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Sampling a porphyry Cu-Au deposit (2 replies)

Oberstorm
10 months ago
Oberstorm 10 months ago

What is the maximum and minimum sampling interval suitable for cores in a porphyry Cu-Au deposit?

Dizzy Flores
10 months ago
Dizzy Flores 10 months ago

The short answer is 'it depends'. If there is a history of a certain sample length to define the geology and there is a lot of data then you may be stuck with that. For example a lot of old drilling one deposit I've worked on had 5 foot samples so there was not much choice other than sticking with 2.5 sampling intervals for new drilling. If a new deposit then you need to consider the likely future mining method and selectivity. If large open pit - perhaps two to three metres would be reasonable depending on geological complexity. A good place to look is stock exchange announcements to see what other are doing.

"All QDM drill core is sampled in continuous two metre intervals".

John Koenig
10 months ago
John Koenig 10 months ago

That is a good question. Porphyry Cu deposits are of very substantial size, and thus are typically mined using large-scale bulk tonnage methods. Consequently, ore blocks are usually estimated that are on the order of 5-10 meters in dimension. This means that samples collected from drill core through such deposits produce the best results if they are 1 to 2 m long (I have also seen lots that are 1.5 m long, but I think that is a consequence of geologists who are still working in the English measurement system; 1.5 m is close to 5 feet). I would probably consider 1 m to be the minimum sampling interval for use in a porphyry deposit, and probably 3 m as the maximum (although 1.5 or 2 m is probably the norm and best for most cases). Obviously, if a major lithological boundary occurs within the core, the adjacent samples should be terminated at the contact (and this typically lengthens and shortens the adjacent samples by a little bit). Geostatisticians can easily deal with the statistical issues associated with different sample sizes, so you need not worry about that.