Holt-McDermott in transition
The major structural breaks in the Abitibi region of northeast Ontario and northwest Quebec are littered with gold and copper mines, often plunging to great depths and producing over many decades. What would make a relatively small, near-surface, 13-year-old gold mine like Holt-McDermott stand out in the Abitibi? Low costs.
At US$148 total cash costs per ounce of gold produced in 2000, Holt-McDermott has a better handle on its expenses than most Abitibi mines, although it is expected to produce the smallest amount of gold (93,000 oz / 2,890 kg) of any of Barrick Gold Corp.’s mines in production this year.
The situation is somewhat different at Barrick’s only other Abitibi gold mine. Bousquet had the second highest total cash costs among all Barrick operations in 2000, at US$219 per ounce Au. (The Tambo mine had higher costs, but it is now closed.) Bousquet produced 165,000 ounces (5,130 kg) of gold last year. With grade dropping and costs rising, the mine is expected to close within two years.
The Holt-McDermott mine in northeast Ontario became a gold producer in June 1988 (see CMJ July 1987, pp. 31-36; CMJ November 1989, pp. 31-37), and has since gone through lean and rich times. Now is a time of transition, with deeper ore being mined at higher costs, while the mill expands to increase custom revenues.
The original go-ahead decision was based on the McDermott Zone, an orebody of modest size and grade above 300 m depth. Exploration diamond drilling frantically searched for more ore, eventually discovering the South Zone underneath. As the late Bob Smith, then-president of the company, noted: “It took you guys eight years to find the orebody.”
“The South Zone was the biggest zone and the best grade ever mined here,” says technical superintendent Jeannot Boutin. “The zone was discovered in 1992 and we began mining it in 1995. We were in the heart of it in 1996-98, and the mine was generating significant cash for the first time. But most has already been exploited. It will end up at 2.8 million tonnes.”
The top half of the central pillar in the South Zone has been mined, and the mining of the bottom half has begun. The west pillar was under development when CMJ toured the mine in March. “The central pillar has held up well,” says mine superintendent Rick Stransky, “because it was mined strategically, with one multi-level retreating vertical face. We have effectively used cable bolting to control dilution during pillar extraction.”
About 136,000 tonnes of narrow remnants of ore in the fringe areas of the South Zone will be extracted over the next few years using a post-pillar mining method.
The next major production sources will be from the New Lower Zones, steep-dipping downward extensions of the South Zone alteration package. These zones were the target of a recent shaft-deepening program completed in 2000. The Lower Zones strike east-west and lie 800 m south of the main shaft between the 775 level and the new lowest level elevation of 1075. (The level number indicates the depth below surface in metres.) These reserves are divided horizontally by a main haulage level at 925 elevation. Above the 925 level there will be three separate mining areas (U100, U102 and U105) collectively called the Upper Zones. The area between the 925 and 1075 levels hosts two mining areas known as C-104 and C-97.
Stopes in all these New Lower Zones will be narrower, with average grades that are lower than South Zone averages. The haulage cycle time will be longer, as the new stopes are deeper and further from the shaft. Between now and the end of this year, Lower Zone ore production, hoisted from the new loading pocket (1145 m elevation), will increase from 40% to 80%. For these reasons, mining costs are expected to rise above those achieved in the South Zone.
The primary mining method at Holt-McDermott has been sub-level longhole stoping, in which overcut and undercut drifts are driven in the ore, usually 25 m apart depending on the ore thickness. The ore is drilled from the overcut, blasted and then mucked using conventional and remote-controlled load-haul-dumpers (LHDs). These transport the ore (or waste) to the internal muck pass systems. The material is then dropped to the main track haulage level where it is trammed 800 m to the main shaft ore and waste pass systems. Tramming is done with eight-car trains (holding 4.3 tonnes per car) powered by two five-tonne locos operating in tandem. The trains are equipped for remote-controlled movement during chute loading.
Ground conditions have been good to date; “SMART” cables, installed in the hanging wall prior to central pillar extraction, record little movement during mining. Tension rebars are being used in the South Zone pillar recovery operations instead of Split Sets to provide a more active support. Cemented rockfill was used in the South Zone primary stopes and will be used in the Lower Zone primaries, allowing for recovery of the adjacent secondary stopes.
The El-Equip and Varis leaky feeder underground radio system provides two-way radio, data and video communications. There is also a wired underground telephone system. A data link to the new surface backfill plant will allow for automated backfill slurry delivery.
Holt-McDermott was developed initially as a “Camflo-style” mine, with pneumatic drills and mucking machines, and underground tracked haulage. The traditional equipment was soon joined by mobile diesel LHDs for flexibility. Since then it has operated as a hybrid mine, with mobile equipment at the working face, feeding a tracked haulage system.
The fleet of Wagner 1.9-m3 LHDs is getting old, although preventive maintenance kept LHD availability high last year at 93%. General maintenance foreman Peter Klimtschuck is currently evaluating the equipment and monitoring operating costs to justify either rehabilitating the fleet or buying new equipment.
The amount and size of diesel equipment that can operate underground is limited by the 6,720-m3/min ventilation system, and the dimensions of the shaft (2.1 m x 8.2 m). Transporting two new 2.6-m3 LHDs last year strained the shaft limits.
The shaft initially bottomed at 420 m, was deepened to 641 m, then 940 m, and was deepened again last year to 1,195 m. With mining moving to greater depths, the hoist could become the production bottleneck, according to Klimtschuck. The hoist recently switched to Notorplast 38-mm-diameter mine hoist rope, because of its higher breaking strength (about 139,000 kg).
There are plans for 7,000 m of lateral development plus backfill raise development this year. An internal ramp system being driven off the 775 and 925 levels will eventually link all internal mine workings between the 775 and 1075 levels. Main level track drifts on 925 and 1075 are being extended east and west along strike of the ore zones to facilitate definition diamond drilling and ore/waste track haulage to the shaft.
Since the beginning, specialty work has been handled by contractors. Boart Longyear Inc. does the longhole production drilling and slot raising, while Forage Garant et Frres of Evain, Que., handles the diamond drilling. Dynatec Corp. is currently raiseboring for the extended rockfill system. Alex MacIntyre & Associates completed the recent shaft extension and is currently doing Alimak raising for the New Lower Zones muck handling and ventilation systems.
Mill and Tailings Expansion
Holt McDermott has a conventional carbon-in-leach processing plant. In 1995, the plant was expanded from 1,400 to 2,000 tonnes per day to treat ore on a custom basis from the adjacent Holloway gold mine (Battle Mountain Canada Ltd.), which would begin commercial operations in October 1996.
A second expansion, completed last December and now in the ramp-up period, increased mill capacity from 2,500 to 2,700 tonnes per day, and is expected to improve gold recovery from 94.5% in 2000 to 96.5% this year. The US$1.7-million cost was shared by Barrick and Battle Mountain / Newmont C
anada Ltd. Since January 2001, all ore from the Holloway mine (now owned by Newmont) has come to the Holt-McDermott plant. The plant throughput this year will include 45% from the Holt-McDermott mine and 55% from Holloway.
“The bottleneck in the plant is the SAG mill,” says mill superintendent Pierre Beaudoin. “We could push 3,300 tonnes through the plant except for the SAG mill.” The recent expansion therefore concentrated on the grinding area. It included decreasing the size of the muck being fed into the SAG mill to 7.6 cm; increasing the transfer size of material from the SAG to the ball mill; and adding a tertiary ball mill and the back-up pumping systems.
As well, a Svedala continuous charge monitor was installed on the SAG mill last year (see CMJ April 2001, p. 9). This is one of only three such systems in the world, and is the only one linked to an expert system. “We always try to push the tonnage. Now we are able to find the sweet spot for the ore charge, which changes depending on the ore diameter,” says Beaudoin.
“We take great care of our mills,” says Alain Bouchard, the mill maintenance co-ordinator. “We need all the availability of the mill to process more ore.” Over a 30-month period, Bouchard has recorded and entered every piece of equipment in the plant into a computer system, and has worked out an effective schedule for inspections and preventive maintenance. “We have made a bypass circuit for everything in the plant [except the SAG mill]. “Also, we have bought substantial spare parts for all critical items, to make sure they don’t shut us down for long.” The mill crews’ diligence on shutdown scheduling as well as system improvements have increased mill availability from 89% in 1999 to 94.7% in 2000, and Bouchard is expecting this to rise to 96% this year.
The Holt-McDermott tailings management system has performed well since the start of operations, dating back to 1988. The method of tailings disposal and liquid effluent management involves the discharge of mill slurry to a tailings basin where the solids settle out and the majority of the process chemicals naturally degrade. In the spring, water is transferred from the tailings basin to a polishing pond. Water is then held in a polishing pond for several months prior to its fall release to the environment. Continuous discharge flow monitoring allows the mine to control discharge at flow rates that are proportional to those of the receiving water flow rates. Mine water pumped up from the underground operations at Holt and Holloway supplements the feed water for mill processing.
With the mill expansion came the need to re-evaluate the tailings storage facility. Two longer term issues had to be addressed: the tailings facility would have to expand to accommodate additional tailings quantities; and a different closure plan was needed to handle the anticipated mineralogical changes of tailings.
Construction of a new polishing pond allowed the existing pond to be used as a tailings settling area, solving the issue of the need for increased capacity. The topography of the downstream area provided a “textbook example of an area-to-depth ratio needed for water polishing,” according to Ron Martel, environmental co-ordinator. The expansion construction will be completed by the end of summer 2001, at an expected total cost of Cdn$3.3 million.
The increased proportion of tails from Holloway (bearing arsenopyrite) was addressed by changes to the closure plan. The plan now has the area of tailings deposition being converted to a wetlands area, with a one-metre water cover to be employed at closure, as opposed to the dry vegetated cover in the previous plan.
The permitting process for the tailings expansion project required approvals from both federal and provincial ministries. Says Martel: “The key to timely success of this process was pre-consultation with all concerned parties to identify and rationalize the issues early in the process.”
At a forecasted mining rate of 450,000 tonnes per year, Holt-McDermott’s current reserves and resources will last through to 2005. However, the mine’s exploration team is attempting to expand those reserves. “We know the favourable alteration continues plunging down-dip and to the west, but finding out how much of the package is mineralized is the challenge,” says chief geologist Craig Todd. This year will see 8,000 m of surface exploration diamond drilling and 5,000 m of underground exploration drilling, plus in-the-hole and surface induced polarization surveys to pin down the ore’s geophysical signature.
If the program succeeds, then the first mine that Barrick built from exploration stage through to production, will keep its doors open well after 2005.