Great Dyke

GREAT DYKE (RU: Великая Дайка) is the intrusive massif within Zimbabwe with the deposits of the chromium ores, which are the greatest deposits in terms of the reserves. The Great Dyke stretches itself (along the azimuth of 30 degrees) for 560 kilometres, with the thickness from 3.2 to 12.3 kilometres. The Great Dyke is embedded among the rocks of the Zimbabwe massif, which is similar to the rocks of the Bushveld complex in terms of composition. The Great Dyke represents by itself the stratified lopolith (the age is 2.5 billion years), which is comprising the three zones: the ultramafic zone (dunites, serpentinites), the transitional zone (harzburgites, olivine pyroxenites, picrites), and the mafic zone (anorthosite gabbro, norites, gabbro, quartz gabbro). The chromium ores form the pseudo-stratified interlayers within the bottom (ultramafic) zone.

There are known 11 ore-bearing horizons, which are alternating themselves with the interval of 6 metres on average, with the slightly inclined dip from the edges to the centre of the structure. The large deposits are situated within the northern part of the massif (the region of the Harare city). The ore bodies are confined to the dunites, harzburgites, and pyroxenites, which are replacing one another successively; the less large ore bodies are within the region of the Shurugwi city. The ore bodies of the vein-alike and lenticular shape are embedded among the serpentinized harzburgites, talc and talc-carbonate rocks. They distinguish the friable metallurgical and massive (lumpy) chemical ores, which are containing from 42% to 50% of the Cr2O3 compound with the ratio of Cr:Fe from 2.0 to 4.2. There distinguish themselves with the most high quality the metallurgical ores, which are confined to the lowest horizons. The total reserves of the chromium ores are estimated at 1.0 billion tonnes (1981).

The deposits are being developed since the start of the 20th century using the opencast method. There conduct the development of the ores the companies: British "Rio Tinto"; American "Rhodesien Chrome Mines Ltd.", "Union Carbide Rhomet"; South African "African Chrome Mines Ltd.". The extraction is 550 thousand tonnes of the metallurgical ores per year (1980). Small deposits of the copper-nickel ores are known near the Bindura and Zvishavane (Shabani former) cities. The ores are of the interspersed type within serpentinites, and of the venulous-interspersed type within amphibolites and norites, with the content of nickel from 0.71% to 1.1%. The ores are being developed using the opencast and underground methods, at the Shangani, Epoch, Trogi, and Madziwa deposits. The concentrates are processed within the Bindura city at the industrial plant with the productive capacity of 7.5 thousand tonnes of nickel per year. The ore manifestations of platinum are associated with the thin ore bodies of the chromium ores within dunites and pyroxenites. The industrial concentrations of the platinoid metals (4.7-6.2 grams per tonne) are associated with the eluvial placer of the Wedza deposit.