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Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands, lb in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2020 oz lb $ / oz $ / lb | Dec. 31, 2019 oz lb $ / oz $ / lb
Silver 2020 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 45 | 2,556
Average price per ounce/pound (in USD per Ounce) | $ / oz | 17.82 | 17.20
Zinc 2020 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 21,330 | 21,550
Average price per ounce/pound (in USD per Ounce) | $ / lb | 0.91 | 1.04
Lead 2020 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 7,441 | 5,159
Average price per ounce/pound (in USD per Ounce) | $ / lb | 0.78 | 0.88
Gold 2020 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz |  | 10
Average price per ounce/pound (in USD per Ounce) | $ / oz |  | 1,481
Silver 2020 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 2,840 | 5,700
Average price per ounce/pound (in USD per Ounce) | $ / oz | 16.00 | 15.73
Gold 2020 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 95 | 130
Average price per ounce/pound (in USD per Ounce) | $ / oz | 1,482 | 1,435
Lead 2020 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 5,842 | 11,740
Average price per ounce/pound (in USD per Ounce) | $ / lb | 0.98 | 0.98
Zinc 2020 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 441
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.13

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