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Tarkwa, Iduapriem

Ghana

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The Tarkwa gold mine (Gold Fields Ghana Limited) is located four kilometres west of the town of Tarkwa, and 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, ~200 km west of Accra and 90 km north of the coastal city of Takoradi (#Location: 5° 18' 27"N, 2° 01' 40"W).

The Iduapriem gold mine (AnglGold Ashanti) is located ~8 km southwest of the town of Tarkwa, and is bordered to the north by Gold Fields Ghana Limited Tarkwa Mine.

Tarkwa

The Gold Fields Ghana Limited Tarkwa operation mines from four open pits: Pepe-Mantraim, Teberebie, Akontansi and Kottraverchy. The mine is some 25 km to the south-west of the Damang deposit in the same district.

The Tarkwaian clastic fluviatile sediments overlie the older Palaeoproterozoic meta-mafic volcanics of the Birimian Series and form part of the West Africa Craton. These sediments truncate 1890 to 2061 Ma granites which intrude the Birimian, and are younger than 1650 Ma mafic volcanics.

Structurally, the Tarkwaian belt has been subjected to moderate folding, with at least five episodes of deformation recognised. The original deposition occurred in a basinal environment with associated low to steep angle normal growth faulting. Subsequent compression and folding led to the development of thrust faults and inversion of previous normal faults. The final stages involved further thrusting in a southwest direction.

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 sequence is as follows, from the base:
Birimian mafic meta-volcanic, volcaniclastic and sedimentary rocks;
angular unconformity
Tarkwaian Group
  Kawere Series, 250 to 700 m thick - quartzites, grits and conglomerates;
  Banket Series, 120 to 160 m thick - quartzites, grits and conglomerates;
  Tarkwa Phyllite, 120 to 400 m thick - chloritic and sericitic phyllites and schists;
  Huni Series, 1370 m thick - quartzites, with minor phyllites.

The Banket Series contains all of the economic gold mineralisation of the Tarkwa District. It can be subdivided into a footwall and hanging wall barren quartzite, separated by a sequence of mineralised conglomerates and pebbly quartzites. The stratigraphy of the individual quartzite units comprises auriferous conglomerate reefs interbedded with barren immature quartzites. The units thicken to the west and current sedimentological parameters indicate a flow from the east and north-east.

The bulk of the mineralisation is 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 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 Fields Ghana Limited (GFGL) 2012, describes sedimentalogical studies that have re-evaluated the formation and geology of these deposits and the recognition of a cycle of events, from initial channel formation and rapid down-cutting of the central channel, through a period of uplift and reworking. Finally, a period of meandering channel bars and flow reduction leading to the development of low-grade conglomerates with silty interbeds. The period of uplift and reworking has been recognised as being the principal episode of gold deposition and concentration within these reefs. The conglomerates developed through this succession have been divided into a series from A to G, where A is the lowest conglomerate band in the Basal Reef. The style of sedimentation in the C, E and G reefs differs from that of the channelised, incised A reefs to a more localised sheet-flood-dominated alluvial fan deposit.

The series of units are as follows, from the base upwards:
AFc - up to 3 m thick, only occurring in the west, where it subcrops against the A1 in the east. It is well sorted with rounded clasts of quartzite and visible gold;
A1 - between 2 and 7 m thick, composed of moderately to poorly sorted conglomerate and thin quartzites with occasional visible gold;
A3 - up to 7 m thick, moderately sorted thin, discontinuous conglomerate lenses within a package of cross-bedded quartzites, with only rare visible gold;
B2 - up to 3 m thick, with very coarse quartzites with thin lags of subrounded pebble conglomerate in the Akontansi Central pit area. Elsewhere, it is poorly developed and not economically extractable;
C, D and E - up to 8 m thick, subdivided into the lower C reef and upper E reef, both of which are conglomeratic and are separated by the D reef quartzite;
F2 - up to 2 m thick, occurring as a variably developed polymictic gravel, essentially a marker horizon, except in the east where it carries low grades;
G - from 2 to 6 m thick, of poorly-sorted conglomerate with clasts of quartzite and phyllite.

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 in 1999 was 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.

Reserve and resource figures quoted by Gold Fields (2007) for their Tarkwa operations as at 31 December 2006 were:
    Measured + indicated + inferred resources - 386.2 Mt @ 1.5 g/t Au, for 584 t of Au,
    Proved + probable reserves - 305.7 Mt @ 1.3 g/t Au, for 390 t of Au.
Ore reserve and mineral resource figures quoted by Gold Fields (2013) for their Tarkwa operations as at 31 December 2012 were:
  Open pit and underground mill ore
    measured resources - 113.0 Mt @ 1.46 g/t Au, for 164 t of Au,
    indicated resources - 142.0 Mt @ 1.24 g/t Au, for 176 t of Au,
    Inferred resources - 30.1 Mt @ 2.92 g/t Au, for 87.9 t of Au,
  Heap leach and retreatment ore
    indicated (South Heap Leach) - 56.0 Mt @ 0.38 g/t Au, for 21.3 t of Au,
    measured (stockpiles) - 4.1 Mt @ 0.74 g/t Au, for 3.01 t of Au,
   TOTAL measured + indicated + inferred resources - 345.2 Mt @ 1.31 g/t Au, for 452 t of Au,
   TOTAL proved + probable reserves - 293.3 Mt @ 1.07 g/t Au, for 313.5 t of Au (included within resources).

Reserve and resource figures quoted by Gold Fields (2015 Annual Report, 2016) for their Tarkwa operations as at 31 December 2015 were:
    Ore reserves - open pits - 144.8 Mt @ 1.25 g/t Au;
              - surface stock-piles - 66.6 Mt @ 0.43 g/t Au;
    Mineral resources - excluding stockpiles (resources are inclusive of reserves)
        measured resource - 70.439 Mt @ 1.47 g/t Au;
        indicated resource - 117.029 @ 1.33 g/t Au;
        inferred resource - 4.752 Mt @ 1.13 g/t Au;
        TOTAL resources - 192.22 Mt @ 1.38 g/t Au;
        TOTAL resources including stockpiles - 258.785 Mt @ 1.13 g/t Au.


Iduapriem

Iduapriem is also located within the Tarkwaian Group of rocks, and hosted in the Proterozoic Banket Series conglomerates as described above. The outcropping Banket Series in the Iduapriem mine lease area forms prominent curved ridges that extend southwards from Tarkwa, westwards through Iduapriem and northwards towards Teberebie. All known gold mineralisation within the Banket Series is associated with the conglomerates and is found within the matrix that binds the pebbles together. The gold content is a function of the size and amount of packing of the quartz pebbles within the conglomeratic units. At Iduapriem, the gold mineralisation is unrelated to metamorphic or hydrothermal alteration events and the gold is coarse grained, particulate and free milling. Mineralogical studies indicate that the grain size of native gold particles ranges between 2 and 500 µm and averages 130 µm. Sulphide mineralisation is present only at trace levels and is not associated with the gold.

Ore reserve and mineral resource figures quoted by AngloGold Ashanti (2012) for their Iduapriem operations as at 31 December 2011 were:
    measured + indicated + inferred resources - 143.5 Mt @ 1.43 g/t Au, for 204.83 t of Au,
    proved + probable reserves - 55.4 Mt @ 1.43 g/t Au, for 79.44 t of Au (included within resources).

The most recent source geological information used to prepare this summary was dated: 2012.     Record last updated: 12/9/2013
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.


Tarkwa, Iduapriem

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Ainoo S  1990 - A Case Study of the Teberebie Project: in   Symposium on Gold Exploration in Tropical Rain Forest Belts of Southern Ghana Minerals Commission, Accra, Ghana    pp 112-119
Hirdes W, Nunoo B  1994 - The Proterozoic Paleoplacers of the Tarkwa Gold Mine, SW Ghana: Sedimentology, Mineralogy and Precise Age Dating of the Main Reef and West Reef, and Bearing of the Investigation on Source Area Aspects: in    Geologisches Jahrbuch   D100 pp 247-311
Klemd R, Hirdes W, Olesch M, Oberthur T  1993 - Fluid Inclusions in Quartz-pebbles of the Gold-bearing Tarkwaian Conglomerates of Ghana as Guides to their Provenance Area: in    Mineralium Deposita   28, (1993) pp 334-343
Netherway D G, Pertzer B A  1990 - Geology and Exploration of the Iduapriem Project: in Barning K (Ed), 1990 Symposium on Gold Exploration in Tropical Rain Forest Belts of Southern Ghana Minerals Commission, Accra, Ghana    pp 130-134
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
Pigois J-P, Groves D I, Fletcher I R, McNaughton N J, Lawrence W S  2003 - Age constraints on Tarkwaian palaeoplacer and lode-gold formation in the Tarkwa-Damang district, SW Ghana: in    Mineralium Deposita   v38 pp 695-714
Wright A J  1997 - The Tarkwa Project: in   World Gold 97, Singapore, 1-3 September, 1997 AusIMM, Melbourne    pp 189-197


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