Waterberg Coalfield Research Articles

Extractability and mineralogical evaluation of rare earth elements from Waterberg Coalfield run-of-mine and discard coal

This study explores the extraction of rare earth elements (REEs) from high-ash run-of-mine and discard coal sourced from the Waterberg Coalfield. Three distinct methods were employed: (1) ultrasonic-assisted caustic digestion; (2) direct acid leaching; and (3) ultrasonic-assisted caustic-acid leaching. Inductively coupled plasma mass spectrometry was utilized to quantify REEs in both the coals and resultant leachates. Leaching the coals with 40% NaOH at 80 °C, along with 40 kHz sonication, yielded a total rare earth element (TREE) recovery of less than 2%. Notable enrichment of REEs was observed in the run-of-mine and discard coal by 17% and 19%, respectively. Upon employing 7.5% HCl, a recovery of less than 11.0% for TREE was achieved in both coal samples. However, leaching the caustic digested coal samples with 7.5% HCl significantly enhanced the TREE recovery to 88.8% and 80.0% for run-of-mine and discard coal, respectively. X-ray diffraction analysis identified kaolinite and quartz as the predominant minerals. Scanning electron microscopy-energy dispersive microanalysis revealed monazite and xenotime as the REE-bearing minerals within the coal samples. These minerals were found either liberated, attached to, or encapsulated by the clay-quartz matrices. Further mineralogical assessments highlighted the increased REE concentrations in coals post-caustic digestion and subsequent recovery during acid leaching. This increase was attributed to the partial dissolution of kaolinite encapsulating the RE-phosphates and the digestion of REE-bearing minerals. Notably, undissolved REE-bearing elements in the caustic-acid-leached coal indicated the necessity of harsh leaching conditions to augment REE recovery from these coal samples.

Characterization and pre-concentration of rare earth elements in density fractionated samples from the Waterberg Coalfield, South Africa

The increased global demand for rare earth elements (REEs) from unconventional sources has led to increased interest and research in coal deposits. South Africa has extensive coal resources, but understanding of the concentration, mode of occurrence, and distribution of REEs in the coals and associated sediments is limited. This study aims to assess selected coal samples obtained from an exploration project underway in the Waterberg Coalfield (South Africa) to determine the concentration and mode of occurrence of REEs, and to gain insight into the behaviour of REEs during density fractionation. Density fractionation of coal is considered a pre-concentration method prior to REE extraction. Samples were selected from zones 8, 7, 4, and 3, spanning the Grootegeluk and Vryheid Formations of the Waterberg Coalfield. Density fractionation was conducted at relative densities (RD) of 1.3 to 1.8, providing float (F) and sink (S) products. Five parent and thirty density-fractionated coal samples were characterized using proximate analysis, petrography, X-ray diffraction (XRD), and the concentration of REEs determined using Inductively Coupled Plasma- Mass Spectrometry (ICP-MS) (after microwave digestion).The samples are classified as medium-rank C bituminous coals with variable maceral composition. The dominant minerals are kaolinite and quartz, occurring as both syngenetic and epigenetic phases. The total REY (REEs + yttrium) concentrations range from 45.1 to 389.2 ppm, higher than for world coals (except sample 8H F1.30, REY = 45.1 ppm), and generally higher than the average for Chinese coals. Float products obtained at 1.40 to 1.80 RD show greater variation of light REY (LREY) compared to medium REY (MREY) and heavy REY (HREY), whereas float fractions at 1.30 and sink fractions at 1.80 RD show no distinct enrichment patterns. Although the results suggest a relationship between the concentration of LREY with float fractions obtained between 1.40 and 1.80 RD, pre-concentration using density fractionation generally shows limited benefits for these coals. Ashing of the coals may be more effective in concentrating the REEs and further investigations are therefore required.

A critical review of initial Resource and Reserve tonnage estimation and reporting

This review was initially motivated by the author's own experience in attempting to reconcile run-of-mine production tons with the Coal Resource and Reserve estimation of mineable, in-situ tonnages. The primary scientific measurements and observations that are collected during exploration at the beginning of the value chain will have a profound effect on the future of the mining operation. Due diligence must be exercised during the planning of a new mine and during the life of an existing mine. The comments provided by the technical and economic assessment group of Venmyn Deloitte confirmed the need for this critical review. They found the reporting of Coal Resources in South Africa to be inconsistent. This was particularly problematic in the Waterberg Coalfield in Limpopo Province. In this coalfield there are two types of coal deposit present. The first is comprised of the thick intercalated, cyclic coal and shale/mudstone sequences of the Volksrust Formation. This overlies multiple coal seams within the Vryheid Formation, each of which is thick enough to be extracted as an individual raw coal unit. On the other hand, the interbedded coal and shale seams of the Volksrust Formation require beneficiation to separate the coal from the shale. The review examines practices and methods, investigates alternatives, provides checks and balances, and tests these against actual production reconciliations. In conclusion, the best estimates of the mineable, in-situ tonnage will be obtained from the air-dry raw material density. These estimates should be adjusted afterwards to allow for free moisture content. The adjustments are derived from reconciliation data. The greatest contributing factor to the over-estimation of Reserve tonnages is the moisture content of the raw material. This fluctuates significantly under varying conditions in situ, as well as upon exposure to the natural environment. The air-dried density of the raw material includes inherent (structurally bound) moisture within the matrix. It provides credible tonnage estimations of raw material available while also providing an indication of voids within the volume of material being assessed. The calculated solids percentage can be used to adjust the specific gravity, which is determined via the Archimedes principle. This will supply a representative estimate of the material to be mined.

Palynostratigraphic correlation of the Springbok Flats coalfield to other coal-bearing successions in the Karoo basins of southern Africa

Strata of the Springbok Flats, Ellisras and Tshipise (Soutpansberg-Pafuri) basins of South Africa are considered to have been deposited contemporaneously with those of the main Karoo Basin (MKB), but lithological differences complicate precise correlation. In this study, palynofloras from recently drilled boreholes in the Springbok Flats Basin (SFB) are used to correlate the Springbok Flats coal zones with other southern African coal-bearing successions. Permian palaeoenvironments of the SFB are reconstructed through a combination of palynology, sedimentology, and coal petrology. The lower coal zone (LCZ) palynoflora best correlates with Palynozone K4 in the MKB (equivalent to the Witbank No. 4 seam, Vryheid Formation) and can also be associated with Biozone D of the Waterberg Coalfield (Ellisras Basin), and the Vereeniging top seam in the New Vaal Colliery, northern Karoo Basin. The upper coal zone (UCZ) palynoflora of the SFB correlates closely with the Gwembe Formation (mid-Zambezi Basin, Zambia), Biozone E of the Waterberg Coalfield (Ellisras Basin), and Biozone KK 3 (Kalahari Karoo Basin, Botswana). A new radioisotopically constrained palynozonation for the MKB allows relative ages to be assigned to the coal zones of the SFB: Kungurian for the LCZ, and Roadian for the UCZ. Palynology is a useful tool in correlating these economically important strata across southern Africa.

Petrology and palynology of select coal samples from the Permian Waterberg Coalfield, South Africa

The petrology and palynology of coals from the Permian-aged Waterberg Coalfield, situated in the Limpopo Province of South Africa, are considered. The Waterberg Coalfield contains a significant portion of the country's coal reserves, but is still comparatively underexplored. Detailed maceral analyses, combined maceral-microlithotype analyses, mean random and maximum vitrinite reflectance analyses, etching, and palynology studies were undertaken on coals from four productive horizons. There is a general upward trend in vitrinite content through the sequence (15–59 vol% mineral matter free), dominated by collotelinite, along with a slight decrease in coal rank. The lower Vryheid Formation coal has a comparatively low mineral matter and high inertinite content, dominated by inertodetrinite. The upper Grootegeluk Formation samples contain above 35 vol% mineral matter, and liptinite contents ranging from 14 to 21 vol%. The use of blue-light fluorescence in conjunction with incident light (single-scan method) revealed that liptinite is intimately associated with inertodetrinite and mineral matter, specifically clays and occasionally pyrite. It is evident that the liptinite maceral content has previously been under-reported in the Waterberg coals. The palynofloras correlate with the well-studied Witbank Coalfield. The petrography, along with the palynology, essentially confirm the palaeoenvironment of the Karoo Basin as progressing from a cooler, more oxidising environment, to a wetter, seasonal environment.

Performance evaluation of microbial desulphurization of waterberg steam coal as a pre-combustion technique

The current study evaluates pre-combustion microbial desulphurization of medium – sulphur type coal containing total sulphur (1.45 wt. %) and pyritic sulphur (≥ 0.51 wt. %) in a laboratory scale. Coal samples used in the current study were supplied by one of South African Power Utility, Eskom power station as received from the nearby Waterberg coalfield in Limpopo in South Africa... After a stipulated hydraulic retention time (HRT), the coal samples were filtered, washed, dried and analyzed for total sulphur, ash content and calorific values. The calorific value of the coal significantly improved from 20.42 MJ/Kg to 24.16 MJ/Kg on the 5th day. The process removed up to 72 % of the total sulphur content in the coal samples at an HRT of day 9th. Furthermore, ash content reduced by 16 %. This study provides a novel breakthrough of an alternative microbial desulphurization process of Waterberg steam coal as a pre-treatment technique in order to meet stringent environmental emissions standards for SO2.

Waterberg coal characteristics and SO2 minimum emissions standards in South African power plants

Key characteristics of coal samples from the supply stock to the newly commissioned South African National Power Utility's (Eskom's) Medupi Power Station – which receives its supply coal from the Waterberg coalfield in Lephalale (Limpopo Province, South Africa) – were evaluated. Conventional coal characterisation such as proximate and ultimate analysis as well as determination of sulphur forms in coal samples were carried out following the ASTM and ISO standards. Coal was classified as medium sulphur coal when the sulphur content was detected in the range 1.15–1.49 wt.% with pyritic sulphur (≥0.51 wt.%) and organic sulphur (≥0.49 wt.%) accounted for the bulk of the total sulphur in coal. Maceral analyses of coal showed that vitrinite was the dominant maceral (up to 51.8 vol.%), whereas inertinite, liptinite, reactive semifusinite and visible minerals occurred in proportions of 22.6 vol.%, 2.9 vol.%, 5.3 vol.% and 17.5 vol.%, respectively. Theoretical calculations were developed and used to predict the resultant SO2 emissions from the combustion of the Waterberg coal in a typical power plant. The sulphur content requirements to comply with the minimum emissions standards of 3500 mg/Nm3 and 500 mg/Nm3 were found to be ≤1.37 wt.% and ≤0.20 wt.%, respectively.

ACID MINE DRAINAGE AND ITS POTENTIAL IMPACT ON THE WATER RESOURCES IN THE WATERBERG COALFIELD, SOUTH AFRICA

South Africa currently ranks ninth in the world for proven coal reserves that have been estimated to last for over 200 years. Coal constitutes 77% of the primary energy needs in the country, with the Waterberg Coalfield estimated to hold about 40% of the remaining South African coal reserves. Coal deposits in the study area are largely composed of shales, mudstones, siltstones and sandstones which host coal-containing clay minerals, quartz, carbonates and sulphides; the most abundant sulphide mineral is pyrite. Once mining operations commence, the sulphide minerals are exposed to atmospheric oxygen which promotes degradation of hydrous sulphides into iron and sulphuric acid causing acid mine drainage (AMD). Acid-base accounting (ABA) was used to determine the balance between the acid-producing potential (AP) and neutralising potential (NP). From the analysis the net neutralising potential (NNP) was determined to classify a sample as potentially acid-producing or non-acid producing. X-ray diffraction (XRD) and X-ray fluorescence (XRF) were used as other sources of characterisation for geological samples where the mineralogical and whole-rock analyses are determined. Acid mine drainage does not only result in the generation of acid but is accompanied by decreased pH and increased values of specific conductance, dissolved metals and sulphate. Inductively coupled plasma analysis was applied to determine the release of the heavy metals as well other constituents which can be detrimental to the environment. Samples were collected from the interburden and overburden as well as the coal seams. Acid-base accounting results show that the interburden and coal samples have the higher risk of producing acid upon oxidation when compared with overburden samples which have a higher concentration of neutralising minerals that can neutralise the acid produced through sulphide minerals as oxidation takes place. Analysis of kinetic test results indicated the long-term behaviour of different samples, with the electrical conductivity (EC) and pH changing over time. Samples with lower pH continued to produce more sulphate, while calcium continued to increase until it was depleted from the samples. The water demand will increase as mining continues in the area, with inter-catchment transfers identified to overcome local water-scarcity issues. Acid mine drainage poses a major threat to water resources, both groundwater and surface water.

THE STRUCTURE OF THE KAROO-AGE ELLISRAS BASIN IN LIMPOPO PROVINCE, SOUTH AFRICA, IN THE LIGHT OF NEW AIRBORNE GEOPHYSICAL DATA

Research Article| December 01, 2014 THE STRUCTURE OF THE KAROO-AGE ELLISRAS BASIN IN LIMPOPO PROVINCE, SOUTH AFRICA, IN THE LIGHT OF NEW AIRBORNE GEOPHYSICAL DATA C.J.S. FOURIE; C.J.S. FOURIE Tswane University of Technology (TUT), Environmental, Water and Earth Sciences Department, Private Bag X680, Pretoria, 0001, South Africa, e-mail: fouriecjs@tut.ac.za Search for other works by this author on: GSW Google Scholar G. HENRY; G. HENRY Centre for Mining Innovation (CMI), Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa, e-mail: ghenry@csir.co.za Search for other works by this author on: GSW Google Scholar L.P. MARÉ L.P. MARÉ Council for Geoscience, Geophysics Division, Private Bag X112, Pretoria, South Africa, e-mail: leoniem@geoscience.org.za Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2014) 117 (2): 193–210. https://doi.org/10.2113/gssajg.117.2.193 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C.J.S. FOURIE, G. HENRY, L.P. MARÉ; THE STRUCTURE OF THE KAROO-AGE ELLISRAS BASIN IN LIMPOPO PROVINCE, SOUTH AFRICA, IN THE LIGHT OF NEW AIRBORNE GEOPHYSICAL DATA. South African Journal of Geology 2014;; 117 (2): 193–210. doi: https://doi.org/10.2113/gssajg.117.2.193 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The Coaltech Research Association funded an airborne magnetic and radiometric survey over the Karoo-age Ellisras Basin in the Northern Province of South Africa in 2008. The Waterberg Coalfield, which is destined to become the major source of energy in South Africa in the future, is situated in this sedimentary basin. Interpretation of the processed geophysical datasets has added significantly to our understanding of the structure of the Ellisras Basin, in addition to providing additional constraints on lithological and structural mapping. The filtered magnetic and ternary radiometric images have yielded abundant information that will be important in future mapping. Two-dimensional modelling of the magnetic data has provided a new confirmation of the half-graben model for the structure of the Ellisras Basin. The Zoetfontein Fault Zone (ZFZ), which forms the northern boundary of the basin against Archaean Limpopo Belt rocks, is block-faulted and generally steeply dipping. The thickest development of the Karoo Supergroup rocks is against this fault, attaining up to 550 m. The sediment thickness decreases gradually to the south, indicating an asymmetric basin fill. The present southern boundary of the basin is formed by the Eenzaamheid Fault Zone, south of which the ~2.0G a Waterberg Group rocks are developed.Despite its economic importance, the Ellisras Basin has not been well-studied, and there is a need to correct this in the near future as South Africa becomes more reliant on energy from the Waterberg Coalfield. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Distribution of selected trace elements in density fractionated Waterberg coals from South Africa

Abstract The Waterberg Coalfield, located in the Limpopo Province (north-western area) of South Africa, contains a significant proportion of the South African coal resource. This Permian coalfield typically contains high vitrinite, high ash coals in the Kungurian Grootegeluk Formation, and high inertinite, low ash coals in the Artinskian Vryheid Formation. Four run-of-mine coals and density fractioned samples were analysed to determine their trace element content. The concentrations of most trace elements in the run-of-mine coals exceed the global averages and certain global ranges, and generally exceed values reported for other South African coalfields. Specifically, Hg concentrations are high to very high in all the coals (up to 2.43 ppm in a sample from the Vryheid Formation), Cd and Se concentrations are comparable to or lower than global averages, and As is very low in the Vryheid Formation (1.57 ppm). A sample from the Grootegeluk Formation is enriched in siderophile elements, and a sample from the Vryheid Formation is depleted in chalcophile elements. Studies of the density fractionated samples indicate that Cd exhibits an organic affinity in the Grootegeluk Formation samples, and possibly a pyritic (or sulphide) affinity in the Vryheid Formation sample. Selenium has a greater affinity for the middle, clay-rich density fractions in all samples. Arsenic reports to the mineral-rich sink fractions, specifically the fractions enriched in pyrite. Mercury reports to both the float and sink fractions, indicating an organic and inorganic affinity in these coals, although there is more enrichment in the sink fractions in the Grootegeluk Formation samples, and a definite organic correlation in the Vryheid Formation sample.

Investigation of potential water quality and quantity impacts associated with mining of the shallow Waterberg coal reserves, west of the Daarby Fault, Limpopo Province, South Africa

From South African and international experience, it is known that coal mining has a pronounced impact on surface and groundwater quality and quantity. The influx of water may be as low as 1% of rainfall for underground mines to as much as 20% for opencast mines. Such differences may influence the quantity and quality of surface water and groundwater resources at the local scale and further afield. The Waterberg coal reserves represent the only area with proven coal reserves which are still available for development in South Africa. These reserves are targeted for large-scale mining in the near future, and are situated in a relatively dry part of South Africa. In view of the low rainfall and limited surface water resources, the necessary level of safeguard measures to ensure the quantity and quality of existing water resources is unclear. Experience from other areas cannot necessarily be extrapolated directly. A scoping level study was performed to consolidate the existing information on the geohydrology and pre-mining water quantity and quality of water resources associated with the Waterberg coal reserves. New data regarding water quality and acid-base potential for the different geological areas (through field investigations) and geology and mining methods were obtained. Findings showed a significant likely impact on groundwater resources, with a potential for generation of acid mine drainage, although low transmissivities will in all probability prevent decant after back-filling is complete.Keywords: Waterberg Coalfield, water quality, water quantity, recharge, acid-base accounting, geohydrological modelling

South Africa’s diminishing coal reserves

South Africa’s coal reserves have been significantly reduced since 2003 and a re-assessment based on the complete statistical history of production from southern Africa has indicated that the present remaining reserve for the entire subcontinent comprises only about 15 billion tonnes or gigatonnes (Gt). South African coal geologists should therefore be mindful of experience in Britain, where reserves were grossly overestimated by conventional techniques and remained a large multiple of future production until very shortly before the effective collapse of the industry in the 1980s. The southern African historical analysis has shown that an impressive leap in coal production occurred between 1975 and 1985, from about 69 million tonnes per year (Mt/yr) to 179 Mt/yr. By 1989, the cumulative production had reached 4 Gt. Despite this doubling since to just over 8 Gt, the underlying pattern has been one of faltering growth. Hubbertarian analysis predicts a peak in production rate of about 284 Mt/yr in 2020, at which stage approximately half (12 Gt) of the total resource (23 Gt) will be exhausted. The Waterberg Coalfield (Ellisras Basin) in South Africa may be a remaining large resource, but structural complexity, finely interbedded coal-shale strata at large depths, low grades, high ash content and water scarcity are likely to inhibit its major development. Given South Africa’s heavy dependence on coal for power generation and electricity supply, an anticipated peak production in 2020 will cause problems for future economic growth.

96/04611 The Grootegeluk formation in the Waterberg Coalfield, South Africa: Facies, palaeoenvironment and thermal history — evidence from organic and clastic matter

96/04611 The Grootegeluk formation in the Waterberg Coalfield, South Africa: Facies, palaeoenvironment and thermal history — evidence from organic and clastic matter

The grootegeluk formation in the Waterberg Coalfield, South Africa: facies, palaeoenvironment and thermal history — evidence from organic and clastic matter

The Grootegeluk Formation of the Waterberg Coalfield is about 70 m thick and consists of relatively thin coal beds interbedded with numerous mudstone and carbonaceous mudstone layers. Maceral analyses of the coal seams and the interbedded carbonaceous mudstones show that vitrinite is the dominant maceral (up to 90 vol%), whereas inertinite, liptinite and reactive semifusinite generally occur in minor proportions. The vitrinite content increases upward in the formation, with a concomitant decrease in inertinite (including reactive semifusinite) from around 60% inertinite at the base of the formation. The upward increase in the vitrinite concentration is associated with an increase in the energy of the depositional environment, which is also considered to have been instrumental in the enhanced preservation of the vitrinite. The repetitive nature of the coal-mudstone succession, the lateral persistence of cycles, and the maceral and ash proportions, indicate that the formation may have resulted from a combination of allogenic controls (tectonism and climate) and authigenic controls (environments of deposition). The vitrinite reflectance (R̄ max) of the Grootegeluk Formation (mean 0.72%) and palynological evidence indicate that the Grootegeluk Formation was subjected to maximum post-depositional temperatures of about 100°C. The lower portion of the Grootegeluk Formation is dominated by kaolinite, quartz and minor amounts of apatite, and the upper parts by quartz, kaolinite and minor amounts of montmorillonite, illite and microcline. The base of the Grootegeluk Formation is considered to have been relatively more distal and the upper portions more proximal to the source of the sediments. At the base of the Grootegeluk Formation a ∼ 2 m thick tonstein zone occurs, which is dominated by well-crystallised kaolinite, organic matter ( ∼40 wt%), siderite, calcite and minor proportions of apatite. The tonstein zone has mineralogical, maceral and chemical characteristics distinctive from the other mudstones. Calcite lenses, which occur predominantly in the upper half of the Grootegeluk Formation, are interpreted as syndepositional. Early diagenetic globular pyrite and spherulitic siderite occur in the coals and organic-rich mudstones of the Grootegeluk Formation. Fine-grained granular siderite is present predominantly in the organic-poor mudstones of the Grootegeluk Formation and the overlying Beaufort Group. Systematic changes in the mineralogy and major element chemistry make major element whole-rock chemistry an ideal stratigraphic indicator tool for the Grootegeluk Formation. The tonstein has anomalously high concentrations of the trace elements Sc, V, Cr, Mn, Ni, Cu, Ge, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ba, La, Ce, Nd, Pb, Th and U, and low concentrations of S, Co, As and Zn, which, in addition to its mineralogical character and correlation throughout the basin, make the tonstein an important chronostratigraphic marker for local and intrabasinal correlation. The overlying Beaufort Group mudstones contain no organic matter. However, interpretation of the mineralogical and chemical data indicates that they were deposited with organic matter which was later degraded. The mineralogy of the mudstones and the concentration of trace elements suggest that the sediments were deposited in fresh rather than in marine waters.

A Profound Meteoric Water Influence on Genesis in the Permian Waterberg Coalfield, South Africa: Evidence From Stable Isotopes

ABSTRACT The coal-bearing sediments of the Grootegeluk Formation of the Waterberg Coalfield contain siderite, ankerite, and calcite of various forms that precipitated during deposition and diagenesis, and have highly variable d18O and d13C values. Spherulitic siderite, which is present predominantly as early diagenetic nodules (1-5 mm in diameter) in coal and carbonaceous mudrock in the lower part of the Grootegeluk Formation, has relatively high d13C values up to +8 (PDB) and d18O value between +9 and +15 (V-SMOW). Granular siderite (< 0.1 mm in diameter) is present in organic-poor mudrock of the upper Grootegeluk and the base of the overlying Beaufort Formation; it has low d13C values (-l9 to -14) and d18O values between +12 and +22. Ankerite is present only in minor amounts (< 1%) and has isotopic values similar to the coexisti g siderite. Calcite lenses conformable with coal and mudrock beds have d18O values between +16 and +20, and d13C values » -12, The oxygen and carbon isotopic values of cleat-filling calcite, which is present only in coal seams, has d18O values between +12 and +22 and d13C values between -15, and -7. Fine-grained calcite, disseminated in carbonaceous mudrock and as inclusions in kaolinite, is present only at the base of the Grootegeluk Formation and has d13C values between -7 and +3 and d18O values » +15. The calcite lenses are interpreted to be synsedimentary and to have precipitated from depositional waters with d18O value of -13. Spherulitic siderite and disseminated calcite precipitated during early diagenesis dominated by anaerobic microbial decomposition of organic matter. Granular siderite formed at higher temperatures (< 100°C) and later than the spherulitic siderite. The uniformly negative d13C values (-12) of the cleat-filling calcite indicate a predominantly organic source for the carbon, but the erratic and wide range of the d18O values do not allow distinction between precipitation daring burial or later during uplift of the sediments. Stable-isotope data of the carbonates are consistent with precipitation from meteoric water, but estimation of temperatures of formation are not reliable because burial processes result in highly d18O-depleted pore water.

The West Waterberg tonstein, South Africa

What is thought to be the first recorded occurrence of a tonstein in the coalfields of South Africa is described from the Volkrust Shale Formation of the Permain Karoo Sequence in the West Waterberg Coalfield. Now, virtually pure kaolin, petrological, mineralogical and geochemical evidence points to its origin being an airfall tuff of acid composition. The concentrations of Ba, Sr, and Pb in the tonstein are unusually high. It is thought the elements were released from the ash and precipitated as phosphates of mixed composition. The phosphate distribution is not uniform and is apparently related to the fragmental texture of the rock.