Art. IV.—Thermal Activity in its Relation to the Genesis of certain Metalliferous Veins.
[Read before the Otago Institute, 13th September, 1904.]
IT is manifest that the whole series of eruptive after-actions will commence at the moment of intrusion of the magma, and continue until the igneous mass has become completely cooled.
Igneous magmas are now admitted by petrologists to contain more or less water together with many constituents of a hydrous or gaseous character. Hence the fusion of magmas is not believed to be pyrogenetic—that is, the result of dry heat alone—but hydato-pyrogenetic—that is, fusion by heat in the presence of water.
According to Arrhenius* water renders the magma more liquid. It has been shown by experiment that magmas which require a temperature of 3,000° Fahr. to produce dry fusion can be fused in the presence of water at 500° Fahr. According to the same distinguished physicist water in a rock magma acts the part of an acid, liberating free silicic acid and free bases.
The activity of water at high temperatures is very great. Barus† has shown that water heated above 185° C. attacks the silicates composing soft glass with remarkable rapidity; and Lemberg has proved experimentally that water at a
[Footnote] * Svante Arrhenius, “Zur Physik des Vulkanismus,” Geol. Fören. Forgh., Stockholm, 1900.
[Footnote] † C. Barus, “Hot Water and Soft Glass in their Thermodynamic Relations,” Am. Jour. Sci. iv, vol. ix, 1900, p. 161.
temperature 210° C. slowly dissolves anhydrous powdered silicates. It is probable that at great depths the pressure will be sufficient to hold the water in the form of a liquid in a superheated condition.* At high temperatures both water and steam possess a great capacity for dissolving mineral substances.
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] * C. R. van Hise, “Some Principles controlling the Deposition of Ores,” Trans-American Institute of Mining Engineers, vol. xxx, p. 27.
[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.
Ohaeawai Cinnabar Deposits.
The Ohaeawai Hot Springs quicksilver deposits, on the mainland some distance north of the Hauraki Peninsula, are
[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.
of great importance on account of the evidence which they furnish in connection with the genesis of solfataric oredeposits
The basement rocks consist of marly clays and greensands of Lower Tertiary of Upper Cretaceous age, which are covered with flows of basalt and beds of scoriæ. It is agreed by all geologists that the basalt constitutes the youngest rock-formation in the district. The surrounding country is studded with old craters, and there is everywhere evidence of former intense volcanic activity.
The hot springs around which the quicksilver-deposits are clustered are situated about two miles south-east of Lake Omapere, which itself occupies the site of an old crater. They occur along the edge of a flow of basalt, which is overlain at this point by deposits of calcareous and siliceous sinter and solidified siliceous and carbonaceous muds, through which sulphur and cinnabar are finely disseminated. There are also deposits of pyrites with or without cinnabar, in some cases containing traces of both gold and silver.* The sinters also contain gold and silver.
The ground around the springs is generally very hot, and all attempts to develop the quicksilver-deposits have been frustrated by the large volumes of hot water encountered at shallow depths below the surface.
The district has been examined at different times by Captain Hutton, Sir James Hector, A. McKay, and the author; but the best description is that of André P. Griffiths, who conducted extensive prospecting and mining operations there in 1895 and 1896. The mining operations and borings disclosed many important details which could not be gathered from a surface-examination.
The iron-pyrites occurs in masses near the basalt, and also filling cracks and fissures in that rock. The thickness of the pyritic masses varies from 3in. to 3ft., but their other dimensions are extremely irregular. Close to the pyritic masses there is a hard white siliceous sinter from 8 in. to 10 in. thick, which Griffiths found to contain gold and silver in places. One assay of the sinter gave a value of £3 per ton, but unfortunately the proportion of gold and silver is not given† The cinnabar generally occurs lining small cavities and cracks in the solidified muds and sinters surrounding the original fissures in the basalt. It also occurs impregnating the sinter in an extremely finely divided form. Sulphur occurs the sinter in larger proportion than either the cinnabar or pyrites.
[Footnote] * André P. Griffiths, “The Ohaeawai Quicksilver-deposits,” Trans. N.Z. Inst. Min. Eng., vol. ii, p. 48.
[Footnote] † André P. Griffiths, loc. cit., p. 50.
The hot springs give off large quantities of H2S, and occasionally a little steam. The gas escaping through the water of the pools and small streams is partially oxidized, liberating sulphur, which imparts a milky-white colour to the pools, locally known as “white lakes.” The beaches of the so called “white lakes” consist of sulphur mixed with magnetic ironsand and a small proportion of alum. Sulphur is also being sublimed at the vents of openings in the rocks from which H2S and SO2 gases escape.
The prospecting-work conducted by Griffiths disclosed some interesting features. A deposit of cinnabar and pyrites crops out at the foot of the hills to the south-west of the main deposits. A shaft was sunk near it, and cut the lode at a depth of 35 ft. The ore was 2 ft. thick, and consisted of small crystals of pyrites cemented by cinnabar. At this depth there was a strong evolution of H2S, and the heat of the rocks increased so rapidly with the depth that mining was extremely difficult.
It is noteworthy that the outcrop of this lode was found close to the charred trunk of a tree partially imbedded in hard siliceous mud. The trunk and roots of the tree were coated with a thin film of cinnabar, as also were some pieces of fossil kauri-gum found near the roots.
A small trench was sunk over a small fumarole; and at a depth of 10 ft. the temperature of the rock was found to be 185° Fahr.
No. 1 borehole, cased with 3in. piping, was put down to a depth of 10 ft. where it encountered the edge of the basalt. At the same time it struck a fissure from which hot mud was projected a height of 60 ft. for about forty-eight hours. The mud was succeeded by boiling water charged with H2S gas, which was found to issue at a pressure of 30lb. per square inch.
Griffiths further mentions that the richest deposits of cinnabar were found in close proximity to the hottest fumaroles, and that at very shallow depths a temperature was soon reached which precluded mining operation being carried on.
The Ohaeawai hot springs cinnabar-deposits, although never likely to be turned to economic account, are of great scientific importance from the light which they throw upon the formation of sulphide ores by solfataric actions. The deposits are still in process of formation, and metallic sulphides have been, and are still being, deposited in underground fissures and at the surface, together with the sinters which form the matrix.
The hot springs and fumaroles owe their existence to the eruption of the basalt, but the basalt is manifestly not the
source of the metals. The source may not be deep-seated, but that it exists at some distance below the flow of basalt is almost certain.
The waters of the Ohaeawai springs were found by Captain Hutton in 1870 to contain zinc, manganese, silica, free sulphuric and hydrochloric acids, but not traces of mercury.* A sample of the water analysed by W. Skey in 1896 gave the following results:—
|Grains per Gallon.|
|Protoxide of iron||2·23|
Abundant evidence of the hydro-thermal origin of veins traversing eruptive rocks is also obtainable in Europe and America.
In several of the mines in the Comstock Lode ascending thermal waters were encountered in the deep workings, and seriously impeded mining operations.† The water which flooded the Gold Hill mines issued from a borehole in the Yellow Jacket Shaft at a depth of 3,080 ft. It had a temperature of 170° Farh., and was heavily charged with hydrogen-sulphide.‡
Baron von Richthofen,§ who examined the Comstock Lode at a time when no abnormal temperature was noticeable, ascribed the origin of the lode to earlier solfataric action.
Sulphur Bank Cinnabar-Deposits.
The quicksilver-mines at Sulphur Bank, in California, furnish important evidence in relation to the genesis of oredeposits by solfataric action. At this place the basements rocks are slates and sandstones overlain by a fresh-water
[Footnote] * F. W. Hutton, “On the Occurrence of Native Mercury near Pakaraka, Bay of Islands. “Trans. N.Z. Inst., vol. iii, 1871, p. 251.
[Footnote] Clarence King, U.S. Geological Exploration of Fortieth Parallel, 1870, p. 87.
[Footnote] ‡ George F. Becker, “Geology of the Comstock Lode,” U.S. Geol. Surv. 1882, p. 230.
[Footnote] § F. von Richthofen, “The Comstock Lode, its Character and Probable Mode of Continuance in Depth,” San Francisco, 1866, p. 54.
formation, which in turn is capped by a flow of basalt. The sandstones and slates are broken and fissured in such a way as to form a breccia. The interspaces are filled partly with a still soft or already indurated siliceous paste, containing finely disseminated metallic sulphides, and partly with cinnabar, for the most part in coherent crusts.* In the same mine the basalt is reduced to a porous mass, and traversed by irregular fissures filled with sulphur and cinnabar.† Hot mineral water and gases carrying H2S force their way through the interstices of the deposit in the fissured sandstones and slates.
The silica-deposits are found in all stages of consolidation, from a gelatinous mass to chalcedony, and alternate with layers of metallic sulphides, consisting of cinnabar and pyrites.
Unfortunately, no information is obtainable as to the nature of the fresh-water formation lying between the Cretaceous sandstone and basalt.
According to Becker, the hot water is rich in chlorides, borax, and sodium-carbonate. The gases liberated from the water consisted of 893 parts of CO2, 2 parts of H2S, 79 parts of marsh-gas (CH4), and 25 parts of nitrogen, in 1,000 parts.
According to Dr. Melville the marcasite associated with the cinnabar contains traces of gold and copper; and in the efflorescence from the mine-workings Becker detected traces of cobalt and nickel.
In the upper zone only sulphur was found; lower down sulphur and cinnabar, and in depth cinnabar and pyrites occurring upon or within deposits of silica.
Steamaboat Springs Cinnabar-Deposits.
In a valley surrounded with eruptive rocks, and underlain by altered sedimentaries believed to be of Archæan age, thermal springs issue from several points from north-and-south fissures. The floor of the valley is covered in places with a sheet of calcareous sinter in which there are many fissures,
[Footnote] * J. Le Conte, “On Mineral Veins now in Progress at Steamboat Springs compared with the same at Sulphur Bank,” Am. Jour. of Science, vol. xxv, p. 404.
[Footnote] † Prof. F. Posepny, “The Genesis of Ore-deposits,” Trans. Amer. Inst. Min. Eng., vol. xxiii, p. 197.
[Footnote] ‡ J. Le Conte, “On Mineral Veins now in Progress at Steam-boat Springs compared with the Same at Sulphur Bank,” Am. Jour. Sci., vol. xxv, p. 424.
[Footnote] § G. F. Becker, “Geology of the Quicksilver deposits of the Pacific Slope,” U.S. Geol. Surv., Washington, 1888, p. 331.
here and there still open, but mostly closed by the deposit of silica on their walls. From some of the springs hot vapours and gases, chiefly CO2 and H2S, still issue.
Becker found in the mineral water small amounts of mercury-sulphide and sodium-sulphide. About a mile to the west of the main group there are similar fissures yielding steam and CO2. In the sinters of these occur several metallic sulphides. Becker analysed the filling of several fissures and found. besides hydrated ferric oxide, lead, copper, and mercury sulphide, gold and silver, and traces of zinc, manganese, cobalt, and nickel.
Thermal Action in Relation to Vein-Formation.
The occurrence of metallic sulphides in the sinters at Sulphur Bank, Steamboat Springs, and Ohaeawai hot springs; the mushroom-capped lodes at Waihi and Great Barrier Island; and the tree-stems replaced by sulphides found in veins at great depths below the present surface, afford conclusive evidence of the filling of veins by hot ascending waters and gases in areas occupied by later eruptive rocks. It is a notorious circumstance that ore-deposits are most numerous in the neighbourhood of extended zones of eruptive rocks, as in Hungary, Transylvania, Nevada, Colorado, and New Zealand, where the vein-bearing rocks are principally andesite, phonolite, and trachyte. In other rocks veins are fewer and more scattered.
For veins in these altered later eruptives Lindgren suggests the name “propylite veins,” but it is doubtful whether the genetic difference between propylite veins and true fissureveins is sufficiently marked to justify the distĩnction. Moreover, the roots of propylite veins will be difficult to distinguish from fissure-veins connected with a plutonic intrusion.
Professor Suess,* speaking of the importance of the rôle played by the waning phases of volcanic phenomena in the formation of mineral veins, says, “Hot springs may be taken as the latest phase of a whole series which led up to the present deposits of ore.”
In Nevada the sulphur-bearing rock occurs in beds lying between limestone and magnesian rocks. In Utah the sulphur occurs associated with gypsum near an old crater.
At Tikitere, in New Zealand, there are extensive deposits of sulphur in an old crater. A large proportion of the sulphur is the black amorphous variety. The heat of the fumaroles and hot springs is too great to permit the excavation of the sulphur to a greater depth than 6 ft. or 8 ft.
[Footnote] * Professor Edward Suess, Lectures, Royal Geographical Journal vol. xx, Nov. 1902, p. 520.
At White Island, in the Bay of Plenty, the deposits of sulphur occur in and around the crater-lake, mixed with gypsum. The crater-water is hot, and highly charged with free hydrochloric and sulphuric acids. The gypsum is deposited in crystalline incrustations on the sides and floor of the crater-lake. The source of the lime has not yet been determined; but the supply must be constant, as gypsum is being deposited continuously. The sulphur is deposited in the water from gas-springs which are seen bubbling everywhere in the floor of the lake; and also from fumaroles around the margin of the crater.