Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Property, plant and equipment
Property, plant and equipment, gross	$ 35,261	$ 33,958
Less: Accumulated depreciation and amortization	(28,753)	(28,438)
Property, plant and equipment, net	6,508	5,520
Mineral properties
Property, plant and equipment
Property, plant and equipment, gross	9,353	9,353
Exploration properties
Property, plant and equipment
Property, plant and equipment, gross	2,418	2,418
Royalty properties
Property, plant and equipment
Property, plant and equipment, gross	200	200
Buildings
Property, plant and equipment
Property, plant and equipment, gross	3,818	3,755
Mining equipment and machinery
Property, plant and equipment
Property, plant and equipment, gross	17,652	16,135
Other furniture and equipment
Property, plant and equipment
Property, plant and equipment, gross	926	890
Construction in progress
Property, plant and equipment
Property, plant and equipment, gross		259
Asset retirement cost
Property, plant and equipment
Property, plant and equipment, gross	$ 894	$ 948