I am not an accountant but under Australian accounting standards...

I am not an accountant but under Australian accounting standards I don't think impairments are made by exclusive reference to stock market values of companies, the basic principle when it comes to assets is whether or not an asset is carried above or below its recoverable amount, ie the amount that can be recovered from that asset through using the asset or selling the asset. You can not carry an asset on your books for more than the recoverable amount.

Here is an Australian government guide explaining the standards in 2007. I'd say the standards have moved on quite a bit since then but the general principles probably wouldn't have changed radically.

https://www.aasb.gov.au/admin/file/content105/c9/AASB136_07-04_COMPapr07_07-07.pdf

"The objective of this Standard is to prescribe the procedures that an entity applies to ensure that its assets are carried at no more than their recoverable amount. An asset is carried at more than its recoverable amount if its carrying amount exceeds the amount to be recovered through use or sale of the asset. If this is the case, the asset is described as impaired and the Standard requires the entity to recognise an impairment loss. The Standard also specifies when an entity should reverse an impairment loss and prescribes disclosures."

That standard is for general assets but exploration and evaluation assets are treated differently.

Here is a guide for these types of assets, explaining the standards, also published by the Australian government in 2015. Impairment starts being covered on page 8.

https://www.aasb.gov.au/admin/file/content105/c9/AASB6_08-15.pdf

Remember the famous Mark Twain quote.

"A gold mine is a hole in the ground with a liar standing on top of it."

I think this can be more generalised nowadays to

"A mine is a hole in the ground with an accountant standing on top of it"

Accounting standards take your pick. Esh