Northparkes E26 L1N Block Cave Progress

doi:10.36487/ACG_repo/2435_O-09
9th International Conference and Exhibition on Mass Mining, Kiruna Sweden 17-19 September 2024
Northparkes E26 L1N block cave – progress to date
L. Snyman, S. Webster and M. Flynn
Northparkes Operations, Parkes, Australia
ABSTRACT
Northparkes underground operations consist of E26 sub-level cave and E48 Lift 1 and the E26 Lift 1
North block caves. Two open-cut mines blend with underground ore in the processing plant to produce
a copper and gold concentrate.
E26 block cave mining began in 1997, and the ability to extend cave footprints and utilise existing
infrastructure has provided continual ore and value, along with several technical challenges. The E26
Lift 1 North block cave extraction level and undercut wraps around the historic E26 L1 block cave.
Development commenced in 2019, and extensive re-design occurred due to the variable ground
conditions of the existing cave and historic development around the footprint. The undercut start-up
sequence against the existing cave commenced in a post-undercut form to overcome substantial pillars
between historical development, then shifted to an advanced undercut as the pillars reduced to an
acceptable size. We present the finding of the inclined saw-tooth undercut design, which allowed a
reduction in the undercut development drives, reducing the duration and cost of the construction. The
production phase commenced in early 2022, and construction was completed five months early
compared to the feasibility study.
The cave extension geometry amongst existing infrastructure and the overlying subsidence crater
made L1N cave growth a unique cave during its cave establishment and breakthrough, posing
monitoring challenges. This paper looks at the progress of the cave from feasibility study,
construction, cave establishment and essential considerations for monitoring the growing cave
through to full production.
1
INTRODUCTION
The E26 Lift 1 North (E26L1N) Feasibility
Study identified that a potential 41.7Mt of low
grade copper and gold ore can be extracted from
E26L1N by means of the block caving mining
method. Now constructed, production from
E26L1N in the Life of Mine Extension Plan runs
from 2021 until 2033 to replace the ore that was
produced by the E48 block cave and the E26
sub-level cave (SLC).
The E26L1N block cave is described as a
remnant mine extension adjacent to the existing
E26 cave (Webster et al. 2020), the first
underground mine developed at Northparkes
Operations. The extraction level at an RL of
9760 sits beneath the E26L1 extraction level
(9800m RL) and above the E26 Lift 2 North
(E26L2N) cave (Talu, 2010). The E26L1N
development area forms part of the larger E26
orebody, which was mined initially as a small
open pit, then underground by block-caving in
two central lifts (L1 and L2), with a subsequent
extension (L2N). Currently a sub-level cave
operation is recovering peripheral ore from the
southern upper portion of E26L2, where cave
propagation was incomplete.
The E26 orebody plunges steeply (83-84°) to the
northeast in line with the strike axis of the
orebody. Copper-gold mineralisation at E26
occurs within stockwork quartz veins,
disseminations and breccia associated with the
intrusion of pipelike quartz monzonite
porphyries.
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E26L1 unexpectedly experienced surface
saprolite and clay on the extraction level some
months after the air blast event in 1999 (Ross
and van As, 2005). Clay was first evident as a
smear then progressed to chunks. The E26L2
began production in 2002 and produced clay on
the level in June 2006, roughly one year after
breakthrough into E26L1 in August 2005.
Initially there was minimal impact of the clay on
the production rates due to the clay handling
issues, with production rates maintained in
excess of 5.5Mtpa. However, large-scale mining
operations were suspended in August 2007
following early dilution of the ore-stream with
surface clays that had been drawn down from
surface through the E26L1 and E26L2 cave
volumes.
Through
experimentation,
operations
established that a clay content of <10-15%
mixed with fresh rock had to be consistently
maintained to ensure downstream material
handling infrastructure did not become blocked
with clay. Once the majority of drawpoints had
reached >15% clay in E26L2 ED1-6, no further
draw from E26L2 cave was possible without
creating repeated blockages of both underground
and surface material handling equipment (that is
crusher, chutes, ore skips and transfer points). In
addition, clay rushes occurred at a number of
transfer points both on the surface and
underground, resulting in significant safety
hazards to mine personnel. Northparkes decided
in July 2007 that attempting to feed ore with
>15% clay material was not sustainable and that
other ore sources needed to be secured.
The strategy for draw control of E26L1N is
predominantly to manage ingress of old cave
material, promote caving and limit clay
reporting to the material handling system and
mills.
2
DESIGN
Northparkes has incrementally mined larger
extensions to its initial block cave footprints in
the areas of E26 lift 2 with three additional
extraction drives, E48 lift one with one south and
two northern extraction drives and now E26 lift
one north with 11 extraction drives. The
challenges of designing a cave footprint amongst
existing infrastructure and caved rock, limits the
orientation options and requires a rethink of
undercut geometry and extraction level
ventilation and loader automation zones. The
design challenges are discussed in Webster et al
2020, whereas this paper focuses more on the
undercut geometry and caveability after the
construction and early years of caving.
2.1 Sawtooth undercut design
The Northparkes Step Change Project (Wyllie
and Webster, 2012) focused on innovations and
efficiencies in block cave design to expand the
production at Northparkes with four new caves.
All undercut designs for these caves involved an
advanced undercut narrow inclined ‘sawtooth’
pattern with a single drill drive above the
drawbell and offset from the extraction drive.
This single drill drive per drawbell reduces the
development metres required when compared to
other undercut designs requiring two drill drives
per drawbell. Major benefits include the speed
of constructing the undercut and reduced time to
first ore production.
The drawbell major apex was designed at 50
degrees which was flatter than previous
Northparkes caves but designed to minimise the
blast hole length which is the limiting factor of
this undercut geometry. As the angle reduction
reduces the volume of the pillars in the major
apex, the separation between the extraction level
and the undercut was increased again from 14m
to 17m. E26L1N however had height restrictions
due to remnant development of lift one,
connections to existing material handling
transfer points and the E26 lift 2 north cave
beneath. For these reasons, the height of the
sublevel was returned to 14m, and the angle of
the major apex reduced to 45 degrees. The
design of the incline sawtooth was adapted to a
long and short configuration to allow a wider
overlap, less prone to bridging. To further
mitigate potential for bridging that can happen
to any undercut shape, the ring designs were
overlapped, and the short holes were over-drilled
by minimum 1.5m into the long side of the
adjacent undercut.
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Figure 1
Long and short undercut shape.
A development since the Step Change Project
design is the implementation of WebGenTM
(wireless boosters) which were first trialed in the
Northparkes E26 Sub Level Cave and have
justified advantages in safety and productivity
gains (Hawkins, 2021). The option of using
WebGenTM, given it is wireless, can be precharged and has long sleep times, allows
optionality of firing of the drill pattern, a feature
that provides options for forming the complex
geometries of an undercut and drawbell.
E26L1N being amongst remnant drives, drill
patterns required bespoke designs and
adjustments to the rings of the undercut, again
WebGenTM provided optionality in these areas.
Advanced undercut geometry was preferred for
E26L1N due to the ability to remove swell
material from undercut without having to rely on
drawbell and undercut slot availability. As the
poor ground condition around the existing E26
cave would potentially prohibit drawbell and
undercut slot establishment at the area directly
adjacent to the cave, the post undercut method
could complicate cave extension initiation. This
was a learning from E48 cave extensions which
utilised a post undercut geometry.
2.2 Extraction level drainage
The gradient applied to the undercut sublevel
and extraction level ranges from 1% to 2% to
allow water to flow naturally from the drifts to
the extraction level access or undercut sublevel
access and eventually down to the sumps. The
sumps are strategically located at the start of the
developed accesses to allow mining uphill,
reducing pumping requirements during
development. The water runoff from the west
extraction level access, northern part of the east
extraction level access and drifts 1 to 6 are
captured by sump 1. Similarly, the water runoff
from drifts 7 to 11 are captured by Sump 2. The
changing gradient direction of some drives
required the undercut ring lengths to be adjusted
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to ensure connection of the undercut between
drives.
being more massive and more elastic.
Hydrofracturing of the BQM was recommended
so as to:
• Increase cave propagation which in turn
reduces ingress of old cave material;
• Facilitate caving at the same rate as the
volcanics on the north;
• Reduce arching over of the caveback;
• Reduce risk of cave stall;
• Reduce oversize reporting to drawpoints;
and
• Reduce magnitude of seismicity within the
hydrofractured region.
Figure 2
Northparkes has performed hydraulic fracturing
at E26 and E48 block caves with success.
Extraction level drainage.
Treatment prior to undercutting is believed to be
the most effective. Numerical simulation results
show that the hydrofracturing improves the
caving in the area with an approximate 5%
increase in material availability.
Figure 3
Changing extraction level heights
and location of undercut rings to
be adjusted.
Hydrofracturing of the BQM was performed
from the eastern side of the subsidence crater as
shown in Figures 4 and 5. The holes were drilled
and hydrofractured by the same contractor under
guidance of the CSIRO.
Figure 2 shows the layout of the E26 Sub level
cave (south in pink), lift two cave (centre) and
E26L1N Feasibility footprint (north in green).
The blue lines indicate the split drainage
directions across the footprint. Figure 3 shows
the changing extraction level heights due to
drainage directions and the locations of undercut
rings to be adjusted.
3
DEVELOPMENT
Stress measurements indicate that the major
principal stress is 30MPa sub-horizontal towards
111deg at the extraction level. No issues with
stress or seismicity were encountered during
development of the extraction and undercut
levels.
Figure 4
From the caving assessment, it was recognised
that Biotite Quartz Monzonite (BQM) has been
difficult to initiate continuous caving due to it
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Aerial view of hole collars on the
east side of the crater for
hydrofracture.
9th International Conference and Exhibition on Mass Mining, Kiruna Sweden 17-19 September 2024
void ratio. The solution was to initiate with a
post-undercut form and transition to an
advanced undercut, utilising pre-charging and
wireless detonation to complete the final firing.
Figure 6 shows a section view through one of the
initial drives with the firing sequence in the
transition from post undercut to advanced
undercut.
Figure 5
As-drilled hydrofracture holes
overlain on footprint.
Developing through pre-conditioned ground,
developing up against the existing cave, and
developing undercut drives into the existing
cave were all well managed and without
incident. The previously pre-conditioned ground
was only a challenge from an operational
perspective – drilling in automation, bulletproof glass and barricading precautions were all
part of the controls. There were expectations that
when developing up against the existing cave
there would be a zone of highly fractured and
loosened rock encountered. This was not the
case. In some areas there were indications that
the cave was close when weathered and oxidized
zones were intersected, but in two of the
undercut drives development cuts were taken
beyond the boundary into the consolidated cave
material – a combination of oxidised clay and
rock that was not too dissimilar rock in
appearance to intact.
4
Figure 6
Sequence of firing as illustrated in Figure was:
• Fire the drawbell from the extraction level,
• Remove enough material for the following
firing from the drawpoint,
• Charge the remaining shots from the
undercut level or abandoned infrastructure
levels, which consists of a conventionally
charged downhole portion and a precharged uphole portion.
• Fire the downhole conventionally,
mucking the material to make void and
then
• Firing the pre-charged uphole shot
wirelessly, before stepping over to the next
drawpoint.
UNDERCUT
The E26 L1N undercut is an inclined sawtooth
shape with a short and long side in each drive,
connecting to form a continuous area.
4.1 Post Undercut to Advanced undercut
transition
The undercut for E26L1N adjoins the E26L1
cave footprint and has abandoned infrastructure
drives running through the undercut design. Part
of designing around this existing infrastructure
caused the drawbell heights to be varied and for
the south-western section they were too high to
be fired in a single firing due to having a low
Sequence of Post-Undercut to
Advanced Undercut transition.
The transition between post and advanced
undercut occurred when the height of the
drawbells were low enough to support a single
firing and at that point the undercut firings were
all conventionally fired with wired electronic
detonators and the drawbell firings delayed until
the undercut had advanced beyond them.
The additional complexity required and
additional risks to be managed at the execution
of the transition compared with changes to the
design of undercut level that could have been
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made to remove the additional complexity
highlights the importance of thorough planning
and risk assessment at the earliest point in the
planning process
4.2 Dealing with hangups and poor material
flow
During the execution of this plan, there were
several issues encountered around the mass
firings where the material became compacted
and hung up. The general strategy applied when
trying to remove these issues was to:
• Fire adjacent rings
• Detonate a small explosive charge in the
drawpoint (bombing the drawpoint)
• Drill slashing rings from adjacent drives
and fire
• Fire the next ring and repeat if required.
Similarly with the angle of the undercut being 45
degrees, the material did not always rill easily
from the left and right side, leaving material that
could not be left behind without fear of losing
the connection between the undercut.
“Bombing” of the drawpoint was successfully
used in most cases to get material to rill from the
drawpoint again.
In the case of Figure 7, a drawpoint was hung for
a significant period of time, remedial actions
taken included firing adjacent rings, bombing
and drilling of slashing rings from the adjacent
drives (which broke through into broken
ground). After consideration of the risks, the
next rings were fired and material was recovered
successfully, indicating that the material was
broken, but too tightly packed to flow and when
that pressure released, material was able to flow
freely.
Figure 7
A CMS of a hung drawpoint after
mass blasting.
4.3 Verification of undercut connection
between drives
At the beginning of undercutting, the rings were
over-extracted to allow for a cavity monitoring
survey (CMS) to be completed, which
confirmed the connection between the short and
long arms of the undercut design as shown in
Figure 8. However, this was only confirmed in a
small set of rings and not rigorous for the entire
undercut - the presence of pillars that could put
load on the extraction level were not able to be
ruled out. Since completion of the undercut in
Q1 2022, there has been no evidence of loading
on the extraction level that has been tied to poor
undercut performance.
This performance may be better attributed to the
favourable ground conditions and setting for
undercutting rather than rigorous quality
assurance and control with the undercutting
process, considering this, future undercutting
should include regular checks for the connection
of the undercut between drives to ensure the
desired performance is achieved, with other
proxy performance indicators used in between.
The extraction target of the rings fired was used
as a proxy for the performance of the blasted
shape, if the desired 30% extraction was not
achieved and the brow opened, the process used
to deal with hangups and poor material flow was
used. If the extraction target was achieved
without the brow opening, the ring's
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performance was considered high enough to fire
the following ring.
Figure 9
Figure 8
Connection of the short and long
arms of the inclined sawtooth
undercut was observed through
CMS.
4.4 Verification of connection to existing cave
As the undercut was designed to butt against the
older E26 L1 cave, there were several areas
where it was required to ensure that no pillar was
left behind, to ensure free flow of material and
avoid the potential for sudden failure of the pillar
at a point in the future. There were two
scenarios:
Rings reaching over to connect
with the existing cave on the left
hand side.
In both situations, the connection to the existing
cave was confirmed by evidence of the E26L1
material at the drawpoint, which was distinct
from the fired and freshly caved material in that
it was very fine material with a distinct red
colour as shown in Figure 10. The larger boulder
sized material was also an indication that caving
was occurring in fresh rock as opposed to the
fired material from the undercut.
• The L1 cave directly ahead of
development
• The L1 cave to the side of development
Where the cave was directly ahead of
development, stand up rings (where the rings
start at 45 degrees and progress to 10 degrees to
build height) were used. Where the development
came alongside the cave, additional rings
reached over to blast the pillar as shown in
Figure 9.
Figure 10 Connection to the existing cave
was
confirmed
through
drawpoint material observations.
4.5 Evidence of caving
The abandoned conveyor drive, CV07, was also
able to be used as an inspection level for the
undercut and confirm that caving was occurring
above the height that was originally fired, before
being sealed with a thick cemented wall used as
an airblast plug to prevent the flow of air into
CV07.
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The amount of value that was gained from these
surveys was key in developing a mass balance
model for the operation and verifying
assumptions about how the cave was
progressing, especially in the initial stages to
mid stage of the undercut progression where the
cave had not progressed high enough to trigger
other
cave
indicators
and
markers.
Consideration should be very high for future
projects to design in ways to gather these
surveys as shown in Figure 11.
growth which matched closely with the
predicted rate.
Figure 12 Undercut fired and open hole
breakthrough
with
cave
interpretation.
Draw call
Figure 11 Evidence of caving observed from
the level above the undercut.
5
The draw call design for E26L1N has three
primary functions:
• Promote caving away from the existing
cave to prevent arching into E26L1
• Prevent the ingress of E26L1 material into
L1N
• Provide good fragmentation through
differential drawdown of material
DRAW STRATEGY
The draw strategy in L1N during and
immediately following the undercutting was to
limit the draw as low as possible to give
sufficient time for the ground to fail naturally
and reduce as low as possible the airgap between
the caveback and muckpile. This gradually
ramped up as more evidence of caving and the
muckpile height growing was observed.
The initial draw strategy used the smart marker
recovery as a guide to caving occurring and the
draw call from the area was adjusted to keep the
airblast risk as low as possible in the early
stages, where little evidence was able to be
collected on the caving progress.
Once the cave progressed high enough to reach
the open holes from the surface (Figure 12), the
same strategy was used to increase the
production rate from the cave and also to
determine an estimate for the rate of vertical
The way that this is achieved is by increasing the
draw call in each drawpoint as the distance from
E26L1 increases, drawpoints close to the cave
are bogged with low calls and the perimeter of
L1N has the highest draw call, as shown in
Figure 13. The exception is only made in one
extraction drive where some convergence was
evidenced but has been kept under control
through increased bogging in the area.
While the highest grade for the block is in the
areas where the lowest bogging is occurring, the
need to ensure complete caving and preventing
the ingress of material from L1 has taken priority
over the head grade. Once the cave has been
established and broken through completely to
the surface, the bogging strategy will change to
give the highest long-term value.
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holes from the surface (6 triaxial geophones)
(Figures 15 and 16).
Figure 15 Plan view of sensors (blue are new
installs for L1N).
Figure 13 Draw call design for E26L1N.
Figure 14 Modelled cave growth of L1N.
6
MONITORING
The unique layout of the L1N cave wrapped
around the existing L1 cave and infrastructure
and located vertically below the eastern flank of
the existing subsidence crater, with no
intersecting underground excavations resulted in
limited ability to monitor the pillar between the
crater and the cave back. Monitoring systems in
place for L1N include a seismic system, open
holes, cave tracker beacons and tiltmeters as
well as surface subsidence monitoring by drone
photography.
The IMS seismic system includes 18 sensors
installed from both underground (6 triaxial
geophones and 6 uniaxial geophones) and in
Figure 16 Section view of sensors.
Only one open hole was initially drilled to
monitor cave growth. It was drilled from north
to south. This hole held water for 5 months after
completion of undercutting and then started
sucking in May 2022. From May to October that
year the caveback progressed steadily every
week until in October a small airgap of no more
than 11m was measured. Unfortunately, the hole
was lost due to the failure of a wireline dipper
down the hole that could not be retrieved. A
further 6 holes were then drilled to provide
coverage across the cave – at an angle steep
enough to allow the camera to run down the
hole, at a safe distance from the edge of the
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crater, but also to a target depth that would give
meaningful caveback information over time.
Two holes were drilled in the north and four in
the east. The open holes are certainly the
simplest and most useful form of monitoring the
caveback as long as they remain open. Weekly
changes to the muckpile sighted down choked
open holes have given confidence that rilling is
occurring within the cave and that an excessive
airgap is not present (Figure 17).
Cave tracker beacons were installed in surface
holes drilled from the eastern side of the
orebody. Of the 64 beacons installed, 17 have
been extracted or stopped spinning to date.
Unfortunately, no beacons have been physically
recovered. The remaining beacons are located
outside of the modelled final cave shape and will
provide information on the lateral growth of the
cave, if any.
The beacons have given valuable flow data
within the muckpile – the harder draw on the
perimeter of the cave to promote cave
propagation and prevent arching has resulted in
the beacons being drawn across to that side
within the muckpile as shown in Figure 18.
Figure 17 Progressive change of muckpile material at bottom of open hole over time.
Smart markers that were leftover from the E48
block cave were installed in holes drilled from
the undercut. Two holes were installed vertically
into the gap formed by the sawtooth undercut
pattern (Figure 19). Each hole had smart markers
installed every 2m from the toe of the hole down.
These markers indicated that the undercut had
progressed beyond the fired ring. Two or three
rings of markers were installed in every second
undercut drive as shown in Figure 20. 228 of the
393 smart markers installed have been recovered
to date.
Figure 18 Cave tracker beacons flow within
muckpile.
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surface infrastructure to monitor the stability of
the pillar as the cave develops and breaks
through to surface. No tilt has been measured at
the time of writing.
A Wingtra fixed wing drone is used to capture
aerial photographs (and surface topography) of
the subsidence zone. There is no safe access to
the edge of the crater and drone photography is
the only means to identify changes within the
crater.
Figure 19 Smart
marker
between undercut
rings.
installation
holes and
Sections drawn across the crater show that the
eastern slope which is directly above the L1N
footprint has progressively subsided over the last
12-16 months. There is no evidence of a
chimney or “glory hole” within the crater at this
stage.
N-S section view across E26 crater
Figure 20 Smart marker
undercut level.
rings
across
NE-SW section view across E26 crater
Figure 21 Placement of smart markers on
9830 level.
57 smart markers were placed on the floor of the
9830 level, as shown in Figure 21, (50m above
the undercut level) to track the caveback
progress through the existing infrastructure.
Several of these markers have been recovered to
date.
The Geo4Sight tiltmeters were installed into a
hole in the pillar between the cave and the
Figure 22 Sections through E26 crater and
plan.
New technology, not available in Australia when
L1N was developed, has subsequently been
installed in two sacrificed open holes. Optic
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fiber cable for micro-seismic monitoring, strain
cable for detection of small rock movements and
geophones have been installed.
Of particular interest is the Gypsum Leached
Zone (GLZ), a highly fractured and broken
rockmass at the top of the stratigraphy. All holes
drilled through this layer have to be cased with
steel casing in order to keep the holes open. It is
therefore not possible to monitor any changes in
the hole conditions in the GLZ section using an
open hole camera, and it is also unlikely that
seismic events would occur in this rockmass.
Core from a diamond drill hole through the GLZ
is shown in Figure 23 to give an indication of
how broken it is and Figure 24 shows seismicity
immediately after undercutting and then a few
monthly snapshots during caving. The GLZ
horizon, extraction level, undercut level and L1
stall height are shown on the graph for reference.
There are opportunities to use new technology
and develop new methods to determine the
location and growth of the caveback in block
caving mines. The use of geophysical methods
is currently being explored which will eliminate
challenges faced with complex geometries.
Figure 23 GLZ core.
Figure 24 Graphical representation of seismic events looking west.
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7
CONCLUSIONS
The key findings in this paper are:
The advance sawtooth undercut is a viable
design for reducing the number of development
drives required to establish an undercut. The
authors note that the performance may be
attributed to the favorable ground conditions and
rigorous quality assurance and control is still
recommended.
Constructability of a cave extension amongst
remnant development and variable ground
conditions brought complexity to drill and blast
that required more detailed planning at the
earliest point in the planning process.
Talu, S, van As, A, Seloka, W & Henry, R. (2010). Lift 2
North extension cave performance. Y Potvin (ed.),
Caving 2010: Proceedings of the Second International
Symposium on Block and Sublevel Caving, Australian
Centre for Geomechanics, Perth, pp. 407–421
Webster, S, Samosir, E & Wyllie, A. (2020). Learnings
from mining cave extensions at Northparkes Mines
and new technology to improve the value of future
cave designs. R Castro, F Báez & K Suzuki (eds),
MassMin 2020: Proceedings of the Eighth
International Conference & Exhibition on Mass
Mining, University of Chile, Santiago, pp. 92–102,
Wylie, A., Webster, S. (2012). Northparkes Mines – Step
Change Project. MassMin 2012: Proceedings of the 6th
International Conference and Exhibition on Mass
Mining, Sudbury, Ontario Canada.
Expectations of cave growth in cave extensions
should remain pragmatic to the prospect of arch
over and draw calls focused on promoting cave
growth take precedence.
Monitoring locations opening into the cave are
not always available due to the installation of
cement plug walls as an air blast risk
management requirement. The cave extension
wrapping around the lift one cave and beneath
the subsidence zone limited locations to install
cave monitoring and redundancy of open holes
should be planned for. New technologies will
certainly help to eliminate the monitoring
complexities experienced with the L1N cave.
ACKNOWLEDGEMENT
The authors would like to thank Evolution
Mining for allowing the publication of the
learnings from the development and execution
of L1N – a unique block cave in our industry.
REFERENCES
Hawkins, E. (2021). Optimisation of blasting practices in
caving operations. Master of Philosophy (Mining
Engineering) thesis, School of Civil, Mining, and
Environmental
Engineering,
University
of
Wollongong,
2021.
https://ro.uow.edu.au/theses1/1233.
Ross, I. & van As, A. (2005). Northparkes mine – design,
sudden failure, air blast and hazard management of the
E26 Block Cave. Proceedings of the 9th Underground
Operators Conference, Australia, Australasian Institute
of Mining and Metallurgy, Melbourne, pp. 7–21.
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