana Project. Ghana Geological Survey
Bulletin, 45.
[5] Efa, E. and Muff, R. (2003-2005) Ghana Geological
Survey Department, Field Mapping.
[6] ORTEC (2002) Advanced measurement technology, In-
corporated Part, 777800.
[7] Amili, R. (1977) Internal structure and mineralogy of the
Gloserheia granite pegmatite, Froland, Southern Norway.
Norsk Geologisk Tidsskrift, 57, 243-262.
[8] Nude, P.M., Shervais, J.W., Attoh, K., Vetter, S.K. and
Barton, C. (2009) Petrology and geochemistry of nephel-
ine syenite and related carbonate-rich rocks in the
Pan-African Dahomeyan orogen, southern Ghana West
Africa. Journal of African Earth Sciences, 55, 147-157.
doi:10.1016/j.jafrearsci.2009.03.010
[9] Muff, R. and Efa, E. (2006) Environmental and Engi-
neering Geology for Urban Planning in Accra-Tema.
Ghana Geological Survey Department Bulletin, 55.