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Exploring for deeply buried ore deposits
Published in Natalia Yakovleva, Edmund Nickless, Routledge Handbook of the Extractive Industries and Sustainable Development, 2022
Raymond J. Durrheim, Musa S.D. Manzi, Glen T. Nwaila, Susan J. Webb
Diamonds are a rare polymorph of carbon that forms at high pressures. Their primary source is kimberlite, an unusual ultrabasic igneous rock that occurs as small volcanic pipes, dykes and sills, which originates in the upper mantle at depths exceeding 120 km (Stachel et al., 2004; De Stefano et al., 2009). In most parts of the mantle, the temperatures are too high for diamonds to survive at such depth; it is only in the mantle keels of Archean cratons that temperatures are low enough for diamonds to be preserved (Rombouts, 2003). Thus, knowledge of the position (and history) of craton boundaries (not only on the surface but also at depths of several hundred kilometres into the mantle, as the boundaries need not be vertical) is important for diamond exploration, enabling regions to be surveyed at a prospect scale. While gravity and magnetic techniques are very useful in defining the surface craton boundaries (e.g. Corner and Durrheim, 2018), the geophysical techniques best able to map the deep structure of cratons are broadband seismic and magnetotellurics (MT).
Precious stones
Published in Francis P. Gudyanga, Minerals in Africa, 2020
Carbon-containing minerals at high temperature and pressure at depths of 140 to 190 kilometres in the Earth’s mantle over periods of 1 to 3.3 billion years are the sources for the formation of natural diamonds. Deep volcanic eruptions result in a magma bringing diamonds close to the Earth’s surface. This magma cools into igneous rocks known as kimberlites and lamproites where the diamonds crystals grow larger with longer residence in the cratonic lithosphere [929].
Phanerozoic Androgenic Magmatism
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
It is difficult to name another igneous rock with equally variable appearance, texture, mineral and chemical composition. The range of composition, together with frequent similarities with subalkaline and alkaline picrites, melilite-bearing volcanics, basanites, ultrabasic lamprophyres and subvolcanic members of alkali-ultrabasic units as well as the constant presence of xenolith material and extensive rock alteration, tend to blur definition of the term “kimberlite”. Nowadays kimberlite is defined as an ultrabasic rock, tending to be alkaline, frequently diamondiferous, with inclusions of olivine, magnesian ilmenite, phlogopite, pyrope garnet, chromian diopside and other compositions, embedded in a fine-grained phlogopite–serpentinite–carbonate matrix (Sobolev, 1974; Laz’ko, 1988c).
Identifying critical parameters in the settling of African kimberlites
Published in Mineral Processing and Extractive Metallurgy Review, 2018
E. T. Boshoff, J. Morkel, N. Naude
Kimberlite is the host rock from which diamonds are mined. There are to date up to 4000 different kimberlites and lamproites intrusions identified, and more are identified each year due to the systematic exploration program carried out by the various diamond exploration companies. Various attempts have been made to classify kimberlites according to the different mineralogical compositions. Mitchell (1986) classifies three groups of kimberlites according to the amount of olivine, phlogopite and calcite, kimberlite (equivalent to basaltic kimberlite), micaceous kimberlite (equivalent to lamprophyric kimberlite), and calcite or calcareous kimberlite. Skinner and Clement (1979) classify kimberlites into five groups according to the predominance of diopside, monticellite, phlogopite, calcite, and serpentine present. Serpentine is formed over centuries during the alteration process of olivine and is one of the main minerals in blue ground. Blue ground is unweathered kimberlite found deeper in the volcanic pipe. Serpentine can account for 20–50% of the groundmass in the mineral and chemically breaks down into the clay minerals of smectite, vermiculite, calcite, chlorite, and talc during the weathering process (Hopwood et al., 1975).