Zimbabwe is perhaps the only country whose colonisation by Europeans was a direct result of its perceived rich and wide variety of gold deposits. The Portuguese occupied parts of the country in the 17th century and traded in gold with local miners. Following the accounts of the Portuguese, and information gathered by 19th century explorers and hunters, a lot of interest on Zimbabwe’s gold was aroused in South Africa and Europe, leading to many fortune seekers visiting the country. Similarly, stories about unlimited gold riches, which were thought to be the provenance of the famous Witwatersrand deposits, led to the invasion of the country by Cecil John Rhodes’ British South Africa Company in 1890.
Ancient miners did a great deal of exploration and mining long before the arrival of the Europeans. The ancients discovered nearly all outcropping auriferous quartz veins and worked them down to the water table. Gold exploration in modern times, which has resulted in the discovery of over 6,000 deposits, was thus greatly facilitated by the numerous old workings. Gold exploration in Zimbabwe has therefore been largely biased at rediscovering quartz veins at old workings at the expense of virgin areas. Quartz vein ore bodies are normally small; consequently some commentators have often regarded Zimbabwe as a country of small gold deposits, a wrong conclusion that is mainly based on historical data, rather than on technical information. Also the sheer number of rediscoveries has led some commentators to wrongly conclude that the country has been over-prospected.
At independence, new mining companies mostly from Canada and Australia brought into the country new ideas and techniques to extend exploration outside the ancients’ workings and to revisit traditional quartz vein-based deposits to assess their potential for low grade, high tonnage propositions. The new techniques have shown positive results, which suggest that Zimbabwe is neither generally over-explored, and nor is it a small mines country.
OUTLINE OF THE GEOLOGY
About 60% of Zimbabwe’s land surface comprises an Archaean age basement known as the Zimbabwe Craton, which is dominated by granitic rocks locally enclosing remnants of volcanosedimentary piles known as greenstone belts. The greenstone belts are renowned for the rich variety of mineral resources, including gold, base metals and industrial minerals.
The famous Great Dyke marking the upper boundary of the Archaean in Zimbabwe, hosts worldclass reserves of platinum group metals and chrome ore.
Metamorphic belts rich in metamorphic minerals surround the Craton: the Archaean Limpopo Mobile Belt to the south, the Proterozoic Magondi to the northwest, and the Proterozoic Zambezi Mobile Belt to the north and northeast.
Several sequences of sedimentary rocks cover the peripheries of the Craton: the Proterozoic strata of the Magondi, the Gairezi and the Sijarira basins, the Permian- Triassic-Jurassic sedimentary and basaltic rocks of the Karoo Supergroup and Cretaceous sediments of the Post Karoo, and finally undeformed Tertiary to Recent sands of the Kalahari.
A COUNTRY OF SMALL GOLD DEPOSITS?
With a production since independence in 1980 of over 400 tonnes of gold from several thousand shear quartz vein-based small mines located on lines of ancient workings, Zimbabwe is generally considered to be a country of small gold deposits. However, to fully understand the mineralisation and nature of ore bodies in Zimbabwe, the history of exploration and mining, as well as current understanding of the nature of gold deposits and controls on mineralisation and future trends of exploration should be considered.
Miners in Zimbabwe rarely estimate reserves more than two years ahead of production. Consequently, resources of the majority of mines remain unknown. Although monthly production returns from such mines would tend to suggest that they are small deposits, in many instances total production at the end of the life of the mine or after further exploration, show that several small mines could in fact be large deposits.
Some of the arguments for classifying Zimbabwe as a country of small deposits include the fact that well explored Archaean Cratons such as the Superior Province in Canada and Yilgarn in Australia have linear greenstone belts hosting world-class gold deposits associated with crustal scale shear zones, whereas the Zimbabwe Craton has arcuate greenstone belts with gold deposits in localised shear zones. Canadian and Australian greenstone belthosted gold deposits are also associated with wide alteration zones, while the Zimbabwean ones have narrow alteration halos.
These features would tend to suggest that the geological environment in the Zimbabwe Craton was not favourable for the development of large gold deposits. However, current exploration is indicating that world-class deposits are not necessarily associated with large alteration zones and crustal scale shear zones, as in the Yilgarn and Superior examples. For instance, Freda- Rebecca, a million ounce deposit located near Bindura, is upon an array of local shear zones not connected to known major shear zones. The same mine has narrow alteration haloes.
Recent work has shown that the Zimbabwe Craton, like other similar geological environments elsewhere, is highly heterogeneous, being cut by crustal scale shear zones that impart a conspicuous control on gold localisation. It is therefore becoming increasingly apparent that structural and lithological controls affecting localisation of gold deposits in Australian and Canadian Cratons are in many ways analogous to the Zimbabwe Craton situation. There is no reason why mines of the same significance as those in other Archaean granite-greenstone terrains should not be found in the Zimbabwe Craton.
A COUNTRY UNDEREXPLORED?
Although the Zimbabwe Craton has one of the highest gold productivities per square kilometre, the available technical information suggests that the country is basically under-explored. As indicated above, the ancients only explored for resistant outcropping quartz vein ore bodies. Exploration that ensued after colonisation of the country by Europeans concentrated on the ancients’ mines, as it was cost effective. The usual methods of exploration by the early Europeans were to get information from local villagers about the existence of old workings, on which prospecting shafts were sunk. It is astonishing to note that this form of exploration became so entrenched in the exploration culture of Zimbabwean prospectors that it has remained basically unchanged up to today. Zimbabwe therefore remains essentially under-explored, especially in virgin areas away from the ancient workings.
Greenstone belts, hosts to most gold deposits, are coincidentally prime agricultural areas as a result of their rich red loamy soils. Mineral exploration is prohibited in cultivated areas or areas registered for cultivation, except with the consent of the landowner, which is not always easy to come by. Exploration has, as a result, mainly been confined to hilly areas that have high incidences of hosting quartz vein-hosted deposits.
LACK OF APPRECIATION OF IMPORTANCE OF GEOLOGISTS Many gold deposits in Zimbabwe were discovered without using much geological modelling as a result of the existence of the ancient workings that simplified the exploration
Exploration models are normally derived from data carefully and scientifically recorded over time from existing mines. However, many mines in Zimbabwe have operated without resident geologists. A lot of information that could be used as a foundation for exploration in similar areas has therefore been lost. Many small mines could be more highly prospective than currently perceived, but the potential cannot be realised when there are no geologists to record the salient features characteristic of bigger ore bodies. Shamva Mine, for instance, has intermittently operated without a geologist before the 1990s and its potential as a world-class deposit only become apparent after geologists were engaged on full time basis. Recent re-evaluation of the derelict Globe and Phoenix Mine, the country’s premier gold deposit, indicates that large tonnages of viable ore were left out in both the hanging and footwalls of the mined out high grade ore body and along extensions that are now sterilised by the Kwekwe town lands. There is probably a similar scenario at several old mines. For instance, strike extensions of many mines have not been investigated because of an entrenched tradition of going underground before fully understanding lateral extensions of the ore bodies – a result of the lack of usage of geologists by many mining companies. Probing of lateral extensions of ore bodies is also prohibited by adjoining mining claims belonging to different individuals.
Several companies engaged in mining and exploration in Zimbabwe, are still practicing the tradition of not making much use of geologists. Exploration geologists are mostly used as field technicians who are not usually involved in the design of exploration programmes. As a result, most local geologists become rigid, and cannot react adequately to unexpected observations encountered during exploration. This results in waste of time and money, and reduction in chances of discovering mineral deposits.
GREAT EXPLORATION OPPORTUNITIES
Small-scale miners lack the necessary financial and technical skills to explore and exploit larger ore bodies, preferring narrow quartz veins that are easier to exploit with rudimentary tools. As a result, a number of potential large ore bodies remain sterilised by small-scale mining. Eureka mine in Guruve and Freda-Rebecca near Bindura were typical small mines exploiting narrow quartz veins through conventional underground methods. However, a new interest in low grade, high tonnage gold deposits saw the reinvestigation of the two mines resulting in them being developed into large open pittable mines.
With over 6,000 small gold deposits having been exploited in Zimbabwe, there are huge opportunities to investigate many of them for potential for large mines. Studies in the Zimbabwe Craton have indeed shown that most large deposits do not occur in isolation, but include apparent small mines with potential for development into larger mines if investigated inclusively. Well known areas such as the Termite- Khanye trend in Silobela, the Dalny-Lily Fault zone in the Midlands Greenstone Belt, the Surprise Fault near Shurugwi and the environs of Motapa Mines, and several other areas in the Zimbabwe Craton, have great potential to host several million ounce gold deposits.
Many areas of small-scale mining such as Tafuna Hill, Makaha, and Mt. Darwin provoke methodical reevaluation as casual observations suggest that small-scale miners in the areas are exploiting discrete rich quartz veins in much wider lower grade shear zones that could be viably exploited as large mines.
USE OF MODERN EXPLORATION TECHNIQUES
Before 1990, technology in mineral exploration in Zimbabwe was estimated to be 30 years behind that of other countries with similar geological environments such as Australia and Canada. Geophysical geochemical and remote sensing techniques that are responsible for discoveries of mineral deposits in virgin areas in other countries were rarely used in Zimbabwe. Many companies have now increased use of these techniques resulting in new discoveries. Bubi mine is a recent discovery made following up geochemical anomalies. Other new significant discoveries made by careful analysis of geochemical and geophysical results are Ipanema and Hungwe in the Belingwe Greenstone Belt, an area traditionally regarded as having small deposits.
Aeromagnetic surveys of most of the country carried out through Canadian government assistance between 1988 and 1990 opened up new ground for exploration. Linear extensions of existing ore bodies that are not obvious on existing geological maps were revealed by the new aeromagnetic data. Extensions of the Jena mines below the Kalahari on the western edge of the Zimbabwe Craton were discovered. Followup high sensitive geochemical surveys on the geophysical anomaly discovered a mineralised body buried below 15 metres of Kalahari Sands, resulting in discovery of Maligreen deposit. Greenstone belt areas covered by the Kalahari sands in the western part of the country have thus become new targets for mineral exploration.
It is now acknowledged that most known large lode gold deposits are associated with pervasive mineralisation in brittle-ductile to ductile shear zones not necessarily associated with much quartz veining or silicification. Where the shear zones are not associated with resistant quartz veins, they form low-lying areas covered by a mantle of soil that is normally under cultivation in most greenstone belts. The ancients who had developed excellent skills at identifying resistant outcropping ore bodies missed most shear zone ore bodies in such areas. Subsequent exploration that was aimed at rediscovering ancient mines also missed buried shear zone-hosted ore bodies. As shear zones are known to persist both laterally and down-dip, they form considerable ore deposits (e.g. Dalny, Globe and Phoenix, Cam and Motor, How, Vubachikwe, Arcturus, Shamva, etc.). There are therefore strong possibilities that a good number of larger ore bodies still remain concealed in the greenstone belts, especially in cultivated areas away from hills. New techniques of geophysics, geochemistry and remote sensing should assist in the identification of these ore bodies.
UNDER-EXPLORED GREENSTONE BELTS
Although it is now established that majority of gold deposits in Zimbabwe are located in greenstone belts and their immediate granitoid surroundings, smaller greenstone belts have generally not been considered for exploration by mining companies despite several known gold mines and occurrences they host. While to date over 1,400 exclusive prospecting orders (EPOs) have been issued to explore various parts of the country, smaller greenstone belts have not been subject to any detailed investigations under the EPOs.
Beatrice Greenstone Belt located about 60 km south of Harare is a good example of a potential area disregarded by large mining companies. This greenstone belt has not been geologically mapped in detail despite hosting three mines, Joyce, Roma and Beatrice, which produced about three tonnes of gold each. These deposits and other smaller mines in the greenstone belt probably had their potential as big mines obscured by the level of operations, which were mostly smallscale. The geology of these deposits that comprises wide ductile shear zones pervasively infiltrated by sulphides typify that of larger deposits. The full potential of these mines, which are now derelict, was not realised because no appropriate exploration was undertaken at the mines as they operated without geologists.
Other overlooked smaller greenstone belts with good potential for hosting big mines include Mt. Darwin, Dindi, Wedza, Felixburg, Makaha and Lower Gwanda.MRA