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.