Abstract
The Kalahari Manganese Field (KMF), located 60 km northwest of Kuruman, Northern Cape Province, South Africa, is the largest commercially viable land-based source of manganese. The KMF is composed of five erosional relics, namely the Hotazel, Langdon, Leinster, Avontuur and Main Kalahari Manganese (MKMD) deposits, with the Leinster Deposit being the most northerly of the five. The KMF deposits are associated with the Hotazel Formation of the Transvaal Supergroup (Griqualand West Sub-basin) which is characterized by three manganese ore horizons interlayered by four higher-type iron formations. The Hotazel Formation’s stratigraphic sequence within the Leinster deposit however has been disrupted by intrusive units heavily disrupted by intrusions (sills), sometimes and incorrectly referred to as “bostonites”.
These intrusions are also present in the Main Kalahari Manganese Deposit, which is currently being actively mined. However, the intrusions here appear as dikes, which only crosscut the Hotazel Formation without disrupting the stratigraphic sequence. In the Avontuur Deposit, the intrusions appear as split thin sills, although the lateral extent of the intrusions is unknown. Due to the degree the intrusive units have disrupted the normal stratigraphic sequence of the Hotazel Formation, it is important that they are properly classified and investigated, as no detailed studies of these intrusions have previously been carried out.
A geochemical and petrographic study was conducted in order to classify and characterize the intrusions associated with the Leinster Deposit The intrusive units of the Leinster Deposit was also related to other intrusions of the KMF. Optical and scanning electron microscopy and X-ray diffraction aided in determining the mineralogy and petrography of these intrusive rocks, while X-ray fluorescence and inductively coupled plasma mass spectrometry were utilized to determine the whole-rock geochemistry.
The intrusive units are dolerites composed of plagioclase and clinopyroxene with minor amounts of quartz. The intrusions show evidence of alteration and metamorphism seen by the presence of chlorite and pumpellyite. Geochemically the intrusions are tholeiitic basalt to andesitic basalts with chondrite-normalized rare earth element (REE) data showing light REE enrichment over heavy REE. Primitive mantle-normalized trace element data show negative anomalies of Ce, Nb and Sr, and positive anomalies of Pb and U.
Previous studies that have been done on the intrusions of the MKMD and Avontuur deposits of the KMF also concluded that the intrusions are tholeiitic basalts to andesitic basalts. The samples from the Avontuur Deposit showed overlapping primitive mantle-normalized trace element patterns, with similar positive Pb and U anomalies, negative Nb and Sr anomalies, and light REE over heavy REE enrichment, to the Leinster Deposit sills.
Abstract
Incompatible trace element ratio diagrams Th/La and Th/Nb vs La/Sm, Th/Yb vs Nb/Yb and Zr/Ti vs Nb/Y also show an overlap between the Avontuur and Leinster deposits which may suggest that these intrusions are related. A few samples of the Main Kalahari Deposit show similarities to the Leinster Deposit with regards to the normalized trace and rare earth element patterns of the Leinster Deposit. However, no overlap is shown with the incompatible trace element ratios. This may suggest that the intrusions of the Main Kalahari Deposit are not related to those of the Leinster Deposit.
It is currently unknown if the intrusive units associated with the Leinster Deposit represent one or multiple magmatic events or are even related to other magmatic events in the KMF or regionally. The first attempt to determine the timing of the intrusions within the Leinster Deposit was done in this study. Using heavy mineral extraction techniques, a representative number of samples were tested for the presence of minerals suitable for geochronology (e.g. titanite, baddeleyite, apatite, zircon) to constrain the age of these intrusions.
Titanite was identified in thin sections using automated scanning electron microscopy and was dated in-situ using laser ablation inductively coupled plasma mass spectrometry. The analysis was done on two samples which yielded discordant ages with lower intercept ages of 1771 ± 140 Ma and 1739 ± 310 Ma, respectively. The lower intercept ages analyzed possibly represent the timing of hydrothermal alteration rather than the age of emplacement of the intrusive units. However, the age does not correspond to any known ages of hydrothermal alteration events (i.e., Wessels, Mamatwan and Smartt events) affecting the Griqualand West sub-basin.
The intrusions affecting the Leinster Deposit were compared to six magmatic bodies found across South Africa and into Botswana to determine if they were related to other magmatic events regionally. Due to the discordant age associated with the Leinster Deposit a comparison was made using geochemical data to intrusions found in South Africa that are within a similar age bracket (ca. 2.4 Ga) to the Leinster Deposit. The comparison was made between the Leinster Deposit intrusions and the Mashishing dike swarms, Machadodorp Western Silverton Lavas, Ongeluk Formation extrusives, Molopo Farm Complex, Black Hills dike swarm, and the Puduhush extrusives. No other magmatic units with known ages in SA overlap with the age ranges found in this study (i.e., ~1.7 Ga). Little to no overlap was observed between the geochemistry of the six magmatic bodies to the geochemistry data of the Leinster Deposit sills. Further investigation would be required to determine if the intrusive units of the Leinster Deposit are related to any other magmatic bodies within South Africa.