Popular Science Monthly/Volume 72/June 1908/The Genesis of Ores in the Light of Modern Theory
|THE GENESIS OF ORES IN THE LIGHT OF MODERN THEORY|
IT is well understood, but often forgotten, that all the constituents of ore deposits are found in some form in the earth's crust, contained in more or less abundance in the rocks, especially in the eruptive rocks; and that they have been in some way collected from their disseminated condition in these rocks, and concentrated in veins, beds or other deposits.
Analyses of fresh eruptive rocks have demonstrated the existence therein of all of the ingredients of our valuable ores and their compounds. Few of them occur native like gold, silver, copper and platinum; and often, because of their minute quantity and fine state of subdivision, it is not possible to determine the precise form in which they are present.
The presence of sulphur, arsenic, antimony and tellurium indicates that there may be many metallic combinations in the eruptive magmas similar to those formed at later periods, nearer the surface.
The average composition of the earth's crust has been approximately estimated as follows:
Copper, lead, zinc, tin, silver and gold, although metals of great importance to man, constitute so small a part that their percentages are expressed by four to eight decimals, that is, between hundred thousandths and billionths of a per cent.
In some eruptive rocks, however, the percentage is much higher, and has been determined to be in the thousandths of a per cent. in the case of copper, lead and zinc, and one tenth to one hundredth as much of silver and gold.
The amount of metallic content found to occur as a primary constituent in unaltered rock is thus seen to be far too small to constitute workable ores, and indeed is often so insignificant as to be determined with difficulty. You all know that several per cent. of iron, manganese, zinc, lead and copper are required to make an ore valuable, the percentage varying, of course, with the locality, complexity of the ore and other familiar factors.
It is therefore apparent that a process of natural concentration is essential for the production of ore deposits, bringing into limited space the material formerly disseminated through ten thousand or a hundred thousand times that extent of ground, or accomplishing the same result by the removal of the admingled rock impurities.
Wherever this concentration is brought about by assembling of solid particles under conditions that admit of freedom of movement, we have placer deposits as of gold and platinum, of tin, iron and chromium ores, and sometimes of precious stones, such as diamonds, sapphires, rubies, garnets and others.
The ores found in veins, in disseminations throughout the rocks and in irregular shaped deposits in soluble rocks can not have been collected in any such manner. Their mode of occurrence and relation to the enclosing rocks make it evident that they have been slowly deposited from solution. And the only solvent of general distribution is water, with its varying content of acids and alkalies under changing conditions as to temperature and pressure.
Water is the magic instrument by which all the copper in Butte's vast mines, all the gold and silver of the Comstock and of Goldfield, were assembled; more potent than the Philosopher's Stone, more universal than the air we breathe; constantly at work, dissolving, transporting and redepositing. With indefatigable zeal and never-flagging industry it searches through the innermost recesses and penetrates the most closely locked chambers of the rocks, removing treasures through their very walls, and often repairing breaches made in the attack so skilfully as to defy detection, or to make the masonry stronger than when first laid. Small wonder that the ancients regarded it as one of the four prime elements!
But, although for several years water has been recognized as the great agent in the formation of ore deposits, geologists are not agreed as to the source of this water, the conditions under which it is most effective, nor the relative importance of its work in ascending and descending movements.
Regarding its source, we have those who believe with John Woodward, Franz Posepny and C.R. Van Hise that the water in the uppermost layers or outer zone of the earth, including the waters on the surface and in the atmosphere, accomplish the formation of ore by means of a perpetual circulation. From the air it falls on the earth as rain; through crevices and fractures it enters the rocks by reason of its head or the weight of more water on top of it, and finds its way deeper and deeper to the very lowest point where the density of the rocks will permit it to penetrate. Down to this depth, which is theoretically not more than five or six miles, the temperature has been constantly increasing, and the water by reason of this higher temperature has been gaining strength as a solvent and picking up alkalies or acids that enable it to hold even the most difficultly soluble substances in solution. Finding no escape downward, and urged on by cooler and heavier waters above, these saturated solutions begin to move laterally and upward, expanding and becoming of lower specific gravity because of the forced deposition of dissolved material as they become supersaturated. Following the directions of least resistance, these metal carriers reach the surface as hot springs or geysers through fractures caused by earth movements. Gradually the walls of these fractures become coated with vein minerals and ores, until the waters stop flowing or the fracture is healed and a vein is formed.
Then there are those like Vogt, Spurr, Weed and Kemp, who maintain that the chief source of underground waters is the unconsolidated magma of molten lava within the earth. These authorities point to the immense volumes of steam emitted from volcanoes; they call attention to the conclusions of European scientists who have decided that many of the hot springs can not be derived from meteoric waters heated and returned to the surface; they remind us that there is so much watery vapor derived from lavas that possibly the oceans themselves were formed from volcanic emissions. They point out the ease with which such waters, thus derived and so heated, could gather metallic substances at great depths and bring them to the places where they are now found. They mention the fact that there is a very general association between the more important mining regions and eruptive rocks; and they raise several serious objections to the premises of the disciples of the meteoric water school.
On this particular point we shall not dwell further; it is quite probable that both theories contain elements of truth; and that ore deposits have been formed by both magmatic and meteoric reascending waters. It is even possible in some cases to determine by the character of the minerals the origin and nature of the causative solutions.
As to the relative importance of the work of ascending and descending waters there is also divergence of opinion. There are few who still doubt the agency of descending waters in the formation of the oxidized ores, such as carbonates, silicates and oxides of copper, lead and zinc and silver chloride, or in the superficial or shallow alteration of the sulphides, arsenides or antimonides. The iron ores of the Lake Superior region, for example, are generally believed to owe their concentration to descending solutions, in this respect differing from many of the Scandinavian iron ores, according to recent descriptions.
It is not, however, the oxidized or "dry" ores alone that are now believed to owe their formation in large part to the action of descending waters; but the base ores consisting of chemical combinations of the metals with sulphur, arsenic, antimony, tellurium and some rarer elements. It is only within the past decade that it has been considered possible that the sulphide minerals are produced by reaction between sulphate or carbonate solutions and undecomposed sulphides or other minerals found in veins. Laboratory experiments have, however, shown that the operation is not only possible, but easily accomplished and duplicated under normal conditions as to temperature and pressure. This is a fact of great importance and wide significance, for it aids in the explanation of many formerly puzzling phenomena of mines and mining geology.
It has long been noticed by the students of ore deposits that by far the greater number of mines become exhausted at comparatively shallow depths; that veins, instead of continuing downward uniform in size and composition, like dikes of diabase and porphyry, become smaller and of lower value with depth, and often disappear altogether. It is noticed also that the shape of many ore deposits and the distribution and paragenesis of the minerals which they contain can often be better explained on the theory of descending than of ascending mineralizers. Moreover, it is apparent that there are changes constantly in progress in those portions of sulphide ore bodies lying nearest the surface of the ground. These changes consist in the oxidation of the sulphides and their solution as sulphates. These sulphate solutions percolate downward into the veins or rocks below along the most open channels; and thus, by degrees, the upper zone of the vein is robbed of most or all of its sulphide minerals, and only a gossan or iron cap remains.
The process of oxidizing and leaching out of the sulphides in the superficial zone of ore deposits tends, first of all, to disguise the nature of the unaltered ore below. In many instances the ore discovered from the outcroppings is gold ore. And gold mills are often erected and operated for years upon such ore, without a suspicion arising that extensive bodies of copper or lead sulphides occur at greater depths. Such was indeed the history of Leadville, Colorado; of Bingham, Utah; of Ely, Nevada, and of Mount Morgan, Australia. The latter is one of the world's greatest gold mines; yet it is now producing copper from its lower levels; and developments have proved it to be a great copper mine. Immense low-grade deposits of copper ore are found below the gossan at Ely and at Bingham, although it is doubtful whether the most experienced geologist or keenest observer of mineralization phenomena would in either place have felt justified in predicting the existence of the wealth below.
In other localities the metal values have either all been removed, or else the primary sulphide ore was too poor in gold to leave oxidized ores of value. In such cases the discovery of the subterranean treasures is purely fortuitous. Butte may be considered the most conspicuous example of this class. The outcrops of its copper veins contain the merest traces of that metal; and there is seldom enough silver or gold in them to justify mining even under the low costs obtaining there to-day. The zone of oxidation is generally from one hundred to two hundred feet deep; and if it had not been for the presence of another system of veins carrying silver, veins of different age and origin, but closely associated geographically, this greatest of copper camps might not yet have been discovered. It was in the search for silver ore that copper ore was discovered here, and one can not help wondering how many more camps equal to Butte may be undiscovered and unsuspected where no outcropping silver or gold mines attract the prospector, and reward the efforts of the miner. Here is surely an important and unexplored field for the geologist. The study of oxidized vein phenomena may yield results thoroughly satisfactory from both material and scientific points of view.
Below the zone of oxidation the chemical reactions which take place between the descending acid solutions and the unoxidized ores result in the formation of more and richer sulphides, down at least to the level of the lower limit of free circulation, and as far as surface waters penetrate. And as erosion of the surface is continually bringing deeper and deeper sulphides within the reach of oxidizing and dissolving surface waters the operation is in constant progress, and these lower-lying ores become more and more enriched until in some cases are formed bonanzas of world renown, and almost inestimable value. It is a fact of much significance that such bonanzas are generally limited to depths where descending waters may have penetrated at one time or another. Indeed the very channels through which the enriching solutions came can often be detected; and peculiarities of shape and position observed which can be explained with difficulty on any other theory.
Practised miners often point to the richness of ore shoots near the junction or crossing of veins. Indeed such pockets and shoots are usually sought and frequently found where two veins come together. This fact alone may not signify the instrumentality of downward moving waters. But when in connection with it we discover that rich ore shoots are also frequently found at the intersection of veins by faults, and zones of movement so recent or of such shallow depth or limited extent that the faults themselves are not veins, and have not been mineralized except near the intersected veins, and when the ore shoots thus formed occur on that side of the fault plane where they could have been formed most naturally by descending waters, and are wanting entirely in the corresponding place on the other side, then, indeed, we recognize beyond a doubt the agency of meteoric waters in both situations.
It is often possible where sulphide ores have been deposited in soluble rocks to distinguish between the products of ascension and descension, and here too the latter are frequently of much the highest grade.
This theory of secondary enrichment which is so frequently referred to in recent mining literature; and is still so little understood, depends, of course, on the existence of a body of primary ore, probably formed by ascending solutions. If there are no ores to be oxidized the downward moving waters will have no metalliferous burden to deposit. But wherever the rocks contain disseminated ore, no matter how small the percentage, there is a possibility of the formation of richer ores through the action of surface waters. And where the primary mineralization was itself comparatively rich, even though not a minable product, there the downward-moving waters may the more readily bring about concentrations of high-grade bonanza ore.
Bearing in mind this conception of the meaning of "secondary enrichment," and admitting that it is frequently accomplished through the agency of descending meteoric waters, let us briefly consider the conditions under which they are most active and efficient:
It is a proposition requiring no argument that if by the aid of mineral bearing solutions the ores occurring in veins are to be enriched, these solutions must enter the veins. And if all the meteoric waters which fall upon the outcrop of a vein or upon rocks containing disseminated ore run off rapidly down the mountainside without remaining to oxidize, dissolve and penetrate the vein with their load of mineral, there can not be any enrichment caused thereby. Furthermore, if the work of the surface waters is chiefly destructive mechanically instead of chemically there will be little opportunity for the deposition of secondary concentrations of ores within the rocks. If, for example, the principal effect of the rains and snows is to erode and wash away the exposed portions of veins with all their contained ores, there will be a scattering and wasting instead of an assembling and storing. In other words, secondary enrichment by descending waters depends first of all upon the ratio of oxidation to erosion. Where erosion is more rapid than oxidation the unoxidized sulphides will be found in the rocks and veins at the surface of the ground, and in the sands rolling down the beds of torrential streams as in Alaska. While if oxidation precedes erosion the uppermost zone of a sulphide ore deposit will be oxidized and leached of its base minerals, as is the case here in Butte, and to varying extent over the larger portion of the temperate zones of the earth. Assuming that the conditions are such as to permit the entrance of surface waters, and that the ground-water level is at some depth, which depth naturally varies from year to year and age to age because of many common geological phenomena, the factors upon which depend the extent of secondary enrichment are: (1) Quantity of water, (2) time, (3) temperature, (4) the physical structure and solubility of the rock containing the primary ore, and of the ore itself.
It is manifest that a large supply of mineralizing solution will accomplish greater results than a small supply, provided it follows the course of the ore. For the metals in solution can hardly escape precipitation by reaction with the primary sulphides present, sooner or later, at some depth; and the oxidizing and dissolving effects will certainly increase with the amount of active oxygen-bearing moisture available. In regions of very little rainfall there may be partial oxidation to the depth of several hundred feet; and yet there may still remain particles of the primary sulphides upon the very surface of the rocks. Chemical activity is great; but the thirsty rocks quickly absorb that part of the water of rains and melting snows which is not evaporated, and the work of oxidation is not so complete as in regions more plentifully supplied with rain. On the other hand, there may be such heavy and constant downpourings of rain, even in tropical regions, that erosion is again the most active agent.
The second of our factors is time; a commodity of which the geologist is accustomed to make most liberal and even extravagant use in his arguments and theories. In this he is frequently justified; and the most astonishing results may be produced by the long continued but slow operation of natural forces in any given direction. Events of the past few years have, however, reminded us forcibly that catastrophic phenomena must not be forgotten in comprehensive reviews of the earth's history.
The time element enters in a variety of ways into the problem of ore formation by descending circulations. Thus an ore deposit formed in its primary, low grade constitution during earlier geological periods, such as the Cambrian or Huronian, and during all of the subsequent ages exposed to the action of superficial agencies unhampered by subsequent covering of later rocks, has a thousandfold the opportunity for concentration of its ores that is presented by similar rocks and ores formed during later geological epochs, say the Tertiary. This is exemplified by the iron ores of the Mesabi range as contrasted with the glauconitic deposits of New Jersey or Texas. During almost all the ages since the Cambrian the iron ore formation of the Mesabi has been exposed to the weather, covered only for a geological moment during a part of Cretaceous time. The result is the largest and purest deposits of iron ore ever discovered, while rocks of similar composition but much more recent formation exhibit only the initial stages of ore formation.
Another way in which time affects ore deposition is in connection with the rate at which the waters move in a vein. Solutions of a given composition may move so rapidly as to produce but little effect, or may move so slowly that they clog up or retard other active waters after their own power is exhausted. Upon a steep drainage slope or mountain the waters may pass off so rapidly, even below the actual top of the ground, as to exert but little influence, or they may move with just sufficient rapidity to accomplish their maximum of chemical effect.
Our third factor, temperature, is of great importance. In the first place, oxidation, which is but another name for combustion, is greatly accelerated or retarded by slight changes in temperature. Sulphides which remain immersed for centuries in water under a glacier in Alaska would be completely oxidized in a few years exposed to the heat of the sun on a southern slope in Colorado or California. In the next place, the rate of solution depends directly upon temperature, increasing as the temperature rises, and, itself a process of heat consumption, is greatly facilitated by heat from external sources. Thus in warm rocks, in mild climates, upon the sunny side of mountains, there will be the most favorable conditions as regards temperature, for the formation of secondarily enriched ore deposits. The experienced prospector will tell you that it is in precisely these localities that they are found, although he never before heard any explanation for it.
Lastly, the physical structure and solubility of the rocks and ores affect their susceptibility to later enrichment for perfectly obvious reasons. A dense rock is not readily entered by mineralizing solutions. Likewise an insoluble one is not easily replaced and does not afford lodgment for ores. And if the ores themselves are not readily attacked by oxidation or by solvents, the quantity, time and temperature may all be sufficient to accomplish great results with more tractable ores, but have practically no effect upon these refractory ones. A good example of this again is found on the Mesabi range where the heat of an eruptive rock has so altered a portion of the iron formation for many miles that it has resisted surface solution and concentration, and is a worthless low-grade mixture of rock and magnetic ore still; while away from the influence of the eruptive, have been formed the iron ore deposits which have given to the iron and steel industry of this country the raw material required to make us preeminent in the markets of the world.
Reduced to more simple language and ideas the foregoing remarks amount to a statement that climate, sun, rain, average temperature, topography, depth of soil or surface debris, erosion, glaciation and other common and often unobserved influences and conditions have decided bearing upon the important question of ore formation.
These are the phases of our modern theory that have received little attention hitherto; and are yet of practical value that can hardly be overestimated. We find few bonanzas of high-grade ore in Siberia, Russia, Alaska, British Columbia, Washington or northern Ontario. Our theory tells us why they are not to be expected, and why such enriched ores as are found seldom extend downward to great depths. We turn to regions of milder climate, less glaciation, gentler topography, and we find the rocks altered and softened and oxidized to some depth below the surface. We find that the veins wear "iron hats"; and beneath them we find bonanzas reaching to great depths. We find our best ore shoots on the sunny sides of the mountains, while the veins on the northern shaded sides where the snow lies till mid-summer and the rocks are cold produce no such rich ore. We begin to realize that our theory is based on fact and proved by observation; and that it justifies us in placing confidence in it, and in acting upon it within reasonable limits. And we marvel that facts so simple and of such easy comprehension and yet of such practical value should receive so little attention from the writers on ore deposits.
- F. W. Clarke, Bull. U. S. G. S., 148, p. 12; J. F. Kemp, Econ. Geol., I., III., 210.
- H. V. Winchell, "The Synthesis of Chaleocite," Bull. Geol. Soc. Am., Vol. XIV., pp. 269-276, 1903.