Abstract
This thesis represents the first comprehensive compilation of geological, mineralogical, petrographical, and geochemical data concerning the Avontuur Deposit of the Kalahari Manganese Field. The deposit, which is the second-largest erosional remnant in the Kalahari Manganese Field, is located ~25 km to the north of the Main Kalahari Deposit, the sole source of manganese ore production for the Kalahari Manganese Field, yielding about 20 million metric tonnes annually.
The Avontuur Deposit, concealed beneath overlying younger strata, was identified in the 1970s by analysing aerial magnetic data and borehole intersections. Its magnetic signature arises from magnetic lithologies within the Hotazel Formation, including manganolutite beds, a diabase sill, and iron formations set within weakly magnetic lithologies. Samancor, now part of South32, and Aquila Resources made borehole data available for this study, including exploration assay data, and gave access to the drill cores sampled for mineralogical and petrographical analyses.
The structural complexity was investigated by constructing fence diagrams from the borehole data. The deposit shares a fundamental structural similarity with the Main Kalahari Deposit, characterised as a wide-open synclinal structure tilted towards the southwest. In the northern section of the deposit, a south-westward-dipping, bowl-shaped configuration is evident, while the southern part displays westward and slightly southward-dipping strata with less prominent open folding. The synclinal structure is intersected by a network of sub-parallel steep normal faults and transverse faults, resulting in a complex mosaic of horsts and grabens. In the northwestern corner of the Avontuur Deposit, over-thrusted blocks associated with the Black Ridge thrust fault system are identified.
An important distinction of the Avontuur Deposit is the thick diabase sill in the lower Hotazel Formation, which is unknown in the Main Kalahari Deposit. This study dated the sill as 2 422 +16/-7 Ma old (baddeleyite, U–Pb ID-TIMS), similar in age to the Ongeluk volcanic rocks. The volcanics must, however, predate the sill, likely indicating the previous misidentification of a sill sample as volcanic rock. The Ongeluk Formation and, by extension, the Hotazel Formation are probably older.
From an exploration perspective, the deposit presents four distinct settings. The highest exploration potential exists in the northwestern quadrant of the Gravenhage area, characterised by shallow, over-thrusted, sometimes duplication-thickened high-grade manganese ore, like
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the Wessels-type ore. The northern and eastern boundary sectors offer favourable conditions due to shallower depths and supergene upgrading, identical to the eastern Main Kalahari Deposit, while the southern parts show exploration promise, although less defined by available drilling. The deepest parts of the synclinal structure have lower exploration priority due to discontinuous, thin, low-grade ore beds at significant depths.
The stratigraphy of the deposit is like that of the Main Kalahari Deposit, containing three cycles of iron formation and manganolutite in the Hotazel Formation, conformably capped by the Mooidraai Formation carbonates. The three sedimentary manganese ore beds are positioned in systematic and cyclical gradational relationships with the host iron formation. These relationships manifest both at the top and bottom of each bed. Each ore bed displays a gradual transition sequence, starting from chert-rich banded iron formation, transitioning into hematite lutite devoid of chert banding, followed by manganolutite, returning to hematite lutite, and eventually reverting to chert-rich banded iron formation. This recurring sequence is consistent throughout the Kalahari Manganese Field.
The manganese ore beds exhibit an internal cyclic pattern characterised by symmetrically arranged mineralogical sub-zones related to the broader sedimentological cycles, with the highest manganese grades towards the centre of each bed. While similarities exist between the Avontuur Deposit and Main Kalahari Deposit due to their proximity, the mineral assemblages in their manganese ore beds exhibit significant differences. Braunite, a silicon-bearing Mn2+/Mn3+-oxide, dominates the Main Kalahari Deposit but is virtually absent in the Avontuur Deposit. In contrast, the Avontuur Deposit contains abundant jacobsite and Mn-silicates (friedelite, tephroite and gageite), with only traces of these minerals in the Main Kalahari Deposit. Both deposits contain manganese carbonates and hausmannite, particularly in the middle sub-zones of the lower and upper manganese ore beds.
These pronounced mineralogical differences necessitate revised paragenetic interpretation and indicate lateral differences in the physicochemical environments of deposition. These findings prompted a re-evaluation of the accepted back-arc depositional setting. A conceptual model is presented in which the Hotazel Formation was deposited in a continental rift within a stratified water column comprising an alkaline, possibly, but not necessarily oxygenated shallow marine water reservoir overlying an acidic and metal-bearing brine pool with a marked chemocline. Comparison to the Red Sea, specifically to the modern sedimentation in the Atlantis II Deep brine pool, supports the continental rift model.
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The implications of these findings are far-reaching – the presence of iron and manganese-rich sediments in the Hotazel Formation is not necessarily evidence for the presence of an oxygenated shallow marine ocean water reservoir in the earliest Paleoproterozoic. Indeed, insight gained from the composition of the Hotazel Formation would then only be of local relevance – and unlikely allow any inference about the global atmosphere-ocean system at ~2.45 Ga.