Geology & GeoMetallurgy

Geology & GeoMetallurgy 2017-04-04T06:58:01+00:00
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Sampling Visible Gold in Diamond Drill Core (9 replies)

Bill Fraser
1 year ago
Bill Fraser 1 year ago

I am looking for some feedback regarding the importance of analyzing a diamond core sample with visible gold - it is a sample I would like to see preserved as it is not yet a common sight in a deposit. However, I am not sure how to then deal with it in a QAQC process to be included in ore resource estimation. It is a high-nugget deposit, so the surrounding sample points cannot be relied on to reflect the bonanza

Any thoughts?

1 year ago
Amar 1 year ago

Preserving even 10m core with VG should not affect your resource estimation of the whole deposit. It is normal to preserve VG part of the core length. You need to preserve only 10cm or 20cm with the VG as a sample unless of course the purpose of your drilling campaign is to preserve VG. VG only does not contribute to nuggety in resource estimation of a deposit, but the spatial distribution or variance of the deposit is the main cause of nuggety. Check your sampling method.

Sugar Watkins
1 year ago
Sugar Watkins 1 year ago

Visible gold (VG) often presents security and QA/QC problems in addition to estimation problems. I suggest photographing the core with VG before splitting and after each size reduction step. I generally prefer saving half-core but if rock quality is good enough a skeleton is probably sufficient. One cannot say for sure without seeing the core and the data but screen fire assay is probably the best way to assay in the presence of VG. It should go without saying that no jewellery is allowed in the sample prep areas, at least two people should be around during sample prep and a video camera doesn't hurt. Best practices are to keep all samples, rejects and pulps of high-grade under lock and key. Whatever is preserved should only be allowed out of a secure area with sign out. (I know of a couple of cases where high-grade intervals have disappeared.)

Victor Bergman
1 year ago
Victor Bergman 1 year ago

What would be your thoughts for my case: I have an ore of 100-1000 ppm with course gold and galena veining? I’ve split the core in halves, crushed one ½ meter and going to screen +0.5mm and concentrate the coarse gold with a gravity table and pulverize and split the -0.58 mm and submit 100 g for fire assay. How should I concentrate the gold if I have some 1-2kg of +0.5 with 10-20% of galena in the sample? What products should I sample and how much material do I need to assay?

1 year ago
Oberfuhrer 1 year ago

From your description, I guess you are on a typical coarse-gold deposit. The presence of coarse gold has a serious impact in assay repeatability and, hence, in analytical precision. Assaying the -0.58 mm fraction by conventional fire assay, even if you use 100 g aliquots, will be surely reflected in a very low analytical precision, which is characterized by very high variability in the assay duplicate results.

I suggest you to use the screen assay (also called metallic assay) method. Any major lab is able to assay gold with this method, but I am including below a brief description:

Pulverize a large split (not less than 500 g, and preferably 1,000 g) to 95% passing 0.106 mm (150 mesh).
Wet-sieve the entire split using a 150 mesh o 200 mesh sieve.
Dry and weight separately both the oversize and the undersize fractions. If pulverization was properly conducted, the weight of the oversize fraction should not exceed 50 g (approximately) in a 1,000 g split.
Fire-assay the oversize fraction in a single fusion to obtain the oversize gold grade.
Fire-assay in duplicate the undersize fraction following the ordinary FA protocol (30 g or 50 g aliquots), and calculate the average undersize gold grade.
The gold grade of the original sample is calculated as the weighted average of both fractions, using the weights and gold grades previously determined for each individual fraction.

As you will have two assays from each undersize fraction, you can use this information to assess the analytical precision. Hopefully, it will be now within acceptable limits. Unfortunately, you cannot replicate the oversize analysis, because you will have only one result for each sample.

The screen assay is expensive, but, to my best knowledge, it is the only way to obtain sufficiently precise fire assay results in coarse-gold situations.

Galena behaves in a more predictable way than gold during pulverization. There is no "coarse galena" effect, so to assess for lead, simply take an ordinary assay aliquot from the originally pulverized (and well homogenized) sample. Notice that this aliquot should be taken prior to any screening.

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

I worked in the same deposit before with most of the time we found visible gold on the core, because our mineralisation’s was controlled by shears zone most of the time the visible gold occurred on the intersection of these shears zone to dealt with the visible gold, normally we sample into minimum sample intervals (in our case 30cm) as this is the smallest volume that fit to our safe sample nomogram chart, then we do screen fire assay which is the best method to capture this visible gold. Then put details note in the description which allows us to recognize when we get into grade estimation/statistics analysis.

Sugar Watkins
1 year ago
Sugar Watkins 1 year ago

The idea behind screen fire assay is that you assay all of the course material so I would suggest going to a finer crush than 0.5mm until the fraction on the screen is an assayable quantity (<100gm) . You should do multiple assays on the material passing the screen to make sure you still do not have nugget problems. Another thing which may result in even more depressing (by depressing I mean higher variance between the two samples) is to split the half core into two quarters and do screen fire assays on the two pieces of quarter core. Doing screen fire assay on quarter core probably provides more information on the variability associated with coarse gold than knowing the assay of the half core. You should also run some material known to be metal free through the sample prep circuit after prepping the VG samples and assay it to make sure you don't have cross-contamination.

Victor Bergman
1 year ago
Victor Bergman 1 year ago

Your method makes a lot of sense. What particular pulverising method would you recommend? Thanks for a very good advice.

1 year ago
Oberfuhrer 1 year ago

As long as you reach the required size target (95% passing 150 mesh, for example), you can use whatever pulverization method you like, although the method should ensure that no gold is lost. The implication of that size target, though, is rather practical than qualitative. The screen assay method requires that you (weight and) assay all the oversize. Let's say that you pulverize 1 kg sample and you do not reach that size target, but only 70% passing 150 mesh. In principle, when you screen 1 kg pulverized sample using a 150 mesh sieve, the oversize will weight approximately 300 g. With such a large weight, you will hardly be able to do this on a single fusion using the regular fire assay crucibles. You will probably require various fusions, making the analysis even more expensive. The undersize, though, can still be assayed in two fusions. If the sample was pulverized to 95% passing 150 mesh, the oversize will weigh approximately 50 g. This weight can be easily handled in a single fusion.

Making a coarse-gold-bearing sample pass through a 150 mesh sieve has an additional potential complication that should be taken into consideration. Some minuscule gold particules (but still relevant at such low grades) may be retained in the sieve holes, therefore altering the assay result. Special plastic sieves are available in the market that can be used to screen the sample and be placed in the crucible together with the oversize. The plastic will burn and no gold will be lost. 

Marshal Meru
1 year ago
Marshal Meru 1 year ago

A target of 98% -150 M appears to significantly reduce the assay variability within the undersize product if coarse gold is present. Blending the pulverized product in disposable sample cups significantly reduces risk of gold lost (or transfer to later samples) via blending surfaces. Blending on roll cloths and then transferring to envelopes appears, with coarse gold present, also to occasionally associate with "minuscule gold particules (but still relevant at such low grades)" causing problems,

In some cases, if the portion pulverized is large enough (300 - 600 g) and the crush before rotary split to generate the pulverize portion is fine enough (4M or 6M), a representative assay can be generated on moderately coarse gold without resorting to a screen fire. This is less expensive (lower labour), but requires careful QC monitoring via spot checks to insure both crushing and pulverizing standards are sustained. Hand "feel" tests, skill-validated against screen tests, are best - the sieve loss/bias of value is very real. If your material is amenable to this alternative to screen fires, is also advisable to apply semi-random pulls of crushed rejects to compare how well splits replicate. (Semi-random means pull the checks from samples for which standard pulp replicate QC assays show higher variability - worse case checking is more likely to spot a concern.)

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