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The Fort Knox gold mine, located in the rolling hills about 25 miles northeast of Fairbanks, spends between $1 million and $3 million a year to combat water that accumulates in the pit. Winter weather makes dealing with the water all the more challenging, requiring heat tape, water pumps and generators that all must keep working in sub-zero weather.
FILE PHOTO/Patricia Liles/For the Journal
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Located in the midst of an Arctic desert in Interior Alaska, the Fort Knox gold mine faces substantial engineering challenges in dealing with water in the ever growing open-pit mine.
Fort Knox's owner and operator, Kinross Gold spends between $1 million to $3 million a year on the mine's dewatering budget, according to Paul Jensen, a mine geologist and hydrologist at Fort Knox. He described the mine's dewatering program on Jan. 14 during a weekly meeting of the Fairbanks chapter of the Alaska Miners Association.
"Fort Knox has had some serious water problems" since Kinross acquired the Fairbanks-area gold mine in 1998, Jensen said, as he showed slides that illustrated accumulated water within the pit, primarily from groundwater seepage.
Hydrology work has identified one major aquifer located around 200 to 300 feet below the existing pit depth. The aquifer produces the greatest amount of water from late August through October, Jensen said.
Other water sources are located in the schist rock surrounding the Fort Knox granite-hosted deposit and the schist is "six times harder to dewater than the granite," according to Jensen's studies.
Additionally, spring runoff contributes to the pit water issues, although lined diversion ditches that funnel water to the tailings impoundment do help with the faster-flowing water, Jensen said.
Current mine plans call for the pit to be dug another 500 feet deeper, which will ratchet up the necessary effort in dealing with groundwater. Additionally, recent exploration drilling has identified more mineralized material within and surrounding the Fort Knox pit.
A majority of exploration spending in Alaska in recent years, including about $1 million in 2004, has been spent on in-pit drilling at Fort Knox to determine whether deeper-buried mineralization would warrant a pit expansion. The company announced plans to complete, starting last November, another 28,000 feet of drilling at the Fort Knox pit.
Kinross is considering an expanded pit beyond current plans that call for a final size to be roughly 265 acres, measuring 5,200 feet by 2,600 feet, by more than 1,000 feet deep.
Mineralized rock remains at depth, although the company must increase its dewatering work and address inversion, or air quality issues in the bottom of the pit, should it be dug deeper, Jensen said.
Increasing the size of the pit to access additional mineralization could add more time to the life of Fort Knox. Existing plans call for the final milling to conclude in 2010, closing the facility that currently employs more than 400 workers.
Initial dewatering efforts in past years included drilling 200-foot drain holes and later installing sump pumps, Jensen said. But water continued to plague miners, and even contributed to a wall failure.
In addition to wet blast holes, crews were digging up and hauling ore with considerable water content. In winter months, "we were sending a lot of ice to the mill," Jensen said.
Since then, the company has drilled and is operating 20 water wells, which produced a maximum of 1,100 gallons of water per minute last summer. Winter water pumping rates have dropped to 550 gallons per minute, he said.
Work completed last summer by contractor Lang Exploration Drilling includes drilling 16,975 feet of drilling and sampling work, 2,745 feet of 8-inch well drilling and development, and 4,510 feet of piezometer construction to monitor groundwater conditions. Lang crews, working on-site 47 days, also cleaned out a caved-in well and changed a pump and motor in a 1,293-foot well, Jensen said.
The full-time Fort Knox hydrology crew consists of Jensen and two mechanics. They maintain 23,400 feet of water line, ranging in size from 3 to 10 inches in diameter, which moves pumped groundwater to the mine's tailings impoundment. Much of the water lines are heat traced and insulated, allowing the water flow to continue during the challenging sub-zero temperatures in winter months.
But those heat trace lines require power, as do water pumps. While some of the lines and water well pumps have been hard-wired to the mill and pull power off of the Interior grid, the cost of installing additional power poles, transformers and cable are cost-prohibitive to wire all of the wells, Jensen said.
Operating up to seven portable generators costs $20,000 a year for each unit, just for fuel, oil and lubricants, Jensen said. Keeping those portable power sources operating in winter is also challenging.
"When a generator goes down, you have about one hour in cold temperatures before the lines start freezing," he said. "We've installed beacons (on generator units) and some are plugged into a backup generator that is heated and ready to roll."
At one point in the midst of winter, he lost more than 2,000 feet of water line when three generators went down. "Winter comes at you from all directions," Jensen said.
To increase the mine's well maintenance ability, Kinross has ordered a new well service rig, which will be able to pull a 24,000- to 25,000-pound well from a depth of 1,400 feet. It's a considerable step up from the existing rig purchased in 1998, Jensen said. That vehicle, which has a 7,000-pound well pulling capacity, "within a year was out-of-date," he said. "We now use it to move pipe around, and use a boom truck to move wells."
