Royal Gold will acquire a 52.25% stake in Mt. Milligan copper-gold mine located in British Columbia, Canada, from Thomson Creek Metals.
The company will make an initial payment of $75m and commit to scheduled quarterly payments until it has paid Thomson $200m.
The acquisition increases Royal Gold's investment in the project to $781.5m.
North American mining company Thompson Creek Metals initially agreed to sell a 40% stake in the mine but has since amended the agreement.
The company will use the proceeds of the sale to finance the construction and related cost of the Mt. Milligan project.
As per the terms of the sale amendment, Thompson Creek will also receive $435 an oz when gold is delivered from the mine.
Thompson Creek chairman and chief executive officer Kevin Loughrey said, "We look forward to the completion of the Mt. Milligan copper-gold mine in the third quarter of 2013 and commencement of commercial production in the fourth quarter of 2013."
Mt Milligan copper-gold project, located 90 miles northwest of Prince George, Canada, is a truck-shovel open pit mine with a production capacity of 60,000t a day.
The mine, with a 22-year life, is projected to produce an average of 81 million pounds of copper and 194,000oz of gold annually.