Africa 2000 - The Gold Deposits of East & West Africa - Deposit Descriptions, West Africa

This MODULE commenced in Accra, Ghana, on the evening of Monday 30 October, 2000 with an Introductory Meeting and Dinner. Module 2 included detailed briefings and visits to the following gold mines and projects, as well as expert workshop/briefings, in the order detailed below:

Damang - Ghana

The Damang gold mine is located some 25 km North-east of the gold mining town of Tarkwa in south-western Ghana, and 200 km to the west of the capital Accra. The ore is hosted by the Lower Proterozoic Tarkwaian clastic fluviatile sediments that overlie the older, metamorphosed Lower Proterozoic greywackes and mafic meta-volcanics of the Birimian Series. Both suites are part of the more extensive Man-Leo Shield which comprises Archaean and Proterozoic rocks that were once part of a larger craton in West Africa and Brazil. The deposit lies on the eastern margin of one of a number of NE trending belts of Birimian volcanics that are un-conformably overlain by synclinal belts of Tarkwaian sediments. Both the Birimian and Tarkwaian have been progressively deformed, culminating in overturning and over-thrusting on both edges of the belt.

Within the district the Tarkwaian sediments are characterised by extensive conformable blankets of mineralised conglomerate ("bankets"), believed to represent metamorphically modified palaeo-placer accumulations (typified by the Tarkwa mine as described below). The Damang deposit is unusual for the Tarkwaian in that it comprises a free milling hydrothermal quartz-vein stockwork. Similarly Birimian lode gold deposits (typified by Obuasi as described below) are not developed to any significant extent in the Tarkwa district, and are geochemically very distinct from the mineralisation in the Tarkwaian rocks.

Most mineralisation at Damang occurs in the core and on the eastern limb of the Damang Anticline, a long attenuated structure, bounded on both sides by strike faults. Gently east dipping to flat lying lenticular quartz veins make up ladder like sets of east dipping mineralised zones. These veins have selvedges of pyrite and pyrrhotite, while disseminated sulphides are found through the hosts in the more heavily veined areas. The hosts are steeply east-dipping quartzite (ex-sandstone) and conglomerate, although in the hangingwall iron rich laminated staurolite-tourmaline meta-sediments and overlying finer grained meta-sandstones are also mineralised. Better grades are found in the vein system where it is developed within the conglomerate units that are the hosts to the stratabound mineralisation (bankets) found elsewhere in the district.

The Damang mine was commissioned in 1997 after an expenditure of USD 135 million. In the 12 month period to June 1999 it produced 8.38 t (0.27 Moz) of gold from 3.73 mt of ore averaging 2.37 g/t Au. The recovery was 95% and cash cost USD 200/oz. In March 1999 the proven+probable reserve was 33.8 mt @ 2 g/t Au at a 1.6 g/t Au cut-off, while the resource was 60 mt @ 2.09 g/t Au at a 1 g/t Au cutoff for 125 t (4.03 Moz) of contained Au. The mine is operated by Abosso Goldfields Ltd which is owned 90% by Ranger Minerals Ltd and 10% by the Government of Ghana.

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Tarkwa - Ghana

The Tarkwa mine is the principal of a number of operations developed within the conglomerates of the Tarkwaian Sequence over the southern Ashanti Belt in south-western Ghana. The mine is some 25 km to the south-west of the Damang deposit described above. In the Tarkwa District the Tarkwaian clastic fluviatile sediments overlie the older Lower Proterozoic meta-mafic volcanics of the Birimian Series. These sediments truncate 1890 to 2061 Ma granites which intrude the Birimian, but are younger than 1650 Ma mafic volcanics. The Tarkwaian Sequence occupies a belt some 250 km long and up to 16 km wide over the Ashanti Belt volcanics. The sequence is generally comprises a basal 250 to 650 m thick suite of greenish-grey feldspathic quartzite, grits, breccias and conglomerates, followed by the Banket Series which is 120 to 600 m thick, composed of sandstones, quartzites, grits, breccias and conglomerates. The Banket Series contains all of the economic gold mineralisation of the Tarkwa District, concentrated in three to four conglomerate bands, each of 1 to 10 m in thickness, concentrated in the lower 40 to 75 m. Each conglomerate is separated by quartzite and grit. 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. The Banket Series is overlain by the 100 to 650 m thick Tarkwa Phyllite and the 1350 m thick Huni Sandstone.

The Banket Series conglomerate units comprise the Sub-basal Reef which is discontinuously developed, up to 40 m thick and only carries low gold values. It is overlain by the main producing unit, the Basal Reef which is 2 to 10 m thick and has been historically mined over widths of 1.2 to 7 m with 3.5 to 14 g/t Au, averaging about 6 g/t Au. The Basal Reef is in turn composed of a number of lesser conglomerate bands. At Tarkwa the highest grade Au is found in the bottom 20 cm of the lower most conglomeratic horizon of the Basal Reef, associated with well sorted, well packed, hematitic conglomerates which are usually less than 2 m thick. It is overlain in turn by the Middle Reef which is 1.5 to 15 m thick with grades generally varying between 1.5 and 2 g/t Au. The uppermost conglomerate band is the Breccia Reef which is 2 to 10, to a maximum of 20 m thick, with low gold values that rarely exceed 1.5 g/t Au.

Gold grade appears to be inversely proportional to the thickness and directly proportional to the hematite content and possibly the diameter of conglomerate pebbles. The intercalated quartzites between the conglomerate bands also contain gold and hematite, although usually <0.5 g/t. The individual payable conglomerates are variable, being present as discontinuous lenses which are of the order of 600 to 1000 m long and 100 to 150 m wide or less. In polished sections of the Tarkwaian ore, gold is present as 10 to 15 µm, generally equi-dimensional grains with irregular outlines. These grains occur as clusters in the vicinity of hematite; lodged within areas containing hematite; in the quartz and sericite matrix of the conglomerate; and near the margins of quartz pebbles. Much of the hematite has been recrystallised as it is euhedral and often porphyroblastic, comprising from 2 to 60% of the matrix to the reef conglomerates.

Historically the Tarkwa District mines have produced around 250 t (8 Moz) of gold since 1880. The Tarkwa mine was taken over by Gold Fields Ltd in 1993 and a new open pit and underground operation brought into production in May 1998. The total resource in the open pit operation is estimated to be 420 t (13.5 Moz) of gold. Production in the twelve months to June 30 1999 was 5 mt of ore averaging 1.3 g/t Au for 6.4 t Au.

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Obuasi - Ghana

The Obuasi gold deposit is located on the western margin of the largest of the ten NNE trending linear belts of Lower Proterozoic greenstones (dated at 2166±66 Ma) found in Ghana and neighbouring West Africa. Each of these belts is predominantly composed of metamorphosed tholeiitic basalts and lesser andesites with intercalated graphitic phyllites belonging to the Birimian System and is separated from the next belt by sequences of black and grey phyllites, schists and meta-greywackes with subordinate volcanics. Individual belts are 15 to 40 km in width. The largest, the Ashanti Belt which hosts the Obuasi deposit is 250 km long and passes under younger Voltaian sediments to the north. The Birimian has been intruded by Eburnian (1.8 Ga) syn- and post-tectonic granitoids. The structure if 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. Both margins of most of the volcanic belts are defined by shears.

The Obuasi deposits are found along the western margin of the Ashanti Belt, at the sheared and overthrust contact between the more competent volcanics to the east and the more ductile sediments to the west. The main rock types in the mine area are siltstones, phyllites, meta-greywackes, schists and meta-volcanics. The major structural trend hosting gold mineralisation at Obuasi extends over a length of 24 km. The most prominent structures are the flat dipping Cote D'or Fissure and the steeply dipping Obuasi Fissure which intersect to form the southerly pitching Main Reef Fissure.

Ore is present as both quartz veins and lenses, and as disseminated sulphide ore formed on the fringes of the shear zones. In the former the gold is free milling, whereas in the second it is locked in sulphides within mafic rocks and in schists. In addition a third ore type, oxidised ore derived from the weathering of the sulphides, is exploited from the surface, while granitoid stockworks have also been recognised. The sulphide ore is principally composed of a disseminated assemblage of arsenopyrite, rutile, pyrite, pyrrhotite, chalcopyrite, stibnite, sphalerite and galena. The gangue comprises quartz, carbonate, chlorite, sericite and carbonaceous matter. Higher grade gold is found in rocks with finer arsenopyrite. The gold itself within the disseminated sulphide ore is present as micron and sub micron grains at crystal surfaces and boundaries. Within the quartz vein type ore, free gold is visible, while minor sulphides are also present. Wall rock alteration fringing the quartz veins includes sulphidation, mainly arsenopyrite, sericitisation and carbonatisation.

As at December 1998, the total estimated resource was 89.4 mt @ 8.1 g/t Au for 725 t (23.4 Moz) of contained gold, including 63.9 mt @ 10.2 g/t Au underground, 19 mt @ 3.2 g/t Au in open pit and 6.5 mt @ 2.5 g/t Au in tailings. This mine is the flagship of the Ashanti Goldfields Limited. The operation has a plant capacity of 10.6 mtpa and uses CIL-bioxidation. Historical production has been in excess of 550 t (17.3 Moz) of gold from 556 mt grading on average 17.6 g/t recovered gold. In 1998 the underground operation produced 2.24 mt of ore at 8.57 g/t Au, while the surface operations yielded 4.32 mt @ 3.22 g/t Au. The open pits will be exhausted in 2000 and the underground production rate increased.

Underground operations extend over a strike length of 8 km and to a depth of 1500 m, served by 12 shafts and 3 declines. The haulage capacity at the end of 1998 was 6.1 mtpa of ore and waste. Several open pits are distributed over the same 8 km strike length.

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Yamfo-Sefwi - Ghana

The Yamfo-Sefwi project is located on the north-west margin of the 2.2 to 2.1 Ga Birimian Yamfo-Sefwi belt of volcanics, dominated by tholeiitic meta-volcanics and meta-sediments, near the sheared contact with the adjacent Sunyani Basin sediments. The latter are mainly volcaniclastic shales, greywackes and siltstones. Eburnian co-magmatic granitoids (the Dixcove Suite) have intruded the volcanics, notably along the contact with the Sunyani Basin sediments. The gold mineralisation has been emplaced close to the hydrothermally altered brittle-ductile shear zones at or near the volcanics-sediment contact. Three deformations are in evidence, namely an early S1 foliation, related to the shears that form many of the lithological contacts in the region; a regional F2 folding and shearing reworking earlier D1. The mineralisation at Yamfo is hosted by shearing of this phase. The third phase is a late brittle episode associated with hydraulic brecciation and minor gold. In the main Centenary Zone there are two mineralised NE trending and SE dipping shears that are mineralised. Gold is also discontinuously developed between the shears. The gold is generally hosted by sheared, altered and brecciated greywacke/siltstone in which the main alteration products are quartz-carbonate and feldspar-chlorite-pyrite. It is usually fine grained, <25 µm, and is associated preferentially with carbonate and quartz-carbonate veinlets and stockworks, together with fine grained pyrite . The mineralised shears average 20 m in thickness but may be up to 70 m wide. Narrow high grade shoots of limited extent are indicated within the main ore zones. These have grades of up to 14 g/t over narrow widths. To the south in the Kenyase Zone, the sediment-volcanic contact is occupied by a dioritic intrusive of the Dixcove Suit. Gold is hosted by the sheared contact between the intrusive and sediments and a mylonite, along with gold bearing quartz vein stockworks in the hangingwall diorite.

Within the Rank area, some 25 km to the SSE, the main contact mineralisation is less continuous, while the hangingwall stockworks are potentially more significant. A second mineralised shear, to the east of and parallel to the main zone is found within a granodiorite, with gold being associated with strong hydrothermal alteration which dip at 70 degrees to the east. Intersections have been indicated over a two km length, ranging from 10 to 130 m (average 30 m) wide, with 2 to 3.9 g/t Au.

The project is currently the property of Normandy Ghana Gold, a wholly owned subsidiary of Normandy Mining Ltd. In mid 1999, the resource estimated for he project area was 64.56 mt @ 3 g/t Au for 190 t (6.16 Moz) of gold using a 0.5 g/t Au cut-off. The project is currently in feasibility, but is as yet un-developed.

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Accra Workshop - "The Setting & Occurrence of Gold in West Africa"

This full day workshop was presented by experienced, Accra based consultant Bob Griffis and included coverage of the following topics:

The Geological and Tectonic Framework of West Africa.
The Distribution, Geological Controls and Characteristics of Gold Mineralisation in West Africa.
Descriptions and details of important deposits not visited and a comparison with those seen.
Exploration for Gold in Ghana, techniques applied, data sets available, particular problems and benefits, legal framework, etc.

Mali Workshop - "The Geology of, & Exploration for, Gold in Mali"

The workshop in Bamako covered the geology of, & exploration for, gold in Mali in general, and at the Sadiola mine in particular, and was presented by Jon Hill of AngloGold, Mali .

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Sadiola - Mali

The Sadiola Hill Gold Mine operated by AngloGold Limited is located in Mali, some 510 km to the north west of the capital Bamako. The deposit falls within the Kenieba-Kedougou window, which comprises an outlier of the early Proterozoic Birimian Series on the north-east margin of the Kenema (or Man) Shield. The Birimian Series is composed of a collage of volcanics to the west, overlain by the younger, dominantly sedimentary domain of the Kofi Formation to the east. These younger sediments are cut by the major Senegalo-Malian shear zone which has a number of associated gold deposits located in its splays.

The gold mineralisation at Sadiola Hill is associated with the near north-south trending Sadiola Fracture Zone, a diorite filled fault zone that separates a greywacke and meta-pelite sequence to the west from an impure marble to the east. The diorite is discontinuously developed within the fault.

Protracted lateritic weathering has produced a deep kaolin rich clay zone, which has decarbonated the marble at and near the surface. Gold has been enhanced in the kaolinitic oxide zone, particularly in the decarbonated marble. This saprolite zone contains the bulk of the resource, and all of the current reserve as a flat lying body which is mostly free digging. It passes down into a soft sulphidic zone below, and then to untested hard sulphide mineralisation.

In the primary zone the gold, with a fineness of 850 to 970, is associated with sulphides which are dominated by arsenopyrite, pyrite and pyrrhotite. The primary mineralisation is accompanied by calc-silicate, potassic and carbonate alteration and by silicification.

Exploration commenced in 1990 when IAMGold, a Canadian junior secured he mineral rights and entered into a joint venture in 1992 with Anglo American, whose interest later passed to AngloGold. A regional geochemical survey in 1987 had rediscovered the deposit when a clear 7000x3000 m anomaly was delineated, centred on Sadiola Hill where artisanal workings in saprolitic oxide ore could be followed for 1200 m along trend. After a capital expenditure of USD 80 million, the first gold was poured in December 1996, with full production being achieved in 1997. By 1999 the throughput had reached 5.2 mt of ore and by September 1999 the operation had produced 40 t (1.28 Moz) of gold. In late 199 the resource totalled 106 mt of ore @ 2.39 g/t Au, using a 1.0 g/t Au cut-off. This corresponds to 254 t (8.2 Moz) of contained gold.

The project is owned by AngloGold (38%), IAMGold (38%), the Mali Government (18%) and the International Finance Corp (6%).

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Morila - Mali

The Morila deposit is located in southern Mali, some 180 km south of the capital Bamako, and 600 km south-east of Sadiola. AngloGold Limited recently purchased 50% of Randgold Resources (Morila) Limited which holds 80% of Societe des Mines de Morila SA. The balance is held by the Mali Government. Construction commenced in late 1999, and the mine is currently in the pre-strip phase and the first gold is scheduled to be poured in late 2000. Production is expected to total around 13 t (0.42 Moz) per year, with a mill throughput of 250 000 t per month, a recovery rate of 90% and cash cost of USD 137 per oz. The total capital expenditure will be USD 85 million.

The mineralisation is present as a quartz-sulphide stockwork and disseminated sulphides hosted in an intrusive contact zone. Up to 60% of the gold is free and can be recovered by gravity. The total resource is estimated to be just over 180 t (5.6 Moz) of contained gold, comprising 1 mt @ 6.7 g/t Au as free digging oxide ore, followed by sulphide ore averaging 4.35 g/t Au

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For more information contact: T M (Mike) Porter, of Porter GeoConsultancy (mike.porter@portergeo.com.au)
This was another of the International Study Tours designed, developed, organised and escorted by T M (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC) in joint venture with the Australian Mineral Foundation (AMF). While the reputation and support of the AMF contributed to the establishment of the tours, after it ceased trading at the end of 2001, PGC has continued to develop, organise and manage the tour series.

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