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Quality control of Screen Fire Assay for gold (4 replies)

Bill Rico
9 months ago
Bill Rico 9 months ago

I have recently been looking at screen fire assays of core samples containing coarse gold. This seems to be a very difficult process for a project to quality control in terms of duplicates due to the in-homogeneity of the coarse fraction - finer fraction no problem. As far as determining accuracy via Certified Reference Materials, I cannot see how it will be possible to homogenize enough to certify the material within any reasonable limits at the coarse fraction. Only way I can think of is to use several different labs and see if there is any overall bias, but live with the poor precision - any ideas on this approach?

John Koenig
9 months ago
John Koenig 9 months ago

About the expected accuracy about SFA of core, I suggest you to refer to the recent paper I presented to Sampling 2012 Conference (http://is.gd/ROeOYa).

The overall measurement error (including sampling error) is so large than the use of CRM cannot give you the accuracy. It will just give you the analytical error/bias which is a small part of the overall error.

9 months ago
Gruppen 9 months ago

Assuming you are doing the assay on 1/2 core, assaying 2 1/4 cores may provide a measure of variability, particularly when you compare that variability to the nugget effect of the downhole variogram for that hole. If this is a vein type deposit you may not have very many pairs in the downhole variogram. If the samples are channels the channel variogram may also provide some information on the nugget. It is also worthwhile to look at the chunks on the screen which go into the screen fire assay. If the chunks are actually visible and there are less than 30 or so then the marbles in a jar statistical approximation may also yield some insights. If you can actually see the gold in the core you may also be able to come up with the average weight of the gold chunks, calculate the number of chunks in the ½ core and again use the marbles in a jar approximation.

Ace Levy
9 months ago
Ace Levy 9 months ago

How much of the coarse fraction is being sampled for the assay? I wouldn't be surprised if bias/scatter is happening at that point. Best to use a rotary sample divider so long as there is enough material to be split among the eight receivers "equally". If not, then the nugget effect will still be evident.

Bob Mathias
9 months ago
Bob Mathias 9 months ago

The best way is to do a combination of screen metallic’s, size-by-size assays, and triplicate analyses of properly sampled coarse fractions. This is a more practical approach and can be augmented by a simple gravity test to reduce the cumulative errors of sampling, sub-sampling, losing gold in pulverizing and subsequent measurement bias.

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