Note 3 - Property, Plant and Equipment - Depreciation and amortization (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Property, Plant and Equipment.
Depreciation of plant and equipment	$ 7,661	$ 5,270	$ 14,428	$ 10,024
Amortization of right of use assets (finance leases)	1,999	1,383	3,881	2,097
Amortization and depletion of capitalized mine development costs and mineral rights	3,896	3,130	7,098	6,342
Total depreciation, depletion, and amortization	$ 13,556	$ 9,783	$ 25,407	$ 18,463