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Note 8 - Shareholders' Equity (Tables)
 6 Months Ended
Jun. 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