Geodinamica Acta
ISSN: 0985-3111 (Print) 1778-3593 (Online) Journal homepage: https://www.tandfonline.com/loi/tgda20
Geochemical constraints on provenance and
source area weathering of metasedimentary rocks
from the Paleoproterozoic (~2.1 Ga) Wa-Lawra
Belt, southeastern margin of the West African
Craton
Daniel K. Asiedu, Magdalene Agoe, Prince O. Amponsah, Prosper M. Nude &
Chris Y. Anani
To cite this article: Daniel K. Asiedu, Magdalene Agoe, Prince O. Amponsah, Prosper M. Nude
& Chris Y. Anani (2019) Geochemical constraints on provenance and source area weathering of
metasedimentary rocks from the Paleoproterozoic (~2.1 Ga) Wa-Lawra Belt, southeastern margin
of the West African Craton, Geodinamica Acta, 31:1, 27-39, DOI: 10.1080/09853111.2019.1670414
To link to this article:  https://doi.org/10.1080/09853111.2019.1670414
© 2019 The Author(s). Published by Informa Published online: 27 Sep 2019.
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GEODINAMICA ACTA
2019, VOL. 31, NO. 1, 27–39
https://doi.org/10.1080/09853111.2019.1670414
ARTICLE
Geochemical constraints on provenance and source area weathering of
metasedimentary rocks from the Paleoproterozoic (~2.1 Ga) Wa-Lawra Belt,
southeastern margin of the West African Craton
Daniel K. Asiedua, Magdalene Agoeb, Prince O. Amponsaha, Prosper M. Nudea and Chris Y. Anani a
aDepartment of Earth Science, University of Ghana, Accra, Ghana; bTechnical Department, Azumah Resources Ghana Ltd, Accra, Ghana
ABSTRACT ARTICLE HISTORY
The Wa-Lawra Belt which is situated in the northern part of Ghana consists of Received 15 April 2019
Paleoproterozoic Birimian fine metasedimentary rocks metamorphosed to greenschist facies, Accepted 17 September 2019
particularly, in the western part. A whole-rock geochemical study of these metasedimentary KEYWORDS
rocks was undertaken to unravel their source area weathering, provenance and tectonic Birimian; geochemistry;
setting. Geochemical characteristics of the studied shales show that they are immature in metasedimentary rocks;
nature and first cycle in origin, with little or no recycled component. Compared to Post- provenance; Wa-Lawra Belt
Archaean Australian Shales (PAAS), the studied shales indicate reduction in Zr, Hf, La, Nb, Th
and Ta being the high field strength elements and evidences of transition metal enrichments
in V, Ni, Sc, Co, and Cr. Major element geochemistry indicates that the shales were subjected
to slight potassium metasomatism after deposition. Pre-metasomatized Chemical Index of
Alteration calculations indicates that weak to moderate degree of chemical weathering took
place at the sediment source area. Co-Th-La-Sc systematics reveals a combination of mafic
and felsic provenances for the shales. Eu/Eu* together with values of Th/U and some
abundances of trace elements show that the shales were mainly derived from juvenile
rocks. Average REE model calculations suggest that the source materials are composed of
about 49% basalt, 16% TTG and 35% granite.
1. Introduction 1992). The processes involved in the growth of the
Paleoproterozoic continental crust have always
The West African craton (WAC) is divided into the
aroused debate amongst various researchers.
Reguibat shield and Leo-man shield to the north and
Workers such as Abouchami et al. (1990) and Boher,
south respectively, comprising Archaean rocks of
Abouchami, Michard, Albarède, and Arndt (1992) have
Liberian age (3.0–2.5 Ga) to the west and the
suggested that within the Birimian, a relationship
Birimian Paleoproterozoic age to the east of the Leo-
does exist between the tholeiitic magmatism and an
Man shield, respectively (Figure 1). The Birimian is
oceanic plateau environment whereas several others
made up of four metasedimentary basins and six
have proposed that the entire Birimian crust grew in
volcanic belts. The volcanics within the belts comprise
an island arc environment (Ama Salah, Liégeois, &
low grade metamorphosed lavas that are mainly tho-
Pouclet, 1996; Beziat et al., 2000; Sylvester & Attoh,
leiitic in composition, ‘belt type’ tonalite-granodiorite
1992). It has further been indicated that the Birimian
intrusions as well as minor felsic volcaniclastics
of the Haute-Comoé was deposited in an intraconti-
(Hirdes, Davis, & Eisenlohr, 1992). The basins consist
nental trans-tensional back-arc basin (Vidal & Alric,
of volcaniclastics, argillites intruded by extensive, late-
1994). This suggests a pre-Birimian basement existed.
kinematic ‘basin type’ granitoid plutons which vary
The Birimian Supergroup in Ghana consists of nar-
from tonalite to peraluminous granite in composition
row sedimentary basins that trend northeasterly as
and wackes which are isoclinally folded (Davis, Hirdes,
well as linear greenstone belts consisting mainly of
Schaltegger, & Nunoo, 1994; Hirdes et al., 1992; Leube,
volcanic, volcaniclastic to clastic series. Both the sedi-
Hirdes, Mauer, & Kesse, 1990).
mentary basins and the greenstone belts (one of
Geochronological as well as geochemical studies of
which is the Lawra belt – the study area) are intruded
igneous rocks indicate that the main Paleoproterozoic
by various generations of granitoids (Figure 2). These
crustal growth events in the WAC are characterised by
volcanic and metasedimentary supracrustal rocks
the formation of huge volumes of juvenile material
occurred during an accretionary period around 2.1
with the involvement of a significantly small Archaean
Ga (Abouchami et al., 1990; Taylor et al., 1992) during
crust component (Abouchami, Boher, Michard, &
the 2.1–2.0 Ga Eburnean orogeny (Bonhomme, 1962).
Albarede, 1990; Taylor, Moorbath, Leube, & Hirdes,
The supracrustal sequence was folded and
CONTACT Chris Y. Anani cyanani@ug.edu.gh Department of Earth Science, University of Ghana, P.O. Box LG 58, Legon, Accra, Ghana
© 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits
unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
28 D. K. ASIEDU ET AL.
Figure 1. Simplified geological map of the Leo-Man Shield of the West African Craton (WAC) indicating the position of Ghana
modified after Carney et al. (2010). Inset showing a simplified map of Africa showing the position of Ghana.
metamorphosed under predominantly greenschist In Ghana, the basement rocks to the Birimian are
facies conditions. To the east of the Birimian not known. Extensive geochemical and isotopic stu-
Supergroup is a relatively younger surrounding unit dies of igneous rocks (i.e. volcanic rocks and asso-
which is the Voltaian Supergroup Figures 1 and 2 ciated granitoids) indicate that the Birimian
(insert). The Voltaian Supergroup fills the Volta Basin constitutes a huge addition of materials from the
(Figure 2, insert) which is made up of Neoproterozoic mantle (e.g. Dampare et al., 2009; Sakyi et al., 2018,
to early Paleozoic strata up to ~5 km thick. The strata 2014) although the terrane may be underlain substan-
consist of a succession of sandstones, mudstones, and tially by Archaean basement. In Ghana, the metasedi-
few proportions of limestone (Affaton, Sougy, & mentary rocks in the Birimian Supergroup have
Trompette, 1980; Anani, Mahamuda, Kwayisi, & received relatively less attention, even though they
Asiedu, 2017; Kalsbeek, Frei, & Affaton, 2008). It covers form the dominant component of the overall
a surface area of ~115,000 km2. Birimian rocks. The crustal evolution of this segment
GEODINAMICA ACTA 29
Figure 2. Geological map of the study area modified after Amponsah et al. (2016), showing the various gold camps (where the
samples were collected). Insert showing a map of northwestern Ghana showing the surrounding rock units modified after
Petersson, Scherstén, and Gerdes (2018).
of the continental crust may be derived by important composition estimates of the upper crust (e.g.
information from these metasedimentary rocks. Condie, 1993; Taylor & McLennan, 1985). This paper
Siliciclastic sedimentary rocks provide an abun- presents the outcome of a bulk-rock geochemical
dance of data about the evolution of the crust. study of Birimian metasediments of the Lawra belt,
Siliciclastic rock compositions have served as the northwestern Ghana; the scanty geochemical studies
source of valuable information regarding source-area of the metasedimentary rocks of the Birimian
and tectonic setting (e.g. Bhatia & Crook, 1986), paleo- Supergroup in Ghana have been restricted to south-
climate (e.g. Fedo, Eriksson, & Krogstad, 1996; Fedo, western Ghana (Asiedu et al., 2017, 2004). This study
Young, & Nesbitt, 1997), as well as average aims at (i) defining the geochemical features of the
30 D. K. ASIEDU ET AL.
metasedimentary rocks, (ii) discussing conditions that half is defined by the mineral assemblage hornble-
prevailed at the source area regarding paleoweather- nde, clinopyroxene and plagioclase (Block et al.,
ing, and (iii) constraining their tectonic setting and 2015).
provenance. A multiphase deformational episode has been
recognised on the Wa-Lawra belt by several workers
(Amponsah et al., 2016; Baratoux et al., 2011; Block
2. Geological setting
et al., 2015). The first deformation event on the belt is
The Wa-Lawra greenstone belt in NW Ghana forms recognised as a result of a sinistral anastomosing
a section of the Eastern Paleoproterozoic Birimian transcurrent shear zone showing steep dips. It is
terrane within the WAC (Amponsah et al., 2016; oriented N to NNW (left lateral sinistral jog) or fault
Block et al., 2015; Griffis, Barning, Agezo, & Akosah, (along the Jirapa and Jang fault) and denotes an ESE-
2002). This Paleoproterozoic Birimian geology WNW and E-W shortening (Amponsah et al., 2016).
extends into countries such as Burkina Faso, Ivory This structural grain resulted from a long-protracted
Coast, Mali, Niger and Ghana. The Wa-Lawra green- deformation episode from 2128Ma to 2086Ma. This
stone belt trends N-S (Kesse, 1985; Somakin and affected all the volcaniclastic sediments, sedimentary
Lashmanov, 1991) in Ghana whilst all other belts and volcanic rocks within the belt (Baratoux et al.,
trends NE-SW. It is the southern extension of the 2011). A N-NNW striking and vertically dipping pene-
N-S trending Bromo belt which runs from Burkina trative foliation (S1) is evidence of this structural grain.
Faso into Ghana (Amponsah et al., 2016; Baratoux Further evidence is a sub-horizontal stretching linea-
et al., 2011). The Wa-Lawra belt in Ghana is tion which trends northwards and plunges 20º and
bounded by the Diebougou-Bouna granitoid parallel to the S0 bedding planes. The second defor-
domain to the west in Ivory Coast. It is fault mational episode is marked by E-W tension gashes.
bounded (i.e. Jang Fault which is a NNW-S splay of They crosscut S1 shear zones and are mostly quartz
the main Jirapa fault) by the 2162 ± 1Ma to 2134 ± filled with E-W direct shortening. The 2020 Ma to 2000
1Ma Koudougou-Tumu granitoid terrane composed Ma last deformational episode in the belt is marked
of gabbro and gneisses. These have porphyritic by F3 isoclinal fold and crenulation cleavages. Vertical
granite intrusions of 2128 Ma to the east (Block fold axes mark the F3 folds as well as axial planar S3
et al., 2015). In the south, it is bounded by the Bole- foliation which is ENE-WSW. This deformational epi-
Nagondi belt and the Bole-Bulenga domain. sode is assumed to indicate N-S shortening.
According to Block et al. (2015) and Amponsah
et al. (2015, 2016), the Wa-Lawra belt is divided
3. Sampling and analytical methods
into two halves, with each half juxtaposed by the
crustal-scale transcurrent sinistral Jirapa fault. Jirapa Forty-one (41) samples were collected from thirty (30)
faults crosscuts the Bole-Bolenga domain and inte- drill holes from the following areas in the western part
grates with the Bole-Nagondi belt in the south of the Wa-Lawra belt: Basabli, Duri, Yagha, Benkpong,
(Block et al., 2015; DeKock et al., 2012). The western Kunche and Butele (Figure 2). Twenty-two (22) shale
portion consists mainly of basalts (2200–2160 Ma; samples devoid of weathering and alterations were
Baratoux et al., 2011), (2139 ± 2 Ma detrital zircon selected. These were subjected to whole-rock geo-
ages; Agyei Duodu et al., 2009), sediments (interca- chemical analysis. The whole-rock major and trace
lated suites of meta-shales, meta-siltstone and element (including REE) analysis was performed by
meta-arenites), volcaniclastic rocks and intrusive ALS laboratory in Vancouver, Canada. Sample pre-
granitoids of varying ages (i.e. 2212 ± 1 Ma; Sakyi paration was done by placing sample (1.0 g) in an
et al., 2014). All the rocks in this half have experi- oven at 1000°C for 1 hour, cooled and then weighed.
enced greenschist metamorphism. P-T conditions The percent loss on ignition was calculated from the
obtained from mica schist in the eastern half from difference in weight.
a chlorite-quartz-water (Chl-Qz-H2O) and phengite- The major elements were analysed by Inductively
quartz-water (Ph-Qz-H2O) equilibria defined Coupled Plasma-Atomic Emission Spectroscopy (ICP-
a P-T space of 310-380ºC at 2 kbar (Block et al., AES). This was first achieved by dissolution of the
2015). Based on micro-thermometric analysis done, grounded samples by lithium metaborate/lithium tet-
CH4-H2O ± SO2 and H2O-CH4 -CO2-SO2 fluid inclu- raborate (LiBO2/Li2B4O7) fusion method. This involved
sions by Amponsah et al. (2016) indicated that mixing lithium metaborate/lithium tetraborate flux
greenschist facies hydrothermal fluids circulated in with a prepared sample (0.200 g) and fused in
the rock at temperatures 310 to 370ºC. The eastern a furnace at 1025°C. An acid mixture containing nitric,
half of the Wa-Lawra belt is composed of amphibo- hydrochloric and hydrofluoric acids was then used to
lite, para and ortho-gneisses, granitoids and rhyo- cool and dissolve the mixture. The solution was then
lites (Amponsah et al., 2016; Block et al., 2015). The analysed by ICP-AES. The results were corrected for
amphibolite facies metamorphism in the eastern spectral inter-element interferences. 0.01% was the
GEODINAMICA ACTA 31
detection limit for the major element oxides (SiO2, Na2 Sr ratios (0.2 to 0.93) are lower than that of PAAS (Rb/
O, Fe2O3, Al2O3, MnO, TiO2, CaO, P2O5, K2O and MgO). Sr = 1.25; Taylor & McLennan, 1985).
The trace element analyses were performed obser- The studied shales have high so-called transition
ving the protocols as for the major element analyses. metals Ni (73 to 120 ppm, average of 94.5 ppm), Co
However, the prepared sample weighed 0.100 g and (21 to 35, average of 27.4 ppm), Cr (150 to 260 ppm,
the analysis was done by Inductively Coupled Plasma – with an average of 190 ppm), V (110 to 180 ppm, with
Mass Spectroscopy (ICP-MS). The base metals were average of 151 ppm), and Sc (16 to 24 ppm, with an
analysed using the ICP-AES by sample (about 0.25 g) average of 19.8 ppm). Generally, the studied shales
digestion with perchloric, nitric, hydrochloric and are enriched in these transition metals relative to
hydrofluoric acids and addition of dilute hydrochloric PAAS (Figure 6).
acid. The detection limits (ppm) for the trace elements The concentrations of the high field strength ele-
are as follows: 1 (Co, Cu, Mo, Ni, Sc, Sn, W), 5 (As, V), 2 ments (HFSE), Zr, Hf, Ta, Nb, Th, U, Y, La range from
(Pb, Zn, Zr) 10 (Li, Tl, Cr,), 0.05 (Th, U, Dy, Cd), 0.1 (Sr, Ta, 103 to 163 ppm, 2.8 to 4.1 ppm, 0.2 to 0.4 ppm, 4.6 to
Ca, Nd), 0.2 (Rb, Hf, Nb), 0.5 (Ag, Cd, Y, Ba, Ce, La), 0.03 7.1 ppm, 2.26 to 3.86 ppm, 0.9 to 1.45 ppm, 9.2 to 22.2
(Pr, Sm, Yb, Er, Eu), 0.01 (Tm, Tb, Cs, Ho). ppm, 11.8 to 23 ppm, respectively. Relative to PAAS
the studied shales exhibit depletion in the HFSE
(Figure 6). The average Zr/Hf value of 36.6 is sugges-
4. Results tive of zircon control (Zr ≈ 40). Th/U values (2.33 to
The results of the geochemical analysis and their corre- 3.14) are consistently lower than that of the continen-
sponding sample locations are shown in Tables 1 and 2 tal upper crust (Th/U ≈ 3.8; McLennan, Hemming,
respectively. McDaniel, & Hanson, 1993).
4.1. Major elements 4.3. Rare earth elements
The studied metasedimentary rocks generally have The rare earth element (REE) data for the studied
SiO (53.9 to 68.6) wt.%, Al O (14.5 to 20.5) wt.%, shales are somewhat variable with total REE (ΣREE)2 2 3
TiO2 (0.55 to 1.0) wt.% and P2O5 (0.11 to 0.21) wt.%
values of 65.7 to 127 ppm, averaging of 101 ppm,
contents similar to that of PAAS (Figure 3). However, which is lower than that of PAAS value of 184.8 ppm
Fe2O3 (6.39 to 10.15) wt.%, MgO (2.2 to 4.5) wt.% and
(Taylor & McLennan, 1985). The shales display REE
Na O (0.94 to 3.63) wt.% contents are enriched patterns that are similar when normalised to PAAS2
whereas CaO (0.24 to 2.21) wt.% and K2O (1.17 to
with slightly depleted Light REE (LREE) (Figure7), posi-
3.70) wt.% contents are generally depleted compared tive Eu-anomaly and fairly flat Heavy REE (HREE). On
to that of PAAS (Figure 3). They have low SiO /Al O a chondrite-normalised diagram (not shown) the stu-2 2 3
values (2.73 to 4.73) indicative of their low maturity. died shales display fractionated LREE patterns (aver-
SiO2 correlates negatively with Al O (r = 0.88), TiO
age LaN/SmN = 2.81), small negative Eu (average Eu/
2 3 − 2
(r = 0.81), MgO (r = 0.88), and Fe O (r = 0.81) Eu* = 0.79) and fairly flat HREE patterns (average Gd− − − N2 3
(Figure 4). P O , K O, MnO and CaO do not system- /YbN = 1.51) which are characteristic of sediments2 5 2
atically vary with SiO . However, Na O (r = 0.19) shows derived from upper continental crust (Taylor &2 2
weak positive correlation with SiO (Figure 4). McLennan, 1985).2
Several workers have used major element whole-
rock geochemisty to classify siliciclastic sedimentary 5. Discussion
rocks (e.g. Blatt, Middleton, & Murray, 1980; Crook,
1974; Herron, 1988; Pettijohn, Potter, & Siever, 1972). 5.1. Heavy mineral accumulation and
By their Fe2O3/K2O versus SiO2/Al2O3 the studied sam- metamorphism
ples classify as both Fe-shales and shales (Figure 5). The The degree of sorting and recycling may be deduced
Fe-shales have comparable SiO2, moderately lower Al2 by evaluating the accumulation of weathering-
O3 and higher Fe2O3 contents than the shales. resistant phases such as zircon and monazite in silici-
clastic sedimentary rocks (McLennan et al., 1993).
Enrichment of zircon, and therefore recycling and
4.2. Trace elements
sorting, can be inferred from the Th/Sc versus Zr/Sc
The concentrations of the large ion lithosphere ele- diagram; the ratio Zr/Sc is an effective index of zircon
ments (LILE) Cs, Rb, Ba and Sr range from 2.59 to 12.1 enrichment while the ratio Th/Sc is a good indicator
ppm, 45.8 to 110 ppm, 218 to 879 ppm, and 110 to of igneous chemical differentiation processes
394 ppm, respectively (Table 1). In comparison to (McLennan et al., 1993). The studied samples on this
PAAS, the studied shales exhibit slight to strong diagram trend in the general provenance-dependent
depletion in Rb, Ba and Cs and enrichment in Sr. Rb/ compositional variation pattern with none of the
32 D. K. ASIEDU ET AL.
Table 1. Geochemical data for the metasedimentary rocks of the Wa-Lawra belt.
SAMPLE BR223 BR258 BR302 BR203 BR263 BR450a BR450b BR265 KR699 KR695 KR700a
SiO2 59.6 58.1 61.9 57.7 61.4 64.3 55.6 62.1 56.1 60.6 64.2
Al2O3 17.0 16.2 16.0 18.2 15.2 14.7 18.7 16.1 19.6 17.7 16.6
Fe2O3 9.08 8.81 8.59 9.08 7.18 7.53 9.79 7.91 8.44 6.70 6.39
CaO 1.11 1.92 0.63 0.62 2.07 0.97 0.99 1.03 2.21 2.07 0.85
MgO 3.68 3.80 3.43 3.70 3.31 2.98 3.87 3.17 3.47 3.21 2.89
Na2O 3.22 2.12 3.45 3.20 3.52 3.29 3.56 3.48 3.05 3.11 3.63
K2O 1.53 2.18 1.33 1.97 1.59 1.17 1.76 1.48 2.95 2.39 2.24
TiO2 0.66 0.66 0.65 0.76 0.59 0.62 0.73 0.71 0.80 0.61 0.64
MnO 0.11 0.09 0.09 0.06 0.09 0.08 0.10 0.06 0.07 0.07 0.06
P2O5 0.11 0.16 0.11 0.12 0.14 0.12 0.19 0.17 0.17 0.11 0.14
LOI 4.51 5.33 4.19 4.35 5.49 3.68 4.66 3.80 3.78 2.94 2.82
Total 100.56 99.32 100.32 99.71 100.53 99.44 99.95 100.01 100.64 99.46 100.41
Rb 58.9 83.2 53.6 72.9 59.6 45.8 71.3 53.1 87.7 80.3 76.0
Sr 282 233 182 241 297 154 178 261 394 355 141
Ba 246 422 222 373 376 218 324 395 720 653 714
Th 2.96 2.97 2.95 2.97 2.76 2.26 3.49 2.91 3.43 2.98 2.77
U 0.99 1.10 0.94 1.06 1.08 0.91 1.27 1.06 1.23 1.11 1.18
Zr 113 112 110 122 111 103 129 118 130 127 131
Hf 3.4 3.2 3.2 3.5 3.3 2.9 3.7 3.3 3.9 3.3 3.5
Nb 5.2 5.2 5.0 5.7 4.7 4.6 5.6 5.4 6.0 4.8 4.9
Ta 0.3 0.3 0.3 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.3
Y 17.4 17.2 16.6 19.9 15.6 14.4 20.9 15.6 19.3 9.2 14.6
Sc 22 20 22 21 17 17 21 18 23 16 16
V 169 151 151 175 131 128 156 136 176 137 140
Cr 170 170 150 190 190 180 180 190 200 210 230
Co 30 26 29 31 24 26 32 25 29 22 24
Ni 100 110 94 102 83 85 105 90 101 73 80
La 13.4 18.0 16.8 16.2 17.1 11.8 21.4 17.4 21.1 18.2 16.0
Ce 29.1 38.2 35.6 36.1 35.4 25.5 45.0 36.3 44.4 37.6 35.4
Pr 3.52 4.78 4.42 4.46 4.43 3.14 5.53 4.58 5.39 4.63 4.32
Nd 14.0 19.4 17.7 18.5 17.7 12.7 22.8 18.3 22.2 18.4 17.2
Sm 2.94 3.98 3.50 4.04 3.58 2.57 4.50 3.54 4.34 3.41 3.40
Eu 0.90 1.02 0.82 0.93 0.94 0.68 1.15 0.85 1.25 0.88 0.80
Gd 2.68 3.39 3.33 3.48 3.03 2.41 3.78 3.05 3.65 2.55 2.84
Tb 0.46 0.51 0.46 0.52 0.48 0.39 0.65 0.48 0.57 0.29 0.47
Dy 2.74 3.10 2.83 3.52 2.79 2.41 3.69 2.80 3.51 1.55 2.58
Ho 0.61 0.64 0.61 0.74 0.59 0.54 0.75 0.58 0.70 0.33 0.53
Er 1.95 1.87 1.70 2.18 1.67 1.57 2.10 1.56 2.07 1.10 1.52
Tm 0.28 0.28 0.26 0.30 0.26 0.23 0.29 0.21 0.30 0.21 0.24
Yb 1.91 1.89 1.79 2.03 1.63 1.55 1.97 1.58 2.11 1.26 1.61
Lu 0.32 0.29 0.27 0.32 0.26 0.23 0.33 0.26 0.32 0.22 0.24
SAMPLE KR7002a KR080a KR0802b KR700b KR7002b KR565 AVA042a AVA042b AVA043 AVA038a AVA038b
SiO2 57.7 58.9 57.7 58.1 56.2 58.1 63.8 68.6 59.3 53.9 59.9
Al2O3 18.2 18.3 18.5 18.3 18.4 17.8 15.7 14.5 17.8 20.5 17.9
Fe2O3 9.67 8.61 9.51 8.87 9.40 8.43 7.90 6.93 8.40 10.2 8.40
CaO 1.86 1.28 1.16 1.02 1.22 1.25 0.30 0.24 0.41 0.60 1.16
MgO 4.50 3.99 4.01 4.17 4.41 3.98 3.01 2.43 3.55 4.47 3.59
Na2O 2.63 2.16 1.91 1.64 1.44 1.73 3.09 0.94 3.31 1.18 2.17
K2O 2.02 2.84 3.02 3.18 3.18 3.01 1.49 2.34 1.66 3.10 2.01
TiO2 0.72 0.73 0.73 0.72 0.69 0.70 0.59 0.55 0.68 0.81 0.77
MnO 0.10 0.09 0.09 0.09 0.09 0.09 0.10 0.05 0.10 0.07 0.07
P2O5 0.20 0.18 0.19 0.16 0.18 0.16 0.16 0.13 0.21 0.19 0.16
LOI 4.34 4.13 4.09 4.59 4.92 4.70 3.85 3.76 4.40 5.25 4.55
Total 101.89 101.21 100.86 100.79 100.08 99.9 99.94 100.47 99.77 100.22 100.68
Rb 73.0 99.5 108 108 103 102 54.0 83.2 60.6 110 74.5
Sr 289 207 154 125 110 139 195 144 145 165 308
Ba 543 463 498 872 879 830 551 729 427 720 545
Th 3.2 3.43 3.34 3.55 3.40 3.38 3.07 2.74 2.98 3.86 3.32
U 1.34 1.30 1.29 1.27 1.24 1.45 1.00 0.90 1.18 1.28 1.17
Zr 135 137 137 139 129 134 132 107 163 156 134
Hf 3.5 3.4 3.7 3.9 3.6 3.6 3.4 2.8 4.0 4.1 3.6
Nb 5.8 5.8 5.7 5.9 5.6 7.1 6.3 4.8 5.3 6.2 6.1
Ta 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.4 0.4
Y 20.1 19.8 20.1 17.7 19.9 17.8 17.3 15.0 18.5 22.2 17.1
Sc 21 21 21 20 21 21 16 18 19 24 20
V 163 163 155 160 159 161 110 126 150 180 146
Cr 180 200 210 180 160 180 160 160 260 220 210
Co 28 30 30 27 27 29 22 21 27 35 29
Ni 101 99 100 94 98 94 80 81 87 120 102
La 21.6 21.3 21.9 18.1 22.7 17.3 22.0 15.1 18.4 23.0 20.7
Ce 47.3 46.7 48.4 39.4 48.4 38.6 46.3 32.5 40.6 49.9 45.2
Pr 5.74 5.85 5.86 4.91 6.02 4.85 5.61 4.01 4.95 6.18 5.57
Nd 24.2 23.0 24.8 20.6 24.1 20.6 22.8 16.1 21.0 26.0 23.0
Sm 4.79 5.00 5.19 4.47 5.42 4.24 4.71 3.61 4.33 5.28 4.68
Eu 0.95 1.18 1.17 0.89 1.01 0.98 1.15 0.87 1.09 0.99 0.96
Gd 3.86 4.44 4.15 3.68 4.34 3.71 3.65 2.80 3.54 4.52 3.60
Tb 0.59 0.63 0.64 0.53 0.66 0.53 0.52 0.42 0.55 0.73 0.54
(Continued)
GEODINAMICA ACTA 33
Table 1. (Continued).
SAMPLE BR223 BR258 BR302 BR203 BR263 BR450a BR450b BR265 KR699 KR695 KR700a
Dy 3.35 3.50 3.56 3.14 3.68 3.20 3.06 2.61 3.25 3.87 2.92
Ho 0.73 0.72 0.78 0.66 0.75 0.66 0.69 0.58 0.70 0.89 0.64
Er 2.18 2.20 2.29 2.03 2.06 1.99 1.75 1.65 1.99 2.42 1.85
Tm 0.33 0.32 0.33 0.31 0.34 0.29 0.30 0.23 0.29 0.39 0.30
Yb 1.93 2.05 2.05 2.10 2.12 2.01 1.80 1.47 2.01 2.35 1.82
Lu 0.34 0.33 0.34 0.31 0.34 0.33 0.28 0.26 0.32 0.37 0.29
Major elements in wt.%, trace elements in ppm.
Table 2. Sample locations of the metasedimentary rocks (Table 1) of the Wa-Lawra Belt.
Sample 1D/Depth Easting Northing Rock name Sample type
AVA038a (211.0–211.5 m) 524,468 1,166,276 SHALE CORE
AVA038b (217.5–218.0 m) 524,468 1,166,276 SHALE CORE
AVA042a (194.0–194.5 m) 524,420 1,166,079 SHALE CORE
AVA042b (201.1–201.6 m) 524,420 1,166,079 SHALE CORE
AVA043 (202.0–202.5 m) 524,765 1,166,074 SHALE CORE
BR203 (251.5–252.0 m) 527,047 1,152,225 SHALE CORE
BR223 (84.4–85.0 m) 527,186 1,151,474 SHALE CORE
BR258 (117.5–118.0 m) 527,046 1,152,173 SHALE CORE
BR263 (144.5–145.0 m) 527,076 1,152,123 SHALE CORE
BR265 (147.7–218.2 m) 527,050 1,152,095 SHALE CORE
BR302 (185.5–186.0 m) 527,022 1,152,224 SHALE CORE
BR450a (228.0–228.5 m) 527,052 1,152,351 SHALE CORE
BR450b (234.0–234.5 m) 527,052 1,152,351 SHALE CORE
KR080a (150.0–151.0 m) 527,028 1,149,108 SHALE CORE
KR0802b (156.0–156.5 m) 527,028 1,149,108 SHALE CORE
KR565 (163.0–163.5 m) 526,845 1,149,499 SHALE CORE
KR695 (229.5–230.0 m) 526,990 1,148,778 SHALE CORE
KR699 (276.0–276.5 m) 526,992 1,148,575 SHALE CORE
KR700a (229.0–229.5 m) 524,765 1,166,074 SHALE CORE
KR7002a (261.0–261.5 m) 526,925 1,149,247 SHALE CORE
KR700b (264.0–264.5 m) 526,925 1,149,247 SHALE CORE
KR7002b (299.0–299.5 m) 526,925 1,149,247 SHALE CORE
samples. A very steep chondrite-normalised HREE pat-
tern is characteristic of monazite which has very high
REE abundances, and even small amounts (<0.01%)
result in significant increases in the chondrite-
normalised GdN/YbN ratio (McLennan et al., 1993). The
fairly flat HREE pattern of the studied shales (Figure 7)
rules out preferential monazite accumulation in the
samples.
Some major and trace elements in metamorphic
rocks are mobilised by interaction with fluids. To
a lesser extent, by solid-state diffusion and melt
generation (Rollinson, 1993). The rocks of the
Lawra Belt have been subjected to up to greens-
Figure 3. Post-Archaean Australian Shale (PAAS)-normalised chist facies metamorphism and therefore there is
major element plots for the studied metasedimentary rocks. the possibility that some of the elements may
PAAS values are from Taylor and McLennan (1985). have been preferentially remobilized (Rollinson,
1993; Taylor & McLennan, 1985). Such element
remobilization would obviously reduce the effec-
samples plotting in the high Zr/Sc range which is tiveness of using them in provenance
characteristic of zircon accumulation associated with discrimination.
sediment sorting and recycling (Figure 8(a)). To minimise the effect of remobilization, samples
Zircon is rich in HREE and its accumulation will result containing any visible hydrous fluid transfer veinlets
in the decrease in the chondrite-normalised LaN/YbN were avoided for this study. Nevertheless, some
ratio. Therefore, a negative correlation between Zr and authentic proofs have been used against large-scale
LaN/YbN would be expected if zircon is concentrated in remobilization of the elements in the studied metase-
the samples. However, there is no such negative corre- dimentary rocks: All the samples have similar and
lation between the two (Figure 8(b)), and it, therefore, smooth REE patterns which would not be expected
rules out preferential zircon accumulation in the during remobilization. In addition, although it is
34 D. K. ASIEDU ET AL.
Figure 4. Covariation of major elements versus SiO2 (wt%) for the studied metasedimentary rocks.
Figure 5. The classification of the studied metasedimentary
rocks using log (Fe2O3/K2O) versus log (SiO2/Al2O3) (after Figure 6. Trace element concentrations of the studied shales
Herron, 1988). normalised to the composition of average PAAS. The normal-
ising values are those of Taylor and McLennan (1985).
possible that some elements such as the LILE may
5.2. Source area weathering and diagenesis
have been remobilized, the covariance among the
HFSE such as Zr, Hf, Nb, Ta, Th, U, V, Cr, Ni, Co, and For most rocks, the Rb/Sr ratio increases with increas-
REEs suggests that these elements have the same ing degree of chemical weathering. This is so because
degree of low mobility during metamorphism. Rb+, a large alkali trace element, remains fixed in the
GEODINAMICA ACTA 35
2.33 to 3.14 (average 2.73) which is well below
upper crustal values, and the U concentrations range
from 0.9 to 1.45 ppm (average 1.15 ppm) well below
that of typical shales (U ~ 3.1) and upper continental
crust (U ~ 2.7) (Taylor & McLennan, 1985). This sug-
gests a low degree of chemical weathering at the
sediment source area.
The Chemical Index of Alteration (CIA) has been
used to quantify the weathering history of sedimen-
tary rocks, primarily to understand paleoclimate con-
ditions (Nesbitt & Young, 1982, 1984). Unweathered
igneous rocks have CIA values less than 50, typical
shales average about 70 to 75, and intensely weath-
Figure 7. Rare earth element abundances of the studied ered rocks have CIA values that approaches 100 (Fedo
shales normalised to PAAS. PAAS values are from Taylor et al., 1996; McLennan et al., 1993). The studied shales
and McLennan (1985). have CIA values that range from 58 to 78 (average,
67.6), which ranges from those of unweathered
igneous rocks to typical shales, indicating low to
weathered residue, in preference to the smaller Sr2+ moderate degree of weathering at the sediment
which is selectively leached (McLennan et al., 1993; source area.
Nesbitt et al., 1980). As a result, the Rb/Sr ratio has The weathering history for the studied shales may
been used to evaluate the intensity of chemical be evaluated using the A-CN-K diagram (Figure 9). In
weathering at the source area; Rb/Sr > 1 is an indica- this diagram, it is expected that the samples will plot in
tion of high degree of chemical weathering whereas a trend parallel to the A-CN join if weathering is the
Rb/Sr < 1 indicates moderate to low degree of che- control of the composition (Fedo et al., 1996; Fedo,
mical weathering. The studied shales have range of Nesbitt, & Young, 1995; McLennan et al., 1993). The
Rb/Sr from 0.20 to 0.93 (average, 0.43) suggesting low studied samples, however, indicate a linear trend
degree of chemical weathering at the sediment (Trend 1). It is not comparable with simple weathering
source area. being the sole control of the composition (Trend 2).
For most upper crustal igneous rocks, the Th/U is The plots suggest the effects of K addition to the
typically about 3.5 to 4.0 (McLennan et al., 1993). samples as a result of metasomatism (Fedo et al.,
Weathering typically results in the oxidation and sub- 1996, 1995). The pre-metasomatized CIA values of the
sequent dissolution of U thereby elevating the Th/U studied samples may be estimated from the diagram
ratios above the upper crustal values, especially for using the method outlined by Fedo et al. (1995). This
shales (Taylor & McLennan, 1985). However, other puts the pre-metasomatized CIA range from 60 to 85
sedimentary processes may result in U enrichment (Figure 9) indicating low to moderate degree of che-
thereby lowering the Th/U ratio; in such cases the mical weathering in the source area of the sediments.
low Th/U will be accompanied by high U content. The occurrence of K enrichment is widespread in
The Th/U values in the studied shales range from Precambrian sedimentary rocks and therefore, Fedo
Figure 8. (A) Plot of Th/Sc versus Zr/Sc for the shales of the Wa-Lawra belt. The compositional variation trend lines are from
McLennan et al. (1993). (B) Plot of La/Yb versus Zr for the shales of the Wa-Lawra belt.
36 D. K. ASIEDU ET AL.
enrichment in transition metals such as Cr, Ni, Co,
Sc, and V, suggesting a significant contribution from
mafic sources (Figure 6). Compared to PAAS the stu-
died shales show less LREE enrichments and less pro-
nounced europium anomalies (Figure 7) also
suggesting significant input from mafic sources.
Co-Th-Sc-La systematics can reveal the mixing
between felsic and mafic sources for sedimentary
rocks (Taylor & McLennan, 1985; Yang et al., 1998).
On La/Sc versus Co/Th and Sc/Th versus Co/Th dia-
grams (Figure 10(a and b)), the studied Birimian shales
plot between the basalt and granite end-members
with cluster towards the mafic end (high La/Sc and
Co/Th, and low La/Sc). The geochemistry of the stu-
died shales can, therefore, be explained as having
Figure 9. Al2O3–(CaO*+Na2O)–K2O diagram for the metase- been derived from a mixture of basaltic rocks (mainly)
dimentary rocks of the Wa-Lawra Belt. and granitic rocks (subordinately).
Following the establishment of diverse possible
source components, we seek to indicate the relative
et al. (1996) proposed the Plagioclase Index of
contribution of three rock types with distinct REE
Alteration (PIA) to evaluate weathering histories it
patterns. These are basalt (BAS), granite (GRA) and
takes care of the influence of K-feldspar. The maxi-
tonalite–trondhjemite–granodiorite (TTG). Kasanzu,
mum PIA value is 100 and unweathered plagioclase
Makenya, and Manya (2008) were adopted to achieve
has a PIA value of 50. The studied shales have a range
the mixing calculations. Modelling for the average
of PIA values from 59 to 88 (average, 72) suggesting
studied Birimian shale accomplished using the follow-
weak to moderate weathering in the source area of
ing REE parameters: Gd /Yb , Eu/Eu*, and La /Yb .
the sediments. N N N N
The REE data of the endmembers (i.e. BAS, GRA, and
TTG) were extracted from Condie (1993). The mixing
calculations were set in a matrix form as:
5.3. Source rock composition 2 
 3 2 32 3
The compositions of fine-grained siliciclastic sedimen- Eu = Eu 0:93 0:36 0:99 x4 5 4 54 5
tary rocks, such as shales, are particularly characteris- LaN = YbN ¼ 11:62 9:19 2:73 y
La = Sm 3:61 3:44 1:81 z
tic of the bulk composition of the source region. The N N 2 3
0:80
abundances of HFSE and the transition metals, and ¼ 4 6:765
REEs have particularly proved useful in discriminating 2:81
the source composition of fine-grained metasedimen-
tary rocks (e.g. Asiedu et al., 2017; Roddaz, Debat, & where x = TTG, y = granite (GRA) and z = basalt (BAS).
Nikiéma, 2007; Yang, Kyser, & Ansdell, 1998). The ratios LaN/YbN and GdN/YbN are chondrite-
Compared to PAAS the studied shales show depletion normalised (Normalising values from Taylor &
in HFSE such as La, Zr, Hf, Th, Ta, and Nb, and McLennan, 1985).
Figure 10. Plots of (A) Co/Th versus La/Sc and (B) Co/Th versus Sc/Th for the metasedimentary rocks of the Wa-Lawra Belt. Also
plotted are average Paleoproterozoic volcanic and plutonic rocks from Condie (1993). BSH, studied shales; BAS, basalt; AND,
Andesite; GRA, granite; TTG, tonalite–trondhjemite–granodiorite; FVO, felsic volcanic rock.
GEODINAMICA ACTA 37
The results obtained from the mixing calculations depleted mantle sources of the arc provenance. The
have shown that a mixture having 16% TTG, 35% low Th/U values of the studied shales (Th/U = 2.33 to
granite and 49% basalt is best for the modelling of 3.14) and the relatively low Th and U concentrations
the studied Birimian shale. compared to upper crustal values and PAAS also sug-
gest that the shales have young undifferentiated arc
provenance (Figure 12; McLennan et al., 1993). The
5.4. Tectonic settings and location of sources above geochemical characteristics, therefore, suggest
that the studied Birimian shales are juvenile crustal
The geochemical compositions of siliciclastic sedi-
material derived from local sources, most probably
mentary rocks have been used to discriminate the
the adjacent volcanic rocks and their associated
tectonic settings of sedimentary basins (e.g. Bhatia,
granitoids.
1983; Roser & Korsch, 1986). Particularly useful for
Our inference that the Birimian shales of the Lawra
Precambrian metasedimentary rocks are tectonic dis-
greenstone belt represent juvenile crustal materials
crimination diagrams that utilise immobile trace ele-
derived locally from the volcanic and associated grani-
ments (e.g. Bhatia & Crook, 1986). On the Th–Sc–Zr
tic rocks places constraints on the evolution of the
and the Th–La–Sc diagrams, the samples of the
Birimian crust. Our present work, together with pre-
Birimian shales fall exclusively in the oceanic island
vious provenance studies on the Birimian metasedi-
arc field (Figure 11(a and b)).
mentary rocks (e.g. Asiedu et al., 2017, 2004) shows
McLennan et al. (1993) defined four different types
juvenile geochemical signatures with only minor con-
of terrane that can be identified from geochemical
tribution of an older crustal component. The lack of
data: Young Differentiated Arc, Old Upper Continental
evidence for incorporation of substantial Archaean
Crust, Young Undifferentiated Arc, and Recycled
Sedimentary Rocks. Compared to PAAS and upper
crustal values the studied shales have (i) relatively low
but variable SiO2/Al2O3, K2O/Na2O, and CIA values, (ii)
lower ratios of incompatible to compatible elements,
such as Th/Sc and Zr/Sc (Figure 3), and (iii) lack of
substantial Eu anomalies and low LREE enrichment
(Figure 7). These geochemical features indicate young
undifferentiated arc provenance for the studied shales
(McLennan et al., 1993). The Th/U ratio is typically
about 3.5 to 4.0 for most upper crustal rocks
(McLennan et al., 1993; Taylor & McLennan, 1985).
Sediments from active margin tectonic settings,
which consist of young undifferentiated crust, typically
have Th/U significantly below 3.5 accompanied by low
Th and U abundances (McLennan and Taylor, 1991;
McLennan et al., 1993). On Th/U versus Th diagram
(Figure 12), the studied shales mainly plot in the Figure 12. Plot of Th/U versus Th for the shales of the Wa-
depleted mantle sources field reflecting geochemically Lawra belt (after McLennan et al., 1993).
Figure 11. Plots of (A) Th – Co – Zr, and (B) Th – La – Sc for the tectonic setting discrimination of the shales from the Wa-Lawra
belt (after Bhatia & Crook, 1986). A, Oceanic Island Arc; B, Continental Island Arc; C, Active Continental Margin; D, Passive
Continental Margin.
38 D. K. ASIEDU ET AL.
detritus, therefore, rules out an intra-cratonic origin of Geological map of Ghana 1:1 000 000. Geological Survey
the sediments as previously proposed by Ledru, Pons, Department of Ghana (GSD); Accra.
Milesi, Feybesse, and Johan (1991). Ama Salah, I., Liégeois, J.-P., & Pouclet, A. (1996). Evolution
d’un arc insulaire oceanique birimien pr´ecoce au Liptako
nigérien (Sirba): Géologie, géochronologie et géochemie.
Journal of African Earth Sciences, 22, 235–254.
6. Conclusions Amponsah, P. O., Salvi, S., Beziat, D., Baratoux, L.,
A whole-rock geochemical study was undertaken on Siebenaller, L., Nude, P. M., . . . Jessell, M. W. (2015). The
Bepkong deposit, Northwestern Ghana. Ore Geology
fine-grained metasedimentary rocks from the Birimian Reviews, 78, 718–723.
Wa-Lawra Belt of northern Ghana in order to constrain Amponsah, P. O., Salvi, S., Beziat, D., Baratoux, L.,
the provenance and source area weathering. The fol- Siebenellar, L., Jessell, M., . . . Adubofour, E. G. (2016).
lowing deductions were made: Multistage gold mineralization in the Wa-Lawra green-
stone belt, NW Ghana: The Bepkong deposit. Journal of
African Earth Science, 120, 220–237.
Anani, C. Y., Mahamuda, A., Kwayisi, D., & Asiedu, D. K.
(1) The fine-grained metasedimentary rocks are (2017). Provenance of sandstones from the neoprotero-
classified as shales on the basis of their major zoic Bombouaka Group of the volta Basin, northeastern
element compositions. Ghana. Arabian Journal of Geoscience, 10, 1–15.
(2) The shales are rst cycle in origin and obtained Asiedu, D. K., Asong, S., Atta-Peters, D., Sakyi, P. A., Su, B.-X.,fi
from materials of mixed mafic and felsic com- Dampare, S. B., & Anani, C. Y. (2017). Geochemical and
Nd-isotopic compositions of juvenile-type Paleoproterozoic
positions. Mixing calculations using the REEs Birimian sedimentary rocks from southeastern West African
suggest a provenance with mixture having Craton (Ghana): Constraints on provenance and tectonic
16% TTG, 35% granite and 49% basalt. setting. Precambrian Research, 300, 40–52.
(3) The shales represent juvenile crustal materials Asiedu, D. K., Dampare, S., Asamoah-Sakyi, P., Banoeng-Yakubo,
derived locally from the associated granitic and B., Osae, S., Nyarko, B. J. B., & Manu, J. (2004). Geochemistry of
Paleoproterozoic metasedimentary rocks from the Birim dia-
volcanic rocks. mondiferous field, southern Ghana: Implications for prove-
(4) The shales were deposited in an oceanic island nance and crustal evolution at the Archean-Proterozoic
arc setting. boundary. Geochemical Journal, 38, 215–228.
Baratoux, L., Metelka, V., Naba, S., Jessell, M. W., Grégoire, M.,
& Ganne, J. (2011). Juvenile Paleoproterozoic crust evolu-
Acknowledgments tion during the Eburnean orogeny (2.2–2.0 Ga), Western
Burkina Faso. Precambrian Research, 191, 18–45.
We thank the management and staff of the Exploration Béziat, D., Bourges, F., Debat, P., Lompo, M., Martin, F., &
Department of the Azumah Resources Ghana Ltd for provid- Tollon, F. (2000). A Paleoproterozoic ultramafic–Mafic
ing the rock samples for this study, and granting us access assemblage and associated volcanic rocks of the
to their property for the fieldwork. This research was funded Boromo greenstone belt: Fractionates originating from
by the Earth Science Capacity Building Project, Department island-arc volcanic activity in the West African craton.
of Earth Science, University of Ghana. Precambrian Research, 101, 25–47.
Bhatia, M. R. (1983). Plate tectonics and geochemical com-
position of sandstones. Journal of Geology, 92, 181–193.
Disclosure statement Bhatia, M. R., & Crook, A. W. (1986). Trace element charac-
teristics of greywackes and tectonic setting discrimina-
No potential con ict of interest was reported by the tion of sedimentary basins. Contribution to Mineralogyfl
authors. and Petrology, 92, 181–193.
Blatt, H. G., Middleton, G. V., & Murray, R. C. (1980). Origin of
sedimentary rocks (2nd ed.). Englewood cliff, N. J., Prentice
Hall.
ORCID Block, S., Ganne, J., Baratoux, L., Zeh, L., Parra-Avila, A.,
Jessell, M., . . . Siebenaller, L. (2015). Petrological and geo-
Chris Y. Anani http://orcid.org/0000-0002-9153-9807
chronological constraints on lower crust exhumation dur-
ing Paleoproterozoic (Eburnean) orogeny, NW Ghana,
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