Figure 1. Two examples of a small suite of specimens collected on a mine visit in November 1995. The mineralogy of manganese (Mn) is very complex, especially in terms of oxides and other O-bearing species. These ores contain both carbonate (pink kutnohorite, aka kutnahorite) and oxide (e.g., fibrous black todorokite) minerals. Left: striking pink, white and black banded ore, with some brecciation on margins of the kutnohorite band. Sample 1760, collected by Gerry Ray in the Smartt Pit. Right: a thick todorokite band in hausmannite (Mn3O4), with mm-scale mottling indicative of formation by replacement of kutnohorite. Sample 1757, Middle Cut ore from the Mamatwan Pit, collected by Rod Thomas. Apparently an unusual sample, as todorokite here is known mostly in the Smartt Pit. A polished thin section is virtually opaque, a testament to the challenge of interpreting these rocks. The sample reacts in dilute hydrochloric acid, indicative of the survival of some of the original carbonate. The thin section reveals the todorokite, crystals in columnar aggregates, grown normal to the banding, while the hausmannite shows a distinct, euhedral tabular crystal habit (see Figs. 2,3). At the boundary of the massive and fibrous layers is a mm-scale interface dominated by pleochroic biotite mica and fine chalcedonic silica, both in sheaves or microfractures parallel to the layering (Figs 3,4). The Mamatwan mine commenced operation in 1963, owned by SAMANCOR.
"Rock of the Month #157, posted for July 2014" ---
from the Kalahari manganese field.
The Kalahari manganese field in Northern Cape Province, South Africa, hosts the world's largest manganese ore resources, beneath an area of at least 1,100 km2. The ores are the largest land-based sediment-hosted manganese resource known. The Mn deposits worked by numerous mines are restricted to the Proterozoic Hotazel Formation, in a sequence of volcanic rocks and chemical sediments, such as banded iron formation. The area is largely covered by Kalahari sand deposits, but Mn ores were first described in 1906, though exploitation did not commence until the 1940s, accelerating after the introduction of geophysical surveys for exploration in the early 1950s.
The main ore type in the basement is Mamatwan-type ore, (Cairncross and Dixon, 1995, pp.60-69). It is a diagenetic to low-grade metamorphic ore, with various oxide and carbonate ore minerals. The ores were upgraded some 1350 million years ago by a major hydrothermal event in the northwest part of the basin, producing some of the exceptional variety of Mn minerals found in the Kalahari basin. The so-called Wessels-type ores that resulted are coarser, and of higher Mn content. The known resources in the basin, quoted by Nel et al., 1986, include 13,204 million tonnes of Mamatwan-type ores (20-40 percent Mn) and 409 million tonnes of richer Wessels-type ores (30 to 44-plus percent Mn).
The mines of the Kalahari field are famous for spectacular specimens of many species, known to mineral collectors worldwide. These are well represented in the Mineralogical Record and related publications, such as Cairncross et al. (1999) and the beautifully illustrated sequel by Cairncross and Beukes (2013), and relevant sections in Cairncross and Dixon (1995). The geology of the mine and region (e.g., Nel et al., 1986) have been supplemented with detailed studies on the associated alteration and mineral paragenesis (e.g., Gutzmer and Beukes, 1995, 1996; Cairncross and Beukes, 2013). In total, roughly 150 mineral species are known from the Kalahari Mn field (Cairncross and Dixon, 1995, p.68, list 140 species; Cairncross and Beukes, 2013, list 155, including 18 minerals for which the Kalahari field is the type locality).
The Mamatwan mine is the largest open-pit Mn mine, in the southeast part of the Kalahari Mn field. Collectors know the mine largely for calcite, quartz pseudomorphs after calcite, and pyrite specimens from a central sulphide-rich fault zone. The lowermost of three Mn-enriched layers within banded iron formations is exploited. This lower layer is in turn subdivided into 3 layers, and the central layer, some 19.7 metres thick, with an Mn/Fe ratio >8 and a Mn content of 38%, is the ore. The high Mn/Fe ratio makes the ore suitable for production of high-Mn (>78% Mn) alloys. The high carbonate content makes the ore essentially self-fluxing, a further metallurgical benefit (Nels et al., 1986). The host rock is fine-grained sediment, manganolutite.
The ores at Mamatwan are composed of such manganese-bearing minerals as:
Figure 2. The following images, with tentative interpretations, show some of the detailed features of the outwardly black, fine-grained ores. Firstly, two photomicrographs of the polished thin section of sample 1757, in plane-polarized reflected light, nominal magnification 100X, long-axis field of view 0.9 mm. Left: textural variation in the fibrous todorokite layer, showing the todorokite fabric (lower right), a lens of fine granular oxide, and the fine silty host rock which contains relict kutnahorite and (seen here) coarse hausmannite. Right: interface between todorokite-rich layer and host rock. Note the tabular, tetragonal hausmannite crystals. Added transmitted, plane-polarized light spotlights the thin layer of brown biotite mica with traces of clear quartz. The monoclinic todorokite is named for a mine in Hokkaido, northern Japan. The fibrous texture is characteristic of this mineral.
Figure 3. Two photomicrographs of the polished thin section of sample 1757, in plane-polarized reflected light, nominal magnification 100X, long-axis field of view 0.9 mm. These are two views of the tabular hausmannite (Mn3O4), crystals, with the polarizer shifted by 90 ° between images, to highlight the strong bireflectance in this oxide. The small but well-formed hausmannite crystals overgrow the todorokite fibres.
Figure 4. Two photomicrographs of the polished thin section of sample 1757, in crossed-polarized transmitted light, nominal magnification 200X, long-axis field of view 0.45 mm. A trace of fibrous to spheroidal chalcedonic silica ("quartz") occurs with biotite mica, parallel to the sedimentary layering. There appears to have been some slip (shearing) between layers, generating small gashes infilled by silica. The fibrous texture of the silica is clear: a sensitive tint plate was added to the right-hand image to accentuate the texture and show the optical orientation of the quartz fibres.
Cairncross,B and Beukes,NJ (2013) The Kalahari Manganese Field: The Adventure Continues. Random House Struik / ASSORE, Johannesburg, 384pp.
Cairncross,B and Dixon,R (1995) Minerals of South Africa. Geol.Soc.S.Africa, 296pp.
Cairncross,B, Beukes,N and Gutzmer,J (1999) The Manganese Adventure: the South African Manganese Fields. Associated Ore and Metal Corporation Ltd, 250pp.
Gutzmer,J and Beukes,NJ (1995) Fault-controlled metasomatic alteration of early Proterozoic sedimentary manganese ores in the Kalahari manganese field, South Africa. Econ.Geol. 90, 823-844.
Gutzmer,J and Beukes,NJ (1996) Mineral paragenesis of the Kalahari manganese field, South Africa. Ore Geology Reviews 11, 405-428.
Nel,CJ, Beukes,NJ and De Villiers,JPR (1986) The Mamatwan manganese mine of the Kalahari manganese field. In `Mineral Deposits of Southern Africa' (Anhaeusser,CR and Maske,S editors), Geol.Soc.S.Africa, 2335pp., 963-978.
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