Download REGIONAL GEOLOGY AND MINERALIZATION

GEOLOGICAL SETTING
REGIONAL GEOLOGY AND MINERALIZATION
TARKWAIAN GROUP
CAPE COAST GRANITE COMPLEX
BIRIMIAN SUPERGROUP
DAHOMEYAN FORMATION
BASIC INTRUSIVE
TOGO SERIES
DIXCOVE GRANITE COMPLEX
The most striking feature of the geology of Ghana is the
parallel disposition of evenly spaced belts of folded
Birimian metalavas. The width of each belt is 40 to 50 km,
the distance between individual belts is approximately 90
km and the belts generally trend northeasterly.
The Birimian System in Ghana is part of the Birimian
supracrustal sequence of West Africa, which extends from
Ghana westwards to Senegal and Mauritania and
northwards into Burkina Faso. The Birimian rocks consist of
interlayered sedimentary and volcanic flow rocks
metamorphosed to low greenschist (a.k.a. greenstone)
facies of Barrovian type. It is subdivided into:
LOWER BIRIMIAN - These are metasedimentary rocks which are divided into volcaniclastic rocks,
turbidite-re-lated wackes, argillitic rocks and chemical sediments.
UPPER BIRIMIAN - These consist mainly of metalavas (metamorphosed basaltic and andesitic
lavas). The Birimian gold deposits and occurrences are set in five parallel volcanic belts several
hundred kilometers long and trending in a north easterly direction. These belts are the Ashanti, Sefwi,
Kibi-Winneba, Bui and Bole-Na Ngodi belts. They consist of tholeiitic lava flows and are separated by
basins containing metasedimentary rocks and granitoids in different proportions. Sediments between
belts consist of volcaniclastic rocks (pyroclastics and epiclastics) and volcanogenic argillites, which are
derived from volcanoes of the belts.
The principal gold producing areas of Ghana are associated with the coeval lower Proterozoic Systems
of the Birimian (approximately 2 billion years ago), meta-volcanic (arc) and metasedimentary (basin)
rocks, along with the younger, unconformably overlying Tarkwaian epiclastic system, developed within
and along the margins of the Kumasi Basin. Erosion of the volcanic arcs during arc/basin development
produced the argillites, turbidite units and tuffs of the Kumasi Basin.
The gold deposits of West Africa largely lie within the Proterozoic domain of the Man Shield, the
southernmost subdivision of the West African (or Guinean) Craton. Lithologies comprising the shield
overlie the largely unknown Archaean Liberian Craton.
The West African Shield represents approximately 45% of the exposed geology of Ghana, being
largely restricted to the northern, western and south-western portions of the country. The shield area
is confined to the southeast by a Proterozoic mobile zone and the central-eastern portion is largely
veneered by late Proterozoic to early Palaeozoic sediments of the Volta Basin.
The Precambrian Birimian System of West Africa can be broadly subdivided into phyllites, tuffs and
greywackes of the Lower Birimian and various basaltic to andesitic lavas and volcanoclastics of the
Upper Birimian. These subdivisions are believed to be coeval and have been deformed and regionally
metamorphosed to grades ranging from lower greenschist to lower amphibolite facies.
The Birimian System has been intruded by two distinctive granitoid types. The larger basin-type
granitoids (and gneisses) are muscovite and/or biotite-rich and are distinctly foliated and deformed,
providing a pre-tectonic appearance. The smaller belt-type (arc related) granitoids are hornblenderich, lack the characteristic foliation of the former and are generally interpreted to be syn or posttectonic. Despite their appearance, the belt-type granitoids are dated as being 60 to 90 million years
older than the larger basin-type granitoids.
The younger Proterozoic Tarkwaian sediments, thought to unconformably overlie the Birimian System,
consist of a thick series of arenaceous and, to a lesser extent, argillaceous sediments believed to be
derived from erosion of the Birimian. Economically important conglomerates and quartzites termed
the Banket Group, comprise the basal portion of the series. The Tarkwaian Series is largely confined
to elongate north northeast trending basins, believed to represent intra-cratonic rifts. The margins of
these basins commonly coincide with major (frequently mineralized) structures representing the
contact between the Upper and Lower Birimian sequences.
Gold mineralization in Ghana (applicable to the majority of the Birimian of West Africa) is found in
three principal settings. The most significant of these deposits are closely related to major structures
at the Upper and Lower Birimian contract. Deposits are of numerous styles, including quartz reefs
hosted within frequently carbonaceous, phyllites and greywackes associated with major semiconformable shear structures and subsidiary oblique faults. Lower grade mineralization may also be
present as disseminations or associated with sheeted quartz veining with tuffs, greywackes and basic
dykes situated in close proximity to major structures.
The second and increasingly significant style of gold mineralization is that associated with sheeted
veins and stockwork zones within granitoids. These deposits are typically lower grade than reef style
mineralization and appear to be confined to the smaller belt-type or Dixcove Suite granitoids and their
regional equivalents.
Banket deposits represent the third significant style of mineralization, hosted by quartz pebble
conglomerate towards the base of the Tarkwaian Series. The gold is thought to be of detrital origin,
derived from erosion of the Birimian Series upon which the Banket Group lie. Epigenetic sheeted or
stockwork quartz veining is, however, being increasingly recognized within lower portions of the
Tarkwaian Series.
Approximately two-thirds of Ghana is dominated by Paleoproterozoic Birimian rocks consisting of five
(5) evenly spaced volcanic belts trending northeast-southwest. The intervening basins between the
volcanic belts are filled by sediments. The remaining one-third is made up of post-Birimian rocks.
The supra-crustal rocks are highly deformed, however, the sedimentary rocks are particularly
characterized by extensive folding.
Tectonic events caused areas to be folded, faulted,
metamorphosed and subjected to igneous activity, sedimentary and erosion processes, resulting in a
series of gold belts.
The lavas are primarily of basaltic composition with the presence of andesitic, dacitic and rhyolitic
rocks. Some pattern of facies distribution is shown by the Birimian sedimentary basins from the
margins towards the basin centers.
The transition zone between the volcanic belts and the sedimentary basins is marked by a chemical
facies, which has of late been found to be the site of much of gold mineralization in Ghana.
Recent studies indicate that the volcanic belts and sedimentary basins are lateral facies equivalents,
both of which are intruded by three types of granitoids differing in age, mineralogy and chemistry;
namely:
h
granitoids in the sedimentary basins (the Cape Coast type) are dominated by two-mica granites;
h
granitoids associated with the volcanic belts (the Dixcove-type) are dominated by hornblendebearing granites; and
h
the late K-rich granitoids (post-Tarkwaian) comprising the Bongo, Tongo and Banso granitoids.
Each of the Birimian volcanic belts contains meta-sedimentary rocks (the Tarkwaian Group) that lie
unconformably over the Birimian. The Tarkwaian consists entirely of conglomerates, sandstones,
phyllites and slates derived from the Birimian country rocks.
The Dahomeyan System occupies the southeastern corner of Ghana and occurs as four alternate belts
of acid and basic gneisses, trending south-southwest to north-northeast direction. A greater part of
the Dahomeyan System forms a monotonous low-lying plain broken by isolated inselbergs and ridges
of mainly ultramafic intrusives and hills forming outliers of Togo rocks. The areas occupied by the
basic gneisses are flat (Accra Plains), while the areas occupied by the acid gneisses give rise to gently
undulating topography.
The Togo Series are made up of meta-sediments and Buem Formation dominated by meta-sediments
and metavolcanics which support a range of hills lying in the eastern corner of the country.
Undeformed, flat-lying Neoproterozoic clastic sedimentary rocks forming the Voltaian System cover
the eastern third of the country.
A strip of Paleo-Cretaceous to Tertiary sediments occurs along the coast. These consist of the
Accraian Series (Accra area), the Sekondian Group (Elmina, Komenda and Sekondi areas), Apollonian
Formation (Tano and Keta areas) and the Amisian Formation (the Saltpond area).