Dredging Silt and Winning Gold

Ohinemuri Regional History Journal 52, September 2008

By Ollie Richardson BA, Dip Jnlsm, Waihi.

In 2004 the possibility of dredging the Coromandel Harbour to deepen it and pay for new wharf facilities and flood protection works by reprocessing the gold-rich silt was aired in the local press.

However, reprocessing this silt (mining tailings) to extract gold left behind by early mining methods was not a new idea. From the late 1800s to the end of the first decade of the 20th century such an operation existed in Waihi and was then transferred to Paeroa. Why was (and still is) the silt considered so rich and why did this practice of reprocessing not continue?

In the early days of the first Waihi mining phase, which began in earnest in the early 1880s, the old miners blasted in tunnels underground, shovelled the gold bearing quartz known as 'ore' into wagons and brought it to the surface.

Then it was taken to the stampers where it was crushed. The stampers, heavy iron blocks which were lifted by cams, fell hard on the ore and were noisy. The ore was crushed until it resembled sand. It was mixed with water and the slurry was then run over a copper plate which had been coated with liquid mercury.

Waihi Paeroa Gold Extraction plant, Waihi

The gold and silver in the slurry amalgamated with the mercury creating an amalgam. However the process was not entirely satisfactory as only 67% of the gold and only 5% of the silver was won. [amalgamation with mercury worked reasonably well on the Thames and Coromandel fields, but not at Ohinemuri. 25 – 30% was the recovery at the Martha Battery on the Ohinemuri River – E]

The first of these treatment plants was operating by 1882. Soon there were many gold mining companies, each with their own battery. Martha, Union, Rosemont, Silverton to name just the bigger ones, operating between 1882 and 1889. [not so. Only two batteries in this period – E]

In the late 1880s ore roasting kilns were constructed to aid gold recovery. On being brought up to the surface, the ore was transferred to these roasting kilns for drying and part-calcining. By slowly burning wood with the ore, it dried out and became more friable and this made crushing easier resulting in less wear and tear on machinery.

Nonetheless, gold and silver recovery remained at the same levels. [the Waihi Battery at Union Hill was the first to install ore roasting kilns, and they used hot pan amalgamation, which gave a recovery of about 60% of the bullion – E] The handling of the dusty kiln-dried ore was also a health hazard and contributed to the miners' disease called Phthisis or silicosis or 'coughing disease' which caused many premature deaths.

The roasting kilns were not a sustainable method of treating the ore as wood to fire the kilns became increasingly scarce because trees were felled indiscriminately to feed them.

The Cyanide Method.

In 1894 [? – E] it was discovered that treating the crushed ore with cyanide would dramatically increase the yield of precious metals. The new method was trialled in the nearby Crown mine in Karangahake in 1896 [1889 – E]. The trials were not proved over anything bigger than small batches rather than on a commercial scale but the method did work.

Once the method was perfected it meant a gold recovery of 92% rather than 67% and silver recovery of 60% instead of 5% . It took a while for this new process to catch on as the patent holders charged a royalty to use their method, which made it not as cost effective to use.

It was the waste or 'tailings' of the early mining phase, still loaded with gold and silver, that the men who worked on this site planned to recover.

In the late 1890s the New Zealand Government bought the rights to the cyanide method on behalf of the mining companies and this tipped the balance in favour of its general use.

Once the cyanide treatment was fine-tuned the smaller operations which had languished became more viable and were bought up by the English funded Waihi Gold (and Silver) Mining Company which was to become the dominant employer in the town and last until 1952.

The Waihi Dredging Company was formed in 1897 by E H Barber with a capital outlay of £5000. He used a primitive drag line to try and recover the rich ore tailings dumped in the Ohinemuri River. Success was limited. In 1902 the site had a new owner called Mr Rich who renamed the Waihi Dredging Company the Ohinemuri River Claim.

By late 1903 Messrs Brown and Thompson bought the plant and renamed it the Ohinemuri River Syndicate. The business changed hands again in 1908 and became known as the Waihi-Paeroa Gold Extraction Company.

Brown invented the Air Agitation Tanks and applied for a world wide patent. The business was not an instant success. Brown and McMillan, over the next few years, perfected the air agitation within the cyanide tanks to give maximum recovery of precious metals and the tanks became known within the gold mining industry as B & M (or Pachuca) tanks.

By 1904 the first tube mill, to crush the tailings ever finer, was erected on the site.

Mine Inspectors' reports

In his March 1903 report for the year ending 1902, Mines Inspector James Coutts said that the Ohinemuri Syndicate "has taken up several claims in the Ohinemuri River with the object of treating the tailings that have been allowed to run away from the Waihi and Waihi-Silverton batteries. They have carefully sampled the tailings, and the results obtained have been of such an encouraging nature that an up-to-date plant for treating the tailings has been designed and is in course of erection between Waikino and Waihi. This should be completed early in the year."

On May 18, 1903 Mr Bush in his Warden's Report for 1902 confirms:

"This is a private enterprise and the promoters have spent between £3000 and £4000 on the plant above. It is estimated that 10 men will be sufficient for the work. About 20,000 tons of tailings are supposed to be accumulated in the dam alone, so that if they are payable the syndicate should be successful in their enterprise. About 50 tons will be treated daily."

However the hopes of easy riches for the early entrepreneurs were not to materialise quickly. Reports of both the mines inspector and the warden at Thames Warden office for 1904 report that the work done in 1903 proved unsatisfactory.

The tailings had to be ground finer still, into 'slimes' to pass through a 200 mesh sieve, and treating this exceedingly fine material meant that perfect agitation in the cyanide tank was essential. Months had been spent in experimenting to devise the best method for doing this.

By 1904 the method had been perfected and 2000 tons of tailings had been treated. Ten men had been employed during that year. To increase the capacity of the current plant so that 50 tons of tailings could be processed daily, alterations in machinery became necessary which meant that further capital needed to be raised.

Waihi-Paeroa Gold Extraction Company's plant on the Ohinemuri River, 1910-1917

While this was in process 1905 and 1906 saw the plant lie idle. Nothing much happened until 1907. By 1908 new machinery had been installed and considerable work was done that year.

Thirty men were employed. In 1909 the company worked continuously with an average of 23 men employed. All machinery worked satisfactorily and 23,950 tons of tailings were treated. The company began to erect a new plant in Paeroa so that it could also treat the tailings from the Karangahake mines downstream.

Last figures for the Waihi site were given for the 18 months preceding March 31, 1910 when the company obtained bullion to the value of £14,000 from 32,000 tonnes of tailings treated.

On March 31 of that year the Ohinemuri River, owing to exceptionally heavy rainfall - 14 inches in 16 hours, rose 20 ft and washed tailings from the bed of the river. This was the death knell for the Waihi operation as the floodwaters brought with them tonnes of barren tailings from further upstream and the company decided to remove their plant to Paeroa forthwith.

At the end of April 1910 tenders were advertised for to dismantle the Waihi plant and remove it to Paeroa.

Paeroa folk pleased

Paeroa residents were pleased the Waihi-Paeroa Gold Extraction Company was coming to their town as once the plant was operating there, many tons of tailings would be taken from the river and this would hopefully clear the river channel thus saving them from the periodic flooding the town had been subject to.

What the Paeroa townsfolk did not realise was that once the tailings had been treated they were returned to the river. When the intended fate of the tailings was realised by the Paeroa townsfolk this became the subject of several heated public meetings, eventually ending with a Royal Commission which began in the Paeroa Courthouse on 23 May, 1910, chaired by Mr William Ferguson of Wellington.

Much evidence against the company was given by legal counsel for the farmers, by Maori representatives and even James Mackay. The Commission then inspected the Thames harbour, the Waihou and Ohinemuri Rivers, followed by a visit to Waihi to see the silting problem for themselves.

However the evidence did not sway Central Government and in December 1910 a bill was passed which maintained the company's right to discharge treated tailings back into the river.

I am indebted to the following sources: Ohinemuri River of Gold by Colin Townsend, published by Patricia Townsend, Timaru, 2002, and Eric Lens of Waihi.