ZIMBABWE MAIN ECONOMIC MINERALS
Gold mining and exploration in Zimbabwe has been going on from ancient times and it is estimated that a third (about 700 tonnes) of all historical gold production was mined locally from the seventh century until the introduction of mechanized mining methods with the arrival of Europeans about a century ago. There are over 4000 recorded god deposits, nearly all of them located on ancient workings.
The occurrence of gold in Zimbabwe is mainly confined to hydrothermal vein and shear zone deposits found exclusively in rocks of the Basement Complex age formed 2400 million years or more ago. The only noteworthy production from outside the Zimbabwe craton has been Renco Mine, which is in ancient deformed, high-grade metamorphic rocks of the Archean age North Marginal Zone of the Limpopo Mobile Belt and a number of mines in the Proterozoic Piriwiri Group of metamorphosed argillaceous sedimentary rocks with minor volcanics. Some gold is also recovered as a by-product from copper deposits in the Deweras Group Sediments. The production of alluvial gold, present along all the major rivers draining the greenstone belts, has largely been the domain of illegal gold panners.
It occurs as native silver in association with other minerals such as gold, copper and lead. With exception of the Osage Mine in Zimbabwe it is declared as a by products from the mining of platinum, gold and copper. Gold mines in the Odzi greenstone belt have the highest silver and gold ratios.
PLATINUM GROUP ELEMENTS
Platinum Group Metals (PGMs) consist of platinum, palladium, rhodium, ruthenium, iridium and osmium and have a high demand worldwide because of their wide variety of uses in industry. Zimbabwe’s Great Dyke, a linear early Proterozoic layered mafic-ultramafic intrusion trending over 550km at a maximum width of about 11kilometres, has the second largest platinum reserves in the world after the Bushveld Complex in South Africa. An estimate of 2.8 billion tonnes PGM ore at 4g/t are estimated to lounge on the Dyke. Notably, PGMs are mined as primary metals only in the Bushveld in South Africa and along the Great Dyke in Zimbabwe.
The occurrence of PGE mineralization in the Great Dyke was recorded in the early 1920s. Following these documentations and the discovery of PGMs in the Merensky Reef of the Bushveld Complex of South Africa, there was a boom in PGM prospecting between 1925 and 1926 that resulted in the discovery of Wedza Mine. Since the 1950s, several companies have undertaken exploration. Currently platinum exploration on the Great Dyke has been carried out by CAMEC (Todal Mining) in the Bougai section in Shurugwi, and Global Platinum Resources in Chegutu.
Mining is currently being carried out at Mimosa, Ngezi and Unki Platinum Mines. Mining operations ceased at Hartley Platinum Mine in 2000, because of bad ground conditions. Demand for PGMs has seen an increase in exploration and evaluation of Zimbabwe’s platinum deposits.
Off the Great Dyke, the potential for PGM mineralization exists in the following areas
¨ Mashava Igneous Suite
¨Bubi Greenstone Belt
Zimbabwe is located within an exceptionally rich diamondiferous metallogenic province. Large areas of the country are covered by the Archaean Craton and the Archaean Limpopo Belt which are likely to have the best developed mantle root and diamond potential along with discoveries of the mineralized kimberlites on the Craton (Murowa, Sese, Colossus) and on the Limpopo Belt (River Ranch) which make Zimbabwe an excellent exploration target with potential for economic kimberlites. The diamond exploration success in the neighbouring Botswana and South Africa, the greatest producers of gem quality diamonds has made a positive impact on the diamond prospectively of the country. More than 120 kimberlites have been discovered but economic grades occur in two deposits the River Ranch and the Murowa Diamond Mines. Currently evaluation is being carried out on several kimberlites in the southern area of the country while some were found to be non-commercial. Of late, the discovery of diamondiferous Proterozoic conglomerates in the Umkondo basin has led to the opening of several diamond mines within the Chiadzwa area, e.g. Mbada, Marange Resources, and Anjin etc.
Zimbabwe has the 2nd largest high grade chromium ores in the world after South Africa with reserves of approximately 10 billion tonnes.
Chrome is mainly mined along the Great dyke of Zimbabwe and occurs as seam/strati form deposits. In greenstone belts off the dyke it occurs as podiform structures in serpentinites, e.g. in Zimasco mine on Shurugwi Mashava. In Mashava chrome is found in greenstone remnants in the Limpopo mobile belt south of Mberengwa. Chrome also occurs as elluvial deposits in the greenstone areas,
Giant crystals of up to 1.5m have been found on the dyke. Chrome is mainly used stainless steel production, as a metal coat, in the chemical industry and in metallurgical processes.
Zimbabwe has vast high grade coal deposits occurring as fossilized carbon. It occurs in lower Karoo sediments. These are the middle Zambezi basin to the north and save Limpopo basin in the south of the country hosts about 12 billion tonnes of good quality coal. About 29 coal localities are known but major producers are Hwange colliery and Makomo resources. The country’s full potential is however yet to be exploited.
In Zimbabwe nickel occurs within the Archean craton in rocks of komatitic composition e.g. at Trojan mine. It also appears layered/unlayered mafic-ultramafic intrusive bodies e.g. Empress, Madziwa Great dyke. It’s also found in nickel laterite e.g. northern part Great Dyke hydrothermal shear zone deposits. There are nickel deposits in several serpentinite areas in greenstone belts with igneous complexes around the country. The country has got huge potential in komatiite and laterite and more than 30 nickel deposits are known. Currently production is at Trojan mine in Bindura.
Asbestos occurs as chrysotile. It’s found in ultramafic complexes e.g. ,Mashava Igneous Complex, in massive serpentinites and slip fibre zones in which shears are filled with matted fibres in the Great dyke e.g. Ethel mine. There are 60 deposits scattered in the Masvingo, Insiza, Gwanda, Mberengwa, and Shurugwi, which have been worked on for chrysotile.
Zimbabwe was once the world’s 3rd largest producer of asbestos before the demand declined. After gold asbestos was once largest income producer in the mining sector. Production ceased with the closure of Gaths Mine and Shabani Mine.
COAL BED METHANE
Coal bed methane is a gas intrinsically associated with coal. Coal is both a source and reservoir for methane gas occurrence in Zimbabwe. It mainly occurs in the middle Zambezi Basin e.g. in lupine concession. It’s also found in the Save Limpopo basin e.g. Save Runde district.
Reserve is still a resources studies to ascertain if the gas could be exploited commercially have not been concluded. Coal bed Methane is used for Electricity generation, Ammonia production for fertilizer and in Iron production.
There are over 70 known deposits in Zimbabwe that have produced copper either as a primary or secondary product. The main producing area has been the Magondi Basin in an area stretching for over 150km. Similar copper deposits are found in the south eastern part of the country in the Umkondo Basin. Several copper prospects also occur in hydrothermal deposits in Archaean Greenstone Belts and in granite e.g. Inyathi, Copper duke. Primary copper production virtually ceased following closure of Mhangura, now being produced as a by-product of other minerals e.g. PGM, Gold, Nickel
Zimbabwe has huge iron deposits associated with banded ironstone formations in greenstone belts. Major deposits are estimated to be over 30 billion tonnes of reserves. Magnetite deposit associated with Carbonatites e.g. Gungwa and Dorowa, Banded Iron Formation in Archaean Greenstone Belts, magnetite schists e.g. Chiredzi deposits (Mongula and Manyoka), layered Igneous complexes source of titanium and vanadium also e.g. ChuatsaBanded Iron Formation, Archaean Greenstone belts e.g. Kwekwe, Buhwa and oxidation of other minerals e.g. Ripple Creek Deposits with high grade ore are found in Buchwa and Ripple Creek. Significant ironstone deposits include the huge Mwanesi deposit west of Chivhu and Nyuni near Masvingo. Manyoka and Mongula and several similar deposits in the Limpopo Mobile belt are also important deposits.
Pegmatites which are ubiquitous in several geological environments especially on the edges of greenstones and in metamorphic belts, are a source of a variety of minerals including tantalite, tin and wolframite, beryl, mica, feldspar, and gemstones such as emerald, aquamarine, chrysoberyl, alexandrite and euclase.
Granites, gneisses, migmatites, gabbro-norites, dolerite, marbles and quartzites, suitable for use as dimension stones are typical rocks belonging to Zimbabwe’s geological environments. The most well-known dimension stone in Zimbabwe is black granite ubiquitous in the north-eastern part of the country which has attracted considerate foreign investors.