Goldex goes green

There is more to being a successful gold mine than separating precious metals from waste rock. The undertaking should include being an ecologically responsible neighbour. At least it does for Agnico-Eagle’s new, 100%-owned, $183-million GOLDEX MINE located in the city of Val d’Or, Que. Goldex has shown its consideration by installing ventilation fans underground, housing the ore stockpile in a dome-shaped building, keeping cyanide off the site, and eliminating the need for a large tailings impoundment area.

Gold mining began in Val d’Or in the mid- 1930s with the startup of the Lamaque and Sigma mines, names that remain among today’s producers. The Goldex deposit was discovered in 1963 and was investigated for decades. A 480-m-deep shaft was sunk and 3,000 m of underground development was carried out between 1985 and 1987. The shaft was deepened to 790 m between 1994 and 1996, and a 100,000-tonne (t) bulk sample was taken from a test stope, but the project was deemed uneconomic and Agnico-Eagle allowed it to flood.

In 2004 Agnico-Eagle rehabilitated the workings in light of stronger gold prices. By February 2005, a new gold resource estimate– 1.16 million oz–was produced, and construction began in July 2005. The decades of work paid off, producing official numbers at the end of December 2007 including 300,000 t at 2.23 g/tAu in proven reserves and 22.8 million t at 2.20 g/tAu in probable reserves. That is a total of 1.63 million oz of contained gold. There is also an indicated resource of 304,000 t grading 2.75 g/t Au.

“The secret to this project is cost control,” said mine manager Yvon Sylvestre. “Over the life of the mine we are forecasting $20 a tonne–all in.”

Goldex geology

Like the nearby LaRonde and Lapa deposits, the Goldex deposit lies in granite-greenstone terrane near the southern boundary of the Abitibi and Pontiac sub-provinces. The Goldex mineralization is hosted within the Goldex quartz diorite sill, within a succession of mafic to ultramafic volcanic rocks. The mafic-ultramafic sequence is overturned, dipping steeply to the north.

The deposit extends from 500 to 800 m below surface and is entirely hosted by the Goldex quartz diorite. The current area of economic interest is the Goldex Extension zone (GEZ) where two extensional shear vein sets occur: the dominant dipping 30 south, and less abundant dipping 30 north. GEZ extends over 300 m vertically and 450 m along strike, varying from 25 m thick along the edges to over 150 m thick in the middle. Additional mineralization is inferred to the east and below the reserves as well as to the east-southeast above 73 level.

The gold occurs as microscopic particles usually associated with pyrite in quartz-tourmaline veins and to a lesser degree in narrow fractures in the sericite-albite-altered quartz diorite. About two-thirds of the gold is recovered in the gravity circuit of the mill.

Underground mine

The Goldex mine is developed with two shafts and stations on levels 38, 60, 66, 73 and 76. Mine superintendent Daniel Par conducted an underground tour for CMJ in mid- May. We entered down shaft No. 1, which is 790 m deep and furnished with a cage as well as two 4-t skips. It is 1 km from the crusher, but “l’autobus” or the train shuttles people back and forth.

The deposit is to be mined as a single stope using longhole shrinkage. Four Cubex 165- mm drills with auxiliary compressors have been chosen as production drills. Each drill is accompanied by a diesel-powered, tracked pipe tub for drill rods. They have automated the job of rod-handling, so Agnico’s drillers are spared the sore backs that so often come from changing rods.

Treating the orebody as a single stope means that there will be a great deal of drilling done to create blasts of a million tonnes or more each. The first such blast is in the works for 2009. Development will be complete to the top of the orebody in 2010.Production drilling will end in 2012 and the final blast will be done in 2013.Mucking will continue until 2017.The orebody is divided into three sections to be mucked in sequence–East, West and Centre–keeping the top of the muck from section to section as level as possible.

The choice of blasting agent depends on what type of work is being done. Anfo is the explosive of choice for development, but when production gets underway, emulsion will be used with Orica i-kon electronic detonators.

Mucking and haulage on level 76 will be done with Caterpillar R2900G XTRA load-haul- dumpers (LHDs). The drift is 5.3 m wide and 4.7 m high, which is enough for the large loaders but not for ventilation tubing as well. Instead, a parallel ventilation drift is being dug. The loaders have a bucket capacity of 11.6-m3 or 20-t with 320-kW engines to propel them. Each R2900G XTRA is 3.5 m wide, 3.0 m high to the top of the roll-over protection enclosure, and 11.5 m long.

Hewitt Equipment supplied three of them. They are by far the largest LHDs this writer has ever seen.

The permanent garage has been excavated on level 76. Meantime, the temporary garage that was cut into the orebody continues to assemble equipment. Level 76 will also be home to the refuge station/lunchroom, including the mine supervisor’s office. The fuel bay will become operational in June, and LHDs will be fuelled with a hose system and quick-fill nozzle similar those used for Formula 1 racing cars. A warehouse for mechanical parts will be built on level 76, and the powder magazine will be housed on level 60.

The 1,675-cm x 2,135-cm Birdsboro Buchanan jaw crusher is installed on level 77 (770 m below surface). The crusher reduces ore to -12.7 cm before it is conveyed to the ore storage bin, and conveyed to the loading pocket. The automated ore crushing and loading system will have a capacity of approximately 6,960 t/d.

Dumas Contracting sunk the 865-m-deep No. 2 production shaft that is 5.5 m diam and concrete-lined. It is fitted with automated Koepe friction hoists that move a pair of 21-t bottom-dump skips. Skipping is to start up shaft No. 2 by the end of June. A maryanne cage will be available. A control room for the hoisting systems will be located on surface.

As much as 25,000 t of crushed ore can be stockpiled in a dome-shaped building on the surface. Enclosing the stockpile cuts down on dust that might not be appreciated by the nearby residential neighbourhood.

The ventilation system is downcast through both shafts, and the two 373-kW Joy M84-50-1200 fans are mounted on level 38 to reduce the noise. There is, however, a natural gas heater and surface fan used to circulate warm air during the winter. The ventilation system has been operating at 4,200 m3/min during development, but it has a total capacity of 11,200 m3/min to meet the needs of the commercial production rate of 6,960 t/d.

The ground is reasonably competent, but sensors are being installed to monitor potential areas of stress. Most areas of the mine need only rockbolting and screening, but Shotcrete is used occasionally. A pair of MacLean bolters are available for bolting and screening.

The mine is relatively dry. Only about 65 L/sec are pumped to the surface.

Goldex mill

The Goldex mill building houses only grinding, gravity and flotation circuits. By trucking the concentrate to the LaRonde mill for gold recovery, planners eliminated the need to have cyanide at the Goldex site and the regulatory and licensing complexities associated with it. The mill is automated to the point that a shift consists of only three people–a team leader, a grinding operator and a helper.

Stockpiled ore is drawn down by three apron feeders and conveyed to the 7.3-m-diam x 3.7-m-long SAG mill. The engineering was done by Farnell-Thompson, but Agnico- Eagle undertook the sourcing of the components and construction management. This type of “direct sourcing”gave the mining c
ompany exactly what it needed.

The grinding circuit includes a bank of hydrocyclones and a 5.0-m-diam x 8.2-m-long ball mill.

“The target grind is 80% -106 microns,” mill superintendent Julie Fournier said. “There is also the potential to coarsen the grind and save energy because the flotation circuit is not very sensitive to the grind size. But the cyanidation circuit at LaRonde is, so the optimum target must be determined.”

Three-quarters of the grinding circuit circulating load enters the gravity circuit, which achieves a 65% gold recovery rate. The slurry passes into one of three identical lines, beginning with a vibrating screen. Underflow from the screens reports to a Knelson centrifugal concentrator with a 1.2-m diameter. Batches are cycled through the Knelson machines hourly. The gold-containing concentrate then passes through magnetic separators and across Gemini tables. With the addition of fluxes, the gold-bearing material is smelted in an induction furnace and poured into dor bars at the Goldex site.

“We’re pushing the gravity recovery now for fast cash flow,” added Fournier.

Material from the grinding circuit not suitable for gravity recovery plus material rejected from the gravity circuit feed a rougher-scavenger circuit. The slurry is conditioned with collector and frother, and then passed through six 38-m3 Outokumpu OK38 cells. Scavenger underflow is recirculated to the conditioner and again to the rougher. Underflow from the rougher is treated in a 2.4-m-diam x 9-m-tall flotation column.

The flotation column produces gold-bearing concentrate that is thickened, then passes through a holding tank and is loaded into tanker trucks for the 60-km trip to the LaRonde mill. There cyanide, lime and lead nitrate are added and the resulting pregnant solution joins the stream of similar material from the LaRonde mine where gold is recovered in a Merrill-Crowe circuit.

Final tailings at Goldex come from the scavenger cells. They are sampled and thickened before final disposal. The tails are nonacid generating, which is a great advantage. A small (5-million-t) auxiliary impoundment area has been built at the Goldex site. The bulk of the tailings will be used to remediate the tails at the former Manitou mine (see sidebar) beginning in July.

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New tails solve old environmental disaster

The old Manitou mine near Val d’Or ceased operations in 1979, at a time before closure plans and water treatment plants were required. Since then its acid-generating tails have fouled the local watershed, eventually reaching the Bourlamaque River.

The situation is an environmental mess begging for a solution, and fortunately there is one. Agnico-Eagle and the provincial government (Ministre des Ressources naturelles et de la Faune, MRNF) have forged a unique partnership to rehabilitate the tailings and clean up the mess, hopefully forever. In short, non-acid-generating tails from the Goldex flotation circuit will be used to cap the old site and stop the migration of the old, acid-prone material.

Project manager Rosaire Emond has worked on the project since Agnico-Eagle first proposed it. After several years of discussion, he said, the province agreed when it realized its own plan would have cost $70 million.

The $49-million cost of the new plan is to be shared, $35 million from the MRNF and $14 million from Agnico-Eagle. Costs are broken down into $40 million for capital projects and a further $9 million for operating costs over the life of the Goldex mine.

Last year 700,000 tonnes of the old Manitou tails were moved, a 1.5-km stretch of Manitou Creek was remediated where it enters the river, and 14 km of drainage ditches have been built to control most of the runoff water. The pumping of Goldex tails is to begin in July, and material will be deposited to a minimum depth of 1.5 m. The layer of Goldex tails will prevent both wind and water erosion of the old Manitou tails. The new cover is also expected to raise the groundwater levels 3-4 m, creating a sub-aqueous situation in some areas and stopping acid-generation. The polishing pond at the Manitou site is dammed to establish controlled drainage.

A 23-km road and pipeline from the Goldex mill to the Manitou tailings will be built. Agnico-Eagle is responsible for construction management, operations and providing the fresh tailings. The province agreed to provide the Manitou site, pipe and road, pumps at the Goldex mill and the necessary permits. The province also retains legal responsibility for the site of the former producer.

The Manitou-Goldex tailings project is overseen by a five-person management committee from MRNF and Agnico plus an independent consultant.

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