Note 3 - Property, Plant and Equipment - Depreciation and amortization (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Property, Plant and Equipment.
Depreciation of plant and equipment	$ 9,069	$ 7,661	$ 17,890	$ 14,428
Amortization of right of use assets (finance leases)	2,883	1,999	5,376	3,881
Amortization and depletion of capitalized mine development costs and mineral rights	3,927	3,896	7,832	7,098
Total depreciation, depletion, and amortization	$ 15,879	$ 13,556	$ 31,098	$ 25,407