The attackers overran a mining camp in the Maru district of Zamfara state, in the latest in a series of similar raids in the region.
The gunmen’s motive is unclear. Zamfara governor Abdulaziz Yari Abubakar called the attack an “act of terrorism”.
In July Nigerian President Muhammadu Buhari sent special military units to fight armed gangs in based in the forests of Zamfara.
Hundreds of people have been killed in villages in the area over the past three years.