Great Dyke

The Great Dyke is a linear geological feature that trends nearly north-south through the center of Zimbabwe passing just to the west of the capital, Harare. It consists of a band of short, narrow ridges and hills spanning for approximately Convert|550|km|mi|abbr=on. The hills become taller as the range goes north, and reach up to 1,500 feet (460 meters) above the Mvurwi Range. The range is host to vast ore deposits, including gold, silver, chromium, platinum, nickel and asbestos.

Geology

Geologically the intrusion is not a dyke, but is lopolithic and Y-shaped in cross-section. It is a layered ultramafic intrusion that extends across Zimbabwe with a strike of about N10°E. The width of the intrusion varies from 3 to Convert|12|km|mi|abbr=on. The Great Dyke is unusual in that most ultramafic layered intrusions display near horizontal sill or sheet forms.

The well-layered lower units of ultramafic rocks comprising the Great Dyke are locally overlain by erosional remnants of gabbroic rock. These mark the centres of the four sub-chambers within the Great Dyke magma system, namely (from noth to south) Musengezi, Darwendale, Sebakwe and Wedza. Each of these sub-chambers has an elongate, doubly-plunging synclinal structure, and was fed by a feeder dyke continuous below almost the entire Great DykeWilson, A.H. and Prendergast, M.D., 1989. The Great Dyke of Zimbabwe-I: tectonic setting, stratigraphy, petrology, structure, emplacement and crystallisation. In: Prendergast, M.D., Jones, M.J. (Eds.), "Magmatic sulphides-the Zimbabwe volume". Institution Mining Metallurgy, London, 1–20. ISBN 9781870706063] . Stratigraphically, each sub-chamber is divided into a lower Ultramafic Sequence of dunites, harzburgites, olivine bronzitites and pyroxenites together with narrow layers of chromitite that constitute the bases of cyclic units and that are extensively mined along the Great Dyke, and an upper Mafic Sequence mainly consisting of a variety of plagioclase-rich rocks, such as norites, gabbronorites and olivine gabbrosPrendergast, M.D. and Wilson, A.H. 1989. The Great Dyke of Zimbabwe-II: Mineralisation and mineral deposits. In: Prendergast, M.D., Jones, M.J. (Eds.), "Magmatic sulphides-the Zimbabwe volume". Institution Mining Metallurgy, London, 21-42. ISBN 9781870706063] .

The dyke lies within the Zimbabwe craton and has been dated at 2.575 billion years old [Oberthuer, T., Davis, D.W., Blenkinsop, T.G., Hoehndorf, A., 2002. Precise U–Pb mineral ages, Rb–Sr and Sm–Nd systematics for the Great Dyke, Zimbabwe—constraints on crustal evolution and metallogenesis of the Zimbabwe Craton. "Precambrian Research", 113, 293–306. [http://www.ingentaconnect.com/content/els/03019268/2002/00000113/00000003/art00215] ] . The Great Dyke acts as a strain-marker for the craton: The fact that it has been undeformed since intrusion (excluding the Musengezi area) shows that the Craton had stabilised by the time the Dyke intruded [Dirks, P.H.G.M. and Jesma, H.A. 2002. Crust–mantle decoupling and the growth of the Archaean Zimbabwe craton. "Journal of African Earth Sciences", 34, 157–166 [http://dx.doi.org/10.1016/S0899-5362(02)00015-5] ] .

Two mafic dykes, the East and Umvimeela, flank the Great Dyke to the east and west respectively. Volcanic surface manifestation of the Great Dyke intrusion have not been recorded and have probably been eroded .

The Great Dyke is a strategic economic resource with significant quantities of chrome and platinum. Chromite occurs to the base of the Ultramafic Sequence and is mined throughout the dyke . Below the Ultramafic-Mafic sequences' contact, and in the uppermost pyroxenite (bronzitite and websterite) units are economic concentrations of nickel, copper, cobalt, gold, and platinum group metals (PGM). The base metals occur as disseminated inter-cumulus Fe-Ni-Cu sulphides within an interval referred to as the base metal subzone, below which is a sublayer enriched in platinum group metals called the PGE subzone. The Base metal and PGE subzones together, make up the Main Sulphide Zone (MSZ) [Stribrny, B., Wellmer, F.-W., Burgath, K.-P., Oberthür, T., Tarkian, M. and Pfeiffer, T. 2000. Unconventional PGE occurrences and PGE mineralization in the Great Dyke: metallogenic and economic aspects. "Mineralium Deposita", 35, 260-281. [http://www.springerlink.com/content/gdu1qdermmy1a6hy/] ] .

Development

The Great Dyke was first reported in 1867 by the explorer Karl Mauch. However the existence of rich ore deposits was not realized until around 1918.

Chromite is mined throughout the Dyke, especially in the Mutorashanga and Lalapanzi areas. Platinum group metals are currently mined at Ngezi Mine (south of Selous), Unki Mine (near Shurugwi) and Mimosa Mine (near Zvishavane).

References

Further Reading

* Guilbert, John M., and Park, Charles F., Jr. (1986) "The Geology of Ore Deposits", Freeman, ISBN 0-7167-1456-6
* "Mining in Zimbabwe" [http://www.zimplats.com/activities/acts.htm Zimplats website]