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Anyone On The Forum With Assaying Knowledge?


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I sent in a sample to two different assayers out of the same sample batch and received two very different results.

One assayer returned results of 0 Au and 0 Ag.  The other assayer returned results of .68 Au/ton and .36 Ag/ton.

Anyone have some serious thoughts on what would cause this?

Both assayers are professionals with 30+ years of experience.

Bugler

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Clay Diggins,

Thank you for responding.  The samples sent were out of the same crushed batch.  The batch was almost crushed to powder

and completely mixed.  8oz. was sent to each assayer and it was fire assayed by each according to each report.

I will call each assayer today.  Not to challenge their integrity but to see what they think could have caused this result.


Bugler

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With .68 gram per ton and an 8oz sample it could easily be the nugget effect if free milling gold is involved. Gold being malleable is particularly susceptible to the nugget effect in small samples.

Your sample was 1/4000 ton so at .68 gram/ton the assay result could be based on a single particle of gold weighing .0011023 of a milligram.

Perhaps only one sample from the split had a tiny gold particle? In which case, assuming excellent and multiple assays, the results would be .34 g/ton.

Fire assays are the standard but generally many assays are done on randomized samples before any assumptions about the quantity per ton of any particular deposit can be made.

I suspect you will hear something similar from your assayers.

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Thank you again, Clay Diggins,

I called both assayers and each has offered to run the samples again at no charge.  Both assayers were professional

and helpful.

Your explanation was very helpful as well.

Thank you,

Bugler

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I have material that runs 1.8 oz. to the ton consistently (quartz/free gold)   I sent in a sample for an assay(I don't know why now) prepared as instructed and it came back with zero gold.

It was a very reputable firm.   When I got the report back, I called them asking why no gold?  They said that is not uncommon. There just wasn't a particle of gold in the sample.

They said the best way to determine the gold content in that type of material was to crush and process a ton (or more) of material.

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Maybe the use of screen fire assaying would be more suitable if coarse gold is typically found in your samples vs the use of normal fire assaying.  I think the charge sample for normal fire assays is quite small, and may not indicate the true amount of coarse gold in the original sample, whereas on a screen fire the coarser material is screened out separately and fire assayed ( including the screen), and the finer portion of the sample is also fire assayed.  I know the mine I worked at had coarse visible gold in quartz veining, and results were often quite low from normal fire assays, whereas many of the screen fire assays often came back with some huge numbers.

Whilst not professing to understand the absolute technicalities behind the methods, just commenting on what I used to see come across my desk over the years with assay results.  Some of the gold/qtz samples we sent off had gold up to thumbnail size, hence those gold bearing sections of drill core in the ore zone were always marked up specifically for screen firing when logged by the geologists.

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Both results are so low that unless you are talking about a mountain of millions of tons of ore to be worked at a rate of thousands of tons per day, this rock is waste. Most ore tests will have some level of minor variances in the results due to just what Clay Diggings observed - an errant tiny gold particle makes it into one sample and the other has no such tiny particle in it.

If you are thinking to work this rock as an individual, both tests confirmed it is waste.

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Thank you all for chiming in.  Reno, this property is up for sale as a building site and it just happens to have an old mine on it.  The seller was  trying to find out if the old mine would or could add value to the sale price.  The property is within 10 miles of the "richest hill on earth". 

The comments about the assays were very helpful for the seller and it certainly gave me a better perspective going forward.

I am a prospector who metal detects and am always keeping my eyes open for geological changes but my heart is with placer.

By the way, Fists Full of Gold is a great publication and is well worth owning!

Bugler

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