Note 13 - Fair Value Measurement - Assets and Liabilities Accounted for at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Receivables from provisional concentrate sales	$ 9,745	$ 6,421
Fair Value, Recurring [Member]
Certificates of deposit and other deposits	1,053	1,025
Total assets	186,164	82,820
Total liabilities	11,756	7,214
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Recurring [Member]
Money market funds and other bank deposits	129,830	62,452
Equity securities – mining industry	19,389	6,207
Fair Value, Inputs, Level 2 [Member]
Receivables from provisional concentrate sales	27,864	11,952
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member]
Metal forward and put option contracts	381	0
Foreign exchange contracts	7,647	1,184
Metal forward and put option contracts	11,737	5,777
Foreign exchange contracts	$ 19	$ 1,437

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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A savings certificate entitling the Entity (that is, bearer) to receive interest at an established maturity date, based upon a fixed interest rate. A certificate of deposit may be issued in any denomination. Certificates of deposit are generally issued by commercial banks and, therefore, insured by the FDIC (up to the prescribed limit). Certificates of deposit generally restrict holders from withdrawing funds on demand without the incurrence of penalties. Generally, only certificates of deposit with original maturities of three months or less qualify as cash equivalents. Original maturity means original maturity to the entity holding the investment. As a related example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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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 portion of liability contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward (delivery or nondelivery) contracts, and swaps entered into.

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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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