Beneath the Surface: Get Down – Canada’s First Diamond Miner Moves Underground
When Ekati went into commercial production in October 1998, it was Canada’s first diamond mine, with reserves in five kimberlite pipes and a plan that called for open pit mining later joined by underground mining (see CMJ October 1998.) The life of the mine is evolving pretty well as planned.
Panda was the first pit to be mined, and it will close in mid-2003. Next was the small Koala North pit (mined from 2000-01), which was not in the original mine development plan. Then came Misery, 29 km to the southeast, just 10 km from the Diavik mine (see p.10 in this issue). Misery has been producing since November 2001. Last year the prestripping of the Koala pit began, and it should be in commercial production in March 2003.
Today the Ekati Diamond Mine produces 4 to 5 million carats of diamonds per year, which is 4% of the world’s diamond production by weight (6% by value). The mine’s 80% owner BHP Billiton Diamonds Inc. has 750 employees at Ekati and Yellowknife plus about 400 contractors. The other owners are Dr. Stewart Blusson (10%) and Chuck Fipke (10%).
There are sufficient reserves for about 18 years more mine life. At fiscal year-end June 30, 2002, reserves of 53 million carats (1.5-mm-diameter cutoff) were reported, consisting of 58 million tonnes with an average grade of 0.9 ct/tonne.
Before building underground mines at Panda (2004) and Koala (2005), Ekati is getting its feet wet at Koala North–North America’s first underground diamond mine. Koala North will adapt “open benching” to a remote, permafrost setting. Open benching was used successfully at De Beers’ very large Finsch diamond mine in South Africa in the early years of its underground life during the 1990s. The portal to Koala North lies between the Koala and Panda pipes, and will provide convenient access to both.
BHP Billiton is providing the management, safety, engineering and logistics support for the project. The project manager leading the underground studies and engineering is Rich Rein. Fortuiously, the underground mine manager, Paul Harvey, used to work at the Finsch mine.
The three-year underground development and mining contract at Koala North is being handled by a joint venture between KeTe Whii Ltd. and Procon Mining and Tunnelling Ltd. KeTe Whii is a mining services company based in the Northwest Territories and is a 50/50 partnership between the Dogrib Treaty 11 communities and the Akaitcho Treaty 8 communities. The operation, under the direction of Procon mining superintendent Larry Perry, employs about 80 people, many of whom were formerly underground gold miners in Yellowknife. The contractors work 11-hour days, four weeks in and two weeks out.
CMJ visited the remarkable test mine in late November with Harvey, Perry and senior geologist Joe Heimbach.
Open Benching Method
Koala North is a small, upright, 52-million-year-old kimberlite pipe with a roughly circular outline surrounded by 2.6-billion-year-old granite. The pipe contains a reserve of 1.5 million tonnes grading 0.45 carat of diamond per tonne.
The last glaciation, from 15,000 to 6,000 years ago, left behind Koala Lake (covering the Koala and Koala North pipes) that had a muddy bottom. The lake was fished clean, and then drained before development could begin.
Open pit mining removed the top 70 m of ore from the Koala North pipe, ending on the 2405 level (405 m above sea level), where the pipe is only 75 m in diameter. The underground development began in late-2001 with the blasting of a trench (200 m north of the pit) to start the portal, which was constructed beginning in February 2002. The underground mine will eventually include 11 levels over a vertical depth of 200 m to the 2205 level.
The 13%-grade ramp (5.5 m tall and 5.5 m wide) leads from the portal (2439 level) to the 2400 level where it spirals downward beside the kimberlite. There are also two fresh air raises and a 1.6-m-wide emergency egress raise now extending from the 2355 level to surface.
Access levels are being driven from the ramp, spaced 15-20 m apart, from which three parallel tunnels (3.8 m wide x 4 m tall) extend into the kimberlite pipe. The three mine tunnels connect at the far side of the kimberlite pipe in a vertical slot, which is open to the bottom of the pit above or to the open bench above. No crown pillar will be left; eventually the mined-out pipe will be a very steep-sided pit essentially the shape of the pipe. In 15 years time, when underground mining is completed at Panda and Koala, all three mines will fill with water to form two deep lakes.
The kimberlite is not as dense or as strong as the country rock. As a result, the workings in the kimberlite have to be kept small. The walls and back in the kimberlite are supported by 2.3-m-long resin-grouted bolts, screen and 50-75-mm-thick shotcrete, with splitsets in the side walls. If the ground monitoring programs detect an increase in stress or change in conditions, more support will be added.
It is also important to keep the kimberlite dry. The country rock is permanently frozen (permafrost) but the kimberlite is not frozen; the Koala and Koala North pipes were covered by a lake, which protected the ground beneath it from freezing. Now that the lake is gone, the kimberlite broken by blasting would freeze if it were wet, which would make it difficult to handle.
Therefore, all the drilling in kimberlite is done in dry conditions, vacuuming away the dust in a two-stage closed circuit dust collection system. As well, ground water in the mine is controlled in two ways. Drain holes are predrilled through the pipe on each level. And this spring, a water collection and pumping system is being installed on surface and in the old Koala North pit to collect water before it reaches the mine.
On the first level, a fan of 9- to 11-m-long blastholes 76 mm in diameter is drilled upward, using a single-boom Tamrock longhole drill. On subsequent levels, full rings will be drilled to lengths up to 20 m. Each ring of blastholes is 2 m apart. Holes are loaded with Anfo and plugged with a birdie. The powder factors on the first level are around 0.5 kg/m in the slot and 0.45 kg/m in the stope rings. On subsequent full retreat levels, powder factors should reduce considerably.
Every second day, four rings are blasted, breaking about 3,000 tonnes of ore, retreating from the slot back across the pipe. The ore is mucked from the three drawpoints on each level by a variety of load-haul-dumpers: two remotely-controlled (an Elphinstone R1700 low-profile 8-yd3 and a Procon 200 6-yd3 machine) and two not remotely-controlled (an Elphinstone R1700 8-yd3 and a Procon 3.5-yd3).
The ore is loaded into two 40-tonne Tamrock Toro 40D fully articulated low-profile mine trucks, modified by Procon, which haul it up the decline to surface, or it is stored in the muckbay on each level for later haulage.
The first kimberlite was mined July 28, 2002. By the end of November, 900 m of decline had been built, four access levels had been driven and the mining tunnels had been excavated on the top two levels. Dilution is estimated to be 10-17% over the life of the mine. There could be up to three mine headings working at once using the single-boom jumbo and up to four more headings for the Boart two-boom jumbo.
Full production from Koala North underground, expected to be reached by April 2003, is targeted at 1,200 tonnes of ore per day, to recover 180,000 ct of diamonds per year over a nominal life of three years.
Koala North Discovery, by Grant Lockhart
Until early 1996, BHP Diamonds exploration focused on finding large kimberlite pipes that would produce the tonnes of ore required to justify building a mine. After five years of intensive searching, this goal was achieved in the spring of 1996 when the Sable kimberlite was bulk-tested.
As the project switched gears to the mine construction phase, the exploration department expanded its quest to include a thorough search of the area around and under the proposed mine infrastructure. In early summer 1996, a high-d
efinition airborne survey was launched from Koala camp, at a scale the world had not seen before. More than 41,000 line-km of data were collected by two helicopters with magnetometer sensors suspended to within 20 m of the ground. From these data, three targets were drilled in the development area, resulting in the discovery of the Beartooth and Koala North kimberlites.
In 1998, Koala North was bulk-sampled. It was then that I had an opportunity to visit the Reno, Nev., recovery plant, where I held three large Koala North diamonds, two of which were particularly beautiful. It was obvious that Koala North possessed the goods, and would one day be mined.Grant Lockhart is senior project geophysicist-geologist with BHP Billiton Diamonds and has worked on the Ekati project since 1992.