Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property, plant and equipment
Property, plant and equipment, gross	$ 8,593	$ 9,102
Less: Accumulated depreciation & amortization	(5,347)	(5,713)
Property, plant and equipment, net	3,246	3,389
Exploration properties
Property, plant and equipment
Property, plant and equipment, gross	2,518	2,518
Royalty properties
Property, plant and equipment
Property, plant and equipment, gross	200	200
Buildings
Property, plant and equipment
Property, plant and equipment, gross	2,603	3,127
Mining equipment and machinery
Property, plant and equipment
Property, plant and equipment, gross	2,392	2,377
Other furniture and equipment
Property, plant and equipment
Property, plant and equipment, gross	$ 880	$ 880