MINERAL PROPERTY INTEREST AND ASSET RETIREMENT OBLIGATIONS - Amortization of Mineral Property Interests and Asset Retirement Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations			$ 1,845	$ 966
El Gallo 1 mine
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations	$ 700	$ 300	1,800	1,000
Amortization of capitalized asset retirement costs	$ 100	$ 100	$ 400	$ 300