Newmont’s Gold Quarry gets new lease on life

The Gold Quarry pit, situated in Nevada’s Carlin trend, faced a shutdown in the second quarter. However, higher-than-anticipated drill assays and the startup of the bio-milling circuit have increased the life of the pit, assuring its survival for many years to come.

Operator Newmont Mining (NEM-N) reports that the expansion project will add 4.6 million oz. to the deposit, which is pegged at 76 million tons averaging 0.06 oz. per ton. The expansion project will increase the size of the 1.5-mile-wide pit to the southeast.

Since it opened, in 1985, the pit has produced more than 12 million oz.

In 1999, Newmont completed 98 core holes, totalling 89,000 ft., which returned gold values 15% higher than results from previous reverse-circulation and mud rotary drilling.

At a recovery rate of 70%, Newmont expects to produce 3 million oz. as a result of the expansion project. Mining costs are anticipated to be US80 per ton. Stripping of 90 million tons of overburden for the expansion project began in April, and the first production is expected in the second half of 2002.

Nearly a quarter of the ore will be processed through oxide heap leaching, with about half that destined for the oxide mill. Another 25% will go to the high-grade refractory mill, but nearly 40% of those recovered ounces will be processed through the newly developed bio-milling circuit.

Newmont has experimented with bio-technology throughout the past dozen years. Eventually, it devised the first commercially workable model for gold production, based on the premise that the biotech application works best as a pretreatment step within already existing technology. Newmont hammered out this technology more than a year ago, though lower gold prices shelved the project until late last year.

Low-grade refractory ore (averaging 0.07 oz. gold per ton and containing 2% sulphur and 0.1% carbon) will be crushed to 80% minus three-quarter inch and stacked on a newly constructed pad, just south of the Gold Quarry pit. The pad has a capacity of 11 million tons, though Newmont began stacking 2.5 million tons of ore on three cells in December 1999.

The bacteria solution (containing thiobacillus ferrooxidans) is first applied during secondary crushing, with additional solution applied from drippers on the pad. The ore stays on the oxidation pad for 150 days, during which time 40-50% of the sulphur content is oxidized.

Each cell was constructed with a solution piping network under a 3-ft. layer of rock. Newmont keeps the process going and heap temperatures under control through a series of blowers, forcing air through the heap.

The material is then removed from the pad and processed through Mill 5, northeast of Gold Quarry, where declining oxide ores have created excess capacity.

In 1999, the 16,000-ton-per-day oxide mill produced 316,400 oz., with most of the ore coming from Gold Quarry. The facility consists of a semi-autogenous grinding unit, a ball mill and a carbon-in-leach circuit.

This year, excess capacity will reach only 50%, creating ample room for the bio-milling material. So far, in the first quarter of 2000, the process has yielded a total of 4,400 oz., and this is expected to increase in the years to come.

Meanwhile, Newmont is marching ahead with other projects at its Carlin operations. Underground development at the Deep Post mine is three months ahead of schedule. Newmont expects to commission the mine by mid-2001. Capital costs for Deep Post are estimated at US$65 million, and the mine should produce 400,000 oz. annually at cash operating costs of US$150 per oz. Total production costs are pegged at US$210 per oz.

Reserves at Deep Post at the end of 1999 stood at 3 million tons grading 0.77 oz., equivalent to 2.3 million oz. gold.

Current development work consists of driving a decline into the deposit from a portal at the bottom of the Betze-Post pit. The company is also gaining access to the deposit through a tunnel from the Deep Star mine, 5,000 ft. to the south. The tunnel, being driven by contractor Small Mine Development, is said to represent excellent exploration potential between the two deposits, and Newmont is cutting out drill stations along the way.

Mineralization occurs in a 200-ft.-wide shear zone, in metamorphic and intrusive rocks, associated with a regional anticline. Because of the compact size of the deposit and the possibility of poor ground conditions, Newmont expects to employ underhand drift-and-fill mining.

Development at Deep Post began last year as the result of a land swap with neighbour Barrick Gold (ABX-T). Newmont essentially traded the Goldbug deposit to Barrick in return for the corridor connecting the Deep Star with Deep Post. The transaction consolidated land holdings for both companies.

Beginning next year, Newmont expects to begin developing the high-grade Leeville deposit, southwest of Deep Star. Startup is slated for 2004, with output providing additional ores for the bio-milling operation at Gold Quarry.

In all, Newmont expects its Carlin operations to contribute 2.9 million oz. in 2000, with average cash operating costs of US$208 per oz. Costs are expected to continue falling in the following years.

Meanwhile, a mile northwest of the main pit, the company has outlined porphyry copper-gold mineralization in the Mike deposit, virtually guaranteeing the company’s involvement for decades to come.

Elsewhere in Nevada, in the Winnemucca region, Newmont is considering expanding the Mega pit at the Twin Creeks operation. Assuming positive feasibility results, stripping could begin in 2002, with production projected for 2003.