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The Pamphlet Collection of Sir Robert Stout: Volume 70

(a) Volcanic Series of Auckland

(a) Volcanic Series of Auckland.

Commencing with the northern portion of the colony the reefs of the Auckland provincial district first claim attention, and with the exception of an isolated patch near Wellington, where quartz reefs have been worked in the Maitai slates (Carboniferous), the North Island contains no other gold-field,

Gold was discovered in Auckland in the year 1852, but from the first the progress of the field was much retarded by opposition from the native owners of the soil, and the subsequent discovery of alluvial gold in great profusion in the South Island caused an exodus of the mining population, which was further intensified by the breaking out of hostilities in the year 1863. In 1867, however, peace having been restored, arrangements were made with the Maories for the opening up of the land, and on July 30th the gold-field was proclaimed. The first results were disappointing; instead of the rich alluvial deposits which had characterized the South Island, but little ground of this nature was found, and when at last a rich leader of quartz was discovered, the gold proved to be of very poor quality. This peculiarity still remains, the bullion yielding generally 54 to 75 per cent, of gold and 40 to 20 per cent, of silver, In spite of these early drawbacks the reefs have turned out fabulously rich; in some cases the yield has been for considerable distances at the rate of 600 ounces to the ton. So far from becoming poorer, as they have been worked to deeper levels, these lodes have maintained their yield, and It was the opinion of scientific men well acquainted with the fields that, as operations were continued to a greater depth, the supply of gold would not only be kept up but would probably be increased, Until recently the claims have depended principally on the rich leaders of quartz which occur, but of late years more attention has been paid to the large bodies of stone, popularly called barren or buck reefs, which exist. These are of enormous size, and give from 3 dwts. to 8 dwts. to the ton, with the possibility of opening up rich leaders, and there is no doubt that with improved appliances they can be made to pay handsomely.

The extremely complex nature of many of the ores renders their treatment very difficult, and year by year improvements in this direction are being made. The ordinary crushing battery and amalgamation processes are practically useless; before the tailings at Te Aroha were preserved for treatment the lowest estimate of the annual loss was £20,000. At Waihi the tailings were found to be worth £15 per ton, page 5 and for many years they were permitted to flow away without any regard to their value. The ones consist of a mixture of silica and various compounds of iron, copper, lead, zinc, antimony, arsenic, silver, sulphur, tellurium, and gold, and no process short of either smelting or chemical treatment will suffice to free the gold from these metals which it is not desired to extract, Unfortunately, in the locality under consideration, fuel and fluxes are scarce, and nothing therefore remains but to separate the metalliferous portion from the matrix and treat the former by a variety of processes. Many plans have, with more or less success, been tried, and at present the Oassell Gold Extraction Company appear to have grappled with the difficulty. Their process depends on the solvent action of cyanides—usually, in practice, cyanide of potassium—on the gold ore in preference to the sulphides of the base metals with which it is associated. The gold Is subsequently precipitated by the action of porous or filiform zinc. The cost of this system is somewhat heavy, and ores ranging below £2 per ton in value cannot be remuneratively subjected to it, but the percentage of bullion obtained is very high, having, during 1890, at the Crown Company's mine at Karangahake, averaged 93 per cent, of the gold and 79 per cent, of the contained silver.

Generally speaking, the machinery in this gold-field is of a class which compares very favourably with that in use elsewhere in the colony. Self-acting surface, and aerial tramways are used for bringing down the quartz, and one of the latter consists of a single span, 1,090 feet in length. At Grahamstown the claims are worked several hundred feet below sea-level, and an 82-inches bull, direct-acting, condensing engine, capable of making a 10-feet stroke, was purchased and erected by the Government, at a cost of £50,000. The pumps are 24 inches in diameter, and the height of lift 500 feet. The cost of working this engine is £320 to £330 per month, with coal from kamo, delivered at 17s. 1½d, per ton. A Drainage Board manages the concern, the Borough and County Councils contributing £35 a month, while the balance is made up by thirteen mining companies.

Water is very plentiful on the field, and is largely used as a motive power. The Pelton water-wheel is generally employed to drive machinery either for crushing, treating, or illuminating purposes. Stamper batteries are used for reducing the quartz, which is frequently first passed through a stone breaker; berdans are very common on the field. Some of the battery plants are of considerable size, that at the Waitekauri Mine consists of 40 head of stampers, and the Wailn Company has recently put up a plant which is stated to have cost about £60,000.

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Wages in the Auckland district vary from £2 5s. to £2 8s. per week for miners, and the cost of living is low when compared with other gold-fields. There were 1,387 miners employed in 1800, or 299 less than in 1889; all these were Europeans, The yield of gold for 1890 was £125,760, and for 1889 £113,191, and the number of stampers in the former year was 599, The total quantity of gold entered for export to December 31st, 1890, was 1,689,357 ounces of the value of £6,122,173.

Under The Mining Companies Act, 1886, and Amendment Act, 1890, registered companies are compelled to publish (at their own expense) certain details with regard to their position and proceedings, This return for 1890 shows that there were during that year in the Auckland district, 91 companies, having a nominal capital of £1,710,730, £1,501,088 of which was subscribed, and £106,684 paid-up, while £295,232 is returned as the value of scrip given to shareholders, and £255,765 as the total dividends paid. If we deduct from these last figures £243,141 paid by five companies there is not much left among the other 86, but most of these were only registered in 1890.

The following table gives the yield from the northern gold-fields:— District. No. of Tons of Quartz or Mullock Crushed or Sold. Yield of Gold in Ounces. Average Yield of Gold per Ton in Ounces. Coromandel, April 1st, 1880, to March 31st, 1891 20,751 66,069 Oz. Dwt. Gr 3 6 8 Thames (including Ohinemuri, up to 1886-1887) April 1st, 1878, to March 31st, 1891 503,144 594,991 1 3 15 Ohinemuri, April 1st, 1887, to March 31st, 1891 20,858 28,563 1 7 4 Te Aroha, April 1st, 1883, to March 31st, 1891 40,320 47,286 1 3 11 Totals 85,073 736.909 1 5 4

This yield is obviously extremely rich when contrasted with many other reefs in different parts of the world. In Victoria, for instance, during the years 1884-85, the average yield from all the quartz-reefing districts was less than 10 dwts. to the ton.

The geology of this neighbourhood and the occurrence of the reefs are of extreme interest. The rock forming their matrix is of volcanic origin, and of a felsitic character, decomposed in parts and freely pyritous.

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It has been called tufanite by Hector, who considers it of Cretaceo-Tertiary age. The whole formation rests unconformably upon the upturned edges of the slates which form the basenient-rocks of the Cape Colville Peninsula. Mr. Cox, who devoted great attention to the mode of occurrence of the gold, has come to the following amongst other valuable conclusions:—that the steepest parts of the reefs are usually the richest; in order that the reefs may be gold-bearing, it is impeartive for them to be passing through a certain class of ground; and a moderately hard tufaceous sandstone has been found to be the most productive.

As is usual in metalliferous mines, ventilation is not a very important consideration, but the water contains free sulphuric acid, and as ealeite is plentifully associated with the reefs, large volumes of carbonic acid gas are produced, which occasion difficulty and not infrequently danger. This free acid acting on decomposed marcasite and pyrite gives rise to the formation of stalactittc melanterite, which is an interesting feature hi the underground workings, but has not been commercially utilized.