Curragh Resources and its Faro, Yukon, complex are in the midst of switching workhorses. Up until the past few years, the Faro deposit — in terms of draft horses, truly a Clydesdale — had fed the 4-million-tonne-per-year Faro concentrator. Only recently did the Vangorda open pit deposit begin to contribute. But by early summer both the underground and open pit portions of Faro will have been put to pasture. Even in retirement though, the vast Faro pit will serve a useful purpose as a tailings pond (see separate environmental story).
Fortunately for Curragh, its stable of deposits on the fertile Vangorda Plateau to the southeast of Faro has still to be exhausted. After Vangorda, which will run out of ore in the first quarter of 1993, Curragh will rely on the Grum deposit, then the Dy deposit and then? Well it will probably be the Swim. But the Grum and Dy have sufficient reserves to concurrently maintain millfeed beyond the turn of the century.
The Faro mine, 200 km northeast of Whitehorse, first began producing ore in 1969 when it was owned by Cyprus Anvil Mining Co. That company ran into financial trouble in the early 1980s. By 1982, Faro had been shut down. Then, seemingly out of nowhere, came Clifford Frame, formerly chief operating officer at Denison Mines, who had incorporated Curragh Resources Inc. in 1985. Armed with a new operating plan (for details, please see the accompanying interview with Frame), Curragh in 1986 whipped the dormant mine and mill into operating shape in the midst of an especially brutal Yukon winter. At the time of the Curragh acquisition, geological reserves in the district were estimated (by D.S. Jennings, a former Cyprus Anvil chief geologist, and Gregg Jilson, now Curragh’s Vice-President, Exploration) to total 96 million tonnes of 8.9% combined Pb/Zn with 51 g per tonne silver.
From 1986 up until the past couple of years, the Faro orebody, a strataform, stratabound, pyritic, massive sulphide deposit, fed approximately 4 million tonnes of ore per year to the concentrator. Last year, production came from Vangorda and the waning Faro open pit and an associated underground mine with its portal near the pit floor.
In that strike-shortened production year, the open pits produced 3,232,118 tonnes, split almost equally between the Faro and Vangorda open pits, and another 605,903 tonnes from the Faro underground mine. Total millfeed, from mine sources and stockpiles, was 4,126,587 tonnes. (In spite of the 10-week strike, non-union staff were able to maintain production at 50% of normal levels.)
The company as a whole in 1991 produced 487,000 tonnes of Pb/Zn concentrates (the figure also includes Sa Dena Hes production), down considerably from the 548,000 tonnes of the previous year. In the Faro pit, the last slice has been taken off the pit wall. The underground portion is also in the midst of closing. From now through to the beginning of next year, Curragh will rely on the Vangorda deposit and, later, the Grum to supply ore.
Vangorda
Mining began in the Vangorda pit in August, 1990. It should run out of ore by the end of the first quarter of 1993. Pb/Zn mineralization occurs in massive pyritic baritic sulphides (high grade), semi-massive pyritic sulphides (low grade), disseminated sulphides in quartzite (low grade) and disseminated sulphides in carbonaceous quartzites (generally low grade). In general, high grade runs up to a combined 8% Pb/Zn, while low grade is less than 5%.
Vangorda, rather than Grum, was chosen to immediately succeed Faro because the Grum deposit required less waste removal. Logistical problems did crop up because of the 16-km haul to the Faro concentrator. In fact, several methods of ore transport were considered, including a conveyor system. Mechanical-drive, 85-tonne Caterpillar 777s were chosen because these trucks are faster than electric drives on the long, level haul, said John Hogg, Faro General Manager. Larger trucks, such as the 240-tonne variety, were also ruled out. They would have chewed up the haul road, which was constructed from the non-acid-generating, waste-rock phyllites mined from Grum and Faro. The material tends to break down under heavy use and in wet conditions. This is especially true come springtime; continual maintenance prevents a severely rutted roadway from slowing traffic.
The ore is stored at a transfer point about 1 1/2 km from the Vangorda pit. It is separated into high-grade, low-grade and refractory ore and each is hauled separately to the plant crusher station. There, it is blended with Faro pit and underground ore to provide a consistent feed to the plant. Once Faro is depleted, the blending will occur at a new transfer point near Grum, 13 km. from the crusher.
Initial haul road construction began from the Faro end as far back as 1988. The use of phyllites was deliberate — acid mine drainage was to be kept to a minimum. Under one portion of the roadway, Faro incorporated a unique rock drain of calc silicate breccia. This obviated the need for a more expensive and elaborate flume or pipe to withstand the pressures of the thousands of tonnes of rock above it.
In Vangorda, 6-m benches are mined. Two Marion M4s and a Bucyrus Erie 45R drill off either 6.5×6.5-m or 8×8-m patterns. The rotary blast hole machines drill 10 5/8-inch-diameter holes. The engineering and geology departments establish blast patterns for each cut. The depth of holes have been altered recently, because the blasts were leaving partiallly broken toes that were causing difficulties for the shovels.
The shovels include three, 15-cu.-yd. P&H 2100 BLs and a recently acquired mammoth, the 34-cu.-yd. P&H 2800 XPA. This shovel can fill trucks in three passes under optimum digging conditions. In-pit haul trucks include 17, 170-ton Euclid R170s, four, 170-ton Unitrig Mark 36s and three, 120-ton Wabco Haulpacs.
In Vangorda, rock composition is poor. Because of this, one section of the slope has been re-designed to 37[degrees] from the original 45[degrees]. It has been necessary to divert a creek close to the top of the pit wall to eliminate potential wall problems.
Grum
The Grum open pit, 13 km from the Faro open pit and concentrator, is to be mined in three stages, all of it incorporating 12-m benches in waste and 6-m benches in ore. So far only 25 m of overburden have been removed in the first stage, the south section of the orebody. Another two to four benches will open up the ore zone, known as Gnome’s Cap. It is closest to surface, about 30 to 40 m below overburden and waste rock. The more northerly portion of the deposit is under up to 150 m of waste rock, mostly calcareous phyllite and unconsolidated overburden. (At press-time, Curragh had been advanced a $5-million loan from the Yukon government to begin stripping the remaining Grum overburden and waste rock. As part of the agreement, Curragh will contribute $5 million. The stripping had been delayed pending government involvement.)
Grum is not a single, homogeneous orebody, but rather several smaller, concentrated orebodies. It has a mining reserve (within pit, dilution included and a 4.0% Pb/Zn cutoff grade) of 25.1 million tonnes of 7.97% combined Pb/Zn, as well as 50 g silver and 0.81 g per tonne gold. Another 166.4 million tonnes will be stripped as waste, leaving a strip ratio of 6.6:1. In-situ reserves total 32.1 million tonnes grading a combined 8.98% Pb/Zn, and 57 g Ag and 0.95 g Au per tonne. The final pit depth, after eight years of mining, will be roughly 220 m.
Beyond the projected pit boundaries and at depth lie several other ore horizons that may be mineable by underground methods once the pit has reached its ultimate depth and lateral extent.
There are only two types of ore in Grum — massive sulphide (38%) and disseminated sulphides in quartzite (62%). A total of 573 drill holes probed the Grum deposit and the former owners, Kerr Addison, drove an underground ramp to confirm surface drilling and take bulk samples. The underground workings are 150-180 m below surface. Grum was first tested as a geophysical anomaly in 1973 by the A.E.X. Syndicate under the direction of Dr. Aaro Aho.
To satisfy concerns about acid mine drainage, Curragh plans to have two waste dumps. The southwest dump, with a capacity of 7 to 8 million tonnes, will be composed of material that does not generate acid. The main waste dump, with a 17- to 18-million-tonne capacity, will have a sulphide core. This core will have on all sides a non-acid generating waste rock and it will rest on calcareous phyllite waste rock. Furthermore, it will be stockpiled in three, 29-m lifts. Each of the first two lifts will be capped with a metre of glacial till. The third and final lift will be covered by 3 m of till. All drainage will be collected in a ditch excavated at the base of the waste pile. The water will be treated if necessary.
Dy
The Dy deposit is 3 km southeast of the Vangorda pit, which would make the haul to Faro roughly 19 km long. The shallowest ore horizon is 450 m below surface, which obviously requires a shaft and all the trimmings for an underground operation.
So far, 77 drill holes have probed the orebody to an average depth of 1,000 m. The deposit dips to the southwest and is structurally complex and therefore, not yet fully defined. It is open in all directions. According to Gregg Jilson, Vice-president, Exploration, it displays “reasonable continuity,” but is complex in grade and widths. The estimated tonnage is 21 million tonnes, grading a combined 12.5% Pb/Zn (9% cutoff). Jilson is waiting for approval on a decline to a depth of 1,800 m. Curragh would then drift along the northern high-grade zone, which is the A zone barite. The B zone is richer in zinc. Production would occur about 1 1/2 years after a go decision.
Faro
Faro Pb/Zn mineralization was contained in six different rock types. The high grade was found in massive pyritic/baritic sulphides, masssive pyritic sulphides and massive pyrrhotitic sulphides; the low grade in semi-massive pyritic sulphides, disseminated sulphides in quartzite and disseminated sulphides in carbonaceous quartzite.
During our visit, rather tight sequencing was required to drill, blast and haul broken ore at the bottom of the Faro pit. There was little room for maneuvring.
Of the planned mining sequence overall at Faro and the Vangorda Plateau, Colin Benner, President, Base Metals, said it will proceed as follows: Vangorda is to run out of ore in the first quarter, 1993; Grum, at 3.5 million tonnes per year, is to take over as main ore contributor early in that year; the Dy underground mine will be developed in 1993 and operational in 1994, contributing by then 1.2 million tonnes per annum. That will give Curragh time to further define and delineate Swim.
All this will fall on the shoulders of John Hogg, who first came on the scene in June, 1991. His first three months were spent acquainting himself with the operation. “We had just come off a 10-week strike, metal prices were low and I could see we had some cost-cutting to do,” he recalls.
The workforce was reduced by 20%. Column flotation cells were more closely monitored so that adjustments to the spargers could be made before recoveries tailed off too dramatically. As well, changes were made to the regrind system.
Faro is also working with Yukon Electric to develop a demand-side management program, which simply means that Faro is creating a computerized system, incorporating a programmable logic controller, that shuts down certain portions of the mill when peak demand occurs. Yukon Electric is a small utility by most provincial standards. When Faro’s power draw rises to peak load, Yukon Electric must fire up an extra generator to provide the power. It would save both the consumer and the utility money to prevent peak loads.
In the mines, maintenance standards on the haul fleet were upgraded to boost availability. It is now 72%-73%, whereas in 1990 it was 68%. The key thing, said Hogg, is to develop a mine plan and stick to it. “Because of the pit size and complexity it is very important. If you go off the plan you can run into miserable problems, like having three trucks lined up awaiting a single loader.”
Daily tonnages have improved recently. For example, the fourth quarter daily tonnage in 1991 was 46,000. In the current year-to-date (to February) it was 57,000 and for the month of March (daily totals), it was 64,000. That includes waste and ore from the open pits, but not underground. “We’re making the right moves,” Hogg said.
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