Property, Plant and Mine Development – net - Summary (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Property, equipment and mine development - net
Property and equipment, gross	$ 55,767	$ 41,724
Accumulated depreciation, depletion and amortization	(13,025)	(9,376)
Total property, equipment and mine development - net	42,742	32,348
Capital expenditures	1,500	1,100
Machinery and equipment
Property, equipment and mine development - net
Property and equipment, gross	19,763	19,398
Mine development
Property, equipment and mine development - net
Property and equipment, gross	21,060	13,393
Furniture, computer and office equipment
Property, equipment and mine development - net
Property and equipment, gross	2,771	2,899
Mill facilities and infrastructure
Property, equipment and mine development - net
Property and equipment, gross	3,062	2,825
Light vehicles and other mobile equipment
Property, equipment and mine development - net
Property and equipment, gross	1,864	1,942
Construction-in-progress
Property, equipment and mine development - net
Property and equipment, gross	6,663	592
Asset retirement obligation costs
Property, equipment and mine development - net
Property and equipment, gross	354	448
Land and mineral rights
Property, equipment and mine development - net
Property and equipment, gross	$ 230	$ 227