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DERIVATIVE INSTRUMENTS (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
DERIVATIVE ASSETS    
Total derivative assets $ 0.2 $ 0.7
DERIVATIVE LIABILITIES    
Total derivative liabilities 26.4 5.5
Unsettled provisionally priced concentrate derivatives, and swap contracts    
DERIVATIVE ASSETS    
Total derivative assets 0.2 0.0
DERIVATIVE LIABILITIES    
Total derivative liabilities 0.0 0.7
Copper price option contract    
DERIVATIVE ASSETS    
Total derivative assets 0.0 0.7
Gold price option contracts    
DERIVATIVE LIABILITIES    
Total derivative liabilities $ 26.4 $ 4.8