Jamestown, Calif. — Most mines today are not tunnels into the bowels of the earth but vast open-pit operations, such as the site here at Jamestown, Calif. The estimated $1.2 billion in gold reserves lies embedded in rocks as tiny specks. It will take nearly 14 tons of raw ore here to eke out just one ounce of gold.
As giant shovels devour the hillside, 13 massive yellow 100-ton dumpsters will feed the rock into the processing plant at a rate of some 3,000 to 6,000 tons a day.
There the ore fragments get slowly hammered to dust. Boulders the size of TV sets drop into the colossal fangs of the ``primary jaw crusher.'' The four-inch chunks that emerge are fed into more steel crushers, reducing the fragments to half-inch gravel.
Large barrel-shaped grinding mills finish the pulverizing job. ``When the ore is crushed as fine as face powder, then the gold is exposed,'' explains Sonora president Orville Anderson, squatting inside one of the empty grinding mills during a tour.
The powder goes to enormous round flotation tubs. Here a frothy brew is created by agitating a solution of water, chemicals, and ore. Gold-bearing grains stick to the bubbles, rise on the foam, and are skimmed off.
``Legend has it, the method was discovered by a miner's wife,'' says Mr. Anderson says. ``She was washing his clothes when the gold flecks appeared in the suds.''
The gold concentrate is mixed with sodium thiourea, an chemical that dissolves the gold, separating it from the pyrite and other waste ore, known as ``tailings.''
Mineral composition and gold concentration make this flotation technique the most economical at Sonora. Elsewhere, ``heap leaching'' -- which bypasses the fine grinding and suds process by dumping a solution over a ``heap'' of ore -- is proving popular.
At Sonora, the last step is to filter gold out of the thiourea solution and smelt it into bullion bars. Waste ore is ``dewatered'' and piled in a gigantic tailings pit, the floor of which is coated and laced with drains to prevent any remaining chemicals from seeping into the water table.