Mont. Code Ann. § 77-3-116

In every mining lease the board shall reserve to the state a royalty which shall, together with other considerations to be paid by the mining lessee, constitute the full market value as ascertained by the board of the leasehold interest conveyed by the lease. This royalty may not be less than 5% of the returns from or of the full market value of the metalliferous minerals or gems recovered by the lessee from the state lands or reserved mineral rights covered by the lease.