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Finance costs and finance revenues (Details 2) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of business combination [Line Items]    
Annual average of future sales of mineral $ 196,801 $ 193,588
Useful life of mining properties 13 years 13 years
Pre-tax Discount rate (%) 10.00% 10.00%