Note 3 - Property, Plant and Equipment - Depreciation and amortization (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Property, Plant and Equipment [Abstract]
Depreciation of plant and equipment	$ 4,326	$ 3,706	$ 12,740	$ 10,142
Amortization of capitalized mine development costs	932	1,647	2,861	4,149
Total depreciation and amortization	$ 5,258	$ 5,353	$ 15,601	$ 14,291