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INVENTORIES - Schedule of Inventory, Current (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Inventory Disclosure [Abstract]    
Concentrates and work-in-progress $ 14,118 $ 13,788
Raw materials and consumables 2,432 2,973
Inventory, net $ 16,550 $ 16,761