What is the use of a classification of mineral deposits? From the days of Agricola, the founder of the science of ore deposits, successive authors on the subject have attempted classifications, none of which have attained unanimous endorsement of miners, engineers, or geologists. The miner and engineer naturally have little regard for a classification that does not help them to find ore. Mining geologists may share this feeling, and deplore the overemphasis that may be placed on classification by their academic brothers. Apparent effects of such overemphasis may be reflected by students or new graduates who have grown to think that each deposit must fit perfectly into some pigeonhole of the more or less arbitrary scheme that has been drilled into them. Diversity in mineral deposits as in other natural phenomena, however, is too great to be fully expressed in the usual tabular classification, and experience is essential for the appreciation of the value or usefulness of a classification.

The succession of classifications has closely followed the growth of the science of mineral deposits and reflects that growth, as well perhaps as the limited or comprehensive viewpoint of each proponent. The earliest classifications were based on metal content, and were superseded by those based mainly on form of deposit. Source of materials also became a basis, and was followed by physical and chemical conditions and processes. In spite of this development it is interesting to note that in many of the latest reports on mining regions the authors adopt special classifications suited for their particular districts. Some of these, like the earliest classifications, are based primarily on metal content, and others are most conveniently based on form, and it may be convenient to modify these by reference to source of material or temperature of deposition.

Some, including the writers, after noting these different usages, have refused to worry much over classification as an ultimate objective, but, in efforts to make geology useful to the mining industry, they frequently find themselves depending on classification as a means of systematic and comparative presentation of facts and interpretations, and find themselves reaching the same conclusion that others have reached during the last 40 years or more: that in a comprehensive study of mineral deposits a classification based on origin has proved to be the most consistent and the best adapted to subdivision according to such less fundamental factors as form.

It is interesting to recall that Stelzner, one of Lindgren’s instructors, was with Von Groddeck one of the first mining geologists to appreciate and apply the genetic principle in the classification of ore deposits, but, as Lindgren says, “The time was hardly ripe for its introduction until conceptions of genesis had crystallized into fairly definite form. Stelzner remarks with good reason, that it is only by standing upon the ground of a genetic theory that the miner finds courage to sink deep shafts or drive long tunnels.” It is common experience to find that prospectors and “hard rock miners” have their own ideas of “how the ore got there” even though their experience has been too localized for them to appreciate the viewpoint of the geologist.

The development of genetic classifications by different writers up to 1894 has been reviewed by Kemp and briefly treated by Lindgren preparatory to a discussion of the requirements of a classification in which the following remarks are especially significant:

It is probably impossible to produce a classification which will win the approval of all. In the ultimate analysis by far the larger number of mineral deposits have been formed by physico-chemical reactions in solutions, whether these were aqueous, igneous, or gaseous. According to this view the only consistent division that can be made is that between deposits formed by mechanical concentration of pre-existing minerals and those formed by reactions in solutions.

The genetic classification should ultimately determine the limits of ore deposition in each class by temperature and pressure. Each deposit should be considered as a problem in physical chemistry, and the solution of this problem, with the necessary geologic data, will suffice to fix the mode of formation of the deposit.

This approach to the study of mineral deposits naturally leads to distinctions and classification even though that objective were not in mind.
We are far from having the complete material for such a classification, but we have at least a few starting points.
Some minerals are “persistent” and others of limited temperature range.

By collecting the data of mineral association, sequence of deposition, and stability range of the component parts of a deposit, it will be possible to ascertain the conditions prevailing at the time of ore deposition.

Perhaps it is well not to expect too much from physical chemistry, magnificent as its services have been. The complications, even in simple systems, become great when, besides temperature and pressure, concentration, mass action, and time must be considered. At the same time it is believed that the direction indicated is the only safe one to take in classifying the complex phenomena of ore deposition.

Lindgren proposes the classification, which consists of two main classes, based on mechanical and chemical processes of concentration, the second of which is divided and subdivided at some length. In his brief discussion of these divisions he shows the impossibility of keeping even the two main classes entirely separate, and, although he mentions the metamorphism and enrichment of ore deposits, he properly excludes these processes of alteration from his tabular scheme. Any deposit is subject to alteration of one kind or another, and a supplementary classification based on processes of alteration might be appended to the classification of original or primary deposits. The processes of alteration, however, are very similar to the processes that form certain of the original deposits, and receive due attention in connection with them.

Lindgren’s classification is the furthest advanced and has continued to be followed generally since 1911. Since that year more detailed classifications have been presented by Niggli and Schneiderhohn that are generally similar to Lindgren’s, although they introduce a few different technical terms and place special emphasis on certain genetic processes; but in so doing they take a stand that Lindgren especially avoids, since “an absolutely consistent genetic classification is at present impracticable for it forces the geologist to take a definite stand on problems which, as yet, have not been solved.” The different viewpoints regarding sources of solutions as expressed in different chapters of this volume emphasize the wisdom of avoiding such a stand at present.

Classifications based on origin that were devised before 1900, except the outline by Vogt in 1893, implied that deep-seated deposits, other than magmatic segregations, were formed wholly or mainly by waters that had descended from the surface, and, after either short and direct or long and devious circulation, during which they dissolved metals and gangue material from country rocks, deposited their contents either in fissures or open bedding planes, in open pore spaces, or by replacement of the wall rock. Even after 1900, when Lindgren, Kemp, Vogt, and a few others began to present convincing evidence that ore-forming waters were derived from the same magmatic sources as the local intrusive rocks, these older classifications continued to be useful in the recognition of processes of ore deposition; but the new interpretation of the ultimate sources of the waters and their contents accounted for certain features of deposits so much more satisfactorily that it rapidly gained in favor and for several years has been generally followed. The outstanding classification during this period is that proposed and used by Lindgren in 1910 in his first year as visiting lecturer at the Massachusetts Institute of Technology and published in the first edition of his Mineral Deposits in 1913. This classification has received general acceptance and is more often referred to than any other, abroad as well as in the United States and Canada. It has undergone slight changes in subsequent editions of Mineral Deposits, mainly as regards the limiting ranges of temperature within which certain classes of ores were deposited.

In view of this situation the only reason for this chapter is to present a discussion that may aid different readers in an appreciation of the significance of Lindgren’s classification and may show how the classification may be affected by changes in conclusions regarding certain classes of ore deposits.

Lindgren Classification

The classification, as presented in the latest edition of Mineral Deposits, is quoted below:

This is a comprehensive classification that may be made to include not only deposits of ores and nonmetallic minerals, but rocks as well, since from a strictly commercial standpoint a rock is regarded as a mineral deposit adapted for some utilization, whether of great or negligible value. It contains a “pigeonhole” for every kind of deposit, although some kinds are very insignificant, and is sufficiently elastic to allow for any advances that may be made in our knowledge of mineral deposits. A deposit that today is generally believed to have been formed by atmospheric waters may be regarded tomorrow as a product from magmatic waters, but such a change in interpretation merely means the shifting from one “pigeonhole” to another. Each “pigeonhole,’” as shown by Schneiderhohn, may be divisible into several smaller “pigeonholes” if it seems advisable to emphasize certain minor differences in composition of ores or even in form of ore bodies. Even the larger classes may sooner or later be divided into portions that will become recognized as major divisions; for example, Graton proposes with good reason that a part of the deposits heretofore included in the great class of mesothermal deposits (formed at intermediate temperature, generally at intermediate depth, and under high pressure) be recognized as constituting a distinct class deposited under somewhat cooler or “milder” conditions than the rest, which he would call “leptothermal deposits.” He also proposes recognition of another class called “telethermal” deposits, since they were formed so far from their magmatic sources that their magmatic derivation is obscure. This second proposal requires an inferred knowledge of origin that some may not be ready to accept, since they have regarded the deposits in question as not formed by magmatic waters. It, therefore, may be premature to recognize a “telethermal” class in an established system, but such a “pigeonhole” may be made by separating it from the epithermal class and leaving the establishment of “telethermal” deposits, as distinct from deposits of non-magmatic origin, for more generally convincing evidence than has yet been presented.

Some readers with origin and logical treatment uppermost in their minds might remark that the foregoing tabular scheme has been arranged backwards, inasmuch as the original rocks were igneous and the magmatic segregations and pegmatites in class IIC are most closely related to them, those of B2bb next closely, and those of 2ba less closely, whereas the remaining classes of deposits are derived directly or indirectly from these rocks and mineral deposits. It is anyone’s privilege to follow such an order if he wishes, but the tabulated order has the advantage of beginning with the most superficial, simplest, and most thoroughly understood deposits and working gradually to those of deeper and deeper seated origin, an understanding of which must depend upon the progress made in the study of the earth’s interior and of processes that take place under high temperatures and pressures.

https://www.youtube.com/watch?v=Q4J_ezrBLaU&list=PLM0tU1_L7mcS0H8_-qQOTRTa7sTt93DUU

Deposits Produced by Mechanical Processes of Concentration

It is most striking that class I in the foregoing scheme is not divided, whereas class II is divided and subdivided at great length; in other words, by far the greatest number and variety of deposits are of chemical origin. Deposits of mechanical origin, however, are susceptible of subdivision if we are sufficiently interested in them; for example, mechanical concentration of talus deposits is due to gravity alone, that of most sedimentary rocks to the work of marine waters, that of others to the work of rivers or lakes, and that of a few others to the work of glaciers, and of still others to the work of wind. Each of these subdivisions may be subdivided according to the velocity of the concentrating agent, and to the source of material; in fact, if Lindgren had been writing a book on structural materials and raw materials for the chemical industries, class I would doubtless have been subdivided in considerable detail. The scope of his Mineral Deposits, however, is such that the only deposits of class I that are given more than summary attention are placers and these are subdivided in sufficient detail to permit a thorough discussion. The present volume is even more restricted to the subject of ore deposits and for this reason the major class of mechanical deposits is rather summarily dismissed.

Before leaving it, however, we should realize that even the mechanical processes involved in class I do not take place to the exclusion of chemical processes. The fineness of placer gold is generally greater than that of the lode gold from which it is derived, and its degree of fineness increases as distance from its source increases owing to the leaching action of streams that transport it; furthermore, the rare occurrence of minute, drusy crystals of gold on nuggets indicates the solution and precipitation of small amounts of gold. Pure silica sand, derived during one or more cycles of erosion from granitic and other siliceous rocks, requires not only the extremely fine grinding but the complete decomposition of such minerals as the feldspars, whose specific gravities are so near to that of quartz that mechanical action alone can not produce the final results. Mechanical processes may greatly predominate in colder climates, but in warm humid climates the materials concentrated by mechanical processes have first been liberated by the thorough chemical decomposition of the rocks and of any mineral deposits contained in them. These facts serve to emphasize the complexity of the geological processes involved, and to show the tolerance with which any classification must be accepted and used.

Deposits Produced by Chemical Processes of Concentration

In Bodies of Surface Waters

In class IIA, deposits produced by chemical processes of concentration in bodies of surface waters, the impossibility of distinction between certain sedimentary rocks and mineral deposits is again apparent, as is the impossibility of recognizing any one process to the exclusion of all others; for example, although the phosphorus that enters guano is gradually concentrated by a succession of organic processes in sea water, the guano itself is eventually a dry-land deposit whose commercial value may be enhanced by the leaching action of surface waters, a process that should be included under class IIB1a. Even if guano deposits, which are exceptions in the realm of mineral deposits, were grouped elsewhere, these would be similar obstacles to an absolutely consistent classification. The pebble phosphate deposits of Florida offer similar difficulties, as they are partly residual, partly attrital, partly marine, and partly fluviatile in origin.

There is also difficulty in distinguishing sharply between inorganic and organic reactions in surface waters; for example, the iron that spring water has dissolved from rock in its underground course may, on entering a stream and after coming in contact with the air, become oxidized and deposited, but it is common to find certain kinds of algae taking part in the process. Limestones, whether formed in marine or fresh water, may owe their origin in part to the loss of carbon dioxide, which may be a simple inorganic process or more likely is influenced or controlled by organic agencies; in part they represent the direct secretions of mollusks and corals, but to a considerable extent they may represent the mechanical breaking down, redistribution, and recementation of those secretions. Most dolomites, which occur interbedded with limestones, have been formed through the replacement of part of the original calcium of limestone by magnesium by prolonged contact either with the water of shallow sea-bottoms or with underground water. They may be grouped in part, therefore, with deposits formed by inorganic reactions in marine surface waters and in part by concentration by ground water of deeper circulation (IIBb). Certain light gray to buff colored dolomites in the West, for example, in the Yellow Pine district, Nevada,8 are most reasonably attributed to the reaction between limestone and waters of magmatic origin that have traveled far from their source. These dolomites should be classified with the epithermal deposits (IIB2ba1) or perhaps with the ‘Telethermal” deposits as recognized by Graton in the paper cited.

Other deposits (certain siderites, cherts, sulphides, and sedimentary iron and manganese ores) discussed by Lindgren in his chapter on deposits produced by chemical processes in bodies of surface water are subject to similar discussion. From this discussion the reader may be led to infer that the tabulated classification is defective; but the point to be emphasized is that, although the processes indicated are distinct, they seldom act alone and thus may only account in part for the complex origin of certain rocks and mineral deposits.

The formation of deposits by evaporation is less subject to involved discussion. There may be many physico-chemical details as well as problems of sedimentation and earth movements to be studied regarding the process, but the deposition of the constituents of sea water or of undrained lakes by evaporation is readily appreciated. Even this process may be accompanied by some mechanical deposition of mud or silt and by some bacterial action; but the effects of these accompanying processes do not obscure the effects of evaporation in the least. That subsequent changes in these deposits may confuse the student of mineral origin is illustrated more and more by recent investigations of the German potash deposits, whose history was formerly interpreted in such simple terms.

In Bodies of Rocks

Concentration of substances contained in the geological body itself

Concentration by Rock Decay and Residual Weathering

Class IIB which includes all deposits formed chemically within bodies of rock, comprises nearly all of the metalliferous deposits, except placers and the sedimentary iron and manganese ores, and also includes several important deposits of non-metallic minerals. Discussion of the processes involved may be somewhat less disconcerting than that on the preceding pages, but few, if any, of the deposits represented can be attributed entirely to one process. Those deposits formed by concentration of substances contained within the geologic body itself may have more complicated origins than those formed by introduction of substances foreign to the rock; thus deposits resulting from rock decay, though dependent mainly on chemical processes, may be dependent, at least to a minor extent, on processes of mechanical disintegration. Chert and barite nodules in the residual soils left by the solution of certain limestones, are simple insoluble residues, but kaolin and limonite that are commonly classed as residual may have a more complicated origin; indeed, even the simplest process imaginable requires that the constituent elements go into either true or colloidal solution and become redeposited with or without appreciable transfer from one point to another. In some deposits, both of kaolin and limonite, small and even large quantities of these materials have been carried in solution for short to considerable distances and redeposited by replacement of the adjoining rock. Both of these minerals have replaced siliceous sandstones and quartzites to some degree and have replaced limestone and dolomite in relatively large quantity. Some kaolin, though not of economic importance, is also attributed to ascending water derived directly or indirectly from volcanic sources. The term, kaolin, as here used includes several minerals of the clay group, some of which may be more restricted in their modes of origin, so that the precise determination of the clay minerals, that is now in progress, together with a careful study of geological conditions, will doubtless result in a more refined interpretation of these deposits. Sufficient has been said here to emphasize the complexity of processes associated with weathering and rock decay.

The oxidation and enrichment of ore deposits involves similar processes. Gold in oxidized deposits remains largely as a residue and iron in pyrite, if converted promptly to an insoluble oxide, also remains largely in its original position; but both of these metals as well as others may under some conditions be largely dissolved, carried downward, and redeposited by reaction with certain rocks or minerals. These processes are discussed more fully in chapter IX.

Concentration by Ground Water of Deeper Circulation

The role played by deeply circulating waters in the concentration of mineral deposits is perhaps less clearly demonstrated than most other processes of mineral deposition. Years ago, when the theory of lateral secretion was more favorably regarded, the importance of deeply circulating waters was generally taken for granted; but now, since most metalliferous deposits that originated below the belt of weathering have been convincingly attributed to magmatic waters, the inadequacy of deeply circulating meteoric waters to form such deposits appears more and more probable, and one wonders just what their true functions are.

Anyone who has seen the thick efflorescences and stalactites in the deep workings of some mines is impressed with the amount of mineral matter contained in waters below the zone of oxidation. A striking example is afforded by the Ibex mine at Leadville, Colorado, in which the floor of a stope was covered by a layer of malachite 5 inches thick that had accumulated within a period of 10 years. This mineral matter, however, was mainly derived by leaching in or just below the zone of oxidation and was redeposited only by the artificial tapping and evaporation of the water. Even in well mineralized districts, where the amount of dissolved material carried down from the oxidized zone is unusually great, the minerals deposited in quantity beneath that zone owe their existence largely to opportunities for reaction between the descending waters and certain minerals—for example, the precipitation of black copper sulphide (chalcocite) as a replacement of chalcopyrite or pyrite and of zinc carbonate by replacement of permeable limestone or other carbonate rock. It is striking and even distressing when we realize how much zinc carried down from the oxidized zone has been scattered and lost in the vast sea of underground water because it did not come in contact with replaceable carbonate rock in or immediately below that zone. The failure to find deposits of commercial interest very far from the source of supply in the oxidized zone emphasizes Lindgren’s comment that “that part of the dissolved substance carried down is also insignificant in comparison with the vast amount of underlying rocks, so that we cannot expect that the material added from the zone of weathering will produce any far-reaching changes in the composition of these rocks.’’

It would appear from the foregoing paragraph that water circulating below the oxidized zone does not contain very powerful reagents. Such strong reagents as sulphuric acid and ferric sulphate were doubtless utilized promptly and thereafter the sulphates together with any bicarbonates and other materials in solution exerted little or no appreciable influence on ordinary igneous, sedimentary, or metamorphic rocks. It is conceivable that with ample time for reaction certain very insoluble minerals like barite may be formed by the leaching of barium from silicate or carbonate rocks by sulphate waters, or that calcite may be formed by the using up of bicarbonate radicles; furthermore, since water in limestone is saturated with calcium carbonate, local fracturing of the rock may cause local relief of pressure, permit the escape of carbon dioxide, and thereby induce deposition calcite in the fractures. It is also conceivable that hornblende or biotite may become hydrated and changed to chlorite after prolonged contact with ground water, but it is a striking fact that in many granites and other igneous rocks that have long been saturated with ground water these minerals have remained unaltered. Where the development of chlorite is pronounced in these rocks they are much fractured and there is at least a suspicion that they have been subjected to attack by waters that contained reagents of magmatic origin.

Besides the extraction of material from rocks and its local redeposition, ground water may follow a long and complicated artesian course, extracting materials at one or more places, and eventually depositing them remote from their sources. Such a process is more probable if the temperature of the waters has been considerably raised, either by deep circulation or by proximity to some local source of heat. Copper deposits in Red Beds, vanadium deposits in certain sandstones, and some zinc-lead deposits in limestone have been commonly attributed to such an origin. It is noteworthy, however, that the mineral composition of such deposits is very similar to that of outlying deposits in districts where a magmatic origin of the ore-forming solutions is generally accepted, and it must therefore be concluded that the physical and chemical conditions that controlled the formation of these deposits were identical, regardless of the ultimate sources of the waters and the deposited minerals.

It is interesting to note how most of those geologists who have studied many deposits of accepted magmatic origin are always on the lookout for evidence that will indicate or prove a magmatic origin for these zinc-lead deposits while others continue to resist such an interpretation. Some recent mineralogical and structural studies tend to throw doubt on the artesian origin of these deposits, but in the absence of any direct proof of a course of circulation from deep-seated magmatic sources definite conclusion remains impossible. The evidence of a magmatic origin for the copper and vanadium deposits is still weaker, but there are some who would like to advocate it, and those who would deny such a possibility are unable to present an altogether convincing explanation of the origin of these deposits. Similar uncertainties would become apparent were the discussion extended to include those non-metallic deposits that may be attributed to ground water of deeper circulation, such as barite, celestite, serpentine, and magnesite. For these reasons this class of deposits, as remarked above, is among the least understood of the entire range of mineral deposits.

Concentration by Dynamic and Regional Metamorphism

Concentration by dynamic and regional metamorphism is a process of only passing interest to the mining engineer and operator, although it must be given due consideration in a comprehensive study of mineral deposits. Metamorphic rocks and any mineral deposits in them reflect, perhaps more than any others, a persistence of old ideas regarding origin and classification that should be modified or discarded in the light of recent advances in petrology. Where some sedimentary rocks have been traced into metamorphic rocks there is obviously no appreciable change in bulk chemical composition, although certain minerals stable under conditions of sedimentation have disappeared and others stable under conditions of high temperature and pressure have been formed. The small amount of water originally in the sedimentary rock may have been sufficient to bring about the recrystallization; if so, the formation of the metamorphic minerals differs in origin from those already mentioned as formed by deeply circulating meteoric water only in the depth and temperature of formation. There are, however, certain inconsistencies in such an interpretation. Field evidence indicates that in some regions well developed schists and gneisses were formed at depths of only a mile or two, whereas in other regions, notably the Appalachians in Pennsylvania, rocks that must have been buried to much greater depths have escaped profound regional metamorphism. In nearly if not quite all areas of intensely metamorphosed rocks intrusive rocks are also known to be present and the heat of intrusion may account for this difference; but, as detailed study of these areas progresses, the influence of emanations from the intrusive magmas and the permeability of the invaded rocks under high pressures are becoming more and more appreciated, and, even where the chemical composition of the metamorphic rock is essentially identical with that of the original sedimentary rock, the possible if not probable influence of water, fluorine, and other emanations from the magma must be recognized. In other words, regional metamorphism may be equivalent to contact metamorphism on a grand scale taking place at the same time as regional compression; indeed, the process of recrystallization may well have rendered the rocks especially subject to intense and complex folding and compression.

Deposits of garnet, graphite, and other minerals that occur typically in crystalline schists and associated marbles are subject to the foregoing consideration. For the most part they occur in certain beds of rock that were especially subject to the reconcentration of their constituents into new minerals, although some introduction of magmatic material may at least have influenced their formation even though it did not become a part of their composition. Deposits of magnetite and sulphides, however, are not so readily understood. Some have been interpreted as deposits that were originally formed before metamorphism and have survived the process with only mechanical deformation. This interpretation implies that the pressure was sufficient to prevent the sulphur from escaping from sulphides even at extremely high temperatures. Other deposits have been regarded as formed during regional metamorphism, either by concentration from the wall rock or by introduction from some other source, and leaves for speculation the question of stability of sulphide minerals under the conditions that existed. There seems no reason to doubt that the rocks could be permeated by emanations of sulphur as well as other volatile matter, and Lindgren suggests that “to such a permeation in the deep zone, many of the most enigmatic ore deposits of the crystalline schists may owe their origin.” As many published accounts of these deposits are based on studies made years ago, it would not be surprising if some of the enigma were to disappear after thorough restudy in the light of recent progress.

Concentration effected by introduction of substances foreign to the rock

Concentration effected by introduction of substances foreign to the rock involves introduction either by atmospheric or meteoric waters or by waters emanating from or activated by igneous magmas, or a combination of both. The reader will feel that these processes have been considered already, and indeed they have. This lack of systematic consideration, however, does not imply a faulty scheme of classification so much as an imperfect knowledge of the deposits themselves and a complication of processes involved. Although there is a distinct difference between the derivation of minerals from materials in the wall rock and their derivation from materials introduced from a remote source, the two processes can not be sharply contrasted. The material may be concentrated from a cubic inch of rock and redeposited, or it may be concentrated from a larger volume of rock and transferred an appreciable distance before redeposition; in the second case the resulting deposit may possess the same structural and textural features as a deposit formed mainly or wholly from introduced material. The vanadium and copper deposits cited in chapter X are generally believed to represent material transferred within the formation, and the zinc-lead deposits to represent material introduced from an outside source, but they both involve processes of deposition so similar that they have been discussed under a single heading and even compared with deposits of ultimate magmatic origin.

Origin Dependent on Igneous Activity

By Hot Ascending Waters Charged with Igneous Emanations

When we restrict ourselves to consideration of deposits whose origin is dependent on the eruption of igneous rocks, the issue becomes more clearly defined, but even here the source of the depositing waters may not be determinable. It may be the same as that of the igneous rocks or it may be water originally of atmospheric origin but charged with emanations from the igneous source. These deposits as a whole comprise most of the metalliferous deposits of the Cordilleran region and prolonged study of them shows that they have been formed over a great range of temperature and pressure and at different depths below the surface. They have accordingly been classified by Lindgren on such a basis into three general groups: those formed at relatively low temperatures (50°-200° C.), moderate pressure, and for the most part shallow depth; those formed at moderate temperature (200°-300° C.), high pressure, and generally intermediate depths; and those deposited at high temperatures (300°-500° C.), very high pressure, and relatively great depth. Here again a necessarily simple statement of fundamental factors requires explanation when applied to individual deposits. Some deposits conform clearly to these factors, but others do not. Some deposits were evidently formed at high temperature but at no greater depth than neighboring deposits attributed to moderate or even relatively low temperatures; the controlling physical conditions, temperature and pressure, have been generally recognized as consistent, however, and in response to a demand for concise terminology, Lindgren has proposed the terms epithermal, mesothermal, and hypothermal to designate the three groups. One might quibble over the exact etymological significance of these terms, but such quibbling is useless since the factors involved in the origin of the deposits are variable. The general adoption of the terms, both in this country and abroad, is sufficient proof of their usefulness.

Certain minerals, on the basis of either experimental data or consistent field relations, are diagnostic and certain others fairly so, but many occur in so many different associations that they are not at all reliable as indicators of the temperatures that prevailed at the time of deposition. The following minerals, if present in relatively large amounts and of hypogene origin, are indicative of low-temperature (epithermal) deposits: cinnabar, stibnite, realgar, tellurides, selenides, argentite, proustite, pyrargyrite, stephanite, polybasite, pearcite, marcasite, alabandite, chalcedonic quartz, adularia, lamellar calcite, rhodochrosite, rhodonite, alunite, and kaolin. Minerals indicative of high-temperature (hypothermal and pyrometasomatic) deposits include several anhydrous silicates, such as the olivines, pyroxenes, wollastonite, vesuvianite, certain amphiboles, garnets, axinite, tourmaline, topaz, and beryl; also micas, notably phlogopite, lepidolite, biotite, and muscovite; magnetite, and other minerals of the spinel group, specularite, and cassiterite. These lists may appear incomplete to some, but few if any other minerals, even including albite, that may be rather commonly associated with those named are sufficiently restricted in occurrence to be included, and, on the basis of present field and laboratory data they cannot by themselves be regarded as diagnostic.

It is noteworthy that native gold and silver and the common sulphides and gangue minerals are not listed. They are present in commercial quantity in deposits that have been assigned to each of the principal groups, and many, therefore, have been termed “persistent ” minerals. Some may indeed be persistent, but others, when their stability ranges under a variety of conditions have been determined, may prove to be more restricted than is now supposed. It is also noteworthy that no minerals surely indicative of moderate or mesothermal conditions have been listed. Some doubtless exist, but, so far as the literature on mineral deposits is concerned, their restriction to the mesothermal range cannot be convincingly demonstrated.

The character of fracturing, and therefore of the deposits that fill the fractures, bears some relation to the depth of formation. The fractures that contain diagnostic low-temperature minerals commonly bear evidence of development under a light load, those that contain diagnostic high-temperature minerals commonly bear evidence of development under very great pressure, and still others, including those that contain many of the most important base-metal deposits, indicate development under intermediate pressures; but the pressures under which fractures were formed are not necessarily indicators of the temperatures that existed during mineral deposition, as will be noted later. High-temperature minerals may be formed close to the surface, and low-temperature minerals may be formed at surprisingly great depths. The diagnostic minerals and the character of fracturing are both of great value in the study and classification of mineral deposits, but neither should be used too dogmatically.

There are certain other questions that cannot be clearly answered in this correlation with respect to physical conditions. Most of the epithermal deposits, those formed at relatively low temperatures and for the most part shallow depths, are not traceable into deposits typical of those formed at greater depth, and some at least of the most productive epithermal deposits in the western United States were formed at a distinctly later age than the mesothermal and hypothermal deposits of the same region. These differences further emphasize the necessity of considering possible differences in composition of ore-forming solutions, structural control, and influence of different kinds of wall rock, in addition to the temperatures and pressures that existed at the time of ore deposition. There are, however, some indications of transition and they will be mentioned at convenient places in the remainder of the chapter.

Epithermal Deposits

The outstanding epithermal deposits are associated with volcanic rocks, and study of local erosion proves that some of them, notably the gold deposits at Goldfield, Nevada, were formed within a few hundred feet of the surface, although some have been mined to a depth of 5,000 ft. below the surface that existed at the time of their formation. Another feature pointing to deposition at shallow depth is a similarity in composition between epithermal veins and hot spring and geyser deposits in volcanic areas, and a corresponding difference between them and deposits formed under mesothermal and hypothermal conditions. These differences have been clearly summarized by Lindgren.

The sources of the solutions that deposited these epithermal ores and the courses followed by them have worried those, who would like to link all classes of metalliferous deposits into a continuous series. Many of the epithermal deposits are veins that cut volcanic flows or beds of tuff, but, although grouped near volcanic centers, are not clearly connected with any volcanic source from which magmatic water could have been derived. It is easily realized that they could have been interpreted, by those who minimized the importance of magmatic waters, as having been formed through the action of atmospheric water on the lava flows, especially where the ore shoots were mainly confined to one particular wall rock. Others have regarded them as the result of extraction of material from surface flows by permeating volcanic emanations, whether or not mingled with atmospheric water. Such a process undoubtedly accounts for much of the alteration in the rocks in and around volcanic vents, but certain constituents, notably the precious metals and such elements as sulphur, antimony, arsenic, and tellurium are most reasonably attributed to a deep-seated volcanic source, as will be shown below.

Lindgren recognized nine varieties of epithermal veins, all of them associated with volcanic rocks, but for present purposes they may be considered in three groups: quicksilver deposits, gold and silver deposits without noteworthy quantities of base metals, and certain base-metal deposits most of which are workable because of their precious-metal content. Another group may be added to the discussion to include deposits independent of volcanic rocks but representing end products of solutions derived from deep-seated intrusive centers.

Quicksilver Deposits

Gold Deposits