Northern alliance; Cominco’s Red Dog mine, in northwestern Alaska,

The Red Dog project in northwestern Alaska is something of an anomaly. It’s more than just one of the richest base metal mines in the world — it’s a socio-economic alliance between the native people of a region and a mining company. Officially opened in August of this year, the Red Dog mine sits in a package of land encompassing about 840,000 hectares (2.1 million acres) given to natives under the Alaska Native Claims Settlement Act of 1971. The NANA Regional Corp. was formed to hold the land. Under the deal with NANA to develop Red Dog, Cominco Alaska, a unit of Cominco Ltd., paid NANA US$1.5 million on signing, plus advanced royalties of US$1 million per year. NANA is entitled to a 4.5% net smelter return, increasing to 25% after Cominco recovers its capital cost and subsequently rising by 5% every five years to a maximum of 50%. Walter Sampson, vice-president of lands for NANA, recalls that after the U.S. Bureau of Mines announced the potential value of the Red Dog deposit in 1975, a majority of NANA shareholders were opposed to the development of a mine. He said the natives viewed the land as urban dwellers might view their local Safeway store. The land was the source of their food, as it had been for centuries. By the late 1970s, this view had altered somewhat. The outside world seemed to be catching up with the people of the area. There were bills to pay (such as telephone and power) that had not existed before, and, perhaps, the want of a snowmobile. As a result, in 1982 NANA signed the deal to develop the deposit with Cominco Alaska. In order to develop the mine, the joint venture had to secure financing from the state of Alaska for the transportation system, including port facilities and an 84-km (52-mile) road to the mine site. Cominco also had to obtain Congressional approval to build the roadway through the Cape Krusensern National Monument, which encompasses about 32 km (or 20 miles) of the route.

In 1985, both Congressional approval and funding were obtained. Alaska agreed to provide the funds for the transportation system in return for yearly tolls of US$12 million and a guaranteed return of 6.5% on the capital invested. Road construction began in 198198nd involved placing a minimum of 1.5 metres of crushed rock over a geotextile mat, providing stability over the permafrost. Since no gravel could be found, all the rock had to be drilled, blasted, and crushed from 15 borrow pits along the road route. About half of these pits are still active, providing a rock source for road maintenance. The total cost of the road was in the order of US$50 million, or about US$1.0 million per mile, including nine bridge crossings and a cumulative total of about six miles (9.6 km) of culverts. The total cost for the transport system, including the port facilities and the road, was about US$150 million.

The seaport facility consists of a shallow water dock, a deep water dock, concentrate storage building, oil storage tanks and accommodation quarters. Fuel oil is stored in four 2.3 million-gallon, double-walled tanks and is trucked by tanker to the mine site. All truck transport is handled under contract to Arrow Transportation International of Seattle, Washington. Concentrate from the mine site is hauled by 75-ton Arrow trucks to the concentrate storage building at the port. With a shipping season of 100 days, the concentrate storage building must have enough capacity to hold more than nine months of production. The storage building is 64 metres wide, 427.5 metres long and 31 metres high, with enough capacity to hold 409,000 tonnes of zinc concentrate, 36,350 tonnes of bulk ISF (international smelter feed) concentrate and 81,800 tonnes of lead concentrate.

Concentrate is transferred from the trucks to storage by a dump, bin, feeder and conveyer system. The trucks are side-dumped from a platform tipped at 90deg. Because of the moisture content of the concentrate, vibrators have been installed on the bin as well as the truck boxes.

During ship-loading operations, concentrate is reclaimed by a Caterpillar 988B loader, which transfers the material to a 36-inch conveyer running the length of the storage building. The conveyer moves the concentrate the 3,700 ft. (1,110 metres) to the shore area and a 3,000-ton storage bin. The bin discharges on to a 48-inch conveyer running to a shiploader on the deep water dock. The shiploader has a capacity of 3,000 tons per hour and loads 6,000-ton barges from either side of the dock, which allows for varying wind and wave conditions. Barging the material to ocean-going vessels is contracted to Foss Maritime of Seattle. Once the concentrate has been barged to the anchored ship, the barge is unloaded utilizing a 988B Caterpillar loader, which transfers the concentrate on to a vertical bucket conveyer up to the ship’s hold. Cominco expects to load about 15 ships per year ranging in size from 13,600 tonnes (15,000 tons) to 63,600 tonnes (70,000 tons).

Eighty-four kilometres (52 miles) to the east, lies the Red Dog deposit itself, the heart of the multi-million-dollar project. It is situated in the De Long Mountains, which are characterized by eight stacked and folded thrust blocks containing sedimentary and igneous rocks ranging in age from Devonian to Cretaceous. The deposit itself occurs in the second-lowest thrust block and is hosted by black siliceous shale and chert of the Mississippian to Pennsylvanian Kuna Formation. The structural complexity of the region is reflected in the deposit, which could be described as a series of interstacked lenses composed of differing proportions of sulphides, quartz and barite.

The deposit is a stratabound accumulation of silica rock, barite and sulphides. Major sulphides in decreasing order of abundance are sphalerite, pyrite, marcasite and galena. Dominant gangue minerals are quartz, barite and minor shale. Silver is associated with both the zinc and lead sulphides but tends to occur on the lattice structure of the galena. Most of the silver, therefore, reports to the lead concentrate. In general, the sulphides are fine-grained although coarse-grained sphalerite does occur in feeder veins at the base and on the periphery of the deposit. Jim Kulas, chief geologist, notes that the deposit is not high in iron content with less than 6% overall.

Mining takes place in the Main zone, which is estimated to contain a mineral resource of 85 million tons (77 million tonnes) grading 17.1% zinc, 5.0% lead and 2.4 oz. silver per ton (82.3 grams silver per tonne). The Red Dog Main zone is a flat-lying, elongated lens of mineralization with a maximum thickness of 500 ft. (150 metres), a length of 4,400 ft. (1,320 metres) and a width varying in thickness from 200 ft. (60 metres) in the north to 1,400 ft. (420 metres) in the central and southern portions. The stripping ratio of the deposit is relatively low at 0.9:1, a factor helped by previous erosion from Red Dog Creek which cuts through the middle of the deposit. The deposit is, for the most part, overlain by a limited amount of overburden except for two ridge tops that cover the deposit with up to 200 ft. (60 metres) of barren shales and chert. The Main zone is also capped by a mantle of weathered ore ranging in thickness from tens of feet to more than 100 ft. (more than 30 metres) in places. The weathered ore has undergone both physical and chemical weathering. The physical weathering is the result of countless freeze-thaw cycles.

Chemically, the sulphides have been oxidized to sulphates, a process aided by naturally acidic ground water. The oxidized zinc sulphides are very soluble, resulting in highly elevated levels of naturally occurring zinc in Red Dog Creek. As a result of the mobile zinc, the weathered material tends to be relatively rich in lead and silver. Ore is categorized as either direct feed, low grade, weathered, or weathered to the extent that it cannot be milled. The extremely weathered ore-grade material cannot be concentrated using sulphide flotation and is therefore stockpiled in the hope that an economic means of recovering the metals will be found.

Waste material is categorized in three main types: competent mineralized, incompetent unmineralized and unmineralized waste. Incompetent waste, generally ice-rich shales, are dumped in a separate site from the main dump, although both dumps sit in the drainage basin of the tailings pond. Unmineralized waste is stockpiled in a separate dump for use as construction material. The mining operation involves drilling and blasting 25-ft. (7.5-metre) benches. Holes are drilled with a 6–t. (1.8-metre) sub, are 8.5 inches (21.6 cm) in diameter and are lined. Drilling operations use two D40K Drilltech drills with 8.5-inch (21.6-cm) bits. Holes are drilled to about 31 ft. (9.3 metres). Shelly Stephenson, mine engineer, says the pattern size varies from 14 x 17 ft. (4.2 x 5.1 metres) to 15×18 ft. (4.5 x 5.4 metres) to 15 x 20 ft. (4.5 x 6 metres), depending on rock type. “If the rock is high in barite, we need a lot tighter pattern,” she says. The geology department has a well-established grade control. All blastholes are sampled in order to make a determination of the material type and, therefore, where it should go. Because of the weathering, an assay determination has been added that is not exactly standard. The cuttings are assayed for non-sulphide lead and zinc, in addition to total lead and total zinc.

The mining department uses Mintec’s MED system mine-planning software package plus a sub-module for compiling blast hole information. Although the mining operation has been virtually trouble-free, Kulas says there has been some trouble with wet holes. He notes that a hole can appear dry when drilled and loaded but, because of the permafrost, can soak later. For this reason, the company now lines all blast holes with 8-inch (20-cm) plastic liners. Another change since operations commenced is the addition of aluminum to the ammonium nitrate fuel oil (ANFO).

Stephenson notes that the mining fleet and support equipment are almost all Caterpillar, with two D9N dozers, two 992C 13-cubic-yard loaders and four 777B 85-ton haul trucks. As the pit expands, it will be necessary to divert Red Dog Creek which runs through the deposit. Steve Hodgson, manager of mining, says the diversion of the creek will take place somewhere between the fifth and eighth year of the mine’s life. Although no firm decision has been made on the engineering of the creek diversion, one option is to drive a tunnel through a hillside up-stream from the pit. The tunnel would have to be about 2,000 ft. (600 metres) long while its height and width are open to calculation. Hodgson says it would have to be at least 15 x 15 ft. (4.5 x 4.5 metres) and, he notes, “there is no room for error.” If the tunnel cannot handle peak flow periods of the creek, the pit would be in jeopardy. The ultimate plan will have the creek put back to its original path at the end of the mine life creating a lake in the pit. Because the mine life is projected at more than 50 years, Kulas doesn’t expect to see the lake.

Pit walls are designed on a 45deg slope except for the ice-rich incompetent shales, which are designed at a 23deg slope. Hodgson says they do not clean the walls since the sloughed material acts as an insulator to the permafrost, keeping it solid. Run of mine ore is hauled the short distance (in the order of 1,000 metres) to the primary crusher where the truck either dumps directly into a 100-ton bin or dumps the ore at a small stockpile. From the rock bin, ore is fed to a 250-hp, 48 x 60-inch jaw crusher, reducing the ore to minus eight inches. The crushed material is carried to a 6,000-ton live stockpile by a 36-inch belt conveyor. Material is drawn from the bottom of the pile from four points by 36-inch Hydrastroke feeders on to two 36-inch conveyers, which move the material to the grinding section of the mill complex. The belt conveyers deliver the crushed product directly to the feed chute of two 2,000-hp, variable speed, primary semi-autogenous grinding (sag) mills. The sag mills are 22 ft. in diameter by eight feet long. At present, the mills are fully autogenous, and Don Charlton, mill superintendent, is cautiously optimistic that they will remain so. Charlton also points out that the mills are fully reversible, allowing some control over liner wear and longer liner life.

The mills each have a capacity of 140 tons per hour and operate in open circuit although trommel screens at the discharge do remove oversize for recirculation through the mills. Sag mill product is pumped to a 72-inch-diameter-by-40-ft.-long simplex spiral classifier in closed circuit with a 10.7 ft.-diameter-by-15-ft.-long, 1,000-horse- power secondary ball mill. Charlton says the ball mill was added to protect the tower mills from both oversize from the sag mills as well as surges. Classifier overflow is pumped to two banks of 1010nch cyclones with the cyclone underflow distributed among six 450-hp tower mills.

The tower mill is something of an anomaly in North America, although it is used extensively in Japan where its primary advantage (a large reduction in power consumption for tertiary grinding) is important. Compared with the use of ball mills for fine grinding, the company expects to realize up to a 40% reduction in power consumption for the tertiary grinding and regrind circuits. On-site power is provided by five diesel generators, each capable of producing about 5,000 kW. Normally only three are running at about 85% capacity, although J.W. Bills, power house operator, says they have been able to run on only two with both sag mills operating. “You have to watch the boys out there pretty closely at that rate though,” he says. Normal load is between 9,000 kW and 12,000 kW with each engine gulping down about 260 gallons of fuel per hour. Exhaust heat from the power house operations heats the entire facility, even when outside temperatures drop down to minus 45deg F.

Cominco is not a newcomer to the use of tower mills, having used them at its formerly owned Con mine and at the Polaris mine, both in the Northwest Territories. To optimize the operation of the tower mills, one mill is equipped with variable speed and a particle size analyzer to monitor the optimum grind size for flotation.

Charlton says that once the grind size has been determined, the other mills will be adjusted to match. Other factors affecting particle size include ball size, feed rates and the density of the material. The tower mill product is 100%-passing 325 mesh. Because of the fine grind required, lower feed densities are needed and a thickener is incorporated into the mill flow sheet to ensure that there is no adverse impact on flotation retention time. Tower mill product reports to a 200-ft.-diameter (60 metres) feed thickener where underflow is treated to a controlled density for flotation. Overflow reports to the water treatment plant. The thickener load could be built up to provide 22 hours of float-feed if the crushing-grinding circuit had to be shut down, although normally the thickener is operated with a moderate load. The flotation circuit uses Maxwell tank and column cells.

Lead flotation is divided into two sections with higher-grade, fast-floating particles removed in the initial stage. The first stage rougher concentrate is cleaned in a single 5-ft.-diameter (1.5 metres) column. Second-stage rougher concentrate is reground and cleaned in two stages of conventional tank cells. Additional cells are included to remove lead-zinc middling product from these cleaners for ISF concentrate production. Charlton notes that one change to the circuit has been the use of two of the lead roughhrs to float the elemental sulphur found in the weathered ore. The zinc rougher concentrate is recovered in conventional tank cells and is reground before undergoing four successive stages of cleaning in four 8-ft.-diameter column cells. Lead and zinc rougher regrinding takes place in the same area as the tertiary grinding, with one 450-hp tower devoted to lead rougher and two 450-hp tower mills for the zinc concentrate. Reagent mixing takes place in one module of the mill with mixing and supply tanks for each of the six major reagents including lime, sodium cyanide, xanthate, zinc sulphate, copper sulphate and frother.

The term module relates to the construction method used in erecting the mill. The mill and support buildings were built off-site (in the Philippines) in modular form, barged to the port facility and trucked to the mine, each module weighing up to 2,000 tons (1,800 tonnes). Once in place, the piping and electrical connections were made to connect the individual modules to each other. The dewatering section of the mill includes four Lasta pressure filters designed to decrease the moisture content of produced concentrates to about 8.5%.

Two of the filters are devoted to zinc concentrate, one to lead concentrate, and the other as a “swing” filter, taking either lead or zinc concentrate. The lead filter also acts as the bulk ISF concentrate filter and is oversized to allow for large batch runs of ISF.

Charlton says operating the filters requires a certain amount of “tender loving care” because of all the moving parts (mostly chain linkages). Although no problems have been encountered to date, he stresses that regular preventive maintenance is necessary to ensure that they operate as smoothly as possible.

Normal daily outputs of concentrate averaverabout 310 dry tons (282 tonnes) per day of lead, 1,540 tons (1,400 tonnes) of zinc and 140 tons (127 tonnes) of bulk ISF concentrate at full production. Yearly, the mine should produce in the order of 560,000 tons (509,090 tonnes) of zinc concentrate grading 60% zinc, 2% lead and three ounces of silver per ton (103 grams per tonne). Lead concentrate production is estimated at 120,000 tons (109,090 tonnes) grading 62% lead, 9% zinc and 16.5 oz. silver per ton (566 grams per tonne) while the bulk ISF concentrate production should average about 50,000 tons (45.454 tonnes) grading 15% lead, 35% zinc and 4.5 oz. silver per ton (154 grams of silver per tonne).

Concentrate is stored on site in an A-frame structure capable of holding just under three weeks of production. The 3-week buffer allows for interruptions in the transport process. These interruptions are generally due to either inclement weather or, more important, the twice-yearly caribou migration. Since the mine’s 84-km-long road to the port facility cuts across the migration path of the caribou, the operation must be prepared to shut down trucking while the animals cross the road.

To the casual observer, the scope of the Red Dog mine is difficult to put in perspective because of its relatively small physical size. However, put in percentage terms, when the mine reaches full production, it is expected to provide in the order of 6% of the western world’s yearly zinc supply. In dollar terms, at current metal prices, Red Dog will generate about US$600 million in revenue per year.

SIDEBAR

FOUND: RED DOG

Cominco’s published history of the Red Dog deposit, in northwestern Alaska, gives credit for its discovery to Irving Tailleur, a geologist with the U.S. Geological Survey (USGS). The story has it that a bush pilot and sometime prospector named Bob Baker noticed a rusty alteration zone in what is now known as Red Dog Creek and brought it to the attention of Tailleur.

Tailleur visited the area in 1968 and was reported to have noticed abundant barite, black chert, siliceous sinter and iron oxide staining. A few rock samples taken from the site graded more than 2% lead and 1% zinc while one stream sample graded over 10% lead. His findings and their similarities to other large zinc-lead deposits around the world were documented in a USGS open file report published in 1970. He recommended a systematic sampling program over the area and coined the name “Red Dog Creek” after Baker’s prospecting company which, in turn, was named after his pet dog, a constant companion on his prospecting flights.

The USGS report on the Red Dog Creek area sparked no staking interest, and more than two years later, the area was withdrawn from staking by the federal government with the passage of the Alaska Native Claims Settlement Act of 1971. In 1975, the U.S. Bureau of Mines (USBM) awarded consulting geologists Watts, Griffis, McOuat (WGM) the contract to conduct a mineral resource survey over some 19 million acres (760,000 ha) of proposed national parks, monuments and wildlife refuges in and adjacent to the Brooks Range in northern Alaska. WGM reviewed all the published and known material on the geology of the area, including the report by Tailleur on the Red Dog Creek area. Upon visiting the site in June, 1975, WGM geologists reported that within minutes they were finding boulders of massive sulphides.

The USBM subsequently issued a press release in late August, 1975, triggering a massive staking rush on lands open to staking to the west of Red Dog Creek. The WGM’S report to the USBM noted that the site was a significant lead-zinc-barite prospect with a possible grade of up to 44.6% combined lead-zinc and a size representative of a surface expression of 2,700 metres by 900 metres.

No actual drilling was done on the site until 1980, when Cominco drilled eight holes, five of which were reported to have returned spectacular results. Ultimately, it remains open to debate, and perhaps to the actual definition “discovery,” as to who discovered the Red Dog deposit — Tailleur, Watts Griffis McOuat, or even Cominco Alaska when it drilled the property.