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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%