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Finance costs and finance revenues (Details 2) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Disclosure of business combination [Line Items] |  |
Annual average of future sales of mineral | $ 193,588 | $ 233,278
Useful life of mining properties | 13 months | 13 months
Discount rate (%) | 10.00% | 10.00%

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