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Exploration
The deposit was
initially explored by the Geological
Survey of Queensland from 1971 to 1982.
Following the granting of the lease to
the Joint Venture in 1984, exploration
by the Joint Venture has comprised
extensive openhole drilling and core
drilling with large diameter cores taken
for detailed analysis and testing.
Limited seismic work has also been
undertaken. To date more than 3000
boreholes have been drilled within the
opencut zone including approximately 750
coreholes. Borehole spacing is a maximum
of 250 metres although over most of the
area the spacing is much closer. All
holes at Ensham have been geophysically
wireline logged.
Ensham’s current opencut mine plan
has been prepared for a period of 25
years (to the year 2019) and the quality
of the coal for this period is well
known from the detailed drilling. All
exploration data is maintained on
computer database and modelled using
MAPTEK’s Vulcan computer system.
The controlling structural features of
the Ensham deposit are the Comet Ridge
to the east, and a broad southwesterly
plunging syncline which trends northeast
across the central part of the area.
Strata dip towards the synclinal axis at
generally less than 5 degrees although
some areas of flat dip have been
interpreted on the flanks of the
syncline. Major folding is not apparent
within the opencut areas.
The faulting identified or inferred
to date has mostly been normal faulting
with throws commonly less than 10m but
with some throws up to 20m. Fault trends
are mostly east-west to
southeast-northwest. Some low angle
thrust faults have been noted in the pit
highwalls with the southern side
invariably thrust over the northern
side. The overthrust section is usually
less than 30m horizontal with the
vertical throw generally less than 5m.
Slickensiding and some very minor drag
effects have been noted in the immediate
vicinity of the fault plane.
The coal seams mined at Ensham belong to
the Late Permian Rangal Coal Measures.
These Coal Measures are recognised
throughout the area and have a thickness
of some 100m.
The seams subcrop over a strike
length of about 80km around the northern
and eastern limits of the area.
The top of the Rangal Coal Measures
is regarded as the transition from
carbonaceous sediments to the fluvial
sequence of green coloured,
non-carbonaceous siltstones and lithic
sandstones of the Triassic Rewan Group.
Underlying the Rangal Coal Measures
is the Upper Permian Burngrove Formation
which comprises light blue-green
coloured sandstones, greenish grey
siltstones and mudstones, and banded
coal seams which are frequently
interbedded with fawn to light brown
coloured tuff and tuffaceous mudstones.
The coal seams developed within the
Burngrove Formation are not currently
mined but are identified as the Virgo
and Leo seams. These seams have a
thickness of 4-10m and raw coal ash
contents of 30-45%. Washability testing
has indicated that the F1.60 fraction
has an ash content of around 13.5%,
specific energy of 6650 kcal/kg (GAD)
and a yield of 40-70%. These seams also
have coking properties with a CSN of 3-5
being achievable for a washed Virgo
seam.
Tertiary sediments unconformably
overlie Permian and Triassic strata and
generally consist of multicoloured
claystones and quartzose sandstones with
bands of ironstone and silcrete forming
cappings on hills and ridges rising up
to 80m above the surrounding land
surface.
Correlation of all seams within the
tenements has been based primarily on
wireline log response. The main seams
developed within the Rangal Coal
Measures at Ensham are, in descending
stratigraphic order, the Aries (A), the
Castor (C), and the Pollux (P) seams.
The splitting and coalescing of some of
these seams gives rise to a number of
other seams. Within the Ensham Central
and Southern areas, the principal seams
developed are the A2C and A22C seams
formed from the coalescence of an Aries
2 seam and the underlying Castor seam.
These seams have an average thickness of
5.3m and 5.0m respectively and have been
identified over a strike length of some
11.5km.
Based on quality parameters, the
seams are subdivided into a top ply
(±1.0m thick), a middle ply (±3.5m
thick) and a bottom ply (±0.4m thick).
The bottom ply is separated from the
middle ply by a mudstone band that
varies in thickness from 0.20m thick to
over 0.40m thick. Where the mudstone
band is greater than 0.3m thick, the
bottom ply is referred to as the C22
seam. Depth of weathering ranges from
10m to 20m.
At Ensham North, the Pollux seam
becomes cleaner over a strike length of
approximately 5.5km. The seam averages
2.6m thick and is currently being mined
from the Yongala Pit. In the north of
Yongala, the Pollux seam coalesces with
the Castor seam to produce a Castor-Pollux
seam with an average thickness in excess
of 4m. Other thin seams are identified
above the Castor seam. The depth of
weathering at Yongala averages 21m.
The quality of the Rangal seams mined at
Ensham is such that they do not require
washing in order to meet export
specifications and therefore Ensham does
not have a wash plant. The flexibility
of the mining operations, the geological
options and the versatility of the coal
characteristics allows Ensham to produce
a number of thermal coal products by a
combination of selective mining and/or
blending.
Although Ensham produces a limited
tonnage of semi-soft coking coal, the
main product is a low ash, low sulfur,
high energy thermal coal. As a guide,
Ensham can produce thermal coals with an
ash content of 10%-16% and with
calorific values of
6000kcal/kg-7000kcal/kg GAD. Total
sulfur content is normally 0.6%-0.9% and
volatiles are usually 25%-27%. The
underground reserve has a very low raw
coal sulfur content of 0.3%-0.4% and
once the underground mine is developed,
it is felt that this feature of the coal
will lead to a ready acceptance in the
market place.
Because of the direct shipping nature
of the operations, quality control of
mining, stockpiling and blending
operations is essential to ensure Ensham
continues to meet its various contract
specifications. Currently Ensham markets
two main thermal products,
differentiated on ash/energy
specifications, and a semi-soft coking
coal. The versatility of the coal and
the simplicity of the mining/coal
handling operation allows Ensham to meet
a wide range of specifications by
selective mining and blending and a
range of the qualities available is
attached in a separate table. All coal
is sampled by automatic sampler as the
coal is loaded onto each train.
Ensham is committed to ensuring that
our products meet customer requirements.
At the minesite extensive quality
control procedures have been introduced
with sampling and analysis of the coal
throughout the processing phases. As a
demonstration of Ensham’s commitment to
quality, Ensham has achieved
accreditation to ISO9002 for all
operations on site and for its Brisbane
office.
Seam thickness in the opencut area
averages 5-6 metres while the
underground reserve generally comprises
one seam 3 metres thick.
It is estimated that there is an
in-situ resource of more than 1 billion
tonnes in seams greater than 2.5 metres
in thickness (up to 6m in thickness)
within the underground mine area covered
by MDL217.
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