Thread by @TheWealthMiner

America's Gold and Silver failed to adequately test Relief Canyon "ore" for preg-robbing material prior to putting the mine into production.

What does this mean?

https://twitter.com/ReadSomeEcon/status/1790883768414171436

To get gold out of "ore" you expose it to a cyanide solution. The gold goes into this solution, and a bunch of other stuff is left out of the solution.

The goal is to take this gold-containing cyanide solution, get the gold out of it in a concentrated form, which can be sold to a smelter or further treated on site.

HOWEVER, before the gold-bearing solution can go to this next concentration process, it remains in contact with the rock it was previously in for some time.

Sometimes there is material in this rock that has chemical/physical properties that allow it to re-adsorb the gold out of the solution. If so, this reprecipitated gold will not go with the solution for further processing, and it is lost to the waste dump.

Such material is called "preg-robbing" material, since it "robs" the "pregnant leach solution" (i.e. solution with the stuff you want). When the material is carbon the phenomenon is called "carbonaceous preg-rob"

So what's up with Relief Canyon? Why do historic metallurgical tests say that this won't be an issue when it turned out to be an issue?

Since conclusions from metallurgical work on Relief Canyon material assert no carbonaceous preg-rob effect and there was one, then clearly the met work was inadequate to test for the phenomenon.

According to the 2018 feasibility study carried out by Pershing Gold (the brainiacs at America's Gold and Silver couldn't be bothered to do their own internal research), met work consists of bottle roll tests and column leach tests.

These tests provide high level information, but they are short duration (i.e. the tests last much less time than the material is expected to be on the leach pad), and incorporate unrealistic physical scenarios (eg there's no "rolling" of ore mixed with cyanide on a leach pad).

There are specific tests that can be done to check for the preg-robbing effect. For instance you can expose the "ore" to a cyanide solution with a known gold concentration. If this gold concentration declines, you've got preg-robbing ore.

It is also a must to carry out a pilot heap leach program that tests the proposed methods on a few hundred tonnes of "ore" in the actual proposed setting of the mine, vs. in a controlled lab setting. As far as I know USAS didn't do this.

There are other tests as well. This is a useful study to go through. Basically if recoveries reach a peak at time t and then decline at time t', t' > t there is a preg-rob effect.

911metallurgist.com/wp-content/upl…

Testing for preg-robbing material is far from an absolute science. Also, there will always be a preg-robbing effect to some extent. So the actual question is to what extent are recoveries being impacted by the presence of preg-robbing material?

So Americas Gold and Silver bought Relief Canyon from Pershing Gold, which was run by known stock promoters. Pershing did a lot of met work, but this was hardly exhaustive in the context of the potential problems that could occur.

USAS believed the results of this met work without doing any independent research. The company got financing from Sandstorm, who was also sloppy in its DD but at least it got itself loan shark terms that allowed it to bleed USAS shareholders to get its money back.