So the internet says that an "unpatented mining claim" is essentially a mineral leasehold on Federal land. 

Personally, I'd treat it as the sale of any other mineral leasehold. I know the rules for sales of oil & gas leaseholds, but if you're looking at Sec 617 (which appears to be the non-O&G equivalent to G&G), presumably it's some other mineral that your guy was hoping to extract.