Manganese ore is mined in the Black Rock area of the Kalahari, in the Northern Cape Province. The majority of the production is intended for export, but a portion is supplied within the Group to Cato Ridge Works in KwaZulu-Natal, and to the domestic market.
Black Rock is approximately 80 kilometers north-west of the town of Kuruman. (Map)
In 1940, Assmang acquired a manganese ore outcrop on a small hillock known as Black Rock. Several large properties underlain by ore were subsequently found and acquired. Manganese ore mining operations were extended and today include 3 underground mining complexes:
- Gloria (commissioned in 1975) and producing medium grade semi-carbonate ore
- Nchwaning 2 and Nchwaning 3 (commissioned in 1981 and 2004 respectively) and producing various grades of high grade oxide ore.
Today the Kalahari Basin is considered to be the largest and richest manganese deposit in the world.
The manganese ores of the Kalahari Manganese field are contained within sediments of the Hotazel Formation of the Griqualand West Sequence, a subdivision of the Proterozoic Transvaal Supergroup. The average thickness of the Hotazel Formation is approximately 40 metres.
The manganese ore bodies exhibit a complex mineralogy and more than 200 mineral species have been identified to date. The hydrothermal upgrading has resulted in a zoning of the orebody with regard to fault positions.
Distal areas exhibit more original and low-grade kutnohorite and braunite assemblages, while areas immediately adjacent to faults exhibit a very high-grade hausmannite ore. The intermediate areas exhibit a very complex mineralogy, which includes bixbyite, braunite and jacobsite amongst a host of other manganese-bearing minerals.
A similar type of zoning also exists in the vertical sense. At the top and bottom contacts it is common to have high iron (Fe) and low manganese (Mn) contents while the reverse is true towards the centre of the seam. This vertical zoning has given rise to a mining practice where only the centre portion of the seam is being mined.
At the Gloria Mine the intensity of faulting is much less, which also explains the lower grade.
Two manganese seams are present. Seam I is up to 6 metres in thickness and approximately 400 metres underground at Nchwaning and 200 metres underground at Gloria.
Mining of Seam 2 started in 2014. A major upgrade of this shaft was recently completed and the underground infrastructure is being expanded in order to increase production of the Seam 2 grade. This second seam is situated above Seam I and is accessed via the Nchwaning 2 shaft.
In February 2014 the Black Rock Project was commissioned to optimise the output of high-grade manganese ore products. This project aims to ensure the mine’s sustainable longevity. Current mining operations produce +/- 3.2 Mt per annum. The expansion project will lead to increased output to 4.3 Mt. A total of R6.7bn will be invested over a period of 5 years to make this expansion possible. (Black Rock expansion project >>)