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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Plant and Equipment (Details)
 12 Months Ended
Dec. 31, 2020
Minimum | Vehicles  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life of property, plant and equipment 5 years
Minimum | Mining equipment  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life of property, plant and equipment 3 years
Minimum | Mobile equipment components  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life of property, plant and equipment 2 years
Minimum | Right-of-use assets - plant and equipment  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life of property, plant and equipment 10 years
Maximum | Vehicles  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life of property, plant and equipment 7 years
Maximum | Mobile equipment components  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life of property, plant and equipment 7 years