Synopsis
Commercial ruby and green grossular garnet deposits in East Africa are mined directly from bedrock. Consequently exploration programmes for new deposits have to be based on a sound knowledge of the geological controls on the growth of these two minerals.
Rubies in, or immediately adjacent to, chromiferous ultramafic bodies in the Mozambique Orogenic Belt formed in areas where the regional metamorphism attained granulite facies conditions. Green garnets are also confined to these areas of high metamorphic grade as disseminations in vanadiferous graphitic schist and gneiss associated with marble. The superb body colours which makes the rubies and garnets so valuable are due to high contents of chromium and vanadium respectively, derived from their host-rocks during metamorphism.
The host-rocks are themselves ideal targets for direct and indirect prospecting methods. Exploration programmes should be based on the use of satellite imagery, geological mapping, soil and stream geochemistry, and airborne or land geophysical methods.
Current exploitation of gemstones, especially the coloured corundums (sapphires and ruby) and the various varieties of coloured garnet, has established East Africa as a major new gemstone province. In this arid area there is only local secondary dispersion of the mineralization: gemstones are mined directly from bedrock after initial eluvial extraction. Therefore the scientific search for new deposits has to be be based on a sound knowledge of the geological controls on the growth of each type of gemstone.
As such the nature of the host rock has to be known as well as the geological- and especially metamorphic-conditions that were necessary for gemstone formation. However, it is relevant to prospecting programmes to note that the first discovery of gem-quality sapphires in SE Kenya, announced in the Annual Report for 1936 of the Geological Survey of Kenya, was from soils and gravels1. These were locally derived from the corundum-bearing wall rock of an ultramafic intrusion.
Gem-quality ruby and garnet in East Africa formed during regional metamorphism within the Mozambique Orogenic Belt. An understanding of the controls on mineral growth is needed in order to effectively plan exploration programmes for new gemstone deposits. Four controls are apparent:11,13,22.
-
1.
Host rock lithology: gemstone varieties are confined to specific lithologies or to specific lithological associations such as the altered contacts of intrusive bodies.
-
2.
Stratigraphy: host lithologies often occur within lithostratigraphic units which are mappable.
-
3.
Metamorphism: each gemstone variety formed within a narrow range of physical conditions (pressure, temperature, activity of CO2 and H2O fluid phases) during regional metamorphism.
-
4.
Chemistry: the body colours of ruby and the garnets are due to the presence of traces of certain transition group elements, especially chromium and vanadium, in the mineral matrix. Of equal importance is the absence of other transition group elements which can spoil the body colour, especially iron, traces of which can produce ugly brown tints in ruby considerably depreciating their value.
Access this chapter
Tax calculation will be finalised at checkout
Purchases are for personal use only
Preview
Unable to display preview. Download preview PDF.
References
Parkinson J. Outlines of the geology of the Mtito Andei — Tsavo area, Kenya colony. Report Geological Survey Kenya, 13, 1947.
The geochronology and evolution of Africa. Lucien Cahen and others. Oxford: Clarendon Press, 1984. 512pp.
The Pan-African belt of northeast Africa and adjacent areas: tectonic evolution and economic aspects of a late Proterozoic orogen. S. El Gaby and R. Greiling (editors). Braunschweig/Wiesbaden:F. Vieweg and Sohn, 1988. 369pp.
Precambrian collision tectonics in Africa. R.M. Shackleton. In: M.P. Coward and A.C. Ries (editors), Collision tectonics. Oxfords: Blackwell, 1986. p. 329–349.
Key, R.M. and others. Superimposed Upper Proterozoic collision controlled orogenies in the Mozambique Orogenic Belt of Kenya. Precambrian Research, 44, 1989, p. 197–225.
Corundum. R.W. Hughes. London: Butterworth-Heinemann, 1990. 314pp.
Pohl W. and Horkel A. Notes on the geology and Mineral Resources of the Mtito-Andei-Taita area (Southern Kenya). Mitteilungen der Osterreichischen geologischen Gesellschaft, 73, December 1980, p. 135–152.
Key, R.M. Geology of the Maralal area. Report Mines and Geological Survey Department, Kenya, 105, 1987. 93pp.
Hackman B.D. and others. Geology of the Isiolo area. Report Mines and Geological Survey Department, Kenya, 104, 1988. 88pp.
Walsh J. Geology of the area south of the Taita Hills. Report Geological Survey of Kenya, 49, 1960.
Pohl W. and Neidermayr G. Geology of the Mwatate quadrange (Sheet 195/2) and the vanadium-grossularite deposits of the area. Report Kenya-Austria Mineral Exploration Project, 11, 1978. 89pp.
Saggerson E.P. Geology of the Kasigau-Kurase area. Report Geological Survey of Kenya, 51, 1962.
Key R.M. and Hill P.G. Further evidence for the controls on the growth of vanadium grossular garnets in Kenya. Journal of Gemmology, 31, 1989. p. 412–422.
Arneth J.D. and others. Graphite content and isotopic fractionation between calcite- graphite pairs in metasediments from the Mgama Hills, southern Kenya. Geochimica and Cosmochimica Acta, 49, 1985. p. 1553–1560.
Sarbas B. and others. Zur genese ostafrikanischer Grossular vorkommen. Zeitschrift der Deutschen Gemmologischen Gessellschaft, 33, 1984. p. 48–61.
Key R.M. and Ochieng J.O. The growth of rubies in south-east Kenya. Journal of Gemmology, 1991 (in press).
Holdaway M.J. Stability of andalusite and the aluminosilicate phase diagram. American Journal of Science, 271, 1971. p. 97–131.
Altherr R. and others. Corundum and kyanite-bearing anatexites from the Precambrian of Tanzania. Lithos, 15, 1982. p. 191–197.
Gems. Their sources, descriptions and identification. R. Webster. 3rd edition. London: Butterworth. 938pp.
Harding R.R. and Scarratt K. A description of ruby from Nepal. Journal of Gemmology, 20, 1986. p. 3–10.
Gubelin E.J. and Weibel M. Green vanadium grossular garnet from Lualenyi, near Voi, Kenya. Lapidary Journal, 29, 1975. p. 402–414 and 424–426.
Bridges C.R. Green grossular garnets (tsavorites) in east Africa. Gems and Gemmology, 1974. p. 290–296.
Baker B.H. Geology of the Baragoi area. Report Geological Survey of Kenya, 53, 1963. 74pp.
Shackleton R.M. Geology of the country between Nanyuki and Maralal. Report Geological Survey of Kenya, 11, 1946. 54pp.
Baker J. and others. Corona textures between kyanite, garnet and gedrite in gneisses from Errabiddy, Western Australia. Journal of Metamorphic Geology, 5, p.357–370.
Warren R.G. and others. Wollastonite and scapolite in Precambrian calc-silicate granulites from Australia and Antartica. Journal of Metamorphic Geology, 5, 1987. p. 213–223.
Oterdoom W.H. and Gunter W.D. Activity models for plagioclase and CO3-scapolites — an analysis of field and laboratory data. American Journal of Science, 283-A, 1983, p. 255–282.
Goldsmith J.R. and Newton R.C. Scapolite-plagioclase stability relations at high pressures and temperatures in the system NaAlSi3O8 - CaCO3 - CaSO4. American Mineralogist, 62, 1977. P. 1063–1081.
Wedepohl K.H. Untersuchungen am Kupferschiefer in Nordwest Deutschland. Ein Beitrag zur Dentung bituminöser Sedimente. Geochimica and Cosmochimica Acta, 28, 1964.
Shacklette H.T. and others. Elemental composition of surficial materials in the conterminous United States. U.S. Geological Survey Professional Papers, 574-D, 1971.
Eade K.E. and Fahrig W.F. Regional, lithological and temporal variations in the abundance of some trace elements in the Canadian Shield. Geological Survey of Canada Paper, 72–46, 1973.
Bridges C.R. Gemstones of East Africa. Proceedings International Gemmological Symposium, 1982. p. 266–275.
Gem Testing. B.W. Anderson. London: Butterworths, 1980, 434pp.
Key, R.M. Classification of the Earth’s oldest rocks. NERC News, 1989, 10, p8.
Author information
Authors and Affiliations
Rights and permissions
Copyright information
© 1991 Institution of Mining and Metallurgy
About this chapter
Cite this chapter
Key, R.M., Ochieng, J.O. (1991). Ruby and garnet gemstone deposits in southeast Kenya: Their genesis and recommendations for exploration. In: African Mining ’91. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-3656-3_12
Download citation
DOI: https://doi.org/10.1007/978-94-011-3656-3_12
Publisher Name: Springer, Dordrecht
Print ISBN: 978-1-85166-654-6
Online ISBN: 978-94-011-3656-3
eBook Packages: Springer Book Archive