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Mineral properties and development costs (Narrative) (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 9 Months Ended
Apr. 10, 2017
Jan. 11, 2010
Oct. 31, 2011
Oct. 19, 2011
Aug. 31, 2017
Aug. 31, 2016
Aug. 31, 2017
Aug. 31, 2016
Mineral properties expense         $ 8,471 $ 3,077 $ 10,407 $ 4,067
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures $ 150,000              
Maximum Subscription Price 5,000              
Maintenance Costs 10,000              
Proceeds from Contributions from Affiliates $ 10,000           10,000 $ 0
Security Deposit         $ 2,700   2,700  
Approximationss [Member]                
Payments for mineral property             73,400  
Acquisition costs             30,600  
Mineral properties expense             $ 104,000  
Bornite Property [Member]                
Payments for mineral property     $ 4,000          
Net proceeds royalty       15.00%        
Discount on consideration       $ 40,000        
Bornite Property [Member] | Minimum [Member]                
Mineral property interest percentage       16.00%        
Net smelter return royalty       1.00%        
Bornite Property [Member] | Maximum [Member]                
Mineral property interest percentage       25.00%        
Net smelter return royalty       2.50%        
Ambler Property [Member]                
Mineral property interest percentage   100.00%            
Mineral property, fair value of consideration   $ 26,600            
Net smelter return royalty   1.00%            
Purchase price of the net smelter royalty   $ 10,000