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I was looking into these specific minerals: arsenopyrite, pyrite, chalcopyrite, barite, calcite, dolomite. I investigate issues with tellurides not showing expected gold concentrations.

There is gold predicted at the site where these samples are. We tried to prove the presence of gold through aqua regia tests, but the resulting gold concentrations are low. My suspicion is that the majority of the gold resides as an inclusion in these minerals, and the aqua regia test won't be able to show that because of its inability to dissolve the host.

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This was also asked in Chemistry Stack Exchange, where it was noted that barium sulfate is insoluble in water and generally insoluble in acids. A quick check reveals that indeed the other minerals can be dissolved in nitric acid and so should yield to aqua regia. Thus barite is the troublemaker.

Barite can be dissolved in hot alkaline chelating solutions, as described here[1]. This, of course, is not compatible with the acidic solvent you intend to use to dissolve and measure the gold. It is not clear from the reference whether a pretreatment with such a chelating agent to remove the barite before placing in aqua regia would work when the barite is intermixed with the alkali-insoluble sulfides listed in the question.

Reference

1. Bader Salem Ba Geri, Mohamed A. Mahmoud, Reyad A. Shawabkeh, Abdulazeez Abdulraheem, "Evaluation of Barium Sulfate (Barite) Solubility Using Different Chelating Agents at a High Temperature", Journal of Petroleum Science and Technology 7(2017), 1, 42-56.

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  • $\begingroup$ What about the sulfides? Especially the arsenopyrite and pyrite? $\endgroup$
    – asdchem
    Dec 2 '20 at 21:17
  • $\begingroup$ Soluble with reaction in nitric acid $\endgroup$ Dec 2 '20 at 21:19
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    $\begingroup$ Sorry, I am writing on behalf of my wife. She makes harsh decisions. $\endgroup$
    – asdchem
    Dec 3 '20 at 22:03

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