The accident occurred in the KOV open-pit mine at the Kamoto Copper Company concession, in which Glencore owns a 75% stake, said Richard Muyej, the governor of Democratic Republic of Congo's Lualaba province.
"It was caused by the clandestine artisanal diggers who have flooded (the mine) and engaged in an anarchic exploitation," he told Reuters.
A Glencore spokesman said officials were looking into the incident and the company would issue a statement soon.
Shares in Glencore were down more than 7% by 1605 GMT, on track for their worst day since July 2018.
Thousands of illegal miners operate in and around mines in southern Congo, which produce more than half of the world's cobalt, a key component in electric car batteries.
The often old or simple practices employed by artisanal miners can compromise the safety of the mines, and accidents are common.
Congo's military deployed hundreds of soldiers last week to protect a copper and cobalt mine owned by China Molybdenum Co Ltd from illegal miners.
