"Florence has a planned average annual production rate of 84 million lbs. of  copper from probable reserves of 345 million tonnes grading 0.36% copper. The facility has an estimated $920 million pre-tax net present value at a 7.5% discount rate. The after-tax NPV was recently downgraded to $760 million at a 7.5% discount rate due to recent changes to the US tax legislation."

Where did you get this quote from?  Sibiu might actually be right that a mistake was made.  Confusing pre-tax and after-tax NPV.  That would be a pretty bad mistake.