Water — an unknown at Carlin’s deep deposits

Nevada geologists refer to it as the unknown factor in the business of mining gold reserves.

In the wake of one thunderous splash, it can put a damper on even the most sophisticated mining plan, or at the very least, force engineers to go back to their drawing boards.

Back in 1948, it forced an entire crew at the Ruby Hill copper mine at Eureka Cty., Nev., to flee for their lives as the entire underground workings 2,250 ft below surface were engulfed in a raging torrent.

Known as H2O or water in most chemistry classes, it is a problem that engineers are attempting to overcome at American Barrick Resources’ Goldstrike mine near Elko, Nev.

On the 45-mile Carlin gold trend where Barrick and Newmont Gold announced a number of major discoveries last year, the water table is thought to lie 400 ft below surface.

While average annual rainfall in Nevada’s Humbolt River Basin usually peaks at 4 in, (the average annual precipitation in Toronto is 29.97 in) water runoff from the surrounding Ruby and Jarbidge Mountain peaks has accumulated in the porous granodiorite sediments where a number of large gold deposits are being mined. Major problems

Barrick hasn’t reported any major problems at Goldstrike where a decline ramp is being driven 930-ft below surface, but since it is the first Carlin producer to attempt a mining operation below 500 ft, Barrick is entering the unknown.

“We are still learning about how much water is in the ground,” said Brian Meikle, Barrick’s senior vice- president development, who estimates that pumping systems could add $1 per ton to the cost of an open pit mining operation designed to churn out 860,000 oz of gold by the early 1990s.

In anticipation of expected outflow, contracts have already been signed with local ranchers who are eager to accept about as much water as Barrick can produce, for cattle grazing purposes.

“The water barely misses being fit for human consumption,” said Meikle.

With ramp construction scheduled to begin this week, Barrick is planning to pump the water into the natural drainage system and allow it to funnel back into the water table.

“Everyone has a different opinion on how much water we are going to produce,” said Goldstrike exploration manager Larry Kornze. “But it is too early yet to make any grandiose conclusions.” Dewatering test

In a bid to gain some insight into the kind of water flow problems that it will face, Barrick is conducting a dewatering test in two wells at the Post pit where a low grade surface oxide deposit is currently being mined.

“We are pumping 200 gallons per minute in one well and 350 gallons per minute in the other,” said Kornze. “P.V.C. monitors in many of the drill holes tell us that we seem to be getting a significant draw-down along a lateral distance of about 1,000 ft.”

Even if various “guesstimates” prove correct and Barrick has to pump out at least 10,000 gallons per minute to keep the pit dry, Edward Hahne thinks the Toronto company ought to be able to cope.

As mine manager at Hecla Mining’s Escalante silver mine near Cedar City, Utah, he is responsible for overseeing a massive 19,000-gallon-per-minute drainage operation which prevents the underground workings from being flooded.

At $1 per oz of silver produced, the cost of keeping the mine workings above water made the operation uneconomic when silver was selling at $4(US) per oz. Last year, Hecla produced 2.3 million oz silver using a stope mining and tram haulage method. South vein

When the mine reopened, Hecla installed 11 dewatering wells along the vein. The company also set up three dewatering pumps which pick up water from the mines’ south vein and send it to a mined-out area 600 ft underground where it is pumped to surface level and drained off into a canal.

“Some of the water is recirculated back into the aquifer (water table) but we can’t tell how much,” said Hahne. According to the Escalante mine manager, the number of pumps needed to keep the water level down, depends on how much water can pass through a given type of rock.

Since Carlin gold deposits tend to lie in porous limestone host rocks, he says Barrick and Newmont stand a good chance of obtaining a broad flat drawdown when pumps are in place.

If, as seems likely, Barrick elects to use an open pit mining method, Hahne says it could surround the pit with vertical turbine pumps and use them to lower the water table to a level where the company can mine.

“The results from our drawdown tests look pretty good,” said Meikle who regards the Carlin water table as “no big deal.” “Water can be a real problem but dealing with it is just one of the costs of doing business,” he said.