A: I work with underground mines, so my comments are restricted to that environment.
For underground mines that have a fuzzy gradational contact you need to know where the dilution is coming from, so blithely expanding a skin in all directions is not applicable. It has to be looked at on a stope by stope basis.
For other types of mines that don’t have a gradational contact, the grade drops off precipitously beyond the contact, here is an example for a narrow vein mine:
Dilution modelling can be done for underground and software providers often provide tools to model the dilution. I emphasize quantifying the amount dilution of correctly as this may be far more important than the actual grade of the dilution and that in fact for mines where the grade drops precipitously at the contact an average background grade can be used. Here is why:
I have seen numerous cases of operating mines as well as mining studies that have been far off base when it comes to the amount of dilution. Consequently, quantifying dilution in operating mines has become an important part of my work.
Compare the following:

  • A desktop professional estimates the dilution of a narrow vein at 15% and the block model gives a background grade of 0.5g/t in a mine that has a cut off grade of 5g/t.
  • A second professional looks in detail at the amount of dilution and finds that 60% is a reasonable expectation for that mine. (Not an uncommon finding by the way) Even if the second professional uses an average value for the background grade, in fact, even if 0g/t is used, that estimate will be closer to the truth.

Remember also that dilution comes from different sources, each with its own impact on metallurgical recoveries. For example :

  • How does the cement or reagents in the backfill affect the recoveries at the mill? How do we factor that into dilution? (remember it has a different density)
  • What about ore contact variability (sinuosity)? What if we didn’t mine the ore because the contact wiggled out of the mining zone and all we got was mostly waste! What dilution factor would you use for that?
  • What about an LHD operator who dumps ore in a remuck bay on top of some waste
  • What about the truck driver who gets confused about where he is taking his load waste dump or ore dump? This happens and frequently…
  • What about loss of fines in the muck?

I think you might see that the exact value of the background grade can quickly become less important than other factors.

– Paul Tim Whillans

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