Mining a Mountain : Technology Pulls Silver, Gold From Nevada Peak

NENZEL HILL, Nev. —  Early in the summer of 1986, prospectors began taking another big whack at this towering mountain.

Nenzel Hill was 7,275 feet high before the whittling away began two years ago. Today the highest point on the mountain is 7,100 feet--175 feet lower. One section of the peak has been lowered 400 feet.

Unlike prospectors who once burrowed their way underground here at the turn of the century, miners today are using modern-day technology to simply carve up and cart away this peak in order to tap its treasure of precious metals. And they seem to be striking it rich.

In fact, silver mining here on Nenzel Hill, 30 miles northeast of Lovelock in the Humboldt Range of Nevada’s great outback, has propelled this state ahead of Idaho as the nation’s No. 1 silver state. Idaho held the title for years. Now, Nevada leads the country in both silver and gold production.

Boost for Production

The Rochester Mine on Nenzel Hill increased Nevada’s silver production by a whopping 40% in 1987, the mine’s first full year of operation. Rochester, owned by Coeur d’Alene Mines Corp. of Idaho, became the second-biggest silver mine in the United States, second only to Asarco’s underground mine in Troy, Mont.

The Troy mine produced 4.2 million ounces of silver in 1987. Rochester’s production last year was a close 4.01 million ounces. A total of 26,821 ounces of gold was also recovered at the Rochester Mine in 1987.

Even as they strip off the top of this old mountain, these 1980s prospectors are finding evidence of those who came before.

From the 1860s to 1930, Nenzel Hill and the surrounding Rochester mining district were alive with hundreds of miners toiling away underground following rich veins of gold and silver. They recovered more than $6 million of the precious metals.

The mining district was named for Rochester, N.Y., the hometown of prospectors who discovered it. At the turn of the century Joseph Nenzel headed up the mining company working the mountain that now carries his name.

Chemical Processes at Work

“As we blast off the mountaintop and haul away the ore with good assay readings to the crushing plant and the rest to dump sites, we continually uncover old underground workings, old timbers, mining tools and ore carts used by the miners in the old days,” noted Gary A. Vanhuffel, 35, chief mine engineer. The mine plans to set up an exhibit of the artifacts that have been unearthed, he said.

The mine’s main plant and headquarters are located at the 6,250-foot level of Nenzel Hill.

Every 24 hours, giant trucks haul 17,000 tons of the mountain a short distance down its slopes to a nearby crushing plant. And from there, complicated chemical processes are used to remove the good stuff.

The success of the big silver and gold mining effort on Nenzel Hill, which employs 290 workers around the clock seven days a week, is due to what Vanhuffel called “the heap leach process.”

After crushing, the pebbles are dumped on a leach pad now piled 60 feet high, 2,000 feet long and 1,500 feet wide. There will eventually be 4.5 million square feet under leach in the heap.

A thick, high-density polyethylene sheet forms a liner at the base of the pad. A network of pipes on top of the heap irrigates the huge pile of ore with a cyanide solution distributed through a drip irrigation process.

The solution trickles through the pile, dissolving the gold and silver out of the host rock. The metals are carried to the bottom of the pile to a central drain trench, then piped to two ponds capable of holding 2.5 million gallons of the metal-laden solution.

Economically Feasible

After further processing, the metals are heated. Gold and silver bullion is poured from the furnace into pots and formed into 65-pound ore buttons, containing a residue of impurities. Each button is valued at $8,000 to $10,000, depending upon current prices. The gold and silver are later separated in a final refining process.

The cyanide solution will be used to drip irrigate the ever-growing size of the heap for the lifetime of the mine, which is expected to be at least another 17 years. It is estimated that 50% of the silver will be recovered from the pile and 80% of the gold. In time, the plan is to have at least 10 million square feet under leach.

“This is a very low-grade mine. The heap leach process makes mining the mountain economically feasible,” explained Vanhuffel. “The cyanide solution is pumped back, reconstituted and returned to the ore heap over and over again.”