AIM-quoted Arc now holds a 47.5% stake in Zaco.
It bought the additional interest from Rémy Welschinger, one of its non-executive directors, for £52,000, which it paid in shares.
Earlier this week, Arc revealed the discovery of West Lunga – a six-kilometre strike that graded up to 463 parts per million copper.
It was identified by the same team that discovered the nearby world-class Kamoa deposit, currently being developed by a consortium led by Ivanhoe Mines.
“Following the exciting news of the West Lunga discovery we are keen to consolidate our interest in Zaco and we are pleased to have increased our interest in Zaco on the same commercial terms as the initial transaction,” said executive chairman Nick von Schirnding.