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Obuasi, Ashanti

Ghana

Main commodities: Au
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The Obuasi gold deposit is located 180 km NW of Accra in Ghana, West Africa (#Location: 6° 12' N, 1° 41'W).

It is situated 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 Supergroup has been divided into two facies grouping, which have been variously interpreted as the Lower and Upper Birimian (Junner 1935, 1940), or a coeval sequence in which the volcano-sedimentary assemblage represents a distal facies of a volcanic belt respectively (Leube and Hirdes, 1986; Leube et al., 1990). The volcanosedimentary (Lower Birimian) package comprises volcaniclastics, wackes, feldspathic sandstones, argillites, and some chemical sediments including Mn-rich formations, while the volcanic belts (Upper Birimian) assemblage comprises basalts and some interflow sediments. The Birimian is unconformably overlain by oxidised coarse-clastic sedimentary rocks, including conglomerates which are locally gold-bearing (Banket series), as well as sandstones and minor shales.

The supracrustal sequences were folded and metamorphosed under greenschist facies conditions during the Eburnian tectonothermal event at ~2.1 Ga. Both the Birimian and Tarkwaian rocks were deformed by a single progressive event which involved an initial regionally penetrative low-strain phase producing S1 foliation, followed by the development of high-strain zones (S1), commonly found close to basin/volcanic belt contacts. σ1 was sub-horizontal with a NW-SE orientation, and σ3 was sub-vertical (Eisenlohr and Hirdes 1992; Blenkinsop et al., 1994). The intrusion of two distinct suites of granitoids, the comagmatic 2180 to 2170 Ma Dixcove- or belt-type granitoids in the volcanic belts, and the late kinematic 2116 to 2088 Ma Cape Coast- or basin-type granitoids in the sedimentary basins (U/Pb ages on single zircons; Hirdes et al., 1992).

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. 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.

The quartz vein type ore occurs as single, massive or laminated veins between 0.2 and 5 m thick (locally up to 30 m), or multiple quartz veins with intercalated, sheared and commonly sulphidised wallrocks. Gold is free milling, with lesser amounts of various metal sulphides containing Fe, Zn, Pb and Cu (as galena, bournonite, boulangerite, tetrahedrite, chalcopyrite and sphalerite). Rare pyrite or arsenopyrite are usually bound to fragments of wall rocks incorporated in the veins. The gold particles are generally fine-grained and are occasionally visible to the naked eye. Wall rock alteration fringing the quartz veins includes sulphidation, mainly arsenopyrite, sericitisation and carbonatisation. This ore type is generally non-refractory. Veins are parallel to bedding, to S
1 and to S2, with many folded, some having cuspate-lobate interfaces with adjacent rock, and subvertical boundinage or pinch and swell.

The sulphide ore, which commonly occupies the same shear systems as the quartz veins, comprises disseminated, locally stringer-like, and rare, rather massive concentrations of sulphides in Birimian host rocks. It can occur as variably sized fragments of host rocks in the quartz veins, interlayers of host rocks in multiple quartz vein sets, selvages to veins, and patchy mineralisation as well as ore bodies up to 50 m wide in variably sheared and metasomatised country rocks. It is principally composed of a disseminated assemblage of arsenopyrite, pyrite, pyrrhotite, marcasite, subordinate chalcopyrite and sphalerite, and rare microscopic gold, with the gold locked in the crystal structure of the sulphides within mafic rocks and in schists. The gangue comprises quartz, carbonate, chlorite, sericite and carbonaceous matter. The gold is often locked in arsenopyrite, with higher gold grades tending to be associated with finer grained arsenopyrite crystals. The gold itself within the disseminated sulphide ore is present as micron and sub micron grains at crystal surfaces and boundaries. Sulphide ore is generally refractory.

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.

Historical production to 1985 was in excess of 800 t (25 Moz) of gold at grades of 15 to 30 g/t Au (Dzigbodi-Adjimah 1993).

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.

As at December 2006, the total estimated resources were (AngloGold Ashanti, 2007):
    150.294 Mt @ 6.1% Au for 916 tonnes (29.45 Moz) of contained gold, comprising:
        Measured - 46.976 Mt @ 5.12 g/t Au; Indicated - 73.230 Mt @ 5.63 g/t Au; Inferred - 30.089 Mt @ 8.75 g/t Au.
    These resources are distributed as:
        Open pit - 8.519 Mt @ 2.73 g/t Au; Underground - 104.419 Mt @ 7.93 g/t Au; plus
        Tailings - 36.846 Mt @ 1.72 g/t Au; Stockpiles - 0.51 Mt @ 2.59 g/t Au.
Total ore reserves, included within the resources, at the same date were:
    81.095 Mt @ 3.34% Au for 270 tonnes (8.705 Moz) of contained gold, comprising:
        Proved - 18.235 Mt @ 3.21 g/t Au; Probable - 62.860 Mt @ 3.38 g/t Au.
    These reserves are distributed as:
        Underground - 34.242 Mt @ 5.97 g/t Au; Tailings - 46.853 Mt @ 1.42 g/t Au.

In 2000 the operation had a plant capacity of 10.6 Mtpa and uses CIL-bioxidation. 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 were largely exhausted in 2000 and the underground production rate increased. In 2006, the mine produced 12.03 tonnes (0.387 Moz) of gold from approximately 2.75 Mt of ore with a head grade of 4.39 g/t Au.

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. This mine is owned and operated by AngloGold Ashanti (2007).

The most recent source geological information used to prepare this summary was dated: 2007.     Record last updated: 1/11/2012
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Obuasi, Ashanti

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Anonymous  1999 - Obuasi: in    Register of African Gold 1999/2000    pp 99-100
Appiah H, Norman D I,   1991 - The Geology of the Prestea and Ashanti Goldfields: A Comparative Study: in Ladeira E A (Ed.),  Brazil Gold 91 Balkema, Rotterdam    pp 247-255
Blenkinsop T, Mumm A S, Kumi R, Sangmor S  1994 - Structural Geology of the Ashanti Gold Mine: in    Geologisches Jahrbuch   D100  pp 131-153
Carranza E J M, Owusu E A and Hale M,  2009 - Mapping of prospectivity and estimation of number of undiscovered prospects for lode gold, southwestern Ashanti Belt, Ghana: in    Mineralium Deposita   v.44 pp. 915-938
Cox J A, Amanor J A  1999 - Exploration Strategy and Applications at Ashantis Obuasi Operations: in    Journal of the South African Institute of Mining & Metallurgy   May/June 1999 pp 123-137
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
Foster R P  1999 - Destination Africa: New Frontiers, New Mineral Exploration Opportunities: in   African Mining: Exploration and Investment Opportunities Conference, Perth, Australia: 16-17 November, 1999 AJM, Sydney    pp 1-11
Fougerouse, D., Micklethwaite, S., Ulrich, S., Miller, J., Godel, B., Adams, D.T. and McCuaig, T.C.,  2017 - Evidence for Two Stages of Mineralization in West Africa’s Largest Gold Deposit: Obuasi, Ghana: in    Econ. Geol.   v.112, pp. 3-22.
Harcouet V, Guillou-Frottier L, Bonneville A, Bouchot V and Milesi J-P,  2007 - Geological and thermal conditions before the major Palaeoproterozoic gold-mineralization event at Ashanti, Ghana, as inferred from improved thermal modelling: in    Precambrian Research   v154 pp 71-87
Mucke A, Dzigbodi-Adjimah K  1994 - Ore Textures and Paragenesis of the Prestea and Obuasi Gold Deposits in the Ashanti Belt of Ghana: An Ore Microscopic Study: in    Geologisches Jahrbuch   D100 pp 167-199
Mumin A H, Fleet M E, Longstaffe F J  1996 - Evolution of hydrothermal fluids in the Ashanti Gold Belt, Ghana: stable isotope geochemistry of Carbonates, Graphite, and Quartz: in    Econ. Geol.   v91 pp 135-148
Oberthur T, Weiser T, Amanor J A, 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   v32 pp 2 - 15
Oberthur T, Mumm A S, Vetter U, Simon K, Amanor J  1996 - Gold Mineralization in the Ashanti Belt of Ghana: Genetic Constraints of the Stable Isotope Geochemistry: in    Econ. Geol.   v91 (1996) pp 289-301
Oberthur T, Schmidt Mumm A, Vetter U, Simon K, Amanor J A  1996 - Gold mineralization in the Ashanti Belt of Ghana: genetic constraints of the stable isotope geochemistry: in    Econ. Geol.   v 91 pp 289-301
Oberthur T, Vetter U, Davis D W, Amanor J  1998 - Age Constraints on Gold Mineralization and Paleoproterozoic Crustal Evolution in the Ashanti Belt of Southern Ghana: in    Precambrian Research   v89 (1998) pp 129-143
Oberthur T, Vetter U, Mumm A S, Weiser Th, Amanor J, Gyapong W, Kumi R, Blenkinsop T  1994 - The Ashanti Gold Mine at Obuasi in Ghana: in    Geologisches Jahrbuch   D100 pp 31-127
Oberthur T, Vetter U, Schwartz M O, Weiser Th, Amanor J, Gyapong W  1991 - Gold Mineralisation at the Ashanti Mine Ghana: Preliminary Mineralogical and Geochemical Data: in Ladeira E A (Ed.),  Brazil Gold 91 Balkema, Rotterdam    pp 533-537
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
Schmidt Mumm A, Oberthur T, Vetter U, Blenkinsop T G,  1997 - High CO2 content of fluid inclusions in gold mineralisations in the Ashanti Belt, Ghana: a new category of ore forming fluids? : in    Mineralium Deposita   v32 pp 107-118
Yao Y and Robb L J,  2000 - Gold mineralization in Palaeoproterozoic granitoids at Obuasi, Ashanti region, Ghana: Ore geology, geochemistry and fluid characteristics : in    S. Afr. J. Geol.   v103 pp 255-278


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