By P. G. Morgan, M.A.

The term “auriferous series” may be applied to the rocks in question as a non-committal name with respect to their petrographical character. There is as yet no very positive evidence available concerning the age of the auriferous series, but it may be assumed as Eocene, or possibly early Miocene.

After the rocks of the auriferous series had been subjected to more or less denudation, and very probably to the internal change known as propylization, a limited amount of material accumulated in valleys or other hollows that were doubtless occupied by lakes. Such material, in the form of well-stratified tuff, is exposed in the Grand Junction A or No. 1 shaft at a depth of 620–640 ft. below the surface, and about 240 ft. below present sea-level.

Soon volcanic activity on a large scale was resumed, and a second great outpouring of andesitic lavas took place. These rocks form high hills to the north of Waihi, and appear on the surface in the open valley west of Martha Hill. They also occur to an unknown extent under the younger rhyolitic rocks at and near Waihi. In point of age these younger andesites and their accompanying tuffs (developed only very slightly near Waihi) may be referred to the Beeson's Island group of McKay, or to Fraser's “Tertiary Volcanic Rocks of the Second Period.” To the Beeson's Island group may also be assigned the great development of tuffs, in places much intersected by dykes, seen along the coast east and north-east of Waihi.

After the eruption of the “Second Period” volcanics a comparatively brief period of rest from igneous activities was followed by the outpouring of acid lavas. Some of these were rhyolites of the peculiar brecciatedlooking type known as “wilsonite” (11,^* vol. 1, pp. 123, 124; vol. 2, pp. 46, 138). This rock forms the greater part of the so-called “Waihi Plains,” and wraps partly round the outcrops of the auriferous series. With it are associated small amounts of tuff.

Between Waihi and the coast there appear spherulitic rhyolites, probably of approximately the same age as the wilsonite, but possibly older. These are well developed east and north-east of Waihi. They are seen on the coast north of Houmunga Bay, at Waihi Beach (where they contain auriferous veins), and at Mount Hikuragi, a conspicuous elevation a few miles south of Waihi.

Breaking through the wilsonite in various places, and therefore of younger age, are light-coloured rhyolites of harsh texture. These are observable mainly in and near the Town of Waihi, and are also well seen near Waikino, four miles to the west.

The various rhyolites may be provisionally regarded as of Pliocene age, and as contemporaneous with the great rhyolitic flows of the central part of the North Island.

Some hornblende-andesites that occur near Waihi are younger than any of the other andesites, but their age with reference to the rhyolites is uncertain.

The recent surface accumulations of the Waihi district, consisting of a little conglomerate, talus, clay, &c., are of litte moment from a purely geologic point of view. The loamy clay on the slopes of the Martha Hill is apparently largely of aerial origin, and might therefore be called loess. Since this material furnishes a cheap and efficient “filling” for the Waihi Mine workings it has considerable economic value.

Previous Petrographic Descriptions.

In 1870 Sir James Hector (1) passed through the Waihi district. Apparently he considered the andesitic rocks of the neighbourhood to be of doleritic and basaltic character. On the Waihi Plains he noted the presence of “trachyte” [? wilsonite]. To the eastward Hector observed hills capped by “true rhyolite” or “quartzose trachyte” [spherulitic rhyolite with quartz phenocrysts], and on the coast-line cliffs of “true trachyte agglomerate.”

In 1882 Mr. S. H. Cox (2), now Professor of Mining in the Royal College of Science and Technology, London made a flying survey of the Ohinemuri Goldfield. Cox mapped the rocks near Waihi as “tufaceous sandstone” [propylitic facies of andesites and dacites], dioritic rock, anamesite (?), and rhyolite.

About 1883 the late Professor G. H. F. Ulrich determined by microscopic study the andesitic nature of the gold-bearing rocks of the Thames district. Ulrich's work, though known only through a communication made to Professor F. W. Hutton (3, p. 19) may be regarded as the most important contribution ever made to New Zealand petrography. Henceforward the auriferous rocks at Waihi and in other parts of the Hauraki Goldfield if resembling those at Thames were regarded as altered or decomposed andesites—an essentially correct conclusion.

In 1897 Professor James Park, apparently on the strength of microscopic examinations specially made by Ulrich (3, pp. 27, 28) referred to the rocks of the Waihi auriferous series as propylite resulting from the alteration of hypersthene-augite-andesite (3, pp. 26, 87).

It ought to be remarked that the two rock-specimens determined by Ulrich were probably both from the andesitic rocks overlying the auriferous series, and hence Park's determination of the latter as altered hypersthene-augite-andesite can hardly be regarded as authoritative. Apparently this writer at that time regarded the Waihi andesites as all of one type and of one geological age.

Park described the other volcanic rocks at Waihi as rhyolites, of which he mentioned there were at least two distinct flows.

In the same year as the publication last cited an important report on the geology of the Cape Colville Peninsula appeared from the pen of Mr. Alexander McKay (4). On page 59 are a few sentences referring to the auriferous rocks in the neighbourhood of Waihi. These are placed in the “Kapanga group,” but, evidently by design, no specific rock-names are given. In the absence of proper microscopic determinations McKay's attitude of reserve may be regarded as being highly correct. The various rhyolites near Waihi are described (pp. 67, 68) as “spherulitic rhyolite,” “a remarkable brecciated rhyolite” (wilsonite), and “an earthy compact rhyolite.”

Largely as a result of the boom in gold-mining that prevailed during the years 1895–97, a number of papers in which the nature of the rocks at Waihi was more or less cursorily mentioned made their appearance, but for the purpose of the present article it is not necessary further to mention these publications, which were nearly all of an ephemeral character.

In 1899 the late Mr. F. Rutley, in association with Professor Park (5), described several rhyolites from the neighbourhood of Waihi, as well as a doubtful silicified rock, a supposed silicified andesite tuff, and a supposed silicified andesite. Rutley was the first to observe tridymite in the rhyolites

valencianite is doubtless due to its derivation from soda-feldspars by the action of the potash-bearing vein-solutions" (14, pp. 634–35).

Finlayson also made a series of instructive analyses of specimens, all except one of which were from a crosscut to the Empire vein at the 850 ft. level of the Waihi Mine. These analyses show clearly the transition from “chloritized hornblende-dacite,” with 58·39 percent. of silica, to “altered dacite,” with 61·78, 69·35, and 76·61 percent. of silica, and finally to “replacement ore,” with 85·65 percent. of silica.

In his latest work Professor Park refers to the rocks of the Waihi Goldfield in much the same terms as in 1897. The auriferous rocks are called “altered andesites and dacites” (15, p. 349).

In a recent paper, entitled “The Waihi Goldfield” (16), Dr. J. M. Bell mentions the Waihi rocks by the same names as in the previous article by himself and Mr. C. Fraser (12).

Special Petrography.

fresher examples an abundance of small feldspar laths crowded together in more or less parallel arrangement is present, and thus the structure is decidedly inclined to be pilotaxitic; but, since calcite and other alteration-products are invariably present to some extent, the original pilotaxitic (or possibly hyalopilitic) structure is partly obscured. More commonly the groundmass consists mainly of a granular mosaic of quartz, feldspar, and calcite, with minor amounts of other minerals.

(b.) Wall-rocks.

The rocks now to be discussed are from the same localities as those coming under (a), except that, as indicated by the heading, the specimens examined were obtained either within a few feet of the larger veins or were taken from the actual walls of the veins.

Macroscopically the wall-rocks are evidently more highly altered than those some distance from the veins. They are lighter-coloured, and do not effervesce so freely with hydrochloric acid. Feldspar and, less commonly, quartz phenocrysts may be observed. Ferro-magnesian minerals, while usually distinguishable, are always intensely affected by decomposition.

Under the microscope the sections show extreme alteration. The feldspar phenocrysts are nearly all so decomposed that the identification of their original character becomes uncertain. In one or two sections fairly fresh binary twins of orthoclase may be recognized. In most of the sections there are feldspars showing traces of repeated albite twinning, and hence it may be concluded that these feldspars were originally plagioclase. In the main, however, albite twinning is absent, and, since the refractive index of any recognizable feldspar is well below that of Canada balsam, the mineral approaches orthoclase or anorthoclase in its characters. The most highly altered feldspars consist of a mosaic of quartz and feldspar, with minor quantities of calcite and “kaolinitic” matter. In some cases the feldspar of the mosaic is evidently secondary, and therefore to be termed “valencianite.” In other instances, however, its secondary nature is less certain, though appearances are generally quite consistent with such a conclusion. The so-called “kaolinitic” matter mentioned above is strongly suspected to be near sericite in composition, but in the absence of chemical tests its exact nature seems indeterminable.

Original quartz grains appear exactly as in the rocks coming under the last heading, except that they are in some instances in optical continuity with clearly secondary quartz, a phenomenon not observed in the less-altered country.

The ferro-magnesian minerals are entirely altered to chloritic material, with a little iron oxide, &c., so that the original species can hardly be more than guessed at. Rhombic pyroxene may perhaps be assumed as having been present. Primary hornblende is doubtfully indicated by the presence of many small magnetite grains in some of the chloritic matter which has outlines possibly refereable to amphibole.

Other original minerals include a little magnetite, often with leucoxene like alteration (? to siderite), and a few small needles of apatite. A section made from a diamond-drill core obtained in the Grand Junction Mine at a depth of nearly 800 ft. from the surface shows a nest of some mineral with the appearance of tridymite.

The groundmass in the sections of wall-rocks is a granular mosaic composed largely of quartz, with probably more or less valencianite, a little

calcite, &c. An almost opaque streaky substance, nearly white by reflected light forms much of the groundmass. Such material is often referred to as “kaolinitic,” but, as previously remarked, is probably near sericite in composition.

Of secondary minerals not already mentioned, pyrite, almost invariably in small cubes, is the most conspicuous. Epidote or allied mineral may very rarely be present.

(c.) Vein-material.

The vein-material of the Waihi mines largely replaces country, and therefore will be discussed to some extent. In the upper levels of the Waihi Mine the vein-material is quartz, with a little clay, iron and manganese oxides, &c. Some of the quartz is crystalline to the eye, and has obviously been deposited in open spaces. Much, however, is of flinty or chalcedonic appearance. Sections of this chalcedonic material show that it is mainly an indefinite kind of mosaic that resembles the quartz mosaic seen in highly altered wall-rock, and it therefore suggests replacement or silicification of country. This view is supported by a study of the ore from lower levels. The following analysis ^* of a representative piece of “oxidized” ore from one of the upper levels shows that the material is far from being pure quartz :—

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Gold, 2 oz. 5 dwt. 17 gr. perton. silver, 10 oz. 2 dwt. 12 gr. perton.

Sections made from a portion of the specimen that furnished the material for the above analysis show ore with replacement characters seamed by tiny veins of crystalline quartz.

In Nos. 5 and 6 levels (445 ft. and 555 ft.) of the Waihi Mine the presence of partly unsilicified country in the veins is easily recognized. In places quite a large proportion of the vein-material is dark-coloured rock, which in some instances is actually less altered than the wall-rock.

Sections of the less-silicified rock from the veins are very like those of the wall-rocks. Feldspar is the chief original mineral recognizable. Some individuals with feldspathic outlines are mosaics of quartz and presumably secondary orthoclase or anoorthoclase, together with more or less kaolinitic, or more probably sericitic, material. In places fresh, easily recognized valencianite with good cleavage and low index of refraction occurs. Binary twins of apparent orthoclas are observable in at least one section. In several slides traces of repeated albite twinning are quite distinguishable in the feldspars, and in one section acid labradorite, and perhaps andesine, can be identified. Some original quartz in rounded grains is present in all the slides. Ferro-magnesian minerals are in general even more altered than in the wall-rocks, and are represented by somewhat indefinite masses of chlorite and other minerals, particularly pyrite, so that the original species

cannot be ascertained. The groundmass is more or less a quartz mosaic. Other minerals present in it are magnetite, pyrite, and calcite.

(d.) Discussion of Nomenclature.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

With the notable exceptions of Sollas, and to some extent of Lindgren, the various writers who have been quoted on previous pages substantially agree in describing the rocks of the auriferous series as andesites, quartzandesites, or dacites. Lindgren hesitates between dacite and quartztrachyte as a name for these rocks; but this geologist examined wall-rock from the Waihi Mine only. It would also seem to be the case that all, or nearly all, Sollas's determinations were made from samples of highly altered rock adjoining large veins. Had Sollas been given an opportunity of examining a series of less-altered specimens taken some distance from the larger ore-bodies it is probable that he would have reached conclusions more in agreement with those of New Zealand workers. ^* As matters stand, however, the presence of much apparent orthoclase has given rise to a difficulty in naming the auriferous rocks.

From the petrographical descriptions given on the preceding pages it will be gathered that there is much evidence for the view that most, if not all, of the supposed original orthoclase is of a secondary character, replacing original lime-soda feldspar, as has already been suggested by Maclaren and Finlayson. Thus the determination of the auriferous rocks as quartz-andesites or dacites (these terms being here used as all but identical in meaning) may be regarded as proved, at least as regards the Nos. 3, 4, 5, and 6 levels (279 ft., 353 ft., 445 ft., and 555 ft.) of the Waihi Mine, the 500 ft. level of the Waihi Extended Mine, and the 494 ft. level of the Grand Junction Mine. There is no reason for supposing that the rocks of the Waihi-Union and Waihi-Silverton Mines are of different character. Samples of the less-altered country from all the Waihi mines show a great similarity, so that they are possibly all of one original type—probably a hypersthene-dacite with some augite and perhaps a little hornblende.

Chemical analyses of the less-altered mine-rocks entirely suport their determination as dacites of a somewhat basic type. The analyses known to the writer show a silica-content ranging from 55 to 61 per cent., with other constituents in proportions normal to an ordinary andesite or dacite that has been somewhat affected by solutions containing carbon-dioxide. The following determinations of silica and water made on rocks from the present lowest level of the Waihi Mine, communicated to the writer by Mr. A. H. V. Morgan, M.A., Director of the Waihi School of Mines, show that the rocks at that level are, if anything, less acidic than in the upper levels :—

I, country in crosscut at 1,000 ft. level towards Royal reef, about 50 ft. from No.5 shaft; II, country in crosscut at 1,000 ft. level from No. 5 shaft, about 50 ft. south of No. 4 shaft.

(e.) Alteration of Vein-bearing Dacites.

From the preceding pages it will be gathered that near the quartz lodes the dacites are partly silicified, especially-in the groundmass. The original lime-soda feldspars have been almost entirely replaced by valencianite, quartz, probable sericite, and other minerals. Chloritization of the ferromagnesian constituents is prominent. In the sulphide-ore zone more or less pyrite is present.

At some distance from the veins chloritization of the ferro-magnesian minerals is perhaps the most noticeable feature. Many, but not all, of the lime-soda feldspars appear to be replaced by valencianite. Calcite as a secondary mineral is more abundant than in the wall-rocks. Other secondary minerals are quartz, a little pyrite, and perhaps some muscovite and sericite.

By the changes outlined in the last paragraph what may be called the propylitic facies is brought about. The name “propylite” for such rocks, as used by Park and others, is a most convenient term that undoubtedly fills a want, and it is therefore to be regretted that the usage is open to the objection that the name was originally intended to indicate a distinct rock species.

Further discussion of the nature of the orthoclastic feldspar of the altered dacites seems desirable. Some is clearly secondary, and is therefore valencianite. Finlayson has shown that this valencianite is a soda-orthoclase or anorthoclase (14, p. 634). Much more abundant, however, are altered feldspar crystals, now consisting partly of untwinned feldspar of low refractive index, partly of quartz, calcite, and other minerals. Reasons for believing that the feldspar in these crystals is not the remains of the original mineral, but is an alkali (potash-soda) feldspar of secondary character, are the following :—

Of the highly altered feldspars, it cannot be asserted that all were originally lime-soda feldspars, but it seems certain that at least the majority were such. There may, however, be reason for thinking that the fresh but rarely seen individuals of apparently pure potash feldspar with binary twinning are primary. It would be difficult to prove the contrary.

If the possibility that some of the alkali feldspar is original be admitted, then it may be supposed either that orthoclase was a constituent of the original andesitic or dacitic magma, or that during or immediately preceding eruption there was some mixing of an andesitic magma with a rhyolitic differentiate. The presence of corroded quartz seems to support the latter hypothesis, which is decidedly an attractive one, however slender a foundation it may rest upon.

(f.) Significance of Propylitic Facies.

It may be asked whether the propylitic facies, as developed at Waihi, does not necessarily involve the formation of secondary alkali feldspar

(2.) Stratified Tuff of the Grand Junction Shaft.

At a depth of about 620 ft. the Grand Junction A or No. 1 shaft passes from andesite into fine-grained volcanic débris or tuff, which at 630 ft. to

640 ft. is in well-marked layers with a dip of 21° to 22° to the southward. At some depth not exactly ascertained by the writer this shaft enters the older rocks of the auriferous series.

Pieces of much-altered wood still retaining about 10 per cent. of carbon were common in the material excavated from the shaft at 620 ft. to 630 ft., or thereabouts. These, evidently derived from shrubs or small trees, averaged less than 1 in. in diameter, and were in no case more than a few inches long. Sections made of the enclosing tuff show its fragmentary character very well, especially by reflected light. The constituents appear to be mainly highly decomposed broken feldspar crystals, with a few grains of quartz and other minerals. The secondary products present are calcite, chlorite, pyrite, and nearly opaque kaolinitic matter (probably mainly sericite).

The well-stratified tuff at 630 ft. appears to have suffered but little alteration comparable to that involved in the propylitic facies of the underlying auriferous series. There is therefore some reason for thinking that propylization of the latter rocks preceded the deposition of the tuffs; but before any definite conclusion, if such is possible, can be drawn from the line of reasoning here indicated a more detailed study of the tuff than that made by the writer will be advisable.

(3.) Andesites and Tuffs of the “Second Period.”

(4.) Rhyolites.

third a light-coloured fine-grained rock with few phenocrysts. This latter rock contains nests of tridymite, and therefore for the sake of distinction will be designated the “tridymite-rhyolite.”

(a.) Spherulitic Rhyolites.

Macroscopically the spherulitic rhyolites show numerous spherulites, many grains of quartz, and occasional small plates of biotite. A little secondary opal is present in some specimens, and may show good “fire.” Under the microscope quartz appears in large grains, with many gas enclosures. Small crystals of plagioclase and biotite may also be present. The greater part of every section, however, is seen to consist of spherulites. The exact nature of the spherulites in Hauraki rhyolites has been discussed by Rutley (5, pp. 451 et seq.) and Sollas (11, vol. 1, pp. 120–22). Rutley regards them as being composed of devitrified glass, whilst Sollas, beyond adducing evidence opposed to the devitrification theory, makes no very definite pronouncement.^* For the present it may be assumed that the spherulites approach orthoclase in composition, and were formed during the final consolidation of the rhyolites.

In a section of rhyolite from the hills about a mile and a half east of Waihi a quartz-grain is seen to form part of a spherulite boundary. Hence in this case the spherulite is obviously of later formation than the quartz. In the same section a phenomenon which may be termed spherulite within spherulite occurs.

At Waihi Beach a beautifully spherulitic rock is traversed by auriferousquartz veins. Sections show that the rock is more or less silicified, much secondary quartz mosaic being present. Feldspar (probably orthoclase) appears in imperfect crystals, one of which is imbedded in a spherulite.

A small vein traversing one of the sections from Waihi Beach consists of a fine quartz mosaic cementing and in part replacing fragments of country. It may from this be inferred that the auriferous veins of this locality, like those at Waihi, are largely replacement veins following fracture planes or zones.

The rhyolites referred to above are more fully described by Sollas (11, vol. 2, pp. 35, 112).

(b.) Wilsonite.

The peculiar rhyolitic rock commonly known as “wilsonite” has been described by Sollas (11, vol. 1, pp. 123, 124; vol. 2, pp. 46, 138). The same rock is mentioned by Park (3, p. 88) and McKay (4, p. 67), and has probably been described by Rutley (5, pp. 460, 461, No. H₁₅).

The freshest obtainable specimens of wilsonite show pinkish or purplish surfaces flecked with numerous black streaks. The fractured rock has a somewhat vitreous lustre. When affected by surface weathering the purplish portion of the rock become nearly white, and the dark portions tend to approach a light-grey colour. Highly weathered rock is of a nearly uniform almost white colour. In places wilsonite tends to pass into a tuff, owing, it is believed, to brecciation of surface portions that solidified before the cessation of flow. Rounded fragments of andesite, usually from ¼ in. to 1 in. in diameter, are very common as inclusions in the wilsonite. These are most noticeable in the highly weathered rock, which also is usually the most tufaceous-looking.

(5.) Andesitic Rocks of Doubtful Age.

Under this heading are included some of the younger andesitic rocks that were not sufficiently studied to enable the determination of their relative ages with reference to one another and to the rhyolites to be made. The rocks thus occurring are hornblende-andesites of a variable character.

The most notable occurrence of such rocks is the andesite forming the Black Hill, east of Waihi. In places it shows well-marked though not perfect columnar structure. The rock is not very uniform in appearance, but in the main is a dark-grey hornblende-andesite, sections of which, besides abundant hornblende, show a little augite, and in places biotite. The feldspar phenocrysts are probably oligoclase-andesine. A little magnetite and apatite in fine needles are present. A nearly isotropic mineral of very low refractive index was not determined. The base contains numerous feldspar microlites, and with a ¼ in. objective exhibits a kind of feathery appearance under crossed nicols.

Literature.