Several mines in world top copper producer Chile are in the throes of tense labor negotiations, including BHP's Escondida and Codelco's Andina at a time when supply is already tight.
The solution caters both for smaller physical users who use official prices in their contracts and financial and larger physical users who focus more on closing prices.
Costs to expand the Mongolian mine, Rio's biggest copper growth project, have ballooned up to $6.75 billion from Rio's original budget of $5.3 billion in 2016.
Energy regulators last month said the Grassy Mountain project, proposed by a unit of Australian billionaire Gina Rinehart's Hancock Prospecting, would not be in the public interest.