CORONA’S NEVADA GOLD A clean heap leacher comes on-stream

The state of Nevada is widely Tknown as a gaming mecca. It’s Talso a magnet for gamblers of another sort — gold explorationists. For example, since 1979, at least 30 heap leach gold mines have been put into production in Nevada. More are sure to come. The latest entry into the producer category is Corona Corp.’s Santa Fe mine. This mine is some 50 km southeast of Hawthorne, Nev., in the Gabbs Valley Mountain Range. The mineable reserve is 8.5 million tons grading 0.037 oz gold and 0.316 oz silver per ton for a mine life of nearly five years. Geology The Santa Fe orebody lies in the Basin and Range geomorphic province of west-central Nevada. The property lies within a major northwest trending fault system, known as the Walker Lane structural zone. The oldest rocks are limestones of the Triassic Luning Formation, which have been intruded by granite and overlain or faulted by Tertiary volcanics. The deposit occurs as fracture-controlled and disseminated bodies in a 50- to 1,000-ft-wide breccia in the limestone at its faulted contact with the volcanics. Ore-grade material (typically a rich, brownish- red hematite) is visible with the naked eye. In the oxidized zone, currently being mined to the north end of the deposit, an occasional block of barren rock will intrude, usually from the footwall side. But this is not a problem.

Low-grade portions of carbonaceous material are also present in rather small quantities. The oxidized orebody extends to 450 vertical ft. The known oxidized portion will be mined out within four years or so, although geophysical anomalies have been picked up in areas within the boundaries of the Santa Fe claims but outside the current orebody.

An area of sulphide mineralization has been probed in the southeasterly portion of the deposit. This pipe-like sulphide structure will be further tested at depth this year and next. Christopher Jennings, Corona’s senior vice-president, exploration, said: “We know we have a sulphide reserve below us. We’ve got a few holes into it and we know it’s a million tons plus.” A budget for the sulphide deep drilling has yet to be established. Mike Fiannaca, Corona’s manager of exploration, western U.S., said he expects it to be in the $300,000 range. Leaching

At a capital cost of $10.5 million ($1.6 million under budget), this operation should produce an annual 50,000 oz of gold and at least three times that amount in silver over the life of the mine. Cash operating costs are $256 (US) per oz of gold produced, 71% of which are for mining and crushing, 21% for milling and 8% for administration.

Brown & Root USA, the mining and crushing contractor for Santa Fe, has loaded about 540,000 tons onto the leach pads in the initial cells of the first 18-ft-high lift. Ultimately, the leach pile will rise 90 ft. A full 8,000 tons of ore are mined per day, along with 18,000 tons of waste, which is about right for the overall 2.3:1 strip ratio.

A 3-stage crushing system reduces the ore to 80% –1 inch. Mine muck is first crushed in a primary Kue Ken 60-inch jaw crusher to –6 inches, then through an Eljay secondary closed circuit to –3 inch and finally through a Nordbery secondary open circuit to the heap-sized particles ready for leaching. For each ton of ore, 14 lb of cement are added to prevent clay from inhibiting leaching on the pads. The cement mixes with the ore not in a separate agglomeration circuit, but as the ore is carried to the pad along 100-ft-long bridge conveyors. The mixing occurs as the ore drops from conveyor to conveyor and on to the pad. A radial stacker piles the ore. The leach ore contains about 14% total water content, 7% of which is added with the cement and another 7%, along with a weak cyanide solution, at the stacker

“We don’t work any equipment on top of the heap.” Mine Manager Gregory Lang told The Northern Miner Magazine during a tour of the operation. “The radial stacker does that, so we don’t compact the ore and inhibit leaching.” Differential compaction caused by heavy equipment leads to less uniform cyanide percolation.

In addition, the 0.5% sodium cyanide leachate is added by drip emitters that are draped over the ore on a 24-inch grid in the manner of oversized fishnets. This rather simple drip system, an alternative to sprinklers, reduces evaporation and keeps the ponding and channeling of leachate to a minimum, Lang said. The application rate per square foot is 0.005 gal per min with a total 1,800 gallons delivered to the pad every minute. Before ore-loading, the leach pads were covered with 2 ft of overliner protect ion. The 80-mil Gundle-fabricated liner covers 2.1 million sq ft and will eventually carry 5.5 million tons of ore.

The gold-loaded solution is gravity- fed into a pond with capacity of 4.2 million gallons and then pumped for processing through a 1,500-gallon- per-minute carbon adsorption plant. After the pregnant solution passes through the five carbon columns, the gold- and silver-bearing carbon is stripped of precious metals and then captured in an electrowinning process. Waste mill water is recycled from a 4.2-million-gallon barren pond. Santa Fe also has a 4.5-million-gallon storm overflow basin equipped with emergency generator pumping to the leach pad.

An early surprise has been the amount of silver coming out of the furnace. The silver-to-gold ratio is running up to 7 oz of silver to each oz of gold. Earlier estimates had placed it at no more than 5-to-1. “I think the ratio will settle down to 5-to-1 one soon,” said Lang. This is not occurring at the expense of gold recoveries in the mill (running as expected at about 70%), so the only negative is that the carbon in the columns must be cleaned more often and the excessive silver content tends to crowd the whole plant system. “If it got up to 8- or 9-to-1, we’d be hurting,” Lang added. Santa Fe has ordered two new electrowinning cells to cope with high silver content. Mining

In the open pit, six 85-ton Wabco trucks (running on 85-ft-wide haul roads with a maximum grade of 8%), two huge Caterpillar loaders and two Ingersoll-Rand 45R drills do the bull work. The 45Rs drill 8-inch-diameter blastholes on a 15×15-ft pattern. For strict grade control, the cuttings from each blasthole are assayed. Mining is done in 20-ft benches, but Chief Mine Engineer Bruce Thieking said Santa Fe incorporates a double-benching method that produces 40-ft-high walls and 40-ft catchments for wall stability and ease of moving fallen rocks off the catchments. About 85 blast holes are drilled every operating day and blasted with ANFO. The ultimate size of the pit will be 1,500 wide, 3,000 ft long and 500 ft deep.

The mine runs a single, 10-hour shift on a 5-day week, while the crushing and stacking crews run two 10-hour shifts on a 5-day week. Corona’s own engineers issue daily mining plans and monitor the work in the pit.

Welsh/Selland of Reno, Nev., prepared the pads, ponds and site. Kilborn Engineering of Vancouver did the plant engineering and construction. Earthwork started on the access road and leach pad at the beginning of February. The first round from the pre-production pit was shot March 6 and the crushing plant started up July 6. The first dore bar was poured Aug 18.

Corona holds a 100% interest in the mine, subject to a 1.5% net smelter return (nsr) held by Euro-Nevada Mining Corp., a 1.23% nsr (at current gold prices) held by Westley Mines, and a 0.27% nsr held by Brican Resources. One of Corona’s predecessor companies, Lacana Mining, became involved in the project back in 1983. A production decision was made in December, 1987. Also in Nevada, Corona holds interests in the Pinson, Preble and Dee mines and is making progress towards production on a project known as the Marigold.