Geddes’ new plan shows less acid drainage likely

A revised mine plan for the Windy Craggy project was recently submitted to the British Columbia government by Geddes Resources (TSE), a junior company which is proposing to develop a major copper mine in the far northwestern corner of the province. The company submitted its stage- one report to the British Columbia Mine Development Steering Committee in early 1990. But after a review by various government agencies, the company was asked to submit a revised mine plan which would “more adequately” address the potential problem of acid rock drainage.

Geddes said its revised mine plan significantly reduces the quantity of waste rock, segregates the potentially acid generating waste rock and disposes of this material in a submerged impoundment.

Mine planning studies examined open pit mining alone, underground mining, and a combined open pit and underground mining operation. It was concluded that initial open pit development of the North zone and the upper portion of the south zone, supplemented by later underground mining of the lower portion of the south zone, would provide the optimum mining method for the project.

The company said underground mining alone could not provide complete extraction, production capacity or unit cost performance for a financially viable project. And it noted that the revised mine plan reduces the quantity of waste rock to about one-half the quantity generated by open pit mining alone as previously proposed.

The mix of mining methods will allow for an initial design capacity of 20,000 tonnes per day, increasing to 30,000 tonnes per day.

Waste rock extracted at the open pit is to be sorted according to its potential for generating or consuming acid, and only waste rock that will not generate acid will be disposed of on glaciers near the mine. Waste rock that has the potential for acid generation will be transported to an impoundment for submerged disposal.

The impoundment dams will be constructed in the Upper Tats Creek valley (about 11 km from the deposit) for submerged disposal of potentially acid-generating waste rock and mill tailings. The company views this site as suitable for control of seepage and surface runoff and for construction of embankments which will be secure during expected seismic events.

The water management system is designed to provide for collection of all water that has contacted potentially acid generating rock. The revised mine plan also incorporates measures for long-term prevention of acid rock drainage after mine closure.

On the milling side, the process plant will incorporate 2-stage grinding by autogenous and pebble mills before the rougher flotation process. It was noted that tertiary grinding of the rougher concentrate will be required to liberate the chalcopyrite from the pyrite and pyrrhotite. Cleaner flotation stages are expected to produce a 28% copper concentrate with copper recovery of 88%.

Metallurgical test work is still continuing, and some of this work will investigate reagents that are more economical and more environmentally favorable than those initially proposed. The company said process development work now indicates that ammonium chloride and oxalic acid will be used instead of cyanide for pyrite and pyrrhotite depression. Other reagents will include a frother and a promoter.

Also, the Windy Craggy deposit contains significant values in zinc, cobalt, gold and silver for which the currently planned process does not provide optimum recovery. Geddes said further metallurgical test work may determine viable alternatives to the planned process, additions to the process, or later retreatment processes that would optimize recovery of these values.

Geddes plans to haul its concentrates to Haines, Alaska, by truck. A concept for transporting the concentrate by pipeline was shown to be economically and probably environmentally preferred. However, discharge of slurry water in Haines was found to be not permissible.

The Windy Craggy mine site is 235 km from Haines. An all- weather, 105-km access road will be required to link the mine site to the Haines Highway.