Birimian Gold Province
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The Birrimian Supergroup hosted gold fields of Ghana lie within the southern half of the West African Craton. This craton is composed of two Archaean nuclei and substantial Lower to Middle Proterozoic volcanics and sediments, fringed by late Upper Proterozoic and Phanerozoic cover. The northern of the two Archaean nuclei is located within the Reguibat Shield, in Morocco, northern Mauritania and western Algeria. The southern, or Liberian Archaean Nucleus, incorporates much of Guinea, Sierra Leone and Liberia, and covers an area of about 150 000 km2. It is composed of granitic gneisses and greenstone belts, and has dated thermal events at 2960 and 2750 Ma (Petters, 1991).
To the east of the Liberian Archaean Nucleus there is a substantial block of Lower and Middle Proterozoic rocks which, together with the nucleus, comprise the Man Shield. This shield is bounded to the south-west and to the east by Pan African (750 to 450 Ma) mobile belts, and is overlain on its western fringes by a flat lying sequence of Upper Proterozoic to lower Palaeozoic Voltaian sediments. To the north, similar Voltaian sediments, succeeded by Palaeozoic and younger cover, separate the Man Shield from the comparable succession in the Reguibat Shield of north Africa (Petters, 1991).
The main gold fields of western Africa lie within the Lower and Middle Proterozoic rocks of the Man Shield, not the Archaean. These Proterozoic rocks comprise the following:
Birimian Sequence, the age of which is constrained by dates of 2156±10 to 2067±12 Ma (Olson, et al., 1992). These rocks fall into two groupings, which despite the nomenclature, are regarded as broadly being time equivalents (Appiah, et al., 1991). These are:
• Lower Series - predominantly volcanic rocks, including andesitic tuffs and tholeiitic basaltic volcanics, with associated basic intrusives, and interbedded graphitic phyllites with 1 to 2% pyrite, volcaniclastics, greywackes and manganiferous units from which manganese ore is produced. The bulk of the volcanics, around 80%, are basaltic. The proportion of pyroclastics and lavas varies between belts, although the highest pyroclastic:lava ratios are in the Ashanti Belt (Leube, et al., 1990; Olson, et al., 1992).
• Upper Series - predominantly sediments, comprising black and grey phyllites, schists and meta-greywackes with subordinate volcanics, all of which have been metamorphosed to greenschist facies (Appiah, et al., 1991; Dzigbodi-Adjimah, 1993). Phyllites and argillites are the most widespread lithology. It has been suggested that the Lower Series may be divided into four lateral facies types outwards from the belts of predominantly volcanic rocks of the Upper Series, to the basin centre. These comprise volcaniclastics, turbidite related wackes, argillitic rocks and chemical sediments. In the transitional zones, adjacent to the belts of Upper Series volcanics, the intercalated finer phyllites and argillites appear to be especially rich in carbonaceous material, and are composed of fine volcaniclastic material. The main source of detritus in the greywackes also appears to have been the Upper Series volcanics, with clastic grains of chert, graphitic schist, quartz and minor lavas, but no Liberian Nucleus granitic fragments (Leube, et al., 1990);
A sub-division of the Birimian Supergroup was originally proposed by Junner (1932, 1935) referring to the sedimentary sequence as the Lower Birimian to the and the volcanic rocks as the Upper Birimian because the Birimian metavolcanic belts were interpreted to be younger than the Birimian
sedimentary basins. However, more recently, radiometric dating indicates the opposite relationship as detailed above. Radiometric dating of 'Belt-type' granitoid rocks that cut the Birimian volcanic rocks limit the age of volcanism to >2186 Ma, whilst detrital zircons in the Birimian sedimentary rocks and the Tarkwaian Group yield U-Pb ages of between 2187 and 2130 Ma, suggesting deposition of both occurred after 2130 Ma. The 'Basin-type' granitoid rocks that intrude the Birimian and Tarkwaian sedimentary rocks yield U-Pb zircon ages of ~2116 to 2088 Ma, implying deposition of both sedimentary packages occurred prior to that date.
The transgressive lode gold deposits of Ghana and Mali are restricted to structurally controlled ore within this series. Gold concentrations within these rocks, as distinct from the lodes, are above crustal averages. Cherts, which generally contain sulphides, average 108 ppb Au, carbonaceous units 22 ppb, phyllites 8.2 ppb and pyroclastics 3.9 ppb. In comparison the Lower Series pyroclastics and greywackes average 2.8 and 1.6 ppb respectively (Appiah, et al., 1991).
These volcanic dominated belts are generally parallel and trend in a north-easterly direction. Individual belts are 15 to 40 km in width and are spaced at approximately 90 km. There are five such belts in Ghana, with a sixth in the far north-west of the country which trends in a north-south direction, as shown on the accompanying plan. The Ashanti Belt is one of the longest, extending for approximately 250 km, although magnetic data indicates extensions of more than 50 km to the north-east below the Voltaian cover (Leube, et al., 1990). Further such belts of mafic volcanics are mapped to the north and west in Burkina Faso, Ivory Coast, Mali, Guinea and Senegal, as shown on the accompanying plan (Olson, et al., 1992). Age dating of volcanics from three belts gave a value of 2166±66 Ma (Appiah, et al., 1991).
In Ghana, the structure of the Birimian is characterised by isoclinal folds with near vertical axial planes; locally developed open symmetric folds in the volcanic belts; axial plane cleavage parallel to bedding throughout the steeply inclined sediments; and by a weak secondary cleavage oblique or perpendicular to the first. Three phases of fold deformation are recognised in Ghana. In the Ashanti Belt high angle reverse faults or upthrusts are found in mines (Petters, 1991).
Eburnian Metamorphics and Intrusives - Much of the Lower to Middle Proterozoic of the Man Shield is occupied by granitoid rocks and associated gneisses. These rocks yield Eburnian age datings between 2081±25 and 1968±49 Ma, within a thermal event that started at 2127±65 Ma. These granitoids and gneisses resulted from the reactivation of Archaean rocks of the Liberian Nucleus and of the Birimian sediments and volcanics during the Africa wide Eburnian event. The granitoids include quartz-diorite, tonalite, trondhjemite, adamellite, granodiorite and granite. The quartz-diorites are largely found cutting the greenstone accumulations, while the granites are developed within the sedimentary basins. The granitoids are divided into two types, namely, i). synorogenic foliated batholiths within the basin centres (the 'Basin-type'), and ii). late orogenic, unfoliated intrusions within the Upper Series volcanic belts (the 'Belt Type'; Leube, et al., 1990).
However, further work has led to the recognition of two discrete orogenic cycles in southwest Ghana: i). an earlier Eburnian I orogeny associated with the eruption of the Birimian metavolcanic rocks, intrusion of Belt type granitoids, and associated metamorphism between ~2.200 and 2.15 Ga. Regional NW-SE extension and development of the Tarkwaian sedimentary basins followed Eburnian, between ~2.15 and 2.116 Ga. The later involved the deformation and metamorphism of Birimian and Tarkwaian rocks and the emplacement of 'Basin-type' felsic intrusions between 2.116 and 2.088 Ga. Clasts of foliated Birimian sedimentary rocks within the Tarkwaian Group rocks suggests that deformation of the Birimian units commenced prior to the deposition of the Tarkwaian rocks (Allibone et al., 2002).
A substantial block of Birimian rocks are developed within the south-western quadrant of Ghana, the eastern half, being covered with the Voltaian sediments. The north-west of Ghana and those parts of the Ivory Coast, Burkina Faso, Mali, Guinea and Senegal not covered by Voltaian sediments or the Liberian Archaean Nucleus are occupied by Eburnian metamorphics and intrusives with narrower belts of Birimian rocks, as illustrated on the accompanying map (Olson, et al., 1992).
Disconformity or Erosional Overlap - possibly, in part, a tectonic contact related to the different deformational properties of the two sequences.
Tarkwaian Sequence, which is up to 2600 m thick in the Tarkwa district. These sediments are present in all of the volcanic belts, being developed in the centres of each, with the exception of the Sefwi Belt (see the attached plan) where they occur on its eastern margin. No Tarkwaian rocks have been found within the main Birimian basins of the Lower Series. The Tarkwaian sediments overlie and truncate granitoids dated at 1890 to 2061 Ma which cut the Birimian (Leube, et al., 1990), but are older than a series of 1650 Ma mafic intrusives (Kesse, 1990). The main Tarkwaian development over the Ashanti Belt is some 250 km long and averages about 16 km in width (Kesse, 1990).
The Tarkwaian Supergroup within the Ashanti Belt comprises the following, from the base:
• Kaware Group, 250 to 630 m thick - greenish-grey feldspathic quartzite, grits, breccias and conglomerates. The conglomerates contain closely packed pebbles principally of silicified Birimian greenstone and hornstone in a matrix of quartz, feldspar, chlorite, carbonate, epidote and magnetite (Vogel, 1987).
• Banket Series, 120 to 600 m thick - all of the sediment hosted gold mineralisation is restricted to this unit. It is composed of sandstones, quartzites, grits, breccias and conglomerates. The upper sections comprise a transitional facies to the overlying Tarkwa Phyllite (Vogel, 1987).
The main feature of the Banket Series is the occurrence of a conglomerate zone in the lower half of the unit. This zone is generally 40 m, but may be up to 75 m thick, and comprises three or four conglomeratic bands. Each of the conglomerates is separated from the next by quartzites and grits. The conglomeratic zone is the host to the gold mineralisation. Otherwise the group is composed mainly of fine to medium grained sericitic quartzites, frequently crossbedded, with small pebble conglomerates, but only rare and thin phyllites. The footwall quartzites are dark grey, feldspathic and argillaceous and do not contain much hematite. The hangingwall quartzites are light pinkish-grey, feldspathic and contain epidote, zoisite, chlorite and carbonate. The quartzites in the conglomerate zone are characterised by hematitic seams and contain sericite, tourmaline, zircon, rutile, garnet and leucoxene, but little feldspar. The quartzites and conglomerates of the conglomerate zone are relatively clean and mature in contrast to the enclosing sediments (Sestini, 1973).
The conglomerates are oligomictic with 90% of the pebbles being (vein) quartz, with the remainder being quartzite and schist. The matrix is principally quartz grains with sericite, hematite and magnetite. Accessories are tourmaline, zircon, rutile, garnet chloritoid, epidote, leucoxene and pyrite. Traces of bornite and chalcopyrite have also been reported (Sestini, 1973).
• Tarkwa Phyllite, 100 to 360 m thick - Green and greenish-grey chloritic and sericitic phyllites and schists which contain porphyroblasts of chloritoid, magnetite and carbonate. The upper sections comprise a transition to the overlying Huni Sandstone (Vogel, 1987).
• Huni Sandstone,1350 m thick - Feldspathic sandstones, grits and quartzites with bands of phyllites. The quartzite contains variable amounts of feldspar, sericite, chlorite, ferriferous carbonate and magnetite.
The structure of the Tarkwaian is different in each of the five belts. In the Ashanti Belt around Tarkwa, the Tarkwaian is folded into an overall symmetric synformal structure and adjacent antiform. The central part of the belt is characterised by open symmetric or gently asymmetric folding. These open folds turn into tight vertical, isoclinal folds only locally. From the centre of the belt towards the east and the west the open folding passes into a band of overturned strata, followed by a zone characterised by reverse faulting with axial planes dipping away from the centre, towards the margin of the belt (ie. beds are overturned and overthrust towards the centre of the belt). This reverse faulting led to repetition of the strata and is followed outwards by a narrow zone of low angle thrusts (Leube, et al., 1990). Within these thrusted margins Birimian rocks have locally been thrust over Tarkwaian sediments (Dzigbodi-Adjimah, 1993).
Younger Intrusives, including:
• Bongo Granitoids, which post date the Tarkwaian.
• Sills and dykes of quartz porphyry and dolerite which cut the Tarkwaian, with larger sills of gabbroic rocks in the Huni Sandstones. These dykes have been dated at 1645 Ma (Kesse, 1990).
Distribution of Mineralisation
Between 1493 and 1600 the Gold Coast (now Ghana) produced 35% of the entire worlds gold production. This dropped to 23% between 1601 and 1700 and to 9% from 1701 to 1800 (Kesse, 1990).
Gold mineralisation within west Africa is largely associated with the Birimian volcanic (greenstone) belts, as either, 1) structurally controlled, transgressive lodes at the transition between the volcanic belts and adjoining sedimentary basins, entirely within Birimian rocks, or
2) as stratabound deposits within the overlying Tarkwaian conglomerates which are developed over the volcanic belts.
The most prolific mines has been in the Ashanti Belt in Ghana, with the main producers being (Dzigbodi-Adjimah, 1993; Vogel, 1987):
• Ashanti/Obuasi - 800 t Au from 1889 to 1992 @ 15 to 30 g/t Au, largely from underground;
• Prestea - 280 t Au from 1885 to 1992 @ 5 to 19 g/t Au, largely from underground;
• Tarkwa Mines - 250 t Au from 1880, at 3.5 to 14 g/t Au, principally from underground;
• Bibiani - 85 t Au from 1900 to 1973 @ 4 to 10 g/t Au;
• Konongo-Obenemase -55 t Au from 1902 to 1979 @ 3 to 5 g/t Au;
• Bogosu - 30 t Au from 1936 to 1955 @ 3 to 5 g/t Au.
In addition however, there has also been production from other countries on the Man Shield, including (Dzigbodi-Adjimah, 1993):
• Mali, where the Kalana mine has historically produced 50 t of Au at estimated grades of 15 to 20 g/t Au, while BHP's Syama Mine is currently producing around 4.7 t of Au per annum at a grade of 3.6 g/t Au from open pit operations.
• Burkina Faso, where the Poura mine produced 25 t of Au between 1900 and 1966 at an estimated grade of 10 to 15 g/t Au.
• Liberia, whose production originates from mines in the Nimba Belt which have output around 25 t of Au in total.
• Sierra Leone, from the Baomahun mine, which produced 15 t of Au between 1930 and 1956 at an estimated grade of 10 to 30 g/t Au.
On a regional scale the vast majority of Birimian gold deposits occur aligned along the flanks of the volcanic belts, although a number, such as those of the Asankrangwa belt are found within the sedimentary basins. Within Ghana the Birimian gold mineralisation is not evenly distributed either in number or size. A high percentage of all of the gold occurrences and almost all of the major gold resources and mines are concentrated along the north-western flank of the Ashanti belt or the south-eastern flank of the Sefwi belt (see the accompanying plan). There is also a close spatial relationship between the distribution of gold and manganese which is developed predominantly on the flanks of the volcanic belts at the transition to the sedimentary facies of the basins. Although no BIF's have been recognised in these corridors, there are chemical sediments characterised by cherts, sulphides, Fe-Ca-Mg carbonates and rocks rich in carbon. The presence of these chemical sediments is utilised as an exploration guide. The individual corridors are up to several hundred kilometres in length, with widths of 10 to 15 km (Leube, et al., 1990).
These corridors also coincide with zones of regionally extensive shearing and faulting represented by sets of parallel, steeply dipping, deeply penetrating and laterally extensive regional fault systems, which are developed at the contact between the meta-volcanic and meta-sedimentary sequences (Dzigbodi-Adjimah, 1993). These faulted corridors may contain up to ten anastomosing shear zones at any one point (Appiah, et al., 1991).
The most recent source geological information used to prepare this summary was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
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Dzigbodi-Adjimah 1993 - Geology and Geochemical Patterns of the Birimian Gold Deposits, Ghana, West Africa: in J. of Geochemical Exploration 47 (1993) pp 305-320|
Leube A, Hirdes W, Mauer R, Kesse G O 1990 - The Early Proterozoic Birimian Supergroup of Ghana and Some Aspects of its Associated Gold Mineralization: in Precambrian Research 46 (1990) pp 139-165|
Oberthur T, Weiser T, Amanor J, Chryssoulis S L 1997 - Mineralogical Siting and Distribution of Gold in Quartz Veins and Sulfide Ores of the Ashanti Mine and Other Deposits in the Ashanti Belt of Ghana: Genetic Implications: in Mineralium Deposita 32 (1997) pp 2-15|
Reisberg, L., Le Mignot, E., Andre-Mayer, A.-S., Miller, J. and Bourassa, Y., 2015 - Re-Os Geochronological Evidence for Multiple Paleo-Proterozoic Gold Mineralizing Events at the Scale of the West African Craton: in Andre-Mayer, A.-S., Cathelineau, M., Muchez, P., Pirard, E. and Sindern, S., (Eds.), 2015 Mineral Resources in a Sustainable World, Proceeding of the 13th Biennial SGA Meeting, 24-27 August 2015, Nancy, France, v.4, pp. 1655-1658.|
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