Importance of Epigenetic Deposits

    in Geology

    While mining geology covers the whole field of mineral occurrence, there is one broad group of deposits about which more has been said and written than about all others put together. This is the group known as epigenetic—that is, ores and mineral products introduced into their surrounding rocks after the rocks themselves had already come into existence. The reason for this emphasis is not that such deposits have been the greatest producers, for a single class outside this group has turned out a far greater bulk of material, measured either in tons or in dollars. If this book seems to place chief emphasis on epigenetic ores, especially veins, lodes, and replacement deposits, it is because this group has been the object of most attention from the profession as a whole.
    Such deposits are, by comparison, hard to find and follow, and, having a higher value per ton, have called for the services of more geologists and compelled the solution o£ more diverse and difficult problems than have deposits of other types.

    The other broad group of deposits, syngenetic (mineral deposits formed along with the rocks that enclose them), are by no means without their geological problems; but those syngenetic deposits that occur in igneous rocks are for the most part either too simple to require geological guidance or too erratic to benefit by it. So they seem now, at least, although better technologies no doubt will be developed to deal with them more adequately. Syngenetic deposits in sedimentary rocks, which account for all of the coal, most of the outstanding deposits of iron and manganese, and the bulk of the non-metallic minerals, involve few problems which do not also face the student of epigenetic deposits— problems of sedimentation and stratigraphy and, in many cases, problems of structure (especially folding and faulting), although here again growing technology may evolve special techniques as the economic incentive becomes more conducive.

    Geology in Mining

    The mining industry in its task of finding, following, and extracting metallic ores has always made use of geology in one way or another. Writings on mining since medieval times all venture into discussions of ore genesis and ore localization; naive and amusing as they may seem to us now, they were the best geology known at the time and were, even then, considered part of the knowledge essential to mining. Until geologists began to take an interest in the specialized problems of mining, each miner or engineer had to be his own geologist, applying as best he could, and often with marked success, the ideas that he gained from science or developed for himself. Only during the last century, and particularly during the last generation, have those aspects of geology that are applicable to mining been developed to such a degree as to form the basis for a separate profession.

    In these days most projects for the exploration and development of metals are carried out under some form of geological guidance, whether it is supplied by professional geologists or by engineers who have them-selves acquired a knowledge of geology, and whether it is based on original investigation or on surveys by government or scientific organizations.

    Of the professional geologists who devote their attention to matters bearing on mining, many, but by no means all, are employed by mining companies. A large group are in government employ, and a few are engaged by organizations concerned with the financial aspects of mining.