MINING IN AFRICA SPECIAL — New mines spring from old

The nation of Zimbabwe takes its name from the ruins of the ancient empire of Monomotapa which thrived there for 1,200 years before European colonization.

The ancients created a beehive of mining activity with several thousand workings producing 622 kg (20,000 oz.) of gold yearly, or 800 tonnes overall. Today, Zimbabwe produces 40 minerals and is a major supplier of ferrochrome, chrysotile asbestos and lithium minerals, as well as a minor supplier of gold, nickel and tin.

The mineral resources of Zimbabwe remain varied and abundant. The spectacular Great Dyke alone is estimated to contain in excess of 10 billion tonnes of chromite and more than 1.6 billion tonnes of platinum reserves. The celebrated Gold Belts carry 800 gold-mining operations, their deposits offering a potential similar to Western Australia, where production has jumped tenfold during the last decade.

These belts are also the source of nickel, asbestos and iron ore. One of them contains the uncommon Bikita pegmatites which are among the largest deposits of lithium, caesium and beryllium in the world.

Twice the area of Great Britain, Zimbabwe is essentially a vast plateau 3,000 to 6,000 ft. in elevation. It is cut on its borders by two mighty rivers, the Zambezi and the Limpopo, the former harnessed by the 1,200 MW Kariba dam. Energy from hydro and coal (the latter fires the 920-MW Hwange plant) is plentiful, and the country’s infrastructure has been designed to assist mineral development.

An average rainfall of 700 mm (28 inches) creates an airy savanna particularly hospitable to man. Occasionally, an epochal drought, like that of 1992, threatens the hydro schemes and leaves the country short of 25% of its energy requirements and the supply of potable water for 300,000 Zimbabwean miners and their kin. However, such distress can readily be avoided; ample generating capacity lies idle in neighboring South Africa and Mozambique.

Although Zimbabwe’s strength is the multiplicity of small mines winning a wide variety of minerals, only major mines can afford an appropriate water supply system, this being the trend of the country’s mining renewal. The boundaries of Zimbabwe encircle precisely the Archean Rhodesian craton or basin, one of the oldest known fragments of the earth’s crust. The bulk of the mineral wealth of southern Africa has come from this basin and its neighbor to the south, the Kaapvaal craton of South Africa.

The 22 gold-bearing greenstone belts (gold belt schists) scattered over 70,000 square miles occur on either side of the north-south-trending Great Dyke. The volcanic and sedimentary piles that make up the greenstone belts fall into three distinctive groups:

* the oldest sequence of basaltic volcanics and banded iron formations known as the Sebakwian Group;

* the felsic and high-magnesium volcanics, iron formation and clastic sediments of the Bulawayan Group; and

* the mainly clastic sediments, arkose and greywacke, with interbeds of felsic flows, of the Shamvaian Group.

Gold deposits occur in all three systems in all types of rocks; the Bulawayan Group hosts the largest number of deposits.

More than 6,000 old gold “mines” of all sizes are associated with the Archean greenstone rocks, from ultramafic to felsic varieties. For the 19 larger gold producers, the metal occurs in quartz-pyrite veins along fractures or in sulphide replacements of volcanic rocks and brecciated banded iron formations. Examples are the Cam & Motor mine and the Wanderer mine, both now depleted, which produced 4 million oz. and more than 1 million oz., respectively.

Nickel-copper, chrome and chrysotile asbestos are associated with ultramafic to mafic volcanics and plutonic rocks within the greenstone belts. The Renco mine is a fascinating case of the occurrence of gold reefs outside the traditional areas of investigation and not previously recorded in granulitic terranes. (“Granulite” refers to a metamorphic rock composed of even-sized, interlocking, granular minerals).

The blue-grey quartz of the Renco reefs shares characteristics with the remarkable conglomerate pebbles of the Witwatersrand which, added to the high fineness of the gold, suggests a much wider distribution of this type of occurrence.

First the ancients, then the 1939 Renco Syndicate, worked the shallow oxide layer. Assays were up to 23 grams gold per tonne (0.67 oz. per ton), but recoveries reached only 50%. In the 1960s, one of Gold Field’s boreholes returned 338 grams over 107 cm (108 oz. over 3.5 ft.). However, recoveries still only reached 60% with conventional cyanidation.

In the 1970s, RTZ undertook pilot production while outlining sizable tonnages. Lab work determined that up to 65% of the gold was in grains of less than 15 microns and 28% was free gold associated with native bismuth. An appropriate extraction technique allowed melting off bismuth (melting point 271C), thus avoiding the combination of gold and bismuth into maldonite (black gold) and obtaining recoveries of 90%.

Drilling revealed a reef with a mean grade of 9.4 grams over a 126-cm width (3 oz. over 4.1 ft.), but underground bulk sampling showed an ore grade 30% higher.

The 2 million tonnes of ore extracted yearly are treated by the carbon-in pulp method; the smelter uses an induction furnace, recovering copper and silver as secondary products.

The most striking feature of Zimbabwean geology is the 530-km-long, 5.6-km-wide Great Dyke which slashes through the middle of the country. This intrusive into Archean granite consists of rock types exhibiting a distinctive layering. A cross-section of the Dyke reveals stratiform chromite seams, confined to olivine-rich rock types, occurring over its entire length. The seams appear as a succession of saucers or a layered syncline. The upper seams contain chromium in chemical grade ores while the lower seams are of metallurgical grade (chrome being an important raw material in the steel industry). The latter have a chromium-to-iron ratio of 2:8 and higher. The 195-mile stretch of the Dyke known as the Hartley complex, which comprises up to 11 seams, shows a ratio of up to 4:2.

The Great Dyke also contains a pseudo-stratified platinum-bearing horizon akin to the famous Merensky Reef of the Bushveld Igneous Complex in the Transvaal. The platinum boom of 1927 caused a staking rush all along the Dyke, but metallurgical tests revealed the metal to be in such a fine state of subdivision as to render extraction unprofitable.

Sixty-seven years later, BHP Minerals has worked out the metallurgy and has started the construction of its Hartley underground platinum project, slated to become operational in 1996.

It will mine 2.19 million tonnes (2.4 million tons) to produce yearly 150,000 oz. of platinum, 110,000 oz. of palladium and 24,000 oz. of gold, along with rhodium, nickel and copper.

— The author is a geologist and writer in Toronto.