Spill shuts down the Omai gold mine

The Omai gold mine in north-central Guyana was shut down when it was discovered that effluent had seeped through the tailings dam and into the Omai River.

The market took instant notice of the spill. Shares of Montreal-based Cambior (TSE), which operates the mine through its 65% ownership of Omai Gold Mines, lost $4.12 on Aug. 21 to close at $14.25, having traded as low as $13.63. Golden Star Resources (TSE), which owns 30%, fell to $6 before closing at $6.75, down $2. By presstime, the price of Cambior was $14.63 and Golden Star, $7.25.

The remaining 5% of the mine is owned by the Guyanese government.

Cambior President Louis Gignac immediately flew to Omai and took direct charge of the situation. Vice-president Normand Bdard, environmental manager Serge Vzina and two specialists engaged as consultants were also on hand, co-ordinating clean-up efforts with the Guyanese government.

Seepage through the main tailings dam started just before midnight local time on Aug. 19. The effluent flowed through two parts of the dam into the mine’s aeration pond, which, in turn, overflowed into the cooling pond and down to the Omai River. On-site staff divertedthe wastewater into the main Fennell open pit and shut down operations at the mine.

The Guyanese Ministry of the Environment was notified about 4:30 a.m., and Cambior and Golden Star said the agency was working closely with Omai staff to contain the spill and limit environmental effects. Staff from the Geology and Mines Commission and the Ministry of Health, together with advisers from the President’s Office and the University of Guyana, were being briefed on the situation.

By the evening of Aug. 21, Omai management had decided on a 4-part response to the spill. Tailings effluent still in the pond was to be pumped to the open pit; a coffer dam was to be built at the foot of the tailings dam to stop the flow into the Omai River; sinkholes in the dam were to be backfilled; and a temporary ditch from the tailings pond to the pit was to be widened and deepened. About 750,000 cubic metres of water remained behind the tailings dam.

Work on the coffer dam and backfilling of the holes were progressing quickly by presstime (late afternoon, Aug. 23). The flow rate to the river was down to 5,000 cubic metres per hour, and Cambior expected it would be completely stopped within a few more hours.

Staff from the company, the national government and the Regional Administration are monitoring the condition of the Omai River, which flows past the mine, and the Essequibo River, Guyana’s largest, which receives the flow about 4 km downstream from the spill site.

Water samples taken Aug. 21, immediately downstream from the mouth of the Omai, had 0.09 parts per million (ppm) total cyanide, compared with the World Health Organization’s guideline for drinking water of 0.2 ppm. (Canadian environmental agencies use guidelines of 0.2 ppm for drinking water and 0.005 ppm for protection of aquatic life.)

The wet season is nearing its end and the high flow volumes have diluted the plume. Later samples from the same area have had lower cyanide concentrations.

In the Rockstone-Sherima area, 75 km down river, a “visible slurry plume” had arrived by 2 p.m., Aug. 21. River waters at Sherima had concentrations of 0.009 ppm total cyanide.

Along the Essequibo, where roughly 18,000 people live, residents were advised not to use river water for drinking, cooking or watering livestock until further notice, and not to eat fish or shrimp caught from the river. As a precaution, the agency has begun to truck in drinking water supplied by Omai.

Guyanese officials had estimated the total discharge at 1.25 million cubic metres of effluent, carrying 28 ppm cyanide, for a total cyanide loading of 35 tonnes.

The government said it was “very disappointed and upset about this incident,” while the opposition People’s National Congress called for a permanent shutdown. The companies expressed their “deep regret” and gave assurances that there was no threat to the health of residents downstream.

Henry Roy, chief financial officer of Cambior, said rehabilitation of the mine site was expected to take two to three months, and would result in a production loss of between 40,000 and 60,000 oz. Omai is a valuable asset to Guyana’s economy, accounting for about a fifth of gross domestic product, and analysts considered it likely that the government would be anxious to bring it back into production.