Tough environmental hurdles faced by MinVen’s Gilt Edge mine

The Gilt Edge mine, currently a heap leach operation, could be producing as many as 200,000 ounces of gold per year beginning in August, 1992. The company proposes mining six million tons of ore per year and as much as 18 million tons of waste. MinVen has submitted its case to environmental agencies and continues to work closely with them to iron out any problems that might crop up during the approval process.

“I’m confident we can get all the permits we need based on my talks with the regulatory people,” MinVen’s project director Doug Stewart told The Northern Miner during a visit to the site a few kilometres southeast of this famous mining town.

In this state, heap leach operators line not only the pads, but even the ground beneath mill buildings to prevent cyanide seepage. Primary, secondary and tertiary liners are a must. And even slight alterations to mine plans require more than a token submission of details to some government department.

At a capital cost of $128 million, the proposed expansion would tap MinVen’s deeper sulphide ores, which were delineated in an exploration drilling program that began in January, 1988. The $3.6-million (US), 160,000-ft program proved up reserves of 32 million tons grading an average 0.045 oz gold per ton. Later drilling, however, showed that as much as 90 million tons may be mineable. The official published reserve now stands at 54.2 million tons of proven and probable ore grading an average 0.041 oz gold per ton at a cut-off of 0.0225 oz per ton. The strip ratio for that tonnage is 3:1 (waste:ore).

While much of this reserve is nominally a sulphide ore, MinVen’s senior exploration geologist James Barron noted that it is less refractory than other such ores because it contains a relatively low fraction of sulphide mineralization (in the order of 3% to 5%). In addition, the gold is associated with pyrite. “We’ll get a lot of the gold with gravity circuits,” he said. “It’s basically a liberation project that will require a very fine grind.”

MinVen has submitted to the U.S. Forest Service, which must approve the plan of operations before local and state permits are sought, two different plans — one for a 60-million-ton pit and another for a 90-million-ton pit.

The latter figure represents a deeper extension of the proven sulphide reserves, but current drill data are insufficient to establish whether profitable mining is possible. The waste-to-ore ratio, for example, may prove too high if drilling on the periphery of the current, projected pit dimensions fails to core mineable values.

“Our ability to go deeper (than the 60-million-ton figure would warrant) is determined by our drilling for more ore on the perimeter,” Barron said.

While MinVen’s expansion proposals are assessed by authorities, it will continue to mine the oxide portion of the orebody which produced a first dore bar in October last year.

This is an open pit, heap leach operation with more than a few wrinkles to make it interesting. For example, experience has shown that a 13×13-ft drill pattern, as opposed to a wider pattern, gives far tighter grade control because rock movement is inhibited. On the downside, production drilling times have climbed and overbreak is an unavoidable redundancy.

But mine engineer Michael Golliher is generally pleased with the orebody. “What we’re finding when we mine is that the ore/waste boundary doesn’t feather out. It’s an abrupt change. I like it that way because it’s cut and dried.” The mine contractors move 7,500 tons of ore per day and between 12,000- 13,000 tons of waste.

The current bottleneck in the operation has been the leach pads. State regulations require that any material subjected to cyanide must be detoxified and neutralized before it is dumped as spent ore in an unlined pit.

So the leach cycle must incorporate several weeks of detoxification to cut the cyanide to 0.5 parts per million. (By comparison, the allowable level of cyanide in drinking water in the state is 0.75 ppm. In Nevada apparently, another big heap leach state, allowable cyanide levels in off-load leached material are 2.0 ppm.)

Detoxification, however, reduces the pad space available for leaching. MinVen had originally projected this stage of the cycle might require four weeks. It has found neutralization and off-loading of the 65,000-ton cells take 8 to 12 weeks.

To shorten the time, MinVen is now crushing the rock to 1-in rather the original 2-in crush. This increases the surface area over which the neutralizing agents (hydrogen peroxide and copper sulfate) can do their work.

“We’re really pioneering a new area with this neutralization,” said Stewart during a tour of the leach pads. He expects Gilt Edge will produce anywhere from 30,000 to 36,000 ounces of gold this year, but not the 40,000 projected earlier. Extreme winter conditions also cut into production, but only a week’s worth of leaching was lost.

During the abbreviated production period last year (leaching began in mid-year), the mine processed 547,772 tons of ore and produced 6,666 oz of gold and 8,072 oz of silver. Because of the high silver content, a Merrill-Crowe zinc precipitation system recovers the gold and silver from the pregnant leach pad solutions. Carbon columns are less effective when silver-to-gold ratios run any higher than 3:1.

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