Musselwhite mine & mill

By Goldcorp’s own admission, the Musselwhite mine is not the most visible gold producer. It is a low-grade underground operation in a remote corner of northern Ontario, 430 km north of Thunder Bay. It is jointly owned by Goldcorp (68% and the operator) and Kinross Gold Corp. Nonetheless, it has enjoyed a productive 10-year history and, thanks to a new structural geology interpretation, its exploration potential is growing.

New geological model

The geology around the Musselwhite mine has been of continuing interest to geologists for four decades, but by the end of 2006 the model of the Musselwhite deposit had changed fundamentally. In the early 1990s the Musselwhite deposit was assumed to lie in a fold closure. A decade later, the deposit was drawn on a faulted model. Then last year, geologists adopted a shear zone model. What they now know is that gold mineralization can occur anywhere in the iron formation but that it is higher-grade where the iron formation is intersected by specific shear zones.

Geologists have determined so far that along every kilometre of strike length the deposit contains about a million ounces of gold. In fact just over 2.0 million oz of gold have been poured from Musselwhite, a further 2.0 million oz of gold are contained in reserves and another million ounces in resources, for a total of 5.0 million oz in 5.0 km of developed or drilled deposit strike length. And it is open along strike for perhaps 1.8 km or more beyond what has been drill-tested.

The T Antiform is currently being mined and has for many years been the mainstay of the Musselwhite mineral reserve, but is still returning core that assays as much 11.0 g/t Au over 6.0 m in a newly identified shear zone.

Farther from the mine workings lies the PQ Deeps area. Some of the drill results have been spectacular–10.2 m of 127.0 g/t Au (uncut) including a 0.7-m section at 1,688.0 g/t Au. Some of the richer sections contain 200,000 oz per 100 m of strike length; in other words, gold is being discovered at twice the rate of the historical resource.

Drilling off barges on the lake, Musselwhite geologists have been able to confirm that the PQ Deeps mineralization continues along strike for perhaps 1.2 km north of the existing reserves.

The new structural model has also spurred the North Shore drill program, on the north shore of Opapimiskan Lake. This is an area between 5.0 and 6.0 km from the Musselwhite portal. Two iron formations (North and South) have been drilled with assays as high as 7.0 g/t Au over 4.6 m.

Musselwhite mine general manager Chris Start says Goldcorp is “encouraged” by the drill program. “Essentially, we are doing step-out drilling, not at 300 m, but at 1,800 m. In the first hole we drilled we found gold mineralization. We are now following up on these early and positive results.”

A series of near-surface quartz veins, referred to as the Kenpat veins, lie directly above these recent drilling results. In a further twist to the Musselwhite story, these are the very same quartz veins in which the Musselwhite brothers made their initial gold discovery.

If the dreamers at Goldcorp had their way, a second mining complex would be built on the Musselwhite property on the north shore of Opapimiskan Lake.

Mining

Access to the Musselwhite mine is through a twin decline system, one providing access for people and materials and the other being the conveyor drift. The operation produced 240,000 oz of gold last year from stopes located mainly (south to north) in the T Antiform, PQ and PQ Deeps areas.

The T Antiform has reached the fourth phase of mining between the 500-m and 670-m levels. Phase 4 mining began in the first half of 2005 and will continue at a rate of 659,000 t/y until 2011. Where mining widths are greater than 15.0 m, transverse stopes are developed. Where mining widths are less than 15.0 m, longitudinal retreat methods are employed.

Production from the PQ zone began in September 2003 at a planned rate of 550 t/d of ore. Conventional longhole and sub-level drifting with longhole stoping suits this structure. It will be mined out in 2007.

Mining of the PQ Deeps zone has begun between the 570-m and 820-m levels. Methods are the same as those in the T Antiform. Production at 400,000 t/y will continue until 2015.

Two other zones are also active mining areas. The Esker deposit above the T Antiform, which extends from the 290-m level to near surface, will begin ore production late in 2007 at a rate of 1,000 t/d. Developing this area had the added benefits of improving ventilation in the T Antiform and PQ Deeps. Mining began in the Snoppy deposit in 2005 from the 30-m level. Because of their location beneath the polishing pond and close to the shop and the warehouse, remaining reserves above that level will be mined from the surface at a later date.

Musselwhite is in the process of replacing its fleet of scoops and trucks with a new fleet of Caterpillar equipment consisting of eight AD45B trucks, six R2900G load-haul-dumpers, three R1700G LHDs and a single R1300g LHD. Trucks will be arriving at site during 2007, and the scoops during 2007 and the start of 2008. Other significant underground equipment includes a fleet of four Sandvik double-boom jumbos and six Sandvik bolters.

The Musselwhite property includes an open pit that was mined in 2004. Ore from this operation was stockpiled and is used to maintain the milling rate at 4,000 t/d.

Ore treatment

Musselwhite ore is crushed underground in a Nordberg jaw crusher, then hoisted to another Nordberg jaw crusher on surface. Final ore reduction to –9 mm is accomplished in a Simplicity cone crusher. The ore is further reduced to 80% –106 microns in a grinding circuit consisting of a Nordberg rod and a Nordberg ball mill. The seven existing 40-mm Technequip hydrocyclones are soon to be replaced with four 510-mm Weir Cavex cyclones.

The gravity circuit includes two Knelson concentrators. The recovered free gold is placed in a ConSep Acadia reactor, also provided by Knelson. This is a patented, high-efficiency batch leaching system and EW reactor. Between 26% and 30% of the gold is recovered in the gravity/Acacia circuit.

Ninety-three per cent of the remaining gold is recovered by conventional CIP means, bringing the total gold recovery rate to 95.5%. The mill houses four stirred-tank reactors in which the slurry is leached, and six CIP tanks. Loaded carbon passes through an elution column using a caustic-cyanide solution to strip the gold. The pregnant solution passes into three EW banks, where the gold is plated onto cathodes. Gold is washed from the cathodes, dried, smelted and poured into dor bars containing 85-93% Au.

Tails from the CIP circuit are washed with reclaim water in a two-stage counter-current decantation circuit. Cyanide is destroyed using the Inco SO2 process. The final slurry is impounded behind a tailings dam. The current plans are to increase the existing tailings disposal capacity using a dry stacking method and backfill system to help accommodate the mine life that could extend up to the year 2026.

Looking forward

Despite low grades and high costs, the future is looking bright for the Musselwhite mine. The deposit has already produced over 2.2 million oz of gold and has the potential to produce over 5.0 million more ounces. The new structural model of the deposit has been the key to unlocking gold where none was known before.

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