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Mineral Properties - Capitalized Investment in Mineral Properties (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Exploration |  | $ 40
Lik Project (Alaska) |  |
Exploration | $ 15,611 | 0
La Promesa (Peru) |  |
Exploration | 6 | 6
Montana Royalty Property |  |
Exploration | $ 40 | $ 40

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