The Geology of South Africa

The geology of South Africa is extremely varied and economically important. Some rocks exposed are among the oldest in the world. Other young formations contain important fossil deposits that have made them world-famous among paleontologist. Some of the largest and richest mineral deposits occur in South Africa, which has led to the country being called a "treasure house" of minerals. Kalahari Manganese Field (Kuruman area) (RSA)  The History: To the west of Kuruman (Northern Cape Province) below the arid shrub savannah and the thick layer of red Kalahari Desert sand one finds the largest land-based sedimentary manganese deposits of the world. These deposits are at least 1100 square km in extent. Dr A. W. Rogers of the Geological Commission of the Cape Colony noted a small hill called Black Rock and reported that the base of the hill and the area around it consisted of large amounts of black manganese ore. Not much interest was given to "Black Rock" until a colleague of Rogers - Dr A.L. Hall developed interest in the area. In 1940 Associated Manganese Mines of SA acquired the manganese outcrop "Black Rock" and soon began to mine. After the Kuruman area was surveyed by Dr L.G. Boardman with geophysical techniques the mines Smartt, Hotazel and others where opened after 1954.1 The manganese deposits are confined to the Hotazel Formation of the Griqualand West Supergroup of the middle Protozoic age. The base of the Hotazel Formation consists of a bright-red banded iron-formation bed (varying from massive to fine-grained specularite and/or euhedral magnetite crystals) overlying volcanic glass breccias and lavas of the Ongeluk Formation. The banded iron-formation units grade into microcrystaline kutnahoritic ovoid-rich braunite rock. The Kutnahorite in this area is concentrated in ovoids, which represent partially compacted, early diagenetic concretions in hematite and braunite rock. The braunite rock bed of the lower section of the lowest of the three sedimentary cycles present in the Hotazel Formation, is between 5 and 45m thick. This is the major ore unit of the Kalahari manganese field with an manganese content varying between 20 and 48 weight per cent. The middle manganese-bearing unit (cycle 2) is a maximum of 2m thick and in not economically viable. The top manganese ore body was mined in previous years. It rarely exceeds 5m thickness. Grey hematitic and manganese minnesotaite rocks are found between the lower and the middle manganese ore bodies. About 1300 million years ago a widespread hydrothermal event occurred in the north-western portion of the Kalahari manganese field which reached temperatures up to 450°C in the Wessels, N'Chwaning and Black Rock Mines. This event decarbonated and desilicated portions of the Hotazel Formation to the north-west and thus upgraded the manganese content of the ore. Furthermore the hydrothermic event is of great significance for from the collectors point of view as a wide range of rare as well as unusual mineral combinations where produced Mamatwan-type ore is the major manganese ore in the Kalahari basin. It is primary a diagenetic to low-grade metamorphic ore, consisting of braunitic matrix and abundant primary carbonates and ovoids of Kutnahorite. Minor minerals also include hausmannite, cryptomelane, jacobsite and hematite. This type of ore is found in the Mamatwan, Middelplaats, Adams, Perth, Smartt, Gloria and Devon Mines.4 The manganese ore bodies in the north-western part of the Kalahari manganese field (Wessels, Black Rock and N'Chwaning Mines) have been termed Wessels-type ore. These ore bodies contrast markedly to the primary Mamatwan-type ore. The ore has been hydrothermically alter and metamorphosed. This resulted in a manganese ore with a coarser grain size with higher manganese content. This ore is braunite-rich and contains other major minerals such as braunite II, bixbyite, hausmannite, marokite and hematite together with minor amounts of calcite. The overall carbonate content of the Wessels-type ore is lower than that of the Mamatwan-type ore. Andradite and barite are common gangue minerals. Additionally minor minerals such as tephroite and rhodochrosite as well as aegirine (in the iron formation above the ore layors)are associated with this ore type. Most of the Wessels, Black Rock, N'Chwaning II and parts of N'Chwaning I ores are of this type.5 The Hotazel outlier is situated in a graben to the east of Black Rock and contains a very high grade ore (60 to 70 per cent average). Hausmannite with lesser amounts of other minerals and a very low carbonate component are the main constituents of the Hotazel supergrade ores. Although the ore is found typically in the Hotazel Mine, parts of the Langdon Annex and N'Chwaning I Mines also contain this ore-type.6 Minerals: The Kalahari manganese field in the Kuruman area must be one of the best known mineral locations in Southern Africa. In particular the N'Chwaning I and II, Wessels, Black Rock and Hotazel Mines have "brought to light" a wide range of minerals that are of interest to the collector. There are two main groups of minerals from the Kalahari manganese field: the first is associated with the ore and often consists of microscopically identifiable minerals; the second group consists of minerals found in cracks, fissures and pockets. A large portion of the minerals are calcium-manganese silicates with lesser amounts of sulphates, carbonates and borates. Potassium, sodium, magnesium and iron are common components of the minerals. Due to the variety of factors controlling the formation and the subsequent metamorphism of the ore deposits, many unusual minerals are found in the mines of this area. A feature that makes the Kalahari manganese field unique is the presence of a large number of rare mineral species. Of the approximately 150 minerals, 10 have to date only been found in the Kalahari manganese field and a further 26 are found at four or fewer other mineral localities worldwide.7 The N'Chwaning I Mine (actually phase 1 of the N'Chwaning Mine) is no longer in production. The mine had two principal mineral suites. The north-western areas, where mining began, were noted for the presence of exceptional rhodochrosite specimens associated with mainly manganite and drusy secondary quartz. The ore consisted here mainly of the Hotazel Supergroup. Later mining shifted to the southern areas. Here well crystallized hausmannite, barite, hydroxyapophyllite, Inesite, brucite and celestine where found and Rhodochrosite became a rarity. The N'Chwaning II Mine is separated from N'Chwaning I Mine by a 60m downfault to the west.8 The N'Chwaning II Mine is known for its sturmanite, brucite, hausmannite, gaudefroyite, hydroxyaophyllite, Thaumasite and andradite. It resembles the Wessels mine particular in the eastern and north-western areas.9 The Wessels Mine borders the N'Chwaning Mines and can be separated into three underground areas. The first, the central-southern area, is particularly rich in carbonates, primarily manganoan calcite, kutnahorite and rarely rhodochrosite. The second, the north-western area, in known for its sturmanite, gaudefroyite and sugilite. Finally the third area, the central eastern area, has sugilite, andradite, Inesite, hydroxyapophyllite, ruizite, barite and celestine.10 Although the Black Rock Mine is close to the N'Chwaning and Wessels Mines, most of the minerals of the latter mines are not to be found in Black Rock. The most common minerals are braunite II, bixbyite, chalcedony, calcite, andradite, hematite and barite. The steep dip of the rocks may have led to a greater loss in of hydrothermal fluids. The strata of the Wessels and N'Chwaning Mines is in contrast more horizontal which might thus have trapped the fluids from precipitating.11 The Hotazel Mine is known principally for its rhodochrosite specimens, which came mostly from the central area of the mine. This ore body was originally mined from an open pit but know mining is done underground.12 Minerals from the Kalahari Manganese Field 13 Buying/collecting: There is a mineral shops in the town Kuruman. It is opposite the "oog", the largest fresh water spring of the the southern hemisphere. The "oog" alone is worth a visit to Kuruman - it is full of fish and sea roses. In the mineral shop you will find any new materials coming form the Kuruman area. Sometimes it is also worthwhile contacting the local miners if they have any material. It is not possible to do any self-collecting in the mines near Kuruman. -------------------------------------------------------------------------------- Footnotes "Minerals of South Africa" by Bruce Cairncross and Roger Dixon: 1: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 60ff. 2: B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 60ff. 3: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 62. 4: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 63. 5: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 63. 6: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 64. 7: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 67. 8: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 64f. 9: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 65. 10: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 65. 11: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 65. 12: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 65. 13: Cairncross, B./Dixon, R.: in Minerals of South Africa, Singapore 1995, p. 68.