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v3.19.1

Other liabilities - Share-based payment arrangements (Details) |  |  |  |  |  |  | 12 Months Ended
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Jun. 14, 2018 USD ($) | Mar. 21, 2018 USD ($) Y | Jun. 20, 2017 USD ($) Y | Jun. 01, 2017 USD ($) Y | Nov. 24, 2016 USD ($) Y | May 29, 2016 | Dec. 31, 2018 USD ($) EquityInstruments Y Option shares | Dec. 31, 2017 USD ($) EquityInstruments Option shares
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Number of instruments granted | EquityInstruments |  |  |  |  |  |  | 1,025,042 | 757,365
Shares subject to performance conditions Outstanding | shares |  |  |  |  |  |  | 875,649 | 737,902
Service condition |  |  |  |  |  | 3 years | 3 years |
Share options exercised | Option |  |  |  |  |  |  |  | 30,000
Grant date of June 1, 2017 |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Number of instruments granted | EquityInstruments |  |  |  |  |  |  | 19,463 | 19,463
Weighted average fair value of share options granted |  |  |  |  |  |  | $ 10.96 |
Employee stock option |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Weighted average fair value of share options granted |  |  |  |  |  |  | $ 18.62 | $ 16.51
Option life | Y |  |  |  |  |  |  | 6.18 |
Share based compensation expense |  |  |  |  |  |  | $ 2,798,000 | $ 2,405,000
Equity incentive plan |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Exercise price |  | $ 0 | $ 0 | $ 0 | $ 0 |  |  |
Grant date share price |  | 15.19 | 10.50 | 10.96 | 11.81 |  |  |
Number of options with no performance obligations |  |  |  |  |  |  | 149,393 | 19,463
Percentage of total shareholder return (“TSR”) relative to a comparator group, vesting condition |  |  |  |  |  |  | 30.00% |
Percentage of TSR relative to S&P Global 1200 Metals and Mining Index, vesting condition |  |  |  |  |  |  | 30.00% |
Percentage of return on invested capital, vesting condition |  |  |  |  |  |  | 20.00% |
Percentage of net operating profit after tax, vesting condition |  |  |  |  |  |  | 20.00% |
Weighted average fair value of share options granted |  | $ 22.56 | $ 15.90 | $ 16.77 | $ 16.66 |  |  |
Expected volatility |  | 49.86% | 43.15% | 43.09% | 44.83% |  |  |
Option life | Y |  | 3.00 | 3.00 | 3.00 | 3.00 |  |  |
Risk-free interest rate |  | 2.48% | 1.52% | 1.44% | 1.39% |  |  |
Performance period remaining at grant date |  | 2 years 9 months 11 days | 2 years 6 months 11 days | 2 years 6 months 29 days | 2 years 1 month 6 days |  | 3 years |
Company TSR at grant date |  | 2.10% | (0.30%) | 4.00% | 40.00% |  |  |
Median comparator group TSR at grant date |  | (6.20%) | (7.20%) | (3.70%) | 56.40% |  |  |
Median index TSR at grant date |  | (8.40%) | 0.60% | 4.80% | 45.70% |  |  |
Equity incentive plan | Grant date of June 14, 2018 |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Number of instruments granted | EquityInstruments |  |  |  |  |  |  | 129,930 |
Exercise price |  |  |  |  |  |  |  |
Weighted average fair value of share options granted | $ 9.34 |  |  |  |  |  |  |
Equity incentive plan | Grant date of March 21, 2018 |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Number of instruments granted | EquityInstruments |  |  |  |  |  |  | 287,080 |
Exercise price |  |  |  |  |  |  |  |
Weighted average fair value of share options granted |  | $ 22.56 |  |  |  |  |  |
Equity incentive plan | Grant date of June 1, 2017 |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Number of instruments granted | EquityInstruments |  |  |  |  |  |  | 382,002 | 455,627
Exercise price |  |  |  |  |  |  |  |
Weighted average fair value of share options granted |  |  |  | $ 16.77 |  |  |  |
Equity incentive plan | Grant date of June 20, 2017 |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Number of instruments granted | EquityInstruments |  |  |  |  |  |  | 17,342 | 17,342
Exercise price |  |  |  |  |  |  |  |
Weighted average fair value of share options granted |  |  | $ 15.90 |  |  |  |  |
Equity incentive plan | Grant date of November 24, 2016 |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Number of instruments granted | EquityInstruments |  |  |  |  |  |  | 189,225 | 264,933
Exercise price |  |  |  |  |  |  |  |
Weighted average fair value of share options granted |  |  |  |  | $ 16.66 |  |  |
Minimum |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Options receivable by participant depending on financial performance of the Company, as a percent |  |  |  |  |  |  | 0.00% |
TSR correlation considered significant |  |  |  |  |  |  | 20.00% |
Maximum |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |  |  |  |  |  |
Options receivable by participant depending on financial performance of the Company, as a percent |  |  |  |  |  |  | 200.00% |

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