Note 3 - Property, Plant and Equipment - Depreciation and amortization (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Property, Plant and Equipment.
Depreciation of plant and equipment	$ 8,063	$ 5,903	$ 22,492	$ 15,928
Amortization of right of use assets (finance leases)	2,432	1,710	6,312	3,806
Amortization and depletion of capitalized mine development costs and mineral rights	3,948	3,822	11,046	10,164
Total depreciation, depletion, and amortization	$ 14,443	$ 11,435	$ 39,850	$ 29,898