Technology News

No. 460 July 1997

U.S. Department of Health and Human Services

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Prototype Borehole Miner Selectively

 Extracts Gold from Permafrost

Objective

Assess the technical feasibility of mining small and/or deeply buried

frozen placer deposits from the surface through boreholes.

Approach

Placer deposits in permafrost often contain up to 15% cobbles and

boulders. Because of erratic loading on the bit caused by elasticity

and heterogeneity of the ice-and-gravel matrix, these deposits are

extremely difficult to excavate. The approach taken in this research

project was to mine only the ore-bearing portion of the deposit

using a high-pressure water jet to thaw the gravels and a downhole

hydraulic-lift-type pump to bring the ore to the surface as slurry.

Full-scale jetting tests in frozen gravel were completed, and the data

were used to design a jet excavator. The excavator will deliver a

water flow of 890 L/min (235 gal/min) through a 15.7-mm (0.62-in)

diam nozzle at 6,900 kPa (1,000 psi). It can excavate -4 C (25 F)

permafrost at a rate of approximately 9 mt/h (10 st/h) and drive a

0.1-m (1-ft ) horizontal heading a distance of 4.6 m (15 ft) in

2 2

approximately 10 min. The prototype slurry pump was designed to

lift minus 15-cm (minus 6-in) gravels to the surface from a depth of

15 m (50 ft).

How It Works

The prototype borehole placer miner is shown schematically in

figure 1. The mining sequence is as follows. Two borehole placer

miners are placed in boreholes approximately 7.5 m (25 ft) apart.

Each miner cuts a long, horizontal passage approximately 0.1 m (1 2

ft ) in cross section toward the other miner until the two passages 2

meet. The cutting jet of one miner forces slurry toward 

the inlet of the slurry pump on the second miner at high velocities,

which keeps the gold in suspension. The slurry pump then lifts the

slurry to the surface. The jetting and pumping sequence alternates

between the two boreholes until the ore-bearing horizon is mined

out. If a large cobble clogs the inlet, a vacuum sensor alerts the

operator, who can fire a projectile to clear the blockage.

Field Test Results

Initial field tests of the borehole mining concept were conducted at

a mine on Tenderfoot Creek, 43 km (27 miles) northwest of Delta

Junction, AK. The gold-bearing gravel at this site is about 0.3 m (1

ft) thick and lies immediately adjacent to bedrock under

approximately 13 m (43 ft) of overburden.

The first field trial of the borehole placer miner involved three

boreholes and took place in two phases. In the first phase, singleborehole mining, the only force inducing the gold to flow into the

slurry pump was pump suction. In the second phase, one

“production” borehole was excavated until a connection was cut in

the pay zone that linked the production hole with an auxiliary hole

4 m (13 ft) away. Then a cutting jet with a 15.7-mm (0.62-in)

nozzle was lowered into the auxiliary hole and directed toward the

borehole miner in the production hole. This jet was operated at

pressures from 700 to 5,500 kPa (100 to 800 psi) and rotated in the

horizontal plane to flush material into the sump of the production

hole. The jet pump in the production hole was operated for the

duration of the test.

The two-hole arrangement produced three times as much gold as

single-borehole mining, although only about 1.5 m (2 yd ) of

3 3

material was excavated from the gold-bearing zone. Thus the total

amount of gold produced (7 gm [1/4 oz]) was small.

Clogging of the slurry inlet and presence of large (30- to 35-cm [12-

to 18-in] diam) boulders were the main problems hindering

production during field testing. These problems will have to be
Figure 1.—Borehole placer miner excavating gold-bearing permafrost gravels.

overcome before borehole gold mining can become commercially Mention of any company name or product does not constitute

feasible.

Present Status

A second field trial was completed and the results published in the

proceedings of the Water Jet Technology Association conference,

August 1995. Research was discontinued when the U.S. Bureau of

Mines closed in 1996. Final reports and a video summarizing the

work are still available from Art Miller and George Savanick at the

following addresses.

Dr. George A. Savanick Mr. Arthur L. Miller

189 Redwood Drive E. 315 Montgomery Ave.

Apple Valley, MN Spokane, WA 99207

(612) 432-7594 (509) 354-8028

endorsement by the National Institute for Occupational Safety and

Health.

To receive additional information about mining issues or other

occupational safety and health problems, call 1-800-35-

NIOSH (1-800-356-4674), or visit the NIOSH Home

Page on the World Wide Web at http://www.cdc.gov/

niosh/homepage.html 

As of October 1996, the safety and health research functions

of the former U.S. Bureau of Mines are located in the National

Institute for Occupational Safety and Health (NIOSH).