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INCOME AND MINING TAXES - Tax Effects of Temporary Differences (Details) - USD ($) $ in Thousands | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Deferred tax assets: |  |  |  |
Net operating loss carryforward | $ 66,085 | $ 57,667 |  |
Mineral properties | 66,038 | 60,299 |  |
Other temporary differences | 30,999 | 14,356 |  |
Total gross deferred tax assets | 163,122 | 132,322 |  |
Less: valuation allowance | (154,298) | (121,212) | $ (124,153) | $ (123,648)
Net deferred tax assets | 8,824 | 11,110 |  |
Deferred tax liabilities: |  |  |  |
Acquired mineral property interests | (12,637) | (16,024) |  |
Total deferred tax liabilities | (12,637) | (16,024) |  |
Total net deferred tax liability | (3,813) | $ (4,914) |  |
Valuation allowances | $ 33,100 |  |  |

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