Segment Reporting (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
USD ($)
segment
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Number of geographic regions | segment 3      
Balance sheet information        
Total current assets $ 58,915 $ 42,873    
Total non-current assets 156,847 62,861    
Total assets 215,762 105,734    
Total current liabilities 29,659 12,085    
Total non-current liabilities 65,353 3,111    
Total shareholders' equity 120,750 90,538 $ 158,058 $ 127,262
Total liabilities and shareholders' equity 215,762 105,734    
Income statement information        
Sales, net 125,196 90,692 120,301  
Total mine cost of sales 88,449 78,205 91,669  
Exploration expenses 4,886 2,485 2,720  
Total other costs and expenses (without exploration) 14,218 10,760 10,413  
Provision for income taxes 9,615 5,573 9,967  
Net income (loss) from continuing operations 8,028 (6,331) 5,532  
Operating Segments | Oaxaca, Mexico        
Balance sheet information        
Total current assets 50,057 34,744    
Total non-current assets 66,756 62,695    
Total assets 116,813 97,439    
Total current liabilities 25,833 11,007    
Total non-current liabilities 1,436 3,098    
Total shareholders' equity 89,544 83,334    
Total liabilities and shareholders' equity 116,813 97,439    
Income statement information        
Sales, net 125,196 90,692 120,301  
Total mine cost of sales 88,449 78,205 91,669  
Exploration expenses 4,813 2,418 2,614  
Total other costs and expenses (without exploration) 3,995 48 1,101  
Provision for income taxes 8,518 2,331 7,612  
Net income (loss) from continuing operations 19,421 7,690 17,305  
Operating Segments | Michigan, USA        
Balance sheet information        
Total current assets 5,528      
Total non-current assets 90,018      
Total assets 95,546      
Total current liabilities 2,459      
Total non-current liabilities 63,438      
Total shareholders' equity 29,649      
Total liabilities and shareholders' equity 95,546      
Income statement information        
Exploration expenses 55      
Total other costs and expenses (without exploration) 1,167      
Provision for income taxes 305      
Net income (loss) from continuing operations (1,527)      
Corporate and Other        
Balance sheet information        
Total current assets 3,330 8,129    
Total non-current assets 73 166    
Total assets 3,404 8,295    
Total current liabilities 1,367 1,078    
Total non-current liabilities 479 13    
Total shareholders' equity 1,557 7,204    
Total liabilities and shareholders' equity 3,403 8,295    
Income statement information        
Exploration expenses 18 67 106  
Total other costs and expenses (without exploration) 9,056 10,712 9,312  
Provision for income taxes 792 3,242 2,355  
Net income (loss) from continuing operations $ (9,866) $ (14,021) $ (11,773)