Art. XL.—Notes on the Geology of Kuaotunu Goldfield.
[Read before the Auckland Institute, 19th June, 1893.]
It has often been asserted that the Hauraki Peninsula is a goldfield from one end to the other, and the numerous discoveries of the last few years would certainly seem to justify this conclusion. The wide gaps which at one time existed between the older-established goldfields are being gradually filled up, while the boundaries of all the fields are being continually extended.
Perhaps the most important find of late years was the discovery of payable gold at Kuaotunu, a little over three years ago. Since that date a large amount of money has been expended in the preliminary work of prospecting and opening the mines, erection of batteries, and other necessary works, and as a result this field has now taken its place as a steady gold-producer.
Kuaotunu is situated on the east-coast side of the peninsula, on the neck of the short peninsula lying between Mercury Bay and Whangapoua Harbour. It is connected with Coromandel by a bridle-track, and there is regular communication with Auckland by a steamer-service twice a week. The port suffers the disadvantage of being an open roadstead; and this is a difficulty which it will be very costly to remedy. The water-way has, nevertheless, been an inestimable boon to the place, and it is doubtful if without this it would have been able to survive the troubles which beset the early stages of its existence.
General Geological Features.
When first I examined this goldfield I was led, from the surface indications, to the belief that its geology would be found to present but little to distinguish it from the geology of the Thames, Coromandel, and other well-known goldfields in the Hauraki Peninsula. A detailed examination on a subsequent occasion showed that, including the Matarangi district, two distinct geological formations are represented on this goldfield. Both contain gold-bearing quartz veins. The younger formation consists of decomposed andesitic tuffs and breccias, which do not occur within the drainage-area of the Kuaotunu River, but are largely developed at Matarangi, whence they extend almost continuously to Coromandel to the west, and Mercury Bay and Makarau to the south.
At Kuaotunu proper the older rocks—of probably Palæozoic age—consist of an upper series of highly-indurated sandstones and greywackes, and a lower gold-bearing series of clay-shales, diabase-ash and breccia beds. The former occupy the low spurs and ridges which form the western boundary of the Kuaotunu basin, while the latter descend northwards from the sources of the river, and form the high bush-covered ranges on the east side of the river-valley.
Classification of Formations.
Recent.—Sand-dunes, alluvial flats, and swamps.
Lower Tertiary.—Andesite tuffs and breccias.
Palæzoic.—(a.) Sandstones, greywackes, &c.
(b.) Clay-shales, diabase - ash and breccia beds.
The sand-dunes extend along the coast from Maori Point to the mouth of the Kuaotunu River, a distance of over a mile. They seldom rise to a height exceeding 25ft. On their inland side they are bounded by a wide extent of low-lying flat and swamp land, which follows the course of the river for over a mile and a half. In places the swamp possesses a width of almost half a mile, but in general it is widest at the lower end of the valley, and gradually narrows in the upper part until it ends a little above the Try Fluke battery.
Lower Tertiary.—Andesite Tuffs.
When travelling from Coromandel to Kuaotunu this formation is first met with near the summit of the main range, on the slopes looking towards Whangapoua Harbour. From there it extends eastwards to Owera and Matarangi, near Kuaotunu. The tuffs decompose readily into yellow and yellowish-brown clays, but in general physical characters they are undoubtedly closely related to the gold-bearing tuffs at Coromandel and the Thames. Their finely-stratified appearance in places would tend to the belief that they were of pyroclastic origin, the materials of which they are composed having probably been derived from submarine or maritime volcanic eruptions of a widespread and violent character.
The coarse andesitic breccias which are everywhere associated with the finer tuffs consist of large angular and rounded masses of hornblende-andesite, enclosed in a matrix of grey or yellowish coloured ash or tuff-like material. No solid flows of andesite were observed in this district associated with the gold-bearing tuffs and breccias.
This district affords no evidence as to the age of these gold-bearing rocks. They rest unconformably on a highly-
denuded surface of the Palæozoic sandstones and greywackes, while no elastic rocks of younger date are found overlying them.
The scarcity of evidence relating to the age of these tuffs is a noticeable feature of the geology of the Hauraki Peninsula, and this circumstance is solely due to the almost entire absence of members of the numerous fossiliferous formations which in other parts of New Zealand render the geological structure so varied, and very frequently so involved and complicated.
The only evidence bearing directly on the age of these rocks, so far as known at present, is found at Waitete, situated on the coast-line a few miles south of Cabbage Bay. Two years ago, when making a reconnaissance geological survey of that part of the coast, I discovered a small patch of the New Zealand brown-coal measures, occupying an area not many square chains in extent. They consisted of the following strata, reading the section downwards:—
1. Hard shelly limestone.
2. Calcareous and marly sandstones.
3. Ferruginous conglomerates.
The conglomerates were about 200ft. thick, and rested directly on the basement rocks, which at this point consisted of blue- and red-banded slaty shales. The shelly limestone, which was the highest and closing number of the series, dipped away to the north-east, and a few chains back from the beach disappeared below a great accumulation of volcanic tuffs, breccias, and solid lava-flows of an andesitic character. These rocks, so far as could be judged from physical characters and general appearance, were in every respect similar to the gold-bearing tuffs and associated rocks in other parts of the peninsula.
On a subsequent occasion I traced these tuffs and breccias without a break as far as Paparoa and Paul's Creek, and thence southwards to the Tokatea Range near Coromandel. Another circumstance which tends to prove their identity with the tuffs and andesites of the Thames and Coromandel is the discovery in them of gold-bearing veins of quartz in the neighbourhood of the limestone deposit.
The Palæozoic rocks on which the coal-measures rest are in several places in the vicinity of Waitete intruded by massive dykes of igneous rock. It is a noteworthy fact that I was unable to find, after a most careful examination, a single fragment of igneous rock included among the materials composing the conglomerates. This negative evidence is of great value as tending to prove that these igneous intrusions took place after the deposition of the Cretaceo-tertiary coal-beds. The whole of the stratigraphical evidence obtainable at Waitete
points to the Post-eocene age of the Thames and Kuaotunu tuffs, which can be traced almost continuously to Coromandel on the west and Te Aroha on the south.
In connection with their economic importance, it is interesting to note that they are the youngest gold-bearing rocks in the Southern Hemisphere, being younger than the gold-bearing rocks of Otago, Reefton, and the different goldfields of Australia, by the whole of the Secondary epoch and the upper part of the Palæozoic. Even in composition and origin they stand unique, and their homologues are found only in two countries in the Northern Hemisphere—namely, Transylvania, in Hungary, and the Pacific States of America; and in these countries the similarity extends also to their gold and silver contents, which are frequently as refractory and difficult to treat as ours, while their free-milling bullion is alloyed with silver to the extent of about 30 per cent., as it is throughout the Hauraki Peninsula.
(a.) Greywackes and Slaty Breccias.—These rocks occupy the wooded spurs and ridges on the west side of the Kuaotunu River, and form the broken rocky headlands and islets between Kuaotunu and Matarangi, and the steep precipitous sea-cliffs north of the mouth of the river.
They consist of hard siliceous greenish and grey-coloured sandstones, interbedded with hard blue slaty breccias and occasional bands of slaty shale. They are always much jointed and shattered, and often streaked with thread-like veins of quartz or hæmatite. Up to the present time no vein containing payable gold has been found in them.
(b.) Clay-shales, Diabase-ash and Breccia Beds.—The clay-shales occupy the lower slopes of the spurs on the east side of the valley, and are well exposed in many of the road-cuttings. Their strike varies from north to north-east, and their dip is always, so far as can be made out, to the eastward. They are soft and crumbling, and form red, yellow, and brown clays. In many places on the spurs behind the township they contain large irregular segregated masses of grey chalcedonic quartz, often streaked or brecciated. On the range between Kuaotunu and Otama there are several very large deposits of this kind of quartz cropping out and forming conspicuous objects in the landscape.
At Otama and Opito the clay-shales are intruded by large masses or dykes of black hornblende-andesite, forming high isolated hills with rounded outlines and steep black sides.
Besides chalcedonic quartz, the clay-shales also contain veins or reefs of crystalline quartz which in many places have been proved to contain payable gold. Among these may be
mentioned the Waitai, Otama, Maori Dream, and Black Jack Mines.
Immediately overlying the clay-shales come a series of diabase-ash and breccia beds. These are principally developed in the high, conspicuous, abruptly-ending ridge which lies between the two main branches of the Kuaotunu River. This ridge extends from the “Junction” at the Red Mercury battery to the sources of the river.
As seen in the low-level drives of the Red Mercury and Great Mercury Mines, these rocks consist of a series of greenish-grey diabase-ash and breccia beds, interstratified with smaller beds of a dark-grey slaty shale and slaty breccia. The ash and breccia beds decompose most readily into red and brown clays, and in the shallower parts of the mines their true nature cannot be determined. In the solid they are intensely hard and tough, and their presence has added greatly to the cost of the development of the reefs in the Red and Great Mercury Mines at the low levels. They are traversed by the Try Fluke, Red Mercury, and other parallel reefs, which follow their course and underlie.
These diabasic rocks are not found on any other part of the peninsula, and their presence here as gold-bearing strata may indicate a greater persistence of the reefs than has been the case in the other goldfields of the Hauraki district. The well-known Try Fluke reef, enclosed in these rocks, has been traced through the leases of the Kapai, Try Fluke, Carbine, Red Mercury, Great Mercury, and Irene. It possesses well-defined walls, and varies in width from 2ft. to 20ft. Its average width is probably about 6ft. Its course is N.N.E.—S.S.W., and its dip easterly at angles seldom under 60°, and more often over 65°. It has been proved to continue downwards in the deepest workings so far undertaken upon it.
The nature of the quartz varies in different parts of the lode. In some places it is hard, cavernous, and stained black and brown with manganese-oxides; in others it is mullocky, friable, and crumbling, and stained rusty-brown with iron-peroxide. At the outcrop, near the Red Mercury low-level drive, the lode-matter possesses a pure white colour, and occurs in peculiar tabular bundles made up of thin layers or laminæ of friable quartz, which look like pseudomorphs after some of the heavy earths.
The gold is alloyed with about 35 per cent. of silver, and it exists principally in an extremely finely-divided state. The patches of rich quartz which are so characteristic of the reefs at the Thames and Coromandel are not known in this reef or in any other reef in this district.