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Note 2 - Long-lived Asset Impairments (Details Textual) $ in Thousands | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Aug. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Sep. 30, 2020 USD ($) | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Asset Impairment Charges, Total |  |  |  |  |  |  | $ 1,799 | $ 77,882
Carlisle Mine [Member] |  |  |  |  |  |  |  |
Asset Impairment Charges, Total |  |  |  |  |  | $ 65,700 | $ 0 |
Mineral Properties, Net, Total |  | $ 1,800 |  |  |  | $ 1,800 |  | $ 1,800
Equipment, Redeployed |  |  |  |  | $ 23,000 |  |  |
Carlisle Mine [Member] | Closing of Mine [Member] |  |  |  |  |  |  |  |
Restructuring and Related Costs, Incurred Cost, Total |  |  |  | $ 1,100 | $ 1,100 |  |  |
Carlisle Mine [Member] | Measurement Input, Discount Rate [Member] | Valuation Technique, Discounted Cash Flow [Member] |  |  |  |  |  |  |  |
Asset Impairment, Measurement Input |  | 0.10 |  |  |  | 0.10 |  | 0.10
Bulldog Reserves [Member] |  |  |  |  |  |  |  |
Asset Impairment Charges, Total |  |  |  |  |  | $ 9,200 |  |
Hourglass Sands [Member] |  |  |  |  |  |  |  |
Asset Impairment Charges, Total | $ 1,800 | $ 2,900 | $ 1,800 |  |  |  |  | $ 2,900
Mineral Properties, Net, Total |  | $ 1,900 |  |  |  | $ 1,900 |  | $ 1,900

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