Sentinel Metals (ASX:SNM) secured an option to acquire the 1% production royalty held by Amazon Mining Company over the 13 patented claims at its Columbia project in Montana, according to a Wednesday filing with the Australian bourse.

Exercising the option would decrease the project's effective aggregate royalty to 3.2% from 3.8% based on current resource weighting, the filing said.

The purchase price is set at 500 ounces of gold, about AU$3.1 million at the current spot rate.

The company can maintain the option for up to seven years through yearly payments of five ounces of gold, with all payments credited against the 500-ounce purchase price, per the filing.

Sentinel Metals shares fell 2% in midday trade Wednesday.