RE:Rio Tinto reached an agreement ! Couple of points here. RIO is paying for the portion IT DOES NOT own or Turquise Hill. This means that any offer from RIO woudl be for 92% of Casion, not 100% as I think someone stated before.
Second, they are paying for 51% of Turquise Hill which owns 66% of the mine. So they are paying $3.1B for roughly 33% of the mine asset. That values NPV of the mine at 9.3B in RIOs mind according to my rough calcs.