Note 3 - Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Less: Accumulated depreciation and amortization	$ (19,897)	$ (15,809)
Total property, plant and equipment, net	156,343	149,205
Plant and Equipment, at Cost [Member]
Property, plant and equipment, gross	113,564	109,911
Construction in Progress [Member]
Property, plant and equipment, gross	16,892	12,066
Mine Development [Member]
Property, plant and equipment, gross	$ 45,784	$ 43,037