Petrographic and geochemical constraints on the origin and post-depositional history of the Hotazel iron-manganese deposits, Kalahari Manganese Field, South Africa

Abstract

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