How to Get Gold Out of Fort Knox

In Alaska the words “Fort Knox” mean gold, and a lot of it. But people aren’t referring to the federal repository. Fort Knox is by far the state’s largest gold producer and its only primary lode gold mine. Operator Fairbanks Gold Mining, 100%-owned by Kinross, is expecting to produce 410,000 oz of the yellow metal this year.

Located about 40 km northeast of Fairbanks in central Alaska, the operation consists of the Fort Knox and True North pits plus the Fort Knox mill, which treats ore from both mines.

This is a large-tonnage, low-grade producer, and both the general manager Rick Dye and mill superintendent John Hollow agree that the low grade, about 1.0 g/t Au, is a huge challenge.

“It’s very low grade,” says Dye, “and you have to work very hard for everything you get here.”

“Typically, grades this low would be a heap leaching operation,” added Hollow. “Sometimes I think we’re running a 40,000-ton-per-day sampling plant.”

Without a dedicated workforce, there would not be 410,000 oz this year. Dye remarked: “The best thing about this operation is the attitude of the people. They are willing to change and make improvements even in such a low-grade environment.”

The history of gold mining near Fairbanks stretches back to 1902 when placer deposits were discovered in Fish Creek, downstream from the Fort Knox deposit. In 1913 lode-mining claims were staked on what is now Fort Knox. A three-stamp mill was built, but there is little evidence that the claims were actually mined.

The area was largely ignored until 1980 when two local prospectors, Joe Taylor and George Johnson, worked the placer deposits. They were confident that gold mineralization was widespread and staked additional claims. In 1986 the Taylor and Johnson claims were leased, and numerous small companies, including Fairbanks Gold, began exploring the area.

Amax Gold bought the Fort Knox project in 1992 and made Fairbanks Gold a wholly-owned subsidiary and operator. The success of the drilling program led Amax to complete environmental and feasibility studies. In 1995, construction of a mill and pre-stripping of the pit began. The first gold from the Fort Knox deposit was poured in December 1996. Kinross acquired the project in its 1998 merger with Amax.

The Hindenburg prospect, central to the True North mine, was acquired by Amax in 1990. Ryan Lode Mines, a wholly-owned subsidiary of La Teko Resources, entered into a joint venture with Amax in 1993. La Teko liked what it found and acquired a 100% interest in the property that year. Newmont Exploration became interested in the True North property in 1995, and it formed a joint venture, with Newmont holding 65% and La Teko the other 35%. Kinross acquired La Teko’s interest in True North through a merger in 1999. The same year it bought out Newmont’s interest. The first gold from the True North mine was poured in 2001.

The geological source of this wealth lies in the Fairbanks Mining District, a southwest-northeast-trending belt of lode and placer gold deposits. The dominant rock unit is the Fairbanks Schist, and it is interlayered with the metamorphic Cleary Sequence. Many northeast-trending faults and shear zones show a prevailing strike/slip movement and are important to the localization of gold mineralization.

In the Fort Knox deposit, gold occurs in and along the margins of pegmatities, quartz veins and veinlets, quartz-filled shears, and fractures within a granitic intrusive. During magmatic doming, fractures were created that became the conduits for mineralizing fluids within the stockwork and shear zones. The stockwork veins strike mostly east-west and dip randomly. Vein density decreases with depth. Shear zones generally strike northwest-southeast and dip moderately to the southwest.

True North mineralization is hosted in carbonaceous sediments and complex structures, and it is associated with small quartz veins. The deposit has a northeast elongated orientation parallel to the Eldorado Fault. It consists of a series of gentle to complex folds (especially between the fault and a subparallel discontinuity), shear zones, breccias, and occasional low-angle faults. Northwest of the discontinuity the mineralized zones dip gently to the northwest, and south of it the mineralization dips to the southeast. Individual mineralized zones between the fault and discontinuity are highly variable.

Fort Knox mine

The Fort Knox mine and mill lie about 40 km by road northeast of Fairbanks. This is a conventional truck-and-shovel operation that runs year-round, seven days a week. The 2003 schedule calls for mining between 94,000 and 130,000 tonnes/day, about 30% of which will be ore.

Blastholes are bored using four Ingersoll-Rand DM45, 16.5-cm-diameter drills. The holes are sampled and assayed for grade control purposes. Anfo is the predominant blasting agent, but emulsion is used in the few wet holes that are encountered.

Mining at Fort Knox began in the middle of the deposit at the top of an existing hill, and proceeded in the classical method of widening the pit through three phases. The fourth phase opened the pit to the west, and the fifth phase pushed out on the east side. The mine plan is designed with 9-m-wide benches (both single and double), variable slope angles, and a 30.5-m-wide haul road with a maximum 8% grade. The haul road is reduced to one-way traffic, 18.3-m-wide and a 10% grade in the lowest benches. The plan includes 14 dewatering wells (with four more to be drilled this year) as mining is now below the water table.

Stripping for the sixth phase, to expand the pit southward, will begin in 2004 on the 2200 bench. Engineers estimate that 50 million tonnes of waste rock will be removed before the middle of 2006 when commercial mining rates are reached.

Depending on the grade, ore is sent to the primary crusher, low-grade stockpiles or waste rock dumps. Fort Knox is in the enviable position of not having acid-generating waste. The production fleet consists of 10 Caterpillar 785 trucks (150 short tons), four Unit Rig MT-3700 trucks (200 short tons), three shovels (Hitachi EX 3600, Hitachi EX 3500 and Caterpillar 5230), and one Caterpillar 994 loader.

Other equipment and facilities at Fort Knox include three Caterpillar D10 dozers, two Caterpillar 16G graders, truck shop, wash bay, fuelling facilities, and a site office. The fleet will be expanded by six trucks and one more loader when stripping begins on the final phase of pit development next year.

True North mine

Mining of the True North deposit began in 2001. The mine yields between 18,100 and 36,600 tonnes of ore and rock per day. Again, the blastholes are sampled and assayed for grade control. The trucks are smaller (85 short tons) than at the Fort Knox mine, but mine development is very much the conventional truck-and-shovel method. The mine plan calls for 6-m-high benches, 45 slope angles, and 24-m-wide haul roads with a maximum 8% grade. The mining licence allows the ultimate pit depth to reach 6 m above the water table.

Depending on the grade of material, ore is stockpiled at the mine or placed in waste rock dumps. Stockpiled ore is trucked about 20 km to the primary crusher at the mill. Waste rock is used to build access roads or placed in dumps both adjacent to and in the mine. This is a side-hill pit with areas of permafrost.

Major equipment includes two Ingersoll-Rand blasthole drills, five Caterpillar 777 trucks, one Caterpillar 5130 shovel, two Caterpillar 992 loaders, one Caterpillar 988 loader, two Caterpillar D10 dozers, three Caterpillar 16 graders, and seven Kenworth OT tractor/trailers. There are also a truck shop, wash bay, fuelling facilities, and site office at the mine.

Neither the True North nor Fort Knox mines are automated. The fleets are relatively small, the sightlines good, and not even a computerized truck dispatch system is used.

Mining at the True North pit will be completed in the first quarter of 2005. Thereafter, the mill will rely entirely on ore from the Fort Knox pit until it is exhausted in 2010. Then the low-grade stockpiles will be treated.

Fort Knox mill

Gold recovery begins when trucks deliver ore to the 3,800-t/hour Nordberg gyratory mill. This year’s budget calls for processing 37,650 tonnes of ore daily, 78% from the Fort Knox mine and 22% from True North. Mill feed is crushed to -20 cm and conveyed 800 m to the mill building. Head grades average about 0.915 g/t Au for Fort Knox ore and 1.65 g/t Au for True North ore.

The blended, crushed ore is conveyed to a 10.35-m-diam by 4.6-m-long Svedala SAG mill. It operates in closed circuit with two 6.1-m-diam by 9.1-m-long Svedala ball mills, each of which has a cluster of 10 Warman 660-mm cyclones. Ore is ground to 68-70% passing 100 mesh.

A portion of the cyclone underflow is passed through a gravity circuit. True North gold is very fine grained, so the gravity circuit is not a factor in recovering it. However, when the coarser Fort Knox material is treated alone, the gravity circuit recovers about 25% of the gold. The circuit consists of two 1,066-mm-diam Knelson concentrators, one running at 135 t/day and the second at 275 t/day. The greater throughput of the second unit is due to the manufacturer’s design modifications. The first unit will be modified in a similar manner, bringing the total circuit capacity up to approximately 590 t/day.

Slurry from the grinding circuit is thickened in an Eimco 33.5-m-diam high-rate unit and then enters a train of seven 17-m-diam by 15.5-m-high leach tanks. It then passes into a series of six 17-m-diam by 13.5-m-high tanks loaded with 10 g/L activated carbon. The advance rate is about 10.9 tonnes/day. After 24 hours’ retention in both trains, the gold load is 2,740 g/t, and the loaded carbon is screened out.

The mill is laid out as a cyanidation/CIP plant, but it operates more like a CIL plant, says Hollow. About 80% of the gold is dissolved in the leach circuit, and the remaining 20% in the CIP tanks. When ore from the True North deposit reached the mill two years ago, adjustments were made to the reagents, notably the addition of lead nitrate because of the ore’s stibnite content. Use of cyanide and lime also increased slightly.

The loaded carbon is stripped using a Zadra process in 9-t batches at 150C and 60 psi. Gold is recovered from the pregnant solution by electrowinning and melted into dor bars containing about 80% Au and 15% Ag. The gold recovery rate is 90%. The stripped carbon is regenerated in an electric Lockheed-Haggarty furnace.

A US$6.5-million tailings wash thickener was added to the mill in October 2002. This is a 27.5-m-diam Outokumpu high-compression unit that produces 65% solids underflow. The importance of this facility is that it recovers cyanide ahead of tailings impoundment. The cyanide (NaCN) is, of course, recycled to the mill. Ideally, all the cyanide will be recovered from the thickener, and the existing Inco SO2/air detox plant will no longer be needed.

The gravity-flow, zero-discharge tailings impoundment has more than enough capacity for planned disposal. Tails are deposited subaerially from May through September and subaqueously in the winter months. The mill uses 7,500-11,500 L/min make-up water. All water from the tailings pond (which includes pit water) is returned to the mill and additional make-up water comes from surface run-off ponds.

The Fort Knox mill was state-of-the-art, computerized and automated when it came on-stream in 1996, and it has been upgraded regularly since then. Process control relies on DCS and PLC using reagent concentration set points and SAG mill throughput rates to keep it running at 37,000 t/day. The plant had an uptime record of 96% last year.

Continuous improvement program

Kinross has a corporately-mandated continuous improvement program (CIP) aimed at just that: every day make something work more productively or at lower cost. The program has had positive results across the company.

At Fairbanks Gold there are always several CIP programs on the go simultaneously. Teams are assembled from among the production personnel directly involved, and they are charged with finding and implementing solutions to problems they have identified. “It’s very much a bottom-up approach,” manager Dye told CMJ, “because they are the people who have to live with the changes.”

In the Fort Knox mill, a team is working to boost SAG mill throughput. “We’re looking at pretty much the same things the whole world is,” said superintendent Hollow. “We are in the process of looking at reduced feed size, opening the grates, liner design changes, and mechanical constraints. We might even run the SAG mill in open circuit rather than closed with the ball mills.”

Whatever the eventual solution, having the mill employees directly involved means that they are willing to implement change, and that they have a stake in its success.

Other CI projects currently underway involve warehouse direct charge management, traffic flow around the primary crusher, and improving grade control.

What the future may hold

Fairbanks Gold is making plans to close its mines and mill within a decade. Reclamation has been carried out concurrently with mining, but, that being said, Dye admitted there is not much reclamation activity now. “Early on Fairbanks Gold did a lot to clear up the activity from the old dredging and placer operations. On closure we will recontour the waste dumps. They may become a granite quarry because of their high quality. The buildings may get a second life either for use by the military or as a school. The tailings area will become a wetland or grassland,” he said.

Those closure plans may be premature, if the Kinross exploration department has anything to say about it. Last year they replaced mined reserves at Fort Knox, and that type of effort will continue.

This year’s US$2.5-million exploration budget will also fund the search for satellite deposits that could feed the Fort Knox mill after 2010. Just north of the True North pit lies the Zeppelin zone, a potential additional ore source; and there are anomalies to the southwest that deserve looking into. West and adjacent to the Fort Knox ground is the NOAA project. The surface rights to this area are owned by the National Oceanographic and Atmospheric Agency (hence the name). The geology is the same as the Fort Knox deposit, and drilling is planned this year. The Ryan Lode deposit, 55 km west of Fort Knox, is being reviewed, but there has been no drilling on it for a couple of years.

Perhaps the most promising project is the Gil property, 12 km east of the Fort Knox pit. This is a joint venture of Kinross (80%) and Teryl Resources (20%), based in Richmond, B.C. Exploration has found two zones, the Main Gil and North Gil, both of which are open to the east. This year’s drill program will concentrate on upgrading the known mineralization and expanding it to the north and northeast. The Skarn Grid zone at the south end of the Gil project is very promising as well and 100% Kinross-owned.

Kinross knows that there is more to Alaska than cold and snow. The company is confident there will be a profitable gold mine there, as well, for many years to come.