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DERIVATIVE INSTRUMENTS - Provisionally priced sales (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
GAIN (LOSS) ON THE PROVISIONAL PRICING OF CONCENTRATE SALES    
Realized $ 0.5 $ (8.9)
Unrealized 1.5 (1.6)
Total gain (loss) 2.0 (10.5)
Gold    
GAIN (LOSS) ON THE PROVISIONAL PRICING OF CONCENTRATE SALES    
Realized 2.2 (1.2)
Unrealized 0.5 1.1
Total gain (loss) 2.7 (0.1)
Copper    
GAIN (LOSS) ON THE PROVISIONAL PRICING OF CONCENTRATE SALES    
Realized (1.7) (7.7)
Unrealized 1.0 (2.7)
Total gain (loss) $ (0.7) $ (10.4)