JOHANNESBURG (miningweekly.com) – The University of the Witwatersrand School of Geosciences' Professor Rais Latypov and his team may have discovered how the country’s chromite layers were formed.
In a statement released on Friday, the School of Geosciences noted that the scientific community still does not possess a complete understanding of the processes that resulted in South Africa’s rich mineral wealth.
One such process pertains to the formation of chromitite layers hosted by layered intrusions – a major source of chromium.
Scientists tried to determine how layers of pure chromite form from magmas that are supposed to be rich in olivine, not chromite.
“It has been widely believed that magmas sourced from the mantle cannot directly crystallise chromite, as the mantle rocks that are being melted are rich in olivine and, therefore, these melts should crystallise olivine, not chromite,” noted Latypov, whose team published a paper in Nature Communications earlier this year.
“Together with . . . my colleagues, I have been trying . . . to find a mechanism that can explain the formation of these large chromite deposits in shallow crustal chambers but it turned out that we have been looking in the wrong place.”
Latypov and his team have been studying layers of chromite in South Africa’s Bushveld Complex, where over 80% of the global resources of platinum-bearing chromite deposits can be found.
They discovered that some basaltic magmas will form chromite after decompression as they rise up from the mantle through the crust towards the Earth’s surface. The reduction in pressure, as the magma rises up from the mantle to the crust, is key to the crystallisation process of the chromite.
“When these magmas arrive into a shallow magma chamber, sitting only a few kilometres below the surface of the Earth, they are already saturated in pure chromite and, on cooling, can crystallise layers of platinum-bearing massive chromitite.
The layers of platinum-bearing chromite that are formed through this process can be seen clearly in the ridges at the top of the Bushveld Complex, near Steelpoort, in Mpumalanga, which became exposed through erosion. Some of these layers can be several metres thick and extend for hundreds of kilometres.
“The Bushveld Complex covers an area of hundreds of square kilometres. It stretches from Steelpoort, in Mpumalanga, in the east, to the Pilanesberg, in the west; and from Bethal, Mpumalanga, in the south to north of Polokwane, in Limpopo,” said Latypov.
He and his team believe that the Bushveld chamber must have been operating as a flow-through system into which magmas were entering and depositing their chromite, before flowing out of the chamber and erupting as basalts through volcanoes, which have since eroded away.
“It seems that the reduction of lithostatic pressure during the transfer of mantle-derived melts towards the surface plays a vital role in the formation of magmas that produce planetary resources, without which modern human society cannot develop in a sustainable manner.”
Further research is being conducted on whether other magmatic deposits around the world, such as vanadium-bearing magnetite in layered intrusions, can also be related to lithostatic pressure reduction.