Indicator role of rare and rare-earth elements of the Northwest manganese ore occurrence (South Africa) in the genetic model of supergene manganese deposits

The authors analyzed the content of a number of rare and rare-earth elements in the rocks of the Northwest manganese ore occurrence, confined to the Neoarchean dolomites of the Malmani series of the Transvaal Basin. 30 samples of manganese ores and host rocks were analyzed by ICP-MS and XRF methods. Average values of Post-Archean Australian Shale (PAAS) were used as the geochemical standard for data calculation and normalization. The concentrations of elements normalized on PAAS reveal the LREE anomaly in manganese ores. Positive Ce and negative Y–anomalies indicate the hydro-genetical type of sedimentation of ore minerals from manganese-bearing aqueous solutions. This is consistent with the ratios of the elements: Ce and Y SN/Ho SN, Ce and Nd, Fe – Mn – (Co + Ni + Cu) × 10. A clear correlation between the contents of rare-earth elements in manganese nodules and the underlying Malmani dolomites indicates their close genetic relationship. Negative Ce and positive Y–anomalies in manganese wad minerals indicate differences in the conditions of their formation. The manganese wad formed earlier than the rest of the ore with the active participation of microorganisms.

these deposits. They described potential sources of ore substances that formed manganese nodules in the lacustrine and oceanic water columns. Manganese nodules and crusts are often enriched in several rare (RE) and rare-earth elements (REE) [22,28,41]. In this regard, many authors drew attention to the variability of the contents of rare-earth and rare elements and used this feature to develop genetic schemes of ore formation [4,29,44], which help to determine the probable ore source. In particular, the RE and REE contents were taken into account to determine the genesis of oceanic ferromanganese nodules and crusts under various oceanic conditions [2,3,24].
Rare and rare-earth elements are often used to determine the conditions for the accumulation of a particular ore type, so an important factor in the accuracy of their diagnosis is the choice of appropriate standards. In the absence of modern and well-founded ideas about the formation of manganese ores in the Malmani area, the authors performed field and analytical work on the area of distribution and vertical sections of the Northwest mineralization (Fig.1). It was taken into account that valid reference standards of these elements are needed for a more reliable analysis of ore matter formed in supergenic zones of ancient manganese rocks. Therefore, in this paper, the authors tried to perform and present the results of the study of the geochemistry of rare and rare-earth elements in manganese ores and host carbonate rocks.
Research method. To complete this work, in 2018-2019 B.L.Pharoe collected manganese ore and host rocks samples at an ore occurrence in the Heifeld region of the Northwest province of South Africa. Samples were taken from a rectangular exploration network consisting of 70 pits located 200 m apart. Samples were collected during the first and second stages of exploration performed by General Nice Manganese Limited. Underlying Neoarchean manganese dolomites and overlying ores (manganese wad and manganese nodules and crusts) were sampled by trenching.
During the first stage of the study, 30 samples were taken from five exploration pits. XRF, ICP-MS, and Powder XRD were used to characterize the chemical and mineralogical composition of the ores. Rare elements were determined using XRF MagiX Fast spectrometer. Analyzes of 31 elements in 22 ore samples were performed on a Perki-nElmer NexiON 300X inductively coupled plasma quadrupole mass spectrometer at the University of Johannesburg (table). The data obtained show anomalous concentrations of rare-earth elements, which were then normalized to compositions of the Post-Archean Australian shale [39]. The obtained analytical results made it possible to calculate the correlation coefficients, the ratios between the contents of manganese, rare and rare-earth elements, and also to establish anomalous concentrations of elements -genetic markers of the source of ore manganese. 1 -area of the Northwest ore occurrence; 2 -quartz arenites, greywackes, shales, siltstones of the Cape Supergroup (P2); 3 -metasedimentary rocks, granites of the Kibaran orogeny (R2); 4 -Neoproterozoic Pan-African metamorphic orogenic (amphibolites, granulites, charnockites Neoproterozoic Pan-African orogenic metamorphic (amphibolites, granulites, charnockite (PR2); 5 -gneisses and granites, formed during the Ubendian orogeny (PR1 2 ); 6 -granite, gneisses, rhyolite, and mylonites of the Renoboth craton (PR1); 7 -granites, greenstone formation, carbonatites, granulites, metagabbro of the Limpopo belt (Ar2); 8 -granite, gneiss and greenstone complexes of the Zimbabwe craton (Ar1); 9 -granite, gneiss and greenstone complexes, carbonatites, komatiites of the Kaapvaal craton (Ar1) Due to the anomalous behavior of La in the oxidation zone, the traditional formula (2) for calculating the Ce anomaly values can lead to an overestimation of the Ce contents. Therefore, P.Joss and M.Lawrence [5] suggested using the following formula: Potential sources of manganese. One of the most important but challenging issues in the formation of manganese deposits is determining the primary source of Mn solutions. Most of the world manganese deposits are classified into four categories [1,26,35]: • Sedimentary manganese deposits are a chemogenic type of ore deposits formed as precipitation from a manganese-rich solution. The source of manganese may be either the result of its dissolution during weathering of terrigenous rocks or hydrothermal solutions accompanying island-arc volcanism in the marine environment.
• Magmatogenic (hydrothermal and contact metasomatic) -hydrothermal ore deposits, which were formed because of sedimentation of manganese-rich aqueous solutions in the vicinity of volcanic vents.
• Weathering crust, residual, infiltration, etc. deposits -near-surface accumulation of manganese oxyhydroxides and oxides as a result of the manganese-bearing rocks weathering and dissolution.
• Metamorphosed deposits are formed because of regional or contact metamorphism of sedimentary or magmatic ore clusters. Usually, such deposits arise as a consequence of metamorphism of pre-existing manganese deposits of any genesis.
South Africa is known to include the giant reserves of the Paleoproterozoic sedimentary manganese ore cluster in the Kalahari region and the karst Postmasburg manganese deposit. It is necessary to pay attention to a number of ideas about the genesis of these manganese deposits in South Africa [21,23,42]. For example, the ore in the Kalahari deposits consists mainly of kutnahorite and braunite, i.e. from the minerals that make up the Mamatwan-type ore, and, in the zone of its secondary alterations, known as the Wessel-type ore [15,16,19]. The proposed ore genetic models range from the formation of manganese ores through metasomatic alteration of iron-manganese formations to chemogenic sedimentary models or volcanogenic sedimentary and transgressive-regressive mechanisms of manganese sedimentation coming from a distal volcanic source and from the underlying Ongeluk formation.
The model of manganese ore genesis of recent years is the model proposed by N.Bix et al. in 2016 [7]. In this case, the example of ore formation of the Postmasburg deposit suggests karst leaching of manganese-rich dolomites and residual accumulation of manganese wad, which subsequently underwent diagenetic alterations. The final process causes the increase in Mn content [40].
Returning to the manganese ores of the Heifeld region, it should be noted that the genesis of these deposits has also been previously discussed [27,32,33]. In contrast to the Kalahari and Postmasburg manganese deposits, the ore minerals here are predominantly composed of manganese oxyhydroxides: romanechite, cryptomelane, pyrolusite, lithiophorite, and vernadite. The predominance of these mineral phases and the absence of any manganese carbonates and silicates are typical signs of near-surface conditions for the accumulation of ore minerals. They were formed as a result of weathering, leaching, dissolution, and residual enrichment of pre-existing manganese-bearing rocks. The primary manganesebearing rocks are mainly dolomites of the Malmani subgroup. These rocks contain up to 8 wt. % of MnO on average. SEM-EDS petrographic analysis of dolomites polished sections showed the presence of manganese carbonates and silicates in small inclusions. The primary minerals here could be kutnahorite, rhodochrosite, and braunite. However, only manganese oxides were noted during the XRD study of these samples. C.Roy [35,36] suggested that the initial presence of rhodochrosite as the primary manganese matrix, being under low temperature and intense oxidation, subsequently leads to the formation of pyrolusite and cryptomelane.
Genetic model of the Northwest manganese ore occurrence. To build a genetic model of the Northwest manganese ore occurrence, the results of analyses of the rare and rare-earth elements contents in ores, as well as petrographic, mineralogical and X-ray spectral analyses were used. The REE contents in manganese ore samples, normalized to the Post-Archean Australian shale, are shown in Fig.2. REESN in ores is characterized by relative enrichment in LREE.
MREESN are slightly elevated in content over the reference with a sharp decrease and alignment of the relative concentrations in the HREESN spectra (Fig.2). The REE spectra show a pronounced positive CeSN anomaly and a negative YSN anomaly. The presence of positive and negative Ce SN and Y SN anomalies, respectively, is a characteristic feature of hydrogenetic/hydrogenic mineral sediments [13,17,25]. These anomalies and ore enrichment with light rare-earth elements (LREE) are observed in samples with increased content of supergene minerals: cryptomelane and hollandite-romanechite.  It is important to note that the manganese wad sample (WAD), in contrast, is characterized by a positive Y and a negative Ce-anomalies, i.e., the composition of the manganese wad is closer to the composition of black coaly shale interlayers of marine origin in the underlying dolomite section. It can probably be concluded that the manganese wad was formed due to the diagenetic transformation of dolomites and coaly shale interlayers simultaneously with the formation of the Post-African I surface uplift and erosion, which preceded the formation of manganese nodules [31,33,43].
Using statistically substantiated genetic diagrams, it is possible to clearly distinguish ores of hydrogenous, diagenetic, and hydrothermal origin [6,7]. The results of analyzes of samples from the Northwest ore occurrence are plotted on a ternary plot with the following vertices: Mn, Fe, and Cu + Ni + Co, and fields of compositions of ores formed under lacustrine and continental shelf conditions. K.Nicholson et al. proposed [30] a binary diagram (Co + Ni) -(As + Cu + Mo + + Pb + V + Zn), which separates the compositions of supergene and hydrothermal ores. The reliability of the conclusions, obtained from these diagrams is supported by data on the distributions of rare and rare-earth elements. It is known that manganese oxides contain a relatively high amount of RE and REE adsorbed from aqueous solutions during the precipitation of minerals. Coherent behavior of rare-earth elements in natural environments allowed S.Sun, F.McDonough [37],    [17], and P.Josso et al. [5] to find additional criteria for the genetic classification of ore matter, taking into account the relationship between Ce and Y anomalies and Nd concentration (Fig.3).
It should be emphasized that the compositions of ores from the Northwest ore occurrence on the ternary plot according to E.Bonnati et al. [8] are dispersed along the Fe-Mn base, which indicates the predominance of these elements and the subordinate content of Co, Ni, and Cu (Fig. 4, a). Hydrothermal samples from the Kalahari manganese deposit are biased toward Mn because of their lower trace element content (Co + Cu + Ni). The K.Nicholson diagram (Fig.4, b) [30] shows the distribution of ore compositions from the Northwest ore occurrence in the field of supergene ores, these ore samples are hydrogenetic, in accordance with the diagrams [5,12]: the * SN SN Ce /Ce -YSN/HoSN ratio (Fig.3, a); Ce /Ce Nd (Fig.3,   g); Na-Mg (Fig.5, a); (Cu + Ni + Co)-Zr (Fig.5, b).
Based on the observed regularities in the distribution of rare-earth elements and ore compositions on genetic diagrams, the model of the Northwest ore occurrence can be determined as follows: Continental shelf

Meteoric waters
Transgressive-regressive cycle • First, at the platform stage, a predominantly dolomite sequence of Neoarchean Malmani dolomites (Ar2) was formed under marine basin conditions, enriched in manganese, magnesium, calcium, and iron cations. Oceanic aqueous solutions were probably enriched in ore from hydrothermal fluids that evaporated during the pre-Ventersdorp volcanic event (Ar1) (2.7 Ga ago). The ore material could be also supplied due to erosion of the nearby continental land (Fig.6, a).
• Later, sea-level rise occurred, which led to regressive-transgressive cycles, recorded in the formation of three-layer manganese ore bodies of the Khotazelskaya suite [18,29]. A transgressive episode of sea level movement has been documented in the Transvaal region, where the Malmani dolomites are deposited over alluvial quartz arenites of the Black Reef Formation [9,10,18]. During the transgressive cycle, the Malmani dolomites came onshore in the typical open Transvaal epeiric sea, which covered much of the South African landscape on the northern and western flanks of the Kaapvaal Craton (Fig.6, b) • In the Late Cretaceous and Middle Cenozoic, two uplifts and erosions of the Post-African land surface were recorded [11,31], which led to the denudation of the Malmani dolomites and coaly shales under the surface chemical weathering by meteoric waters. These erosional events are known as Post-African I and Post-African II uplift and erosion surfaces [38]. The exhumed manganese-bearing dolomites underwent a process of intense chemical weathering and dissolution, which led to the leaching of ore components (Fe, Mn, Ca, Mg, etc.) into the environment. The uplift of the territory gave rise to the formation of the Post-African I erosion surface about 77 Ma ago [43]. The formation of manganese wad began due to the destruction of coaly shales (Fig.7, a). This process was reflected in the sections of the ore deposit of the Northwest ore occurrence, where clay interlayers are visible in the manganese wad. XRD analysis revealed elevated contents of clay material and lithiophorite in the manganese ore. The REE spectrum (Fig.7, b) with a negative Ce-anomaly provides additional information on this subject. This indicates the early formation of a manganese wad on the Post-African I peneplain surface, preceding the formation of manganese nodules. The material from which the manganese wad is formed is black coaly shale of the lower Malmani subgroup.
• The formation of manganese nodules with ore matter formed due to leaching and dissolution of manganese-bearing dolomites of the Malmani subgroup in a lacustrine setting occurred during the second event -regional erosion (Fig.7, c). Ore components entered the overlying water column in the form of manganese colloids. This can be seen in the REE spectra, where the compositions of dolomite samples perfectly coincide with the compositions of manganese nodules (see Fig.2).
Conclusions. Indicator properties of rare and rare-earth elements in the studied manganese nodules and wad allowed restoring the sequence of formation of manganese ore deposits within the Northwest manganese ore occurrence. The data obtained on the basis of a representative volume of samples, analyzed by SEM-EDS, XRF, and ICP-MS, allowed to substantiate a genetic model of the multi-factorial process of manganese ore genesis in the Northwest province of South Africa.
In this regard, it becomes possible to use the indicator properties of rare and rare-earth elements to restore the genetic history of the new, not yet studied, manganese ore occurrences by analogy with the results obtained in this work. The properties of such indicator elements as Cu, Ni, Co, Pr, Ce, La, and highly charged Y and Zr, provide a reliable determination of manganese sources in the study of new promising deposits.
Supergene manganese deposits are characterized by the presence of manganese oxide phases, which are the products of secondary enrichment as a result of near-surface weathering and dissolution of primary manganese-bearing rocks. Like other supergene deposits, the Northwest manganese ore occurrence consists mainly of manganese oxides of the high valence state Mn 4+ . These minerals are known for their physical and chemical properties, which make it possible to extract relatively high concentrations of rare and rare-earth elements from aqueous solutions during precipitation. The peculiarity of the geochemical behavior of these elements can be used to determine the ore source.