Art. VI.—Notes on the Origin of the Metal-bearing Solutions concerned in the Formation of Ore-deposits.
[Read before the Otago Institute, 8th August, 1905.]
Ore-deposits may be divided into two genetic classes—namely, those deposited by metal-bearing solutions, and those formed by direct magmatic segregation in an igneous magma. The former include the majority of ore-deposits of economic value; the latter are comparatively rare and unimportant.
It is manifest that the waters concerned in the formation of ore-deposits of the first class must be either (a) meteoric and descending, or (b) magmatic and ascending.
Descending waters are believed to gather their mineral contents from the rocks through which they percolate. They are said by Van Hise and other writers to descend to great depths by gravitative stress, and to move laterally towards open channels and fissures, where they deposit mineral matter. In this we find the basis of the theory of lateral secretion.
Hot ascending solutions are held to be genetically connected with igneous masses—that is, they are either directly magmatic, or they are liberated from sedimentaries in the form of gases and aqueous vapour. In this conception we have the fundamental basis of the theory of ascension of solutions.
Theory of Lateral Secretion.
According to this theory it is assumed that meteoric waters percolating through the country rock, by the aid of carbondioride and alkalies dissolve out certain constituents, which are afterwards deposited in fissures and cavities.
The origin of the theory is unknown, but it is certain that Delius, in 1770, Gerharde, in 1781, and Lasius, in 1789, were supporters of it, the latter basing his argument upon a careful examination of the veins of the Hartz Mountains.*
In 1847 Professor Bischof, of Bonn, a distinguished geologist and chemist, in his fascinating “Text-book of Chemical and Physical Geology,” discusses the chemical processes which take place when meteoric waters and different kinds of aqueous
[Footnote] * Georg. Lasius, “Observations on the Hartz Mountains,” Hanover, 1789: “Ores and Minerals,” Vol. ii.
solutions come in contact with rocks. His work created a new scientific basis of research in this branch of economic geology. He contended that ores were obtained by leaching from the rocks traversed by the veins, and suggested the possibility of the vein-constituents being found in the adjacent rocks.
In 1855 Forchhammer, the famous chemist of Copenhagen, found traces of lead, copper, and zinc in the roofing-slates of North Wales, a discovery which was held to afford conclusive proof of the origin of ore-veins by processes of lateral secretion.
In 1873 Professor F. Sandberger, of Wurzburg, dissatisfied with the meagre results obtained from the examination of sedimentary rocks, directed his attention to a systematic chemical investigation of the rocks traversed by ore-veins, and of the vein-stuff itself, in different mining centres in the Black Forest. In clay-slate he discovered copper, zinc, lead, arsenic, antimony, tin, cobalt, and nickel; in sandstone, lead and copper. Titanic and phosphoric acids were found to be present everywhere in small quantity.
Sandberger's results showed that a close relationship existed between vein-contents and the country rock; but he was by no means satisfied as to the origin of the heavy metals. He accordingly extended his investigation to an examination of the constituents of igneous rocks. He crushed large samples of rock and separated the constituent minerals by solutions of different densities. Large samples of the individual crystallized silicates thus isolated were subjected to careful analysis. In this way he found all the usual elements formed in metalliferous veins. Thus, in olivine he found iron, nickel, copper, and cobalt; in augite, copper, cobalt, iron, nickel, lead, tin, and zinc; and in the micas, many base metals. Gold, mercury, and tellurium were not sought for.
In 1880 Sandberger announced his belief that the mineral contents of veins were derived not from some unknown depth, but from the immediate wall-rock.
Gold-bearing veins are common in slates and sandstones of marine origin; and, as sea-water, according to the announcement of Sonstadt* in 1872, contains somewhat under a grain of gold to the ton, it is held by the exponents of lateral secretion that the sea is therefore the source of the gold in veins traversing marine sedimentaries. It is maintained that when sediments are deposited on the floor of the sea they must necessarily entangle a certain proportion of sea-water, and that when these sediments become consolidated the gold must remain in them.
[Footnote] * Confirmed by Professor Liversidge in 1893 and Sir William Ramsay, F.R.S., in 1905.
The theory of lateral secretion received a new impulse from the researches of Sandberger. It seemed competent to explain the origin of many ore-veins, and, although strongly opposed by Professor Stelzner of Freiburg and Professor Posepny of Przibram, it found much support in America, in a more or less modified form.
Thus, Emmons,* discussing the manner in which he considers the Leadville ore-deposits were produced, summarises his views on ore-formation in general as follows:—
Ore-deposits have been deposited from solution, rarely in open cavities, most frequently by metasomatic interchange.
Solutions do not necessarily come directly upwards, but simply follow the easiest channels of approach.
The material was derived from sources within limited and conceivable distance, very often the older intrusive rocks.
Emmons, while supporting the principle of lateral secretion, disclaims the narrow views of Sandberger, who limits the source of the vein-contents to the wall-rock in immediate contact with the vein.†
In the critical discussion which followed the publication of Professor Posepny's paper on the genesis of ore-deposits, in 1893, Blake and Winslow reaffirmed their belief that the zinc and lead ores of Wisconsin were framed by lateral secretion.‡
Becker,§ while strongly dissenting from Posepny's view that metasomatic replacement was incapable of producing Such pronounced ore-bodies as those at Leadville, makes a clear statement of the supposed operation of metasomatic processes. He says, “Replacement, like solution, must occur along fissures or channels, and metasomatic ore-bodies will present analogies in form to the open spaces of caves of solution.”
Rickard∥ discusses the problem of ore-formation from a wide standpoint, and is not a dogmatic supporter of the extreme doctrines of either ascension or lateral secretion. He affirms that there is no ground for the belief in the existence of a reservoir of water at great depth, and maintains that all ascending water must at one time have been descending water.
This last can only be true in regard to meteoric waters. So far as the existence of deep-seated water is concerned, his view
[Footnote] * S. F. Emmons, “The Genesis of certain Ore-deposits,” Trans. Am. Inst. M.E., Vol. xx, p. 125.
[Footnote] †“The Genesis of Ore-deposits,” New York, 1901, p. 199.
[Footnote] ‡ Loc. cit., p. 188.
[Footnote] § Loc. cit., p. 200.
[Footnote] ∥ Loc. cit., pp. 190 and 211.
is not in accord with the hydro-fusion theory of modern petrologists.
Rickard, who possesses a personal knowledge of the goldfields of Australia and New Zealand, discusses the probable origin of the veins of the Thames Goldfield, in the latter country, and in the main agrees with Captain Hutton that they were formed by processes of lateral secretion by thermal waters.
Professor J. Le Conte* in a carefully prepared thesis combats the extreme views of both Posepny and Sandberger. He makes an earnest attempt to combine what is true in each, and reconcile their differences. It is manifest, however, that he leans favourably to the side of lateral-secretion processes, although not defined as such. He considers both sides right and both wrong. Ascensionists, he thinks, are right in deriving metals mainly by ascending solutions from great depths, but wrong in imagining these depths to be an exceptionally metalliferous barysphere, and wrong in not allowing subordinate contributions by lateral currents from the wall-rock higher up. The lateral-secretionists, on the other hand, are right, he thinks, in deriving metals by leaching from the wall-rock, but wrong in not making the thermosphere the main source.
Le Conte succinctly summarises his views in the following terms:—
Ore-deposits, using the term in its widest sense, may take place from many kinds of waters, but especially from alkaline solutions; for these are the natural solvents of metallic sulphides, and metallic sulphides are usually the original form of such deposits.
They may take place from waters at any temperature and pressure, but mainly from those at high temperature and under heavy pressure, because, on account of their great solvent power, such waters are heavily freighted with metals.
The depositing waters may be moving in any direction—up-coming, horizontally moving, or even sometimes down-going—but mainly up-coming, because by losing heat and pressure at every step such waters are sure to deposit their contents abundantly.
Deposits may take place in any kind of water-way—in open fissures, in incipient fissures, joints, cracks, and even in porous sandstones, but especially in great open fissures, because these are the main highways of ascending waters from the greatest depths.
[Footnote] * J. Le Conte, “The Genesis of Ore-deposits,” p. 270.
Deposits may be found in many regions and in many kinds of rocks, but mainly in mountain regions and in metamorphic and igneous rocks, because the thermosphere is nearer the surface, and ready access thereto through great fissures is found mostly in these regions and in these rocks.
Professor C. R. van Hise, in a classic paper on “Some Principles controlling Deposition of Ores,”* defines his views in the following sentences:—
That the greater number of ore-deposits is the result of the work of underground water.
That the material of ore-deposits is derived from rocks within the zone of fracture.
That by far the major part of the water depositing ores is meteoric.
That the flowage of water underground is caused chiefly by gravitative stress.
That the waters which perform the first work in the genesis of ore-deposits are descending waters.
Lateral secretion is therefore an essential step in the first concentration of ore-deposits.
That sulphide ores are generally deposited by ascending waters in trunk channels.
That the majority of ore-deposits, if not all, are partly deposited in pre-existing openings, and are partly replacements of wall-rocks.
It is manifest that he attaches too little importance to the genetic connection existing between ore-deposits and eruptive processes, and places too much dependence upon the formative power of meteoric waters.
Professor Kemp† contends that mining operations in America show conclusively that mines become drier with increasing depth; and deep mining in South Africa, Australia, and New Zealand adds confirmation to this view. The dryness of mines in depth seems to destroy the foundations of Van Hise's main contention respecting the underground circulation of meteoric water.
“Van Hise admits that there are ore-deposits which have a direct igneous origin, but thinks they are or limited extent. In his rejoinder to Kemp he somewhat modifies his former concep-
[Footnote] * “The Genesis of Ore-deposits,” p. 282; also Trans. Amer. Inst. M E., Vol. xxx, 1901, p. 27.
[Footnote] † J. F. Kemp, “The Rôle of Igneous Rocks in the Formation of Veins,” “The Genesis of Ore-deposits,” 1901, p. 681.
tion with respect to the action of meteoric waters, and admits that the rôle of igneous intrusions may be very considerable.*
It has been suggested by the opponents of lateral secretion that the metals contained in the silicate minerals of eruptive rocks are not primary but secondary constituents. According to their view, lateral secretion is only a process of concentration.
Ascension of Solutions.
According to this theory it is assumed that the material which fills a lode has been brought in solution from great depths, and not derived from the rocks in the immediate vicinity of the lode.
In his classic memoir on “The Genesis of Ore-deposits,” the late Professor Posepny, an ardent supporter of the ascension hypothesis, laid great stress upon the occurrence of sulphur and cinnabar at Sulphur Bank, California, impregnating a decomposed basalt, and still mildly in process of formation from gaseous emanations and hot mineral waters.
Similar conditions exist at Steamboat Springs, in Western Nevada, where we have an example of a mineral vein in process of formation. The matrix is banded siliceous veinstone, containing iron and copper sulphides, sulphur, and metallic gold.
Sandberger, who was an equally strenuous supporter of lateral secretion, objected to this view on the ground that he knew of no spring which deposited mineral incrustations on the walls of their channels. He regarded the Sulphur Bank and Steamboat Springs phenomena as exceptional.
Becker, who made a special examination of the deposits at Sulphur Bank and Steamboat Springs, strongly opposed the views of the extreme ascensionists. And with regard to the origin of the deposits he expressed the following views: “The evidence is overwhelming that the cinnabar, pyrite, and gold of the quicksilver-mines of the Pacific Slope reached their present positions in hot solutions of double sulphides which were leached out from masses underlying the granite, and the granite itself.”† Further on he says, “I regard many of the gold-veins of California as having an origin entirely similar to that of the quicksilver-deposits.”
Becker's views postulate a new hypothesis lying midway between the ascension and lateral-secretion theories, and now
[Footnote] * C. R. van Hise, “The Genesis of Ore-deposits,” New York, 1901: Discussion, p. 763.
[Footnote] † G. F. Becker, “The Geology of the Quicksilver-deposits of the Pacific Slope,” U.S. Geol. Survey, Vol. xiii, 1888, p. 449.
receive more general support than the extreme views of Posepny and Sandberger.
According to the definition of lateral secretion by the latter, the descending waters became charged with mineral matter by leaching the rocks in the region of vadose circulation. On the other hand, Posepny assumed that the ascending waters became charged at great depths by coming in contact with a deep-seated repository of metalliferous matter, the existence of which is necessarily hypothetical.
The modification suggested by Becker leans towards the ascension theory, and differs only from lateral secretion in assuming a deeper source for the mineral contents of the vein-matter.
From the data recorded in the preceding papers we may deduce the following conclusions respecting the genesis of ore-deposits:—
That the majority of ore-deposits are genetically connected with igneous rocks.
That circulating underground water and gases are the principal agents concerned in the dissolution, primary concentration, and deposition of vein-matter.
That ore-deposits do not necessarily occupy pre-existing fissures and cavities.
That ore-deposits were in many cases formed by meta-somatic replacement.
That vein-filling waters are ascending waters, in most cases probably of magmatic origin.
That the mineral contents are derived from rocks contiguous to the zone of fracture or zone of metamorphism.
That the principal agents of dissolution are water and gases aided by heat and pressure.
That precipitation from the ascending waters takes place in orderly horizontal zones in accordance with the natural laws governing solution and precipitation.
That secondary enrichment is in the majority of cases due to the migration of mineral contents from a higher to a lower level, through the agency of descending meteoric waters.
The theories of lateral secretion and ascension of solutions are based on the fundamental assumption that the mineral matter filling cavities was deposited from circulating waters. Their differences lie principally in the different conceptions as to the direction and operation of the circulating liquids.
The lateral-secretion theory supposes—
The ascension theory assumes—
Many writers assume that ascending and descending waters are merely units in what may be termed a hydraulic circuit, the interchange being caused by gravitation assisted by capillary, and the difference of temperature of the ascending and descending waters. Much of the movement, it is claimed, is necessarily lateral and towards channels filled with ascending waters.
It was maintained by Professor Posepny and Dr. Raymond that descending waters were merely oxidizing, and incapable of depositing sulphides. This contention has, however, been successfully disproved by Emmons, Becker, Van Hise, and other American geologists, who have shown the existence of secondary sulphides both above and below water-level, or in what may be termed the zone of vadose circulation.
Professor Daubree always maintained that metallic sulphides could not be deposited without the agency of organic matter; but Skey, as far back as 1870, proved experimentally that from its solution of carbonate of soda, potash, and ammonia, gold is reduced by sulphides, but not from it a solutions in alkaline sulphides.* He found that 1 grain of iron-pyrites was able to reduce 8.5 grains of gold.
[Footnote] * W. Skey, Trans. N.Z. Inst., Vol. iii, 1870, p. 226.