The valuations (assuming they're done by a reputable company - which I haven't verified) will be based on an estimate of what viable deposits are likely to be found in the areas they have the rights to.  Those estimates will be made based on known actual results of other areas with similar geology.  They're an educated guess basically.

As I don't invest myself (only trade) I have no interest in doing the background research necessary to validate this IPO - but a key part of that is ensuring that the valuations are done by a credible company not by a sock-puppet one which produces (unrealistic) valuations to order.  I have no reason to believe the latter is the case here - but ensuring it isn't the case is the very first thing I'd do if I was considering investing.

EDIT: The detail of the specific find mentioned isn't that critical.  Even if it's a tiny quantity the important thing is that it verifies the presence of useful ore and so supports the likelihood of further larger deposits being found when a full survey is conducted.