It's an audit. In an audit you contact the people verifying the asset, not the people who own it, as they are not impartial parties since they own it, and are likely not qualified anyway.
When my company is being audited, the auditors look at our books and see what we are valuing our accounts receivable at(what we say our customers owe us), they then contact a sample of those customers to verify what our books show.
Inventory works the same, only they physically count our inventory to prove what we say on our books matches what we actually have.
The mine owners could say whatever they want, and have whatever docs they want to prove what they say, but if it's not from an accredited third party source, then it isn't worth the air to say it, or the paper it's printed on.