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Property, Plant and Mine Development, net - Narrative (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 Mine Development, net        
Depreciation and amortization expense $ 3,521 $ 4,340 $ 11,299 $ 13,537