Abstract
Ph.D.
The economically important sedimentary manganese deposits of the Paleoproterozoic Kalahari and
Postmasburg manganese fields, are situated in close geographic vicinity to each other in the
Griqualand West region of the Northern Cape Province, South Africa. This thesis describes aspects
of mineralogy, petrography and geochemistry of the manganese ores with the purpose to establish
genetic models for genesis and alteration of manganese ores of both manganese fields.
The Kalahari manganese field, situated some 60 km northwest of Kuruman, is the largest known
land-based manganese deposit. Manganese ores occur interbedded with iron-formations of the
Hotazel Formation of the Voelwater Subgroup of the Late Archean-Paleoproterozoic Transvaal
Supergroup. The sediments of the Voelwater Subgroup are preserved in five erosional relics, of
which the Kalahari manganese deposit is by far the largest and the only one of economic importance.
Two types of ore are mined, low-grade sedimentary Mamatwan-type ore and high-grade Wesselstype
ore. Mamatwan-type ore is represented by microcrystalline laminated braunite-lutite composed
of kutnahorite, Mn-calcite, braunite and hematite, modified by the occurrence of late diagenetic or
metamorphic hausmannite, partridgeite, manganite and calcite. Mamatwan-type ore contains up to
38 mass % Mn and constitutes about 97 % of the ore reserves in the Kalahari manganese deposit.
High-grade Wessels-type ore, with a manganese content of between 42 to 48 mass % Mn (on
average), constitutes about 3 % of the ore reserves. It occurs only in the northwestern part of the
main Kalahari deposit, and in small deposits at Hotazel and Langdon, in association with a system
of north-south striking normal faults. The Wessels alteration event is thought to be related to the
Kibaran orogenetic event (about 1.1 Ga). Fault zones are ferruginized and alongside faults
sedimentary Mamatwan-type ore has been hydrothermally upgraded to Wessels-type ore.
Metasomatic fronts are defined by changing mineral associations. These associations clearly
illustrate that decreasing degrees of alteration relate to increasing distance from the fluid feeders.
Areas of unaltered Mamatwan-type ore are preserved in the core of fault blocks. Wessels-type ore
consists mostly of hausmannite, bixbyite, braunite II and manganite and subordinate gangue minerals
such as clinochlore and andradite but the mineral assemblage associated with the Wessels alteration
event is unusually diverse. More than 100 minerals have been identified, amongst them 8 new
mineral species and an unusual, ferrimagnetic, Fe-rich variety of hausmannite.
Mass balance calculations illustrate that the upgrading of the Wessels-type manganese ore is a
consequence of leaching of CaO, MgO, CO 2, and Si02 from a low-grade Mamatwan-type precursor. This metasomatic process results in increasing secondary porosities, compaction of the orebody to
two thirds of its original thickness and consequently residual enrichment of manganese in the ores.
Three younger alteration events are observed in the Kalahari manganese deposit. These are only of
minor economic importance. Wallrock alteration associated with the Mamatwan alteration event is
characterized by reductive leaching of Fe and Mn around syntectonic veins and joints with pyritechalcopyrite-
carbonate mineralization. The alteration is explained by infiltration of epithermal
solutions that were introduced along veins or joints. The timing of the alteration event has tentatively
been placed into the Pre-Karoo era. The Smartt alteration event is associated with intensive faulthosted
brecciation and replacement of braunite and carbonates of the Mamatwan-type ore by
todorokite and manganomelane, a process that causes considerable upgrading of the manganese ore
next to a fault breccia at Mamatwan mine, and the formation of stratiform cross-fibre todorokite
veins at Smartt mine. The Smartt alteration event postdates the Mamatwan alteration event and has
tentatively been correlated with Pre-Kalahari groundwater circulation. Supergene alteration of the
ores took place in Kalahari and Post-Kalahari times. It is characterized by the occurrence of
cryptomelane, pyrolusite and other typically supergene manganese oxides along the suboutcrop of
the Hotazel Formation beneath the Cenozoic Kalahari Formation.
The Postmasburg manganese field is situated about 120 km to the south of the Kalahari manganese
field on the Maremane dome. Two arcuate belts of deposits extend from Postmasburg in the south
to Sishen in the north. Two major ore types are present. The ferruginous type of ore is composed
mainly of braunite, partridgeite and bixbyite and occurs along the centre of the Gamagara Ridge, or
Western belt. The siliceous type of ore consists of braunite, quartz and minor partridgeite and occurs
in small deposits along the Klipfontein Hills (or Eastern belt) and the northern and southern
extremities of the Gamagara Ridge.
Geological and geochemical evidence suggest that the manganese ores represent weakly
metamorphosed wad deposits that accumulated in karst depressions during a period of lateritic
weathering and karstification in a supergene, terrestrial environment during the Late
Paleoproterozoic. The dolomites of the Campbellrand Group of the Transvaal Supergroup are host
and source for the wad accumulations. Contrasting geological settings are suggested for the
accumulation of the siliceous and the ferruginous types of ore respectively. The former originated
as small pods and lenses of wad in chert breccia that accumulated in a karst cave system capped by the hematitized Manganore iron-formation of the Transvaal Supergroup. The cave system finally
collapsed and the hematitized iron-formation slumped into the sinkhole structures.
The ferruginous type of ore accumulated as mixed wad-clay sediment trapped in surficial sinkhole
depressions in the paleokarst surface. The orebodies are conformably overlain by the Doornfontein
hematite pebble conglomerate or aluminous shales belonging to the Gamagara Formation of the Late
Paleoproterozoic Olifantshoek Group. Well preserved karst laterite paleosol profiles, described from
the basal section of the Gamagara Formation, provide a strong argument for the terrestrial, supergene
origin of the manganese ores. The manganese ores in the Postmasburg manganese field were affected
by diagenesis and lower greenschist facies metamorphism. Metamorphism resulted in
recrystallization to braunite in the siliceous ores of the Eastern belt, and to massive or mosaic
textured braunite and idioblastic partridgeite in the ferruginous environment of the Western belt.
Secondary karstification and supergene weathering are evidence for renewed subaerial exposure of
the manganese ore and their host rocks. The metamorphic mineral assemblage is replaced by
abundant romanechite, lithiophorite and other supergene manganese oxides.
Comparison between the Kalahari- and the Postmasburg manganese field shows that sedimentary
manganese accumulation took place in entirely different depositional environments and owing to
different mechanisms. Their close geographic relationship appears to be coincidental. Apparent
similarities arise as a consequence of regional geological events that postdate the deposition of the
manganese ores. These similarities include the lower greenschist facies metamorphic overprint, an
event tentatively related to thrusting and crustal thickening during the Kheis orogenetic event, and
syn- to Post-Kalahari supergene alteration. The correlation of structurally controlled hydrothermal
alteration events in the Kalahari manganese field and the Postmasburg manganese field remains
difficult due to the absence of the necessary geochronological constraints.