Tidal,
Just a comment regarding "so doubling the amount of extracted material increases the NPV by 200%"
NPV calcuations are the present value of discounted future cash flows. The further out the cash flow is, the less its worth in todays money. If they were to double the amount of resource utilized, it would impact the NPV, but anything past 20 years would have a greatly reduced value. The real increase in NPV would be to increase the mine size from 30K to 50K as you'd have a much larger FCF per year, while maintaining roughtly the same mine life.
-Garth
