Kennecott’s gravy train

The first year of mining the open-pit Ridgeway deposits in So Carolina was, to borrow a term employed by Engineering Manager Peter Evans, “gravy.” The soft oxides allowed blasthole spacings of 20 ft. (six metres) rather than the 14 ft. (4.2 metres) required for harder sulphide rock, which, a couple of years after start-up, is the main gold ore now. Also in that first year, the semi-autogenous grinding (sag) mill ran some days at better than 20,000 tons (18,200 tonnes) per day and averaged 15,400 tons (14,000 tonnes) per day as compared with its 15,000-ton-per-day capacity (13,635 tonnes per day). Gold recoveries were 89% as compared with the first-year forecast of 88%. Life-of-mine gold recovery should average 82% as a result of the sulphide content of the orebodies.

Kennecott Ridgeway Mining Co. is a joint venture between Kennecott Corp. and Galactic Resources. Evans has been on the project since the exploration stage, when drilling proved up 56 million tons grading 0.032 oz. gold per ton in the two depooits, sitting one mile apart. But Evans indicated that because of the project’s location and the low grade, the deposit was not just discovered, it was made. The discovery hole was a 100-ft. (30-metre) intercept that graded 0.05 oz. gold per ton (1.7 grams per tonne). It was a section of a hole being drilled for a water well as a promise to the landowner.

Understandably, dilution control is critical when the average grade is 0.032 oz. gold per ton (1.2 grams per tonne). Blastholes are drilled using two Driltech D40K track-mounted drills and each hole are assayed and the blasted material is flagged as waste or ore. Ten million tons of lower-grade ore (averaging 0.021 oz. gold per ton or 0.720 grams per tonne) are being stockpiled for processing at the end of the project’s life, while higher-grade ore is loaded by three 12-cu.-yd. 992-C Caterpillar loaders into seven 85-ton 777-B Caterpillar trucks and hauled to the carbon-in-leach plant. Generally, with the exception of a 12-hour shutdown during Hurricane Hugo, the mine and mill have run almost exactly as projected in the feasibility study. Both mining and milling proceed 24 hours a day, 350 days a year, moving 5.2 million tons (4.7 million tonnes) of ore and a similar amount of waste.

Since the pits are below the local water table, an elaborate system of perimeter and in-pit water wells is used to maintain dry conditions. During the wet season, up to 550 U.S. gallons (2,090 litres) per minute are pumped from the ground, and the water is rerouted to replace the loss from evaporation that occurs in the tailings pond. Area water wells are monitored by a third-party consultant to check that pit dewatering is not affecting them. The geohydrology of the area is such that the surrounding water resources have not been affected to date, and no adverse effect is expected in the future.

The mine is in a transitional stage from mining the softer, higher-throughput oxide ore to mining the harder, lower-recovery sulphide ore. The difference was felt quite early during the traasitional period by the slowing of the grinding circuit. An oxide/sulphide blend was processed until May. Since then, mill feed has been coming solely from the sulphide zone of the North pit. That phase will end in early 1992, after which mining of the lower-grade Phase II ore of the South pit will begin and continue until 1995, followed by Phase II of the North pit through 1998. The 10-million-ton (9.1-million-tonne), low-grade stockpile will be processed from 1998 to the year 2000.

According to Galactic Resources, operator Kennecott’s 48% partner, the average 1989 cash operating costs were US$140 per oz., excluding royalties, which are significant. Ridgeway’s land position is 2,380 acres (952 ha). There are nine different owners of the producing areas, with varying percentage royalties attached to each area. The cash cost was calculated by subtracting the silver credits from the production costs. The mine poured and shipped 166,000 oz. (5,160 kg) of gold in 1989 and another 6,500 oz. (200 kg) was adsorbed on to carbon at year’s end. Silver production was about 71,000 oz. (2,210 kg), but the ratio will increase as the sulphides are processed to about 60:40 gold to silver over the life of the mine. Silver recovery averages 35% to 40%. For the first quarter of 1990, the mine and mill were on schedule, producing 40,000 to 42,000 oz. (1,240 to 1,310 kg). And while the 1990 projection of 160,000 oz. (4,975 kg) can probably be met, the harder ore will require another ball mill to maintain production levels in 1991 and beyond. Mill expansion, anticipated in the original feasibility study, will include a new ball mill and a recycle crushing and conveying circuit off the SAG mill.

The mine represents a total investment of about US$100 million, which includes land acquisition, exploration and reserve development, and mine and mill construction. The zero-discharge tailings facility, the largest structure on the property at 210 acres (84 ha), is a double-lined facility with compacted clay and a 60-mil high-density polyethylene liner. Clay waste overburden was used to construct the tailings embankment.

The impoundment is enclosed by a chain-link fence and patrolled by security to frighten away animals that might be harmed by the process solution. Passive hazing techniques are employed and, if necessary, a hovercraft can be dispatched to patrol the area and chase off any wildlife. Two churches were moved from the mine area and three miles of state highway and county roads were rerouted to allow for development of the North pit and the tailings facility.

How do you mine and process more than 38 short tons of ore to produce one ounce of gold for US$140? And do it 400 times a day? A low stripping ratio is one answer. Both orebodies outcrop at the surface and the overall stripping ratio is less than 1:1.

Mining in the South pit began with 30-ft. (9-metre) benches, but after the pit reached 150 ft. (45 metres), the bench heights were changed to 20 ft. (six metres) to improve grade control and equipment utilization. At the time of our visit, Phase I of the South pit was almost completed and the pit was nine benches down to a depth of about 220 ft. (66 metres). The North pit was just taking form, with four 20-ft.(6-metre) benches from Phase I having been developed.

A simple orebody without impurities and a simple milling process ar other reasons cited for the low costs. After resolving start-up problems in late 1988 and early 1989, the mill ran quite smoothly. The Ridgeway plant was designed by Fluor Daniel and Minproc of Australia

The fast, efficient operation speaks well of the employees. Most of the 150 or so Kennecott employees and the 50 to 60 workers contracted for security and equipment maintenance were hired locally and trained for their specific jobs. Managers, by and large, were recruited from other mining operations.

Because of local concerns about how the operation will affect the surrounding environment and because of a general lack of local knowledge about mining, extensive effort and expense have been directed toward informing the public about the history of gold extraction in the region.

In 1969, John Chapman discovered gold in a creek near the town of Ridgeway and reported his find to the South Carolina Geological Survey. The state drilled test holes that were subsequently reviewed by Amselco Minerals. Amselco was impressed enough in 1981 to open exploration offices and begin drilling. During the next four years, more than 100,000 ft. (30,000 metres) of drilling defined two deposits; the South (Chapman) deposit wiih 23.7 million tons grading 0.029 oz. gold per ton (2.15 million tonnes grading 9.95 grams per tonne) and the North deposit with 32.5 million tons grading 0.034 oz. gold per ton (29.5 million tonnes grading 1.17 grams per tonne). Extensive exploration of a 10-sq.-mi. (26-sq.-km) area has failed to define additional deposits.

Galactic earned a 48% interest in 1986-87 prior to the feasibility work. Final permits were received in September, 1987 and con
struction began. A challenge by a local citizens group led to a work stoppage late in the year, but after a series of discussions, work was renewed in early 1988.

Mining began in June, 1988, the mill started up on Dec. 5, 1988, and the first pour was recorded on Dec. 9, 1988. Production is expected to average 130,000 to 135,000 oz. (or 4,040 to 4,200 kg) annually through to 1999. Higher production in the first few years will facilitate capital repayment.

Topsoil, stripped from the orebody and stockpiled, will be spread back over the disturbed areas at the end of mine life. The tailings pond will be sealed and replanted and the pits will be contoured and reclaimed to freshwater lakes.

SIDEBAR

HISTORY AND GEOLOGY

The first gold discovered in the United States was in North Carolina in 1799. Twenty-one years before the California gold rush, Colonel Benjamin Haile found the precious metal in South Carolina, 40 miles (65 km) northeast of the Ridgeway mine. Piedmont Mining Co. resumed operating the Haile mine in 1985 as a 1,000-ton-per-day (910-tonne-per-day) heap leach operation. In the same area, the Brewer mine is also producing gold, but neither is as large as Ridgeway.

The deposits are part of the Carolina Slate Belt, which extends from northern Virginia to eastern Alabama. The Ridgeway project covers a major east-west-trending contact zone between metavolcanic and metasedimentary rocks. The rich, dark-red clays of the turbidite host rock were altered by hydrothermal fluids that remobilized the iron to the edges of the orebody. Gold mineralization occurs in widespread alteration consisting of silicification, quartz veining, and pyrite development. About 20% of the gold in the sulphides is inextricably encapsulated in pyrite, but the ore is remarkably free of clay or other impurities. Some uneconomic molybdenum is present in the ore.