The volume of water behind the dam has been rising and concerns had been raised in recent weeks about what could happen as snow melts and its capacity is exceeded.
The miner expects to lower production costs to $1.08 per pound during the 2017 financial year, from a projected $1.21 per pound in the year ending June 2016.
After a year and a half of lower crude prices, there is little expectation that Opec, under pressure from dominant force Saudi Arabia, will change course at this week’s meeting.
World's five largest mining companies together have lost an astonishing $570 billion in market value since 2011–the worst period of performance since 1966.