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Note 8 - Shareholders' Equity (Tables)
 9 Months Ended
Sep. 30, 2017
Notes Tables  
Schedule of Potential Per Share Dividend Amounts Quarterly Price Levels [Table Text Block]
Quarterly average realized silver price
per ounce
    
Quarterly dividend per
share
    
Annualized dividend
per share
  
$ 30     $
0.01
     $
0.04
  
$ 35     $
0.02
     $
0.08
  
$ 40     $
0.03
     $
0.12
  
$ 45     $
0.04
     $
0.16
  
$ 50     $
0.05
     $
0.20