Corona launches its first wholly-owned U.S. mine

Corona Corp. (TSE) has established its first, wholly-owned beachhead in the United States. The Santa Fe open pit, heap leach operation, several miles southeast of here, was officially launched as a gold producer during a ceremony that included about 200 guests, senior Corona officials and the governor of Nevada.

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 of silver over the close to five years of mine life. Current proven and probable reserves stand at 8.5 million tons grading 0.037 oz gold per ton and 0.316 oz silver.

“This heap leaching operation is not only up and running fast,” Corona President and chief executive Peter Steen told assembled guests, “it is up and running well.” The mine boasts a small profit in only its second month of operation. Cash operating costs are $256(US) per oz of gold produced, (71% of which are charged to mining and crushing, 21% to milling and 8% to administration.)

Brown & Root USA Inc., the mining and crushing contractors for Santa Fe, have 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 above the surrounding sage brush. A full 8,000 tons of ore are mined per day, along with 18,000 tons of waste, which is about right for the over-all 2.3:1 strip ratio.

A 3-stage crushing system reduces the ore to 80% –1 inch. For each ton of ore, 14 pounds 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 onto the pad. A radial stacker piles the ore.

“We don’t work any equipment on top of the heap. The radial stacker does that, so we don’t compact the ore and inhibit leaching,” mine manager Gregory Lang told The Northern Miner during a tour of the operation. 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 an oversized fishnet. 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 gold-loaded solution is gravity-fed into a pond 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.

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 ounce 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 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.

In the open pit, six, 85-ton Wabco trucks, 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.

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.

The Santa Fe orebody lies within a major northwest-trending fault system. The oldest rocks are limestones of the Triassic Luning Formation, which have been intruded by granite and overlain or faulted by tertiary 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, Thieking said. But this is not a problem.

Low grade portions of carbonaceous material are also present in rather small quantities. The fracture-controlled, oxidized orebody is mostly confined to the breccia and extends to 450 vertical feet. 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, manager of exploration, western U.S., said he expects it to be in the $300,000 range.

Corona holds a 100% interest in the mine, subject to a 1.5% net smelter return held by Westley Mines. 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.