The men are believed to have drowned after water from a burst dam flooded the shafts and underground tunnels they were working in.
Police said rescue teams were trying to pump out the water and search for bodies and any survivors.
According to reports, illegal miners in Battlefields, 175 km west of Harare, had entered shafts on land owned by RioZim and another firm in search of gold.
It added that the two mines, called Silver Moon Mine and Cricket Mine, are run by registered companies.

Trump says Iran ceasefire 'on life support' after he rejects Tehran's response
India's Modi to begin five-nation tour, including UAE
EU ministers agree on sanctions targeting violent West Bank settlers
Last six passengers leave Hantavirus-hit ship as captain hails their patience
Suspect in Trump attempted assassination pleads not guilty
UK's Starmer promises to be bolder to try to rescue his job
Philippine lawmakers to vote on impeachment of presidential hopeful Duterte
Six people found dead in boxcar in Texas, police say
