Kalahari Manganese Field Research Articles

A volcanic-exhalative origin for the world's largest (Kalahari) Manganese field

The ∼ 2220 Ma Kalahari Manganese field of South Africa is the world's largest Mn resource and a major producer. Current models for its origin rely on those developed for Phanerozoic deposits, invoking a submarine redox boundary and water movements across a continental shelf, precipitating Mn oxides from the sea. Here we report the discovery of major hydrothermal alteration in the thick andesitic volcanic pile beneath the Mn ore. This and other evidence shows that the Kalahari manganese is actually a volcanic exhalative deposit, analogous in some respects to those forming at present day mid-ocean ridges. Important differences in depth and oxygen supply account for the smaller area and high grade of the Kalahari Manganese Field, compared with the widespread but thinly developed modern ocean floor Mn deposits.

Effenbergerite, BaCu[Si4O10], a new mineral from the Kalahari Manganese Field, South Africa: description and crystal structure

AbstractEffenbergerite, ideally BaCu[Si4O10], structure determined by single crystal X-ray methods in space group P4/ncc, a = 7.442(2)Å, c = 16.133(5)Å, V = 893.50 Å3 Z = 4, is a new mineral from the Wessels mine, Kalahari Manganese Field, South Africa. It is associated with native copper, calcite, quartz and clinozoisite within pectolite veinlets, embedded in a matrix of braunite, sugilite and hausmannite. Effenbergerite occurs as transparent blue platelets with perfect cleavage parallel to {001} in sizes up to 8.0 × 8.0 × 0.1mm. It has a pale blue streak, subconchoidal fracture, a calculated density of 3.52gcm−3 and an estimated Mohs' hardness of 4–5. Effenbergerite is uniaxial negative with ω = 1.633(2), ε = 1.593(2), strongly pleochroic from intense blue (ω) to nearly colourless (ε). The strongest lines in the X-ray powder diffraction pattern (with refined lattice parameters a = 7.440(1)Å, c = 16.133(2)Å) are: (dobs/Iobs/hkl) (8.0624/100/002), (4.0325/39/004), (3.5443/29/104), (3.1998/44/114), (2.6892/21/006), (2.3943/41/116), (2.0169/34/008), (1.9466/22/108) and (1.4802/21/2.0.70).Effenbergerite is the natural analogue to synthetic BaCu[Si4O10], isotypic with SrCu[Si4O10] and CaCr[Si4O10] as well as with the minerals cuprorivaite, CaCu[Si4O10] and gillespite, BaFe[Si4O10]. The structure consists of silicate sheets [Si8O20]8− parallel (001) formed by corner-linkage of silicate 4-membered rings. The copper(II) atom is nearly planar 4-coordinated; the barium atom has a distorted cubelike environment of oxygen atoms. The mineral is named for Dr. Herta S. Effenberger of the University of Vienna, Austria.

Gageite from the Kalahari manganese field, South Africa

Gageite from the Kalahari manganese field, South Africa

Physicochemical environments for the formation of quartz-free manganese oxide ores from the early Proterozoic Hotazel Formation, Kalahari manganese field, South Africa

Physicochemical environments for the formation of quartz-free manganese oxide ores from the Hotazel Formation of the Kalahari manganese field, northern Cape Province, South Africa, are discussed quantitatively on the basis of the phase relations in the system Mn-Fe-Si-O and qualitatively in the system Ca-Mn-Fe-Si-O. The phase relations involve major oxide minerals of the ores, namely, braunite, braunite II, hausmannite, bixbyite, jacobsite, and hematite, and were constructed under quartz-deficient conditions, using existing and estimated thermody-namic data and mineral assemblages from the ores. Standard enthalpy delta HM 0 (sub f.298) and Gibbs free energy delta G 0 (sub f.298) values of braunite are estimated to be -1,011.9 + or - 5 and -937.3 + or - 5 kcal/ mole, respectively, on the basis of published experimental data and predicted entropy and heat capacity. The low-grade (diagenetic to very low grade metamorphic) Mamatwan-type ores, consisting mainly of braunite, hausmannite, jacobsite, hematite, kutnohorite, and calcite, are altered by hydrothermal solutions into the high-grade, high-temperature Wessels-type ores consisting of braunite II, bixbyite, hausmannite, hematite, calcite, and andradite with trace amounts of braunite. The peak temperature of formation of the high-grade ores appears not to have exceeded about 300 degrees to 400 degrees C. The ores are oxidized from oxygen fugacity levels defined by the assemblage hausmannite-jacobsite-hematite to that defined by the assemblage bixbyite-hausmannite, with increasing temperature. Under hydrothermal quartz-deficient conditions, the stability of braunite is enlarged to higher temperatures with increasing silica activity (a (sub SiO (sub 2 aq ) ) ). Because braunite is much less abundant in the high-grade Wessels-type ore than in the low-grade Mamatwan-type ore, the maximum a (sub SiO (sub 2 aq ) ) ) in the solutions could be estimated at 10 (super -4) to 10 (super -3) at 300 degrees to 400 degrees C from the stable assemblage bixbyite-hausmannite-braunite. Kutnohorite, and to some extent calcite, was a major source of Ca reacting with silica released from braunite to form braunite II and andradite.

Gem-Quality Friedelite from the Kalahari Manganese Field Near Kuruman, South Africa

T h e comparatively rare mineral friedelite was first reported in 1876 by Bertrand. This occurrence was in the manganese mine at Adervielle, in the Vallke du Luron of the Hautes Pyrenees, France. The mineral was named in honor of Charles Friedel, the French chemist and mineralogist. Since then, friedelite has been recorded as occurring sporadically in other manganese silicate deposits worldwide, including the Hartig mine near Pajsberg, Sweden (Lindstrom, 1891); the Sjo mine near Orebro, Sweden (Ingelstrom, 1891); the mines near Veitsch, Austria (Hoffmann and Slavik, 1909); the Buckwheat mine and Parker shaft, Franklin Furnace, Sussex County, New Jersey (Palache, 19 10); the Taylor mine, Franklin Furnace, and the mines at Sterling Hill, Sussex County, New Jersey (Palache, 1935); the Fe-Mn deposits at Dshumart and Kamya, Central Kazahkstan, U.S.S.R. (Kuyapova, 1960); and the deI posits of the Atasui region of Central Kazahlzstan (Kuyapova, 1968). It appears that only the occurrences at Franklin Furnace and Sterling Hill have produced gemquality friedelite (Sinkankas, 1959; Sinkankas, 1962; Arem, 1977; Webster, 1978). The suitability of this comparatively rare mineral for gem cutting has been mentioned by Sinkankas (1968)) Vargas (1969), and Vargas (1979). In November 1980, a little more than 100 years after its original discovery, a new occurrence of gem-quality friedelite was discovered, at the Kalahari manganese field in the Republic of South Africa. A brief description of the mineralization of this rare gem material and the circumstances that led to its discovery is presented here. MINERALIZATION The friedelite was found during shaft-sinking operations at the Middelplaats mine in the Kalahari manganese field, near Kuruman, northern Cape Province, Republic of South Africa (figure 1). It is associated with the lower manganese ore zone indicated in figure 2. The ore itself is a fine-grained mudstone into which the ore mineral, braunite, is incorporated in a highly disseminated state (Middelplaats Manganese Ltd., 1981). Koekemoer, a mineral dealer, relying on information supplied by the miners who removed the friedelite from the shaft, originally placed the occurrence in the upper portion of the 25-m-thick ore zone at a depth of about 395 m below the surface and a distance of about 75 m from the main shaft (personal communication, 1981). Subsequent follow-up investigations by Zaayman, the resident mine geologist, placed the friedelite locality at the base of the lower ore body near the old ventilation shaft. Unfortunately, the precise underground position could not be determined, inasmuch as that portion of the mine, once it was worked out, had been converted into maintenance workshops and all the rock faces covered by shotcrete, a cement aggregate applied at high pressure. Nevertheless, small fragments of friedelite were subsequently found in this vicinity (Zaayman, personal communication, 1982). On the basis of information received from the miners, it is estimated that originally two to three tons of low-grade manganese ore were collected, from which about 100 kg of friedelite (then thought to be a variety of rhodochrosite) and gan-