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Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) | 12 Months Ended
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Dec. 31, 2020 oz lb $ / oz $ / lb | Dec. 31, 2019 oz lb $ / oz $ / lb
Silver 2021 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 1,282 |
Average price per ounce/pound (in USD per Ounce) | $ / oz | 25.00 |
Silver 2020 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz |  | 2,556
Average price per ounce/pound (in USD per Ounce) | $ / oz |  | 17.20
Gold 2021 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 4 |
Average price per ounce/pound (in USD per Ounce) | $ / oz | 1,858 |
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
Zinc 2021 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 23,314 |
Average price per ounce/pound (in USD per Ounce) | $ / lb | 1.19 |
Zinc 2020 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 21,550
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 1.04
Lead 2021 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 4,905 |
Average price per ounce/pound (in USD per Ounce) | $ / lb | 0.90 |
Lead 2020 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 5,159
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 0.88
Zinc 2021 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 41,557 |
Average price per ounce/pound (in USD per Ounce) | $ / lb | 1.17 |
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
Lead 2021 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 30,876 |
Average price per ounce/pound (in USD per Ounce) | $ / lb | 0.88 |
Lead 2020 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb |  | 11,740
Average price per ounce/pound (in USD per Ounce) | $ / lb |  | 0.98
Zinc 2022 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | lb | 18,519 |
Average price per ounce/pound (in USD per Ounce) | $ / lb | 1.28 |

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