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Note 12 - Derivative Instruments (Tables)
9 Months Ended
Sep. 30, 2018
Notes Tables  
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]
September 30, 2018
 
Ounces/pounds under contract (in 000's)
   
Average price per ounce/pound
 
   
Silver
   
Gold
   
Zinc
   
Lead
   
Silver
   
Gold
   
Zinc
   
Lead
 
   
(ounces)
   
(ounces)
   
(pounds)
   
(pounds)
   
(ounces)
   
(ounces)
   
(pounds)
   
(pounds)
 
Contracts on provisional sales
                                                               
2018 settlements
   
1,221
     
6
     
10,362
     
4,685
    $
14.56
    $
1,211
    $
1.17
    $
0.94
 
December 31, 2017
 
Ounces/pounds under contract (in 000's)
   
Average price per ounce/pound
 
   
Silver
   
Gold
   
Zinc
   
Lead
   
Silver
   
Gold
   
Zinc
   
Lead
 
   
(ounces)
   
(ounces)
   
(pounds)
   
(pounds)
   
(ounces)
   
(ounces)
   
(pounds)
   
(pounds)
 
Contracts on provisional sales
                                                               
2018 settlements
   
1,447
     
5
     
21,550
     
4,740
    $
16.64
    $
1,279
    $
1.45
    $
1.11
 
                                                                 
Contracts on forecasted sales
                                                               
2018 settlements
   
     
     
32,187
     
16,645
     
N/A
     
N/A
    $
1.29
    $
1.06
 
2019 settlements
   
     
     
23,589
     
18,078
     
N/A
     
N/A
    $
1.33
    $
1.09
 
2020 settlements
   
     
     
3,307
     
2,866
     
N/A
     
N/A
    $
1.27
    $
1.08