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INVENTORIES - Schedule of Inventory, Current (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Inventory Disclosure [Abstract]    
Concentrates and work-in-progress $ 14,190 $ 14,118
Raw materials and consumables 2,637 2,432
Inventory, net $ 16,827 $ 16,550