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PLANT AND EQUIPMENT, MINE DEVELOPMENT AND CONSTRUCTION IN PROGRESS - Additional Information (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Minerals Property Interests [Line Items] |  |  |  |
Impairment charges |  | $ 711 |  | $ 50,600
Mine development costs |  | 3,837 | $ 3,866 | $ 1,169
El Gallo 2 project |  |  |  |
Minerals Property Interests [Line Items] |  |  |  |
Total cost of deposit |  |  | 1,500 |
Refund of deposits |  |  | 1,000 |
Impairment charges | $ 2,000 | $ 700 |  |
El Gallo 2 project | Mine development costs |  |  |  |
Minerals Property Interests [Line Items] |  |  |  |
Mine development costs |  |  | $ 500 |

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