ODDS ‘N’SODS — Payoff at Sheep Trail

Exploring for gold is a risky business and sometimes, as when Homestake Mining sank $700,000 into a single 10,200-ft.-long exploration hole a few years ago, the gamble pays off. Thanks to a persistent, decades-long exploration program, the company hit pay dirt at 6,350 ft. below Sheep Tail Gulch, north of Central City, S.D.

There are more than 500 miles of underground workings in the 8,000-ft.-deep Homestake mine. The North Homestake exploration project (or “North Drift,” as it is called) is a little mine-within-a-mine, according to the district geologist.

Of the 47 workers assigned to the project, 26 were hand-picked and given special training. Homestake announced the find and authorized the project in March, 1990, and work started the following month. During that year, Homestake contacted several manufacturers of drilling equipment in order to acquire a drill suitable for the project.

The criteria for the drill were outlined by the regional exploration manager of the Homestake mine in consultation with Tonto Drilling of Salt Lake City, Utah. The drill had to have a capacity of no fewer than 5,000 ft. (N-size wire line) and be capable of drilling throughout 360 without disrupting the steady movement of ore trains hauling from the face.

The JKS Boyles B-20 was chosen. The standard model B-20 is powered by a 100-hp electric motor; however, because of the increased depth required at the Homestake mine, it was necessary to bump this up to 125 hp and run slightly larger hydraulic pumps. The drill was commissioned in June, 1991, and the first hole was 50 up, using “H” wire line equipment. Drilling up-hole at any time requires considerable skill and concentration, and using “H”-size tools is even more of a challenge.

Drilling this first hole began without a hitch. But 500 ft. into the hole, water temperature increased to more than 118 F and pressures rose to 500 lb. per square inch. The hot water, which may have come from isolated fracture zones in the rock, flowed out at 300 gallons per minute and stood a foot deep on the floor before being diverted into a pipe system and pumped out of the mine. The pooled water raised the temperature in the drift to nearly 110 F and the humidity to 100%. The sultry air quickly wore down the workers, who had to take breaks every 20 to 30 minutes. Special ventilation and cooling systems were installed to make conditions more tolerable.

The drill hole was temporarily halted at this point so that crews could grout the aquifers with cement to seal off the water flow. A consulting firm was brought in and several hundred gallons of grout were pumped into the bore hole. This method proved unsuccessful, however, and it was decided to, instead, try to drill through the fracture zone.

Drilling was again started and continued to 800 ft. of “H” wire line. At this point, the hole was reduced to “N,” leaving the “H” rods in the hole as a casing through the fracture zone. Drilling continued for the next 79 shifts until the hole reached 4,975 ft. As depth of drilling increases, the daily footage drops off proportionally; coring rates over the depth of this hole were as follows:

1,000 to 2,000-ft.: 100 ft. per shift

2,000 to 3,000-ft.: 71 ft. per shift

3,000 to 4,000-ft.: 56 ft. per shift

4,000 to 4,975-ft.: 41 ft. per shift

The overall average for the hole was 62.9 ft. per shift.

The rock formations cored were Proterozoic and consisted of quartzite, pelite, iron formation and basalt. Since 1991, when the drill was commissioned, JKS Boyles has been in constant communication with the mine’s administrators. The drill has only required servicing on one occasion, and that was a routine maintenance check-up. After a few minor adjustments, the drill was ready for another 30,000 ft. of drilling.

— Gary Pipher is vice-president of U.S. sales for JKS Boyles International of Orillia, Ont. The above is from a case study he presented at this year’s convention of the Canadian Diamond Drilling Association.