Summary:
South African manganese deposits are currently confined to the Early Proterozoic Transvaal Supergroup, Northern Cape Province. They can be grouped into two major categories:
1. Syngenetic carbonate-rich manganese deposits interbedded with the banded iron formations (BIF) of the Hotazel Formation, colloquially called the Kalahari Manganese Field.
2. Karst-fill type manganiferous material from residual Fe-rich manganese developed in dolomites of the Campbell Rand Subgroup in the Postmasburg Fe-Mn Fields.
The Kalahari Manganese Field is situated within a structural basin, known as the Dimoten Synclinorium, plunging at an angle of 3° to 8° to the north and northwest. It extends from Mamatwan Mine in the south for approximately 40 km to Wessels Mine in the north, with an east-west extent varying between 5 and 15 km.
Mamatwan Mine
The manganese mineralisation at Mamatwan is hosted by the Hotazel Formation of the Postmasburg Group (Transvaal Supergroup) and occurs as stratiform bodies interbedded with banded ironstone formation. This Formation is characterized by three manganese-rich horizons, called the Lower, Middle and Upper Bodies respectively, separated by BIF.
The lowermost of the three units, the Lower Body (LB), is the only unit exposed and currently mined at Mamatwan Open Pit Mine. The Middle Body is poorly developed, often ferruginized, and carries no economic value. The Upper Body (UB) is considered to be potentially economic, with manganese scattered throughout the unit. The Kalahari Manganese Field, with specific reference to the Mamatwan type ore (rich in braunite–kutnahorite mineral assemblages) represents approximately 97% of the total manganese resource in the region.
At Mamatwan Mine the LB is 37.5 m thick on average. It is subdivided into an uneconomic 19.5 m thick upper horizon (the Top Cut) containing ±31% Mn, with a Mn/Fe ratio of 5; a central horizon (the Manganese Cut) which is on average 19.7 m thick and contains ±37.5% Mn with a Mn/Fe ratio of 8.5; and a 6 m thick Basal horizon. These are subdivided into smaller seams (Zones), based on different mineralogical compositions. The Zones are visually distinguishable from each other and are identifiable by a combination of geological logging and chemical analysis. The M, C, and N Zones of the central horizon are high grade while the lower grade material of the Top Cut is represented by the V, W, X, Y and Z Zones. The Basal horizon is low grade and not considered to be potentially economic so is not part of the Mineral Resource.
The ore at Mamatwan consists of banded, very fine-grained braunite-kutnahorite lutite, containing concretionary ovoids, laminae and lenticles of Mn-calcite with which hausmannite is commonly associated. Subordinate amounts of hematite, jacobsite and rhodochrosite are also present. The relatively high Mn/Fe ratio of the economic manganese ore makes it suitable for the production of high manganese (76% Mn) alloys. The relatively high carbonate content of the ore, reflected in the CO2 content of 12 to 16%, makes it virtually self-fluxing.
Wessels Mine
Wessels Mine extracts high grade (above 47% Mn) manganese ore. The ore is typically coarse grained, shiny, massive or vuggy, with the principal minerals being hausmannite and braunite II (Wessels-type ore), as opposed to primary, lower grade braunite further to the south in the Kalahari Basin (Mamatwan-type ore). The current interpretation is that the Wessels-type ore was formed by hydrothermal alteration of primary Mamatwan-type ore and that the alteration took place along a system of post-Olifantshoek, north-south and east-west striking normal faults.