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Accounts and Other Receivables (Schedule of Trade and Other Receivables) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Accounts and Other Receivables [abstract]    
Trade receivables from concentrate sales $ 34,250 $ 23,185
Advances and other receivables 1,249 1,095
Value added taxes recoverable 871 707
Total accounts and other receivables $ 36,370 $ 24,987