MINING IN AFRICA SPECIAL — Ghana’s Ashanti belt famous for

The following are excerpts from a paper co-authored and presented by A.H. Mumin at a recent seminar on the geology and mineral deposits of the West African Shield. The seminar was sponsored by the University of Toronto Student Chapter of the Society of Economic Geologists.

The Ashanti gold belt of southwestern Ghana is the principal gold-producing region of West Africa. It is unique for its abundance and variety of gold deposits, historical gold production and early Proterozoic, accretionary tectonic setting.

The gold belt extends for 250 km, from the Atlantic coastline at Axim to the northeast, where it is overlain by mid-Proterozoic, Voltaian sedimentary basin. It consists of volcanic, sedimentary and igneous rocks which intrude and overlie Birimian marine sedimentary rocks.

Historical production of gold dates back at least 1,000 years. From the 14th to the 18th century, the Birimian of West Africa was the world’s premier gold-producing region.

Southwestern Ghana is the most important gold-rich area in the Birimian, but other areas include western Mali/ eastern Senegal, northeastern Guinea, southwestern Mali, south-central Burkina Faso/ northwestern Ghana and southeastern Ivory Coast.

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The Ashanti gold belt is one of about 20 structurally controlled, supracrustal belts in the Birimian province of West Africa. It consists of four major units aligned in a north-south-to-northeast-southwest manner. The four units are marine sedimentary rocks, volcanic greenstones, granitoid intrusives and Tarkwain clastic sedimentary rocks.

Evidence suggests the marine sediments accumulated on an accretionary prism. Isoclinal folding, metamorphism, intrusions and mafic rocks, together with mineralization along structurally controlled belts, transformed the Birimian to its present-day configuration.

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The Ashanti structural belt, situated along the western margin, disrupts the Birimian rocks to a width of 1 km. It is intruded by abundant (mostly mafic) dykes of variable composition ranging from felsic to ultramafic. Individual structures within the belt strike N40E and dip from 60 to 80 to the northwest.

Iron-rich tholeiitic volcanic rocks are abundant in the northern and southern regions, and geochemical investigations indicate the intrusives were most likely feeders for the overlying volcanics.

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Four principal types of gold deposits are being mined along the Ashanti belt: * primary, mesothermal, shear-hosted deposits, including (a) crack-seal quartz vein lodes and (b) disseminated sulphide lodes hosted by either (i) sedimentary rocks or (ii) mafic intrusives and/or volcanic rocks; * Witwatersrand-type paleoplacer deposits;

* near-surface oxidized deposits from either (a) primary mesothermal lode deposit or (b) paleoplacer deposit; and

* modern river and fluvial plain placer deposits.

Quartz vein lode deposits along shears hosted within sedimentary rocks are the most important source of gold in the region. Examples include the Prestea, Konongo and Ashanti mines, where discontinuous lenses measuring more than 15 metres in width are mined. Underground workings extend 8 to 10 km along strike and to a depth of 1.7 km.

The most common sulphide is arsenopyrite; also present, but in lesser amounts, are chalcopyrite, sphalerite, tetrahedrite and galena. At Ashanti, underground grades may exceed 16 grams gold per tonne.

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Estimates of historical production and modern small-scale mining vary widely. However, it is thought that during the past 100 years the Ashanti gold belt produced about 40 million oz.: 2 million oz. from Konongo; 22 million oz. from the Ashanti mine at Obuasi; 1.3 million oz. from Bogosu; 6.2 million oz. from Prestea; 1 million oz. dredged from the Ankobra River; and about 8 million oz. from the Tarkwa area mines.

Estimated reserves for the area are said to be substantial, with 1993-94 figures from Ashanti Goldfields’ mines at Obuasi being 87 million tonnes averaging 7.1 grams.

In 1993, Ghana produced 1.3 million oz. (41.4 million grams). — A.HMINING IN AFRICA SPECIAL — . Mumin is a post-doctorate fellow with the Department of Earth Sciences, University of Western Ontario.