Note 12 - Fair Value Measurement - Assets and Liabilities Accounted for at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Receivables from provisional concentrate sales	$ 3,851	$ 6,421
Fair Value, Recurring [Member]
Total assets	115,657	82,820
Total liabilities	19,061	7,214
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Recurring [Member]
Money market funds and other bank deposits	75,923	62,452
Equity securities – mining industry	12,162	6,207
Certificates of deposit and other deposits	1,053	1,025
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member]
Receivables from provisional concentrate sales	26,003	11,952
Foreign exchange contracts	295	1,184
Metal forward and put option contracts	221	0
Foreign exchange contracts	9,174	1,437
Metal forward and put option contracts	$ 9,887	$ 5,777

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of equity securities categorized neither as held-to-maturity nor as trading.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value portion of asset contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward contracts, and swaps.

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Fair value of financial and nonfinancial obligations.

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Amount due from parties in nontrade transactions, classified as other.

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