I'm not an accountant so I've probably made a mistake in my calculation (happy to be corrected) but I think it's consistent with Argonaut's assumptions, which still resulted in a post-tax IRR of 244%
The agreement looks like it's 50% of the after-tax margin for a concentrate (for example, if the margin is 70%, it's 50% x 70% x after mineral tax income).
Using Argonaut's assumptions on pg4 of their first report.
Revenue = $710m
Production cost = $241m
Mineral tax @ 4% = $28m
Income after mineral tax = $682m
After mineral tax margin = 65%
Production sharing = 50% x 65% x $682m = $221m
Assume income tax of 25%
Gross profit = $710 - $241 = $469