Homestake assembling pieces of rich, intricate Eskay puzzle

Few discoveries this past decade have sparked more speculative interest than the Eskay Creek project, a polymetallic deposit on a sub-alpine plateau some 80 km northeast of Stewart, B.C. When it was being explored in the late 1980s, spectacular gold values generated such excitement that concerns about the remoteness and metallurgical complexity of the deposit were completely overshadowed.

But those logistical and technical issues are now front and centre for Vancouver-based Prime Resources Group (VSE) which has advanced its 50%-owned project to the feasibility stage. Homestake Canada, a unit of New York-listed Homestake Mining and Prime’s largest shareholder, manages the joint venture and has a 28% direct interest (or 55% total interest). The remaining 22% is held by Placer Dome (TSE).

Homestake expects to complete an in-house feasibility study and submit its application report in support of a mine certificate by mid-summer. The long-term goal is to begin production in early 1996 and thereafter produce an estimated 250,000 oz. gold and 11 million oz. silver annually. The mine would provide 235 direct jobs and be an economic boon to northern communities and local aboriginal groups.

The amount of metal to be produced over the 8-to-10-year mine life is impressive indeed, considering that the operation will be relatively small, averaging only 360 tonnes per day. But there is nothing modest about the grade of the minable reserve: 1.08 million tonnes grading 65.5 grams (2.1 oz.) gold and 2,930 grams (94.2 oz.) silver per tonne, plus 5.7% zinc, 0.77% copper and 2.89% lead (diluted by about 27%). The centre of the deposit contains a high-grade core containing 40% of the total precious metals in only 25% of the tonnes, or about 250,000 tonnes grading more than five ounces of gold equivalent per tonne.

“We expect Eskay Creek to be a high-capital/low-operating-cost mine,” says Jack Thompson, president of Prime Resources, adding that preliminary capital cost estimates are in the range of $230-290 million. “I hesitate to estimate gold production costs per oz. at this stage, although we expect they will be in the low US$200s.”

Homestake says its choice of mining method — drift-and-fill — was dictated by high ore grades, the awkward geometry of the deposit, and relatively weak rock. Rock bolts and screening will be used in most areas requiring support, although it is expected some areas will require shotcreting. “The objective is to remove 100% of the ore, or close to that,” says project manager John Turney. The ore will be mined in drift-sized openings (typically three metres wide and three metres high), using conventional drilling and blasting techniques. Access will be via existing portals at the 917-metre and 780-metre elevations. The mine will be highly mechanized and use diesel-powered load-haul-dump vehicles and trucks. Backfill will consist of development rock and cemented sand and gravel.

The mine has already been developed by about 1,260 metres of decline, plus crosscuts, drifts and raises which bring the total existing development to about 4,600 metres. Homestake plans to carry out an additional 3,500 metres of waste development in the pre-production period, as well as develop five of the seven stopes that will be required to sustain an average mining rate of 360 tonnes per day.

Because development rock will mainly consist of a rhyolite unit, which has potential for acid generation, waste rock will be kept in a sub-aqueous condition.

An underground program provided bulk samples for metallurgical testing. (Five types of ore have been classified; blending will ensure consistent mill feed.) After evaluating several techniques, Homestake selected a mill process which provides good recoveries (more than 90% for gold and silver) with “satisfactory” effluents. The flowsheet incorporates grinding (80% minus 270), an acid pre-leach circuit, a pressure oxidation circuit, a counter-current decantation for separation and recovery of copper and zinc, precious metal recovery by cyanidation using carbon-in-leach, hot cyanide treatment and a Merrill-Crowe process. The plant will be designed with maximum re-circulation of process water.

In order to enhance silver production, Homestake opted to use a “jarosite formation” pressure oxidation circuit similar to the one it pioneered at the McLaughlin mine near Napa, Calif.

Treatment steps were tested first on a bench-scale-batch basis, followed by several integrated pilot runs using all circuits at Hazen Research in Colorado. “The pilot runs turned up no surprises and gave us confidence the process is suitable for Eskay Creek ore,” Thompson says, adding that four engineering firms are now preparing bids for mill construction. The proposed process involves pre-leaching ground ore with recycled acid before oxidation in the 4-compartment autoclaves. Sulphides are oxidized to sulphuric acid and copper and zinc go into the solution.

The slurry is washed to separate the liquid and pulp. The liquid contains copper and zinc and is treated by copper cementation and subsequent solvent extraction-electrowinning of zinc. Homestake expects to produce one million lb. of copper cement (which will require further refining) as well as 12 million lb. of cathode zinc each year.

Barren solution and slimes, containing ferric hydroxide and metals co-precipitated or absorbed on to ferric hydroxide, will be recycled back to the autoclaves. The solids will be neutralized before carbon-in-leach, a conditioning step prior to cyanidation.

Because of the high silver content of the ore, autoclave conditions conducive to silver jarosite formation will have to be maintained in order to prevent silver from leaching in the carbon-in-leach circuit. This will then ensure maximum gold recovery.

Following carbon-in-leach, a step involving hot alkaline cyanide conditioning is required before additional cyanidation. This breaks down the jarosite, releasing the silver in a form suitable for cyanidation. Silver and gold are then recovered from the liquid using the Merrill-Crowe zinc dust process. The barren slurry is then subject to cyanide destruction before discharge into the tailings impoundment area.

The amount of tailings will be relatively small — about 1.4 million tonnes during the life of the project. They will not be acid-generating (since the ore is oxidized), but will contain metals such as iron, lead, antimony, arsenic and mercury.