Grade Control Paper 2

Grade Control in the Southern Middleback Ranges —
A Case Study
D Crawford1
, P Leevers2
, S Parsons3
, G Solly4
and S Nielsen5
ABSTRACT
Hematite ore has been mined in the Southern Middleback Ranges as blast
furnace feed for the steelworks at Whyalla, by what was then BHP, since
1991. With the floating of BHP Integrated Steel as OneSteel in 2001 and
the subsequent loss of key mining personnel, breakdowns in grade control
methods and procedures occurred. These were compounded by changes
in orebody characteristics and grade distribution leading to poor reserve
recovery and poor grade control, both which resulted in loss of control of
ore specification and severe deviation from the mining schedule.
As a result, a total review of grade control, mining method and
scheduling procedures was undertaken as well as a review of contractor
quality assurance. It was found that sampling methods were poor and
unrepresentative, grade control blocking processes were outdated and did
not reflect the geology of the deposit, the ore mining method was
outdated and did not respect the geometry of the orebody, reconciliation
methods were being completed globally and were not reflective of
individual areas of the deposit, and that deviations from the mine plan
were occurring due to access to high-grade areas outside the schedule in
order to bring stockpiles back on grade.
Based on these findings, updated sample splitter systems were fitted to
drill hole rigs, double bench sampling was implemented (2 × 4 m samples
as opposed to 1 × 8 m sample) and a three-dimensional cellular grade
control system introduced to better reflect the geology and orientation of
the deposit. At the same time ore mining was changed from front end
loaders mining on 8 m benches to excavator mining on three lifts on 8 m
benches; this aided selectivity and respected the geometry of the
orebodies aiding cleaner recovery. Reconciliation is now completed on a
blast-by-blast basis, which allows for quick and easy identification of
poorly performing areas of the resource model that can then be
incorporated back into resource model updates. Regular daily and weekly
meetings are also held with emphasis on the mining schedule to ensure no
deviation from the schedule in the long term.
These improvements have resulted in a lift in reserve recovery both
globally and locally associated at the same time with improvement in
grade.
INTRODUCTION
Hematite ore has been mined in the Middleback Ranges for well
over 100 years and in the South Middleback Ranges (SMR) for
the Integrated Steelworks at Whyalla since 1991.
The floating of BHP Integrated Steel Long Products Division
as OneSteel Pty Ltd in 1999 led to the subsequent loss of key
mining personnel, breakdowns in grade control procedures and
mine scheduling over the following years. These issues were also
compounded by changes in orebody characteristics and grade
distribution that led to poor reserve recovery, loss of control of
ore specification and severe deviation from the mining schedule.
With a return of mine personnel between 2002 and 2003 an
internal review of all aspects of the grade control process, mine
planning process, mining methods and mining contractor
management was undertaken in early 2004.
The findings that came out of this review and were considered
critical to the process are outlined below:
1. sampling methods were not representative and exhibited
poor repeatability;
2. ore blocking and interpretation procedures and processes
were not reflective of the geological characteristics of the
orebody;
3. geological information collected during the course of
operations was not being used in the grade control or mine
planning process;
4. ore mining methods being employed were lacking in
selectivity and did not respect the geometry of the orebodies
being mined and there was a lack of communication between
the mine owner and the mining contractor;
5. short-term mine planning was poor to non-existent with
daily operations focusing on short-term goals with no
reference to the long-term mining schedule; and
6. reconciliation was completed on a global basis and not
completed on a local (shot to shot) scale, meaning that poor
performing areas were not being recognised.
This paper details the changes that have been made over the
past 18 months and the improvement that these have had on ore
quality, variability and reserve/resource recovery.
OPERATIONS
The Southern Middleback Range hematite resources are located
55 km south-west of Whyalla (Figure 1). The SMR is also the
location of OneSteel’s Iron Magnet deposit.
These deposits are wholly owned by OneSteel Pty Ltd.
Hematite ore is mined from three main pits: the Iron Duke, Iron
Duchess and Iron Knight. 1.5 Mtpa is mined as feed for
OneSteel’s Blast Furnace and steel making operations, combined
with this is one million tonnes of high-grade lump and fines
produced from the ore beneficiation plant. 1.6 Mtpa of feed are
sourced for this plant from low-grade ore from the Iron Duke,
Duchess and Knight Pits and Dumps. A further 1 Mtpa of
high-grade fines is exported. A flow chart of mining and
blending operations is illustrated in Figure 2.
Mining is completed on 8 m benches, with blast sizes typically
in the order of approximately 120 000 BCM. Mining is
completed over three flitches, two of 3 m height and one of 2 m
plus heave, using 240 tonne excavators and 130 tonne dump
trucks. Prior to the conversion to full-time excavator mining in
ore, the main load-out machines used were Caterpillar 992
loaders mining the full 8 m bench in one pass.
Ore is either direct fed (straight from pit to crusher) or
stockpiled prior to blending through the primary crusher.
Blending onto final stockpiles is completed using a windrow
stacker, constructing stockpiles of 50 000 tonnes. These are
railed to Whyalla as either feed to the Steelworks or stockpiled
for export. Due to limited space and environmental
considerations, limited blending is carried out in Whyalla.
Iron Ore Conference Fremantle, WA, 19 - 21 September 2005 1
1. Mine Geologist, OneSteel Pty Ltd, PO Box 21, Whyalla SA 5600.
Email: crawfordd@onesteel.com
2. MAusIMM, Principal Geologist, OneSteel Pty Ltd, PO Box 21,
Whyalla SA 5600. Email: leeversp@onesteel.com
3. Mine Manager, OneSteel Pty Ltd, PO Box 21, Whyalla SA 5600.
Email: parsonss@onesteel.com
4. Senior Mine Geologist, OneSteel Pty Ltd, PO Box 21, Whyalla SA
5600. Email: sollyg@onesteel.com
5. Technical Analyst, OneSteel Pty Ltd, PO Box 21, Whyalla SA 5600.
Email: nielsens@onesteel.com

GEOLOGY
The hematite deposits of the Middleback Ranges have been
formed by supergene enrichment of the Lower Middleback Iron
Formation within the Hutchison Group, which forms the
Southern Middleback Ranges. All the deposits occur along the
western flank of the north-south trending ridges, which rise up to
150 m above the coastal plain.
The Hutchison Group sequence is comprised of a sequence of
clastics, dolomite and banded iron formation (BIF). This package
has been highly deformed, by at least four recognisable phases,
and metamorphosed to upper amphibolite facies. Various phases
of igneous intrusion have also occurred throughout the ranges.
The structural setting and relationship of the igneous intrusives
have been important controls on the supergene enrichment
process. All the hematite deposits occur up-dip from the
carbonate and silica facies BIF.
Mineralisation forms as a discontinuous zone along the
western flanks of the ridges, with a variable easterly dip from 30°
to almost vertical (90°). Hematite is the dominant ore mineral
with significant goethite and minor limonite present. Ore widths
are highly variable from greater than 50 metres in the central
areas of the Iron Duke and Iron Duchess to less than five metres
in some of the eastern areas of the Iron Duke and Iron Knight.
2 Fremantle, WA, 19 - 21 September 2005 Iron Ore Conference
D CRAWFORD et al
FIG 1 - Location map of the hematite deposits of the Middleback Ranges.
FIG 2 - Flow chart of mining operations at SMR.

The igneous intrusives, which are altered to amphibolite and
generally weathered to clay, cut through the orebody and have a
north-south strike and a steep westerly dip. Individual dyke
thicknesses vary from less than a metre up to 15 m. Dykes can be
quite continuous and some have been recorded over the entire
length of the Iron Magnet and Iron Duke deposits (greater than
2000 m), while others pinch and swell rapidly or bifurcate and
rare folding has been observed. The amphibolites are generally
devoid of mineralisation but can be slightly hematised,
containing low-grade iron. They are the typical source of Al2O3
and TiO2 contamination within the SMR deposits.
Other contaminants that occur within the deposits include
phosphorous (P), often associated with goethite, silica (SiO2)
from incomplete leaching of the host BIF and manganese (Mn)
in the form of pyrolusite infilling cavities and fractures within
hematite zones. Both phosphorus and manganese are restricted in
occurrence but are generally higher in the Iron Knight and very
low in the Iron Duchess.
Towards the base of the hematite zones levels of calcium (Ca)
and magnesium (Mg) are also occasionally elevated. This
elevation in carbonate is interpreted to be from incomplete
leaching of the original carbonate-rich BIF host.
GRADE CONTROL
Blasthole sampling
Traditionally sampling had been carried out at the SMR
operations from the cuttings at the blasthole collar using a
hand-held trenching tool. The sample supposedly being
representative of the 8 m bench being drilled. Subdrill was
estimated by eye and a portion of the collar was scraped away to
take this into account.
Initial investigation found that sample repeatability was poor
when two samples were taken from the same cone at different
points. The allowance for subdrill was subjective and therefore
inaccurate and composite sampling did not reflect orebody
geometry (ie holes through contacts would return as low grade
rather than high grade and waste). This in turn gave the
impression of variability within the hematite orebodies that was a
function of the sample quality not the orebody itself, leading to
poor reconciliation and recovery.
One possible solution considered was a move to reverse
circulation drilling as the grade control drilling and sampling
method. When trialled, this proved excellent in terms of sample
representivity and depth flexibility. However, the cost of reverse
circulation drilling proved prohibitive to the operation.
As a result, in cooperation with Henry Walker Eltin (HWE are
the current mining contractors at the SMR) it was decided to trial
a blasthole sample system mounted on the main blasthole rig
(Figure 4). The system was supplied by SDS Australia Pty and
has the ability to provide up to four samples automatically over
the eight metre sample depth.
Initial concerns with representivity due to ultrafines (less than
90 microns) were tested by comparing the blasthole sample, with
the ultrafines from the pulse collector (Table 1). A positive
variation does exist, with ultrafines being higher in iron grade
and lower in contaminant grade. However, the bias and flexibility
GRADE CONTROL IN THE SOUTHERN MIDDLEBACK RANGES — A CASE STUDY
FIG 3 - Cross-section through the Iron Duke orebody, showing the changing geology with wide ore zone at the top west of the pit and
narrow lenticular ore pods at the base and to the east.
FIG 4 - SDS sampling system fitted on Ingersol Rand blasthole rig
at the SMR.

of the using a blasthole sampling system when compared to the
previous sampling method outweighed any sampling concerns.
As the Iron Duke, Iron Knight and Iron Duchess pits are very dry
(less than 3.5 per cent moisture), water is not an issue until the
pits drop at least another 100 mRL.
The effect of the improved sampling was to improve contact
delineation, as it was better able to follow the dip of the orebody
and grade estimation through a more rigorous sampling
procedure. Figure 5 illustrates the improved contact definition
and sampling accuracy between the old collar sampling method
and the use of the blasthole sampling system. To represent the
old sampling method samples have been composited to 8 m.
Grade control interpretation
Grade control interpretation was originally carried out using
polygonal estimation from shapes created while looking at
drill hole grades in plan view. Each shot was looked at in isolation
and no allowances were made for orebody continuity or geological
inputs. As a result mark-outs generated did not represent the
orebody geology. This lead to major corrections to mark up once
blocks was laid out or during mining, leading to excessive dilution
and ore loss and inaccurate estimations of block grades.
In conjunction with the updated sampling process the grade
control system has been changed to a block modelling system
based on sectional interpretations. The main features of the
system are outlined below:
1. assays are imported into Datamine and checked to
standards and blasthole logging;
2. interpretations are then completed on a sectional basis
using all available blasthole data, mapping and resource
drilling data, including blasthole data from above and
adjacent benches;
3. the blast outline is overlain to the resource model and
bench geology derived from mapping and, if needed,
corrections made to confirm the geology (Figure 6);
4. a three-dimensional (3D) block model of the blast is then
generated using ordinary kriging (Figure 8 shows a typical
variogram in the hematite ore) or inverse distance squared
from all blasthole data;
5. the model is then sliced to the mid point of each planned
flitch and mark-out generated based on like mining ore
categories (Figure 7); and
6. destinations are assigned to ore blocks and areas where
the geology is interpreted to be difficult are flagged for
particular attention during ore mining.
MINE PLANNING AND ORE MINING
Mine planning
Prior to 2002, short-term scheduling meetings were held on a
weekly basis with no formal meeting process recorded on a daily
basis to review the progress of the week’s plan. No reference to
the long-term mining schedule was made during the weekly
meeting. This combined with poor ore predictability lead to an
emphasis on satisfying short-term grade requirements, leading to
a severe deviation from the mining schedule and essentially a
‘high grading’ of the hematite orebodies.
One of the first improvements made was to introduce daily
planning meetings. The agenda of the daily meetings is outlined
below:
1. review the previous 24 hours’ production;
2. analyse any deviation from plan;
3. highlight any Qa/Qc issues encountered in the mining,
crushing or blending process;
D CRAWFORD et al
Sample ID Type Fe SiO2 Al2O3 LOI P CaO MgO Mn S TiO2 Na Zn K2O
1 Sample 63.81 3.03 1.78 1.67 0.03 0.11 0.900 0.143 0.005 0.152 0.057 0.007 0.014
Ultrafines 64.4 2.82 1.78 1.6 0.03 0.13 0.780 0.130 0.005 0.072 0.060 0.012 0.012
2 Sample 64.11 1.21 0.73 4.55 0.13 0.02 0.168 0.699 0.011 0.076 0.043 0.010 0.042
Ultrafines 65.4 1.02 0.68 3.4 0.1 0.02 0.140 0.550 0.018 0.062 0.030 0.004 0.033
3 Sample 68.45 0.50 0.29 0.76 0.02 0.02 0.161 0.077 0.005 0.032 0.010 0.003 0.006
Ultrafines 68.8 0.48 0.3 0.6 0.02 0.01 0.160 0.040 0.005 0.005 0.010 0.003 0.003
4 Sample 53.02 8.61 4.44 4.51 0.07 0.13 1.029 1.979 0.021 1.450 0.197 0.012 0.423
Ultrafines 56.2 7.22 3.73 3.7 0.05 0.03 0.770 1.160 0.023 1.181 0.170 0.008 0.334
5 Sample 65.39 2.67 1.00 1.62 0.04 0.03 0.289 0.186 0.010 0.215 0.089 0.004 0.028
Ultrafines 66.8 1.79 0.95 2.2 0.05 0.02 0.150 0.150 0.014 0.097 0.090 0.003 0.017
6 Sample 67.55 2.28 0.14 0.55 0.05 0.03 0.048 0.066 0.005 0.006 0.016 0.003 0.004
Ultrafines 67.7 1.88 0.28 0.7 0.05 0.02 0.040 0.090 0.007 0.005 0.040 0.003 0.004
TABLE 1
Ultrafines versus blasthole sample representivity.
FIG 5 - Double bench sampling system is illustrated in the second
set of blastholes. Note the improved delineation – compared to the
upper set of blastholes, which were completed as a composite.
Numbers represent iron grades as per cent Fe.

FIG 6 - Interpreted geological bench plan showing the thin hematite zones typical of the southern area of the Iron Duke pit. The overlay
shows the blast outline from Figure 7.
FIG 7 - Block model view of blast showing mining block outlines.

4. communicate the next 24 hours’ production and outline the
expectation to all production personnel involved (this
ranges from drill and blast to crushing and blending); and
5. bring together members of HWE and OneSteel
management in a formal way to ensure communication is
open and that all parties are working towards a common
outcome.
The targets for the daily production meetings are derived from
the weekly planning meeting.
The weekly meeting communicates the short-term mine
planning goals with reference to the long-term mining schedule.
Any deviations from the previous week’s production are
discussed and these are reconciled to the long-term planning
position.
The net effect is to tie in the short term, medium-term and
long-term planning goals. Any deviations that are occurring can
be analysed quickly and the effects to long-term planning targets
known prior to any major schedule deviations occurring.
Ore mining
Mining in the SMR has in the past been carried out using
Caterpillar 992 front-end loaders on 8 m benches. To allow for
the low break-out of the front-end loaders, blasts were designed
with short extent and an emphasis placed on ‘throwing’ the blast
forward to allow for easy digging. The impact of which was
excessive ore movement.
Due to short shot length and the use of loaders mining from the
floor upwards, mining was completed along the strike with no
respect for the geometry of the orebody. High-grade blocks were
frequently slotted or mined from footwall to hanging wall, leading
to excessive dilution, ore loss and poor grade reconciliation. This
in turn led to frequent deviations from mining schedule to
compensate for poor grade prediction/recovery.
As part of business improvement plans implemented between
OneSteel and HWE in 2003 a Liebherr 994 excavator was
brought to site for wall control and waste removal, with a view to
use the excavator for ore mining, the main advantage being the
improved selectivity that excavators provide. After completion of
the grade control improvement review, ore mining using the
excavators was quickly implemented and a second excavator
(Liebherr 994) arriving within two months of the decision to
utilise excavators instead of loaders being made.
The review and change from front-end loaders to excavators
had impacts on all aspects of the mining process, including
staffing, drill and blast, scheduling and blending. Shot size had to
be increased six-fold from an average strike length of 25 m to
150 m in length in order to have enough room to use excavators
effectively and efficiently. Choke blasting become the normality
rather than the exception leading to less ore movement during
firing.
The conversion to larger shots was hampered initially by drill
and blast design, assay turnaround and scheduling as enough
blasted stocks had to be generated to allow for different grade
options to be available in the ore mining process. This also meant
a total review of the short-term and medium-term mining
schedules to account for larger blasts, limited mobility of
excavator mining fleets and the use of multiple flitches instead of
single bench mining.
The change to excavator mining was also combined with an
increase in the amount of contractor supervision during mining
and a change to mining respecting the geometry of the orebodies.
This meant a move to mining from hanging wall to footwall and
the mining along contacts rather than slotting along or across
them.
The hematite orebodies of the SMR have excellent visual
control. This combined with the limited mobility of the excavator
has also led to a change from partial to seven-day grade control
coverage and the scheduling of thin ore zones on day shift only.
The new mining methods combined with the new blocking out
procedures and the increased supervision have to date resulted in
much lower dilution and ore loss during mining and of course a
much more predictable quality of ore.
STOCKPILE CONSTRUCTION
Traditionally the construction of stockpiles has been limited by
restrictions on the quantity of ore allowed from each pit. These
restrictions were originally placed because of concerns of grade
predictability as well as textural variation. In general a stockpile
was built from direct feed (direct from pit to crusher) as much as
possible. Stockpiles were normally built with available pit
material first and then cleaned up in a ‘reactive’ manner.
The result of this type of building was to have a very small
supply of high-grade clean ore on the ROM pad at any given
time and also a large amount of ‘offspec’ ore (ore that was high
in one or more contaminants). Figures 9 and 10 show the effects
of poor grade predictability during stockpile building.
D CRAWFORD et al
FIG 8 - Typical iron (Fe) variogram. Major orientation set at 0° towards 355°. Dashed line represents sample pairs.

Time on the x-axis for Figures 9 and 10 is the period in days
over which the stockpile was completed. Predicted and Actual is
the predicted and actual grade of the contribution that was added
to the pile on a specific day. Pile grade is the cumulative grade of
the pile at a particular point in time. The pile grade on day 17 is
the final build grade of the pile.
There is significant variation between the predicted grade of
the pile contribution and the actual grade of the contribution.
This poor reconciliation contributed to a lack of confidence when
performing stockpile blending. This is evident in the ‘spiked’
nature of the contributions to the stockpile during the build
process. As the average grade of the pile moved towards the
upper limit or lower limit of the spec, ore that was well below or
well above the current grade of the pile was added. Ore of the
same grade was not added in case the predicted grade of the
contribution was incorrect, putting the pile out of spec.
This type of stockpile building was not optimising ore stocks.
This led to high-grade Iron Duke and Iron Duchess ore was being
used to blend down highly off spec ore at the expense of
moderate grade/moderate offspec material. The net effect of this
was a rapid depletion of high-grade ore, leading to severe
deviation from the mining schedule that had the potential to
shorten mine life.
Short-term scheduling was therefore difficult due to the lack of
accuracy in grade control predictions, meaning that variation in
predicted grades against actual grades forced material to be
FIG 9 - Fe build grades versus stockpile grades.
FIG 10 - SiO2 build grades versus stockpile grades.

sourced outside the weekly and monthly schedule in order to
satisfy blending requirements. Inter-pile variability also suffered
due to the above problems and it wasn’t uncommon to see SiO2
grade have a variance of in excess of 20 per cent (relative
difference) between piles (Figure 13).
The new mining methodology means that very little material is
now direct fed. The stockpile building process as it stands now is:
1. Ore from the pit is stockpiled (and blended) on the ROM
pad.
2. Ore is reclaimed from the ROM pad and blended through
the primary crusher by loader and/or truck.
3. -140 mm ore then passes through the secondary crushers
and is crushed to -75 mm.
4. For Whyalla stock and export stock this material then
passes through the ISO standard sample tower, which takes
a lump and fines sample every 1000 t. This material is then
blended onto 50 000 t stockpiles for railing to Whyalla.
5. Low-grade ore is taken from the LGO dumps and blended
through the primary, secondary and tertiary dry crusher
(producing -32 mm material), through the sample tower
and into 30 000 t stockpiles as feed for the low-grade
beneficiation plant.
D CRAWFORD et al
FIG 11 - Recent Fe build grades versus stockpile grades.
FIG 12 - Recent SiO2 build grades versus stockpile grades.

Increased grade predictability and cleaner mining methods
have caused high-grade stock levels on the ROM pad to increase
and ‘offspec’ ore levels to drop. This is due to better grade
predictability (Figures 11 and 12) provided by accurate
sampling, applying geology to blockouts and making the more
selective production machines work with the geology of the
orebodies. Variation between piles has also benefited from the
new processes and for example SiO2 variation has now been
reduced to nine per cent (relative difference) (Figure 13).
Another item to be implemented after the review is the
submission of standards during the stockpile building process.
Although an ISO standard sampling station has always been
used, the accuracy of the laboratory Qa/Qc was not checked on a
daily basis. The introduction of standards has increased the
confidence in the grade of the final build piles. Prior to the
introduction of standards and duplicates there was a culture to
‘blame the laboratory’ and not review the sample process, or look
at outlying samples in an objective manner.
RECONCILIATION
Prior to the grade control review, reconciliation was completed
month by month purely as a tonnage balancing operation. Survey
monthly pickups were cut against each other (current month
versus previous month). These areas were then used to cut a
section of the resource/reserve models to compare with
production tonnes. If tonnages were similar and they were
generally within ten per cent, the pit was considered reconciled.
Review of the procedures identified that although the
reconciliation appeared fine it only gave results for the global pit,
whereas certain areas of the pit performed better than predicted
while other areas had very poor recoveries. As a result, the
reconciliation method could be described as indicative at best,
and did not highlight critical issues with the grade control and
ore mining processes.
Reconciliation is now completed on a blast-by-blast basis
(Figure 14). Wireframes for each blast are cut against the
resource and reserve models to give an accurate representation of
what had been predicted and comparison made. The updates to
the reconciliation process have aided in the following:
1. Making it easier to monitor digging practices and their
effects.
2. Identify mining areas that have poor recovery and grade
reconciliation and also identify areas that have yielded
higher than predicted.
3. Used to mark areas for further work with resource modelling
and/or particular attention when digging in the future. Areas
that have been identified to be under performing or over
performing in the resource and reserve models can then be
targeted for further drilling or interpretation updates in future
resource models.
4. Identify blasts that may require more intense supervision.
5. Continuos monitoring of the grade control process, allowing
for updates to grade control systems and mining methods.
CONCLUSION
The improvements in grade control implemented at the Southern
Middleback Ranges have been driven by a need to update
systems in the face of a changing orebody and increased
customer demands.
None of the changes or systems implemented are extremely
advanced. All improvements made are in common use in most
other open cut metalliferous mines in Australia and were
completed with minimal expense and form a logical progression
from the initial definition stage through to the blending and
long-term planning stages.
The key improvements made are:
1. decrease in variability for the key elements (SiO2, Al2O3,
Fe, P) to 0.07, 0.14, 0.3 and 0.002 respectively from 0.11,
0.18, 0.34 and 0.003 respectively;
2. increase in reserve recovery from 88 per cent prior to
implementation to 107 per cent after implementation of
improvements;
3. the ability to stick to a mining schedule with proactive
long-term goals based on customer demand rather than a
reactive approach; and
4. a continuous improvement approach to processing or
mining based on orebody conditions.
FIG 13 - SiO2 standard deviation between export shipments showing the reduction in variability after the introduction of the new grade
control procedures.

ACKNOWLEDGEMENTS
The authors wish to thank HWE for their help implementing the
improvement process; OneSteel Management for permission to
publish data presented in this paper and OneSteel Iron Duke
Staff for their assistance in preparation of this paper and Andrew
Waltho for review of this paper.
D CRAWFORD et al
FIG 14 - Reconciliation of a completed shot in an area of thin ore zones. Note: Ore blocking = production (indicating clean digging).
Mining through thinly banded areas (reserves are much lower than resource). Production = ore blocking = resource (new methods
are able to completely and cleanly mine the whole resource).

Grade Control Paper 2

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