Back at desk and updated my original calculation to include more detail from the SS. And of course this only includes the starter pit.
Project NPV (pre tax) = $1.6b
IRR (pre tax) = 117%
The killer is the PSA, hence why they probably included that comment about negotiating the PSA. Based on the best information we have on that, project payback is about 3 years, after tax and royalties.