An overview of the Permian (Karoo) coal deposits of southern Africa

On the fundamental difference between coal rank and coal type

“…Time, climate, vegetation, and geological setting are inter-related and inter-dependent prerequisites for coal formation, and coal rank progresses irrespective of, and unaffected by, coal type and grade (Cairncross, 2001). Maceral composition is controlled by peat-forming vegetation type, as well as by the Eh and the pH conditions, and the microflora of the peat, which all are, in part, controlled by the climatic conditions and sedimentary substrate upon which the peat develops, as well as the sedimentary influx into the peat; all factors are interrelated rather than independent (Cook, 1975).…”

“…Superimposed burial metamorphism determined the current rank of the coals, with the easterly increase related to the fairly steep geothermal gradient towards the east. In many locations across southern Africa, the dolerite intrusions resulted in devolatilization of the coal (fairly extensive in the Witbank Coalfield, South Africa), or pockets of semi-anthracite (Zimbabwe) and anthracite (Namibia) (Cairncross, 2001), but these tends to be localized. Natural graphite occurs between the coal and the dolerite intrusions in some localities (Fig.…”

“…Beyond the stable cratonic shelf, which typified much of the Karoo Basin in South Africa, localized intra (such as the Waterberg Coalfield, or the Tanzania coalfields) or intercratonic (such as the Livingstonia Coalfield, Malawi) graben or half-graben basins enabled the accumulation of sediments and vegetation for longer periods of time in embayments (Cairncross, 2001;Falcon, 1989). These embayments resulted in more extensive sequences of coal seams, frequently with higher vitrinite contents, towards the mid-to-late-Permian.…”

“…Coal rank ranges from subbituminous to bituminous medium rank A-D in these embayments, with a predominance of medium rank C bituminous coals, and isolated coking coal deposits (such as Wankie Coalfield, Zimbabwe; Tshikondeni Coalfield, South Africa; Moatize Coalfield, Mozambique). Regional metamorphic controls on rank were a result of higher geothermal gradients, due to thinner crust, across the intracratonic belts and rifted basin coalfields (Cairncross, 2001). In addition, localized contact metamorphism caused significant localized changes in coal rank.…”

Assessment of Permian coalbed gas resources of the Karoo Basin Province, South Africa and Lesotho, 2016

“…As the Cape fold belt developed, thrust loading caused lithospheric subsidence, forming an asymmetric foreland basin that was deepest in the south and shallower to the north. In the southern foredeep, the Permian section is dominated by deep-water shales and sandstones, whereas to the north and northeast the Permian section contains fluvial-deltaic conglomerates, sandstones, shales, and coals (Cairncross, 2001;Hancox and Götz, 2014). The focus of this study is the potential for coalbed gas resources in the coal-bearing sequences of the Vryheid Formation of the Ecca Group ( fig.…”