Emerald Ex is buying the 18.48% stake for cash with the transaction set to close on 25 February.
Nick von Schirnding, Arc’s executive chairman said: "The sale of Andiamo represents a step forward in delivering on our strategy of selling non-core assets.
“The proceeds of the transaction will be deployed to our current exploration programme at our Zamsort copper-cobalt project.
“We are also progressing discussions with a number of interested third parties in respect of our other non-core assets."
Arc’s attention is focused on the Zamsort copper deposit in Zambia and gold acreage at Misisi in the DRC.