sampling australia 2013 - brisbane - may 14-15 putting sampling theory in practice best-practice...

Sampling Australia 2013 - Brisbane - May 14-15

Putting Sampling Theory in Practice

Best-PracticeBlast Hole Sampling

Dominique Francois-Bongarcon

Agoratek International Canada



Plan of Talk• Introduction: some aspects

of grade control

• Precision issues

• Bias issues

• QA-QC aspects

• BH versus RC holes



INTRODUCTION

Some Aspects• Grade Control (GC): How to maximize ore body

representation (Precision and Unbiasedness)

Precision: What is an acceptable margin of error? What is involved? How important is it?

Unbiasedness: How do we achieve unbiased sampling?

• Data Quality: QA-QC• Alternatives: BH versus RC drilling



BH Sampling Precision Issues



Precision Considerations• The kriging variance of a grade-control model block

based on a 25m RC sample grid will generally be at least 2-3 times higher than that derived from BH samples. The higher resolution provided by BH drilling is paramount!

• Many GC Samples will usually be combined into a block estimate, their individual precision is not as important as often believed.

• Ore Heterogeneity studies can help control sampling precision (sample mass and preparation protocols should always be optimized first).

Precision



BH Sampling Bias Issues



Testing for Biases• Whatever sampling method is selected, it should be

subjected to sampling bias test to make sure statistically significant biases cannot be evidenced

• Only a comparison to a full BH pile reference grade is meaningful

• Several sampling methods should be tried• All important elements should be compared (for

instance, it is not uncommon to see auger samples to show a bias for Cu and not for Au)

• Precision will come as a by-product

Bias



Testing for Biases

• Circular segregation around the pile seems to be a larger priority problem than sample correctness (provided biases are within acceptance levels)

• Practical issues should be addressed too (safety, implementation, turn-around)

• The use of a radial collecting pan is deficient on both these accounts

Bias



BH Main Sampling Methods

• Pie-shaped collector

• Cylinder type (spear, auger)

• Shovel

• Rocket sampler



Pie-shaped collector

• Pro: good size distribution representativeness

• Con: does not handle the circular segregation around the BH pile

Methods



Cylinder type (spear, auger)

• Pro: handles the circular segregation around the BH pile

• Con: poor size distribution representativeness

Methods



BH Sampling QA-QC Issues



QA-QC ASPECTS

• Both sampling and assaying QA-QC need to be used to ensue the GC data are reliable enough

• Sampling QA-QC should consist of duplicate field samples

• These should also be used to monitor proper, day-to-day implementation of the sampling procedure

QA-QC



Blast Holesversus

RC Holes



The truth is...• Blast hole (BH) samples are not as bad as

they have sometimes been portrayed

• RC samples are not always as good as they have sometimes been portrayed

• Today, as always, BH, unlike RC samples are the norm in most large mining houses of the world for open-pit grade control

BH vs. RC



General• For grade control, the USER of the sample

data wants Resolution and Unbiasedness more than anything else.

• The user is the mine geostatistician, not the sampling expert, nor the rig manufacturer.

• Precision (reproducibility) is not as big an issue as some say.

BH vs. RC



Comparative Pros

Superior resolution

Lower cost

No smearing

Blast Holes RC Holes

BH vs. RC



Comparative Pros

Several benches

Precise intervals

Favorable logistics

Consistent sampling

Easy automation

Angled drilling


Superior resolution

Lower cost

No smearing

BH vs. RC



Comparative ProsBlast Holes RC Holes

Superior resolution

Lower cost

No smearing

Several benches

Precise intervals

Favorable logistics

Consistent sampling

Easy automation

Angled drilling

BH vs. RC



Comparative Cons

Subdrill

Difficult sampling

Poor logistics, safety

No automation


BH vs. RC



Comparative Cons

Very high cost

Poor resolution

Downhole smearing

Incorrect duplicates

Sampler leveling

Dust suppression


Subdrill

Difficult sampling


No automation

BH vs. RC



Comparative ConsBlast Holes RC Holes

Subdrill

Difficult sampling


No automation

Very high cost

Poor resolution

Downhole smearing

Incorrect duplicates

Sampler leveling

Dust suppression

BH vs. RC



BH vs. RC: in conclusion

• To a large degree, the Australian ‘fad’ for grade control (GC) RC sampling is slowly fading away

• The rest of the world mostly ‘sticks’ to BH sampling as it always did

• Finally, the two methods do not compare directly, as they are really best for different applications:

Short- term GC for BH Medium-term mine planing for RC

BH vs. RC



BH vs. RC: in conclusion• Clearly, in an ideal world, if we could afford to drill RC

holes on a 5-7m grid spacing for grade control, this would be, by far, the best (or least bad) solution!

• However, hoping for it is simply naive: RC are expensive BH have no drilling cost Unlike what is sometimes believed, we know how to

ensure BH samples are unbiased And precision, again, is not a critical issue

BH vs. RC



BH vs. RC: in conclusion• It is true however, that what is recovered from a

BH is not always the entirety of the drilling interval, so that BH indeed are ‘imperfect’

• However, RC downhole smearing and wall collapsing are also common issues that can easily be quite as critical

• As a result, this is not a good choice argument

BH vs. RC



BH vs. RC: references• On RC better than BH:

Pitard, F. 2008, Blasthole Sampling for Grade Control – The Many Problems and Solutions. Sampling 2008, AusImm-CSIRO, Perth. May 2008.

• On the reverse:François-Bongarçon, D. 2010, Are Blast Hole Samples That Bad And RC Samples That Good For Open Pit Grade Control - Story Of a Trade-Off. Sampling 2010, AusImm-CSIRO, Perth. May 2010.

• Dominique François-Bongarçon; AGORATEK International San Mateo, CA - USA

• [email protected].

BH vs. RC



Talk Conclusions• The use of BH samples for grade control is widespread

and has been satisfactory until now when well controlled.

• Adequate control implies: choosing a method among several (during or

after drilling) testing for biases then controlling quality using duplicate samples



Talk Conclusions

• In spite of definite qualities, RC drilling is not necessarily a more desirable alternative, as costs limit its efficiency

• If RC drilling is also used (e.g. for medium-term mine planning), then it can be combined with BH information to enhance GC predictions



THANK YOU!

sampling australia 2013 - brisbane - may 14-15 putting sampling theory in practice best-practice...

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