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.
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