The incident in the town of Gai, in the Orenburg region, occurred during the laying of explosives to extract copper, a Russian news agency reported today.
"Four people were killed. Two were injured and are in hospital in the city," a spokesman for Gaisky GOK, which owns the mine, told the Ria Novosti news agency.
Work in the mine was suspended while a team was dispatched to determine the cause of the incident, he added.