A massive tailings dam breach at Imperial Metals Corp.’s open-pit Mount Polley gold-copper project in British Columbia has been stabilized after a subsequent spill into nearby waterways; no injuries or fatalities were reported at the site some 250 miles northeast of Vancouver and the cause remains unknown.

Government officials said 10 billion liters of water with 4.5 billion cubic meters of tailings may have polluted Polley Lake; Minister of Energy and Mines Bill Bennett said the tailings may have also affected nearby Quesnel Lake. Watershed impacts are also unknown and water and tailings discharge quantities are undetermined. The taiilings contain a 8.5 pH alkaline average and are non-acid generating, said company reps.

“Our first priority is the health and safety of our employees and neighbors,” company officials said in a statement. “And we are relieved no loss of life or injury have been reported – we are deeply concerned and working to mitigate immediate effects and understand the cause.” Hazeltine Creek, an outflow of Polley Lake, was a four-foot stream, but is now 150 feet wide, local government authorities said in a separate statement quoted by Reuters.

The dam, added Imperial officials, is an independently engineered structure operated within design limits and specifications. On-site monitoring instruments and observations of personnel yielded no indication of an impending breach prior to the spill.

Imperial said the mine, its No. 1 project, is now on care and maintenance, having produced 12 million lb copper, 11,867 oz gold and 33,813 oz silver in Q2 2014. In all, Imperial produced 16.6 million lb copper, 13,867 oz gold and 59,289 oz silver. The company also operates the Huckleberry open-pit copper mine in British Columbia and Sterling gold mine in Nevada.