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INVENTORIES - Schedule of Inventory, Current (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Inventory Disclosure [Abstract]    
Concentrates and work-in-progress $ 12,088 $ 14,118
Raw materials and consumables 3,150 2,432
Inventory, net $ 15,238 $ 16,550