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.

At least 73 injured after Houthi rebels attack Saudi civilian and energy facilities
Nepal rescue focused on workers trapped in three tunnels
Indonesia reopens Jakarta airport, four others after Anak Krakatau ash clears
Russia hits Kyiv after brief pause for US envoys' visit, killing two
Kenya offers temporary amnesty to undocumented East Africans amid crackdown fears
7 Indonesian airports remain shut due to volcanic ash from Anak Krakatau
Bolivia confirms 9 deaths after military base explosion
Israeli strikes on southern Lebanese town kill 11, state news agency saysÂ
