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Mine restoration provisions - Schedule of Mine Restoration Provisions (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Reconciliation of changes in other provisions [abstract]    
Reclamation spending $ (1,040) $ (320)
Current portion of mine restoration provisions (3,170) (1,819)
Non-current portion of mine restoration provisions 114,051 96,627
Mine restoration provisions    
Reconciliation of changes in other provisions [abstract]    
Balance, beginning of year 98,446 81,162
Reclamation spending (1,040) (320)
Accretion expense 2,641 1,855
Change in obligation 17,174 15,749
Balance, end of year $ 117,221 $ 98,446