THE GEOLOGICAL STRUCTURE OF-THE THAMES GOLDFIELD.

New Zealand Herald, Volume XXVIII, Issue 8508, 7 March 1891, Page 1 (Supplement)

THE GEOLOGICAL STRUCTURE OF-THE THAMES GOLDFIELD.

FUTURE PROSPECTS. [BY JAMES PARK, F.G.S., DIRECTOR THAMES SCHOOL OF MINES.] [Read at the meeting in Melbourne of the Australasian Association for the Advancement of Science.] Although over twenty-two yea'fs have elapsed since gold .was first discovered at the Thames, the geology of this goldfield jilways been a subject of much discussion among New Zealand geologists, and even at the present time' the most opposite and divergent views are held by different authorities, both as to the structure and true character of the rocks themselves. The mining operations of this field have so far been confined to an area little more than a . square mile in . extent, and as the more accessible and ireadily obtainable gold is being rapidly -worked out, the question of deep-sinking and going further afield must sooner or later claim the attention and serious consideration of mining men and those dependent upon the production of gold. It is abundantly evident that the time has arrived when the future prosperity of the field must depend upon the successful development of new ground, and in order to accomplish this, we must possess an intelligent knowledge of the structure or arrangement of the gold-bearing rocks, and the character of the reefs. When we are in possession of these, as well as the experience accumulated during twenty years of successful mining, wo will be in a position to direct prospecting operations so that they may be conducted in the most favourable places, and the money devoted to this purpose expended to" the best advantage. With this object in view, in the beginning of September of this year I commenced a detailed survey of the line of section extending across the field from Tararu to Hape Creek, the results of which are embodied in this paper. GENERAL STRUCTURE. The rocks of this goldfield, as disclosed by the above line of section which supplies the key to their structure, divide themselves into three distinct formations as follows :— 1. Slaty shales and silicious mudstones. 2. Felspathic and tufaceous sandstones, passing into breccias, with gold-bearing veins. 3. Coarse volcanic breccias and tuffs, | with coal and coaly shales at base. The slaty shales and associated rocks form the old floor of the district, upon which rest unconformably the two succeeding formations, between which no marked break or unconformity exists. Between Shellback and Hape Creeks, these younger formations are arranged as an anticline, the dome of which, formed by the coarse volcanic breccias and tuffs, has been largely denuded, thus exposing the gold-bearing series below. Near the core of the anticline, which is situated between the Saxon mine and the old Queen of Beauty shaft, the strata are inclined at high angles, being much disturbed, but towards the sides of the anticline they are lying flatter, the dip varying from 30 to 50deg. SECTION FROM UONEMILL CREEK TO HAPE CRKKK. The first rock seen on this line of section after passing Bonemill Creek is a tough, grey-coloured, felspafchic rock, containing nests and disseminated grains of iron pyrites. It decomposes to a great depth from the surface into a rusty-coloured tufaceous-looking rock. It is intersected by a number of parallel joints, sometimes showing slickonsided surfaces. The joints run N.N.E.-S.S. W., and are almost vertical.

About fivo chains south of the mill, following the beach in the direction of the Thames, this folspathi<?rock is followed by an intensely hard groeniah-coloured volcanic breccia, often containing largo rounded boulders or masses of andesite, numbers of \s;hich occupy tho beach for a further distance -of five chains. This breccia frequently passes imperceptibly into a finegrained green tufaceous sandstone, which, at a casual glance, might be mistaken for a solid lava or dyke rock. Like the underlying grey felspathic rock near the mill, these rocks decompose to a considerable depth into rusty-colourod sands and clays. About three chains past tho second point the breccias rest upon a highly-denuded surface of blue and grey-ribboned slaty shales. Near tho point of contact the breccias contain small rounded fragments of blue shale and jasperoid quartz, as well as numbers of largo well-dovoloped crystals of iron pyrites. The old rocks consist of hard blue and grey slaty shales, showing very distinct lines of stratification. Their strike is somewhat irregular, the dip varying from S.S.W. to W.S.W. angles of 26deg or 30 dog. They are exposed on the beach at highwater mark, their outcrop measuring about 60 by 100 feet. At low-water mark they are .seen to be interbedded with, and to pass upward into, grey and yellow-ish-coloured silicious mudsfcones, which dip S.S.W. at angles varying from 45 deg to 70 deg. The actual point of junction can be seen to great advantage on the, beach, between high-water and low-water marks. At their base the mudstones are of a bluishgrey colour in the solid, weathering to a pinSish grey on the surface. They are highly pyritous, and are interlaminated with thin layers of grib, consisting of small rounded particles of hard mudstone, mostly of uniform size. The ribboned shales contain similar grit layers near their upper surface.

Passing southward, fcho lino - grained silicious mudsfconos rise into steep rugged cliffs, and form Rocky Point itself. About three chains past the point they terminate abruptly, and are followed unconformably by very hard green tufaceous sandstones, containing small angular blue slaty fragments. At the mouth of Waiolianga Creek may bo rfaen largo boulders of these sandstones, containing angular masses of blue shale. These boulders were, no doubt, brought down by that stream, which cuts across the outcrop of the older rocks in the upper part of its course. On the beach near Waiohanga Creek the tufaceous sandstones are intersected by a number of parallel veins, many of which are filled with material derived from the enclosing rock. They strike N.E.-S.W., and are generally standing vortical. The two largest veins are four inches and six inches in width respectively. The smaller of these divides near highwater mark into two distinct veins, one of which joins the larger. A few chains further along the beach, still going in the direction of Tararu, veins of pyritous quartz and large segregated masses of culcite are plentiful. In the road cutting under Tararu Cemetery the tufaceous sandstones dip S.S.E. at an angle of 40cleg., and soon aftpr passing Tararu Creek the dip changes to the northwest, the strata thus forming a Syncline, by means of which the whole of the sandstones and breccias just described are again repeated before the boundary of the goldfield proper is reached. A few yards post Magazine Point the coarse volcanic breccias and associated rocks rest upon the upper member of the auriferous series, which is well exposed at) Kuranui Creek. There does not appear to be any unconformity or stratigraphical break between the two formations, but the line of junction is very plain, the character of the sediments of each being very dis- 4 tinct. ■

The auriferous series consists of grey and yellowish-grey and sometimes ferruginousaandstones, which alternate with wide belts of hard greyish-blue, coarser-grained sandstone, often of a felspabhic nature. The former are of moderate hardness, are generally highly decomposed, and at the surface look as if at one time' they had been permeated , in every direction by thermal waters. The gold-bearing veins are almost exclusively confined to the softer decomposed sandstones. In places the harder sandstones pass imperceptibly into angular breccias, which weather on the surface into bright - coloured clays, being very subject to* decomposition by atmospheric agencies. From Kuranui Creek to the Waiotahi the strata dip to the north-wqpt at angles varying from 40deg to 50deg, but after passing the latter place the dip rapidly steepous, and soon after passing tire Saxon

shaft becomes vertical, and then changes to south-east, the core of the anticline passing ' through a point between the Saxon mine and Waiokaraka Gully. From the latter place to Ilape Creek the whole of the auriferous rocks are repeated. The low flat spur lying between the Karaka and Waiokaraka is composed of a series of flatlying bedded clays, sands, silts, and coarse cemented gravels, which, from their situation, must have been formed by the former during Pleistocene times. At Hape Creek the goldbearing strata disappear below the coarse volcanic breccias and tuffs which follow in the order of their superposition. » [To be concluded next Saturday. J''