The chemical properties and derived sedimentation patterns of the coal seams in the Witbank-Highveld area

Abstract

From available records and data of colliery product samples, an area within the Witbank and Highveld coalfields was selected for this study. The petrographic and elemental nature of the coal seams have been derived from the relationship between the volatile natter content and the calorific value, both expressed on a dry ash-free basis (Snyman et al, 1983; Barnard, 1987). These derived values were complemented by ash_ analyses of colliery product samples, paleotopographic data and seam thickness. These data provided information on the paleoenvironmental conditions during peat accumulation and deposition. Computerized subsurface mapping techniques were applied to record various chemical and petrographic parameters. The existence of major channel complexes culminating in outwash streams towards the south is postulated. During the formation of the basal seams these trunk channels were up to 6 km wide and trended generally from east to west and from north to south along loT.v gradients. The channels follow topographical lows and skirt topographical highs. The vegetationstabilized channels aggraded upwards at a rate consistent with peat accumulation. The channels were fed by melt-water which originated from the northwards receding glacial ice-cap. Low energy currents led to the wide lateral distribution of autochthonous coal seams. The channels led to the transportation and redeposition of inertodetrinitric material of hypautochthonous origin. The coal seams associated with channel areas are generally thinner and the resultant coals are usually inertinite-rich. These coals have a brittle character and tend to become more shaley towards the south. A close relationship was established between the mineral matter characteristics and coal type of the No. 2 Seam. The inertinite-rich coals within the major channel areas contain mineral matter which is richer in acidic minerals. These minerals are quartz, normative rutile and kaolinite. Coals which formed in interchannel areas and swamps have generally higher vitrinite contents. The mineral matter of these coals contains relatively more pyrite, carbonate minerals and illite. This relationship between mineral matter and coal type is indicative of different paleoenvironmental conditions in which the peat was formed. Vitrinite-rich coals resulted from peats which were subjected to brackish water conditions where moderately high pH and a lower redox potential prevailed. The inertinite-rich coals were derived from peats formed in acidic waters -in an aerobic environment. Fluvial channel environments had a control on seam geometry and the petrographic profile which characterizes coal seams. Most of the vitrinite-rich vitrinite-rich coal seams base which have grades a distinctive cyclic pattern: a upwards into inertinite-rich coal. This petrographic pattern may be repetitive within the seam and is mostly reversed in the coal seams which were deposited in channel areas where a vitrinite-rich cap grades downwards into an inertinite-rich base. The influence of the topography of the pre-Karoo basement is evident throughout the succession of major depositional units, especially in the Witbank coalfield. Nevertheless, the paleoenvironments which existed during the formation of the various seams differed in extent. The different seams each have their own areal distribution, seam geometry and petrographical and chemical characteristics.Van beskikbare inligting asook data van steenkoolprodukte is n studiegebied in die Witbank- en Hoeveldsteenkoolvelde uitgesoek. Die petrografiese en elementsamestellings- eienskappe van die steenkoollae is afgelei uit die verband tussen droe as-vry vlugstof en hittewaarde (Snyman et al, 1983; Barnard, 1987). Hierdie afgeleide waardes is aangevul met assamestellingsanalises van steenkoolprodukte, paleotopografiese data en lac:tgdiktes. Uit hierdie data is inligting oor die toestande wat tydens veenvorming en afsetting geheers het, ingewin. Gerekenariseerde ondergrondse karteringstegnieke is gebruik om verskeie chemiese en petrografiese eienskappe aan te dui. Die bestaan van hoofkanaalkomplekse wat na die suide uitspoelstrome tot gevolg gehad het, word veronderstel. hierdie hoofkanale tot Gedurende die vorming van die onderste lae was 6 km breed en het die roetes met n matige gradient oor die algemeen van oos na wes en van noord na suid gestrek. Die kanale volg topografiese laagtes en om topografiese hoogtes. Die plant-gestabiliseerde kanale het geaggradeer teen n tempo wat met veenvorming tred gehou het. Die voedingsbron van die kanale was smeltwaters afkomstig van die ·noordwaarts terugbewegende gletserbedekking. Lae-energie steenkoollae strome tot het n gevolg wye laterale gehad. Die verspreiding van kanale het die autochtone vervoer en herafsetting van inertodetrinitiese materiaal van hipautochtoniese oorsprong te weeg gebring. Die steenkoollae wat met die kanaalareas geassosieer is, is oor die algemeen dunner en hierdie steenkool is gewoonlik inertiniet-ryk. Die steenkool is ook bros en is geneig om suidwaarts skalierig te word. n Goeie verband tussen die mineraaleienskappe en steenkooltipe van die No. 2 Steenkoollaag is vasgestel. Die inertiniet-ryke steenkool wat in die hoofkanaalgebiede voorkom bevat mineraalbestanddele wat ryker is aan suur minerale. Hierdie minerale is kwarts, normatiewe rutiel en kaoliniet. Steenkool wat in tussen-kanaalareas en moerasse ontstaan het, het oor die algemeen n hoer vitriniet-inhoud. Die mineraalbestanddele van hierdie steenkool bevat relatief meer piriet, karbonaatminerale en illiet. Hierdie verwantskap tussen mineraalbestanddele en steenkooltipe is aanduidend van verskillende paleo-omgewingstoestande waarin die veen gevorm het. Vitriniet-ryke steenkool het ontstaan uit veen wat aan brakwatertoestande redokspotensiaal onderwerp geheers het. was waar matige Inertiniet-ryke hoe pH en n steenkool word veenvorming in suur waters onder oksiderende toestande geassosieer. laer met Rivierkanale het n beheer op laagverspreiding en die petrografiese profiel wat kenmerkend van steenkoollae is, uitgeoefen. Die meeste van die vitriniet-ryke steenkoollae het n kenmerkende patroon: n vitriniet-ryke basis wat opwaarts gradeer na inertinitiese steenkool. Hierdie petrografiese profiel mag herhaal word in n steenkoollaag en is meesal omgekeer in steenkoollae wat in kanaalareas afgesit is waar n vitriniet-ryke bocnste gedeelte van die laag afwaarts gradeer na n inertiniet-ryke basis. Die invloed van die topografie van die voor-Karoo vloer is deurgaans in die opeenvolging van die hoofafsettingseenhede waarneembaar, veral in die Witbank-steenkoolveld. Nietemin het die paleo-omgewings wat tydens die vorming van die verskeie steenkoollae geheers het, van mekaar verskil. Die verskillende lae het elk sy eie oppervlakteverspreiding, laaggeometrie en petrografiese en chemiese eienskappe.