Note 3 - Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Property, plant, and equipment
Less: Accumulated depreciation and amortization	$ (114,822)	$ (111,937)
Total property, plant and equipment, net	457,564	429,842
Plant and equipment
Property, plant, and equipment
Property, plant and equipment, gross	251,002	232,885
Mining property and mineral rights
Property, plant, and equipment
Property, plant and equipment, gross	120,533	120,760
Construction in process
Property, plant, and equipment
Property, plant and equipment, gross	35,039	34,698
Capitalized mine development costs
Property, plant, and equipment
Property, plant and equipment, gross	$ 165,812	$ 153,436