Abstract
D. Phil.
The Kalahari Manganese Deposit (KMD) is the largest of five erosional relics of the Hotazel
Formation that are located near Kuruman in the Northern Cape Province of South Africa.
Manganese ores are exploited from the lowermost of three manganiferous beds that are
interbedded with banded iron-formation (BIF) and hematite lutite, that together constitute the
Hotazel Formation. Two major ore types have been delineated previously, viz. low grade
braunite lutite of the Mamatwan-type, and high grade oxidic ores of the Wessels-type, with
the latter spatially restricted to the northern KMD. Genesis of the ores was temporally
distinct, with the Mamatwan-type ore considered as a sedimentary-diagenetic precursor to the
hydrothermally altered Wessels-type ore.
Drill core samples from the Nchwaning-Gloria area of the northern KMD were analysed, with
the aim to better characterise ore genesis, with emphasis on ore alteration. A second part of
the study aimed at the application of mineralogical and geochemical information to aspects of
ore smelting for the production of Mn alloy for use in the steel industry. Methods employed
were drill core logging, X-ray diffraction (XRD), petrography, electron probe microanalysis
(EPMA), major and trace element (including REE) analysis (employing artificial neural
networks for evaluation of elemental trends), and stable isotope (C and O) analysis.
Significant effort was invested in method development for quantitative mineralogical modal
analysis using Rietveld refinement of XRD data.
The study shows that a number of ore types can be differentiated in the northern KMD on the
basis of mineral assemblage, grade, texture and geochemical characteristics. The ores are
broadly classified into least altered (LA), partially altered (PA) and advanced altered (AA)
types. The LA ores are low grade (<40 wt%Mn) Mn lutites, with dolomite-group carbonate a
significant component in addition to braunite. Serpentine is a ubiquitous trace mineral, and
boron is a characteristic trace element hosted predominantly by braunite in these ores. Ores of
the PA type comprise either braunite-hausmannite-calcite or hausmannite-calcite
assemblages, are fine to coarse grained, and display intermediate Mn grades (40-45 wt%Mn).
They exhibit a transitional trace element signature. Advanced altered ores may be classified
into five different types, based on mineral assemblages that contain hausmannite and/or
braunite as significant minerals. Carbonates occur predominantly in the form of calcite,
present in minor to trace proportions. Textures vary from fine to very coarse grained, and high
Mn grades (typically >45 wt%Mn), are recorded. Trace elements of significance include Zn,
associated with hausmannite, B, associated with massive braunite and a number of trace
minerals, and P, typically present in trace quantities of apatite.
In terms of ore genesis, mineralogical, geochemical and geological considerations suggest
that Mn (and Fe) originated from submarine hydrothermal vents, from which it travelled in
hydrothermal plumes, prior to rapid deposition ~2.2 Ga ago. Diagenesis followed soon after
deposition, through redox reactions involving organic matter and higher oxides of Mn to
produce the braunite-carbonate assemblage primarily observed in LA ores. The
carbonate:oxide ratio and nature of the carbonates varied slightly depending on fluctuations in
organic matter flux to the sediment, as well as marine bicarbonate concentrations.
Metamorphism, in relation to diagenesis and metasomatism, is poorly understood, but is
perceived to have resulted in serpentine formation, as observed in LA and PA ores.