1. Best-Practice Blast Hole Sampling
Dominique Francois-Bongarcon
Agoratek International Canada

2. Plan of Talk Introduction: some aspects of grade control
Precision issues
Bias issues
QA-QC aspects
BH versus RC holes

3. 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

4. BH Sampling Precision Issues

5. 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).

6. BH Sampling Bias Issues

7. Testing for Biases
Bias
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

8. Testing for Biases
Bias
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

9. BH Main Sampling Methods
Pie-shaped collector
Cylinder type (spear, auger)
Shovel
Rocket sampler

10. Pie-shaped collector Pro: good size distribution representativeness
Methods
Pie-shaped collector
Pro: good size distribution representativeness
Con: does not handle the circular segregation around the BH pile

11. Cylinder type (spear, auger)
Methods
Cylinder type (spear, auger)
Pro: handles the circular segregation around the BH pile
Con: poor size distribution representativeness

12. BH Sampling QA-QC Issues

13. QA-QC ASPECTS
QA-QC
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

14. Blast Holes versus RC Holes

15. The truth is...
BH vs. RC
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

16. General
BH vs. RC
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.

17. Comparative Pros Blast Holes RC Holes Superior resolution Lower cost
BH vs. RC
Blast Holes RC Holes
Superior resolution
Lower cost
No smearing

18. Comparative Pros Blast Holes RC Holes Several benches
BH vs. RC
Blast Holes RC Holes
Superior resolution
Lower cost
No smearing
Several benches
Precise intervals
Favorable logistics
Consistent sampling
Easy automation
Angled drilling

19. Comparative Pros Blast Holes RC Holes Several benches
BH vs. RC
Blast Holes RC Holes
Superior resolution
Lower cost
No smearing
Several benches
Precise intervals
Favorable logistics
Consistent sampling
Easy automation
Angled drilling

20. Comparative Cons Blast Holes RC Holes Subdrill Difficult sampling
BH vs. RC
Blast Holes RC Holes
Subdrill
Difficult sampling
Poor logistics, safety
No automation

21. Comparative Cons Blast Holes RC Holes Subdrill Very high cost
BH vs. RC
Blast Holes RC Holes
Subdrill
Difficult sampling
Poor logistics, safety
No automation
Very high cost
Poor resolution
Downhole smearing
Incorrect duplicates
Sampler leveling
Dust suppression

22. Comparative Cons Blast Holes RC Holes Subdrill Very high cost
BH vs. RC
Blast Holes RC Holes
Subdrill
Difficult sampling
Poor logistics, safety
No automation
Very high cost
Poor resolution
Downhole smearing
Incorrect duplicates
Sampler leveling
Dust suppression

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. THANK YOU!