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Finance costs and finance income - Unobservable valuation inputs (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Finance costs and finance income    
Annual average of future sales of mineral $ 193,972 $ 222,238
Useful life of mining properties 14 years 14 years
Pre-tax Discount rate (%) 9.70% 9.30%