Ruby and garnet gemstone deposits in southeast Kenya: Their genesis and recommendations for exploration

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.