INCOME AND MINING TAXES - Tax Effects of Temporary Differences (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforward	$ 70,830	$ 66,085
Mineral properties	59,426	66,038
Other temporary differences	29,009	30,999
Total gross deferred tax assets	159,265	163,122
Less: valuation allowance	(149,921)	(154,298)	$ (121,212)	$ (124,153)
Net deferred tax assets	9,344	8,824
Deferred tax liabilities:
Acquired mineral property interests	(9,344)	(12,637)
Total deferred tax liabilities	$ (9,344)	(12,637)
Total net deferred tax liability		$ (3,813)