ALASKA’S RED DOG

Contrary to what some may think, the Red Dog mine in northwestern Alaska has not been built on a grand scale — four 85-ton trucks, a couple of loaders and drills. Practically any medium-sized heap leacher in Nevada stacks up impressively against Red Dog’s equipment list. So why all the commotion? In a single word . . . grade. For here is an 85-million-ton deposit with a zinc grade of 17%. Open pit. Low strip ratio. Beautiful!

Former Cominco Alaska President H. M. Giegerich has called it “one of the world’s great mineral deposits . . .” In relation to other zinc deposits, Red Dog is the third-largest ever discovered, smaller than the Brunswick deposit in Bathurst, N.B., and smaller than the original Broken Hill deposit in Australia. However, it is quite possible that, when the area is adequately explored, Red Dog will be Number One.

Red Dog is a shale-hosted zinc-lead-silver deposit in Alaska’s De Long Mountains, western Brooks Range. The De Long Mountains are characterized by eight stacked and folded thrust blocks containing sedimentary and igneous rocks ranging in age from Devonian to Cretaceous. The deposit is in the second-lowest thrust block, hosted by black siliceous shale and chert of the Mississippian to Pennsylvanian Kuna Formation. A distinctive, interbedded, light gray calcarenite and dark gray calcareous shale, known as the Kivalina unit, is the stratigraphic footwall to mineralization.

The Red Dog discovery is credited to the late Robert Baker of Kotzebue, Alaska, and Irving Tailleur of the U.S. Geological Survey (usgs). A bush pilot, Baker spotted a rusty, altered zone while flying over Red Dog Creek. Tailleur visited the site and found abundant barite, black chert, siliceous sinter and iron oxide staining. Rock samples graded 2% lead and 1% zinc. A stream bed sediment sample yielded 10% lead. These findings were included in a usgs open file published in 1970. However, native land claims put a halt to staking and Cominco cooled its heels for a time. Finally, by the early 1980s, Cominco, through the nana Regional Corp. was given permission to proceed with the project.

According to a fact sheet produced by Cominco, Red dog will annually produce zinc concentrates of 560,000 tons (509,000 tonnes) grading 60% zinc; lead concentrates of 120,000 tons (109,000 tonnes) at 62% lead, 9% zinc and 16.5 oz silver per ton (565.9 g per tonne); and bulk concentrates of 50,000 tons (45,450 tonnes) grading 15% lead, 35% zinc and 4.5 oz of silver per ton (154.3 g per tonne). So that’s why the zinc market-players are awaiting the full impact of Red Dog on their world playground.

Red Dog will be mined at 6,000 tons (5,450 tonnes) per day, for an annual production figure of 2.1 million tons (1.9 million tonnes). During mine construction, 2.5 million tons (2.3 million tonnes) of overburden and waste will be stripped from the orebody. Competent, non-mineralized waste will become road-building material when it isn’t hauled off to a waste dump. The strip ratio begins at 1.2:1, then gradually falls to 0.8:1.

Bench heights will be 7.5 m. Pit slopes will vary depending on the material present in the wall. The slope through the competent waste and sulphide packages was set at 45^o (deg). This will flatten to 26^o (deg) through overburden and Upper Kivalina shales. The latter are an ice-rich shale whose carbonate matrix has been leached out and which exhibits little or no strength as it thaws. A 100-ft containment berm will be left in the wall below overburden and Kivalina.

The ore will be drilled off with rotary and in-the-hole drills. Blasthole diameters will vary from eight to nine inches. Hole spacings will also change according to the density of the ore. Apparently, ore density fluctuates with grade. Blended and loaded on site, Amonium nitrate fuel oil (anfo) will be practically the sole blasting agent.

The process flow sheet is relatively simple, but it does include a number of innovations. Built in the Philippines, the $410-million (us) mill is a modular construction designed by Ralph M. Parsons Co. of Pasadena, Calif. There were nine modules, up to 1,800 tons (1,640 tonnes) in weight, moved on-site. Other innovations include tower mills, column cells and pressure filters. After the ore is crushed to minus eight inches, the semi-autogenous grinding (sag), ball and tower mills grind the ore. The tower mills have served to reduce grinding power requirements by 40%. Because of the ore’s fine-grained nature, Cominco opted for column cell flotation rather than conventional, large-diameter, flotation tanks. The final zinc concentrate will be ground to 80% passing 30 microns and the final lead concentrate will be 80% passing 20 microns.

By early December, 1989, the mill was in the midst of commissioning. Ralph Hargrave, the current president of Cominco Alaska, told The Northern Miner Magazine that the commissioning was proceeding smoothly. “We have ore feed going into the mill, we have one of the two sag mills operating, we have a ball mill running, half the column cells and two of the four pressure filters. We’re in the process of adjusting the reagents and testing the systems controls.” The only hitches in commissioning have been “nuisance details,” such as connecting pipe between modules and minor electrical difficulties. “When a mill comes in modules and you have to marry them up, piping can be a problem.” Hargrave was anticipating a first production of concentrates by mid-December. Of course, with shipping restricted to summer months, the first boatload of Red Dog concentrates won’t be leaving harbor until July.

Transportation to a seaport is the key to Red Dog. Port facilities on the Chukchi Sea are 84 km from the mine. The road and port carried a hefty price tag of $150 million. (The Alaska Industrial Development and Export Authority financed road and port construction costs.) It also required delicate negotiations between the company, government and environmental groups. A 1.5-m layer of crushed rock placed over a geotextile mat ensures road stability over the permafrost for the 75-ton (82-tonne) trucks carrying concentrate to port. During the 100-day shipping season, barges will float the concentrate from the shallow-water port to ships anchored several kilometres offshore. Concentrate will be shipped to Cominco’s Trail, B.C., smelter via Burlington Northern Railway’s Vancouver terminal.