MINERAL PROPERTY INTEREST AND ASSET RETIREMENT OBLIGATIONS - Amortization of Mineral Property Interests and Asset Retirement Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations			$ 1,144	$ 644
El Gallo 1 mine
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations	$ 800	$ 300	1,100	600
Amortization of capitalized asset retirement costs	$ 100	$ 100	$ 100	$ 100