The deluge shortly after midnight swallowed 15 employee dormitories and three residential houses in Shangluo, in the northern province of Shaanxi, the city government said in a statement.
Around 40 people were missing, it said. Police, firefighters, mining rescuers and medical staff had been sent to the site and residents in the area evacuated, it added.
The mine's operator was identified by the official Xinhua news agency as Wuzhou mining company, which according to its parent company is mainly a vanadium producer.
Separately, a coal and gas burst on Tuesday night in a mine in the southwestern province of Guizhou killed 10 people and left one more missing, provincial work safety authorities said in a statement.
Rescue efforts were under way, the statement Wednesday said.
China -- the world's largest producer of coal -- is grappling to improve standards in the sector, where regulations are often flouted and corruption enables bosses to pursue profits at the cost of worker safety.
Accidents in Chinese coal mines killed 931 people last year, a top work safety official said in March.
The official number of mining fatalities is declining but some rights groups argue the actual figures are significantly higher due to under-reporting.