Hot rock/mineral question

[Boarteats]

Wanted to ask a question to help think through issue of hot rocks/mineralization.

BACKGROUND:
I have access to a bunch of heavily mineralized quartzite. Full of iron oxides, apparent sulfides, etc. Geology and history of area suggests that quartzite conceivably could contain gold. Rocks are ridiculously hard, so have to roast them at 2000 F for an hour. Makes them much easier to break up.

USING FALCON MD20:
So, here’s the challenge. I’m working on pulling together samples for testing. Falcon MD20 scan of crushed samples is first check. It’s a fast fail test. No response indicating possible nonferrous metal, gets sample tossed into junk pile.

Of course, a “positive” response only means that gold or other nonferrous metal MIGHT be present in sample. Unfortunately, rocks likely contain maghemite, sulfides, and possibly other problematic mineralization. As a result, false positives are a big problem. I’d very much like to reduce false positives, since next step in sample testing is chemical analysis using very strong acids.

So, here’s what I’m doing.

– Sulfides: problem addressed with initial roasting.

– maghemite and other ferromagnetic mineralization: Use strong rare earth magnet to remove from sample.

– electrically conductive salts: dry samples before detector scan.

QUESTION:
Tried the above on a couple samples, seems to help. Question: is there a class of problematic mineralization that I am missing, i.e., minerals that cause detector to give false positive? Are any of my stated assumptions incorrect? Any other issues to consider?

Thanks in advance

 

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[DDancer]

Non-magnetic minerals like lead, copper, silver and what not along with decimated iron minerals that wont react to a magnet are possibilities. If your roasting all you samples to eliminate sulfides your likely going to create new compounds that simply cant be eliminated without chemical treatments. They may or may not be false positives. A detector is generally not the best tool to discriminate with unless you have very high grade samples and your likely tossing out some good material.
Otherwise some of your assumptions are ok in as far as what your doing -sulfides -salts. My thoughts.

 

[Boarteats]

@ddancer, Appreciate the response!

Certainly, there are minerals like copper, etc. that will result in false positives. Nothing that
I can do about that. Best that I can do is to reduce the probability of getting a false positive from other sources.

Excluding other metals like copper, lead, etc, the only false positive minerals that I've been able to discern are sulfides, graphite (large amount of it here), and maghemite. I'm sure that I'm likely missing something. Hence, the question.

I don't see how I'd get a false NEGATIVE with MD20 if samples are prepared as described in original post. This very well could be due to my own ignorance. Roasting addresses the two ways that I know of that gold can hide from a very sensitive detector, gold locked up in sufide compounds and (I think) gold dissolved into solution via interaction with manganese dioxide.

Did you have any specific examples in mind WRT potential for false negatives or were you only providing a general caution? I'm very much against throwing gold away.

 

[DDancer]

I'm not sure what your referring to about false negatives. I'll guess you mean that your detector is not registering anything from a given sample that has a known gold content. If thats the case I'll refer you back to my original, and general, warning : A detector is generally not the best tool to discriminate with unless you have very high grade samples.

When dealing with ore's gold is usually very disseminated thru the sample and depending on the treatment and concentration of the sample usually not detectable. An example is panning for flour gold. You pan and see the gold in the pan, scatter it about the bottom of the pan and you will not be able to detect it, concentrate enough of it in the bottom edge of the pan and eventually you can detect it. But you'll detect it very weakly because the gold is still not very concentrated as a solid piece.

The same occurs with sulfides. Sulfides are generally not detectable unless there's a good amount of free metal in the sample. Sulfides are usually a grab bag of metals so even after treatment, roasting, concentrations may become detectable for the free metals but you wont know what you have without further assay or chemical treatment. Roasting in its self does not concentrate the metals, it just removes the sulfides that render the metals non-conductive until removed. But just like flour gold scattered in the pan you'll still get a negative response on the detector unless there's some solid pieces big enough to register. Even then you still have a positive that grab bag needs to be refined.

As to positive hits on the detector I was simply pointing out other items that can make what you call false positives. As you said they can not be eliminated for the most part when it comes to metals but salts, ferrous materials and graphite can be largely removed. So your not really missing much with your question on positives. The problem comes with that grab bag of material you get when you have a positive, and when you get that negative you'll likely toss the baby out with the bath water. If your just high grading you'll still have to take the steps to assay what you get. If your going for it all then the detector is not going to be your friend in this case.