Mining the gold where Cromwell once stood

Press, 19 December 1986, Page 21

Mining the gold where Cromwell once stood

Story by

STAN DARLING

Photographs by DAVID ALEXANDER

Some old buildings still stand along Melmore Terrace, the remains of Cromwell’s old business district, but they will not last for long. The crunch will come before the promontory’s point, above the Clutha and Kawarau rivers, is drowned when the new Lake' Dunstan fills up behind the Clyde Dam in late 1988. Gold-mining excavators will be chewing their way through the top of the bluff where the town’s main businesses once sat. Old Cromwell Mining, Ltd, has three partners, all from Christchurch, who are also behind mining ventures nearby, in the Shotover and at Gibbston, up the Kawarau Gorge. Last week, Noel Becker, the on-site manager, and his workers prepared to get the machinery — with a large rotary screen at its centre — cranked up and the job underway. “There is just under half a million cubic yards to be moved here,” he says. The machinery is on a bench just above the place where the rivers join, in a cut made on land that was Cromwell’s former war memorial site. Mr Becker, a geologist, is a former Ranfurly farmer who has been working as a mining engineer in the area for more than three years. “We hope to be putting through 1200 to 1500 cubic yards of material a day,” he says. “We could get up to an ounce of gold

each hour, but it will be something less than that. The material we’re looking at has about three grains of gold per cubic yard.” There are about 480 grains of gold in an ounce. “At times,” says Mr Becker, “we could come out very lean for the day.” Bore tests taken last summer, however, surveyed a lot of ground, but they were done when about 75 per cent of the commercial district’s buildings were still standing. The mining company has 18 months to move up Cromwell’s old main street. “We will go as far back as it is economically possible to go,” says Mr Becker, “but we can’t go beyond a certain contour. Because of ground stability, we can’t go beyond a certain slope." In about nine months, the mining excavation will be getting close to the Golden Age Hotel, which is still doing business until it finds a new home higher up in the borough. “With alluvial gold, unfortunately the risk is always there,” says Mr Becker, “because you only do a few test pits. But some parts could be stripped off if they are too lean.” Gold found in the digging will be retorted in Queenstown and sold in bars. If the product is similar to the gold found in the Shotover venture, the bullion should be about 94 per cent gold. Under the company’s licence, mining can be done 12 hours a

day, from 7 a.m. to 7 p.m., six days a week. The local council has been uncertain about the mining equipment’s noise, and how much it might disturb nearby residents. Mr Becker thinks the noise could be less than is produced by trucks along the highway across the Clutha River. “Once we are operating, we’ll show how quiet the operation is and make approaches through the correct channels” to mine longer hours, he adds. Excavated material goes through a large screen at the start which cuts out anything wider than 200 millimetres. Stockpiles of the "reject” material could be used as rip-rap rock by the Ministry of Works for future lakeshore protection. Material that passes through the first screen drops into a bin, then is fed at a constant rate up a conveyor belt into the rotary screen. It drops into a hopper, with “heaps of water” coming on to it. After going through a short scrubbing section, it enters the rotating screen, where it is screened to 10 millimetres. At the end of the screen is a short section of 16 millimetres in case nuggets are found in the gravels. That section is just a trial, since nuggets are not expected. Material larger than 10 millimetres will be bladed flat in the initial mining stages. If it is not needed by the Ministry of Works, it could be trucked away.

Material smaller than 10 millimetres drops into a bin and is pumped through a flexible hose to the hydro-cyclone, a dewatering device used to get a better pulp density. Overflow water from there is recycled to the hopper. Then the material is split five ways to primary jigs, which are the main gold recovery devices. Gravel comes over a bed of steel shot, and gold filters through. Material from there goes into a secondary jig, where it is processed in the same way. A final device slows down the volume of water remaining, and gold drops into a Knudson bowl, “basically a centrifugal riffle table,” says Mr Becker, for final recovery.

Gold and black sand drop into the bowl, and the gold stays in the bottom of the riffles, which capture gold down to a tenth of a millimetre or less. Each of two bowls could hold up to 30 ounces of gold, “but we never let it go beyond three or four,” says the site manager. The Clyde Dam is scheduled for completion in October, 1988. Mining has to stop before that so that the lake’s future hydro edges can be formed before it is filled. Filling is expected to take three months, and the schedule still calls for the dam’s first power to be generated in January, 1989. At the end of the first week in December, the dam was 65 per cent finished, and its powerhouse was 80 per cent complete.