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PLANT AND EQUIPMENT AND MINERAL PROPERTIES (Narrative) (Details) - USD ($) shares in Thousands, $ in Thousands |  | 12 Months Ended
--- | --- | ---
Aug. 14, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Property, Plant and Equipment [Line Items] |  |  |  |
Abandonment of mineral properties |  | $ 0 | $ 287 | $ 1,036
Nichols Ranch |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Depreciation |  | $ 2,050 | $ 3,170 |
Nichols Ranch, Hank and Doughstick Properties |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Total purchase price | $ 3,740 |  |  |
Number of shares issued (in shares) | 1,100 |  |  |
Minimum |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Percentage Of sliding-scale gross proceeds production royalty | 6.00% |  |  |
Maximum |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Percentage Of sliding-scale gross proceeds production royalty | 8.00% |  |  |

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