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INVENTORY (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Classes of current inventories [abstract]      
Process material stockpiles $ 4,486 $ 3,566 $ 2,605
Concentrate inventory 3,095 3,437 1,896
Materials and supplies 1,650 2,099 1,303
Inventories $ 9,231 $ 9,102 $ 5,804