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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).
Stratigraphy
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Most continents contain cratons, stable parts of the continental interior where little tectonic activity has occurred for a long time. As sea level rises and falls, cratons may be periodically flooded and sediments deposited. Thus, the sequence of sediments deposited on a continental interior during transgressions and subsequent regressions of an epeiric sea form a cratonic sequence. When seas are absent, sediments and sedimentary rocks are, most of the time, exposed to weathering and erosion. So, the tops and bottoms of cratonic sequences are unconformities, like the unconformities above and below the Tonto Group in the Grand Canyon, which, as described earlier in this chapter, formed during a marine transgression.
The Earth Through Time
Published in Aurèle Parriaux, Geology, 2018
Afterwards, the mountains disappeared as a result of continental erosion and several phases of glaciation. Sediments were created, followed by sedimentary rocks. Internal and external geodynamic cycles were underway. Throughout the three billion years of the Precambrian, orogenies created continental plates that would form the jigsaw puzzle of the beginning of the Paleozoic. Today’s world geologic map shows many old Precambrian basements that constitute the backbones of continents (Fig. 3.17). These cratons are characterized by their near horizontality because they have not been modified by significant deformation after their emplacement. They are often overlain by unaltered Precambrian sedimentary series that have remained in a quasi-horizontal position for more than a billion years (Fig. 3.18).
Assessing the Lithium Potential of the Paleoproterozoic Rocks of the West African Craton; the Case so Far
Published in Geosystem Engineering, 2023
Ghana is located in the southeastern part of the West African Craton. Ennih and Liégeois (2008) defined a Craton as a stable part of the continental lithosphere which has not been deformed for over a billion years or since the Precambrian era. The West African Craton was formed during the Archean and the Paleoproterozoic during the Eburnean orogeny. A period of quiescence during the Mesoproterozoic from 1.7 to 1.0 Ga enabled this large area to develop into a Craton. The West African Craton is made up of an Archean or Paleoproterozoic basement overlain by mainly Neoproterozoic sedimentary deposits. The thickness of the WAC lithosphere is depicted in Figure 2 to be between 150 and 200 kilometres (Roussel and Lesquet, 1991). Archean and Paleoproterozoic strata are exposed over a 4.5 million square kilometre area.