The avalanche occurred in a remote part of the province late Tuesday night, Xinhua reported.
Hu Jianzhong, head of Heicigou Antimony Ore Prospecting Station in Subei county, said the avalanche engulfed his team's camp site. Ten people were believed to have been buried while 11 others managed to escape.
One severely injured survivor was taken to a hospital for treatment, Hu said.
Rescuers are still searching for the seven unaccounted for workers.
Gansu province has been hit with heavy rainstorms rarely seen in the region and the local observatory said the area got about 20 inches of rain in the 23 hours prior to the avalanche.