Mineralogical variation and changes in the South Walker Creek coals, Bowen Basin, Queensland, Australia

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Abstract
The Permian coals of the South Walker Creek area, with a vitrinite reflectance (Rvmax) of 1.7 to 1.95% (low-volatile bituminous to semi-anthracite), are one of the highest rank coals currently mined in the Bowen Basin. Studies of the mineralogy of this coal seam have identified unusual mineralogical assemblages in the vertical sequence, with illite-chlorite assemblages resembling a metamorphic association in some parts of the seam and kaolinite rich assemblages of more normal sedimentary origin in others. A range of analytical techniques, namely XRD, XRF, coal geochemistry, polished section petrology, SEM-EDS, and 3D X-ray tomography, have been used to investigate the nature of this mineralogical variation, including the effects of geological process on several aspects of coal petrology and geochemistry, and especially the impact of rank advance and other post depositional process on the mineralogical variation within the main coal seam of the South Walker Creek area. The South Walker Creek coal occurs in a single seam 10.5 to 14 m in thickness, split in places, and is dominated by dull and bright (40-60% bright), and dull with numerous bright bands (10-40% bright) lithotypes, along with minor dull (< 1% bright) coal. The duller lithotypes are mostly associated with a high proportion of inertinite as seen from organic petrology studies. Overall, the coal seam is dominated by inertinite (mainly semifusinite) with minor vitrinite, together with a small proportion of liptinite (exudatinite and bituminite) macerals. X-ray diffraction analysis shows that the mineral matter of the coal is dominated by kaolinite and interstratified illite/smectite in the top and bottom of the seam, along with typically a small proportion of quartz. In the middle of the seam, however, this is replaced by an NH4 illite-chlorite assemblage, with no quartz. Diaspore also occurs in some parts of the mid-seam interval. Some carbonate minerals (calcite and ankerite) and other minerals, namely rutile, anatase and apatite, have varying abundance in the vertical sequence, but another carbonate, siderite, and also apatite, appear to be most abundant near the top and bottom of the seam. Correlation of borehole sections shows that the kaolinite-rich intervals at the top and bottom of the seam and the illite-rich zone in the middle can be traced along strike through the deposit. Intervals with abundant diaspore in the non-carbonate mineral matter, however, appear to have a more restricted distribution, bounded by faults and seam splits and limited to the structurally lower parts of the mine area. Local changes, such as assemblages with paragonite, dickite and increased proportions of I/S also appear to be associated with fault zones. The minerals in the coal represent three phases of mineralization, syngenetic, diagenetic and epigenetic. Quartz, siderite, kaolinite and I/S occur as syngenetic phases, mainly derived input of detrital sediment into the peat swamp during organic matter accumulation. Some minerals, namely chlorite, illite, phosphates, diaspore and rutile or anatase, may have formed as early diagenetic phases. Epigenetic minerals such as calcite and ankerite, as well as dickite, and nacrite, along with additional kaolinite, illite, chlorite, diaspore, phosphates and rutile or anatase, were precipitated in cleats and fractures at a later stage as a result of epigenetic activity, probably associated with fluid movement in the coal seam strata after compaction and (possibly) rank advance. The mineralogical assemblage in the seam varies both vertically and laterally, partly controlled by the presence of faults and related structural features. Integration of these and other observations suggest that the mineralogical variation within the seam resulted from a combination of processes, including the sediment input to the peat swamp during deposition, changes associated with rank advance or metamorphism, and changes associated with hydrothermal effects due to late-stage fluid migration through the coal.
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Author(s)
Permana, Asep Kurnia
Supervisor(s)
Ward, Colin R
Gurba, Lila W
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Publication Year
2011
Resource Type
Thesis
Degree Type
Masters Thesis
UNSW Faculty
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