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Note 3 - Property, Plant and Equipment (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Property, Plant and Equipment [Table Text Block]
   
December 31,
  
(In thousands)
  
2018
    
2017
  
Plant and equipment
   $
109,911
     $
80,455
  
Construction in process
    
12,066
      
7,626
  
Capitalized mine development cost
    
43,037
      
30,776
  
Less accumulated depreciation and amortization
    
(15,809
 )    
(3,406
 )
Total property, plant and equipment, net
   $
149,205
     $
115,451