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Property, plant and equipment (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about property, plant and equipment [line items]    
Balance $ 80,548  
Impairments 204
Balance 100,301 80,548
Cost [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 185,055 178,057
Asset additions 19,557 7,972
Property purchase option acquired 7,108  
Change in decommissioning provision 1 (891)
Disposals   (83)
Reclassification  
Balance 211,721 185,055
Accumulated depreciation and depletion [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 104,507 97,182
Depreciation/depletion for the year 6,709 7,388
Impairments 204  
Disposals   (63)
Balance 111,420 104,507
Carrying value [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 80,548  
Balance 100,301 80,548
Mining interests [Member] | Cost [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 67,571 63,954
Asset additions 5,233 4,569
Property purchase option acquired  
Change in decommissioning provision (37) (952)
Disposals  
Reclassification 31,595  
Balance 104,362 67,571
Mining interests [Member] | Accumulated depreciation and depletion [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 31,410 28,298
Depreciation/depletion for the year 3,438 3,112
Impairments  
Disposals  
Balance 34,848 31,410
Mining interests [Member] | Carrying value [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 36,161  
Balance 69,514 36,161
Non-producing properties [Member] | Cost [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 77,390 75,746
Asset additions 5,526 1,583
Property purchase option acquired 7,108  
Change in decommissioning provision 38 61
Disposals  
Reclassification (31,595)  
Balance 58,467 77,390
Non-producing properties [Member] | Accumulated depreciation and depletion [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 50,502 50,502
Depreciation/depletion for the year
Impairments  
Disposals  
Balance 50,502 50,502
Non-producing properties [Member] | Carrying value [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 26,888  
Balance 7,965 26,888
Plant and equipment [Member] | Cost [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 40,013 38,196
Asset additions 8,795 1,817
Property purchase option acquired  
Change in decommissioning provision
Disposals  
Reclassification  
Balance 48,808 40,013
Plant and equipment [Member] | Accumulated depreciation and depletion [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 22,566 18,305
Depreciation/depletion for the year 3,261 4,261
Impairments 204  
Disposals  
Balance 26,031 22,566
Plant and equipment [Member] | Carrying value [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 17,447  
Balance 22,777 17,447
Corporate Office equipment [Member] | Cost [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 81 161
Asset additions 3 3
Property purchase option acquired  
Change in decommissioning provision
Disposals   (83)
Reclassification  
Balance 84 81
Corporate Office equipment [Member] | Accumulated depreciation and depletion [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 29 77
Depreciation/depletion for the year 10 15
Impairments  
Disposals   (63)
Balance 39 29
Corporate Office equipment [Member] | Carrying value [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Balance 52  
Balance $ 45 $ 52