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Inventories, net - Impairment of value of inventory (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Inventories, net      
Beginning Balance $ 23,163 $ 11,603 $ 8,386
Provision for impairment of finished and in progress goods, note 21(a) 7,329 4,640  
Reversal for impairment of finished and in progress goods, note 21(a) 9,472 119 0
Provision for impairment of spare parts and supplies, note 26 15,703 11,704 4,814
Reversal for impairment of spare parts and supplies (discontinued operations), note 1(e) (11,641) (4,665) (2,370)
Transfer from mining units held for sale 0 0 0
Provision for impairment of inventory of spare parts and supplies 843 0 0
Reversal for impairment of spare parts and supplies, note 26 (523) 0 (1,345)
Final balance $ 25,402 $ 23,163 $ 11,603