Examination of the iron formations results in elimination of large parts of them, because their metamorphic condition is not favorable to ore concentration. In the remaining areas more intensive methods are followed. It is scarcely possible to summarize briefly all of the structural and stratigraphic methods used in locating the ore bodies. These have often been described in print.[41] Comparatively recent advances in this phase of exploration work have been in the more detailed application of stratigraphic methods to the iron formation. The group characteristics of the iron formation are fairly uniform and distinctive as compared with all other rocks; yet within the iron formation there are so many different kinds of layers represented that it is possible to use these variations with great effectiveness, in correlating favorable horizons for ore deposition, in interpreting drill records, and in other ways. Another method of approach, employed chiefly on the Mesabi Range, relates to the slumping of the ore layers which results from the leaching of silica during the concentration of the ore. This slumping can be measured quantitatively, and has been used to much advantage in exploration, in correlation of ore horizons, in preparation of sections and ore estimates, etc.
Early geologic explorations in the Lake Superior country were based on the assumption that the ores were concentrated by waters working down from the present erosion surface; but recognition of the fact that the waters which did the work were related to a far older and different erosion surface, under conditions which allowed of a far deeper penetration, has modified exploration plans for certain of the districts like the Marquette and Gogebic.
Notwithstanding the complexity of the geologic factors involved, their net result has been to concentrate iron ores in a surprisingly uniform ratio to the mass of the formation in different parts of the region,—with the result that on an average it may be predicted for any district, in an exploration of sufficient magnitude, how much ore is likely to be cut in either vertical or horizontal dimension. Thirteen per cent of the productive area of the Mesabi iron formation is iron ore. For the remainder of the Lake Superior region five or six per cent is the factor. These figures mean that, if a person could explore a broad enough area of iron formation, any miscellaneous group of drill holes or underground openings would tend to yield these percentage results. Such percentages are amply sufficient to pay a large profit on the exploration. The question may be raised why the application of geology is required, if such average results can be secured from miscellaneous undirected work. The answer is that seldom is it possible to conduct an exploration on a sufficiently large scale to be sure of approximating this average, and that geologic study has made it possible in many cases to secure a better percentage result. If the geologist is able to raise the percentage ever so little, the expenditure is amply justified. He is not expected to have 100 per cent success; but he is expected to better the average returns, and in this on the whole he has not failed.
Applying this method specifically to the Gogebic Range, it appears that up to January 1, 1918, exploration and development had covered 3,650 acres of iron formation, measured along the dip in the plane of the footwall, within the limits of the area in which the formation is in such condition as to allow concentration of the ore. The total area of the footwall to a depth of 3,000 feet is approximately 9,650 acres. The range, therefore, was 38 per cent developed to this depth. In the developed area, 160,000,000 tons of ore had been found, or approximately one ton per square foot of footwall area, or 43,800 per acre of footwall explored. The total area of ore measured on the footwall was 785 acres. The ratio of ore area to total explored area, measured in the plane of the footwall, was 21-½ per cent. This may be taken in a rough way to indicate the average exploring possibilities in new ground, where local conditions to the contrary do not exist. This means that over the whole range about one drill hole or cross-cut in five will strike ore on an average. Or, looked at in another way, about 200 feet of drifting in every 1,000 on the footwall will be in ore. Applying this factor to the unexplored area, amounting to 6,000 acres, the range had an expectation on January 1, 1918, to a depth of 3,000 feet, over and above ores already discovered, of approximately 262,800,000 tons. This was sufficient to extend the life of the range by about forty-four years. Knowing the average cost of development of ore per foot in the past, and knowing the annual output and its rate of acceleration, it is possible to figure with some accuracy how much expenditure should be planned for annually in the future in order to maintain a safe margin of reserves against output.
Such quantitative considerations in the Lake Superior region serve not only to guide the general conduct of the exploration and development work, but in some cases as a basis for valuation both for commercial and taxation purposes.
Development and Exploitation of Mineral Deposits
The search for new ore bodies is closely related to the development, extension, and mining of ore bodies already found. In this field the geologist finds wide application of his science. Here he may not be so much concerned with the economic factors or with the broader methods of geologic elimination; his study is more likely to be based mainly on the local geologic conditions.
Some of the larger and more successful mining companies, perhaps the greater number of them these days, have geologists whose business it is to follow closely the underground operations, with a view to advising on the conduct of the development work. This requires the most precise and intensive study. For instance, the Anaconda Copper Mining Company has a staff of several geologists, who follow the underground work in the utmost detail and whose approval must be obtained by the operating department in the formulation of any development plan. The complexity and fault relations of the veins in this company's mines are such that the application of these methods has abundantly justified itself on the cost sheet.
Too often mining companies leave the planning and execution of the underground development work to the local management, commonly to the underground mining captain, without geologic consultation. This procedure does not eliminate the economic geologist; for when the development fails at any point, or new and unexpected conditions are met, the geologist is likely to be called in. In such cases the practice of a geologist is like that of the ordinary medical practitioner; he is called in only when his patients are in trouble. The use of adequate geologic advice in the planning stages is about as little advanced in some localities as the practice of preventive medicine.
The work of the economic geologist may not be ended by the finding and development of the ore; for the moment this is accomplished, he should again consider the economic phases of the problem—the grade of his ore, its probable amount, and other features, in relation to the general economic setting. In his enthusiasm for physical results, he may be carried into expenditures not justified by the economic factors in the problem. Some one else may and usually does look out for the economic elements, but the prudent geologist will at least see to it that someone is on the job.