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Note 11 - Derivative Instruments - Summary of Forward Sales Contracts (Details) oz in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 oz $ / oz | Dec. 31, 2018 oz $ / oz
Silver 2019 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 710 | 842
Average price per ounce/pound (in USD per Ounce) | $ / oz | 15.46 | 14.69
Gold 2019 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 3 | 4
Average price per ounce/pound (in USD per Ounce) | $ / oz | 1,316 | 1,260
Zinc 2019 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 19,952 |
Average price per ounce/pound (in USD per Ounce) | $ / oz | 1.22 |
Zinc 2018 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz |  | 18,450
Average price per ounce/pound (in USD per Ounce) | $ / oz |  | 1.15
Lead 2019 Settlements for Provisional Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 2,646 | 2,700
Average price per ounce/pound (in USD per Ounce) | $ / oz | 0.92 | 0.89
Zinc 2019 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 26,180 |
Average price per ounce/pound (in USD per Ounce) | $ / oz | 1.25 |
Lead 2019 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 1,653 |
Average price per ounce/pound (in USD per Ounce) | $ / oz | 0.96 |
Zinc 2020 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 276 |
Average price per ounce/pound (in USD per Ounce) | $ / oz | 1.26 |
Lead 2020 Settlements for Forecasted Sales [Member] |  |
Ounces/pounds under contract (Ounce) | oz | 551 |
Average price per ounce/pound (in USD per Ounce) | $ / oz | 0.96 |

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