Gunmen attacked the mine in Ajata Aboki village in Shiroro region of Niger state on Wednesday afternoon, the local commissioner for security, Emmanuel Umar said in a statement.
"A yet to be ascertained number of workers in the mining site, including four Chinese nationals, were reported to have been abducted," the statement said.
It said some of the attackers were also killed.
"Security forces mobilized reinforcement for the manhunt of the remaining terrorists."
Umar blamed the attack on "armed bandits/terrorists". Nigeria recently listed bandit gangs operating in the northwest as terrorist groups, in a move to give the military more flexibility in operations against them.
The statement gave no casualty figure, but said the state government commiserated with the security agencies and families of "slain personnel".
It did not name the mining business or say what material was being extracted.
Kidnapping for ransom has become a major problem in northwest and central Nigeria, where heavily-armed criminal gangs known as bandits target communities, schools and businesses for abductions.
Chinese employees working on mines and large infrastructure projects in remote areas are often targeted for kidnapping, and usually released after a short period in captivity.
Attacks by bandit militias in Nigeria are ravaging parts of the country with daily newspaper headlines describing gunmen raiding villages, ransacking rural communities and kidnapping residents.
Violence from the militias has intensified with several thousand people killed each year, hundreds kidnapped and nearly one million people displaced by attacks and intercommunal clashes in the region.
Nigeria's overstretched security forces are also fighting a grinding 13-year conflict against jihadists in the country's northeast, where 40,000 people have been killed and another 2.2 million displaced since 2009.