Petrochemical characterization of dolerites and their influence on coal in the Witbank Highveld Coalfield, South Africa
Du Plessis, Johannes Jochemus
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A study was firstly conducted on the mineralogy and geochemistry of the dolerites, secondly on sedimentological controls (syngenetic) on coal deposition and diagenesis to gain a better understanding of the environment of the coal deposition and thirdly, the metamorphic influence of dolerite intrusions (epigenetic) on coal. The Ogies Dyke is the only intrusion found in the study area at Optimum Colliery. The absence of dolerite sill intrusions in this area made it possible to study coal deposition and diagenesis. It was to investigate the behaviour of a 20m thick bifurcating dolerite sill (the Witbank sill) and its associated metamorphic influence which occurs in the other Koornfontein, Bank and Goedehoop Collieries. The most prominent structure, the Ogies Dyke, forms the northern limit of the study area and forms a very important part of the geochemical and mineralogical study. Thin section investigations revealed the involvement of plagioclase in both the Witbank and Sasolburg dolerite fractionation assemblages indicates that the fractionation processes must have occurred within the crust although within different depths. The absence of pyroxene phenocrysts in the B5 sill (Sasolburg) indicates that the fractionation took place at a pressure significantly higher than that at which the plagioclase and olivine microphenocrysts have formed. The high percentage olivine in the B4 sill (Sasolburg) indicates that these two sills originated from different magma sources. Plagioclase microphenocrysts in the B5 sill as oppose to the macrophenocrysts of the B4 sill concludes that the fractionation processes of the B5 sill must have happened deeper within the crust. This study engage with dolerites that crystallised rapidly, intermediately and slowly as the crystal sizes are directly related to magma cooling. Fine crystalline dolerites like the chilled margins and bifurcations tend to be more susceptible for alteration as opposed to the medium and coarse crystalline dolerites. The 40m thick, fine crystalline B4 sill has undergone the most alteration comparing to the B5 sill, Witbank sill and the Ogies Dyke. The differences identified during this study distinguish the Sasolburg dolerites from the Witbank sill and the Ogies Dyke. X-ray fluorescence techniques were used to analyse the dolerite samples from the study area. All the dolerites are falling in the “basic” group. The B4 – Sasol dolerite sill is a high-MgO (picritic) basalt while the rest are basalts. The chilled margins of the bifurcations have an arithmetic mean of 4.89% MgO and the Ogies Dyke has 4.9% MgO and can be classified as evolved basalts. Lower MgO and Ni values in the Witbank bifurcations comparing to the Witbank sill indicate that the bifurcations are more evolved. The basaltic and evolved basalts can further be divided into low and intermediate K2O concentrations. A higher K2O concentration is placing the Ogies Dyke in the intermediate-K2O group whilst the Witbank sill (interior and chilled margins), and the Witbank bifurcations (interior and the chilled margins) are all falling in the low-K2O group. Two of the Witbank bifurcations (interior) having intermediate-K2O concentrations and are associated with the Ogies Dyke. The picritic B4 sill (Sasol) is also classified as a low- K2O dolerite. Considering K2O and MgO element concentrations the samples are falling in three categories, from evolved to picritic with the majority in the basaltic field. Borehole information was used to conduct isopach and isopleth maps of the pre-Karoo topography, floor elevation and thickness distribution, coal parameters and statistical data of various coal seams to underpin the sedimentological controls (syngenetic) on coal deposition and diagenesis The undulated platform onto which the No. 2 Upper Coal Seam formed at the Optimum study area had a major influence on coal grade. Thicker coals were deposited in the lower lying areas while they were thinning towards palaeohigh areas. Significant values indicate that the thinner coals are higher in ash (air-dry), lower in VM (daf), lower in CV MJ/kg (air-dry) and higher in relative density comparing to the coals deposited in the lower lying areas of the palaeovalley. Lithological descriptions from boreholes and structural interpretations in geological crosssections revealed the presence of a green 20m thick, bifurcating dolerite sill that intruded into the Vryheid formation of the Karoo Supergroup. It is associated with ±20m displacement and metamorphism on coal which is putting major constraints on coal mining in general. The metamorphic influence of the coal is largely restricted to the width of the contact aureole. The nature of the aureole depends on the geometry, variation in thickness and bifurcation of the sill. It is also found that the metamorphic contact aureole is much more extensive in the displaced and uplifted coal seams comparing to those beneath the sill. Moisture (ash-free) of the proximate analyses, volatile matter (daf), CV (daf) and approximated ash yield (AD) isopleth maps show that the dolerite sill caused a localised increase in rank. Areas of high moisture (proximate analyses moisture content) correspond to devolatilised areas, which are higher in ash and therefore having lower CV’s and are adjacent to known intrusions.