(Reuters) - Aurizon Mines Ltd ARZ.TO said on Thursday there had been a "minor discharge" at its Casa Berardi gold mine's tailings facility in Quebec.
Aurizon said most of the spilled material, discovered during an inspection on Wednesday evening, had remained inside a containment area, and that it did not expect any impact on operations.
But Stéphanie Lemieux, a spokeswoman with Quebec's Ministry of Sustainable Development, Environment, Wildlife and Parks, said the spill was a "category two" incident.
Environmental emergencies in Quebec are classified as category one, two, or three, where three is the most serious.
"The solid tailings were almost entirely confined between two dams," said Lemieux. "But the external dam could not contain all the liquid waste, and part of it spilled to the environment."
Lemieux said the company had estimated that 55,000 cubic meters of liquid had escaped, but its chemical makeup was not yet known.
An incident is classed as category two where it could have a significant impact on the environment, where the consequences are important and hard to identify, where the incident is complex and requires particular measures, or where material losses could be significant and people's health could be affected.
Aurizon shares fell earlier on Thursday after the ministry said a dam had broken at the site, spilling some 150,000 cubic meters of liquid and 15,000 cubic meters of solid material.
"The Casa Berardi Mine has four tailings containment ponds and the material flowed from one tailings pond into another tailings pond, neither of which are in use for current operations," the company said in a statement.
"The company is engaged with the environmental authorities, but at present has no further comment beyond the statement issued earlier," the company said later.
Shares of Aurizon closed down 0.7 percent at C$4.23 on the Toronto Stock Exchange.
Aurizon agreed in March to be taken over by U.S.-based silver producer Hecla Mining Co (HL.N) in a friendly deal that values Aurizon at about C$796 million ($789.41 million).
The company briefly suspended operations at Casa Berardi later that month after a contractor was killed in a vehicle accident.
($1 = $1.01 Canadian)
Reporting by Allison Martell; Additional reporting by Euan Rocha and Randall Palmer; Editing by Janet Guttsman and Phil Berlowitz