In short, before actual field exploration is begun, intelligent consideration of the economic factors may go far toward narrowing the field and toward converging efforts along profitable lines. Looked at broadly, this result is usually accomplished by the natural working of general laws of supply and demand; but there are many individual cases of misdirected effort, under the spell of provincial conditions, which might easily be avoided by a broader approach to the problem.
Geologic Factors in Exploration
Coming to the geological aspects of exploration, the procedure in its early stages is again one of elimination. Oil and coal, for instance, are found in certain sediments of certain ages, and one would not look for them in an area of granite. For every mineral resource there are broad geologic conditions of this sort, particularly the genetic, structural, and metamorphic conditions, which make it possible to eliminate vast areas from consideration and to concentrate on relatively small areas.
After the elimination of unfavorable areas, there comes the hunt for positively favorable geologic conditions—for a definite kind of sediment or igneous rock, for a definite structure, for the right kind of mineralogic and metamorphic conditions, or for the right combination of these and other geologic elements. The geologic considerations used in exploration for the various mineral deposits are so many and so diverse, and they require so much adjustment and interpretation in their local application, that one would be rash indeed to attempt anything in the nature of an exhaustive discussion. It is hardly practicable to do more than to outline, for illustrative purposes, a few of the geologic factors most commonly used in exploration.
Mineral Provinces and Epochs
Mineral deposits may be similar in their mineralogic and geologic characters and relations over a considerable area. They may give evidence of having developed under the same general conditions of origin; perhaps they may even be of the same geologic age. The gold-silver deposits of Goldfield, of Tonopah, the Comstock Lode of Virginia City, and many other deposits through the Great Basin area of the southwestern United States and Mexico have group characteristics which have led geologists to refer to this area as a "metallogenic" or "metallographic" province. The gold-silver ores on the west slope of the Sierra Nevadas, for nearly the entire length of California, likewise constitute a metallogenic province. The Lake Superior copper ores on the south shore of Lake Superior, the silver ores on the north shore, miscellaneous small deposits of copper, silver, and gold ores to the east of Lake Superior, the nickel ores of Sudbury, and the silver-nickel-cobalt ores of the Cobalt district are all characterized by similar groups of minerals (though in highly differing proportions), by similar geologic associations, by similar age, and probably by similar conditions of origin. This area is a metallogenic province. The lead and zinc ores of the Mississippi Valley constitute another such province. The oil pools of the principal fields are characterized by common geologic conditions over great areas (p. 149), which may likewise be considered as forming mineral provinces; for them the term "petroliferous provinces" has been used. The list might be extended indefinitely. Knowledge of such group distributions of minerals is a valuable asset to the explorer, in that it tends to localize and direct search for certain classes of ores in certain provinces; also, within a province, it tells the explorer what is to be normally expected as regards kinds and occurrences of mineral deposits. In searching for minerals of sedimentary origin, the explorer will use stratigraphic methods in following definite sedimentary horizons. In searching for ores related to igneous intrusions he will naturally hunt for the intrusions, and then follow the periphery of the intrusions for evidences of mineralization, taking into account possible features of zonal arrangement of minerals about the intrusives (see pp. 42-44), and the preference of the ores for certain easily replaced horizons like limestones, or for certain planes or zones of fracturing.
Just as minerals may be grouped by provinces, they may be grouped by geologic ages. Such groupings are especially useful in the case of minerals which are closely related to certain stratigraphic horizons, such as coal, oil, and iron. The greater number of the productive coal deposits of the United States are of Carboniferous age, and the distribution of sediments of this age is pretty well understood from general geologic mapping. The Clinton iron ores all follow one general horizon in the lower-middle Paleozoic. The Lake Superior iron ores are pre-Cambrian, and over three-fourths of them occur at one horizon in the pre-Cambrian. Gold deposits of the United States were formed mainly in the pre-Cambrian, the early Cretaceous, and the Tertiary. Copper deposits of the United States were formed chiefly in pre-Cambrian, Cretaceous, and Tertiary time. While there are many exceptions and modifications to general classifications of this sort, they seem to express essential geologic facts which can be made very useful in localizing exploration.
Classification of Mineral Lands
In recent years there has been considerable development of the practice of classifying mineral lands in given areas for purposes of exploration and valuation, or for purposes of formulation and administration of government laws. This has been done both by private interests and by the government. These classifications take into account all of the geologic and economic factors ascertainable. The classes of mineral land designated vary with the mineral, the district, and the purpose for which the classification is made.
Common procedure for commercial exploration purposes is to divide the lands of a given territory into three groups—(1) lands which are definitely promising for mineral exploration, (2) lands of doubtful possibilities, and (3) lands in which the mineral possibilities are so slight that they may be excluded from practical consideration. Each of these classes may be subdivided for special purposes. Another commonly used classification is, (1) proved mineral lands, (2) probable mineral lands, usually adjacent to producing mines, (3) possible mineral lands, and (4) commercially unpromising mineral lands.