Sciency Thoughts: Kaapvaal Craton

Showing posts with label Kaapvaal Craton. Show all posts

Saturday, 6 July 2019

Microfossils from the Archean of South Africa.

The first Micro-organisms appeared some time during the Archean Eon (4000 to 2500 million years ago), and began to make changes to their environment, culminating in the Great Oxygenation Event, which is taken to mark the boundary between the Archean and the Proterozoic, 2500 million years ago, when oxygen levels built up to the point at which the Earth switched from having a weakly reducing atmosphere to a weakly oxidising one. These terms derive from the negative charge of electrons, that mean that when an atom accepts an extra electron it's overall charge is reduced, while oxygen tends to accept extra electrons, so atoms forming a bond with it can donate an electron and raise their charge, thus an atmosphere dominated by gasses which tend to donate electrons when they react with other substances is reducing, while one dominated by gasses which tend to accept electrons is oxidising). Fossils of Micro-organisms from this period are therefore highly sought after by palaeontologists studying early life, but to date very through have been discovered, with all known fossils from this period coming from just 15 sites around the world.

In a paper presented at the Astrobiology Science Convention in Washington DC on 24 June 2019, Andrea Corpolongo and Andrew Czaja of the Department of Geology at the University of Cincinnati, describe a new source of Archean microfossils from the Gamohaan Formation of the Kaapvaal Craton in South Africa.

The Gamohaan Formation is a ~110 m thick succession of granular and laminated limestones that forms part of the Neoarchaean Campbellrand Subgroup of the Transvaal Supergroup. The Gamohaan limestones contain ripple cross-lamination near their base, which is taken as indicative of an energetic shallow-water environment, while higher up it is dominated by laminated limestones with a high organic matter content, and structures associated with microbial matting. The formation is brecciated in places, and contains thin dolomite layers and iron sulphide nodules.

The microfossils were three-dimensionally preserved in black chert within a small, domal Stromatolite that formed in a shallow marine setting on a carbonate shelf system at 2.52 billion years ago (Corpolongo and Czaja cite this as being only 200 million years before the Great Oxygenation Event, though the exact date of this is unclear, with many authors putting it as late as 2400 million years ago. Stromatolites are formed by layers of micro-organisms forming biofilms on the surface of sediments in shallow water environments. Typically such films are buried by sediments periodically, with a new biofilm forming on the surface. Over time this builds up to a distinctive structure with layers of organic and inorganic material, a Stromatolite. The earliest Stromatolites appear in the fossil record of Earth about 3.5 billion years ago, though it has been argued that these could form abiotically (i.e. without micro-organisms).

(A)-(D) Photomicrographs depicting some of the large, morphologically unique microfossils observed in petrographic thin sections made from black chert collected in the Gamohaan Formation, South Africa. (E)-(F) Two of more than 200 coccoid/oblong microfossils found amidst morphologically similar microfossils in one petrographic thin section of Gamohaan Chert.

Tuesday, 2 January 2018

Assessing the potential for low-enthalpy geothermal energy generation in South Africa.

South Africa is the largest producer of greenhouse gasses on the African continent, and one of the largest producers worldwide. This is largely due to the country's dependence on coal to produce energy, with over 80% of electricity in South Africa being produced from coal burning power stations. Aware of the problems associated with greenhouse gas production, the nation has set itself the ambitious target of generating 40% of its energy needs from renewable sources, such as wind, solar, or hydroelectric plants, by 2030. This target is made harder by rising demand for electricity, with a rising population and many populations denied electricity under the former apartheid regime now wanting to be connected. Geothermal energy, energy generated by geological processes, is not usually considered as a viable option in South Africa, as the nation lies far from any active volcanic field, the most obvious source of geothermal energy, but the country could still potentially generate some of its power from low-energy geothermal sources, which rely on temperature gradients within deep rocks, rather than active heat eruption at the surface.

In a paper published in the South African Journal of Science on 29 December 2017, Taufeeq Dhansay of the South African Council for Geoscience and the Africa Earth Observatory Network at Nelson Mandela University, Chiedza Musekiwa, Thakane Ntholi, Luc Chevallier, and Doug Cole, also of the South African Council for Geoscience, and Maarten de Wit, also of the Africa Earth Observatory Network at Nelson Mandela University, present the results of a study into the viability of low enthalpy geothermal energy production in South Africa.

Low enthalpy geothermal energy production relies on temperature differences within geological formations rather than direct heating by magma or hydrothermal sources, and can produce useful amounts of electricity at temperature gradients as low as 40°C per km. This does not necessarily require the presence of plutonic intrusions from the Earth's interior, in many cases the presence of radiogenic elements such as uranium within the rock strata produces sufficient heating for low enthalpy geothermal plants to operate..

Schematic illustration of a binary fluid enhanced geothermal system related to surrounding fracture-controlled geological features. Dhansay et al. (2017).

Much of South Africa is underlain by the Kaapvaal Craton, an ancient subcontinental mass of lithospheric material that averages 40-50 km in thickness, but is 250 km deep at its thickest. This ancient mass at first seems to have rather poor potential for geothermal energy production of any kind, but the craton is in fact made up of several large blocks of more ancient material that were fused together during the Archean Eon (4.0-2.5 billion years ago). Thus, while the majority of the Kaapvaal Craton is highly thermally stable, something which has discouraged investigation of the potential for geothermal energy in the area, it is cross-cut by a number of ancient geological sutures, with rather different properties.

These sutures are the remains of ancient orogenic belts, bands of volcanic activity similar to that seem around the Pacific Rim today. The geological process associated with such structures tend to concentrate heavier elements within certain strata. This has led to the high concentrations of gold and other precious metals that drive the South African mineral industry, and also higher concentrations of radiogenic metals such as uranium, which at high enough densities can raise the temperatures of the deposits that host them.

Within the Kapvaal Craton these include the Cape Granite Suite (part of the Cape Fold Belt, which formed during the assemblage of the supercontinent of Gondwana) which contains uranium concentrations of up to 34 parts per million, the Namaqua-Natal Belt that formed during the assemblage of the supercontinent of Rodinia, and has uranium concentrations of between 10 and 54 parts per million, the Thabazimbi-Murchison Lineament, which marks the boundary between the ancient Kaapvaal Greenstone Belt and the slightly younger Limpopo Mobile Belt, with uranium concentrations of up to 30 parts per million, and traces of older Archean granite-gneisses around Mombela and Johannesburg, with uranium concentrations of 20-28 parts per million.

Overview of the major tectonic structures and zones across South Africa with the locations of significant earthquake focal mechanisms and inferred structures related to these events. Locations of the various data sources used within this study (e.g. hot springs and temperature measurement points) and high heat producing plutonic rocks are also highlighted. Note that the Namaqua-Natal Belt probably continues beneath the Cape Fold Belt as far as the offshore Agulhas Fracture Zone. Dhansay et al. (2017).

Dhansay et al. obtained measurements of temperature in South African rocks at depths of between 2 and 5 km from previous studies, as well as inferring deeper temperatures from hot springs at the surface, and the presence of deep faults from Earthquake data, and used this data to build up a map of temperature gradients across the country.

Graphical overview of the calculated geothermal gradients across South Africa. Map includes major tectonic contacts and structures, seismic activity and earthquake focal mechanisms and hot spring locations. Dhansay et al. (2017).

Using this data Dhansay et al. were able to identify a number of regions with geothermal temperature gradients high enough to support potential low-enthalpy geothermal energy generation, most notable along the the Colesberg, Thabazimbi-Murchison and Makonjwa Lineaments, ancient tectonic zones associated with early stages of the assemblage of the Kaapvaal Craton, which are also associated with a number of recent seismic events, and a number of hot springs, which may be due to reactivation of ancient faults with a northeast to southwest orientation.

(a) Potentially viable low-enthalpy geothermal investigation regions (1–5); based on (b) high heat producing plutonic rocks and overlying volcanosedimentary rocks; and (c) approximate groundwater yield. (d) Regional seismicity. Dhansay et al. (2017).

Sunday, 7 February 2016

Three still missing following collapse at South African gold mine.

Three people are still missing following a collapse at the Vantage Goldfields operated Makonjwaan Lily Gold Mine near Low's Creek in Mpumalanga at about 8.40 am local time on Friday 5 February 2015. The incident was reportedly caused by the failure of an underground pillar, which in turn led to a roof collapse, trapping 87 miners below ground. All of these workers have now been rescued successfully with only miner injuries, however the collapse also led to the formation of a sinkhole (surface hole created by the formation of a void bellow ground)which swallowed a mobile office building in which two women and a man were working, It is these three mine employees, understood to have been involved in issuing miners with safety equipment, that are still missing and are still being actively sought by rescue teams. All production at the mine has ceased until further notice, and workers are being offered trauma counseling.

Friends and relatives of the missing mineworkers waiting for news at the Makonjwaan Lily Gold Mine. AP.

The Makonjawaan Lily Gold Mine is a shallow pit mine with an estimated reserve of about 0.35 million ounces of gold and an annual production of about 35 000 ounces per year. It accesses an ore body within the Barberton Greenstone Belt, an ancient (more than 3 billion years old) section of basement rocks underlying parts of South Africa and Swaziland, which forms the eastern part of the Kaapvaal Craton, one of the ancient blocks from which the continent of Africa was assembled.

The approximate location of the Makonjwaan Lily Gold Mine. Google Maps.

Wednesday, 28 May 2014

The potential for geothermal energy in Limpopo Province, South Africa.

South Africa is the largest producer of carbon dioxide on the African continent, and as such has committed itself to ambitious emission reduction plans, aiming to cut production of the gas by 40% by 2050. At the same time the country is still attempting to expand electricity production rapidly, in order to provide energy to sections of the population which were denied it during the apartheid period. To this end it has recently commissioned the building of two new large-scale coal-fired power stations, which will double the countries energy production, but also sharply increase its emissions of carbon dioxide. In order to reduce emissions in the longer term, South Africa is planning the expansion of its solar, wind, hydroelectric and biofuel sectors, as well as the introduction of a carbon-tax (which opponents claim will undermine the country’s economy).

One option that has received little consideration in South Africa is geothermal energy, largely due to the presence of the Kaapvaal Craton beneath much of the country. This is an ancient, thermally inert, section of continental lithosphere which extends as much as 250-300 km beneath the surface, effectively acting as a thermal insulator and preventing heat from the Earth’s interior reaching the surface. However, while the keel of the Kaapvaal Craton prevents geothermal energy reaching the surface above it, it also tends to deflect rising heat into the lithosphere on either side of it,with the effect that these areas do have the potential for geothermal energy generation.

In a paper published in the South African Journal of Science on 27 March 2014, Taufeeq Dhansay of the Council for Geoscience in Polokwane and the Africa Earth Observatory Network at Nelson Mandela Metropolitan University in Port Elizabeth, Maarten de Wit, also of the African Earth Observatory Network at Nelson Mandela Metropolitan University and Anthony Patt of the International Institute for Applied Systems Analysis in Laxenburg, Austria and ETH University in Zürich, Switzerland, discuss the possibility for a geothermal energy plant near Makuleni Village in Limpopo Province, South Africa.

Geothermal potential map produced using available heat flow data created using inverse distance weighting within Quantum GIS. Major tectonic boundaries and geological features are highlighted. Dhansay et al. (2014).

Dhansay et al. look at a possible Enhanced Geothermal Systems plant, a system which exploits latent heat in deeply buried crustal rocks, unlike the systems build around hydrothermal or volcanic sites, which exploit heat direct from the Earth’s convective mantle. Such a plant would pump a water-based ‘working-fluid’ into a deep geothermal reservoir, using the heat of the rocks to raise the temperature of this fluid, which is then pumped back to the surface where it is passed through a heat exchange system transferring heat to a second (organic based) fluid with a lower boiling point. This second fluid flashes to steam, driving the turbines which produce electricity.

Schematic illustration of the hypothetical enhanced geothermal system plant in the Limpopo Province. The blue line indicates the influx well and the red line indicates the outflow/production wells. Also shown is the depth, geological profile, geothermal gradient and detailed binary production system. Dhansay et al. (2014).

In order to work efficiently such a project needs access to buried crustal rocks with a high content of decaying radioactive elements, thick enough to have a homologous mineral composition; typically a granite batholith (a volume of mantle derived magma in the lithosphere, which is slowly cooling allowing the formation of large mineral grains). Makuleni is located about 20 km north of the Kaapvaal Craton, on the boundary between the central and southern marginal belts of the Limpopo Belt. The surface rocks are from the 1.85 billion-year-old Soutpansberg Group, which comprises volcanic and sedimentary formations. The area has hot springs, producing water at the surface as warm as 70˚C, and the temperature is thought to be in the 100-200˚C range 2-3 km beneath the surface.

Overview of the Siloam Valley and Makuleni Village: the site of the hypothetical enhanced geothermal system. Dhansay et al. (2014).

Dhansay et al. calculate that an Enhanced Geothermal Systems plant located at Makuleni would have the potential to produce energy at a cost of 14US¢/KWh (fourteen US cents per kilowatt hour), somewhat more expensive than the current 714US¢/KWh achieved by coal in South Africa, but comparable with other forms of renewable energy. However it is likely that the introduction of a carbon tax in South Africa will substantially raise the cost of coal-fired generation, making such projects more economically viable within the foreseeable future.