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Best-Practice Blast Hole Sampling

Best-Practice Blast 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.

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Best-Practice Blast Hole Sampling

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  1. Best-PracticeBlast 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. INTRODUCTIONSome Aspects • Grade Control (GC): How to maximize ore bodyrepresentation(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 Precision • 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 isparamount! • 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 Methods • Pro: good size distribution representativeness • Con: does not handle the circular segregation around the BH pile

  11. Cylinder type (spear, auger) Methods • 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 HolesversusRC 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 BH vs. RC Blast Holes RC Holes Superior resolution Lower cost No smearing

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

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

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

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

  22. Comparative Cons 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: in conclusion BH vs. RC • 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: in conclusion BH vs. RC • 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: in conclusion BH vs. RC • 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: references BH vs. RC • 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 • dfbgn@att.biz.

  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!

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