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.

The giant Palaeoproterozoic manganese deposits of the Kalahari manganese field (KMF), Northern Cape Province, South Mrica, have been a world renowned resource of manganese ore for many decades. In recent years, the mineralogical composition, geochemistry and genesis of these deposits have been the objects of many geological investigations, yet their origin remains contentious up to the present day. A characteristic feature of the Kalahari deposits is the intimate association of manganese ore and iron-formation of the Superior-type, in the form of three discrete sedimentary cycles constituting the Hotazel Formation. This striking lithological association is an almost unique feature on a global scale. From that point of view, the present study is effectively the first attempt to shed light on the origin and post-depositional history of the Hotazel succession, using as prime focus the petrographic and geochemical characteristics ofthe host iron-formation. Petrographic and whole-rock geochemical information of iron-formation from the southern parts of the KMF, suggests that the Hotazel iron-formation is almost identical to other iron-formations of the world of similar age and petrological character. The rock exhibits essentially no high-grade metamorphic or low-temperature alteration effects. Mineralogically, it contains abundant chert, magnetite, subordinate amounts of silicate minerals (greenalite, minnesotaite, stilpnomelane) and appreciable concentrations of carbonate constituents in the form of coexisting calcite and ankerite. Such mineralogical composition is indicative of processes occurring in a diagenetic" to burial (up to very low-greenschist facies) metamorphic environment. Bulk-rock geochemical data point towards a simple composition with Si02, total Fe-oxide and CaO being the chief major oxide components. Whole-rock rare-earth element data suggest that the iron-formation precipitated from a water column with chemical signatures comparable to modern, shallow oceanic seawater. The virtual absence of positive Eu anomalies is a feature that compares well with similar data from Neoproterozoic, glaciogenic iron-formations of the Rapitan type, and suggests but only a dilute hydrothermal signal, poten!ially derived from distal submarine volcanic activity. Carbon and oxygen isotope data from iron-formation and Mn-bearing carbonates as well as overlying ferriferous limestone of the Mooidraai Formation, compare well with the literature. The former exhibit variable depletion relative to seawater in terms of both BC and 180, while the latter have signatures comparable to normal marine bicarbonate. Isotopic variations appear to be related to fluctuations in the amount of co-precipitated marine carbonate, in conjunction with processes of coupled organic matter oxidation - FelMn reduction in the diagenetic environment. Oxygen isotope data from quartz-magnetite-calcite triplets suggest that crystallisation took place under open-system conditions, with magnetite being the most susceptible phase in terms of fluid-rock isotopic exchange. Data also suggest that the calcite-magnetite pair may constitute a more reliable geothermometer than the quartz-magnetite one, mainly due to the interlinked diagenetic histories between calcite and magnetite. Iron-formation from the northern parts of the KMF can by categorised into three main classes, namely pristine, altered and oxidised. Pristine iron-formation is identical to the one seen in the southernmost parts of the field. Altered iron-formation corresponds to a carbonate-free derivative of intense oxidation and leaching processes at the expense ofpristine iron-formation, and contains almost exclusively binary quartz-hematite mixtures. The rock appears to have lost essentially its entire pre-existing carbonate-related components (i.e., Ca, Mg, Sr, most Mn and Ba) and displays residual enrichments in elements such as Cr, Th, V, Ni and Pb, which would have behaved as immobile constituents during low-temperature alteration. The low temperature origin of altered iron-formation is supported by oxygen isotope data from quartz-hematite pairs which indicate that isotopically light hematite would have derived from oxidation of magneftte and other ferroussilicate compounds in the presence of a low-temperature meteoric fluid, while quartz would have remained isotopically unchanged. Occasional occurrences of acmite-hematite assemblages suggest localised metasomatic processes related to the action ofNaCI-rich fluids at the expense of altered iron-formation. The conditions of acmite genesis are very poorly constrained due to the very broad stability limits of the mineral in environments ranging from magmatic to surface-related. Oxidised iron-formation constitutes a distinct rock-type and shares common attributes with both the pristine and the altered iron-formation. The rock contains hematite as an important constituent while the amount of magnetite is substantially reduced. With regard to carbonate nlinerals, calcite contents are clearly very low or absent, having being replaced in most instances by a single, Mgenriched, dolomite/ankerite:type species. Oxidised iron-formation contains somewhat higher amounts of iron and reduced amounts of Sr and Ba relative to pristine iron-formation, whereas enrichments in elements such as Ni, Th, Pb, Cr, and V are seen, similar to altered iron-formation. Oxidised iron-formation appears to have originated from processes of dissolution-mobilisationreprecipitation of solutes derived primarily from leaching that produced altered iron-formation. It is proposed that the Hotazel iron-formation and associated manganese deposits were formed as a result of episodic sea-level fluctuations in a stratified depositional environment that gradually evolved into a shallow carbonate platform. A critical parameter in the development of manganese sediment may include regional climatic patterns related to a glacial event (Makganyene diamictite) prior to deposition of the Hotazel strata. This suggestion draws parallels with processes that are believed to have led to the formation of worldwide iron-formations and associated manganese deposits subsequent to Neoproterozoic episodes of glaciation. Submarine volcanism related to the underlying Ongeluk lavas appears to have had very little (if any) metallogenic significance, while evidence for a sudden rise in the oxygen contents of the atmosphere and ambient waters is lacking. With regard to later alteration processes, combination of geological and geochemical data point towards the potential influence of surface weathering prior to deposition of rocks of the unconformably overlying Olifantshoek Supergroup, possibly coupled with fault- and/or thrustcontrolled fluid-flow and leaching of the Hotazel succession during post-Olifantshoek times.

In the Middelplaats mine area of the Kalahari manganese field, two drill holes (MP53 and MP54) intersected anomalously high-grade manganese ore sitting stratigraphically just above an igneous body (likely a dike or sill). Manganese ore located within approximate 5 meters of the contact with the underlying igneous rocks has been substantially metasomatically upgraded from 25 percent manganese, to over 40 percent whilst the dominant manganese species within the ore has been altered to hausmannite. This report demonstrates the metasomatic alteration is related to devolatilization (removal and/or remobilization of H₂O, CO₂ and CaO) due to contact metamorphism caused by the underlying igneous rocks. The Middelplaats mine is situated in the southwest corner of the Kalahari manganese field where the paleo basin shallows out and ends. Within the mine area, several stratigraphic units pinch out or are truncated by the side of the basin. This pinching out of lithological formations has led to the underlying Ongeluk Formation being in contact with the much younger units of the Hotazel Formation. Therefore, geochemical investigation into the nature and source of the igneous rocks was also undertaken to see if the rocks from the two drill holes were related to one another and/or the underlying Ongeluk Formation. Results of these geochemical studies have demonstrated that the Middelplaats igneous rocks (dolerites) from the two drill holes (MP53 and MP54) share a co-genetic source region. There is also reasonable geochemical evidence that the source region of the Middelplaats igneous rocks was substantially similar to the source region of the Ongeluk Formation. This may indicate that the source region of the Ongeluk Formation was reactivated at some later stage resulting in the emplacement of doleritic dikes or sills in the Middelplaats mine area. The Middelplaats igneous rocks were also found to have undergone a slight but pervasive potassic alteration; with most of the original plagioclase feldspar showing some level of replacement by a potassium enriched feldspar. Although no source for this potassic fluid was found, the devolatilization reaction within the manganese ore appears to have released some potassium into the surrounding rocks. This additional potassium may be responsible for some localized potassic alteration.

The Postmasburg Manganese Field (PMF), located in the Northern Cape Province of South Africa, once represented one of the largest sources of manganese ore worldwide. However, the discovery of the giant manganese deposits of the Kalahari Manganese Field (KMF) led to the gradual decline in manganese mining activity in the PMF. Two belts of manganese ore deposits have been distinguished in the PMF, namely the Western Belt of ferruginous manganese ores and the Eastern Belt of siliceous manganese ores. Prevailing models of ore formation in these two belts invoke karstification of manganese-rich dolomites and residual accumulation of manganese wad which later underwent diagenetic and low-grade metamorphic processes. For the most part, the role of hydrothermal processes in ore formation and metasomatic alteration is not addressed. The identification of an abundance of common and some rare Al-, Na-, K- and Ba-bearing minerals, particularly aegirine, albite, microcline, banalsite, sérandite-pectolite, paragonite and natrolite in the PMF ores studied in this thesis, is indicative of the influence of hydrothermal activity. Enrichments in Na, K and/or Ba in the ores are generally on a percentage level for the majority of samples analysed through bulk-rock techniques. The discovery of a Ba-Mn arsenate/vanadate similar to gamagarite may also indicate that the hydrothermal fluid affecting the ores was not only alkali-rich but also probably contained some As and V. The fluid was likely to be oxidized and alkaline in nature and is thought to have been a mature basinal brine. Various replacement textures, particularly of Na- and Krich minerals by Ba-bearing phases, suggest sequential deposition of gangue as well as oreminerals from the hydrothermal fluid, with Ba phases being deposited at a later stage. The stratigraphic variability of the studied ores and the deviation of their character from the pigeon-hole-type classification of ferruginous and siliceous ores in the literature, suggests that a re-evaluation of genetic models is warranted. The discovery of hydrothermallydeposited alkali-rich assemblages in the PMF and KMF provides grounding for further investigation into a possible regional-scale hydrothermal event at least re-constituting the ores. Some shortcomings in previous works include disregard for the highly variable nature of the PMF deposits, the effects of hydrothermal activity of the ores and the existence of stratigraphic discrepancies. This study provides a single, broad model for the development of all manganese deposits of the PMF. The source of metals is attributed to all formations that stratigraphically overly the Reivilo Formation of the Campbellrand Subgroup (including the Reivilo Formation itself). The main process by which metals are accumulated is attributed to karstification of the dolomites. The interaction of oxidized, alkaline brines with the ores is considered and the overlying Asbestos Hills Subgroup BIF is suggested as a potential source of alkali metals.