Mineral properties and development costs (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended		6 Months Ended		8 Months Ended	49 Months Ended	134 Months Ended
	Oct. 31, 2011	Jan. 31, 2011	Jan. 31, 2010	May. 31, 2015	May. 31, 2014	May. 31, 2015	May. 31, 2014	Aug. 05, 2012	May. 31, 2015	May. 31, 2015	Jan. 31, 2012	Oct. 19, 2011	Aug. 05, 2011	Jan. 11, 2010	Dec. 31, 2009
Payments for mineral property				$ 0	$ 0	$ 0	$ 0		$ 4,116
Consideration for Ambler property														$ 29,000
Mineral properties expense				$ 291	$ 489	$ 618	$ 1,069		$ 54,387
Approximations [Member]
Mineral properties expense										$ 54,400
Ambler Property [Member]
Mineral property interest percentage														100.00%	51.00%
Mineral Property, Shares Issued, Shares			931,098
Mineral Property, Shares Issued, Value			$ 5,000
Payments for mineral property		$ 12,000
Consideration for Ambler property											$ 29,000
Fair value of cash payments to acquire mineral properties		$ 21,400
Discount rate used		8.00%
Mineral property, fair value of consideration								$ 26,500
Transaction costs													$ 100
Net smelter return royalty														1.00%
Purchase price of the net smelter royalty														$ 10,000
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%