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Inventories - Summary of inventories (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Classes of current inventories [abstract]    
Ore stockpiles $ 6,327 $ 3,859
In-process inventory and finished goods 81,120 81,282
Materials and supplies 88,824 78,093
Inventories $ 176,271 $ 163,234