Petrology and Geochemical Relationship of Birimian Gabbro-Diorite-Granodiorite Dykes in Basaltic Rocks at Butre Area, SW Ghana ()
1. Introduction
Granitoids in contact with host rocks are associated with different types of hydrothermal alterations. Example Irizar granite of Kukri Hills, Antarctica which was intruded by dolerite intrusives showed biotite altered to chlorite; and hornblende to chlorite and haematite (Craw & Findlay, 1984) . At Galore Creek of Canada, diorite/monzonite intruded into sandstone, siltstone and chert and introduced hydrothermal potassic-calcic alterations (Micko et al., 2014) . In the Tavsanlı area in Turkey, the Egrigöz granitoids intrusion into various gneisses and schists caused silicification, argillic-sillicic, and phyllic alterations along fault/shear zones at the contacts of granitoids (Kumral et al., 2016) . In India, Rajastan, metavolcanic rocks intruded by granitoids have undergone mineralogical changes: oligoclase to albite, and biotite to annite, while rocks unaffected by hydrothermal activities remain intact (Kaur et al., 2012) . Oxides of Si, Al, Na, K, Mg, Mn, Ca and Fe were encountered. Si2O wt. % varies inversely to oxides of Al, Mg, Mn, Ca and Fe but almost directly to that of K2O (Rajah et al., 1977) .
The Birimian is a portion of the Man Shield, located south of the West African craton, comprising basins of metasedimentary rocks (commonly slates, phyllites, greywacke) disconnected by belts metavolcanic rocks (basalt, andesite, agglomerates, tuff) (Leube et al., 1990) . The age of the Birimian as estimated from zircons in the metavolcanic rocks of the Birimian Supergroup shows ages between 2162 ± 6 Ma and 2266 ± 2 Ma, while detrital zircon grains from the metasedimentary rocks gave ages between 2180 and 2130 Ma (Oberthür et al., 1998; Perrouty et al., 2012) . Three main types of granitoids are identified within the Birimian - sedimentary type (Cape Coast), volcanic belt type (Dixcove) and K-rich (Bongo) granitoid types (Hirdes et al., 1992) . Minerals of the volcanic belt type are non-foliated quartz and hornblende diorite, while K-rich granitoids are associated with hornblende and microcline which are also non-foliated (Kesse, 1985) . However, the sedimentary basin type is associated with well foliated marked by biotite, microcline, beryl etc. characterised by schist and gneiss enclaves (Perrouty et al., 2012) . The belt type and basin type granitoids in Ghana were emplaced at different times (Leube et al., 1990) . Loh & Hirdes (1999) stated that the age of belt granitoids in the southern portion of the Ashanti belt constrains those of the metavolcanic rocks and so are comagmatic with ages between 2145 - 2190 Ma, while the basin-type is regarded as late-to-post-kinematic and predates the Tarkwaian with ages between 2090 - 2125 Ma (Oberthür et al., 1995) .
At Mpohor area, SE of the Ashanti belt, intrusions have led to the chloritisation of gabbro and diorite (Tetteh & Effisah-Otoo, 2017) . Also at the Prestea mine, SW of the Ashanti belt, alteration of pyrite to marcasite is common (Perrouty et al., 2012) while at Bekpong area, NW Ghana, intrusions have led to the chloritisation and sericitisation of shale (Amponsah et al., 2016) . Mineralisation includes Au deposits (Salvi et al., 2015; Allibone et al., 2004; Perrouty et al., 2012) . Within the Birimian, hydrothermal mineralisation in the basin granitoids is characterised by alterations of quartz, sericite, calcite etc. assemblage but alterations diminish away from mineralised zones with albite, pyrite, chlorite assemblage (Smith et al., 2016) . Alteration halos of phyllite/basin type granitoid contact at Obuasi mine is smaller (Yao & Robb, 2000) compared to that of the basalt and volcanic sediments in contact with K-rich granitoids at the Julie deposit of Wa-East District (Amponsah et al., 2015) . Tarkwaian Group overlies the Birimian rocks of the Ashanti belt (Eisenlohr & Hirdes, 1992) as clastic sediments that were deposited at an approximated constrained age between 2102 and 2097 Ma (Perrouty et al., 2012) .
Three granitoids namely, tonalite in the south, granodiorite in the centre and gabbro/norite at the northern part of Mpohor town occur near metavolcanic rocks at Butre with xenoliths present in tonalite considered as contact metamorphosed, metasomatised basaltic rocks (Loh & Hirdes, 1999) . The alterations and textures related to geochemistry of the granitoids and metavolcanic rocks have not been studied. This study therefore explored the geochemical relationship between the intrusives and their host metavolcanic rocks (Figure 1).
2. Geological Setting
2.1. Regional Geology
The West African craton is represented by rocks of lower Proterozoic timespan from 2500 to 1800 Ma. To the southernmost region lies the Man shield, made up largely of rocks of early Proterozoic age (Wright et al., 1985) . The Man shield is made up of Archean rocks of Liberian age (3.0 - 2.5 Ga) which lies to the west and eastern portion made up of rocks of early Proterozoic age known as the Birimian rocks (Leube et al., 1990) . To the southeastern portion of the Man shield, the Birimian rocks outcropextensively in La Côte d’Ivoire, Burkina Faso, western Ghana, southern Mali, west of Niger and Senegal (Abouchami et al., 1990) .
The Birimian of the Man shield has been folded, metamorphosed and intruded by various pluton suites (granitoids) and is believed to have occurred during the Eburnean age at 2.1 Ga (Leube et al., 1990) . The Birimian rocks of Ghana are composed of belts of metavolcanic rocks that have been disconnected by basins of sedimentary rocks (Leube et al., 1990) . The metasedimentary rocks of the Birimian are commonly made up of slates, phyllites, metagreywacke, tuffaceous shale, siltstone and Mn rich chemical sedimentary rocks (Eisenlohr & Hirdes, 1992) . The metavolcanic rocks on the other hand are made up chiefly of basalt and andesite with interflow of some pyroclastic rocks (Loh & Hirdes, 1999) . In Ghana, the Birimian metasedimentary basins are located at the south-central (Cape Coast basin), southwestern (Kumasi and Sunyani basins) and the northern (Maluwe) respectively (Smith et al., 2016) . Marginal to the belts are found various sedimentary rocks including wacke, turbidites, volcaniclastics, chemical sediments etc. (Hirdes et al., 1993) . In general, the Birimian metasedimentary rocks progress into each other and divided into volcaniclastic rocks, turbidite related wacke, argillitic rocks and chemical sediments (Leube et al., 1990) .
Birimian metavolcanic belts also known as the greenstone belts strike almost parallel to each other in a southwesterly to northeasterly direction and covers hundreds of kilometers. From Griffis et al. (2002) , the belts have approximated widths of 20 to 70 km for larger belts and 10 to 20 km for smaller belts, which mostly occur in the north. There were previously six belts in the Birimian of Ghana (Leube et al., 1990) . However, Smith et al. (2016) outlined the following, named from southeast to northwest, Kibi-Winneba Belt; Ashanti Belt; Manso-Nkwanta-Asankragwa Belt; Sefwi Belt; Bui Belt; Bole-Navrongo Belt; Wa-Lawra Belt and Julie Belt as the major belts in Ghana. Tholeiitic lavas mostly of metamorphosed basalts and andesites are common within the belts together with some mafic rocks and interbedded dacite, rhyodacite and pyroclastics (Berge, 2011) . Although lava/pyroclastic ratios are variable amongst the belts of the Birimian in Ghana, the highest ratios are observed in the Ashanti Belt whiles it is least in the Sefwi Belt (Leube et al., 1990) .
In Ghana, the Tarkwaian Group overlies the Birimian rocks and is perceived as fragments of the Birimian that uplifted and eroded after the Eburnean Tectono-Thermal Event (Eisenlohr & Hirdes, 1992) . The Tarkwaian is much matured within the Ashanti and the Bui Belts with approximated thickness of 2500 and 9000 m, respectively (Griffis et al., 2002) .
![]()
Figure 1. Geological map of SW Ghana showing study area.
2.2. Local Geology
Butre area lies on the far south of Ashanti Belt within the Birimian metavolcanic portion (Figure 1). This part of the belt is composed of volcanic rocks of mainly metamorphosed basalt and andesite together with pyroclastics and sedimentary rocks and have been intruded by granite-diorite suites (Dampare et al., 2008) . There also occur mafic-ultramafic intrusives of various sizes, the largest having been described by Attoh et al. (2006) as complex ophiolite of Paleoproterozoic age and occurs in a supra-subduction zone. At Butre area, the metavolcanic rocks of the Birimian include pillow basaltic lavas. These rocks are in contact with three granitoids namely: tonalite in the south, granodiorite in the centre and gabbro/norite at the northern part of Mpohor town. Xenoliths present in tonalite are considered to be contact metamorphosed, metasomatised basaltic rocks (Loh & Hirdes, 1999) . Metamorphism to amphibolite facies was observed in the rocks especially at areas in proximity to granitoids (John et al., 1999) .
3. Methods Used
The area was mapped and locations determined using GPS and rocks described in hand specimen. Thirteen (13) representative samples were selected from fresh rocks of different types and sampling was across intrusive/metavolcanic rock contact. Sample locations are summarised in (Figure 2). Thin and polished sections were prepared and studied under transmitted and reflected light microscopy at the Petrology Laboratory of the Geological Engineering Department, University of Mines and Technology (UMaT), Tarkwa using the Leica DM 2700P microscope. These minerals were identified and characterised based on their textural relationship taking into account mineral associations and alterations. Eight (8) representative and uniform samples were analysed for whole rock geochemistry at Associated Laboratory Services Minerals, Canada. Each sample was weighed, crushed using mechanical crusher to about 70 % below 2 mm and split using the riffle splitter. It was then pulverised using the agate mortar to about 85 % and made to pass through <75 µm to obtain powdered samples. From each powdered sample, glass discs were prepared using lithium borate flux composed of 50 % lithium tetraborate (Li2B2O7) and lithium borate (LiBO2). 0.9 g of ignited powdered samples were added to 9.0 g of lithium borate flux, mixed well and fused in an auto fluxer at about 1100˚C. From the molten melt, a flat pellet was prepared and subjected to X-ray fluorescence spectroscopy for major oxides using the XRF-06 equipment, the upper and lower limits being 100% and 0.01% respectively. For trace elements analysis, 0.2 g of each sample was digested using perchloric (HClO4), nitric (HNO3), hydrofluoric (HF) and hydrochloric (HCl) acids. The resultant constituents were topped up with dilute hydrochloric acid and analysed for trace elements using Inductively Coupled Plasma atomic emission spectroscopy (ICP-OES), correcting spectral inter-element interferences. The authenticity of the analytical procedure (from accuracy to standards) is obtainable at Associated Laboratory Services Minerals, Canada.
![]()
Figure 2. Geological Map of Study Area showing sample locations.
4. Petrography
4.1. Gabbro-Diorite
The rock is a coarse grained, weakly deformed porphyritic pod in metavolcanic rock which is cut by gabbro 2 (younger), containing granular plagioclase phenocrysts (Figure 3(a)). The matrix is made up of irregularly aligned medium grained plagioclase, quartz and amphiboles and shows chilled margin with metavolcanic rock (andesite). Medium grained version occurs as veins in host rocks with irregularly aligned porphyritic plagioclase in association with granular quartz and disseminated pyrite in quartzo-feldspathic veins occurring in the matrix (Figure 3(b) and Figure 3(c)).
In thin section, the rock is fine to medium grained, moderately foliated and porphyritic. Modal percentages of the minerals are shown in Table 1. A weak alignment of plagioclase is inter-foliated with amphiboles. Amphiboles are being replaced by opaque minerals with overprints of chlorites whiles plagioclase is overprinted by sericite (Figure 4(a)). A quartzo-feldspatic vein which is clouded by sericite almost completely replaces a fine to medium grained matrix, made up of plagioclase and amphibole (Figure 4(b)). Amphibole 2 which could be metamorphic occurs close to a quartzo-feldspatic intrusive whiles amphibole 3 appears to have come along with this same intrusive (Figure 4(c)). A weakly deformed intrusion of medium to coarse grained, irregularly aligned subhedral plagioclase and amphibole that are strongly altered to fine sericite and epidote, respectively, occur in fine to medium grained matrix of plagioclase and amphibole may suggest two generations of diorite (diorite and gabbro-diorite) in relation to the cross cutting structure observed in the field (Figure 4(d)) and (Figure 4(e)). A cluster of aligned coarse grained, granular amphibole that is surrounded by fine grained, irregularly aligned amphibole and plagioclase suggest an amphibolite xenolith caught up by the rock. This coarse grained amphibole is overgrown by altered garnet (Figure 4(f)).
4.2. Diorite
In the field, the rock occurs as an intrusive in contact with metavolcanic rock (Figure 5(a)) with chill margins (Figure 5(b)) and as quartzo-feldspathic dyke (Figure 5(c)). In hand specimen, the rock is light green melanocratic, coarse grained and porphyritic. Plagioclase is granular and occurs as phenocrysts. The matrix is made up of irregularly aligned medium grained plagioclase, quartz and amphiboles.
![]()
Figure 3. Field photographs showing: (a) Gabbro-diorite pod in metavolcanic rock, cut by diorite dyke. (b) Gabbro-diorite veins in fractured host rock. (c) Gabbro-diorite veins in pillow lava.
![]()
Figure 4. Photomicrographs of studied Gabbro-diorite showing: (a) Alignment of plagioclase and amphibole. (b) Quartzo-feldsparthic vein replacing rock. (c) Elongated amphibole in association with a quartzo-feldsparthic vein. (d and e) Intrusion of medium grained irregularly aligned amphibole and plagioclase into fine grained plagioclase and amphibole matrix. (f) Amphibolite xenolith with overgrown weathered garnet. Abbreviations, Plagioclase (Pl), Amphibole (Amp), Pyroxene (Px), Garnet (Gr), Sericite (Ser), Epidote (Epi), Tourmaline (Tur), Dolomite (Do), Pyrite (Py), Magnetite (Mag), Arsenopyrite (Apy), Pyrrhotite (Po).
In thin section, the rock is fine to medium grained and porphyritic made up of phenocrysts of carbonates and pyroxenes with irregular alignment of fine to medium grained subhedral amphiboles and plagioclase in the matrix. Modal percentages of the minerals are shown in Table 1. Carbonate phenocrysts in the rock is partially replaced by pyroxene. Pyroxene has inclusions of subhedral plagioclase that extincts at 15˚ (oligoclase composition) (Figure 6(a)). Dolomite phenocryst is partially replaced by epidote and plagioclase which show albite twinning at extinction angle of 37˚ (Figure 6(b)).
Later amphibole is coarse grained, granular, elongated and cuts foliation marked by fine grains of plagioclase and amphibole (Figure 6(c)). Other versions have undefined outlines but are replaced by earlier versions and later
![]()
Table 1. Modal Percentages of studied rocks from Butre area.
![]()
Figure 5. Field photographs showing: (a) Diorite intrusive into host metavolcanic rock. (b) Chill margin of intrusive and host metavolcanic rock. (c) Fractured diorite dyke in fractured host metavolcanic rock.
![]()
Figure 6. Photomicrographs of studied Diorite showing: (a) Carbonate phenocrysts overprinted by pyroxene, sericite and epidote. (b) Dolomite with overprints of amphibole at the edges. (c) Partially aligned plagioclase and amphibole with later overprint of coarse grained amphibole. (d) Coarse grained anhedral amphibole with later overprint of tourmaline. (e) Quartzo-feldsparthic vein overprinted by hydrothermal ore mineral. (f) Aligned amphibole and plagioclase due to shearing.
overprint of tourmaline (Figure 6(d)). Hydrothermal activity captured as hydrothermal opaque minerals overprint on amphibole, plagioclase and quartz (Figure 6(e)) whereas shearing has led to alignment of amphiboles and plagioclase (Figure 6(f)). Opaque minerals include pyrite and arsenopyrite that are partially replaced by magnetite and haematite. A cluster of anhedral pyrite is corroded by gangue mineral and partially replaced by spots of magnetite (Figure 7(a)). Elsewhere, pyrite is subhedral, isotropic, porphyroblastic and partially replaced by magnetite and exsolution of arsenopyrite (Figure 7(b)). Fine grained exsolution of arsenopyrite is overprinted by later arsenopyrite (Figure 7(c)). Also, blurry shaped pyrite is corroded by light gangue and pitted by magnetite infilling (Figure 7(d)).
4.3. Granodiorite
In the field, the rock is light green leucocratic, coarse grained with visible quartz, sulphides, porphyritic plagioclase and amphiboles and occurs as quartzo-felspatic veins filling irregular fractures in andesite (Figure 8(a) and Figure 8(b)). In thin section, the rock is medium grained with moderate alignment of amphibole and plagioclase. Modal percentages are given in Table 1. Earlier amphiboles and plagioclase are parallel to foliation and overprinted by later amphiboles (Figure 9(a) and Figure 9(b)). Also, earlier fabrics of rock reveal the alignment of plagioclase and amphiboles (Figure 9(c)). Ore mineral include pyrite and magnetite. Pyrite is anhedral, isotropic and occurs in gangue. It is partially replaced by pyrrhotite and later magnetite (Figure 9(d)).
![]()
Figure 7. Photomicrographs of studied Diorite showing: (a) Anhedral pyrite with spots of magnetite. (b) Subhedral pyrite with partial overprint of magnetite and exsolution of arsenopyrite. (c) Fine grained exsolution of arsenopyrite with later overprints of arsenopyrite. (d) Blurbs of pyrite with infilling of magnetite.
![]()
Figure 8. Field photographs showing: (a) and (b) Sheared granodiorite intrusive into host metavolcanic rock.
![]()
Figure 9. Photomicrographs of studied Granodiorite showing: (a) and (b) Alignment of fine to medium grained plagioclase and amphibole with overprint of later coarse grained amphiboles. (c) Earlier fabrics of rock with aligned plagioclase and amphiboles. (d) Anhedral pyrite partially replaced by pyrrhotite and magnetite.
5. Geochemistry
Results
Eight (8) representative samples were analysed at Associated Laboratory Services Minerals, Canada using X-ray fluorescence and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). From whole rock geochemistry of rocks studied, the intrusives were grouped according to their silica content which ranges from (52.6 - 62.7 wt %). Whole rock geochemical results are presented in Table 2. Generally, Al2O3 is enriched for all rock types with range from (11.10 - 15.45 wt %). Na2O is generally low in gabbro-diorite (0.13 - 1.94 wt %) but intermediate in diorite (2.78 - 4.11 wt %). CaO is high in gabbro-diorite (8.29 - 15.35 wt %) but generally low in diorite (3.03 - 6.95).
Total FeO is generally higher in gabbro-diorite (9.49 - 11.25 wt %) but low in diorite (5.56 - 9.28 wt %). Cr generally decreases from gabbro-diorite (470 - 590 ppm) to diorite (160 - 280 ppm). Rb is highest in diorite (59.3 ppm although a sample had lower Rb content of 0.4 ppm) and gabbro-diorite had (0.6 - 1.8 ppm). Zr is fairly enriched in diorite (109 ppm) although a sample recorded low Zr content of (60 ppm) but lower in gabbro-diorite (35 - 67 ppm). Line graphs of FeOT and CaO were plotted against SiO2 to predict possible fractionation trend of the studied rocks (Figure 12). Trace elements triangular graphs to show the tholeiitic and calc-alkaline trends of the various rocks are also given (Figure 11(b)). Fractionation trend is supported in reverse by K2O and Na2O against SiO2 wt % plots respectively (Figure 13).
![]()
Table 2. XRF analysis of intrusives from Butre area.
In thin section, chlorite and sericite alterations were observed. The general depletion of K2O and enrichment of MgO and FeOT in the rocks were investigated using the alteration index and the alteration box plot given by Ishikawa alteration index (AI) and the chlrorite-carbonate-pyrite index (CCPI). The Ishikawa alteration index is given by
(1)
The chlorite-carbonate-pyrite index
(2)
The combination of some trace elements and major oxides were plotted against each other to predict probable tectonic settings and possible magma sources of the rocks (Figure 17 and Figure 18).
6. Discussion
The major rocks in the study area show massive to fractured and deformed pillow structures. These rocks were deduced to be basalt and andesite (Figure 10).
According to Sylvester & Attoh (1992) , pillows and massive lava flows occur at the middle of the volcanic section in the area. The major rocks are intruded by minor rocks of different thickness and textures which are medium to coarse grained and porphyritic with irregular alignment. A cross cutting structure observed in the field suggests generational relationship between the intrusives. Given their low quartz but high mafic mineral contents, the cross cutting intrusives (G6 and G7) were initially perceived as two generations of gabbro. Hence the first generation (G6) was given gabbro 1 and the later (G7) as gabbro 2. However, from geochemical plots, they were classified as gabbro-diorite (first generation) and diorite (second generation). The coarser grained and sheared intrusive (G14) which was previously diorite was also confirmed as granodiorite (Figure 11(a)). The primary minerals of the intrusives comprise of plagioclase of labradorite to andesine composition (An52 - An48), amphibole and pyroxene
with tourmaline, actinolite, apatite and garnet as accessory minerals. The gabbro-diorite is moderately foliated with primary fine grained plagioclase and amphiboles occupying the foliations. The diorite is porphyritic with medium to coarse grained plagioclase and amphiboles. The coarser grained granodiorite has aligned medium grained plagioclase and medium to coarse grained amphibole.
Generally, quartz, sericite and carbonate are alteration products of plagioclase. Amphibole is partially altered to chlorite and epidote. The gabbroic diorite which contains xenolith of amphibolite with overgrowth of garnet suggests an interaction with host rock during emplacement. John et al. (1999) are of the opinion that majority of the rocks within south of the Ashanti belt have been subjected to greenschist facies metamorphism; retrograde from amphibolite facies which is particularly observed in rocks near the belt granitoids.
In thin section, opaque minerals were seen overprinting amphiboles and in some cases, in quartzo-feldsparthic veins. Some of these opaque minerals could probably be alteration products of iron bearing silicates. Hatch et al. (1972) elaborated that opaque minerals, typically magnetite are generated from the conversion of amphibole to pyroxene which is facilitated by the decomposition of the hydroxyl group at elevated temperatures. The resulting opaque minerals may be as a result of oxidation and sulphidation of iron components of amphibole.
Both CaO and FeOT against SiO2 show negative fractionation trends. FeOT is enriched in all rock types but highest in gabbro-diorite (Figure 12(a)). CaO is enriched in gabbro-diorite and granodiorite but fairly enriched in diorite (Figure 12(b)).
K2O and Na2O against SiO2, on the other hand, show positive fractionation trend with K2O generally depleted in all rock types except two samples of diorite which were fairly enriched (Figure 13(a)). Na2O is enriched in diorite but depleted in gabbro-diorite and granodiorite (Figure 13(b)). The characteristic < 0.5 wt. % K2O content and the tholeiitic signature of the studied gabbro-diorite may suggest petrogenic features rather than alteration. According to Dampare et al. (2005) some mafic intrusives are typically hybrid rocks which may have formed by the interaction between granitoid magma and the host mafic rocks. Cr against SiO2 show negative fractionation trend (Figure 14(a)) but Hf, Th and Nb show positive trends (Figure 14(b), Figure 15(a) and Figure 15(b)) respectively. All rock types from this study are LREE depleted (La/Sm chondrite normalised ratios of gabbro-diorite = 0.9 to 2.05; diorite = 1.95 to 3.1; granodiorite = 1.66).
Diorite is fairly enriched in HREE (La/Yb chondrite normalised ratios of diorite = 4.96). Gabbro-diorite and granodiorite are depleted in HREE (La/Yb chondrite normalised ratios of gabbro-diorite = 1.51 to 3.75; granodiorite = 4.10). It is therefore postulated that the general fractionation trend is from tholeiitic gabbro-diorite, granodiorite and diorite (Figure 13).
The alteration box seeks to represent graphically alteration captured under two main indices; the Ishikawa alteration index (AI) and the chlrorite-carbonate-pyrite index (CCPI). These indices are useful in the determination of the extent of sericite and chlorite alteration defined by Ishikawa et al. (1976) .
![]()
Figure 12. Binary Geochemical Plots of studied Intrusive showing variation of (a) FeOT with SiO2. (b) CaO with SiO2.
![]()
Figure 13. Binary Geochemical Plots of studied Intrusive showing variation of (a) K2O with SiO2, (b) Na2O with SiO2.
![]()
Figure 14. Binary Geochemical Plots of studied Intrusive showing fractionation trends of (a) Cr with SiO2, (b) Hf with SiO2.
![]()
Figure 15. Binary Geochemical Plots of studied Intrusive showing fractionation trends of (a) Th with SiO2, (b) Nb with SiO2.
Figure 16(a) shows replacement of sodium (Na) in albite by potassium (K) in the formation of sericite (Eastoe et al., 1987) and Figure 16(b) shows a replacement of potassium (K) in sericite by magnesium (Mg) and iron (Fe) to form chlorite (Schardt et al., 2001) . The CCPI was formulated by Large et al. (2001) to cater for two main limitations of the AI.
The AI does not consider carbonate alterations which usually lead to lower values in AI although the intensity of alteration is high and AI does not discriminate between sericite and chlorite alterations when used alone. Hence the index quantifies the increase in MgO and FeOT linked to Mg-Fe chlorite formation with an additional advantage of its sensitivity to Mg-Fe carbonate alteration and magnetite and haematite enrichment.
In summary, the box plot suggests a general dolomite-ankerite-chlorite-calcite ± pyrite for gabbroic diorite, epidote-calcite-dolomite-ankerite for granodiorite and dolomite-ankerite-sericite ± epidote ± calcite for the diorite (Figure 16(c)). The chlorites are alteration products of amphibole and sericite after plagioclase feldspars related to greenschist facies metamorphism. Two types of metamorphism (amphibolite facies and greenschist facies) were determined. Amphibolite
![]()
Figure 16. Alteration Box Diagrams showing (a) Trend of Na2O wt% against Ishikawa Alteration Index (AI). (b) Trend of K2O wt% against Ishikawa Alteration Index (AI). (c) Relationship between Ishikawa Alteration Index (AI) and CCPI index.
facies stage introduced amphibole + plagioclase + garnet + epidote + quartz + magnetite. The greenschist facies stage introduced minerals such as amphibole + epidote + chlorite + sericite + carbonate + haematite + pyrrhotite. Hydrothermal activity introduced carbonate + quartz + magnetite + pyrite.
The variation of K2O, Na2O, MgO, CaO with SiO2 and Cr, Hf, Nb and Th with SiO2 suggest the rocks are comagmatic and were emplaced in a volcanic island arc setting (Figure 17). However, gabbro-diorite and granodiorite were possibly deposited by mantle fractionates and diorite prior to plate collision (Figure 18).
7. Conclusion
The dykes at Butre area are gabbro-diorite, diorite, granodiorite comagmatically emplaced in a subduction zone into basalt and andesite on a tholeiitic differentiation trend from basalt through to diorite from mantle fractionates with diorite intrusion occurring before plate collision. Relict amphibolite facies metamorphism preceded greenschist facies. Arsenopyrite and pyrite accompanied the emplacement of diorite and granodiorite, respectively; earlier metamorphism introduced magnetite; haematite and pyrrhotite accompanied general metamorphism. The rocks are LREE depleted with La/Sm chondrite normalised ratios from 0.8 in basalt to 3.1 in diorite. There are also commonly negative anomalies of Eu and Ce with basalt, andesite and diorite weakly enriched in HREE with depletion in gabbroic diorite and granodiorite. The trace of gold in the lithologies is not surprising, as mineralisation in the Birimian of Ghana is associated with chloritisation, sericitisation carbonatisation and also composition of arsenopyrite, pyrite and pyrrhotite.
Future work should use microprobe analysis to examine mineralogy and its relationship to the stages of magma crystallisation. Fluid inclusion studies should aim at distinguishing hydrothermal from greenschist metamorphic alterations.