5.1. Diamond Drill Core SLT-015
Stratigraphic units in drill core SLT-015 have been distinguished based on hand specimen observations coupled with detailed petrographic, XRD, and microprobe analyses.
Clast-supported chert breccia. Clasts are predominantly composed of chert as interlocking aggregates. However, coarse (up to 2 mm), polyhedral quartz grains are also present as well as chalcedonic textures that typify void-filling processes (
Figure 3A). The matrix predominantly consists of microscopically laminated hematite with minor hydrous iron oxides. Laminations tend to wrap around quartz and chert grains.
Matrix-supported, vuggy breccia.
Vugs contain sérandite, microcline (var. adularia), witherite and quartz, as well as the rare species noélbensonite (BaMn
23+(Si
2O
7)(OH)
2∙H
2O), tokyoite (Ba
2Mn
3+(VO
4)
2(OH)) and the first discovery of As-rich tokyoite (see [
29] and
Figure 3B). In places, pseudomorphous replacement of sérandite by witherite (inset
Figure 3C) and apparent replacement of sérandite by noélbensonite and As-rich tokyoite (
Figure 3C) can be observed.
The matrix consists of fine-grained, manganese-bearing hematite and, to a lesser extent, aluminium-bearing braunite. Subordinate unidentified hydrous iron oxide phases are also present. This matrix is mostly finely laminated but botryoidal textures also exist, as well as hematite micro-concretions enveloped by manganese-bearing, hydrous iron oxide. Micro-analyses of hematite across the entire sampled section of drill core SLT-015 reveals varying compositions, particularly for Mn
2O
3 and Fe
2O
3, which tend to substitute for one another in the hematite structure (
Table 2).
Aegirine-augite-bearing, clast-supported breccia. Matrix-supported breccia grades back into clast-supported breccia. Micro-crystalline chert and coarse-grained, anhedral to subhedral, zoned quartz are present here. The matrix consists of platy hematite and hydrous iron oxide exhibiting a variety of colloform textures. A mineral of the aegirine-augite solid solution series occurs toward the top of the unit as extremely fine-grained aggregates in small vugs, and as fine-grained, acicular, aggregated grains growing outwards from the walls of quartz-filled vugs.
Aegirine-rich, matrix-supported breccia. Clast-supported breccia gives way to matrix-supported breccia with an abundance of vug-filling minerals. Chert is absent in this zone but coarse-grained quartz is present in vugs. Fine-grained (<50 m) aegirine aggregates line the boundaries of most vugs, with quartz generally forming the inner core and aegirine blades radiating outward from aggregations (
Figure 4A). Aegirine compositions from Wessels Mine in the KMF are very similar to those in drill core SLT-015 (
Table 3). In aegirine-rich samples, the matrix contains approximately 10 wt. % sodium. Given the amount of silicon in the matrix (ca. 36 wt. %), almost all of the sodium can be accounted for by 68% modal aegirine. In other vugs, microcline forms the outer zone with sérandite in the centre (
Figure 4B). However, the bulk of sérandite found in SLT-015 occurs as radial aggregates, often pseudomorphously replacing euhedral quartz grains (
Figure 4C). Aegirine-dominated vugs are rare. The size of these vugs appears to control the grain size of the aegirine within them (
Figure 4D). Coarse-grained aegirine appears to only occur in the centre of larger vugs. The presence of albite with inclusions of sérandite suggests precipitation of albite after sérandite (
Figure 4E). Coarse-grained (average 200 μm), anhedral albite also occurs in this unit, with single grains filling small voids and taking on the void shape. Noélbensonite is also present, predominantly occurring as very fine-grained (<20 µm) aggregated needles (
Figure 4F).
Vuggy manganese ore. Overlying the aegirine-rich, matrix-supported breccia unit lies approximately one meter of extremely vuggy manganese ore (
Figure 2E). In this unit, braunite and hematite are the dominant ore minerals, with vugs filled with various silicate (sérandite, feldspar, quartz), sulphate (barite) and carbonate (witherite, strontianite) phases. Fine-grained strontianite can be found as inclusions in barite vugs and veins and replacing witherite along grain boundaries. Sérandite also occurs in vugs and as a replacement of quartz. Minor modal amounts of aegirine occur as coarse-grained (up to 1 mm) vug-fills. One sample in this unit contains large, angular vugs that have cores of coarse-grained (up to 2 mm) calcite and rims of finer-grained albite (~500 μm) in a matrix of crystalline braunite (
Figure 5A and inset). Here, albite and natrolite can also be found as the only minerals occupying smaller vugs.
The highly porous matrix is characterised by unfilled vugs in the interstices between hematite plates as well as between and within braunite and partridgeite grains. Concentrically zoned manganese concretions identified in this unit have a partridgeite inner and outer zone and an intervening zone of braunite (
Figure 5B). These localised concretions occur in a matrix of crypto-platy hematite adjacent to massive, crystalline braunite and partridgeite. Where vugs consist of calcite and albite, some vug-fills are either partially or completely replaced by fine-grained braunite (
Figure 5C). The entire replacement process is seen in this image, from unaffected gangue phases to partial replacement and finally to complete replacement of gangue vug-fills by braunite.
Massive ferromanganese ore. Reflected light microscopy reveals that this unit is microscopically laminated in places and that vugs are still present but not abundant. Parts of this unit show coarse-grained crystalline hematite. However, the most common textural feature is that of extremely finely laminated, crypto-platy hematite and hollandite (
Figure 5D). Hollandite is a Ba–Mn oxide that is generally associated with supergene alteration but has also been found in contact-metamorphic manganese ores [
30]. In this ore type, monomineralic clasts of braunite can be found in a matrix of laminated crypto-platy hematite and hollandite (matrix grain size <10 µm), where laterally discontinuous laminae have draped around the clasts. Microplaty hematite appears to be replacing the braunite-rich clasts in this case. A bulk rock powder X-ray diffraction spectrum of the matrix of this unit confirms the presence of hollandite, hematite and braunite (
Figure 5E).
Fine-grained breccia. A 60 cm thick unit of fine-grained breccia overlies the massive ferro-manganiferous unit (
Figure 2F). What appear to be breccia clasts in hand specimen are shown to be mineral-filled vugs under a microscope. Quartz is one of the major constituents of the vugs, where it generally occurs as the central core surrounded by natrolite (
Figure 6A). Although Gottardi and Galli [
31] classify natrolite as a fibrous zeolite, Vlasov et al. [
32], for instance, point out that various crystal habits of the mineral exist in the Lovozero massif. These habits range from prismatic, to fibrous, to acicular and even to chalcedonic [
32]. Vlasov et al. [
32] also describe a fine-grained granular habit for natrolite, similar in appearance to that found in drill core SLT-015. In this study, it is also found as medium-grained, anhedral crystals in some vugs where it is the dominant phase. In this case, it can contain fine sérandite inclusions.
Many vugs display a distinct phase zonation where sheaf-like aggregates of sérandite grow inward into a core of quartz (
Figure 6B). The outer zone consists of fine-grained, aggregated natrolite and albite. Barium phases are present in this unit, forming part of the complex mineralogy of these vugs. The principal Ba-phase is barite which occurs as euhedral, blocky to columnar grains (
Figure 6C). Very fine-grained braunite (
Figure 6C) and hematite occurs in the matrix of this unit. Calcite occurs in some vugs but is not abundant. Another accessory phase that occurs in this unit is the manganese-bearing epidote piemontite which appears as fine-grained, reddish brown aggregates at vug boundaries in contact with the opaque matrix (
Figure 6D)
Piemontite typically occurs in rocks which are associated with low-grade metamorphism but it can also be produced by hydrothermal activity within manganese deposits [
34]. Piemontite, as well as Sr-piemontite, is also found in the Kalahari Manganese Field [
35]. Piemontite in this unit has low Fe content and high Mn and Al content (
Table 4). Other phases that are present in the microbreccia unit include minor pectolite and barytocalcite. These accessory phases occur only within vugs. Pectolite tends to occur as fine, blocky aggregates or laths that appear to grow from the vug boundaries toward the centre. The pectolite in this study is easily distinguishable from sérandite in the thin section as it is brown in plane-polarized light. This unit contains the highest abundance of Ba-rich species in drill core SLT-015. Witherite and barite occur in more than one unit in drill core SLT-015 but barytocalcite has only been found in the microbreccia unit.
Siliceous hematite lutite and laminated ferromanganese ore. Two units exist in the upper portion of the study interval in drill core SLT-015 (
Figure 2). The first unit consists of hematite lutite. Transmitted light microscopy (
Figure 7A) shows that fine-grained (<10 µm), aggregated mica is also present. Bulk rock XRD analysis reveals peaks at 2θ angles consistent with paragonite (
Figure 7C). However, the intensities of the respective peaks are not consistent with data from the literature. Average chemical analyses of the mineral are given in
Table 4. Sodium and aluminum concentrations are lower than typical paragonite [
36] but this could be explained by a non-stoichiometric structure for the paragonite in this study, with vacancies in the Na site. This also explains the irregular peak intensities for paragonite in SLT-015. Paragonite is typically associated with greenschist-, amphibolite- and blueschist-facies metamorphic rocks, occurring as fine-grained aggregates, but can also be found in fine-grained sedimentary rocks [
34]. Hematite contributes the bulk of the opaque matrix in this unit.
The second unit in the upper portion of SLT-015 consists of laminated ferromanganese ore with intercalated lenses of brick-red, hematite-rich (ferruginous) lutite. Reflected-light microscopy reveals similar features to those seen in hand specimen. The broad textural appearance of this unit consists of massive, fine-grained, crystalline braunite laminations which alternate with extremely fine-grained, hematite-bearing lutite (
Figure 7B). The very fine-grained (<10 µm) hematite accounts for the brick-red colour of the matrix. Some parts of this unit show alternating monomineralic laminations of fine-grained (<50 µm) hematite and braunite. Discrete silicate phases cannot be observed under the microscope here, but bulk-rock XRD analysis of the clay matrix from a representative sample reveals the presence of both hematite and paragonite.