The commencement of De Beers Canada’s Snap Lake diamond mine operation in the Northwest Territories represents a number of firsts for the company and for the industry.
The De Beers-owned and operated Snap Lake mine is a first in a number of ways. To start with, the project is De Beers’ first mine outside of South Africa and, consequently, the company’s first experience in Subarctic operations. Significant also, the deposit type is a first for De Beers in its 120-year history. The Snap Lake diamonds are contained not in the more typical carrot-shaped kimberlite pipe, but rather in a dyke — an irregular, undulating formation of varying thickness and with occasional breaks in the ore body. The dyke ranges from several metres to an average of 2.8 metres in thickness. This means that the kimberlite at Snap Lake travelled as molten rock from deep underground, finding its way up through cracks in the earth’s crust.
The technical challenges of mining this type of deposit led to another first: Snap Lake will not have an open pit component. Most diamond operations begin their life as open pit projects and, if the ore body justifies the expense, transition into an underground phase. However, the dyke at Snap Lake dips an average of 12 to 15 degrees from the northwest shore down below a lake, presenting further challenges to mining the deposit from the surface. This combination of factors has resulted in Snap Lake being the world’s first major underground-only diamond mine.
It officially opened on July 25, 2008 and is currently processing very close to 3,150 tonnes of ore per day said Chantal Lavoie, De Beers Canada’s senior vice president of operations. That will translate into an annual production of 1.1 million tonnes of ore and 1.4 million carats. The capital construction cost for the project was $975 million. The new accommodations building is now under construction and when it is finished in 2009, total construction capital will come in just under $1.1 billion.
Between a rock and a hard place
“One thing we realized is that the front-end of the plant can handle more material,” Lavoie added. “We are modifying the recovery aspect to have more flexibility with the two types of rock we have, which will allow us to increase throughput.” Between 70 and 80 per cent of the rock surrounding the kimberlite dyke is granitic. Lavoie explained that granites are easy to separate and discard in processing because they are easily distinguished from kimberlite, being very different in colour and density. The challenge facing Snap Lake lies in the remaining material. “The top layer is made up of metavolcanic material, which is very difficult to distinguish from kimberlite and diamonds,” he said. “Early in the design phase, we looked at an optical sorting approach, but that technique relies on colour. Because metavolcanics are very similar in colour to kimberlite, the technique proved to be inefficient.”
Further complicating the issue is the high density of the metavolcanics, making the gravimetric sorting approach more prone to error. “This means the metavolcanics tend to go with the diamonds and not the waste,” Lavoie explained. The increased volume that has to be sorted in the later processing stages is not one the plant was designed to take and resulted in a bottleneck in recovery, which reduces throughput.
Lavoie said the mine is in the process of adding more X-ray and laser sorting machines to address the added flow of metavolcanic rock to recovery. The goal, he added, is to increase throughput by 10 to 15 per cent over the next two years. At that point the mine will be operating at its peak capacity. “We feel that when we reach 3,500 to 3,700 tonnes per day, that is all that the ore body can sustain,” he said.
The ups and downs of automation
The Snap Lake operation currently employs a total of 660 employees, but most of them never see the inside of the processing plant. The plant was designed with a very strong focus on automation, requiring only one operator per shift in the recovery area. Without this automation, Lavoie explained, the plant would need as many as 10 people to operate it. The flip side of the advantages realized by the requirement for fewer workers is that the operator must be extremely skilled.
“Early on, we hired a very small group of people to run the bulk sample plant,” Lavoie said. “Within that group, we identified a few who would become the recovery operators long-term and started training them at that point.” Already skilled in diamond processing, this small group still required an additional two years of training. “Some of them were even sent to South Africa for six months to be trained in operations and maintenance.” Maintenance is nearly as crucial as the operations themselves because of the complexity of the system, Lavoie added. The current operators started training as far back as 2005, and the mine started commercial production with four operators and two maintenance technicians.
Lavoie acknowledged that such a small pool of skilled operators poses a potential risk for the mine. “There is a concern that with so few operators, if you lose one, your performance could suffer,” he said. To address this concern, Snap Lake has expanded the original group to eight operators and four technicians and is planning to continue building capacity as it goes.