Diavik delivers

Is it true that Yellowknife’s sidewalks are studded with diamonds? I don’t think so. But Canada’s newest diamond mine, Diavik, is already spitting out gemstones. The first ones passed through the process plant in late November. It will take two years before the mine achieves planned full annual production of over 6 million carats from 1.5 million tonnes of ore, but the signs so far are good. Diavik came in right on the Cdn$1.3 billion budget, and nearly three months early.

The Diavik diamond mine is an un- incorporated joint venture between two Yellowknife-based companies, Diavik Diamond Mines Inc. (60% interest and operator) and Aber Diamond Mines Ltd. (40% interest). The former is a wholly owned subsidiary of London, U.K.-based Rio Tinto plc, and the latter is a wholly owned subsidiary of Aber Diamond Corp. of Toronto.

The new mine is big enough to be important to Rio Tinto, which is the world’s largest mining company. According to Jonathan Leslie, chief executive of diamonds and gold at Rio Tinto in London, “The mine has produced its first diamonds and once up to full production should approximately double Rio Tinto’s diamond revenues. Diavik is expected to deliver the greatest value per tonne of any diamond asset in the western world.”

“The debt financing that Aber did to pay for its share of Diavik was the largest ever project debt financing for a Canadian junior mining company,” Aber vice-president of marketing Matt Manson told CMJ, “And was the only-ever debt financing for a minority interest in a diamond project unsupported by an pre-sales or 100% off-take agreement for the product. The importance of this is the financing precedent that this project has set for other Canadian single-asset juniors putting their own diamond projects into production without having to involve the diamond majors.”

The pipes–where it all starts

You don’t have to go far back to find the beginning of the story. In November 1991, joint venture partners BHP and Dia Met announced the discovery of a diamond-bearing kimberlite near Lac de Gras. (Their discovery of additional economic pipes would see the creation of the Ekati diamond mine–Canada’s first–seven years later.) The resulting major rush led to the staking of more than 20 million hectares, encompassing most of the Slave Province, within three years.

The original 325,000-ha Diavik property was staked in late 1991 and early 1992 by Aber Resources Ltd. and partners. By 1995, four –A154 North, A154 South, A418 and A21–high-grade diamond-bearing kimberlite intrusions had been discovered that together comprise Canada’s second diamond mine. (By 2003, 63 kimberlites had been discovered on the Diavik-Aber property, half of them are diamondiferous.)

Diavik’s kimberlites are small (<2 ha), steep-sided pipes 55 million years old, hosted in a complex of 2.7-billion-year-old (Achaean) granitoids and metasedimentary rocks, making them among the youngest pipes in the Slave Craton. Subsequent glaciation moved and removed the weathered kimberlite. The discovery of the A154 North, A154 South, A418 and A21 pipes in 1994 and 1995 was the result of drill-testing electromagnetic and magnetic targets, which were associated with glacial trains of diamond indicator minerals.

The A154 South and A418 pipes were bulk-sampled using underground drifting. The 5,935-t sample was processed in 1997 and yielded 21,086 ct of commercial-size diamonds, indicating a very high-grade resource.

Development project

The kimberlite pipes are under about 20 m of water in the east end of Lac de Gras, next to East Island. The Diavik team put together an ambitious plan to build a series of dikes from East Island to surround the pipes and then pump the water out of the enclosed area. The pits will be excavated in succession: first A154 (for both the North and South pipes), then A418, and finally A21. The higher value A154 South and A418 pipes will subsequently be mined by underground block-and-cave methods. When mining is complete, the pits will be flooded, and the dikes breached to create an island environment similar to that pre-mining. All infrastructure, waste rock storage and processed kimberlite storage will be confined to the island.

The environmental assessment review began in March 1998 under the Canadian Environmental Assessment Act, and in September 2000, Diavik received all the necessary permits and licences to begin development. In December Rio Tinto and Aber approved the $1.3 billion capital expense to build the mine.

Construction began the next month. Supplies were brought in over the winter ice road, the same one that links Yellowknife to the Ekati and Lupin mines. The dike for the A154 pit was complete in July 2002. By September, the area inside the dike was sufficiently dewatered to allow pre-stripping of upwards of 30 m of lake bottom sediment and glacial till. Access to the A154 South pipe was gained in November 2002, allowing commissioning of the process plant and first recovery of diamonds.

Meanwhile the surface structures linked by Arctic corridors were being built at what is called the “South Camp”. The largest building is the process plant. As well there is a 13.2-MW power plant and a backup boiler plant. The admin-maintenance complex houses offices, warehouse, and 10 service bays. There are three 18-million-L diesel fuel storage tanks at the site.

The permanent accommodation complex is made of prefabricated modules arranged in four, three-story wings containing 264 single occupancy rooms and central common areas. Nice touches include a wood-floored gymnasium with a running track, a squash court, and a march-style cafeteria.

Diavik’s workforce at the fly-in operation currently numbers 500. The rotation for hourly workers is two weeks in and two weeks out, while staff work four days in and three days out. The majority of workers commute from Yellowknife and nearby communities. The mine has a 1,600-m-long airstrip suitable for Boeing 737 jets and Hercules transport aircraft.

Involving the neighbourhood

The construction and 20-year mine life of Diavik are seen by people in the region as attractive job and business opportunities. Tom Hoefer, Diavik’s manager of public and government affairs, says that Diavik has been very successful at providing local work and benefits. “Throughout construction, our workforce averaged 800, of which 44% were northern residents. This exceeded our expectations. And on the business front, we nearly doubled our local business expectations. Of the $1 billion let in contracts, nearly $750 million was with northern companies, and $500 million was with aboriginal companies or joint ventures.”

Diavik began consulting with northern communities early, and in March 2000 formalized its commitment to protect the environment through an agreement with aboriginal groups as well as the federal and territorial governments. It has concluded separate participation agreements with five aboriginal groups–the Dogrib Treaty 11 Council, the Yellowknives Dene First Nation, the North Slave Metis Alliance, the Kitikmeot Inuit Association and the Lutsel K’e Dene Band. It has also concluded a socio-economic monitoring agreement with these groups and the territorial government. Through these accords, the company has committed to providing significant training, employment and business opportunities for northern residents and aboriginal people.

Massive Water Diversion Project

Chief dike engineer John Wonnacott doesn’t look at the Diavik mine the same way as other people. He sees it as a granite mine that has diamonds as a by-product. Six million tonnes of that granite have gone into building the 3.9-km-long A154 dike that is holding back the waters of Lac de Gras. The dike averages 10-m high, but is 28-m tall at its deepest. There are 1,600 sensors throughout the dike to measure temperature, pressure and movement.

When the dike was completed, the enclosed part of the lake was fished out. In late July, the pumps were turned on, and by September the bulk of the water had been removed, allowing access
road construction and pre-stripping. Initially, the water was pumped directly back to Lac de Gras. As the water became muddy, it was pumped to holding ponds on the island, and then to the North Inlet water treatment facility. Designed to treat both dike seepage and pit water, the facility uses coagulation, flocculation, clarification and direct filtration techniques to treat up to 30,000-m3/day.

The A154 dike is designed to withstand a 1-in-100-years windstorm and maximum lake flood levels. It has a central flexible concrete cut-off wall above pressure-grouted bedrock and overlapping jet-grouted columns in the till beneath the wall but above the bedrock. The wall is protected by granite fill. Large (-900 mm) run-of-quarry rock forms the lake side, with a 4-m vertical head and long slope to absorb wave energy. The central part of the dike, through which the cut-off wall was installed, is made of compacted -50-mm material. The pit side of the dike is covered by -200-mm fill.

The dike incorporates several small islands in the lake. To maintain the permafrost underlying them, thermosyphons were installed. These function passively in winter, utilizing the frigid Arctic air, and in summer are complemented with electric refrigeration.

For Wonnacott the challenging part of the whole project has been the dike because of its many engineering aspects. The technology of building water-retaining dikes in permafrost is not new, but Diavik’s dike has different standards than tailings dams. The latter are meant to contain solids suspended in liquid, and the solids actually help support a tailings dam, but that’s not the case with a water dam.

There is no such thing as 100% impermeability; a small amount of water will invariably leak through any structure. In theory, water running through a dike can erode tiny pathways that get bigger. In practice, this is only a problem if the erosion happens over a short period. Wonnacott is estimating 500-1,000 m3 of water leaking per day for the whole A154 dike, but the system could easily handle up to 5,000 m3/day. The leakage is collected and treated.

Mining Begins

CMJ visited the A154 pit in late November, when light grey overburden was still being removed and the first blackish kimberlite was being accessed.

The A154 South kimberlite is very small–just over 100-m diameter–but very rich, so it is extremely important to keep the ore separate from overburden and country rock. A geologist was stationed in the pit throughout dayshift in the weeks overburden was removed, to ensure no disposal of ore.

Two Driltech D75EX blasthole drills are used for production drilling. Blastholes 251-mm in diameter and 10-m long are loaded with Anfo in emulsion. In early 2003, Diavik was blasting about 150,000 t at a time, every two days. With pit development still underway, ore is being hauled once a week, approximately 20,000-30,000 t at a time depending on the processing plant’s requirements.

The blasted ore and country rock are loaded by three Hitachi EX3600 excavators with 20- m3 buckets and a 35-t payload, and one LeTourneau front end loader of the same capacity. Currently, the ore and rock is hauled by seven 218-t Komatsu 830E trucks each with rubber-lined boxes; and four 90-t Komatsu HD785 trucks can be called on when required.

With a strip ratio of 15:1, Diavik will have to mine 22.5 million t of country rock to get at an anticipated 1.5 million t of kimberlite each year. The waste rock is being placed on East Island in two engineered piles that will eventually reach 70-m and 60-m tall.

The A154 pit will ultimately be 800 m in diameter and 285-m deep. It will be mined until about 2012 with the A418 pit coming on stream two years prior.

Processing the Kimberlite

Diavik’s public affairs officer Arnold Enge explained how the processing plant works.

The 4,100 t/d rated capacity process plant has three modules: a small run-of-mine building, a the main dense media separation (DMS) plant, and a smaller recovery plant.

Ore is trucked to a storage area outside the process plant. It is then fed through a grizzly and larger material reduced by a remote-control jack hammer. The kimberlite is crushed to -40 mm by a jaw crusher and conveyed to the DMS plant. Oversized ore may also be sent to a primary roll crusher.

The DMS plant feed passes through a primary scrubber, which mixes the feed with hot water to thaw the kimberlite. The ore is screened into three size fractions, -40 to +25 mm (oversize), -25 to +1 mm (optimum size) and a -1 mm fraction (fines). The oversize passes through a cone crusher that reduces it to -25 mm. The crushed kimberlite is conveyed to a secondary scrubber where it is again mixed with water and sized. Scrubber fines are mixed with water, treated, and discharged into the processed kimberlite containment (PKC) area for permanent storage.

The PKC area is confined to a valley in the centre of East Island, bounded by lined dams. The base of these dams was built in winter such that permafrost would not be disturbed. Process water, piped to the PKC area is treated and recycled through the plant. Upon completion of mining, the PKC area will measure 1 by 1.3 km. and contain up to 40 m of processed kimberlite. The material will be covered with waste rock and glacial till and vegetated with local plant species.

Optimum-size Kimberlite from either scrubber is conveyed to a 1,000-t DMS ore bin. Kimberlite is mixed with water and ferrosilicon sand then cycloned. Diamonds and other heavy minerals migrate to the outer edge of the vortex forming the concentrate. Less dense material is displaced towards the centre of the vortex forming the float. The float is screened into ore (-25 to +6 mm) and coarse rejects. Rejects are either conveyed to a discharge bin and transported to the PKC area or re-directed to an audit plant to confirm the DMS circuit is maximizing diamond recovery. The ore goes through a high-pressure roll crusher where the kimberlite particles are flattened into “pancakes”. The pancakes are conveyed to the secondary scrubber and returned to the DMS circuit.

Concentrate is conveyed to the fully automated recovery building, and the diamonds are separated from other heavy minerals. Concentrate is first sized into four fractions. It is dried using infrared heaters, conveyed by rare earth element magnets to remove kimberlite minerals, and passed through a dry x-ray sorter to liberate the diamonds.

Diamonds fluoresce when exposed to x-rays. The sorter can detect this fluorescence and then triggers a jet of air, knocking each diamond into a collection box. The rejects or non-fluorescent diamonds are passed over a grease table. All remaining non-diamond material is transported to the PKC area.

The diamonds are sorted into 11 size ranges and placed in separate plastic boxes. The boxes are heat-sealed and put in steel cases before delivery to the Diavik Production Splitting Facility in Yellowknife. There, government representatives evaluate the diamonds for royalty purposes.

Marketing the Diamonds

Under the terms of the Diavik joint venture, each partner has the right to receive and market its share of diamonds, with 60% going to Diavik Diamond Mines Inc. (DDMI) and 40% to Aber.

DDMI’s portion is repackaged in Yellowknife and shipped to Rio Tinto Diamonds in Antwerp, Belgium. According to Hoefer, the quality of the Diavik diamonds has garnered enormous interest from potential customers. “Rio Tinto Diamonds is completing its process of selecting DDMI’s customers. We expect to have 10 sales per year, and there will be no difficulty finding customers.”

Aber’s portion of rough is shipped to the company’s sorting facility in Toronto. It has signed a 10-year agreement to sell at least US$50 million of rough diamonds per year to Tiffany & Co. of New York, beginning this year. The rest of Aber’s diamonds will be sold through Aber Overseas, a rough marketing venture in Antwerp. While Aber Overseas will also sell 10 times a year, its client base will be broader than in a traditional sightholder system. Says Manson, “We expect that the Diavik production will be an
eagerly sought after item in the rough and polished diamond markets when sales begin.

For a table comparing the Diavik and Ekati diamond mines click here.

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