Solfataric—i.e., Formed by Thermal Solutions Aided by Steam and Gases.
It is well known that during and after volcanic eruptions there are emitted enormous volumes of steam, also hydrogen-sulphide, sulphur-dioxide, carbon-dioxide, as well as compounds of chlorine, fluorine, and boron. These gaseous and aqueous emanations come from the same source as the igneous magma, accompany the magma in its ascent, and may possibly be one of the contributing causes of the eruption.
Volcanic phenomena can be studied in many parts of the world, but perhaps nowhere with more advantage than in New Zealand. In the volcanic region of the North Island there are thousands of square miles in which volcanic activity can be seen in every stage and phase; there are active, intermittent, and extinct volcanoes, besides innumerable geysers, fumaroles, and hot springs, active, decadent, and dead. The active and intermittent volcanoes discharge their lavas and fragmentary matter from single pipes, or from lateral vents apparently connected with the main pipe, and from fissure rents. The volcanic eruption at Rotomahana in 1886 was from a fissure rent over six miles in length, extending from the summit of Mount Wahanga southward into the basin of Lake Rotomahana, and thence across the rhyolite plateau to Lake Okaro.† The whole length of the rent was the scene of great activity for some weeks after the first great outburst. The geysers, hot springs, and fumaroles occur in isolated groups, or along a line of fissure which often runs along the floor of a valley, or lower flanks of a range of hills. The geyesers deposit siliceous and calcareous sinters, mostly the former; and the fumaroles native sulphur. Everywhere the air is pervaded with the smell of sulphur-dioxide. The solfataric action is active, waning, or dead. With the latter the vents are closed up by crustification. Where the
[Footnote] † (1.) Sir James Hector, “On the Recent Volcanic Eruptions at Tarawara,” N.Z Reports of Geol. Explorations, 1886–87, p.243. (2.) S. Percy Smith, “The Eruption of Tarawera,” Wellington, 1886. (3.) Prof. F. W. Hutton, “Report on the Tarawera Volcanic District,” Wellington, 1887. (4) Prof. A. P. Thomas, “Report on the Eruption of Tarawera,” Wellington, 1888.
hot springs overflow on the surface they form thick, mush-room-shaped mounds of silica. The silica is sometimes soft and porous. and often dense, hard, and chalcedonc. In all cases the hot springs and geysers are grouped around the volcanic vents, and along fissures in lavas near the point of emission. The waters range from strongly alkaline to acid; and at Rotorua, alkaline and acid springs exist side by side. The ascending deep-seated waters are strongly alkaline; while the source of the acid waters is the superficial deposit of pumice which overlies the rhyolite. The pumice in some places contains disseminated marcasite pyrites, and where the alkaline waters come in contact with the pyrites they are oxidized and reach the surface either neutral or acid, according to the degree of oxidation.
In the Hauraki gold-mining area, which adjoins the northern end of this volcanic region, the country rocks consist of a vast pile of andesitic lavas, tuffs, and breccias of younger Tertiary age, resting on slaty shales and greywacke of probably Triassic age. The gold-bearing veins traverse both the andesites and tuffs, but are only productive in the former. They are fissure-veins; but, strictly speaking, they do not conform to the usually accepted definition of a true fissurevein, since they are generally confined to the igneous-rock formation. Near the borders of the andesites the veins are small and unimportant, and generally die out when they reach the underlying basement rock. On the other hand, the larger and more productive veins are grouped around the old vents, and there seems to be no reason why they should not descend to great depths. In opposition to this view Professor beck* states that it is inconceivable that mineral deposits could be made from solutions at great depths. The country rock on the walls of the ore-veins is propylitised to a moderately hard grey rock. When two or more veins run parallel with each other, as they do in all the Hauraki mining centres, the country rock between the veins is often entirely altered, or propylitised.
In the Thames district the distance between the numerous parallel veins which traverse the goldfield seldom exceeds 200 yards, and in almost every instance the veins are separated from each other by a narrow belt of hard unaltered andesite. These hard bands, of “bars” as the miners term them, possess the same general strike and dip as the veins, and in cross-section present the appearance of lenticular and hourglass-shaped masses. They vary from a few feet to 30 yards in width. The country rock has been found to be propylitised down to a depth of nearly 1000ft. below sea-
[Footnote] * Prof. Beck, “Lehre von den Erzlagestatten,” 1901, p. 139.
level, which is the greatest depth reached by mining operations up to the present time. The propylitisation of the andesites is not widespread, but confined to small areas grouped around the old volcanic vents. Away from the eruptive centres the andesites have suffered surface-decomposition, but are not propylitised. The propylitisation was apparently effected by the fissures, which are now veins, having served as channels for the circulation of the hot mineral waters. From these fissures the waters acted on the rock on each wall, and where the fissures were near each other the metasomatic processes operating from one fissure met those coming from the other. Where the processes of alteration did not meet, narrow irregular sheet-like masses of unaltered rock—the “bars” of the miners—were left between the vein fissures.
At Waihi and surrounding districts the veins are chiefly composed of chalcedonic or micro-crystalline quartz, possessing all the characteristics of solfataric origin. Some of the larger lodes can be traced on the surface for a distance of 16,000ft., but the length of the majority is under 5,000ft. Besides veins having linear extension, there are many huge mushroom-shaped masses of chalcedonic quartz, closely resembling in form the siliceous deposits now forming in the volcanic regions around Rotorua and Lake Taupo.
At Kuaotunu and Great Barrier Island there are many mushroom-shaped deposits of chalcedonic quartz of great size, in some cases covering hundreds, in others thousands, of acres. At Kuaotunu they are more or less circular in shape, and seldom exceed 20ft. in thickness.
At Great Barrier Island the largest deposit is of an unusual character.* It is nearly two miles long, half a mile wide, and from 50ft. to 700ft. thick. The pipe is completeley filled with mineral matter. It has been intersected in four mines in a distance of a mile, and opened up by levels for many hundreds of yards. It varies from 12ft. to 40ft. in width, and is filled with very dense banded chalcedonic quartz, in which iron and silver sulphides are sparingly distributed. The evidence furnished by the mine-workings implies that the overlying mushroom or umbrella of quartz was deposited on the surface from thermal water issuing from a long fissure or rent in the andesite.
The molybdenite deposits at Jeff's Camp, in the Hodgkinson Goldfield, in Queensland, are described by W. E. Cameron† as roughly circular or oval-shaped outcrops of
[Footnote] * J. Park, “The Geology and Veins of Hauraki Goldfields, “Trans. N.Z. Inst. Min. Eng., vol. i, 1897, p. 137.
[Footnote] † Walter E. Cameron, “Wolfram and Molybdenite Mining in Queensland,” Geol. Survey Report No. 188, Brisbane, 1904, p. 7.
quartz, or “blows,” carrying wolfram and native bismuth. The “blows” when followed down develop into irregular pipeshaped masses surrounded on all sides by granite, which is the country rock. When the quartz is extracted there remain only empty pipes or vents. These pipe-like ore-bodies possess a peculiar genetic interest. They appear to closely resemble the siliceous pipes formed in rhyolite by the hot springs in the Rotorua volcanic region, and the mushroom-shaped quartz blows at Kuaotunu.
There are near Waihi in New Zealand several massive deposits of chalcedonic quartz which are stated by Rutley to be replacements of the andesitic country rock.*
A similar replacement of andesite by silica is described by Spurr as occurring at Monte Cristo district in Washington.† He mentions that the silicification has proceeded until most of the rock is made up of quartz, which, he says, varies from coarsely to very finely crystalline in structure, and contains sulphides, chiefly blende, pyrites, and chalcopyrite. Spurr continues, “Thus we have a complete and gradual transition from andesite to a sulphide ore with quartz gangue, by the progressive replacement of the original materials by silica and metallic sulphides.”
In 1894 and 1896 I made an exhaustive examination of the Hauraki andesites for gold and silver. The samples subjected to examination were selected by myself in situ. The analyses were conducted by the cyanide test, on samples ranging from 2lb. to 5 lb. in weight. The pulverised material was leached in glass jars with a 0·3-per-cent. aqueous solution of pure potassium-cyanide for seventy-two hours. The cyanide solutions and washings were evaporated, fluxed with a little pure litharge and borax, and the resulting button of lead cupelled. Simultaneous tests were made so as to check the purity of the litharge and fluxes. All the andesites examined were found to contain gold at the rate of 1 gr. to 1·5 gr. per ton, and silver varying from 3 gr. to 30 gr. per ton of rock. The augite-andesite, at 3,000 ft. from the mouth of the Moanataiari tunnel, contained 1 ½ gr. of gold and 3 gr. of silver to the ton; and the hypersthene-augite-andesite, from the waterfall in Waiotahi Creek, near the Fame and Fortune Mine, 1 ½ gr. of gold and 30 gr. of silver.‡
A petrological examination§ of the rocks showed that the
[Footnote] * J. Park and F. Rutley, “Notes on Rhyolites of the Hauraki Goldfields,” Quart. Jour. Geol. soc., London, 55, 1899.
[Footnote] † J. E. Spurr, U.S. Geol. Survey, Twenty-second Annual Report, p. 833.
[Footnote] ‡J. Park, “The Geology and Veins of Hauraki Goldfields,” Trans. N.Z. Inst. Min. Eng., 1897, p. 52.
[Footnote] § J. Park, “Some Andesites from the Thames Goldfields,” Trans. N.Z. Inst., vol. xxxiv, p. 435.
feldspars and pyroxenes sometimes showed signs of alteration. The samples were selected from the least-altered rocks obtainable, and in no case did they contain visible pyrites.
The evidence is by no means conclusive that the gold and silver are primary constituents. Whatever the source of the gold may be, I am inclined to agree with Percy Morgan* that the quantity of gold and silver in the veins is too great to be accounted for by the traces existing in the andesite.
Dr. J. R. Don,† in the preparation of his excellent thesis on “The Genesis of certain Auriferous Lodes,” made an interesting examination for the presence of gold in the andesites and propylites of the Thames Goldfield. He states that his tests were made upon the concentrates obtained from large samples, by the method of crucible fire assay. His results, in the case of the unaltered andesites, were negative, from which be concluded that these rocks contained no gold. The question that will naturally suggest itself to the mind of the metallurgical chemist, accustomed to the estimation of infinitesimal quantities of gold in cyanide solutions and residues, will be, is the method of crucible or pot assay capable of sufficient refinement to indicate the presence of gold in the proportion of a grain or two to the ton of rock?
My early tests of the Hauraki andesites in 1894 were made by the crucible-assay method. The results, however, were often discordant and unsatisfactory, chiefly on account of the many sources of possible error inherent to the method—errors that it was found impossible to entirely eliminate. Believing that trustworthy results could not be obtained by the pot assay, I adopted a method of leaching the pulverised rock with dilute solutions of potassium-cyanide. By this process larger samples could be tested than by fire assay, and the possible sources of error were reduced to a minimum. The crucible assay is clumsy, laborious, and, in my experience, incapable of the refinement required for the estimation of minute traces of gold even in the hands of the most skilful manipulator.
Luther Wagoner,‡ of San Francisco, who in 1902 made a number of tests for gold and silver in sea-sediments, sandstones, syenite, granite, basalt, diabase, &c., by the cyanide method used by me in 1894 and 1896, arrived independently at the same conclusion. Discussing the assay of rocks, he
[Footnote] * Percy Morgan, “Notes on the Geology, Quartz Reefs, and Minerals of Waihi Goldfield,” Trans. Aust. Inst. Min. Eng., vol. viii, 1902, p. 164.
[Footnote] † J. R. don, “The Genesis of certain Auriferous Lodes,” Trans. Am. Inst. Min. Eng., vol. xxvii, 1898, p. 564.
[Footnote] ‡Luther Wagoner, “The Detection and Estimation of small Quantities of Gold and Silver,” Trans. Am. Inst. Min. Eng., vol. xxxi, 1902, p. 198.
says,* “The statement of Dr. Don that country rocks can be assayed by panning down a quantity and assaying the residue has been tested, as well as the statement that pyrites must be present in order to find gold; and my experiments show that both statements are incorrect—or, at least, not in accord with my experience.”
At Te Aroha, near the northern boundary of the central volcanic region, there are in the andesites hot springs; twenty-five miles distant, soda-water springs; and at the Thames, ten miles further north, gas springs which discharge enormous volumes of carbon-dioxide.
In the mines in the north end of the Thames Goldfield the CO2 issues with great force from cracks and fissures in the rocks. The mine-shafts are situated near the foreshore, and descend to depths varying from 500ft. to 900ft. below sea-level. In close muggy weather in summer, with a low barometer, the gas rises in the mines, and, flooding the workings, drives the miners before it. Sometimes the gas rises up to the top of the shafts and overflows at the surface. Notwithstanding the special precautions employed to effect ventilation and to warn the men of danger, several fatal accidents have taken place in the past thirty years.
In the Big Pump shaft the CO2 escapes with such force as to cause violent boiling all over the surface of the water in the well. The depth of the shaft is 64ft., but the workings are flooded up to the 500ft. level, in consequence of which the gas escapes against a head of 150ft., equal to hydraulic pressure of 65lb. to the square inch. The commotion at the surface of the water at the 500ft. level is caused by the escape of the gas which is not dissolved by the water. The pump has been raising water from this shaft for over a quarter of a century at the rate of 750 gallons per minute. The water is so highly charged with gas as to often cause trouble in working the pumps.
At Waihi, Kuaotunu, and Great Barrier Island there are huge veins of quartz, mostly chalcedonic, many of which are still capped with wide mushroom-shaped “quartz blows.”
The evidence favours the conclusion that the propylitisation of the andesites and formation of the lodes were the result of hydro-thermal action.
Posepny† mentions the remarkable occurrence of treestems changed to galena in the Vesuvian Mine, Freihung, in Bavaria. In these the fibre and annular rings can be easily recognised, being extremely plain on polished surfaces. In the tuff-beds associated with the gold-bearing andesites masses
[Footnote] *Loc. cit., p. 808.
[Footnote] † Prof. Franz Posepny, “The Genesis of Ore-deposits,” 1901, p. 129.
of wood partly or wholly silicified and spangled with nests and veins of iron-pyrites are of common occurrence throughout the Hauraki region.
The Martha Lode and its numerous ramifying branches, the Silverton, Union, and Amaranth Lodes, at Waihi, are all contained in an area of about a square mile. The huge lodes, wide zones of silicified andesite, and extensive propylitisation of the andesite, prove that Waihi was an area of intense hydro-thermal activity some time prior to the eruption of the later rhyolite-flows which now form the plains and wrap around the isolated outcrops of andesite containing the Martha and Silverton veins. The propylitisation has already been shown by the Waihi Mine workings to extend to a depth of nearly 800ft. below present water-level—that is, some 500ft. below sea-level. Obviously the alteration of the andesite was due to the action of ascending and laterally moving thermal waters.
At Thames and Coromandel some of the most productive veins do not reach the surface of the enclosing rock, and the mine-workings at Waihi have disclosed a similar feature in connection with a few valuable veins in the Waihi company's property.*
In 1888 Captain F. W. Hutton, as the result of a petrographical examination of the Thames Mining District, concluded that the veins were of hydro-thermal action.†
T. A. Rickard, a well-known American geologist who examined the same goldfield in 1891, when discussing Professor Posepny's paper on “The Genesis of Ore-deposits,” describes the characteristic-features of the district with the view of springs and later eruptive rocks.‡ He states that his examination of the ore-occurrences and vein-structure, though incomplete, led him to conclude that the deposition of the gold and its associated minerals had followed certain lines of altered country rock which had been exposed to the effects of dying but lingering solfataric agencies.
[Footnote] * P. C. Morgan, “Notes on the Geology, Quartz Reefs, and Minerals of the Waihi Goldfield,” Trans. Aust. Institute of Mining Engineers, vol. viii, 1902, p. 168.
[Footnote] † F. W. Hutton, “On the Rocks of the Hauraki Goldfields,” Trans. Aust. Assoc. Adv. Sci., vol. i, 1888, p. 245, and “Source of Gold at the Thames,” N.Z. Journal of Science, Vol. i, p. 146.
[Footnote] ‡ T. A. Rickard, “The Genesis of Ore-deposits,” Discussion, New York, 1901, p. 222.