The Economic Aspect of Geology Part 3

Survey_, 1920, p. 35.

[2] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol.

Survey_, 1920, p. 33.

[3] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol.

Survey_, 1920, pp. 22-23.

CHAPTER III

SOME SALIENT FEATURES OF THE GEOLOGY AND CLa.s.sIFICATION OF MINERAL DEPOSITS

VARIOUS METHODS OF CLa.s.sIFICATION

Mineral products may be cla.s.sified according to use, commercial importance, geographic distribution, form and structure, mineralogical and chemical composition, or origin. Each of these cla.s.sifications is useful for some purposes. The geologist usually prefers a cla.s.sification based on origin or genesis. In the following chapters on mineral resources, however, such a cla.s.sification is not the primary one, because of the desire to emphasize economic features. The mineral commodities are treated as units and by group uses. Some mineral commodities have so many different kinds of origin in different regions that to distribute them among several genetic groups in description would make it impossible to preserve the unity necessary for consideration of the economic features.

While in the descriptive chapters many references are made to origin, it may be difficult for the reader to a.s.semble them in perspective; for this reason we summarize at the outset some of the salient features of origin of mineral deposits and of their geologic cla.s.sification.

To the layman the reason for emphasis on origin is often not clear. The "practical" man frequently regards this phase of the subject as merely incidental to the immediate economic questions--a playground for harmless theorists. The answer of the economic geologist is that in no other way than by a knowledge of origin is it possible to arrive at an understanding of conditions which so well enables one to answer many practical questions. In the exploration for mineral deposits, it is obvious that an understanding of the kinds of geologic conditions and processes under which a given type of deposit is known to develop results in the elimination of much unpromising territory, and the concentration of work on favorable localities. In forming any estimate of mineral deposits beyond the ground immediately opened up,--for instance, in estimating depth, form, change in values, mineralogical character, or interruptions due to faulting,--it is difficult to form any intelligent conception of the probabilities unless the history of the deposit is understood. If, for instance, the ore is known to be formed by hot waters, a.s.sociated with the cooling of igneous rocks, different conditions are to be expected below the zone of observation than if the ore is formed by surface waters. If the ore body is formed as a single episode under simple geologic conditions, the interpretation of the possibilities in the situation may be quite different from the interpretation applied where the history has been more complex. If the surface conditions suggest possibilities of secondary enrichment of the ores, the interpretation of the conditions underground will be different from those applied where there is no evidence of such enrichment.

Where a mineral deposit is completely opened up in three dimensions, it is often possible to work out economic questions of tonnage, grade, shape, and values, without the aid of geology. Also, where conditions are comparatively simple and uniform throughout a district, the local knowledge of other mines may be a sufficient basis for answering these questions for any new property developed. Empirical methods may suffice.

However, it is seldom that the conditions are so simple that some geological inference is not necessary. Even where problems are settled without calling in the geologist, geological inferences are required in the interpretation of, and projection from, the known facts. It is often the case that the practical man has in his mind a rather elaborate a.s.sortment of geologic hypotheses, based on his individual experience, which make the so-called theories of the geologist seem conservative in comparison. The geologist comes to the particular problem with a background of established geologic principles and observations, and his first thought is to ascertain all the local conditions which will aid in deciphering the complete history of the mineral deposit. There is no fact bearing on the history, however remote from practical questions, which may not be potentially valuable.

With this digression to explain the geologist's emphasis on origin of mineral products, we may return to a consideration of a few of the principles of rock and mineral genesis which have been found to be significant in the study of mineral products.

In the preceding chapters it has been indicated that mineral deposits are mere incidents in the ma.s.s of common rocks; that they are made by the same processes which make common rocks, that none of the processes affecting mineral deposits are unique for these minerals, and that most common rocks are on occasion themselves used as mineral resources. These facts are emphasized in order to make it clear that the study of mineral deposits cannot be dissociated from the study of rocks, and that the study of the latter is essential to bring mineral deposits into their proper perspective. Absorption in the details of a mineral deposit makes it easy for the investigator to forget or minimize these relations.

Nevertheless, in the study of mineral deposits, and especially deposits of the metallic minerals, certain geologic features stand out conspicuously against the common background indicated above. Our discussion of these features will follow the order of rock genesis indicated in the description of the metamorphic cycle.

NAMES

Any cla.s.sification of mineral deposits on the basis of origin is more or less arbitrary. The sharp lines implied by the use of cla.s.s names do not exist in nature. Mineral deposits are so complex and so interrelated in origin, that a cla.s.sification according to genesis indicates only the essential and central cla.s.s features; it does not sharply define the limits of the cla.s.ses.

It is practically impossible for any geologist to present a cla.s.sification which will be accepted without qualification by other geologists, although there may be agreement on essential features.

Difficulties in reaching agreement are increased by the inheritance from the past of names, definitions, and cla.s.sifications which do not exactly fit present conceptions based on fuller information,--but which, nevertheless, have become so firmly established in the literature that it is difficult to avoid their use. In the progress of investigation many new names are coined to fit more precisely the particular situation in hand, but only in fortunate cases do these new names stand up against the traditional currency and authority of old names. The geologist is often in despair in his attempt to express his ideas clearly and precisely, and at the same time to use terms which will be understandable by his readers and will not arouse needless controversy.

As ill.u.s.trative of the above remarks reference may be made to a few terms commonly used in economic geology, such as _primary_, _secondary_, _syngenetic_, _epigenetic_, _supergene_, _hypogene_, _protore_, etc.

The most commonly used of these terms are _primary_ and _secondary_. It is almost impossible to define them in a way which will cover all the conceptions for which they have been used, and yet in their context they have been very useful in conveying essential ideas. An ore formed by direct processes of sedimentation has sometimes been called primary, whereas an ore formed by later enrichment of these sediments has been called secondary. An ore formed directly by igneous processes has been called primary, while an ore formed by enrichment of such primary ore by later processes has been called secondary. It is clear, however, that these terms are merely relative, with application to a specific sequence, and that they do not fix the absolute position of the ore in a sequence applying to all ores. For instance, ores deposited directly as sediments or placers may be derived from the erosion of preexisting ore bodies,--in which case it may sometimes be convenient to refer to the sedimentary ores or placers as secondary and the earlier ores as primary. Or a sulphide deposit originating through igneous agencies may undergo two or three successive enrichments, each successive one secondary to the preceding, but primary to the one following. In spite of these obvious difficulties, the terms primary and secondary may be entirely intelligible as indicating relative order of development under a given set of conditions.

The term _syngenetic_ has been used for mineral deposits formed by processes similar to those which have formed the enclosing rocks and in general simultaneously with them, and _epigenetic_ for those introduced into preexisting rocks. In certain cases _syngenetic_ may be roughly synonymous with _primary_, and _epigenetic_ with _secondary_, and yet a primary ore may be epigenetic. For instance, zinc sulphides in the Mississippi valley limestones (pp. 54-55) are epigenetic, and yet are primary with reference to a later enrichment. The two sets of terms are meant to convey somewhat different ideas and are not interchangeable.

Ransome[4] has suggested, especially for vein and contact deposits, a series of names which has the considerable advantage of definiteness:--_hypogene ores_, formed in general by ascending non-oxidizing solutions, perhaps hot; _supergene ores_, formed in general by oxidizing and surface solutions, initially cold and downward moving; and _protores_, or metallized rock or vein substances which are too low in tenor to be cla.s.sed as ores, but which would have been converted into ores had the enriching process been carried far enough.

In this connection Ransome defines primary ore as unenriched material that can be profitably mined. In view of the general use of the terms primary and secondary as expressing a sequential relation of ore development, it is doubtful whether this more precise definition will supersede the older usage. Also it may be noted that commercial conditions might require, under these definitions, the designation of an ore as a protore at one time or place and as a primary ore at another.

Hypogene ores are dominantly primary, and supergene ores are dominantly secondary, but either may include both primary and secondary ores.

The terms of these several cla.s.sifications overlap, and seek to express different aspects of the same situation. While almost synonymous in certain applications they are not in others.

In this text the writer has certainly not escaped the difficulties in regard to names above referred to, nor in fact has he made any exceptional effort to do so. His chief purpose is to convey, in somewhat elementary terms, an understandable idea of the central features of economic geology. In the main, the most widely accepted terms are used.

Almost at every turn it would be possible, in the interests of precision, to introduce qualifying discussions of names,--but at the expense of continuity and perspective in the presentation of the princ.i.p.al subject-matter. The writer does not wish to minimize the necessity for careful and precise nomenclature; but he regards it important that the student focus his attention on the central objective facts of the subject, and that he do not become misled by the sometimes over-strenuous advocacy of certain names or cla.s.sifications in preference to others. If the facts are understood, he will ordinarily have no difficulty in judging the significance of the variety of names proposed to express these facts. If, on the other hand, the student approaches the subject with a ready-made set of names and definitions learned by rote, he is in danger of perceiving his facts from one angle only and through a distorted perspective.

MINERAL DEPOSITS AS MAGMATIC SEGREGATIONS IN IGNEOUS ROCKS

In this cla.s.s are included deposits which crystallize within the body of igneous rock, almost, if not quite, simultaneously with the adjacent rock. These deposits form one of the main types of _syngenetic_ deposits.

The t.i.taniferous magnet.i.tes const.i.tute a widely distributed but at present commercially unavailable cla.s.s of iron ores. The magnet.i.te crystals of these deposits interpenetrate with the other const.i.tuents of an igneous rock, commonly of a gabbro type, and the deposits themselves are essentially igneous rocks. Their shapes are for the most part irregular, their boundaries ill-defined, and their concentration varying. While their magmatic origin is clear, there is little agreement as to the precise conditions which determined their segregation in the molten rock. There is often a tendency for the ores to follow certain primary sheeted structures in the igneous ma.s.s, a fact for which the reason is not obvious.

The Sudbury nickel ores, of Ontario, Canada, the princ.i.p.al source of the world's nickel, lie mainly within and along the lower margin of a great intrusive igneous ma.s.s of a basic type called _norite_, and locally the ores project beyond the margin into adjacent rocks. Their textures and their intercrystallization with the primary minerals of the igneous rock have suggested that they are essentially a part of the norite ma.s.s, and that they crystallized during some segregative processes which were effective before the magma had solidified. Near the ores there are likely to be granitic rocks, which, like the ores, seem to be segregations from the norite magma. Locally both the ores and the a.s.sociated granitic rocks replace the main norite body in such a fashion as to indicate their slightly later crystallization. However, the intimate a.s.sociation of the ores with the primary minerals in the magma, together with their absence from higher parts of the norite and from the extraneous rocks far from the contact, indicate to other investigators that they were not brought in from outside in vagrant solutions which followed the intrusion of the main magma, but that they were segregated within the magma essentially in place. The occurrence of these heavy ores near the base of the norite naturally suggests that they were segregated by sinking to the bottom of the molten magma, but this conclusion implies certain physical conditions of the magma which have not yet been proved. Again the precise nature of the process and the part played in it by aqueous and gaseous solutions are subject to some doubt and controversy. The settlement of this problem awaits the solution of the more general problem of the origin and crystallization of magmas.

In this general cla.s.s of igneous deposits may be mentioned also diamonds, platinum, chromite, corundum, and other mineral products, although for the formation of commercial ores of many of these substances further concentration by weathering and sedimentation has been required.

Pegmat.i.tes are coa.r.s.ely crystalline acid dike rocks which often accompany a large igneous intrusion and which have obviously crystallized somewhat later than the main igneous ma.s.s. They may const.i.tute either sharply delimited dikes or more irregular bodies which grade into the surrounding igneous ma.s.s. They have a composition roughly similar to the a.s.sociated igneous rock, but usually a different proportion of minerals. They are probably the result of the differentiation of the parent magma. The pegmat.i.tes are of especial interest to the economic geologist because of the frequency with which they carry commercial minerals, such as the precious stones, mica, feldspar, ca.s.siterite (tin ore), and others. They show a complete gradation from dikes of definitely igneous characteristics to veins consisting largely of quartz in which evidence of igneous origin is not so clear. The pegmat.i.tes thus afford a connecting link between ores of direct igneous sources and ores formed as "igneous after-effects," which are discussed in the next paragraph. Aplites are fine-grained acid igneous rocks of somewhat the same composition as the pegmat.i.tes and often show the same general relations to ores.

MINERAL DEPOSITS WITHIN AND ADJACENT TO IGNEOUS ROCKS WHICH WERE FORMED IMMEDIATELY AFTER THE COOLING AND CRYSTALLIZATION OF THE MAGMAS THROUGH THE AGENCY OF HOT MAGMATIC SOLUTIONS.

These deposits are closely a.s.sociated in place and age with igneous rocks, either intrusive or extrusive, and are usually considered to have come from approximately the same source; and yet they afford distinct evidence of having been deposited after the adjacent igneous rocks were completely crystallized and fractured. They are thus _epigenetic_ deposits. They are not themselves igneous rocks and they do not const.i.tute pegmat.i.tes, but they often grade into pegmat.i.tes and belong to the same general stage in the sequence of events. They include deposits formed by contact metamorphism. They are sometimes designated by the general term "igneous after-effects"--a term also applied in some cases to pegmat.i.tes. Some geologists discriminate between "deep vein"

deposits (p. 43) and "contact-metamorphic" deposits, but the two are so closely related in place and origin that for our purposes they will be considered together.

The ores of this cla.s.s are clearly deposited from vagrant solutions which wander through openings of all kinds in the igneous rock and outward into the adjacent country rocks. They also replace the wall rocks; limestone is especially susceptible. This is a phase of contact metamorphism. Some of the most important metalliferous deposits belong in this cla.s.s, including most of the gold, silver, copper, iron, lead and zinc ores of the western United States and the copper deposits of Lake Superior.

In general, ores of this cla.s.s are more abundant about intrusive igneous rocks, that is about igneous rocks which have stopped and cooled before reaching the surface,--than in a.s.sociation with extrusive igneous rocks which have poured over the surface as lava flows--but the latter are by no means insignificant, including as they do such deposits as the Lake Superior copper ores, the Kennecott copper ores of Alaska, some of the gold-silver deposits of Goldfield and other Nevada camps, and many others.

There is general similarity in the succession of events shown by study of ore bodies related to intrusives. First, the invasion of the magma, resulting in contact metamorphism of the adjacent rocks, sometimes with, and often without conspicuous crowding effects on the invaded rocks; second, the cooling, crystallization, and cracking of both the igneous rock and the adjacent rock; third, the introduction of ore-bearing solutions into these cracks,--sometimes as a single episode, sometimes as a long continued and complex process forming various types of minerals at successive stages. This order may in some cases be repeated in cycles, and overlapping of the successive events is a common feature.

One of the interesting facts is the way in which the igneous ma.s.s has invaded and extensively altered the country rocks in some mineral districts,--in some cases by crowding and crumpling them, and in others without greatly disturbing their structural att.i.tudes. The latter is ill.u.s.trated in the Bingham district of Utah and the Philipsburg district of Montana. In such cases there is so little evidence of crowding of the country rocks as to raise the question how such large ma.s.ses of intrusives could be introduced without greater disturbing structural effect. This leads naturally to consideration of the general problem of the manner of progress of magmas through adjacent rocks,--a subject which is still largely in the realm of speculation, but which is not thereby eliminated from the field of controversy. Facts of this kind seem to favor the position of certain geologists that magmas may a.s.similate the rocks they invade.

EVIDENCE OF IGNEOUS SOURCE

No one ever saw one of these deposits in the process of formation; the conclusion, therefore, that they originated from hot solutions, either aqueous or gaseous, or both, which were essentially "after-effects" of igneous activity and came from the same primary source as the a.s.sociated igneous rocks, is an inference based on circ.u.mstantial evidence of the kind below summarized:

(1) The close a.s.sociation both in place and age with igneous rocks. This applies not only to individual deposits, but to certain groups of deposits which have common characteristics, and which const.i.tute a metallogenic province; also to groups of the same geologic age, which indicate a metallogenic epoch (pp. 308-309). The a.s.sociation with igneous rocks in one place might be a coincidence but its frequent repet.i.tion can hardly be so explained. A zonal arrangement of minerals about intrusives is often noted. Geologic evidence often shows the processes of ore deposition to have been complete before the next succeeding geologic event,--as for instance in the Tonopah district of Nevada (p. 236), where the ores have been formed in relation to certain volcanic flows and have been covered by later flows not carrying ore, without any considerable erosion interval between the two events.

(2) The general contrast in mineralogical and chemical composition, texture, and mineral a.s.sociations, between these ore minerals and the minerals known to be formed by ordinary surficial agencies under ordinary temperatures. The latter carry distinctive evidences of their origin. When, therefore, a mineral group is found which shows contrasting evidences, it is clear that some other agencies have been at work; and the natural a.s.sumption is that the solutions were hot rather than cold; that they came from below rather than above.

(3) The contrast between the character and composition of these ores (and their a.s.sociated gangue) and the character and composition of the wall rocks, together with the absence of leaching of the wall rocks, favor the conclusion that the ore minerals are foreign substances introduced from extraneous sources. The source not being apparent above and the processes there observed not being of a kind to produce these results, it is concluded that the depositing solutions were hot and came from below.

(4) The fact that many of the ore minerals are never known to develop under ordinary temperatures at the surface. For some of them, experimental work has also indicated high temperature as a requisite to their formation.

Quartz, which is a common a.s.sociate of the ores and often const.i.tutes the princ.i.p.al gangue, serves as a geologic thermometer in that it possesses an inversion point or temperature above which it crystallizes in a certain form, below which in another. In deposits of this cla.s.s it has often been found to crystallize at the higher temperatures.

The quartz sometimes shows bubbles containing liquid, gas, and small heavy crystals, probably of ferric oxide, as in the Clifton-Morenci district of Arizona. It is clear that the ore-bearing solutions in these cavities, before the crystallization of the heavy mineral inclusions, held dissolved not only much larger quant.i.ties of mineral substances than can be taken up by water at ordinary temperatures, but also a substance like ferric oxide which is entirely insoluble under ordinary cool conditions.