I personally don't understand why a 5% discount rate is used in...

I personally don't understand why a 5% discount rate is used in all the gold DFS's, but even the west African's ones seem to do it. I think the justification is there is no off take risk.

Really NPVs in DFS are kind of rubbery anyway, but where they do have value is in comparing projects. You can use the NPV of projects and the current MCs and workout what the sort of SP discount you should expect at the various stages of development.