3 million (US$85.3 million) in cash to an unnamed buyer.
The deal is expected to close later this month.
In August, RMP launched a strategic review that envisaged significant asset sales. The Ante Creek sale was a result of the review, RMP said.
GMP FirstEnergy co-ordinated and led the process.
With proceeds from the sale, RMP said it will be able to pay down all of its bank debt.