Wildly different assay results on split sample

[BentFunky]

Covid and life had gotten in the way of my gold hunting hobby over the past year or so. Picking up where I left off: concentrates assay and general state of confusion. Anyone else ever get wildly different assay results from a split sample?

Starting Point: a split sample. One half went to Company A. The other went to Company B. Both used FA/AA to generate results.

The Results:
1. Company A, approximately .31oz gold per ton.
2. Company B: approximately .004oz gold per ton.

The Control: panning 400g saved sample yielded about .01 grams of gold. If I’m doing the math correctly, this equates to roughly .68 oz per ton.

Company A is in ballpark. Company B is in a different universe.

Edit: I removed assay company names from post. I don’t want to disparage anyone given my amateur status and limited experience.

 

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[Clay Diggins]

Split samples on free milling gold is not a reliable method for assay. The nugget effect will bite you every time. The results you show are not surprising.

Assaying concentrates doesn't provide you with a reliable per ton assay. Concentrates do not equal ore. I'm unsure why you would ever want to assay your concentrates unless you are operating a recovery plant and need to track recovery efficiency across methods/batches.

Assays do not determine grade. Assays are best used to determine the composition of in situ deposits. Even then hundreds of assays are needed to form an educated opinion about the nature of the deposit.

It's all about what you can recover with free milling gold - if you can't recover it the assay doesn't matter. You need to do bulk processing of your ore as mined to determine your grade. Placer gold is not calculated in ounces per ton, ounces per cubic yard is the method for describing placer deposits.


Here a good explanation of the process of determining grade.
Placer Examination, Principles and Practice

Heavy Pans

 

[BentFunky]

Appreciate info HP. Book looks great and will probably answer quite a few questions.

My primary interest WRT assays was identifying platinum group metals. Gold assay results were really just a sanity check.

I have a rough idea what gold values should be. Suspect gold values in assay report make all other values suspect.That was the idea, at least .

 

[russau]

Again , My Land Matters shows what a valuable asset it is to anyone prospecting & wanting to know what's up !

 

[BentFunky]

It's all about what you can recover with free milling gold - if you can't recover it the assay doesn't matter. You need to do bulk processing of your ore as mined to determine your grade. Placer gold is not calculated in ounces per ton, ounces per cubic yard is the method for describing placer deposits.

Heavy Pans

The ebook, Placer Examination Principles and Practices, was a great read.

Thinking out loud here ...

I'm still mulling over one comment in the book that indicates that one shouldn't grind or pulverize sample material. No rationale was provided. However, makes sense to me on one level, at least. Gold, pressed out into a foil will more be more difficult to separate by gravity methods.

However, depending on particle sizes (post grinding), seems that flattened gold could be recovered by passing ground material through a fine mesh. Gold foil wouldn't pass through mesh given it's surface area. Just about everything else should.

Perhaps the counter argument is that value of gold recovered by above method would be less than the cost of recovering it and/or other recovery such as leaching would be more efficient.

 

[arizau]

The ebook, Placer Examination Principles and Practices, was a great read.

Thinking out loud here ...

I'm still mulling over one comment in the book that indicates that one shouldn't grind or pulverize sample material. No rationale was provided. However, makes sense to me on one level, at least. Gold, pressed out into a foil will more be more difficult to separate by gravity methods.

However, depending on particle sizes (post grinding), seems that flattened gold could be recovered by passing ground material through a fine mesh. Gold foil wouldn't pass through mesh given it's surface area. Just about everything else should.

Perhaps the counter argument is that value of gold recovered by above method would be less than the cost of recovering it and/or other recovery such as leaching would be more efficient.

Google "randy clarkson rod mill" and several articles will show up discussing a method that is likely in use now similar to what you mentioned above. There were also a couple of threads on this forum that were pretty active for a while too.
I'm glad Clay chimed in about your assay questions....

 

[Clay Diggins]

The ebook, Placer Examination Principles and Practices, was a great read.

Thinking out loud here ...

I'm still mulling over one comment in the book that indicates that one shouldn't grind or pulverize sample material. No rationale was provided.

The rationale is that free milling gold doesn't "grind". Grinding gold for the most part doesn't make the pieces smaller it just deforms and eventually tears the gold particles. "Grinding" gold is kind of like grinding cold butter. Gold is malleable and the ore for gold is not. Also grinding creates unnecessary muck when wet processing. Muck cuts down on your recovery rates. The larger the gravel size (within reason) the more effective the gravity separation of the gold.

Placer deposits are not worked by grinding the rock contained in the placer deposit. Ideally the free gold in a placer is readily enriched by gravity sorting of the placer material. In a perfect world with copious water and mechanical scrubbing the process is very efficient. Even if water is not available grinding is not considered an alternative or enhanced recovery method for placer material.

Separation by specific gravity in a wet system is not linear in effect. The smaller the gold particles the less efficient the gravity separation is. The long used solution to that problem was recovery by amalgamation with mercury. Unfortunately that simple inexpensive recovery method takes skill and learned knowledge. Combine those drawbacks with the fact mercury is seen by many as the COVID of the metallic elements these days and you end up with a lot of deposits going unmined and a bunch of old miners scratching their heads over what the big deal is.

Heavy Pans

 

[BentFunky]

Combine those drawbacks with the fact mercury is seen by many as the COVID of the metallic elements these days and you end up with a lot of deposits going unmined and a bunch of old miners scratching their heads over what the big deal is.

Heavy Pans

It's the "this is why we can't have nice things effect."

Consider the plethora of yahoos using crock pots filled with aqua regia to extract small bits of gold from old computer components. Sure, some people are careful, safe, and dispose of their hazardous waste appropriately. However, for every one such person, there are 10 who hurt themselves or damage the environment by dumping hazardous waste into streams, storm drains, etc.