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Biased sampling for exploration (4 replies)

8 months ago
Oberfuhrer 8 months ago

In exploration the goal is often to deliberately bias the sample to find trace concentration of target minerals. The classic example is panning concentrates in search of diamond indicator minerals. I'm interested in comments from members on the more successful stream sediment and soil sampling methods they have used for targeting base metal and gold deposits.

John Koenig
8 months ago
John Koenig 8 months ago

The goal (in metals exploration) is more commonly a consistent sample, rather than a biased sample in my opinion. This is because you need to be able to compare the analytical results across a large number of samples to identify the anomalies. We may sieve to a particular fraction in soils for example, to avoid fractions that may be foreign to the site (e.g. windblown material), but this is in order to raise the concentration target elements above the analytical detection limit. If the sieving doesn't produce a consistent sample, such as when the soil type changes dramatically, then it's value starts to decline <--- unless you level for soil type effects of course :)I don't see that many panned cons taken for routine analysis as such, more for the ability to identify key phases or give an indicative result that some mineralization may be around. That's a significant difference in application.

Carl Jenkins
8 months ago
Carl Jenkins 8 months ago

I was on a sampling crew that sampled over the Gerrit Canyon, Snow Canyon and North Fork/Big Springs, Nevada gold deposits. Our sampling was excellent, the QC/QA was spot on, but our company missed those deposits.

When the results would come back from the labs, the Project Geo would take a red pen and arbitrarily cross off all the Au numbers he "felt" were too high or aberrant.

Later, as a consultant, I had the opportunity to re-sample the North Fork/Big Springs area once again. We sampled on 60m centers over a 15.5 square kilometer area.

That time, the Project Geo was upset and disillusioned with our work as very few of the assays returned Au. However, It was pointed out to him the As anomaly was several orders of magnitude higher than the regional norm. A new 'model' was put into place and a major gold discovery was made.

We utilized a very standard practice of consistently sampling from the "C" Zone, field screening the samples to a -40 mesh after which the lab would screen to a further -80 mesh and then assay with either Fire or AA or both.

Quite often, the methodology of sampling is not the most important factor in discovering a blind ore body, but rather, the interpretation of the results against an unworkable model can be the 'killer' element.

Maya Rothman
8 months ago
Maya Rothman 8 months ago

You summarizes the key points effectively, and as points out it's all about pattern recognition and contrast. For actual information and case studies (albeit with an Australian flavor), check out the CRC LEME website which summarizes the decades of work by people like Ravi Anand and Charles Butt. There are some great documents about how certain elements behave in the regolith and therefore sample selection criteria.

I can't put my name to any discoveries, but I think I can say the team I worked with in WA in the 1990's missed a few because of our lack of understanding of the importance of putting assay results in a landform/regolith context.

8 months ago
Unterstarm 8 months ago

Thank you for a very interesting discussion. About diamonds and panning I think, it is matter of a standardization of concentration method. It can be manual panning with people who pan the same, better one person or a separation device. But at the end Geologist who analysis the data is the most important link in the chain as it was depicted by. That is why using semi quantitative analysis can be better than precise and expensive techniques.