Features of the mineral and chemical composition of the Northwest manganese ore occurrence in the Highveld region, South Africa

The Northwest manganese ore mineralisation is located at a relative distance from traditionally known manganese mining areas in a new manganese-bearing region (Highveld) in the Northwest Province, Republic of South Africa. The ore occurrence was studied on farms: Buchansvale 61 IQ, Weltevreden 517 JQ, Rhenosterhoek 343 JP and Kafferskraal 306 JP. The data obtained from studying the geology of the area pointed out to interests regarding the development criterias for search of similar ore mineralisations in the northwest region of South Africa. The ore occurs predominantly in the form of powdered manganese wad, manganese nodules and crusts, confined to the karstic structures of the upper section of the dolomites. X-ray powder diffraction (XRD), Scanning electron microscopy with energy dispersive link (SEM-EDS) and X-ray fluorescence were utilized to unveil the mineral and chemical composition of the ore samples. The present study therefore presents the results on both chemical and mineral composition of manganese ores, and their depth and longitudinal distribution. Karstic areas causing an increased local thickness of the ore body were identified. The geochemical and microspcopic study of the ores indicates their supergene nature. The main ore minerals includes cryptomelane, lithiophorite, purolusite, hollandite and romanechite associated with impurity components of Ba, Ce, Co, La, Cr, Zn and V.

It is known that manganese oxide mineralization in the Highveld region is associated with ancient erosional surfaces -African I and African II, which began to develop in the Late Cretaceous in the post-Gondwana period [4,[32][33][34].
The tectonic structure of the region is determined by the development of graben-induced depressions and eroded horsts [6]. The average surface relief is 1,550 m above sea level. The relief is formed by an ancient tertiary system of river paleodolines arranged in a fan-shaped pattern. The paleodolines are composed of alluvial deposits framing the proposed watershed [10,13]. The watershed runs between the rivers draining into the Indian and Atlantic Oceans, and extends from Johannesburg to Lichtenburg [8,22,28]. This palaeo-watershed is a relic of the weathered ancient post-Gondwana landmass of the Highveld region and consists of neo-Archean manganese-bearing carbonates, manganese nodules and wad, as well as outcrops of quartzite [5,14,15] (Fig.2). 1exploration pits; 2paleodolines; 3faults; 4siltstone (Р1); 5manganese-bearing dolomites of the Malmani series (Ar2 m ); 6quartzite of the retinue of the Black Reef (Ar1); 7siltstone with interbedded sandstones (Ar1); 8gold-bearing conglomerates with interbedded sandstones(Ar1); 9unsorted terrigenous sediments, conglomerates and sandstones (Ar1); 10elements of occurrence; 11karst funnel(P): depth 6-20 m; 12minesplaces of field development; 13sampling points; 14exploration pits; 15manganese nodules (N); 16powdered manganese wad with a intercalants of clays (K2?); 17manganese-bearing dolomite of the Malmani series (Ar2 m ) Tectono-magmatic events in the region are indicated by the presence of Proterozoic magmatic intrusions and, to a lesser extent, by the impact metamorphism of rocks caused by the Vredefort meteorite impact [13]. Uplifts forming graben structures are shown on the Rz-256 seismic profile which runs through the carbonate platform of the Malmani subgroup Ar2 m (Fig.3).
Research methodology. Geological exploration was carried out in the Highveld covering most of Mn mineralisation in the north-western province of South Africa. An exploration grid consisting of 70 exploration pits intersecting mineralized bodies was developed at the Buchansvale 61 IQ. At the first stage of the study, 30 samples from five exploration wells were studied (Fig.4). 20 thin sections and 15 polished sections were prepared for studying rocks and minerals under petrographic microscope both in plane polarized and reflected light. During material preparation, the ore samples were first washed with distilled water and dried in a furnace for approximately 24 hours. Parts of the sample were cut off with a diamond saw, glued onto slides, and smoothed using finer abrasive chips to a thickness of 30 microns (thin sections). Polished sections of ore samples were glued together in cups with a diameter of 22 mm and a height of 10 mm. They were additionally polished to the desired smoothness of the polished section surface. This method involved the use of a Michel -Levy interference color diagram.
Identification of mineral phases and phase ratios was performed on an XRD 6000 Shimadzu X-ray diffractometer at the Collective Use Center (CUC) of the Saint Petersburg Mining University. To identify the morphological and structural-chemical properties of the samples, optical and electron microscopy methods of 20 thin sections and 15 polished sections were used (Table 1). Petrographic studies were performed on an Olympus BX-51 microscope. Morphological features of samples were studied using scanning electron microscopes JSM-6460LV and JSM-7001F in the modes of secondary electrons and composite contrast. Chemical analyses  were performed using X-ray fluorescence spectrometry on an XRF MagiFast OCP-MS spectrometer. The electron microscopes were equipped with X-Act and X-MAX80 energy dispersive spectrometers, respectively. The analysis of the results was performed at the CUC of the Saint Petersburg Mining University.
Analyses of 31 impurity elements (Table 2)  between the contents of manganese, micro-and rare-earth metals were calculated, and anomalous concentrations of elementsgenetic markers for determining the source of ore manganesewere established ( Table 3).
where SN is the average composition of the post-Archean Australian shale PAAS [29]. Mineral composition of the ore. The ore forming Mn minerals in the area consist predominantly of a group of minerals with a common crystal structure: [A +(2+) (   Mn oxide phases occur mainly in the form of thin concentric shells around the cores or fragments of other rocks together with heterogeneous cementats (Fig.6, a, b). The formation of manganese oxide mineral phases indicates an early stage of ore formation, characterized by precipitation of manganese oxyhydroxides from ore solutions. The increased degree of oxidation and pH levels of the basinal solutions ascribed to weathering and dissolution of manganese-bearing dolomites led to prevalence of high valence state manganese oxides (Mn 4+ ) forming at the late stage of oregenesis: lithiophorite, cryptomelane, and romanechite [2,3]. The formation of minerals at this stage of ore genesis occurred mainly through direct precipitation from manganese-rich aqueous solutions. In particular, the development of romanechite over pyrolusite is observed, apparently caused by a change in the composition of mineral-forming solutions as a result of an increase in their pH (Fig.6, c, d, e).
The process of replacing previously formed mineral phases with manganese oxides is another characteristic feature of the ore. For example, detrital quartz grains in clastic rocks are replaced by cryptomelane. Lithiophorite occurs mainly in the form of thin films with acicular morphology around the pore spaces (Fig.6, f) [9,11,30].
To understand the spatial distribution of elements in mineral samples, their element mapping at the microstructural level was performed by scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDS). Figure 7 shows the profile (A-B) passing through various parts of the sample. Scanning in the direction of the strike along contacts of ore and nonmetallic minerals, as well as across the intramineral zones, allowed to establish the sequence in which the composition of ore solutions changed. The light areas of the ore (Fig.7, a) correspond to a high concentration of manganese and barium (Fig.7, b, d), while the darker ones are mainly characterized by a high intensity of aluminum, iron, and silica spectra (Fig.7, c, e, f ).The positive correlation between the intensities of the manganese and barium spectra reflects the predominance of romanechite. Areas with high concentrations of aluminum, silica, and sodium reflect the composition spectra of clay minerals. The low content of silica, which is evenly distributed over the intramineral zones, is due to the presence of detrital quartz residues and, possibly, chert grains in the manganese oxide cement. The high intensity of the silica spectrum near the end of the profile represents the silicate core of the sample.
Geochemical features of the ore occurrence. The contents of basic and rare-earth elements in ores are shown in Tables 1 and 2. The MnO content ranges from 0.8 to 19 (aver. 12 wt. %). The manganese concentration decreases in the uppermost layer of manganese nodules (TC_08, TC_09; TC_10; TC_11; TC_12; TC_13; TC_14), it has a higher SiO2 content, which is explained by the high content of grains and quartzite fragments in this layer. The upper layer consists mainly of fine -grained manganese nodules.
The content of metal oxides increases in this layer in the following sequence, in weight percent (wt.  Both strong positive and negative correlations are observed between these elements. The Mn content shows a strong positive correlation with the contents of La, Ce, Cu, Ba, Y, Co, Ni, and K, which probably indicates the sorption of these elements from aqueous ore solutions into the tunnel structure of Mn oxides during ore precipitation. A significant similarity is noted in the content of all trace elements, with the exception of Sr (Fig.8, а). Ore minerals may have precipitated from the same ore aqueous solutions.
Another striking feature is the coincidence of the elemental compositions of ore samples and samples of Neoarchean manganese-bearing dolomites of the Malmani subgroup (GNR DD and DOL) (Fig.8, b). In contrast, however, manganese wad represented by sample: WAD indicates a difference in composition of mineral-forming solutions (Fig.8, b).
The Co/Zn ratios in nodules are relatively close in values, varying in the range of 1.01-2.84 (average 2.00), and the Mn wad has a different valueabout 0.24. This indicates a higher degree of sorption of micro-and rare-earth elements in nodules than in wad.The Y/HoSN ratios in the ore are Conclusion. The northwestern manganese ore occurrence represents near-surface accumulation of secondary manganese oxides in weathering crust underlain by Neoarchean manganese-bearing dolomites of the Malmani series, Transvaal basin. The ore occurrence has the following features: 1. The ore mainly consists of manganese wad, nodules and crusts. Manganese wad is preserved in typical karst structures formed as a result of weathering and dissolution of the underlying dolomites. Manganese nodules are confined to the overlying lateritic alluvial part of the ore section.
2. The mineral composition of the ore occurrence is mainly characterized by romanechite, cryptomelane, hollandite, pyrolusite, lithiophorite and vernadite. Nonmetallic mineral phases include quartz, calcite, ilmenite and zircon inclusions. Manganese oxide minerals occur mainly in the form of thin concentric accretions around fragments of calcite, quartzite, sand, etc. These mineral associations are similar to those found in other hypergene manganese deposits [12,20,35].
3. The ore is enriched with a number of rare earth and trace elements: Ce, La, Ba, V, Ni, Cr, Cu and Zr. Most of them positively correlate with the manganese content. The mechanism of sorption of these elements from aqueous ore solutions into the tunnel structure of Mn oxides is assumed [18,19,26].