A Cyanogen sulfide leak was detected at Yuanshishan ferronickel mine in Ping'an county of Haidong City in Qinghai province, the provincial safety administration said in a statement.
The miners who fell ill were immediately rushed to area hospitals by rescuers, state-run Xinhua news agency reported.
Cyanogen gas is colourless with a pungent odour. It can lead to headache, nausea, loss of consciousness, convulsions and death, if inhaled.