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Significant accounting policies - Disclosure of detailed information about estimated useful life or depreciation rate (Details)
12 Months Ended
Dec. 31, 2019
Leasehold improvements [Member]  
Significant Accounting Policies [Line Items]  
Useful life of property, plant and equipment Lease term
Furniture and office equipment [Member]  
Significant Accounting Policies [Line Items]  
Useful life of property, plant and equipment 3-5 years
Exploration equipment and facilities [Member]  
Significant Accounting Policies [Line Items]  
Useful life of property, plant and equipment 3-20 years
Mining plant and equipment (development) [Member]  
Significant Accounting Policies [Line Items]  
Useful life of property, plant and equipment 5-20 years