Ann: Korbel Gold Project Starter Pit Scoping Study, page-403

I think you may be getting the discount rate mixed up with an anticipated margin (profit/risk factor)

In net present value calcs, the discount rate is being applied every year.
ie, gold sold in year 15 at $1750/oz is given a value today.
At a 5% discount rate, that means you would value the gold you are selling in year 15 at $842/oz today
At a 20% discount rate, you would be valuing 1oz of gold being sold in 15 years time at $113 today. this would be discounted way too much and you wont find any mine in the world that would make sense using such a high discount rate.