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REVENUE - Provisional Copper and Gold Sales (Details)
ozt in Thousands, lb in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
$ / pound
$ / ounce
ozt
lb
Dec. 31, 2019
USD ($)
$ / pound
$ / ounce
lb
ozt
Disclosure of Disaggregation of Revenue [Line Items]    
Percent change in commodity prices used for sensitivity analysis 10.00%  
Disclosure of major customers [line items]    
Minimum percentage of entity revenue for major customers 10.00%  
Copper    
Disclosure of Disaggregation of Revenue [Line Items]    
Volumes subject to final pricing Copper (millions) Gold (000s) | lb 49 39
Impact on net income before taxation of 10% movement in market price $ 16 $ 11
Average price of sales subject to final settlement (copper in dollars per pound; gold in dollars per oz) | $ / pound 3.17 2.80
Gold[member]    
Disclosure of Disaggregation of Revenue [Line Items]    
Volumes subject to final pricing Copper (millions) Gold (000s) | ozt 22 15
Impact on net income before taxation of 10% movement in market price $ 4 $ 2
Average price of sales subject to final settlement (copper in dollars per pound; gold in dollars per oz) | $ / ounce 1,899 1,524
Customer 1 [Member]    
Disclosure of major customers [line items]    
Percentage of entity's revenue 23.00%  
Customer 2 [Member]    
Disclosure of major customers [line items]    
Percentage of entity's revenue 14.00%