Geology of Ghana

5/24/2018 Geology of Ghana

1/49

Geology of Ghana Compiled by Nana Asamoah

GEOLOGY OF GHANAIntroductionGhana falls mostly within the Pre-Cambrian Guinean shield of West Africa. The main Pre-Cambrian rock units existing in Ghana are the metamorphosed and folded Birimian, Tarkwaian,

DahomeyanSystem, the Togo Series and the Buem Formation.

The Precambrian rocks are overlain by late Proterozoic to Paleozoic rocks of the Voltaian System.Rocks units, which are younger than the Voltaian System and occur at several places along the

coast include the Early or Middle Devonian Accraian series, Mid Devonian-Lower CretaceousSekondian Series, Upper Jurassic to Lower Cretaceous Amisian Formation, Upper

Cretaceous, Apollonian Formation,Tertiary to Recent unconsolidated marine, coastal, lagoonal,fluviatile sediments and deposits. Intruded into the Birimian rocks are large masses of granitoids

known as the Cape Coast and Winneba G1 rock types, Dixcove G2 rock types and smallermasses of granitoids found mainly in the northern part of the country.


2/49


Geological Map of Ghana

The Geological Provinces of Ghana

Ghana is divided into five geological domains or provinces on the basis of age, tectonics andlithologiccharacteristicsof the supra-crustal rocks. These are;

1. The Western Unit, which lies at the eastern margin of the Precambrian Western Shield or

Craton.2. The South Eastern Unit, which is at the southeastern part of the country belonging to the

Precambrian mobile belt.3. The flat lying Central Unitmade up mainly of the sediments of the Voltaian System.

4. The Coastal Basins5. Tertiary to Recent deposits.


3/49


1. THE WESTERN UNIT

Almost 45% of Ghana's area belongs to the shield area. This part consists of the Birimian System

which was deformed, metamorphosed and intruded by syn-and post granitoids during the

Eburnean orogenywhich occurred 1800 million years ago. In elongated basins, which follow thenortheasterly trending Birimian belts, the molasse type sediments of the Tarkwaian were

deposited.

THE BIRIMIAN SUPERGROUP

The rocks of the Birimian system were deposited on an unknown Archean Liberianbasement.They crop out at the north, west and southern parts of the country. The Birimian has been folded,

metamorphosed and in some places assimilated by granitoid bodies.The folding is intense withdips commonly on the order of 30

o-90

o along NE-SW axis (70o-90o being more common than

shallower dips). The metamorphism is considered to be low-grade greenschist facies. Howeverthose of amphibolite faciesand grades of granulite faciesare common in several localities.


4/49


Faulting tends to follow the strike of the folds and trends perpendicular to the latter. Jointing inthese rocks has many orientations, but most commonly is parallel to fold and fault directions and

in a N-S direction. The Birimian rocks have been intruded by granitoids during and latter stages ofthe Eburnean orogeny at or after the end of the Birimian deposition. Thus the Birimian is

considered to be somewhat older than 2000 million years.

Birimian Stratigraphy

The Birimian system in Ghana is subdivided into a Meta-sedimentary and a Meta-volcanic Series.

(i)The Meta-Sedimentary Birimian Series

This makes up 55% of the area occupied by the whole Birimian System. It is predominantly of

pelitic origin consisting of muds and silts with beds of coarser sediments. It is most often

considered to have been derived from Liberian type rocks as found in the nucleus of the WestAfrican Craton. These rocks no longer exist in Ghana and it is possible that they were destroyed oraltered beyond recognition during the Eburnean orogeny.

The series is now represented by great thicknesses of isoclinally folded, steeply dipping,

alternating slates, phyllites, greywacke and argillaceous beds with some tuffs and lavas. Close togranitic intrusives, the slates and phyllites have commonly been altered to quartz-biotite schist

while the impure sandstones have changed to granulitesand quartz schists.There is evidence fora shallow water depositional environment for the Lower Birimian rocks.

Divisions of the The Meta-Sedimentary Birimian Series

These series have been subdivided into 5 stages

Stage Composite Lithology

1. Upper Arenaceous Yellow brown and sometimes purple, massive meta-greywackes andminor thin beds of meta-siltstone.

2. UpperArgillaceous Predominantly yellowish-brown colored rock assemblages of

phyllite, siltstone and their tuffaceous varieties.

3. Mid Arenaceous Meta-greywacke, meta-siltstone phyllite assemblage which ischaracteristically rhythmically bedded in the lower parts and is also

typically tuffaceous and manganiferous in the middle parts.

4. Lower Argillaceous Predominantly black, grey and dark grey phyllite interbedded withgreenish grey and buff-coloured tuffaceous phyllite.

5. Lower Arenaceous Lithic assemblage of meta-greywacke meta-sandstone, meta-


5/49


siltstone, phyllite and tuffaceous varieties of these rock types.

(ii) Meta-volcanic Series.

The Meta-volcanic Series unconformably overlies the lower Birimian series and takes up 20% of

the area occupied by the whole Birimian system. The series consists of great thicknesses ofbasaltic and andesitic lavas, beds of agglomerates, tuff and tuffaceous sediments. Pillow lavas

have been observed frequently in the metavolcanic Birimian.

The basic volcanics and pyroclastics have been altered largely to chloritized and epidotised rocksthat have been loosely grouped together as greenstones. Where the greenstones have been

subjected to dynamo-thermal metamorphism, they have been converted to hornblende schists andamphibolites. Impure arenaceous sediments which have been recrystallised and resemble very fine

grained diorites grade with increasing grain size into diorites.

The meta-volcanic Birimian series is believed to represent a late phase of eugeosynclinaldeposition but it is also possible that some of the acid volcanic sub-series are related to island arc

type volcanic vents in association with relatively shallow water sediments that include meta-conglomerates, quartzites, calcareous chlorite schists and graphitic schists.

Divisions of the Meta-volcanic Birimian Series

Presently the rocks of the Upper Birimian Series have been subdivided into 3 main units onlithologic basis as follows.

Sub Series Composite Lithology

Basic volcanic Makes up the meta-volcanic Birimian and is further divided intonormal greenstones (metabasalt and metadolerite), amphibolite

greenstones spatially related granite intrusive of greenschists andactinolite-chlorite greenschists.

Acid Volcanics Meta-rhyolites, quartz feldspar porphyry, felsites and quartz-

chlorite-schists.

Sedimentary Volcanics Meta-tuffaceous greywacke, quartzites, and schistose conglomerateand grit.


6/49


The Post Birimian (Eburnean) granitoids (2200-1850 my)

Intruded into the Birimian are large masses of granitoids of uncertain ages but probably post-Birimian or pre-Tarkwaian age.

There are 3 main types.

1) The Cape Coast and Winneba (older, G1 granitoids)

2) Small masses of granitoids known as the Dixcove type (G2)3) The rare Bongo potassic granitoids found mainly at the northern part of the country.

These granitoids are related to the later stages of the Eburnean orogeny at or after the end of the

Birimian deposition.

(1) Cape Coast Granites

These granitoids are at times well foliated, often migmatic, potash rich granitoids which take the

form of muscovite-biotite granite, granodiorite, porphyroblastic biotite gneiss, aplites andpegmatites. The granites are characterized by the presence of many enclaves of schists and

gneisses. They are generally associated with Birimian metasediments. The Cape Coast granitecomplex is believed to represent a multiphase intrusion consisting of four separate magmatic

pulses.

Small intrusive bodies related to the Cape Coast granite complexesIn coastal areas eg near Saltpond, over 80 pegmatite bodies occur and are clearly related to the

margin of the batholith from which they radiate for about 12 km. General mineralogicalcomposition includes quartz, muscovite, biotite, microcline, tourmaline, albite, almandine, beryl,

spessartitte and kaolin. At present Kaolin is exploited by Saltpond Ceramics for china-ware.

(ii) Dixcove granite complex

This complex consists of hornblende granite or granodiorite grading locally into quartz diorite and

hornblende diorite. This complex forms non-foliated discordant and semi-discordant bodies in theenclosing country rocks, which are generally Upper Birimian meta volcanics. The Dixcove granite

is intruded along deep seated faults in three distinct phases which follow one another from basic toacid; gabbro-diorite-granodiorite. The Dixcove complex has lower SiO

2 and Al

2O

3 but slightly

higher CaO contents than the Cape Coast granite. Another remarkable feature is its higherNa2O/K2O ratio. Unlike the Cape Coast granite, the Dixcove granite is free of lithophile elements

such as Li, Be, Sn.

(iii) The Bongo Granite

These are porphyritic, hornblende-microcline plutonic granites that are locally found in northerneastern Ghana. They are thought to be younger than the Dixcove granite.


7/49


The age of the granites falls into 2 well-defined groups. The granodiorite massives that intrude theBirimian rocks give ages of about 2100 million years while the most abundant granites (Cape

Coast type) cut both the Birimian and Tarkwaian giving ages of about 1800 my.

MINERALS IN THE BIRIMIAN

In terms of mineral deposits, the Birimian rocks are the most important in Ghana for minerals suchas gold, diamond, bauxite, manganese and iron are all associated with this system.

1. GOLD

The following gold belts and gold districts are associated with the Birimian rocks.a). Prestea Belt

b). Akropong Beltc). Obuasi Belt

d). Obuom and Konongo Beltse). Asankragwa-Manso Nkwanta Belts

f). Tokoase and Bibiani Beltsg). Sefwi and Sunyani district Belts

h). Sekondi, Axim and Tarkwa Beltsi). Cape Coast, Saltpond and Winneba Belts

Occurrence

There is a wide spread of gold in the Birimian and Tarkwaian rock systems of Ghana. The threemain types of auriferous deposits are:

i) The reef, vein or lode-type gold deposits.

ii) The auriferous quartz-pebble conglomerates.iii) Recent alluvial and eluvial deposits all associated with rocks of the two systems (Birimian

and Tarkwa)

(i)

The Reef, vein or lode-type gold deposits

This type of gold is associated with quartz veins or any gold bearing lode or dyke. This type ofgold deposit is associated with rocks of the Birimian System. It occurs in five distinct forms as

follows:


8/49


(a) Auriferous quartz veins or reefs which cut the Birimian SystemThis type is the most important source of gold in Ghana and occurs as intrusive veins in phyllites

and greenstones in the Birimian. The veins occur at two main places.

near or/at the contact of the Lower and Upper Birimian rocks. as reefs within the Birimian (Prestea -Obuasi- Konongo belt)


9/49


The lodes or reefs range from a fraction of a metre to 30 metres or more in width and a few metres

to several metres in length. The lodes consist essentially of quartz with little ankerite{CaCO3(Mg,Fe,Mn)CO3}, pyrite, arsenopyrite, sericite, graphite, galena, pyrrhotite, sphalerite and

gold. The sulphides and the gold often occur in fractures in sheared and shattered quartz. The bulkof the gold is carried in the smoky grey sheared and laminated quartz. Rocks are usually dull black

phyllites which are originally found to be carbonaceous muds. The reefs generally rich in gold arelenses which are long and wide. They are poor where they are short and narrow. The lenses are

often a few 100m apart longitudinally and are up to 1000m or more in depth.

The most payable reefs occur in fractures, in sheared and shattered quartz which readilydisintegrate on weathering and rarely form outcrops. The Barren quartz often makes prominent

outcrops.

b) Veinsand stockworksin granite porphyrieswhich intrude the Birimian

Auriferous quartz veins in Dixcove granite and porphyry and in granodiorite and diorites eg

Dunkwa-Mpasatia Mine near Kumasi.

c) Sulphide ores which have arisen through mineralization of the country rocks in theBirimian

Mineralised country rocks of the Birimian consisting of tuffaceous phyllites within the green

stones series have been found to be an important source of sulphide ores. The tuffs can be up to30m or more in width of range from a few 100m to many 1000's in metres in strike length and

carry disseminated and thin stringers of pyrite and arsenopyrite with small amounts of gold e.g. inPrestea and Obuasi.

Primary lode gold deposits of the Birimian Supergroup along a set of parallel and extensive steep

dipping, deeply penetrating and laterally extensive regional faulting system (shear zones) locally

referred to as fissure zones at the contact between the metavolcanic and metasedimentary rock

sequences. This type of gold mineralisation along shear zones seems to be characteristic features

of many greenstonehosted gold occurrences in the world.

Two types of gold mineralization are recognized. They are

1. Quartz Vein type2. Disseminated Sulphide Mineralization

d) Oxidised oresThese have been concentrated by chemical and mechanical weathering of gold-bearing veins.

These oxidised ores are very close to the surface in the residual above the water-table within anaverage depth of about 30-80m. They are highly lateritic full of iron oxide, clays, weathered mica


10/49


and quartz pebbles. The gold is free milling and very fine, eg. Pepe (Tarkwa), Blackies & Tom

Collins (Obuasi) & Obenemasi quarry.

e) Pegmatitedykes associated with the granitic rocks in the Birimian

Lode gold can be found in pegmatite dykes a few metres or less in thickness. The gold occurs in

the free state or is mixed with pyrites eg Mankwadze - Winneba district.

Distribution of veins & lode type deposits

This type of deposits are found commonly

i) in the vicinity of the boundary between the Upper and Lower Birimian rocks

ii) where Upper Birimian greenstones are intruded by Dixcove granite or porphyry

iii) where Upper Birimian greenstones are in close proximity to manganese deposits

iv) where the Lower Birimian is intruded by Dixcove granites.Generally the older granite (Cape Coast) and the more highly metamorphosed Birimian

rocks do not contain gold deposits.

Areas of smoky and bluish grey mineralised quartz, containing partings, streaks & fragments of

the altered wall rocks are more favourable for gold mineralization than areas where white andglassy quartz are found. Arsenopyrite needles and galena are good indicators of gold but in some

ores, the gold is associated with pyrite or tourmaline also. Gold frequently occurs in fractures insheared and shattered quartz reefs, commonly as elongated flat bodies of irregular shape and

thickness that lie along and within shear zones.

Genesis of vein and lode-type gold deposits

The mineralised reefs are associated with deep-seated overthrust fault channels and shear zones

along the margins of synclines and particularly along the contact between meta-volcanic Birimiangreenstones and metasedimentary Birimian phyllites. The quartz reefs contained in this zone were

unquestionably formed after the initial period of shearing, but shearing continued after the reefswere formed. The gold mineralization came from external sources for, in all the mineralized reefs,

there is a very close association of gold and pyrite or arsenopyrite. It is shown that the gold ispresent as minute specks within the sulphides - the gold and sulphides were formed during the

period of mineralized solutions permeating upwards through the shear zone from a deep seated &probably magmatic source were responsible for mineralization of the shattered zones. Thus the

degree of mineralization depends not only on physicochemical factors such as alkalinity andstrength of the solution, temperature and lithology of the wall rocks, but also on the rate of flow of

the solvent through the zone it was mineralizing. In the relatively open fracture and crushed zones,the rate of flow would have been greater than through zones of sheared, less competent rocks such


11/49


as phyllites and greenstones.

THE TARKWAIAN GROUP

Rocks of the Tarkwaian Group are concentrated mainly at the S. Western part of Ghana in the

Tarkwa area where they outcrop in a NE-SW trending belt. The belt stretches from near Axim tothe edge of the Voltaian basin near Agogo, a distance of about 250km. It has a width of about

16km. Elsewhere, the Tarkwaian occupies a portion of the Bui Syncline parallel to the Ivory Coastborder at 8 N latitude. This other belt running from near Bepoasi in Brong-Ahafo Region to

Banda-Nkwanta in the Northern Region is about 140km and of average width of 0.8km. TheTarkwaian rocks consist of thick series of argillaceous and arenaceous sediments (mainly

arenaceous) in the lower members of the system.

The Tarkwaian Group is considered to be of shallow water continental origin derived from the

Birimian and associated granites. It is believed that the rocks were deposited in elongated intra-cratonic basins bordered by granite-greenstone belts of the Birimian SuperGroup The sedimentswere deposited in high-energy alluvial fans entering a steep-sided basin filled with fresh water.

They consist of coarse, poorly sorted, immature sediments with low roundness, typical of abraided stream environment.

The Tarkwaian is thought to rest unconformablyon the Birimian, though in some places, the met-

asedimentary Birimian and the Tarkwaian are inter-folded due to post Tarkwaian orogenic activity.In some localities no angular unconformity can be observed b/n the Birimian and the Tarkwaian

Systems.

The Tarkwaian sediments have been subjected to low-grade metamorphism i.e. middlegreenschists to middle almandine-amphibolite facies. The higher grades are uncommon and often

associated with intrusive rocks. The common minerals are chlorite, sericite, zoisite, calcite, quartz,limonite and chloritoid. The sediments must be regarded as integral part of the Eburnean oregenic

cycle of which they represent the final molasse stage.

Divisions of the Tarkwaian Group


12/49


Descriptions of the various Tarkwaian Units

a) The Kawere Group consists typically of shallow water greenish grey, feldspathic,carbonate-spotted quartzites, grits, breccias and conglomerates. The most conspicuous of the group

are the conglomerates with inter-bedded grits and quartzites.

The conglomerates normally consist of predominantly of closely packed pebbles of very finegrained, silicified Birimian greenstones in a matrix of quartz, feldspar, chlorite, carbonate, epidote

and magnetite. The quartzites and grits are normally greenish grey in colour and poorly bedded.

b) Banket Series represents a fluviatile series with a thickness varying between 120-600m

being greater south and west of Tarkwa. It is essentially an accumulation of high energy, coarseclastics, represented by conglomerates, grits, quartzites, which have suffered low-grade

metamorphism. Four reefs or conglomerate bands have been identified typical in the western andsouthern parts of the Tarkwa Goldfields. These are

Three of the units are persistent i.e. breccia reef, middle reef and a unit of basal conglomerates.

The Basal or Main reef is the most persistent conglomerate bed in the Tarkwa Goldfields area andis by far the richest in gold. Furthermore it is generally better sorted than the other reefs and more

uniform in thickness, composition, size and in distribution of pebbles. The matrix of theconglomerates consists principally of quartz and sand (mainly hematite with ilmenite, magnetite


13/49


and rutile), minor constituents are sericite, chlorite, tourmaline, garnet, zircon and gold. Epidote

and pyrite are rare except near dykes, faults and quartz veins.

c) Tarkwa Phyllitesare divided into those with and those without chloritoid. The chloritoid

phyllites may or may not contain porphyroblasts of carbonate. The phyllites without chloritoidvary from sandy to fine-grained lustrous types and may contain abundant magnetite and/or

hematite. Colour banding is common and is due to alternating bands of sericite or chlorite.Another type of banding is due to alternating sandy and fine-grained material.


14/49


d) Huni Sandstone and Dompim Phyllites: The sandstone is the weathered representation offeldspathic quartzites, which are in general finer grained than the Banket Series quartzites. They

are grey, greenish or bluish in colour in outcrops and weather to pale grey and green. They oftenshow distinct magnetite banding and may be cross-bedded. Dendritic growth of Manganese oxide

is commonly seen.

Dompim phyllites and Dompim quartzites are now known to form part of Huni Sandstone and thewhole formation is over 1200m in thickness. The Dompim phyllites are separated from the Tarkwa

phyllites by quartzite and sandstone always.

Post-Tarkwaian Intrusions

The Tarkwaian System is associated with hypabassal acidic-basic igneous rocks which make up

approx. 20% of the total thickness of the system. Most of them are in the form of conformable or

slightly transgressive sills and a small percentage occur as dykes. The intrusives originally consistof typical medium to coarse-grained gabbro essentially composed of pyroxene, plagioclase andilmenite.

Gold in the Tarkwaian

Gold in the Tarkwaian occurs as auriferous quartz-pebble conglomerates (see the section on the

occurrence of gold in Ghana)

The gold in the Tarkwaian System is generally believed to be of placer origin. It is not uniformlydistributed through the Banket and very often, has its highest values along the contact. This is in

accordance with the placer origin hypothesis, since the gold would become concentrated in thebottom parts of the pebble and shingle beds over which the river flowed. Other factors such as the

river flow rate in different parts of its course, uneven distribution at bands and sudden floods dueto torrential rain will have played their part in the patchy and uneven occurrences of the values

which are quite characteristic of the Banket deposit. The Banket reefs are essentially low gradeand mining has to be carried out in a very large scale, at a high level of efficiency if an

economically viable operation is to be achieved.

Evidence favours the source of the gold as having been derived from the Birimian schist which

forms the basement of the Tarkwaian System. No clear origin has been defined for the abundanthematite that is intimately associated with the gold. Uplift of folds to the east of the Tarkwaian belt

created a westerly paleo-slope toward depressed tracts developed over synclines. Erosional debrisfrom the positive areas accumulated in the synclinal basins and at one particular interval of time,

the process of sedimentation led to the reworking and winnowing of sand-supported gravels toproduce significant concentration of gold in the matrices of the graves.


15/49


DIAMONDDiamond is the crystalline form of the element carbon developed by nature under critical

conditions of heat and pressure.There are four principal diamondiferous areas in Ghana.

1). The Birim diamond field 2). The Bonsa diamond field3). The Dunkwa/Jimi field 4). The Nyafomang field.


16/49


The Birim diamond field is at present responsible for almost all the diamonds won in Ghana and

are therefore treated here. The Birim diamond field is located in the Birim valley in the AkimAbuakwa and Western Akim traditional areas in the Eastern region. The producing mines are all

situated within 10km of the Birim River and occur along 80km stretch between Kade and theBirim river confluence with the Pra River. More than 98% of all diamonds produced in Ghana

have been won from the alluvial deposits in the Birim Valley and the remaining Birim valleydiamonds have been won from deposits found in rather patchy zones and a stretch from Oda to

beyond the Pra-Birim confluence. Apparently, the deposits extend nowhere more than 3-5km fromthe Birim River.

Geology

The rocks of the diamond field may be classified as follows, in order of age, the oldest being at the

bottom of the table.

Superficial deposits (Pliocene to Recent) Gravels, sands, clays, laterite and soils

Basic intrusives of uncertain age Dolerite, epidiorite other basic sills and dykes

Acid intrusives of post Birimian Older and younger granite, aplite, prophyry,pegmatite and quartz veins.

Lower Birimian Greywacke, phyllites, tuff, schists and basic

hypabyssal and extrusive green rocks

For most of its course, the Birim river flows in a more or less open valley developed in the Lower

Birimian phyllites, schists, tuffs and greywackes. The strike of the rocks in the diamond fieldvaries from N to E and averages a few kilometers before the Birim river joins the Pra river, the

rocks have been recrystallised and the foliations tend to conform with the direction of the contact.Majority of dips are between 70 90

OSE.


17/49


The Birim River in mining terminology is divided into 3 sections.

i) The Upper Birim from the Supong confluence up to the northern limit of the just aboveKade.

ii) The Middle Birim from Aduasa Narrows up to the Supong confluence.iii) The lower Birim from Pra confluence to the Aduasa Narrows.

For the past 60 years, mining of diamonds in Akwatia areas has been concentrated in the UpperBirim area. Three layers can be found in the Upper Birim diamond field:

1) The overburden or superficial deposits

2) The gravel horizon3) The bed rock which consists of metamorphosed sedimentary, pyroclastic and igneous

rocks of Birimian age.


18/49


Origin of Birim diamonds

It has been established that the diamonds are of alluvial origin and their source could be local with

the approximate origin of the diamonds being identified with the greywackes of the greywacke-phyllite series of the upper part of the Lower Birimian rocks. Several types of host rocks have been

identified. The most important ones are the breccias or greywacke with large sedimentaryfragments and a tuffaceous greywacke. Another type of host rock is the highly micaceous

greywacke.

Field and analytical work has shown that rocks which have previously been called ultra-mafic inthe Birim area are actually intensively hydrothermally altered and regionally metamorphosed

kimberlitic rocks that have now become actinolite schists. The major and minor element chemistryfit well with kimberlite that is characterised by high concentrations of rare earth elements (REE)

enriched in the light REE. The rock appears to be a composite of kimberlite and country rock as doother kimberlite intrusions. The name meta-kimberlite is therefore applied (Appiah et al, 1996).

The better diamond values and large sizes tend to occur in the coarse sedimentary bands whichcould be a meter thick and irregular. The main concentration of the diamonds occurred in theshallow tributary valleys and associated inter-fluviatile deposits in the Akwatia area, but many

other tributaries and terraces of the Birim River also contain workable deposits.

MANGANESE

Three types of manganese deposits are identified in Ghana and are all associated with rocks of the

Upper Birimian Series.

I) Manganiferous phyllites of fine schists with subordinate siliceous phyllite Nsuta-Dagwin and Butre River (Western Region), Odumasi (Ashanti Region), Hapa

(Upperwest Region).

ii) Spessartite quartz rocks with or without rhodonite in association with biotite-schists, andamphibolite.

Adansi, Yabio (north of Sekondi) and Nsuta.

iii) As segregated deposits formed by weathering of the first and second types. The Manganeseappears to have been leached from manganiferous phyllite and other rocks deposited at

lower levels most of the first and second types.


19/49


The Nsuta Manganese Deposits

The Nsuta manganese deposits are the most important deposits known in Ghana. Located at Nsuta6.5 km from Tarkwa, they occur on five hills oriented along two lines with bearings of 015

O-020

O.


20/49


They are named hills A, B, C, D and E. They are connected by saddles and some of the hills are

divided into two parts namely, north and south crests, being between 60-90m above thesurroundings.

The Nsuta manganese overlies rocks of Upper Birimian series striking approximately NNE-SSW

and dipping from 60-90

O

to the east. The greenstones and phyllites are cut by a series of faultsboth along and across strike. Overlying the Birimian are rocks of the Tarkwaian System. Large

Cape-Coast and Dixcove granite complex rocks intrude the Birimian. Thus in descending of agethe geological succession in the area is as follows:

Superficial deposits (Tertiary to Recent)

Alluvium, laterite, lateritic clays, Mn ore and soilTarkwaian System (PE)

Huni Sandstone, Tarkwa Phyllite, Banket, KawareUpper Birimian (PE)

Lavas, tuffs, greywacke with minor manganiferous sediment of phyllite.

Intrusives Gabbro, dolerite, epidote, granite, etc.

Detailed stratigraphic divisions of Upper Birimian at Nsuta are

D) Detrital ores

C) Quartz veinsB) Post-Birimian intrusives

A) Upper Birimian series Upper greenstone (460-660m) Upper argillaceous tuff (150m)

Manganese horizon (50-60m) Lower argillaceous tuff (50-90m)

Basal greenstones (460-600m)

The ore lies conformably within the manganiferous zoneand a section through the ore bodies isas follows from the top.

a. A very hard, strongly cemented, reconstituted lateritic cap of MnO2

b. High grade, porous, black orec. Average grade porous ore

d. Weathered beds of phyllite and tuffe. MnCO

3

Types of ore at Nsuta

Three principal Manganese ores are found at Nsuta.

1. Lenticular, bedded ore-bodies which have been folded and faulted together with the


21/49


Birimian rocks in which they are enclosed and have been considerably modified by

lateritization and weathering in Tertiary to Recent times.

2 Detrital ores which consist of rounded and nodular masses of MnO2ranging in size frommere pellets to boulders several metres in diameter embedded in soft, red, lateritic soil or

clay.

3. Carbonates ores (MnCO3)

The manganese ores occur in lenticular bodies up to 300m in length and 30m in thickness which

vary greatly in size and conform to the fold structure of the enclosing beds.

Origin(First and second Directors of the Geological Survey Department Kitson and Junner.)

The rocks are metamorphosed sediments. Before metamorphism, the sediments consisted ofargillaceous matter, free-silica and MnO2in varying proportions with small amounts of lime and

oxides of Fe, Mg, Titanium, P, Na, K. The composition of the original sediments determined thenature of the final rock. Where the sediment is manganiferous clay, spessartite was readily formed.

If all the MnO and Al2O3 were used up in the process, any excess silica would crystallize asquartz. If after the formation of spessartite, there was an excess of MnO and SiO2, MnO2 and

quartz or possibly some rhodonite would form where Al2O3 was in excess of the MnO necessaryfor garnet, kyanite formed. The ores were always associated with and usually formed by

enrichment of metamorphosed manganiferous muds and fine sands typically represented bymanganiferous phyllites in the Upper Birimian greenstones. The Nsuta ores were deposited

contemporaneously with the sediments and were not introduced by circulating waters (from eitherbelow or above). Evidence in support of the contemporaneous deposition of the manganese in the

original mud of the phyllites is afforded by the occurrence of the manganese deposits along nofewer than six distinct lines, parallel though widely separated from each other. Examples of these

are that the deposits have a close relationship to each other and form portions of the same orassociated bed, deposited originally over a wide extent of ocean and they owe their present

position to the folding of the beds.

[Fourth Director of the Geological Survey Department. Mr. Dennis Bates (1951-1962)]

Mr. Bates was of the view that the manganese oxides in the original sediments (which were

metamorphosed argillaceous sediments usually phyllite with interstitial manganese oxides or bothfound in the Birimian rocks) where derived from submarine volcanic emanations and were

precipitated at the same time as the sediments were deposited. He further suggested that thepresence of the enriched manganese ore deposits is directly related to the structural and

geomorphological conditions for at the end of the Paleozoic era, Ghana experienced a long periodof peneplanation and in the course of this erosion, manganese oxide, in beds of manganiferous

sediments, migrated downwards with percolating waters and were re-deposited in the rocks below.As the land surface became richer in MnO, concentration of MnO reached optimum where


22/49


downward percolation of solution was impeded by intrusive body. The hills of manganese ore now

in existence are merely enriched residuals of the manganiferous sediments, which once extendedfar above them.

So from the origin as explained implies that the ores are limited in depth and therefore in quantity,

as they have a definite base below which no oxide was likely to be found.

Composition of the ores

Two types: Oxides and carbonates.

Oxides

Chief minerals are pyrolusite (MnO2) and psilomelane ((Ba.H2O)Mn5O10)but manganiferousgarnet (spessartite) is very widely distributed in the lower grade ores and where weathered is

frequently represented by pseudomorphous (one mineral occurring in crystalline form of another)

manganese oxides. The central parts of the main ore bodies consist of friable, black oxidescontaining a large proportion of voids and occurring as small scale concentric, nodular, stalactiticand botryoidal (mineral occurring as aggregates with rounded surfaces) growths which prove

reconstitution by percolating waters. The ore is usually dull and shows no crystalline form, butwell-developed crystals of pyrolusite are sometimes found on the walls and stalactitic growths in

the cavities (45-53%).

2. Carbonates

The carbonate rocks are mainly rhodochrosite (MnCO3) and everywhere underlie the oxide body.The MnO2has a step dip to the east and there is a straightforward relationship between the CO3

and O2ore-bodies with the manganese content decreasing with depth. The CO3 contain varyingamounts of silica and Alumina. The average manganese present in eight carbonate rock samples

was 34.16%.

DEVELOPMENT PROJECT

The high grade manganese oxides ores which have being mined for about 70 continuous yearsnow has a limited life span, Hence the GNMC in the early 70's embarked on two major projects

which on completion would enable the continuous production of MnO2and ferro-alloys for manyyears. The projects are based on the extensive MnCO3ores (28 million tons) which occur below

the oxides. The projects are(i) the nodulization(ii) Ferromanganese-silicomanganese projects.

The Nodulization Project

The first project consisting of the heating of 470,000 tonnes of MnCO 3 ore in rotating kiln to

produce 300,000 tonnes of marketable MnO2 nodules every year. Basically nodulization entailsheating MnCO3ore and in the process driving off CO2. In the process low grade (42% Mn) MnO


23/49


is produced.

A contract agreement for the construction of the Nodulization Plant at Nsuta was signed on 3

March 1978 by GNMC and Fuller Company of Pennsylvania, USA

IRON

There are three main iron ore deposits of potential industrial or commercial importance in Ghana.These are:

1. Shieni sedimentary iron ore

2. Opon-Mansi lateritic iron ore3. Pudo titaniferous-magnetiferous iron ore

1. Shieni Iron Ore Deposits (Northern Region of Ghana)

The deposits occur almost entirely in the Northern Region about 160 km east of Tamale. The

deposits form a N-S range of hills, which rise about 60m above the surrounding plain stretchingmore than 36 km. The deposits are divided into Northern and Southern groups with a subsidiary

group further south. Accessibility to the area is rather poor but with the development of the VoltaLake, the Oti River draining the area should be navigable to less than 80km from the Shieni

deposits.

Part of the area west of the Shieni Hills is underlain by the Lower Voltaian which comprisesgently folded arenaceous sediments. The Shieni Hills themselves are composed of ferruginous

tillites which are the same or younger than the Lower Voltaian. To the east of the hills, the countryrock is quartzite of the Buem formation.

Analysis shows iron content of the more completely replaced tillite averaging 30-40%. Silica is

present in high to very high. Phosphorous content is higher than is desirable. The ore is haematiteand hydrohematite. Total estimated reserves 1.270 million metric tonnes.

2. Opon Mansi Iron Ore Deposits (Western Region)

The Opon-Mansi iron ore deposits are located on top of the range of fifteen hills which extendover a distance of 24 km from Opon-Valley in the Western Region in the south to Dunkwa,Central Region in the north. The hills on which the iron ores occur have an average height of

400m and the Wuowuo hill, the largest and the highest is 450 m above sea level. The range is in aforest reserve derives its name from its location between two rivers, the Opon and Mansi, both

tributaries of the Pra river. The Wuowuo hill, 1 km wide, 2 km in length, stands in prominence inthe surrounding country.

The deposits overlie folded rocks of the Tarkwaian and Birimian Systems. The Geological

MnCO3 (Heat) MnO2 + CO2


24/49


Succession, with youngest rocks at the top is as represented below:

Superficial deposits Alluvium and soils, including sands, bauxitic and ferruginous

Laterites and ironstones.Tarkwaian System (As treated elsewhere)

Birimian System (As treated elsewhere)Intrusives Dykes and quartz amphibolites)

The lateritic iron cappings forming the Opon-Mansi deposits are restricted to the area underlain by

the Upper Birimian phyllites and quartzites with embedded meta-volcanics mainly tuffs. TheTarkwaian System is missing on Wuowuo hill.

The ore overlies steeply folded, weathered Upper Birimian phyllites which are approximately9.5m in thickness. The ore is divided into the following categories starting from the top:


25/49


Origin of the Ore

The Opon-Mansi iron ore is a product of lateritization, a process typical of tropical weatheringwhereby the upper layers of the weathered mantle become enriched with sesquioxides(ratio 2:3

i.e. Mn2O3) of elements such as iron, aluminium, manganese while silica, lime, magnesia and thealkalis are generally removed in solution. The end product of lateritization, depending on the

parent rock could be Al-rich laterite (Bauxite), Fe-rich laterite such as Opon-Mansi's or Mn-richlaterite such as Nsuta's. The Opon-Mansi ore is an iron-rich laterite. The average chemical

composition of Fe2O3 is 75.14% with the Fe content ranging 34-45%. Thickness of lateritic oreranges 9-27m. About 147 million tonnes of ore indicated area 4 square kilometers with Fe content

52.5%.

Opon-Mansiiron ore for steel production

In 1972, Fried Krupp GmbH, a West German company submitted a proposal to the Government

for the investigation of the possibilities of utilizing the Opon-Mansiore as basis for an iron andsteel project in Ghana. A contract covering the investigation was signed and testing of samplesbegan in January 1975. The assessment was directed towards a project, which was to make Ghana

to a large extent

i. Independent of steel imports and for Ghana to export steel

ii. Another independent objective was to secure locally, as much as possible, the necessaryraw materials for the production of steel and also raw materials for the existing cement

industry, which has been dependent on foreign imports.

The estimated reserves of ore in WuowuoHill are large enough to supply the steelwork for periodof 18 years (18 million tonnes). On the basis of other close by deposits (80-90 million tonnes), the

period could be considerably prolonged.

After the publication of a favourable first assessment report the government established theintegrated iron and steel commission to see to all work on the project on behalf of the government.

Krupp GmbH was entrusted with the execution of a feasibility study of the project. It decided to

use 97-98% local raw materials. After being awarded the contract, Krupp had to work with theCommission, Geological Survey Dept., Survey Dept., Ghana Highways Authority and local

labour. A total of 94 holes were drilled on the Wuowuo Hill in grid pattern 100m, some holesrepeated. The samples collected sent to Krupp Labs in W. Germany.

In June 1979 Krupp presented feasibility report the iron and steel complex was to be located at

Ashiem, 15 km north of Takoradi on the Takoradi-Kumasi railway line. The iron and steel workswere to consist of

(i) Raw material handling plant(ii) Raw material processing plant


26/49


(iii) Electric reduction furnace plant

(iv) Basic O2furnace steel making plant(v) Converter slag plant

(vi) Rolling Mill(vii) Slag factory

(viii) General services

The raw materials were to be 97% locally obtained from places not more than 150 km fromTakoradi. The raw materials are

1) 600,000 tonnes per year of washed iron ore from 1 million tonnes of raw ore from Opon-

Mansi.2) 450,000 tonnes per year of washed limestone from Nauli deposit and conveyed from the

project harbour near Bonyere by the sea to Takoradi to Ashiem.3) 100,000 tonnes of charcoal per annum produced at Benso-Wassaw are conveyed by rail to

the center.

4) 10,000-15,000 tonnes of silica sand per year from tailing from Tarkwa Goldfields5) 15,000 tonnes per year of caustic soda produced at Pehi near Elmina from salt pans andconveyed by road to Asiem.

At Ashiem three electric furnaces shall smelt the washed down iron ore and the polish processingplant will disintegrate and separate the slag from the furnace into their composite materials namely

alumina, clinker, fertilizers. The following products are expected:

Production (Out put) Tonnes Per year

Pig iron 200,000Slag 234,000-305,000

Crude liquid steel 200,700Billets 193,700

Billets for sale to other steel works in Kumasi and Tema 98,000Billets for Ashieni rolling mill 95,000

Rolled finished products 90,000Burnt lime 206,000

Alumina 85,000Cement clinker and course slag 325,000

Charcoal fines 30,000Low grade fertilizer 100,000

Opon-Mansi vrs Shieni iron ore depositsIn terms of future industrial exploitation purposes, the Opon-Mansi deposits have several

advantages over the Shieni haematite ore deposits.

(1) These deposits are favourably situated near the Western Railway line betweenTakoradi and Kumasi and Wuowuo hill is only 6.4 km from the Opon Valley railway


27/49


station.

(2) The iron ore could be smelted without beneficiation.

(3) The Volta hydroelectric power line from Dunkwa to Tarkwa is close to the deposit.

(4) The iron and steel complex at Ashiem will be close to the sources of raw materials,

i.e. Limestone from Nauli, Manganese from Nsuta, charcoal from Benso and sand fromTarkwa Goldfields Ltd.

(5) The nearness of Ashiem (15km north of Takoradi on the Takoradi-Kumasi railway

line) to Takoradi harbour, will facilitate the movement of machinery and finished productsto and from Ashiem.

3 Pudo Titaniferous-Magnetite ore deposits (Upper-West Region of Ghana)These iron ore deposits occur in two distinct zones north and south of Pudo, a village in the Tumedistrict in the northeastern part of the Upper West Region. The main magnetite-bearing zoneoutcrops 1.20 km NW of Pudo and extends for 5.50 km NE.

Geology

The rocks in the area classified as shown below with the oldest below:

(i) Quartz dolerite(ii) Altered hornblende-biotite granodiorite(iii) Hornblende biotite granodiorite and tonalite.

Unfoliated porphyroblasitc biotite granodiorite and adamellite.

Biotite gneiss(iv) Altered noriteThe ore bodies occur entirely within altered norite, which is usually strongly weathered.

Origin

The magnetite in the Pudo area is thought to be the result of a magmatic segregation in a norite or

hypersthene gabbro, which has subsequently, been almost entirely altered to epidiorite. Themagnetite bands must be a result of some type of vertical injection with the injection having taken

place while the norite was still in the plastic state.


28/49


BAUXITE

A material is considered to be bauxite if the percentage of alumina, which can be recoverable in

the refining process, is 32% or more. Some ores with alumina content higher than 32% cannot beclassified as bauxite because Al2O3 is so closely combined with other elements than recovery of

the alumina commercially impracticable

Bauxite can be white, grey, yellow or reddish in colour. It could be clay-like, hard and rock-like in

texture. It can be porous or compact, pisolite (spherical or sub-spherical rock particle which hasgrown by accretion around a nucleus of size 3-6mm) earth looking or homogeneous in structure.Almost any combination of these types can be found in the same deposit.

Chief impurities in bauxite are iron oxide (geothite, Fe2O3.H2O), hematite (Fe2O3), anatase T1O2,

rutile (T1O2) and silicate impurities. The silicate impurities in bauxite are chiefly quartz (SiO2) andkaolinite. Quartz is the most objectionable impurity in aluminium production as it combines with

alumina during reduction.


29/49


The minerals in bauxite that contain aluminium are gibbsite or hydrargillite(Al2O3.3H2O) with65.4% Al2O3and 34.6% H2O and boehmite and diaspore(Al2O3.H2O) with 85% Al2O3and 15%

H2O. Bauxite with a tenor of say 54% Al2O3contains only 28% Al of which, generally, less than25% is recoverable. 4-7 tonnes of bauxite depending on Al content are required to produce 2

tonnes of Al2O3and 2 tonnes of Al2O3to produce 1 tonne of Al.

The Ghana bauxite deposits belong to the so-called blanket deposits and occur as cappings on theflat tops of hills or mountains whose heights are generally greater than 600m above sea level.

These bauxites can also be classified as lateritic silicate bauxite since they have been formed undertropical weathering conditions with their sections built up from top to bottom as follows:

Top soil

Bauxite

Lithomargic clay (rock like)

Occurrences

The main occurrences of bauxite in Ghana are at Sefwi Bekwai (Awaso) {Western Region} Aya-Nyinahin( Ashanti Region), Atewa range near Kibi and at Mt. Ejuanema both in the Eastern

Region.

Sefwi-Bekwai 19 million tonnesAya-Nyinahin 350 million tonnes

Kibi 152-180 million tonnesMt Ejuanema 5.1 million tonnes

Reserves

The total estimated reserves are approximately 19m tonnes average 49% alumina and 3.3% silica.This figure could reach 50m tonnes on one hill alone, it is estimated to have 21m tonnes of which

11m tonnes could be mined.


30/49


Origin

Genetic and mineralogical investigations have revealed that all bauxite deposits of Ghana are as

result of indirect bauxitization processes, for the minerals of the weathering parent rock have notbeen transformed into gibbsite. In all cases kaolinitic lithomargic clay can be recognised.

Normally weathering of rocks starts with brecciation. This is followed by the formation of iron

oxide, which is precipitated around detrital fragments. Then an amorphous Fe-Al-oxide-hydroxide phase is formed. It is this so-called neomineralisation process in this amorphous

material which results in the formation of the predominantly gibbsite or trihydrate bauxite inGhana.


31/49


At present only the deposits at Sefwi-Bekwai are exploited. These deposits are known as theAwaso bauxite deposits. They are located on a dissected range of hills stretching from south of

Bibiani Southwestwards to Sefwi-Bekwai for more than 16 km.

Geology

All the hills are between 300-500m above sea level and have a nearly flat capping of bauxite and

laterite. The capping varies in thickness from 6 to 20m.

The bauxite rests on a layer of kaolin or lithomarge, which separates it from the underlying LowerBirimian phyllites and slates. The lower Birimian strikes at N40 E to N80 E with steep dips to the

NW. In places the slates contain a good deal of pyrite and it is believed that the sulphuric acidproduced in the oxidation of the pyrite assist in the break down of the silicates resulting in

lateritization of the rocks.

Quartz veins and stringers are common and may contain gold and silver.

Reserves

The total reserves are approximately 19 million tonnes averaging 49% alumina and 2.3% silica.

This figure could reach 50 million because the Inchiniso hill alone (in all eight hills) is estimatedto contain a total of 21million of which about 11million tons could actually be mined.

THE MOBILE BELT

The rocks that make up the mobile belt of the eastern and southeastern Ghana consist of theDahomeyan System, the Togo Series and the Buem Formation.

DAHOMEYAN SYSTEM

The Dahomeyan System occupies the southeastern corner of Ghana, roughly that part of a line

drawn north-north east from Accra to intersect the Ghana-Togo boundary near Agome in Togo.The system underlies the Accra Plains and has a total area of approximately 7,000 sqkm.

The system occurs as four alternate belts of acid and basic gneisses and all four belts trend in a

south-south west to north-north east direction from the coastal plains and enter Togo.


32/49



33/49


The great bulk of the System forms a monotonous low-lying plain broken by isolated inselbergs

and ridges of ultrabasic intrusives and hills forming outliers of Togo rocks. The areas occupied bythe basic gneisses are especially flat and areas occupied by acid gneisses tend to give rise to gently

undulating topography.

The Dahomeyan System consists mainly of hornblende and biotite gneisses, migmatites,granulites, schists, some of which are rich in garnet and a little marble. Intruded in the Dahomeyan

are granites, nepheline, syenite and dykes of porphyry, aplite and dolerite. The rocks have sufferedat least two phases of metamorphism. They have been intensely folded with the fold axes striking

SSW-NNE. Dips are generally high and are to the east.

The Dahomeyan System is considered to consist of Birimian or even younger (Mid Proterozoic)formations which have been involved in the Pan African thermo-tectonic event. This event is

responsible for the extensive cataclasis affecting most of the system.

Stratigraphic succession of the Dahomeyan System

Togo outliers Sericite-quartzite schist & sericite schist Thrust contact.

Acid Dahomeyan Occurs in alternate belts. One lies to the immediate east of the

Togo range. Maximum width is 35km near Accra and extends inNNE direction to Kpong where the width is 4km. It continues

further to the northeast into Togo and encloses a series of Togooutliers. The second belt is sandwich between the two basic

gneissic belts and stretches north-northeasterly from east ofPrampram to Togo. Its average width is 27km. In general, rocks of

acid Dahomeyan are composed principally of hornblende, garnet,quartz, feldspar, epitote and mica. The most prominent rock type

in the first belt is the foliated biotite-muscovite-augen gneiss with

porphyroblasts of pale pink feldspar, intercalated with muscovite-biotite schist. The second belt is composed of biotite gneiss,

hornblende-biotite augen gneiss and migmatite.

Alkali gneiss Predominatly feldspathoidal but also includes feldspathic and quartzbearing types which occur as 7-20m thick layers near the contact

with the basic gneiss. It crops out discontinuously in several separatesegements for > 60km near Somanya, Kpong. The rock types are

classified in two distinct units:i) Nephline gneiss crops out in an almost continuous band for

70km.ii) Kpong conglomerate is found intimately associated with the

rocks of the nepheline gneiss consisting of calcite and biotitematrix enclosing mechanically rounded inclusions of albite,


34/49


alkaline gneiss and amphibolite crops out almost in

continuous bands from near Niflo river west ward acrossKpong and beyond the Tema Highway.

Basic Dahomeyan

Basic Intrusives Such as norite, pyroxenite and diorite occur as sills, stocks, dykes

and minor intrusive bodies within and marginal to the metabasicrocks. These intrusives form well defined topographic features -

mainly linear ridges rising nearly 100m high in distinct scarp faceson the western side and gentler slopes to the east. They have N-S

strikes and dip to the east at 15O-30O. Chromite is associated withthe pyroxenites.

Meta-basics This group of rocks are generally well exposed on the inselbergs.

They consist of a thick series of mafic rocks rich in ferro-magnesianminerals consisting of such rock types as garnet-hornblende gneiss

(dominant), garnet-hornblende-pyroxene gneiss with minor biotite

schists and gneiss at the base. The rocks are typically dark in colourand strongly colour banded. The whole group has been subjected tointense dynamo-thermal metamorphism as a result of which rocks

are re-crystallized. The rocks weather into black or dark greycalcareous clay and silt and usually contain white, grey and nodular

carbonate concretions.Age

Some geologists refer to the Dahomeyan as the oldest rocks in the West Africa but no absolute age

determinations have confirmed this. Studies in Togo and Benin give 2 age groups. Firstly, wholerock isochrons on ortho-gneiss yielded Birimian ages of 1700-2050 my.

On the other hand, biotite age determination on gneisses, migmatites and intrusive granites have

yielded a full range of Pan-African ages from 450-580 my. Thus at present there are no clues to theactual age of the Dahomeyan System.

THE TOGO SERIES

The rocks of the Togo Series form a range of mountains and hills trending in northeast from the

Volta River between Kpong and Anum to the Ghana-Togo border near Palime. To the southwestof the Volta, the range continues as far as west of Accra. The Ho-Abutia hills and a few other

small hills to the east of the main range are also composed of rocks of the same age and areoutliers of the Togo rocks. The Togo Series originally consisted of alternating arenaceous and

argillaceous sediments which have now been converted into phyllites, schists and quartzites exceptin a few places where unaltered shales and sandstones are seen. Quartzite, quartz-sericite, sericite-

quartz schist, sericite schists and phyllites are predominant rocks but hornstones, jaspars andhematite quartz schist also occur.

The Togo range is bounded by two major thrust faults; one with the Dahomeyan contact at its

eastern margin and the other at its western contact with the Cape Coast granite complex rocks, theVoltaian and Buem sediments. The thrust fault along the Western flank has been referred to as the


35/49


western boundary fault and that along the eastern margin as the eastern boundary fault. (It is stated

that the contact at the Togo Series and the underlying Dahomeyan was originally sedimentary.About 1.5m thick conglomeratic sandstone has been observed to occur at this contact in several

localities. Apart from this the contact is largely tectonic and it is represented by the Easternboundary fault). The Togo beds have been subjected to intensive directed pressure metamorphism

resulting in intense folding, fracturing and faulting. Isoclinal folding with the axial planes of thefolds inclined to the east-south-east at 30-60. Recumbent folds with dips less than 30. sometimes

occur. The general structure appears to be geosynclinal in the central and western flanks of theTogo range. There is no evidence of intrusives. Metamorphism in the Togo Series ranges from

greenschist to amphibolite facies.

Although the Togo Series is in some places, strongly veined by quartz, particularly in the schistoseargillaceous types, there is scarcity of gold in the gravels of streams that drain the series

suggesting that the quartz veins are generally barren.

Classification of the Togo Rocks

Stratigraphic division of the Togo Series:

Basic group (youngest) Serpentinites occur in association with quartz schist and quartz

chlorite-schists in a few localities at Anum. They also occur alongthrust plane of the Western boundary fault at the base of the

Togo.

Arenaceous group Consists of two members:i) The cataclastic quartzites make up the bulk of the Togo

Seriesii) The micaceous quartzites occur east of the East boundaryfault as outliers of the Togo: Legon Hill, Achimota RailwayCrossing are examples.

Argillaceous group Made up of phyllonites (that is a mylonite in which recrystallization

or growth of new minerals has taken place and a mylonite is a finegrained rock typically found associated with great zones of tectonic

distortion in which the shearing and dislocation process causingcataclasis is prolong and intense with the result that the individual

crystals in the rock becomes fractured, the whole rock becomingmore fine grained), phyllites and chloritic schists. They are the

lowest members of the series.

THE BUEM FORMATION

The Buem Formation crops at the northern and northwestern parts of the Togo Series. It consists

of calcareous, argillaceous, sandy and ferruginous shales, sandstones, arkose, greywacke andconglomerate, with basaltic, andesitic and trachytic lavas, agglomerates, tuffs and jaspars. The


36/49


formation is highly folded along N-S lines and the beds generally dip to the east at 10 -90O

with an

average of 60 -65O

.

Normally, the Buem Formation is unmetamorphosed but in the fault zone along the Buem-Togocontact, it is frequently sheared and schistose. Small masses of basic igneous rocks (dolerite &

gabbro) intrude the formation in the vicinity of the Buem-Togo contact.

Stratigraphic Sucession of the Buem Formation

Unit Max. thickness Description Locations

B7 Upper massive 6000m Massive Ss with horizons SantrokofiSs, ironstone of Jaspar & sed. ironstone hills

Jaspar

B6 Upper. Shales 14000m Red brown shales with Dayi Valley

horizons of Ss

B5 Lower massive 8000m Massive feldspathic Ss Kpando hills

sandstones sometimes conglomeratic Togo plateau

B4 Limestone/Jaspar 3000m Magnesian limestone, jaspar Kpando hillsJaspar & tillite formed from limestones. Thin & Togo plateau

volcanic horizon at Top& Thick Ss at base.

B3 Volcanic group 5000m Basal pillow lavas, a Aveme to Tapa

few rhyolite flows,Agglomerates and tuff.

Some intercalatedsandstonse

B2 Lower Shale 5500m Brown or purple shale & Mostly under

Sandstone lake

B1 Arkose 250m Well rounded clasts, mostly Islands andconglomerate green arkose Ss and peninsula

quartzite, few pebbles of along centrechert, Jaspar, quartz, of lake

phyllite granite porphyryand limestone.


37/49


THE VOLTAIAN SYSTEM

The sediments of the inland Voltaian Basin cover an area of 103600 sq km and forms the third ofthe geological divisions of Ghana. Almost one-third of the area of Ghana is covered by these

horizontal sandstones, shale, mudstones and conglomerates considered to be late Precambrian to

Paleozoic age.The system has a total thickness of between 3000-4000 m and rest unconformably on the lowerProterozoic Birimian System and related granitoids and on the lower to middle Protozoic-

Tarkwaian System. The latter two have been strongly eroded into a peneplain at the time of theVoltaian transgression.

Stratigraphic Divisions

The classification of these sediments have been difficult due largely to

i) Apparent lack of fossils in the sediments as only some decorticated fragments of plants andworm-tracks are observed.

ii) Lateral facies changes and overlaps resulting from movements during deposition.

However, the Voltaian sediments have been subdivided on the basis of lithology and fieldrelationships into Lower, Middle and Upper units as follows:

Voltaian Units Composite Lithology

Upper Voltaian Massive, cross-bedded Ss series in places, beds of

shale & mudstones.

Middle Voltaian Red, greenish grey argillaceous and conglomeratic

series.

Lower Voltaian Basal sandstone Series.

Depositional history and age of the Voltaian

The Voltaian sediments were deposited on the stable West African Craton which represented the

fore land of the Pan-African Dahomeyan orogeny. Epirogenetic uplifts and glacial erosion haveresulted in two unconformities. The environment of deposition points to quite shallow marine one

with marginal lagoonal areas periodically cut off from the sea for long periods.

The thickness of sediments is about 3000-4000 m. The basin gradually deepens towards theeastern margin. The age is quite controversial but the system is said to range from Upper

Proterozoic to Paleozoic (620-1000 million years).

The Voltaian System and the Buem Formation


38/49


Some geologists point out that the Buem formation is synchronous with the Middle Voltaiansediments but the following reasons tend to dispute this

1. The Buem formation is made up of eugeosynclinal sediments consisting of greywackes,

feldspathic sandstone, shale and volcanics (basalts, andesites, agglomerates and tuffs) whereas theVoltaian System is made up of (unmetamorphosed) siltstone, sandstone, shale, limestones,

deposited in an interior cratonic basin.

2. The Buem formation is completely folded whereas the Voltaian System is only gentlyfolded.

3. In the northeast of Ghana, the Buem feldspathic sandstone, mudstones and shales dips at50-90

to the east and are overlain unconformably by thick beds of coarse conglomerates.

4. The average dip of the Voltaian is 5O

whereas that of Buem formation is 45O

.

5. There is a complete lack of fossils in the Buem Formation.

THE COASTAL SEDIMENTARY BASIN

At several places along the coast from Aflao in the extreme SE to Newtown in the extreme SWcorners of Ghana respectively, are coastal sedimentary basins. These include from the extreme east

to the extreme west, the following;

a) Keta Basin b) Accraian Seriesc) Amissian Formation d) Sekondi Series

e) Apollonian Formation (Tano Basin)

1. Keta Basin

The Keta Basin lies at the extreme SE corner of Ghana adjoining Togo. It is the westmostextremity of the coastal basin that extends westward from the Niger delta into eastern Ghana.

The rocks of the basin comprise mainly sands, gravels, siltstones, shales, and clays with layers of

fossiliferous limestone. The rocks near the surface have been found to have a gentle dip of about 2towards the SE. The rocks of the basement are unknown but assumed to be the Dahomeyan,

similar to that cropping out to the north of the basin.

The Keta Basin has on the whole a tectonic block structure bounded by a fault or fault systems onits northern flank. The trend is essentially NE-SE but in the western half of the area becoming

NNE-SSW and at the Southern part E-W.

Three onshore and on offshore oil wells have been drilled in the Keta Basin. Hydrocarbon showswere recorded in two of them.


39/49


Stratigraphy

Unit Lithology & thickness Age

I Beach deposits of sands & gravels (30-60m) Recent

UnconformityII Glauconite, fossiliferous clays (180m) Miocene

UnconformityIII Calcareous clays interbedded with Eocene

fossiliferous limestone (250-700m)IV Bentonic clays, fossilferous (120-240m) Paleocene

V Bluish grey clays, fossilferous,interbedded with limestone (45-60m)

UnconformityVI Brown, reddish brown, grey, fine to medium

grained sandstone (400-550m)

VII Grey, greyish white, coarse to medium Cretaceousgrained sandstone, gravels interbeddedwith mudstones, shales (370m)

VIII Greenish grey poorly sorted sandstone,siltsone, shale (580m)

Dolerite 70mIX Dark grey micaceous shales, siltstone, Devonian

fossiliferous (610m)

2. Accraian Series

The Accraian Series covers an area of about 11.7 sq km in the vicinity of Accra andunconformably overlies the Dahomeyan basement complex. The series consists of quartz-grits,

gentle folded sandstone, shale and mudstones. The total thickness of the series is unknown due tothe extensive faulting in the area which has obscured the relationship of the various rock units.

Fossils found in the series were attributed to Middle Devonian on the resemblance to North-American fauna of that age.

The Accraian is subdivided as follows:

a) Upper Sandstone & shale formation.

b) Middle shale formation.c) Lower sandstone formation with basal grit.

(i) Lower Sandstone formation

This extends from the headland near Osu Fishery in the east to 300m west of Christiansborg

Castle. The rocks are essentially sandstones with subordinate amounts of coarser materials such asgrits, breccias and pebble beds as well as shales. Beds are conspicuously current bedded and some

bedding surfaces are either massive or thinly bedded. The sandstones dip S-SW at 30 and its


40/49


contact with the Dahomeyan is an unconformity.

(ii) Middle Shale formation

The shales have yielded fossils which consist almost exclusively of trilobites and lamellibranch.These are represented by cast and impressions.

(iii) Upper Sandstone-Shale formation

This interbedded formation consists of thin, fine grained quartzitic sandstones alternating withargillaceous shales. The individual beds are never greater than 30 cm in thickness.

Age: from Lower-Middle Devonian

3. Amisian Formation

The Amisian outcrops at a number of places along the coast near the mouth of the Amisa River

which flows into the sea between Saltpond and Winneba. The Amisian consists of a series ofinterbedded, soft pebbly grits, conglomerates, micaceous sandstones, arkose and greenish grey

clay. It has a general dip of 5 to 20 to the NNW. The deposits are poorly sorted and in placesstrongly cross-bedded with the cross-bedding showing that the source of the materials was from

the sea. The sediments are largely derived from weathered granite, Birimian phyllites, gneisses andTogo quartzites and pebbles.

AgeFossil remains of brachiopods and crustaceans as well as scattered plant remains indicate Upper

Jurassic to Lower Cretaceous age.

4. SEKONDI SERIES

The Sekondi Series consists mainly of sandstones, shale with conglomerates, pebble beds, grits &mudstones resting with a major unconformity on a complex of granites, gneisses and schists. The

series occur as several disconnected outcrops along the coast between Cape Coast and the mouthof the Butre River near Dixcove. It extends inland for a distance varying from 3 to 6 km and

covering an area of approximately 200 sq km. The total thickness of the series is about 1200-1350m.

Stratigraphy

Six formations are now recognized and the general occurrence is as below at Sekondi-Takoradi.


41/49


S6Sekondi sandstone Thickness

* Upper-pebby argillaceous and feldspathic sandstoneand conglomerates.

* Lower-massive quartzose and grits with subordinate 305m shale and mudstones

S5Effia Nkwanta Beds* Upper thin bedded siltstone, shale, shaly, Sandstone 25 and some coarse sandstone with

nodules, bands and lenses of chert.* Middle friable sandstone, well bedded and massive, 95

with interbedded mudstone and shale* Lower-cross bedded, soft, fine grained, pale, 90 purple sandstone.

S4Takoradi Shales

* Black and grey carbonaceous shales, sandy shales and shaly sandstone withinterbedded grit and fine 200

grained sandstone with nodules of siderite and

pyrite.

S3Takoradi Sandstones

* Massive and bedded friable ferruginous sandstone 150with coarse grained beds, breccia conglomerates

and interbedded shales.* Thin bedded, brittle, micaceous sandstone, with 30

sandy shale and some clay shale.

S2Elimina Sandstone* Chocolate and purple feldspathic, micaceous 300-370

sandstone with coarse sandstone, conglomerates,shale and mudstone near the base.

S1Adjua Shale

Varied shales, sandy shales, and sandstonecontaining scattered boulders and pebbles with 40-60

a coarse boulder bed at the baseMajor Unconformity

Hornblende granite of Dixcove typeBiotite granite of the Cape Coast type.

S1Adjua Shales

These are the lowest formation of the Sekondi Series and rest directly on underlying crystalline

rocks. They consist of thin bedded, black or grey shales, with arenaceous laminations and beds ofgrit. The formation becomes more sandy at the top. Underlying the typical shales is a basal series

which is up to 4-6m in thickness and consists of boulder beds, conglomerates, shales andsandstones. Scattered pebbles and boulders occur throughout the formation.


42/49


There occurs throughout the formation, coarse, clastic fragments of rocks of all types - granite,diorite, quartz porphyry, quartz, Birimian greenstone and tuff, Tarkwaian quartzite and sandstone.

These boulders vary greatly in size and shape from round to sub-angular and angular. The shalesare varied with the varves compacted under the layer pebbles & boulders. The shales are strongly

ripple marked. No fossils are observed though.

S2Elmina Sandstone

The Elimina Sandstone is a uniform, hard, massive, medium grained sandstone with a

characteristic chocolate or chocolate purple colour due to the pink feldspars and the dark brownlimonitic cement. It is poorly bedded, well jointed and strongly cross-bedded. It tends to be coarser

grained at the base of the formation while it is thin bedded and somewhat shaly at the top.S3Takoradi Sandstone

This forms the lower part of the Takoradi beds. Where fresh, these massive and beddedsandstones are cream coloured and dark, rusty brown. The mineral content is predominantly quartzwith angular grains but well sorted. The sandstone displays perfect subaqueous cross-bedding. The

base is fossiliferous yielding poorly preserved brachiopods, lamellibranchs and fish remains.

S4Takoradi Shales

Takoradi shales form the upper part of the Takoradi beds. These shales are hard, compact, black or

very dark grey, fissible or sandy shales rich in carbonaceous matter. The shales contain thin bandsof brittle bitumen and traces of oil. (A slight oil smell can sometimes be detected if the shales are

heated or struck with a hammer) They have been found to contain unusual sulphate minerals suchas jarosite and halotrichite. They also contain discoidal nodules of compact finely granular, grey

siderite or clay ironstone. Calcite and gypsum occur as vienlets traversing the shale and sideritenodules and along the bedding planes. The shales are fossiliferous yielding lamellibranchs,

brachiopods, gastropods and fish remains.

S5Effia Nkwanta Beds

These rocks are a variety of rock types and are characterised by bright colours. They are dividedinto lower, middle & upper beds.

Lower Effia-Nkwanta beds are fine grained, soft, cros-bedded and ripple marked

sandstone with a characteristic floury texture on weathered surfaces.Middle Efia-Nkwanta beds are friable, quartzitose sandstone, well bedded, massive with

interbedded mudstone and shale. They are generally brighter pink or orange.Upper Effia-Nkwanta beds consist of well bedded, purple, pink, grey and green shales

and siltstone with some mudstone, fine grained sandstone and a few coarse grained beds.Characteristic feature of the beds is the presence of grey and white chert which occurs in


43/49


thin bands or as rounded nodules. In either case the minerals are distributed along bedding

planes.

S6Sekondi Sandstones

are divided into Upper and Lower beds.

Lower sandstone is thick, massive, cross-bedded, orange pink. It contains abundant

scattered fragments of chert, frequently angular flakes irregularly oriented. Upper sandstone is soft, argillaceous, feldspathic with pebbles: generally of a chocolate

or pink colour. The pebbles are generally well rounded consisting of white quartz, lesseramounts of greenstone, green quartzite, phyllite and chert. The rocks are poorly graded and

are sufficiently resistant to weathering.

THE APPOLLONIAN FORMATION

These are Cretaceous-Eocene marine sedimentary rocks which occur in the eastern portion of a

crescent-shaped basin along the Atlantic coast that occupies southwestern corner of Ghana and alarger portion of southeastern Ivory Coast. The Ghana portion also known as the Tano Basin

covers 1,165sqkm between the mouths of the Ankobra River in the east and the Tano River in thewest (96km).

The rocks consist of alternating sands, clays and limestones. In depth, the sands and clays are more

compact and pass into sandstone and shale. Nodules of pyrite or marcasite are common in theclays and shales.

The only prominent stratigraphic marker of the area is a series of thin, highly fossiliferous

limestone. Boreholes drilled show the sediments dipping to the south or southwest at 1-3 and oilshows have been recorded from this area.

Stratigraphy

Unit No. Lithology & thickness AgeI Beach deposits of loose sands with Recent

intercalation of clays and shaly clays.(Thickness 100-215m)

UnconformityII Fossiliferous limestone with interbedded

black shales (45-120m)Unconformity

III Sandstone with minor shales and limestones(610-915)

IV Main conglomerates (23-76m) CretaceousUnconformity


44/49


V Grey-green, fine-medium grained sandstone with

minor shales (300-325m)VI Black carbonaceous shales (100-450m)

UnconformityVII Siltstone with numerous pebbles & cobbles of

igneous & metamorphic rocks (225m)VIII Greyish to greenish sandstone and shale (1200m)

PETROLEUM POTENTIAL OF GHANA

Geological conditions necessary for Hydrocarbon accumulation are:

a) The existence of favourable conditions for the development of hydrocarbons. These

include parent rocks in a sedimentary basin of a marine environment in which organic siltand clayey substances have been deposited. The marine environment assists in the

transformation of these deposits into hydrocarbons through a series of chemical and

physical processes.

b) The existence of some impermeable rocks such as clay, marl or compact limestone

overlying the accumulation to act as a cap rock and preserve the hydrocarbons.

c) The existence of some favourable structural, tectonic or stratigraphic conditions to trap thehydrocarbons and there also must be reservoir rocks to give the oil opportunity to

accumulate. (It is also necessary to have accumulations in sufficient quantities and undersuitable conditions with respect to pressure and depth to ensure the profitability of their

exploitation)

Main areas of exploration in Ghana

The source rocks for petroleum and natural gas are mainly sedimentary rocks of marine origin.

Nearly one-half of Ghana's total area, about 135,000sqkm is covered by sedimentary rocks whichare found mainly in four different parts of the country and the search of oil is concentrated in these

areas of the country:

i) Tano Basin 1165 km2ii) Keta Basin 2200 km

2

iii) Volta Basin 103600 km2

iv) Continental Shelf 27560 km2

1) Tano Basin

Despite the uncertainty of finding oil in commercial quantities in Ghana, it was among the firstcountries in Africa to attract the attention of oil companies with early efforts concentrated in the

Tano because oil seeps and saturated superficial sands were noted in the Ahanta-Nzima area in the19th Century.


45/49


Geology See the Apollonian Formation.Prospecting

The occurrence of oil and gas in sands in the Tano near Bonyere and Takinta has been known and

between 1896 and 1925 at least twelve boreholes were drilled in the Appollonian to locate oilpools and one produced five barrels of oil per day (Drilled by the West African Oil and fuel

Company in 1896-1897). Drilling by the Societe Francaise de Petrole 1909-1913 produced sevenbarrels of oil per day in one of their five wells drilled. African and Eastern Trade Company 1923-

1925 also drilled with oil shoes.

The Gulf Oil Company (1956-1957) drilled four wells at various places in the Tano and some noncommercial oil shows were observed in all of them. In 1962, Romanian experts in oil explored the

area using gravity survey to estimate the thickness of sediments and map their topography. Amajor fault suitable for trapping oil was found Kangan and Ahonjuri fault. The fault trends NW-

SE with the upthrown side or northern part not as thick as the downthrown side where the

thickness increased towards Ivory Coast. The following conclusions can be drawn with regards tothe oil and gas potential of the Tano Basin.

a) The maximum thickness of the sedimentary rocks in this basin is greater than 3000m alongthe coast increasing to Ivory Coast.

b) The oldest rocks encountered in the survey were mid cretaceous age.c) Rocks of marine origin were found to a maximum depth of 1770m and were separated

from non-marine rocks by an angular discordance.d) Two horizons with indications of oil are known; one near the surface Nauli limestone

horizon and the other at greater depth the black shale horizon.e) The most promising area for oil accumulation lies immediately south of the major fault

indicated by the gravity survey.

2 THE KETA BASIN

It is thought that this basin forms part of the Nigerian sedimentary basin. Gravity survey and two

wells drilled by Romanian experts showed that;

a) the sedimentary rocks in this area are at least 2133m thick.

b) that the rocks are marine and non-marine, rich in organic matter and therefore a potentialsource of hydrocarbons.

c) that the sand, sandstones and limestones present in the sediments are potential reservoir

rocks.

d) that there is a general thickening of marine sediments in a down dip direction towards thesea and this trend may be expected to continue offshore. Of late Texas Pacific Ghana

Incorporated has acquired concessions and had completed seismic survey prior to drilling.


46/49


3 VOLTAIAN BASIN

This is an expansive sedimentary basin covering an area of 103600sqkm2and drained by the Volta

river and its tributaries. The basin is underlain by the rocks of the Precambrian to Lower Paleozoicepi-continental Voltaian system. They comprise a thick sequence of marine and continental

sediments.

Refer to the geology of the Voltaian System.

Prospecting

A Soviet Geological Survey team in 1961-66 carried out a hydrological survey to study

groundwater conditions for the basin. The significant facts revealed by four boreholes are that theLower members of the basin have been folded and dips of up to 40 were recorded. It was also

known that the Voltaian contains limestone and sandstone which could serve as cap rocks for the

accumulation of hydrocarbons. These indications brought in a Romanian oil prospecting team toevaluate the petroleum prospects of the Basin. Rapid gravity and magnetic surveys to revealapproximate depths of the basement and the configuration of the buried structures in the basin

were conducted. Shell Oil Company (Ghana) Ltd also carried out airborne magnetic survey. Afterthese oil companies showed interest and the Government divided the area into 39 blocks for the

companies to prospect. Shell Exploration and Production was granted licence for five yearscovering 10 blocks in the southern part of the basin. No trace of hydrocarbon was found in this

section. But traces of bitumen were encountered in the 1962-65 drilling programme of the SovietGeological Survey team and this has given hope that the basin might prove to be an oil province.

The bitumen is usually found in dolomite intercalated with Mid-Voltaian shale/sandstonesequence. Further examination of the occurrence of bitumen in the Voltaian showed that they were

not distributed in the natural bedding but always in fracture (mostly transverse to bedding).Therefore the opinion is that the bitumen had seeped downwards from a younger overlying

formation which has since been eroded.

Oil Potential in the Voltaian

The basin is an old one and no significant structural closures exist. There are no source rocks in

juxtaposition to allow possible inward normal hydrocarbon migration. Accumulation of oil incommercial quantities was considered unlikely although naturally not impossible, since at present

only one deep oil well has been drilled.

1. Tano Basin 2. Keta Basin 3. Voltaian Basin 4. Continental helf


47/49


4 The Continental Shelf

In 1968, the Government invited private foreign oil companies to undertake oil prospecting work,

most of them were interested in the Continental Shelf and this was divided into 22 concessionblocks which was shared among seven companies (May-Volta Petroleum, Mobil Oil, Texas Gas

Exploration, Texaco Oil, Union Carbide, Signal Oil and others).

In June 1970 Signal Oil Company discovered oil in a well about 14.4km south of Saltpond (block10) drilled at 2967m. Two producing horizons were located; one presumably in the Cretaceous and

the second zone at 2590.9m in the Devonian. Signal estimated 7!million barrels.

Agri-Petco International Inc. (Tuba Oklahoma) was granted prospecting licence in 1976 coveringblocks 10 and 13 and drilled three holes which were encouraging and therefore developed the

field. Results showed that daily production was to be 5000-6000 barrels a day. Between 1978-1983, 3.1m barrels were produced due to difficulties (reservoir consists of seven thin pay sands

with several different oil/water and gas/oil interfaces).

In 1977, Ivory Coast announced that she has struck oil 45km east of Abidjan. The significance of

the find is related to the basinal structure of Ivory Coast, for Ivory Coast is framed by three faultswhich co

Geology of Ghana

Documents

Transcript of Geology of Ghana