Summary:
Tarkwa lies within the Tarkwaian sequence of the Ashanti Belt, a folded and weakly metamorphosed sedimentary package overlying Birimian volcanic rocks. Gold mineralisation is hosted in laterally extensive quartz-pebble conglomerates of the Banket Series, interpreted as fluvial palaeoplacers. These units are gently folded and faulted, with gold concentrated in the matrix of coarse-grained sediments. Unlike orogenic systems, hydrothermal alteration is minor, and mineralisation is associated with detrital and finely disseminated native gold.
The Tarkwa orebody consists of a series of planar meta-sedimentary palaeoplacer deposits (2,132-2,095 Ma old), similar to those mined in the Witwatersrand Basin of South Africa (2,900-2,700 Ma old) with haematite being the dominant heavy mineral at Tarkwa in place of pyrite seen within the Witwatersrand deposits.
The mineral association of the reefs is consistent throughout all the open pits. The predominant gangue minerals are quartzite (83-90 wt%), muscovite (6-12 wt%) and kaolinite (1-3 wt%). The presence of kaolinite is an indication of the degree of superficial weathering of the orebody. Iron oxide, predominantly haematite, is the most common nonsilicate gangue mineral found in most parts of the deposit, varying between 2-3 wt%, along with minor magnetite. There are also trace amounts of todorokite (a hydrous manganese oxide, < 1 wt%) and copper. Sulphur exists in very minor quantities, especially associated with quartz veining at Pepe North and the Pepe West Limb area, most of which is already mined out. The concentrations are too low to have any detrimental effect on gold cyanidation or the environment. The reefs are interbedded with barren quartzite zones. The major gold bearing horizons are:
• AFc – up to 3.0 m thick, only occurs in the west and subcrops against the A1 in the east. Well sorted with rounded clasts of quartzite and visible gold. The AFc sub-outcrops against the A1 package at Akontansi East and is therefore not developed in the Pepe-Teberebie area.
• A1 -between 2-7 m thick, 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 stratified quartzites, visible gold is rare. In the Pepe-Teberebie area, the A2 quartzite unit is too thin to be mined separately, and the A1, A2 and A3 units are mined as a combined A reef package.
• CDE – up to 8 m thick and can be subdivided into the lower C reef and upper E reef, both of which are conglomeratic and are separated by the D reef quartzite.
• F2 – a variably developed polymictic gravel up to 2 m thick, essentially a marker horizon, except in the east where it carries low grades; and G - varies from a 2-6 m thick poorly sorted conglomerate with clasts of quartzite and phyllite.