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Inventories - Summary of Inventories (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Classes of current inventories [abstract]    
Ore stockpiles $ 3,859 $ 1,620
In-process inventory and finished goods 81,282 59,974
Materials and supplies 78,093 76,291
Inventories $ 163,234 $ 137,885