Gold by the bucket

Dredging was New Zealand’s greatest contribution to gold mining technology, and it continues today on the Grey River.   BY HUGH DE LACY

When Allan Birchfield got his 3500-tonne gold dredge working again on the banks of the Grey River on the West Coast earlier this year, he brought back to life a form of gold mining that this country pioneered a century and a quarter ago.

Birchfield’s floating behemoth, the Kanieri, is the only one of its kind that is still working in New Zealand, and one of only a few left in the world. Since June the Kanieri has been working through old tailings near Blackball on the north side of the Grey, after being parked up for five years on the south bank as a tourist attraction at Ngahere.

As gold bursts through the US$1000-an-ounce mark and, according to many commentators, heads for US$1500, Birchfield can congratulate himself on twice rescuing the Kanieri from retirement in defiance of local pundits who reckoned he was mad to even try. It was with no little satisfaction that he was able to host delegates from the West Coast annual mining forum at Greymouth in July on his monstrous machine.

The 170-metre-long and 30 metre-high dredge is working mining permit 41933, an 873 hectare tenement that includes a small worked-over area near Blackball, as well as larger greenfields areas on both sides of the river. It encompasses an inferred recoverable resource of 170,000 ounces and dwarfs the original 294 hectare permit on which Birchfield first got the dredge operating.

With a staff of about 15 it works two eight-hour shifts a day, seven days a week, its 104 in-line buckets digging up 60,000 cubic metres or 120,000 tonnes of rock and gravel a week from the bottom of the 20-metre-deep pond on which it floats. Birchfield won’t say how much gold it is recovering, but when it was running round-the-clock it was producing 5000oz to 7000oz a year.

It’s possible the Kanieri could resume 24/7 operations, but Birchfield isn’t keen on the idea, saying that opting for shorter hours was “a lifestyle decision”.

“It’s a very difficult machine to operate, and I sometimes wonder whether I should have [brought it out of retirement] or not,” he told Q&M. “You can imagine the wear-and-tear on the thing, and it’s not an easy operation to run.”

The dredge is of all-steel construction, is powered by two megawatts of electricity and moved around its 300-metre-wide pond by five computer-controlled positioning winches operated through DC electric motors. Each of the buckets has a capacity of 0.57 cubic metres or about a tonne of gravel, filling at the rate of 25 buckets a minute.

The bucket-line is powered by two 1000hp DC motors, and it washes 850 cubic metres or 1700 tonnes of gravel an hour through a 3.6-metre-diameter screening trommel lined with polyurethane. Any material bigger than 12mm is rejected as coarse tailings that are carried by a conveyor belt to a tailings stack behind the dredge, while the rest is washed through two circular jigs mounted on either side of the screening trommel. The gold-bearing concentrate then passes into a hopper from where it is pumped to secondary and tertiary jigs, and finally to the concentrate bin.

The dredge and its pond advance at the rate of about three metres a day, and ultimately the tailings are bedded down under quality dairying pasturage.

The dredge floats on a pontoon 80 metre long and 36 metre wide that was part of the original machine built by the Kaniere Gold Dredging Company on Lake Kaniere in Westland in 1938, and between then and 1953 recovered 175,000oz of gold from the Hokitika area. In 1957 it was relocated to the Taramakau River, from which it recovered 202,000oz until being laid up in 1978.

With the gold price rising in the early 1980s, the Australian mining company Giant Resources bought the Kanieri (it was always spelt that way rather than with an “e” on the end like the lake and river of the same name) and dismantled it. The salvageable machinery, including the pontoon, was then used to build a new state-of-the-art dredge, costing $30 million, at Ngahere. For all the money that was sunk into it, the new dredge didn’t work too well, and in 1989 was mothballed after only eight months.

Giant Resources itself then crashed into receivership and was de-listed and de-registered from the Australian Stock Exchange in late 1990. Amid dire local predictions that he would follow Giant into oblivion, Birchfield bought the dredge from the receiver. He stripped out much of the fancy gear and got it working efficiently again in 1992, and kept it operating round-the-clock for the next 12 years.

In 2004 the Kanieri was laid up again, but booming gold prices saw Birchfield seek new resource consents and crank the old monster back into life. Its current consents could see it working for a further 20 years.

Bucket-dredging for gold on the rivers of New Zealand was pioneered by Charles Sew Hoy, a Cantonese miner who had successively chased the Calfornian, Victorian and Otago rushes after being born about 1837 in the village of Sha Kong in Guangdong Province, China. He arrived in New Zealand in 1868, and a year later opened an importing and merchandising firm in Dunedin, serving the first waves of Chinese miners who had arrived two years before him.

The business thrived, and in 1887 he bought 56 hectares of the gold-bearing Big Beach flat on the Shotover River and formed the Shotover Big Beach Mining Company, whose major asset was a dredge he had had specially built for mining by Kincaid and McQueen of Dunedin. The design was based on the dredge that kept Dunedin harbour open to shipping, and it could work both the riverbed and the flats. It proved so spectacularly successful that it sparked a gold-dredging boom.

Spoon-type dredges had been tried with mixed success on the Clutha River as early as 1863, but Sew Hoy’s bucket dredge became the prototype, and by the turn of the century there were no fewer than 228 of them in Otago and Southland, and more being built all round the world. There were about 150 of them on the West Coast, and some were also used in the North Island, notably in the Firth of Thames into which gold had been washed from the Waihi fields by the Waihou River.

Birchfield’s Kanieri was the result of a second dredging boom that began during the Depression years of the 1930s and lasted until the mid-1950s. The new dredges were much bigger and more powerful than Sew Hoy’s, and the Kanieri was typical of them.

And if the gold price continues rising the way it has been, it’s quite possible there could be a third boom in the use of dredges for mining.