Fort Knox Mine Description

Return To: Fort Knox Mine Home

PROJECT DESCRIPTION

The Fort Knox Mine is an open-pit gold mine, located approximately 26 miles northeast of Fairbanks, Alaska.  The operator proposes to add a heap leach gold recovery facility in the Walter Creek drainage.  The mine was originally permitted in 1994, and currently produces about 330,000 ounces of gold annually.  The mine site is located primarily on lands owned by the State of Alaska and the Mental Health Trust. FGMI employs 400-425 people at the mine and mill, which operate on two shifts, 24 hours per day, 365 days per year.

The valley fill heap leach will be located in the upper end of the Walter Creek drainage immediately upstream from the existing tailings storage facility (TSF). Excluding the haul road and access roads, the heap leach pad with an in-heap storage embankment and base platform will cover approximately 310 acres and will have a total capacity for leaching 165 million tons of ore. The haul road to the pad will cover approximately 40 additional acres. The pad is to be constructed in five stages, which are illustrated on Figure 1.2 of the Heap Leach Facility Project Description. Table 1.0 provides the area and tonnage planned for each stage of development. The clearing of brush and trees, initial earthwork in preparation of liner construction, the site access road, the portion of the in-heap embankment outside the pad limit, and establishment of drainage control will occur over the entire 310 acres at the beginning of the project. Within that area, 130 acres of lined pad will be constructed for the first two stages and loaded with ore. Each of the three additional stages will be constructed as needed for loading ore.

Table 1.0: Area and Tons for Each Stage of Development

 

Lifts

Pad Area
[Square Feet]

Pad
Area
[Acres]

Cumulative
Leach Pad
Area [Acres]

Capacity
By Stage
[Thousand Tons]

Cumulative
Capacity
[Thousand Tons]

Stage 1

1-4

3,034,731

70

70

13,716

13,716

Stage 2

5-7

2,362,850

54

124

21,659

35,375

Stage 3

8-10

2,422,613

56

180

31,567

66,942

Stage 4

11-13

2,392,773

55

234

36,049

102,991

Stage 5

14-19

2,030,238

47

281

58,202

161,193

Ore for the heap leach pad will consist of run-of-mine rock from the Fort Knox Pit as well as lower grade ore that was previously placed in various stockpiles. The Barnes Creek and Fish Creek stockpiles currently contain 29 million tons of lower grade ore that will be loaded on the heap leach pad. The ore is characterized by relatively high permeability that will promote efficient flow in the heap for rapid solution recovery and drainage and for rapid rinsing at closure.

In-heap storage of process solution and storm water will be accomplished uphill of an embankment at the downstream toe of the heap. The rock that will be used to construct the embankment for the in-heap storage pond is sound, durable, and of high strength similar to the quality of the rock that has been used to construct the downstream random fill for the Fort Knox Mine Tailing Storage Facility (TSF).

The valley fill heap leach pad will be constructed with a 12-inch prepared subbase with a coefficient of permeability of less than 1×10-5 cm/sec overlain by a geomembrane liner of 80-mil thick LLDPE or similar material. Above the geomembrane liner, there will be an overliner consisting of 3 feet of crushed rock containing a network of pipes to promote rapid drainage. The overliner will protect the geomembrane liner during ore loading and will help promote leachate collection and maintain a low head on the geomembrane liner.

The facility will utilize in-heap storage to collect pregnant (gold-bearing) solution. In addition to providing the necessary operating capacity for pregnant solution, the in-heap storage pond will be sized to contain: (1) solution from a 24-hour drain down, plus (2) the runoff from the 100-year/24-hour storm event.

Beneath the in-heap storage pond, a Leachate Collection and Recovery System (LCRS) will be constructed between an overlying primary geomembrane liner and an underlying secondary geomembrane liner underlain by a 12-inch-thick layered prepared subbase. The LCRS will consist of a drainage layer that will report to a pump back system to return any solution passing through the primary liner back to the in-heap storage pond.  The LCRS constructed in conjunction with the double liner in the area of the in-heap storage reservoir will provide leak monitoring and collection. A Process Component Monitoring System (PCMS) will be constructed under the main header lines for the solution collection system, outside of the LCRS, providing additional leak detection. An underdrain system consisting of a network of drainage channels containing drain rock will route water from baseflow in Walter Creek, and other seeps and springs under the subbase to the tailing impoundment providing a third level of leak detection.

Barren (non-gold-bearing) solution will be applied on the heap leach using drip emitters, or possibly sprinklers during the warm months. The solution will flow through the run-of-mine ore. Pregnant solution will flow to the in-heap storage reservoir, which will have an operating capacity of about 68 million gallons, or 9.1 million cubic feet. The pregnant (gold-bearing) solution that collects in the wells in the in-heap storage reservoir will be pumped to the Carbon-In-Columns (CIC) plant using vertical pumps located in the solution collection wells. Barren solution and pregnant solution will be pumped in pipes between the pad and the CIC plant. Loaded carbon will be processed in the existing Fort Knox mill facilities.

The heap leach pad will be located immediately upstream of the tailing impoundment. The tailing dam is a zoned, earth-filled structure designed to hold tailing and process water from the mill as well as surface runoff water. The dam is designed and will be maintained to contain the 100-year/24-hour storm event and the average 30-day spring breakup plus provide 3 feet of freeboard. During the time of operation of the heap leach pad, the tailings dam will remain operational and the minimum allowable freeboard will be increased to include the full volume of the in-heap storage pond, for the extremely unlikely event of a catastrophic failure of the heap leach pad embankment dam.  The tailing impoundment is a zero discharge facility. The mill recycles water from the tailing impoundment for reuse in the beneficiation process. The water in the tailing impoundment will also be utilized for the heap leach process.

Return to top