Ohinemuri Regional History Journal 21, June 1977
By GARY STAPLES
[for previous article see Journal 20: Karangahake Goldfield (continued) - E]
The extraction of gold and silver from ore is in itself a technical subject and only the aspects that were particularly relevant to the Karangahake mines are covered in this article.
Gold and silver at Karangahake was generally difficult to extract because it was combined with other minerals. Even the gold bearing rock (ore) was not easily mined and there was little gold which could be panned, none of commercial value. The recovery rate from the early batteries such as the Hauraki and Ivanhoe was very low, often only 40 - 50%. The value of gold and silver washed down the river with the waste crushed rock (tailings) was as high as £3 worth per ton and in the 1880's £3 was a lot of money to lose in each ton of rock mined.
After the Woodstock took up the ground previously held by the Maria Company, they soon realised that the ore was not suitable for ordinary treatment. About this time, the New Zealand Herald printed an article about the difficulties being met in treating ore at Karangahake. This article was noticed by Mr. J.D. Le Monte [La Monte – E] who was at that time en route from San Francisco to New South Wales where he had a contract to erect furnaces. Mr. La Monte, accompanied by a Mr. Davie and Mr. Humphries, visited Karangahake on 24 May 1885 and made a careful examination of the reefs in the Woodstock and Crown areas. It was his opinion that his furnaces would satisfactorily treat the ore. A furnace capable of treating 150 tons per week would cost £2500 to build and treatment was expected to cost £2 per ton. After holding meetings the mining men formed a syndicate which met Mr. La Monte again on 1/6/85 and purchased the patent rights for the whole of New Zealand at a cost of £10,000. In order to obtain the necessary funds to build the furnace the owners of the Woodstock claim decided to form a company. This was done with Mr. John McCombie as General Manager.
The erection of the furnace commenced in July 1885 on a site opposite the Karangahake township and just down stream from the present "Gold Camp". A ninety-five per cent recovery rate was claimed for this process but because of the high percentage of silica in the ore the process was unsatisfactory. The ores that had been treated overseas in La Monte Furnaces contained a good deal of base metals but because the Karangahake ore lacked this a lot of flux had to be used to make the slag run freely. Only 38 tons of ore was treated which returned 600 oz. of bullion but the cost of smeltering was £13-5-0 per ton. The cost of flux was process's downfall and the furnace was not used again.
Following this failure, Mr. J. Railey, an engineer built a battery of 10 stampers. It was erected in November 1886 one and a half miles up the Waitawheta River on a small flat just beyond the gorge. Briefly, his process consisted of crushing the ore with the stampers then passing the ground ore into large pans, 4' 8" in diameter and 18" deep. Mercury was added to the pans which also contained 12 grinding shoes. The pans were covered and a jet of steam passed through the cover to maintain a temperature of 2100 F. More mercury was added during the grinding process and after three-quarters of an hour the pulp was run into separators so that the mercury could be collected and later retorted. Ore from many Mines was treated but the recovery rate was only about 45% and expensive. Proving a failure the plant was dismantled in 1888.
In 1888, a Mr. Chambers had another smelting works built at Karangahake, the patents of this process belonging to the Parke's Gold and Silver Ores Smeltering Co., London. For a while many mining people had great hopes as Mr. Parke estimated that the fluxes would cost 12/6 per ton but it was found that, in fact, the cost ran between ₤18 and £20 per ton. One ton - 12 cwt. of ore from the Eclipse Mine, Waitekauri was bought by Mr. Chambers in 1888 for £30 but he could not extract the bullion. So ended another hopeful idea. The only other battery operating during this time was the old one belonging to the Ivanhoe Company. It was very out-dated and using the usual battery process was making poor recovery rates.
In Glasgow in 1886 experiments led to the perfection of the MacArthur-Forrest process for dissolving the gold and silver content of pulverised ore in potassium cyanide. It was the introduction of this process to the Karangahake Goldfield (1889) enabling the Mines to prosper. In view of the very considerable importance of the cyanide process a brief outline is given here.
Early methods of recovery had relied on golds strong attraction to mercury. In this method the very finely crushed rock was washed over tables of copper, covered in mercury. The gold was attracted to mercury which could later be retorted away leaving the gold. Another method was to concentrate the gold mechanically by panning on a larger scale by vibrating tables. These methods were still often used in conjunction with cyanide process. Potassium cyanide is a solvent of gold and silver, and is able to dissolve the gold out of the finely crushed rock. The solution thus obtained was then run over zinc turnings and the gold came out of the solution as a black mud. This precipitation of the gold was the result of chemical reaction between the cyanide and zinc. The gold was later refined whilst the cyanide was also recovered and used again. This description is only a simple outline of what took place. The plant required was extensive and a detailed description was not within the scope of this article.
We will now look at the 3 large batteries: the Woodstock, Talisman and Crown.
Crown Battery.
At first the Crown Mine ore was treated at Railey's Battery. When this was dismantled in 1888, the Crown Company erected a crushing plant on the same site. This consisted of 2 Lamberton Mills. The crushed ore was then treated by the Cassell [spelt: Cassel – E] Company who held the patent rights to use the MacArthur-Forrest potassium cyanide process. It was in 1889 that the process was first used in New Zealand and that was to treat the Crown Mine ore.
The original crushing plant was unsatisfactory as was the rather inaccessible site, 1½ miles up the Waitawheta River. Crushing had stopped for awhile during 1890 but resumed in July 1891, only to stop again in August when a flood washed away a dam which supplied the water power to drive the mills. This was repaired but finally after crushing a total of 614 tons of ore the dilapidated machinery broke down beyond repair in August 1892. So different were the trials and troubles of these early years from the success which was to come soon.
By 1892 there were 6 cyanide plants on the Ohinemuri field plus others at Thames and Coromandel. The process was a great success as recovery of gold and silver was 85 - 93% instead of the previous 40 - 50% by earlier methods. The Crown Company purchased the local rights to the cyanide process in 1892. They paid £666. 16. 0 for the plant and over £15,000 for the right to use it. Later the Government purchased the patent rights as small concerns were unable to pay the fee demanded by the Cassell Company.
In 1891, work commenced to erect a new Crown battery near the site of the present rail/road bridge. This was the well known Crown battery, the massive concrete foundation of which can still be seen. The ore was bought from the Mine by horse-drawn tramway trucks. Initially, the battery consisted of a Lamberton rock breaker and 20 stampers plus cyanide plant. Thirty tons a day could be treated but progressively the battery was extended until by 1911 one hundred tons per day was being treated. By 1898 there were 60 stampers in operation all driven by water power (Pelton wheels). In 1902, an auxiliary steam engine was installed to keep the battery going in dry seasons. Further boilers were erected in 1909 to drive a 550 HP steam engine and generator to produce 550 kw of electricity. This power house was erected near the Government railway line and commenced operations on 10 July 1909. The tunnel through which the ashes from the boilers were discharged can still be seen near the river level. The Crown Battery ceased operations in 1916.
Woodstock Battery.
This battery consisting of 10 stampers, purchased by Mr. H. Adams from Waiorangamai [Waiorongomai - E], was erected at the base of Taukani Hill in 1894. Originally, the battery crushed the ore dry but after being enlarged to 40 stampers in February 1897, changed to wet crushing. Many batteries in the early 1890's crushed the ore dry after roasting it in large kilns, but they all changed to crushing wet. This saved the expense of roasting the ore, reduced the dust, and enabled improved cyanide treatment methods to be used.
Further improvements to the battery were carried out in 1901 when a new pipe line to drive the pelton wheel was installed. In 1903, an auxiliary steam plant was installed which drove a 250 HP Corliss engine.
After the Talisman Consolidated Company took over the Woodstock in 1904 the battery building was used to house further compressors. In late March 1910, the battery was damaged by flood waters and then on 10 September of the same year, the buildings were destroyed by fire.
Talisman Battery.
The first battery consisting of 10 stampers and a cyanide plant was put into commission in 1895, but was most unsatisfactory and little used until 1897 after it had been remodelled. This battery was built on the side of Taukani Hill near the Waitawheta River. At first water power was used to drive all the machinery and the ore was crushed drive [dry – E].
A completely new battery was built and was opened on the 31/7/01 although regular crushing did not commence until mid-September. This battery used wet crushing, a significant change from the old dry method.
The new battery was designed by Mr. Alfred Curtis of Messrs Bewick, Moring and Company. Ore was received from the Mine, via an aerial tramway into an ore-bin of 220 tons capacity. The ore first passed through two Blake Crushers then after passing through a screen was delivered to the stampers of which there were 50. The cyanide plant was very extensive and in addition there were amalgamating plates which recovered 40% of the gold content before the ore was passed to the cyanide tanks. The complete cyanide process took about 6 days. Bullion recovered was cast into 1,000 oz bars.
During 1909 - 10 the battery was improved and 3 tube mills installed. They were brought into use on 12/9/1910. A new assay office and up-to-date smelting room was also built. In 1911 two grinding pans for grinding the concentrates were installed to deal with the concentrate which had previously been sent overseas for final processing. The Talisman battery ceased operations in 1918.
For a few years after the batteries closed the machinery was kept in repair as it was hoped new sources of payable ore would be found but during the early 1920's they were dismantled. In later years the remaining scrap iron was progressively removed until now all that remains are the massive concrete foundations hidden by gorse and tea tree.
During 1937 - 38, the Talisman/Dubbo Company erected a Mill on the site of the Former Talisman power House. This only operated a year or two and was then dismantled. The machinery was all driven by electric motors and it did not have "stampers" but crushers and tube mills. The cost of electricity, supplied by the local power board was considerably more expensive than the water power of the old batteries, thus contributing to its unprofitability and hence its closure.