Note 3 - Property, Plant and Equipment - Depreciation and amortization (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Depreciation of plant and equipment	$ 4,484	$ 4,326	$ 13,354	$ 12,740
Depreciation of right of use assets	413		540
Amortization of capitalized mine development costs	1,854	932	4,967	2,861
Total depreciation and amortization	$ 6,751	$ 5,258	$ 18,861	$ 15,601