Mineral products may be classified according to use, commercial importance, geographic distribution, form and structure, mineralogical and chemical composition, or origin. Each of these classifications is useful for some purposes. The geologist usually prefers a classification based on origin or genesis. In the following chapters on mineral resources, however, such a classification 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 assemble 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 classification.
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, associated 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 assortment 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 mass 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 classification of mineral deposits on the basis of origin is more or less arbitrary. The sharp lines implied by the use of class names do not exist in nature. Mineral deposits are so complex and so interrelated in origin, that a classification according to genesis indicates only the essential and central class features; it does not sharply define the limits of the classes.
It is practically impossible for any geologist to present a classification 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 classifications 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.