Waihi Borough Council Diamond Jubilee Booklet 1902-1962
By G. H. WORTH
In this short write-up I will try to describe the terrific impact the cyanide treatment had on the fortunes of the Martha Gold Mining Co. [Waihi Gold Mining Company – E], the town of Waihi, and surrounding districts in general. It was the answer to mining men's dreams, and revolutionised the whole concept of quartz mining, particularly refractory and other sulphide ores. Before the cyanide process was introduced the Martha Mine was in the doldrums, not paying its way and in danger of closing down. Under the old method of treatment, although the assay value of the quartz was reasonably high, the extraction rate was only about 7/6 per ton. The quartz being complex and containing sulphides would not respond to quick-silver treatment. Also the quartz was not ground finely enough.
About this time a new method of treating quartz began to interest mining men. It was known as the cyanide process and it was reputed that it would treat sulphide ores successfully. The pioneers of the cyanide process in Waihi was the Cassel’s Company. They erected a pilot plant on a flat at the southern end of the Union Hill and began the treatment of tailings from the Union Battery [the Cassel Company erected a tailings plant, below the tailings impoundment’s of the Waihi Battery at Union hill – E]. The Martha Company [Waihi Gold Mining Company – E],, knowing that it was losing a lot of gold and silver in treatment, had paddocked thousands of tons of tailings in three paddocks just to the south of the Union Battery [Waihi Battery – E]. It has been said that all residues went to waste in the Ohinemuri River, but if an estimate of 18,000 tons treated is correct, then very little of the rich tailings of the Martha Company was lost in the river. Of course, a certain amount was, but this was unavoidable. A logical reason for the paddocking of the tailings was that the company was hoping that a method of treating the Martha ores would eventually be found, and it is possible that Martha Company being an English company, with shrewd directors, with their fingers right on the pulse of mining, was well aware of the fact that a new process was being developed. (Actually it was first used in New Zealand at Karangahake). They certainly welcomed the Cassell’s Company, with open arms when they brought the process to Waihi [no, they didn’t – E].
By the time the paddock was finished, it had proved a tremendous success. My father told me the recovery of quicksilver alone paid all expenses of the Cassell’s Company, leaving the gold and silver as profit. It also solved the problem of the Martha Gold Mining Co. [Waihi Gold Mining Company – E], in respect of complex ores, with special emphasis on sulphides. From that time the Martha Co., never looked back and soon converted the Union Battery [Waihi Battery. Cyanide process established in 1894 – E] to the new process. The new treatment, being a wet treatment [no, the Waihi Battery remained a dry crusher until 1902 – E], automatically did away with the curse of dust, from which scores of men had died, having inhaled dust in the old dry crushing days and developed Miners' complaint.
The main secret of ore extraction is fine grinding, and to give the cyanide a chance to do its work properly, which is, of course, the dissolving of the gold and silver into liquid form, it became necessary to find some method of grinding the quartz much finer than was possible with stamper grinding. This was solved by the introduction of tube-mills. These were huge revolving steel drums partly filled with flint pebbles. As the ground quartz came from the stamps it was introduced into the tube-mills, which ground the quartz until it was practically liquid, and let the cyanide do its work properly. The ground-up quartz [the cyanide solution minus the ground ore – E] containing gold and silver in solution was then passed over zinc shavings to which the gold and silver adhered, and it was then a matter of retorting at the refinery. The introduction of the cyanide treatment meant new life for the Martha Gold Mining Company [Waihi Gold Mining Company – E], and the despised sulphide ores really came into their own, as they were richer in values than the oxidised ores.
Now the Martha Company could not get enough sulphide ore and where ever it could be worked men were busy stoping it. Previously, tens of thousands of tons of sulphide ore had been left and only the oxidised ore removed. But the Martha Company was lucky. The sub-strata of the mine subsided and hundreds of thousands of tons of sulphide ore began to crush down. After it settled the Company found new methods of working the crushed ore and it became known as the "Milking Cow". I would say, at a rough guess that the best part of a million tons of ore came out of the "Milking Cow" and I would also venture to say no cheaper ore was ever won from an underground mine. With development, winning the ore, trucking to No. 6 shaft and delivering to Waikino Mill, the cost would be no more than seven or eight shillings per ton. This included thousands of tons of surface filling put down the mine under the old system of stoping to fill in where the quartz was taken out. The filling went down the mine not worth a penny a ton, but when trucked out again was worth between two and three pounds per ton. This came about by the grinding and pounding of the crushed sulphides impregnating the mullock filling.
The approximate position of the "Milking Cow" is under the lake which has now formed on the Martha Hill, a home for trout and wild duck. Almost the last chapter in the Company's history was the shovelling up and delivering to the Waikino Mill of the two paddocks of tailings that had lain overgrown with vegetation of all kinds for 60 to 70 years.