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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Ore Stockpile Inventory (Details)
 12 Months Ended 16 Months Ended 28 Months Ended
Dec. 31, 2015
Dec. 31, 2013
Dec. 31, 2014
El Gallo mine      
Stockpiles, Material on Leach Pads, In-process Inventory, Precious Metals Inventory and Materials and Supplies      
Cumulative metallurgical recovery rate for gold production (as a percent)   56.00% 56.00%
Minimum      
Stockpiles, Material on Leach Pads, In-process Inventory, Precious Metals Inventory and Materials and Supplies      
Recovery percentage of leach pads of the recoverable ounces in the first year of leaching 50.00%    
Maximum      
Stockpiles, Material on Leach Pads, In-process Inventory, Precious Metals Inventory and Materials and Supplies      
Recovery percentage of leach pads of the recoverable ounces in the first year of leaching 95.00%