Table of Contents
- Geology and Deep-Seated Mineral Deposits
- Classification of Ore Deposits
- Microscopic Study of Ores
- The Source of Ore Material
- The Theory of Lateral Secretion
- Gold in Sea Water
- The Agencies of Gold Transport
- The Circulation of Ore-Depositing Waters
- The Deposition of Ores
- Surface Enrichment
- The Depths of Ore Formation
When asked to lecture at Bendigo on the geological factors that determine the depths of ore deposits, I hesitated for a little before agreeing to lecture on the theory of deep ores in the city world-famous as the pioneer of deep gold mining. If there be one place, wherein the conditions of deep-seated ore deposits should be sufficiently well understood, it is the city where the 180 mine made its bold plunge to previously neglected depths, and where the record of that famous mine has been broken by the skill of the managers, and the pluck of the proprietors, of the New Chum Railway and Lazarus mines.
Geology and Deep-Seated Mineral Deposits
In the face of such practical triumphs, theoretical discussion may seem a waste of time, especially since it is often declared that the existence of such deep-seated ores was denied by geological theory. This assertion is, however, startling since the oldest and best supported geological theory of ore deposits, has taught for more than a century that ores must descend far below the regions of practical mining. This charge against Victorian geologists is generally based on opinions attributed to the late Sir FREDERICK McCOY. It has been repeatedly affirmed that he declared that gold cannot occur below a depth of 300 feet. But so far I have not been able to discover that McCOY ever made any such assertion or ever held such a view. He expressed his opinion on this subject before two parliamentary commissions. He first appeared as a witness before the commission and one answer on that occasion, in emphasizing the then greater mercantile value of the superficial deposits, appears to give some basis for the charge. He said, in answer to one question:— “Just so; that the richer deposits are nearer the surface of the ground, and being so found, experience shows that those, veins of quartz and other materials containing gold in various parts of the world, have been already scientifically and liberally worked in other, countries, and the gold has been found invariably to fail the lower you descend ; and, on the other hand, that the silver and copper have increased under the like circumstances. Therefore I merely wish to allude to the greater mercantile value of the alluvial deposits and to show that machinery had been applied to the quartz reefs and solid rock where they are manifestly auriferous; that it would be wrong to take the quartz reefs in all places as indicative of a large amount of gold, and to advise the planting of expensive machinery on the idea that it would pay well by perseverance.”
McCoy’s use of the word “fail” was open to misconception ; but he no doubt meant diminution and not absolute disappearance. The reply has to be read with its context, when it is seen to be a protest against the view, then widely advocated in Victoria, that all quartz reefs are necessarily auriferous; that if the quartz at the surface does not pay, all that is necessary is to erect costly machinery and work downwards when payable ground must inevitably be found. McCOY’S statement was a warning against BRACHE’S declaration, that there are 200 square miles of quartz reefs in Victoria; that the whole of the quartz is worth on an average at least $10 per ton; and that the gold supply of Victoria could be safely estimated at $260,783,000,000 which would keep 100,000 miners busy and prosperous for 300 years.”
McCOY’S caution was directed against such over-sanguine estimates and the disastrous consequences that would result if they were acted upon. That this is so, we may clearly see from the carefully worded paragraph, in which the commission of which McCOY was the scientific member, discussed the question of variation of quartz veins in depth. The commission stated that it had especially made enquiries as to the variation of reefs in depth and that “ so extraordinary a uniformity has been found in the answers, that, even giving all weight to the exceptions adduced, there can be no shadow of doubt as to the almost universal general truth of the fact (taking the circumstances on a large scale and without reference to small irregularities or exceptions) of the cap of the reef, or most superficial portion, being the richest. We here desire to make clear, that no special importance is attached to the actual surface of the ground, and no particular number of feet is contemplated as likely to mark the limit of depth from that surface at which gold may be found; but we wish to fix attention on the simple fact of the general ratio of the richness of the vein diminishing, in most cases, with the depth from the cap of the reef, at whatever depth from the surface.”
It is by the deliberate opinion expressed in the report of the commission, that McCOY must be judged. Compare it with the recent verdict of LINDGREN in his monograph on the gold-quartz veins of Nevada city. “ It is well known,” says LINDGREN, “ that the ores above the water level are, from causes of local concentration, richer than those not altered, and an impoverishment may generally be expected below the water level.” So McCOY’S conclusion is repeated by one of the most eminent of American Mining authorities. In fact, in McCOY’S official opinion there is not one sentence that with the light of forty years further evidence, requires serious correction or revision. Surely every mining man will admit that, allowing for local irregularities and certain well-known exceptions, it is a general rule that quartz veins are richest and pay best at the surface; that ores are poorer and less docile at greater depths; and that the advice given by the commission was the wisest advice that could, have been given to the young mining industry of this State.
Classification of Ore Deposits
A similar misconception as to the teaching of geologists was widely expressed eighteen months ago, when the occurrence of gold in the “granite” at Mt. William was described as a staggering blow at geological theory, although two of the oldest of European gold fields have been worked in granite for centuries and centuries.
These misconceptions as to the teaching of geology are due to the wide adoption of a classification of ore deposits founded entirely on a practical and not a theoretical basis.
The classification, which has found such favour, among leading English authorities, that it is sometimes described as the English classification—divides ore into the following groups.
This classification is stratigraphical and it rests on the form of the deposits: The merits of this classification are that it is comprehensive; it is readily understood; it is easily applied. But it has one fatal drawback. It is merely descriptive; it is not genetic; it simply states facts without making any attempt to explain them. Such a classification which takes no account of the origin of an ore, can give us little guidance as to its distribution. And because this stratigraphical classification is an empirical statement of facts and ignored theory, it has done little to advance the science of ore deposits and has been a fertile source of error.
The editor of a country newspaper once incurred unpopularity by attributing local agricultural depression to the fact that the clumsy farmers, instead of carefully picking the turnips by hand knocked them off the trees with sticks. That journalist relied on practical classification of agricultural produce. A theoretical classification of vegetables would have told him that a turnip results from the aldermanic development of a root and has nothing in common with an apple. Knowledge of a theoretical classification would have saved that journalist much practical inconvenience. And it is no paradox to say that for a classification of ore deposits to be practically useful, it must rest on a theoretical basis.
Microscopic Study of Ores
Recent progress has come from the recognition of this view. It is now recognised that in describing ore deposits, it is not sufficient merely to describe the external form of the deposit. The internal structure and mode of origin must also be determined. The study of rocks has been revolutionized by the application of the microscope to the examination of transparent rock slices. The same method has enabled great progress to be made in the study of ores.
According to Sir ARCHIBALD GEIKIE “A rock may be defined as a mass of matter composed of one or more simple minerals, having usually a variable chemical composition, with no necessarily symmetrical external form, and ranging in cohesion from mere loose debris up to the most compact stone.
Accordingly an ore is as much a rock as a sandstone or a bluestone, and ores must be investigated along the lines that have proved most fruitful in the case of rocks.
The study of a rock involves three lines of inquiry:
1st. The characters of the materials of which the rock is composed.
2nd. The determination of the agents by which the materials were transported to the place where the rock was made.
3rd. The nature of the processes by which the materials were deposited to form the rock.
If a geologist be asked to report as to the probable range of a rock, of which he is given a hand specimen, he first determines the nature of its materials. Suppose the rock consists of grains of quartz, felspar and mica. These materials are all of igneous origin; they are the three constituents of granite. But he cannot say whether the rock be granite until he knows how the materials were transported and under what conditions they were deposited. Were they raised to their present resting place in a molten state, from below ? Or were they carried by wind and stream from the flanks of an old granite range ? The microscope will answer these questions ; it will tell whether the rock be a primary igneous rock or whether it be a secondary deposit. It will show whether the rock specimen was part of a vast mass that had consolidated at a great depth —granite, or came from a thin wall-like sheet that solidified nearer the surface—was deposited on the surface as a sandstone.
These three points having been determined, the rock can be named, and the geologist can report as to its probable vertical and horizontal extent. Similarly with an ore; it is convenient to study it from the three points of view, the source of materials, their transport and their deposition at the site of the ore.
The Source of Ore Material
Whence then came the materials of which ores are composed ? All the materials of the earth’s surface, with some insignificant exceptions, have come from one of two sources. They were either derived from material dissolved in the sea or in the waters of lakes and rivers ; or else from molten rocks, raised from beneath the earth’s surface. Take a sandstone for an example ; its grains may have been derived from some older sandstone and that in turn from a yet older sandstone, or from the deposit of a mineral spring. But whatever vicissitudes those sand grains may have experienced, the silica of which they are composed must have come originally from the substance of igneous rocks.
Similarly with a clay, the ultimate source of its materials is the minerals of igneous rocks. Again with a limestone, its carbonate of lime must have come either from matter dissolved in the sea, or from lime salts forming part of some igneous rock.
The first modern speculators as to the source of ores, without hesitation attributed them to igneous rocks, at great depths below the earth’s surface. It was at first a safe appeal to the inaccessible. But a scientific basis to this “Pluto’s Hoard” was given, when it was found that the earth as a whole weighs twice as much as it would do, if it were composed throughout of material only as heavy as the surface crust. Somewhere below the surface there must be material much heavier than the rocks that form the crust. Accordingly it seemed reasonable to assume that the extra weight of the interior is due to the material there, containing extra abundance of metallic constituents.
Hence, when ELIE DE BEAUMONT and DAUBREE assigned ore veins to emanations from deep layers heavily laden with metals, they had mathematical support to their theory.
This view however, was never without opponents. Various facts suggested that the ores came from above or from the sides rather than from below. Veins become thinner or even pinch out altogether as they are followed downwards, as is shown with unfortunate clearness at some mines at Castlemaine. In such cases it is easier to explain the entrance of materials that fill the veins from above than from below. Then again lodes often diminish in their yield of metal as they are worked deeper, of which sad fact we have cases in every Victorian gold field; whereas the ores might be expected to become richer as they approach the supposed source of supply. Further, those veins known as ‘ gash veins ‘ are confined to one bed of rock and must necessarily have been filled from it; yet these veins, in other respects, are identical with the larger veins.
Another powerful argument is derived from the character of the vein-stuffs, the non-metallic minerals which fill the veins and through which the ores are scattered. These vein-stuffs often vary with the nature of the rock beside the vein. Where the vein traverses sandstone, there is a preponderance of quartz; where it traverses limestones, calcite is usually the chief mineral in the vein-stuffs. It looks therefore as if the vein-stuffs had come, not from below, but from the rocks beside the vein. And the ores and vein-stuffs are so intimately associated that if seems most probable that they must both have been introduced by the same agency and been derived from the same source.
The Theory of Lateral Secretion
Hence arose the theory that the ores were superficial in origin and that they entered the vein fissures from the adjacent rocks, from which they were leached by slowly percolating water.
The evidence of many mining fields gave strong support to this hypothesis. In Cumberland the lead lodes are broad and ore bearing in limestones ; they are thin and unproductive where they cut across sandstone and shale. Here then is one instance of the connection of the lode-contents and the country rocks. Another case, which illustrates this fact, is furnished by the copper field of Thuringia. There the country consists of a basement of granite and mica schist, covered by a red sandstone (Rothliegende), a fetid limestone (Zechstein), a thin seam of slates containing copper (Kupferschichten), and finally a bed of marl slate. The country is traversed by veins mainly filled with barite. Where the veins cut through the copper slates they contain ores of copper, nickel and cobalt. It was claimed that the copper had been once in solution in the sea in which the slates were deposited. The fossil fish found in these slates are very contorted, which was attributed to their having died in the agonies of copper poisoning. Then the copper was precipitated on the seafloor, in the mud that since hardened into the copper slates. Water percolating through the slates dissolved out the copper and redeposited it in the veins.
This view of the origin of mineral veins by processes of ” lateral secretion ” was not the work of one man. It was given more scientific expression by the great German chemist BISCHOF, who urged that the ores were obtained by the leaching action of water on the rocks traversed by the veins. BISCHOF’S advocacy gave the theory a fresh start and attempts, were, made to demonstrate the existence in various sedimentary rocks of their supposed, metallic contents. Elaborate rock analyses were made for this purpose. The search was not wholly in vain. Eight years after BISCHOF’S opinion was published, FORCHHAMMER discovered, traces of lead, copper and zinc, in the roofing slates of, North Wales; and it was therefore claimed that the mineral veins of that area might have been derived by lateral secretion from the slates. On, the whole, however, the results of, the search were disappointing and it; was not until later, that the theory made any important progress. Then a distinguished German mineralogist, FRIDOLIN SANDBERGER, who was dissatisfied with the previous attempts to test BISCHOF’S theory, published the results of a detailed research on the ores at Schapbach, in the Black Forest. He made an extensive series of analyses of the ores, veinstones and country rocks of that mining field. His analyses showed conclusively that the veinstones varied in composition with the variation of country rocks; but he could find no definite connection between the ores and the country rocks.
Then a bright idea struck SANDBERGER, regarding the probable distribution of the metallic constituents which BISCHOF assumed to exists in sedimentary rocks. Such rocks usually contain broken fragments of minerals derived from igneous rocks. Such fragments are known as accessory, constituents. Now it occurred to SANDBERGER that the ores might occur in these accessory constituents. If so, a trace of metal, that might be appreciable in proportion to the accessory minerals that contain it, might escape detection when diluted by the barren material that forms the main bulk of the rock. SANDBERGER accordingly crushed considerably quantities of various sedimentary rocks; he separated the accessory constituents and then analysed these concentrates for traces of the metals.
The result was the discovery that small but appreciable quantities of various metals are scattered-through the sedimentary rocks of all ages and all classes, but especially in the slates of the older rocks.
After this epoch-making discovery, the main difficulty in the theory of lateral secretion appeared to be removed. The theory at once sprang into popularity. It is always a pleasure to explain a problem by the substitution of known agents, acting under conditions that can be watched, on materials that can be handled and analysed, in place of unknown forces acting under conditions that can never be artificially imitated, and on materials that must be for ever inaccessible. So the theory was soon adopted for many different types of ores on widely distant mining fields. EMMONS attributed the silver lead ores of Leadville to the lateral secretion from the over lying porphyrites. BECKER derived the gold of the Comstock bonanzas from the adjacent diorites and andesites. CHAMBERLAIN argued that the galena filled gash-veins of the Wisconsion lead fields could only have come from the surrounding carboniferous limestones; and KENDAL adopted the same explanation for the lead deposits of the North of England.
The application of this theory to gold was delayed by one gap in the evidence. SANDBERGER’S analyses themselves gave no direct proof that gold ores could be formed by “ lateral secretion,” for, in his experiments, he had not tested for gold, tellurium or mercury. But that gold must occur in minute traces in sedimentary rocks was maintained as a necessary consequence of its supposed existence in the waters of the sea.
Sea water was first thought to contain gold upon indirect evidence. But SONSTEDT announced that he had positive proof that gold occurs in the sea to the amount of something under one grain of gold to the ton of water. Gold is an element that is very readily precipitated from solution. If therefore it occurs in the sea, it must also occur in the sediments deposited on the sea floor. Gold quartz veins often occur in slates that were formed as marine sediments. SONSTEDT’S discovery, therefore, suggested a source for the gold, which lateral secretion could concentrate in quartz veins.
The amount of gold in a ton of sea water is no doubt minute; but the most impressive fact about sea water is that there is such a lot of it, and even a grain of gold to the ton would make up a prodigious quantity. Prof. LIVERSIDGE, who has found from half a grain to one and a-half grains of gold to the ton of sea water off the coast of New South Wales, has calculated that one grain of gold to the ton of sea water would yield 75,000 millions tons of gold, or almost 75 tons of the precious metal for every man, woman and child on the globe. With this inexhaustible treasure to draw upon, the sea could, yield all the gold of our quartz reefs and be but little the poorer. If this amount of gold be present in the sea, then the advocates of the theory of lateral secretion are not hampered by difficulties as to paucity of supply.
Nevertheless, the fascinating lateral secretion hypothesis has proved a disappointment. It has been untenable in the cases of at least the great majority of ore deposits. For instance, the mining field of Butte, in Montana, is a great mass of granite containing mineral lodes of two sets, one of copper, the other of silver. Now, if the ores had been derived by lateral secretion from the granite, why should this same granite have contributed copper to one set of lodes and silver to the other ? The differences in the mineral contents of these lodes cannot be due to differences in the country rock; so we must seek for some agency outside the granite in which the lodes occur.
Hence the lateral secretion theory is inadequate to explain all the facts. A second fatal objection is that it appeals, to sources of supply which are still unproved, and may be non-existent. That metals occur in the silicates found in slates cannot be denied. But the opponents of the lateral secretion hypothesis maintain that these metals are not primary constituents of the silicates, but have been introduced into the silicates after their formation by hot solutions, or vapours. If so, then the source of these metalliferous solutions and vapours and not, the rocks on which they acted was the original home of the metals. For instance, when visiting the Mount William rush in August, 1900, I found that colours of gold were being obtained from the decomposed plutonic rock, and some more gold from thin quartz veins in the same rock. On the lateral secretion hypothesis, the gold in the plutonic rock was primary, and that in the quartz veins secondary. But there is no evidence of such a difference, and it is equally probable that the gold in the grano-diorite and the quartz came up together in solution from below.
Chemical investigations render it most probable that the metals discovered by SANDBERGER in the silicates are secondary constituents, as they are in mineral lodes. That is to say, the silicates are only banks wherein the metals rest while in circulation—the silicates are not the mine whence the metals originally came. Hence, though lateral secretion may explain the concentration of metals into lodes, it does not tell us the original source of the metals.
Gold in Sea Water
Let us turn therefore to the second possible source of the metals, the material floating in solution in the sea. The asserted discovery of gold in sea water dates from 1,856, when it was found that some Muntz metal plates, that had been part of a ship’s sheathing were richer in gold than similar Muntz metal, that had not been exposed to the sea. FIELD suggested that the excess was gold that had been in solution in the sea and had been deposited on the Muntz metal by electrolytic action.
Since then several chemists have claimed to have detected gold in appreciable quantities in ordinary, sea water. Prof. LIVERSIDGE, one of the most distinguished of Australian chemists, has found from half to one and a half grains of gold to the ton of sea water on the coast of New South Wales.
Many patents have been taken out for processes, invented to exploit this vast mine of idle gold.
Other chemists, however, have failed to confirm these sanguine expectations. DON, of Otago, found either no gold in sea water or only the minutest trace, viz. .07 of a grain per ton, or one part of gold to two and a quarter million parts of water, or .000,000 42%. To deduct such infinitesimal quantities is impossible except by chemists of exceptional skill in manipulation and working with chemicals of almost miraculous purity. So before we take the existence of gold in sea water as established we must consider whether its existence there is reasonably probable. The most plausible hypothesis as to the form in which the gold may occur in sea water represents it as occurring in what is described as a double chloride of gold and potassium, and that salt is very readily decomposed ; the chloride is broken up and its gold precipitated;
“ It would be quite inconceivable,” says Prof. Louis, M.A. “ that metallic gold should not be deposited from this sea water, together with the sedimentary rocks forming on the sea, bottom, if it is allowable to take the ordinary laboratory re-actions of gold as our guide.”
Thus even if a supply of gold solution were actually poured into the sea, it is difficult to see how it could stop there, in the presence of the many agents tending to cause its steady elimination. Moreover, careful analyses prove that sea- deposits, even under conditions favourable to gold deposition, contain no gold. Thus DON found no trace of gold in muds now forming on the New Zealand coast. DON has also proved that the marine sedimentary rocks of Victoria are free from gold, except when they contain minerals that have been subsequently introduced. Thus at Ballarat the slates only contain gold when they also contain pyrites; and the agents that introduced the pyrites probably also introduced the gold.
We may at any rate safely conclude, even if sea water does contain gold, that it is quite unproved and is highly improbable that this source supplied the gold of our quartz veins.
Hence neither the metalliferous silicates, nor the waters of the sea can supply us with the primitive source of the metals. We are driven in our quest from the possible superficial sources to the deep belt below the earth’s crust.
The Agencies of Gold Transport
If the gold originally came thence, we must enquire what are the transporting agents, that have brought it from its deep subterranean vaults.
Three distinct groups of agents have been invoked.
- Gases charged with metallic vapours.
- Molten rocks injected as igneous dykes.
- Hot waters carrying ores in solution, which are deposited when the waters are cooled by reaching the cold upper zone of the earth’s crust.
Each agency has had its champions, and each agent has probably taken some part in “ ore formation.” Ascending vapours may account for the tin ores of Glen Wills. The igneous origin of the gold quartz veins was long a popular belief among Victorian miners, and HOWITT has shown that there are good grounds for this belief in the case of some quartz veins. But such injected veins are scarce and economically unimportant, and there can be no doubt that the overwhelming majority of gold quartz lodes and other mineral veins is due to the action of pecolating water.
The Circulation of Ore-Depositing Waters
Hence it follows that the depth to which ores can extend will depend on the depth to which water can exist and act. Therefore, we must consider what are the factors that control the action and circulation of subterranean water.
Whence comes this ore-depositing water ? According to one school of geologists, including all the lateral secretionists, it is rain water that has worked its way down through cracks and fissures and pores to a great depth below the surface. The water is pulled down partly by the attraction of gravity, but mainly by that impulse which causes a current to spread through the full width and depth of the channel open to it.
If we pour water into a vessel at one side and let it escape at the opposite side, the water does not flow as a narrow stream, direct from its point of entrance to its exit. It flows most quickly along that line ; but every drop of water in the vessel takes part in the movement. The current spreads sideways and downwards till, in the technical phrase, it utilises the whole available cross section. Accordingly, as has been shown in a valuable and suggestive memoir by VAN HISE, if we take the earth’s crust and let water enter at one point and flow out at another, the current is not confined to the shortest route between the two points. It spreads out laterally and extends downwards. If there were no countervailing agent, the water would accumulate until the whole of the rocks below the surface belt would be sodden with heated waters.
The possible existence of this deep reservoir of ore-secreting water is an interesting conception. If it be true, then there is beneath us a sea, broader, deeper, and more powerful than the oceans upon the earth’s surface ; for this subterranean sea would be literally boundless, underlying alike ocean and continent, ranging unbroken from the equator to both the poles; its least depth would be greater than the deepest of known oceanic abysses, and its superheated waters would have far greater chemical power than that of our comparatively cold and inert seas.
But is this subterranean sea a reality? Does the water work down to the required depth and there accumulate ? It is doubtful. Below the depth of 10,000 feet the pressure is so enormous, that the heated rock must be plastic, and open cavities and water channels cannot exist. The largest possible spaces are capillary apertures. But that capillarity would suck water downward far below the level to which gravity alone would take it, has long been claimed on the faith of a famous experiment by DAUBREE.
DAUBREE mounted a glass tube containing water over a slab of porous stone covering a chamber full of steam under pressure. In spite of the opposing pressure of the stream, the water in the tube worked its way downward through the slab. Hence it has been claimed that water can work its way by capillarity against pressure, and thus could reach great depths below the surface. But that experiment can prove nothing which has any practical bearing on ore formation. As Prof. KEMP has reminded us, O. FISHER has shown that the water in that experiment worked its way downward because there was a free air space below it. Otherwise the descent of water would not have taken place. As there is nothing in the earth corresponding to the free space in DAUBREE’S experiment, that, does not prove the conclusions that have been based on it.
Not only is there no proof that capillarity will carry water to great depths, but there is evidence that the supposed world-wide subterranean ocean does not exist. If it did, then water should occur all over the earth at a certain depth, and we need only sufficient perseverance to find water at whatever spot we may choose to seek for it. On the contrary, deep mines and deep bores are generally dry. KEMP has called attention to many cases in Europe and America where bores have reached a vast thickness of dry ground beneath a laye containing water. Australian experience supplies additional; illustrations. The deeper levels of the Lord Nelson mine at St. Arnaud yield barely sufficient water for battery purposes; and the deeper levels at Bendigo meet with but little water, or only with alkaline springs of deep-seated water.
The experience of both the hydraulic engineer and the miner show that there is some force that overcomes the downward tendency of meteoric surface waters, and confines them to comparatively shallow depths. The nature of that counteracting force can be most simply explained by reference to the action of artesian wells.
According to the conventional interpretation of an artesian well, the water works its way downward along a water-bearing stratum between two impermeable beds. The water accumulates in the deeper part of the permeable water-bearing rock under the pressure of the water in the higher part. If the upper rock layer be pierced by a bore, the water imprisoned below it rises to the surface owing to hydrostatic head of the water in the higher part of the water-bearing bed. This explanation would be satisfactory within certain limits if the porous bed consisted of a layer of open tubes. But in nature artesian wells occur where there are no tubular spaces, and where the water works its way along minute crevices and capillary pores. Under such conditions the pressure of the water in the higher part of the bed is soon sopped up by the friction of the water against the sides of the channels. The hydrostatic head is lost long before we reach the bore.
The conception that the water in the artesian wells at Lake Eyre rises to the surface owing to the pressure of the water in the Queensland hills is a fine exercise for the imagination. But the real cause of the ascent is local. It may best be explained by reference to the action of geysers. The old notion of geyser action explained it as due to the periodic discharge of a subterranean cavity by a siphon. But the sip on is as unnecessary as is the hydrostatic head in the case of most artesian wells. The real action is simpler. A geyser is a vertical shaft into which hot water slowly percolates. The temperature gradually rises until the surface water is boiling and the water at the bottom of the well is above the boiling point, but is kept liquid by the pressure of the overlying water. At length the lowest water becomes so hot that the tension is sufficient to overcome the pressure of the upper layers; the superheated bottom water explodes into steam and the mingled mass of water and vapour is hurled from the geyser throat.
Artesian flow is probably in most cases due to the same force—the earth’s internal heat. The temperature below the surface rises one degree F.—according to the generally accepted average—for every 55 feet of descent. Hence at the depth of 8250 feet the temperature is that at which water boils under the normal surface pressure. But the pressure increases even more rapidly than the temperature, and prevents the boiling of the water. If the pressure be relieved, then the water bursts into steam; or, at a less depth than 8250 feet, it would allow the expansion of the water and its contained gases. The artesian bore through the upper impermeable strata relieves the pressure on the heated water, and accordingly its expansive force and imprisoned gases compel it to rise to the surface.
This force is always, working and steadily resisting the downward movement of the surface waters, which are thereby confined to the surface zone. Accordingly many geologists hold that the ore-forming waters are those given off from the igneous rocks, which everywhere underlie the sedimentary rocks of the earth’s surface. Nearly all igneous rocks contain much water and give it off slowly as they cool. Moreover, the character of the water in deep mines suggests that it has been derived from this source. Thus, the temperature of the water at the Comstock lode is far hotter than would be expected , from its depth, while the water in the deepest mines is generally a strongly alkaline solution.
Hence the water below the earth’s surface is of two origins, meteoric, derived from above, and plutonic, derived from below. The depth to which the meteoric water can descend is doubtful, but probably less than 8000 feet; below that depth, where the processes of ore formation are most important, the water is probably alkaline water derived from cooling masses of igneous rock. But even this water has a definite limit in depth ; for water cannot exist above the temperature of its critical point, viz., 687° F. and that temperature will occur somewhere about the depth of 37,000 feet. Below that line water as such cannot exist. It is dissociated into its elements, hydrogen and oxygen. Accordingly we may tabulate the water zones of the earth’s crust as follows : —
The Deposition of Ores
Let us now turn from the distribution of the ore-depositing water to the factors that determine the deposition of ores.
In the first place, these deep-seated waters are superheated, and superheated water is a solvent of unequalled power. The well-known experiments of BARUS, e.g., show that water at a temperature but little above the boiling point of water, will dissolve glass and some silicates. The greatest solvent effect is at a temperature below 400° F., so that water
will dissolve the greatest quantity of material at a depth of about 18,000 feet.
Secondly, the deeper waters will be under great pressure, and in the case of most substances, pressure will increase the amount a given quantity of water will dissolve.
Thirdly, water given off from igneous masses is generally alkaline, and alkaline water is the natural solvent of sulphides, which are the commonest ores.
Hence it follows that as soon as the alkaline waters are set free from a plutonic magma, that, being under great pressure and heat, they begin to dissolve ores and especially sulphides. After passing the belt where water has its maximum solvent power it passes into zones of lower temperature and pressure, wherein the water begins to deposit the material it has dissolved from the zone below.
Now, ascending waters given off from cooling plutonic masses naturally arise through vertical fissures and fault planes, caused by the injection of the plutonic rocks. The extension of a plutonic mass not only raises the temperature around it, but fractures and faults the overlying sediments. The escaping waters naturally rise up these fault planes and deposit in them the vein-stuffs and ores leached from the lower zone. We thus understand the constant association of mineral fields with plutonic masses and metamorphic rocks. The plutonic rock gives off the alkaline solutions that dissolve the metallic sulphides, it supplies the heat that forces the waters to ascend, and it makes the fissures in the overlying rocks, in which the cooling waters can deposit their load of ores.
So far we have used the term ore in its geological and not in its commercial sense. To a geologist, a rock is an ore whether it contains two ounces or two grains of metal to the ton ; whereas to the miner the former is a first rate ore and the latter only a second rate road metal. To the miner the metals uniformly distributed through a quartz vein are of little interest ; he is concerned with the ‘bonanzas,’ pockets, shoots
or chimneys where the ore is concentrated. There is no time this evening to go into the questions of shoot-formation, which may be due to a rock neutralizing the solution and causing the precipitation of the dissolved ores ; thus where a current containing sulphides traverses a bed of limestone, the ore is precipitated as a vein of galena or blende, or where a transverse fissure permits two solutions to mix the chances are a precipitate will be formed and deposited as an ore. Shoot formation is to be regarded as a result of chemical precipitation, whereas the uniform distribution of ores in vertical fissures is rather the result of mechanical deposition from solution by lowering below the point of saturation.
There is one type of ore concentration to which I must refer because it is so closely associated with range in depth. It is that “ surface enrichment ” of gold quartz veins which leads to the general rapid decrease in ore yield below the surface. It is often said that the yield of lodes dwindles from so many ounces to as many dwt. in a descent of 500 feet. It was therefore, once thought probable that the yield would continue to dwindle from dwt. to grains as we go still deeper.
Geologically there is nothing to support that dread, because the superficial enrichment is a secondary concentration in the surface belt, and once below that belt the ore may be expected to keep a more average composition. To illustrate this let us consider a possible case.
An auriferous pyritiferous quartz vein is being slowly formed by deposition from a solution using from below; the solution undergoes a fall in temperature of one degree for every 55 feet of ascent.
The solution will deposit its material regularly and uniformly, as its point of saturation is steadily reduced; but when the solution reaches the surface, its temperature falls rapidly to the air temperature and the whole balance of its matter will be deposited. Hence the primary condition
of such a vein, is uniform ore distribution in the lower zone, and with possibly a richer block on the surface. The vein thus formed is then slowly attacked by surface agents and its level lowered by denudation. The pyrites in the vein will be removed as a ferrous solution, which can dissolve a certain amount of gold. The gold therefore, originally present in the surface zone is dissolved and carried down. At a deeper level the character of the solution changes; it can no longer keep gold in solution, so the gold is all re-precipitated at a lower depth. The removal of another fifty feet of the surface by denudation will lead in turn to the concentration of the gold originally scattered through the uppermost 150 feet into one 50 feet depth of the vein. It will be at least three times as rich as the original vein. This secondary superficial enrichment may be repeated indefinitely until a Londonderry pocket may cap a comparatively worthless lode.
The Depths of Ore Formation
We have, therefore, now considered the general principles that control ore formation and can briefly summarise the limits in depth at which ores can be expected to occur. A genetic classification is here indispensable.
There are five main classes of ore deposits.
- Deposits in which ores are mechanically introduced, e.g., alluvial placers and drifts, such as the auriferous gravels of Victoria, and possibly the banquet of the Rand. In these cases the limit of depth is simple. The ore will not extend deeper than the bed in which it occurs.
The range of the bed is a simple problem of field geology.
- Ores of igneous origin—they are mainly masses of ores due to segregation of the basic materials in slowly cooling igneous rocks. Such deposits are relatively unimportant. There are none such in Victoria and they can only be expected where denudation has laid bare or deeply eroded into great masses of basic igneous rocks.
Where such ores occur they may be expected either as central lenticular masses, the depth of which may be inferred from the size and shape of the available exposures, or they may occur in sheets on the cooling surface of the igneous rock, in which case their downward extension, for practical purposes, is beyond calculation.
- Ores due to sublimation and the allied process of pneumatolysis, such as the impregnations of pegmatites by tin ores, or the alteration of granites into tin-bearing greissens.
In such cases, again, the amount of the downward extension is difficult to estimate, but is no doubt great. In the case of the pegmatites, the vapours probably come from the deeper parts of plutonic masses.
There is one case belonging to this category in which a limit can be definitely assigned, and that is where the ores occur in those altered lavas known as propylitic. The thickness of the lava can generally be approximately estimated, and the ore must be limited to that amount.
The limit in this group is not the source of supply, but the depth at which the ore-bearing current reached a temperature and pressure at which deposition would begin. That depth will be far below the reach of practical mining, though the exact depth in figures may be at present beyond estimation.
- Contact ore deposits occur near the junction of igneous and sedimentary rocks, especially limestones. Such deposits are formed by the replacement of part of the sedimentary, rock by solutions acting under the high temperature due to the igneous rock. This method of ore formation is limited to a narrow belt along the contact between the two rocks, and its depth is restricted to that of the shallower rock.
- The last class of ores is the most important. It includes those deposited by aqueous action in veins, whether fissure veins or replacement veins.
The limits of depth are threefold.
a. In the case of veins wherein the minerals occur in regular layers parallel to the walls, there is one rigid limitation. Such veins have been usually found in open spaces, and they can only be found at depths at which fissures and open spaces can exist. A probable maximum depth for them is 10,000 feet.
b. In the case of veins with irregularly distributed, minerals, the extreme limit of depth is that at which water can exist, viz., the iso-geotherm of 687°. At what is generally regarded as the normal rate of underground increase, this temperature occurs somewhere about 37,000 feet.
c. The depth at which water ascending from below can begin to deposit the materials it is carrying in solution. This will be immediately above the iso geotherm of maximum solution. For most bodies this is the isogeotherm of 400° F., at the depth of about 10,000 feet.
Geologically, there is nothing to forbid the expectation that the metalliferous veins will continue downward to that depth.
The previous estimates are all approximate. The figures will vary with different metals and veins and with the local circumstances. Each case must be determined separately, and after the genesis of the deposit has been determined.
Geologically, the depths of many types of veins may be expected to continue far below the limit of practical mining. But that limit itself may be greatly extended. It is only a matter of expense. Little by little the available limit of mining has been pushed downward. Once the assumed limit was 3000 ft.; then it stayed for a long time at 4000 ft.; now it is taken in the Rand at 5000 ft. And even that estimate is not final. The great obstacle in vein mining—the temperature—can be overcome by ventilation with compressed air. Liquid air gives the miner a power to which it is difficult to set a limit. If bores could be sunk to it, any enterprising mining engineer might take a contract to reduce to a cool and comfortable temperature the hottest recesses of Hades itself.
How far it may be commercially possible to follow ores to plutonic depths and what are the indications from which a superficial ore deposit may be expected to reach them, I have not to-night considered. I have tried to explain the principles that determine the depth at which ores are formed, and not the conditions that determine their economic value. This neglect of the latter question is not due to indifference; for the interests of geological theory and mining practice are inseparable. As our knowledge of the surface rocks grows more complete, the attention of the geologist is more and directed to the deeper zones of the earth’s crust. For access to them he is mainly dependent on the excavations of the miner, whom he can repay by help in tracking the ore shoots in their irregular subterranean courses. Accordingly, the geologist and the miner should in future work in closer co-operation than they have done in the past; and then the miner, guided by the insight of the geologist, and armed with the invincible ingenuity of the engineer, will compel deeper, and ever deeper, layers of the earth’s crust to yield to our service, their long hidden secrets and their well buried stores of wealth.