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Mineral resource exploration and evaluation expenditure (Details Textual) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of Mineral resource exploration and evaluation expenditure [Line Items]      
Construction in progress $ 375,316 $ 207,830  
Expense arising from exploration for and evaluation of mineral resources 5,537 5,864 $ 5,322
Exploitation [Member]      
Disclosure of Mineral resource exploration and evaluation expenditure [Line Items]      
Assets arising from exploration for and evaluation of mineral resources $ 8,645 9,791  
Metallic exploration [Member]      
Disclosure of Mineral resource exploration and evaluation expenditure [Line Items]      
Useful life measured as period of time, property, plant and equipment 10 years    
Expense arising from exploration for and evaluation of mineral resources $ 165 0  
Mt Holland [Member]      
Disclosure of Mineral resource exploration and evaluation expenditure [Line Items]      
Assets arising from exploration for and evaluation of mineral resources 30,475 11,298  
Property, Plant and Equipment [Member]      
Disclosure of Mineral resource exploration and evaluation expenditure [Line Items]      
Construction in progress 12,841 10,292  
Non-current assets [Member]      
Disclosure of Mineral resource exploration and evaluation expenditure [Line Items]      
Assets arising from exploration for and evaluation of mineral resources 6,576 5,099  
Current Inventories [Member]      
Disclosure of Mineral resource exploration and evaluation expenditure [Line Items]      
Assets arising from exploration for and evaluation of mineral resources 1,367 $ 2,028  
Other non-current financial assets: [Member]      
Disclosure of Mineral resource exploration and evaluation expenditure [Line Items]      
Assets arising from exploration for and evaluation of mineral resources $ 3,433