A Tectonics Article By Angell Pearce
Mineral Deposits
The African Plate is home to 30% of the world's mineral resources (Sharaky, 2011). Its beginnings in the Archean are hosted by the Kalahari, Tanzanian, Kasai and West African cratons (see figure 1) that host many of Africa's major mineral deposits (Taylor et al., 2009). The cratons are characterized by greenstone belts, tonalites and high grade metamorphic lithologies (Taylor et al., 2009) and the major ore deposits are gold, antimony, iron, chromium, nickel-copper and copper zinc (Hammerbeck and Veselinovic-Williams, 2007). Major orogenies border these cratons due to collisions during the formation of Gondwana, (and subsequently Pangaea), and during this time large sedimentary basins formed across the plate (Taylor et al., 2009). Placer deposits, such as the gold of the Witwatersrand in South Africa, were then able to accumulate in these basins.
The Proterozoic gave way to cycles of platform sedimentation and volcanism, in some cases creating stratabound ore such as the copper of the Piriwiri-Lomagundi basin in NW Zimbabwe (Hammerbeck and Veselinovic-Williams, 2007). The orogenies continued and induced magmatism that lead to the large chromium, iron, titanium, vanadium, nickel-copper and platinum -group metal deposits of the Bushveld Complex (South Africa) and the Great Dyke (Zimbabwe). Finally, Pangaea break-up began around 160 Ma, and is still separating today. Mineral deposits associated with the Somali plate (and consequently the Seychelles and Madagascar microplates) are: bauxite, copper, gypsum, iron, salt, tin, uranium, cement, gold, platinum, potash, tantalum, aluminum, cement, fluorite, garnets, gemstones, ilmenite, limestone, soda ash, trona, diamonds, nickel, phosphate, silver, chromite, graphite, ilmenite, mica, quartz, and scandium (Taylor et al., 2009). The Arabian plate also hosts many valuable minerals, particularly gold, zinc and copper, typically associated with volcanic activity. |
Witwatersrand, South Africa
The Witwatersrand deposit of South Africa is the world's largest gold reserve covering an area of 350 x 200 km (Taylor et al., 2009). It is an alluvial fan-shaped placer deposit that came from streams that were flowing over granites and mafic/ultramafic rocks in the area (Frimmel et al., 2005; Taylor et al., 2009). The influx of the placer deposits can be attributed to the larger-scale tectonics of the region. The basin originated as a result of rifting in the area (3074 Ma) and post-rift thermal subsidence (Beach and Smith, 2007). The craton then experienced compression and the thrusting surrounding the basin allowed for an increase in sedimentation (2950 Ma).
The Witwatersrand deposit of South Africa is the world's largest gold reserve covering an area of 350 x 200 km (Taylor et al., 2009). It is an alluvial fan-shaped placer deposit that came from streams that were flowing over granites and mafic/ultramafic rocks in the area (Frimmel et al., 2005; Taylor et al., 2009). The influx of the placer deposits can be attributed to the larger-scale tectonics of the region. The basin originated as a result of rifting in the area (3074 Ma) and post-rift thermal subsidence (Beach and Smith, 2007). The craton then experienced compression and the thrusting surrounding the basin allowed for an increase in sedimentation (2950 Ma).
Diamond-Bearing Kimberlites, South Africa
Africa holds 46.8% of the world's production of industrial diamond, and 50% of its gem diamonds (Taylor et al., 2009). Diamonds are often mined from kimberlites, many of which are found in the Archean cratons of the African plate. Additionally, there are many diamond placer deposits distributed around the African plate (see figure 2). The largest diamond mines can be found in southern Africa in the Kalahari craton, and many scientists believe there is a link between tectonics and kimberlite distribution. According to Jelsma et al. (2004), there are four dominant trends on which clusters of kimberlites can be found in this area - NE 039-058, E 080-112, SE 134 and S 163-186. These parallel fractures or shear zones found deep in the crust that likely originated from the break-up of Gondwana. The kimberlites from southern Africa have been dated to the early growth of the Indian Ocean and the opening of the Atlantic (Jelsma et al., 2004). |
Hydrocarbons
The Arabian plate currently holds 48% of the world’s oil reserves and 43% of the world’s natural gas reserves (BP, 2013). The fertility of the plate with respect to oil and gas can be attributed to its tectonic history. Prior to separating from the African plate, the Arabian plate was part of the northern passive margin of Gondwanaland (Beydoun, 1998). It shifted from areas of high latitude where clastic sedimentation occurred, to tropical areas where carbonates were deposited and organic productivity was high (see figure 3). These shifts in geography supplied both the required source and reservoir rocks for hydrocarbons. Furthermore, evaporates – an effective hydrocarbon seal – were able to accumulate during periods of transgression. Sediments were laid out by epeiric seas for a characteristically long and nearly uninterrupted period of time – roughly 570 Ma of sedimentation – and this gave way to the prolific petroleum region the Middle East is today (Beydoun, 1998).
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Beydoun ZR. 1998. Arabian plate oil and gas; why so rich and so prolific? Episodes 21(2):74-81.
Frimmel HE, Groves DI, Kirk J, Ruiz J, Chesley J, Minter WEL. 2005. The formation and preservation of the witwatersrand gold fields, the world's largest gold province. Society of Economic Geologists, Littleton, CO.
Hammerbeck E and Veselinovic-Williams M. 2007. Status of metallogenic mapping in the world today; with special reference to the digital metallogenic map of africa. U.S.Geological Survey Circular :115-24.
Jelsma HA, de Wit M,J., Thiart C, Dirks PHGM, Viola G, Basson IJ, Anckar E. 2004. Preferential distribution along transcontinental corridors of kimberlites and related rocks of southern africa. South African Journal of Geology 107(1-2):301-24.
SHARAKY, ABBAS M. "Mineral Resources and Exploration in Africa." Egypt: Cairo University.
Taylor CD, Schulz KJ, Doebrich JL, Orris GJ, Denning PD, Kirschbaum MJ. 2009. Geology and nonfuel mineral deposits of africa and the middle east. Open-File Report - U.S.Geological Survey :246.