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PLANT AND EQUIPMENT AND MINERAL PROPERTIES (Narrative) (Details) - USD ($) $ in Thousands |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Aug. 14, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Property, Plant and Equipment [Line Items] |  |  |  |  |
Abandonment of mineral properties |  | $ 0 | $ 0 | $ 0 | $ 287
Minimum |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Percentage sliding-scale gross proceeds production royalty | 6.00% |  |  |  |
Maximum |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Percentage sliding-scale gross proceeds production royalty | 8.00% |  |  |  |

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