The General Relations of the Geologist To Exploration And Development

The economic geologist is more vitally concerned with exploration and development than with any other phase of his work. This comes closest to being his special field. Here is a fascinating element of adventure and chance. Here is the opportunity to converge all his knowledge of geology and economics to a practical end. The outcome is likely to be definite one way or the other, thus giving a quantitative measure of the accuracy of scientific thinking which puts a keen edge on his efforts. It is not enough merely to present plausible generalizations; scientific conclusions are followed swiftly either by proof or disproof. With this check always in mind, the scientist feels the necessity for the most rigid verification of his data, methods, and principles.

The general success of the application of geology to exploration and development is indicated by the rapid increase in demand for such service in recent years, and by the large part it plays in nearly all systematic and large-scale operations. The argument is sometimes made that many mineral deposits have been found without geologic assistance, and that therefore the geologist is superfluous. The answer to this argument is that there are often hundreds of "practical" explorers in the field to one geologist, and that in proportion to numbers the story is quite a different one. The very fact that many large mining organizations, as a result of their experience, now leave these matters of exploration and development largely in the hands of geologists, is a tribute to the usefulness of the science. Also, it is to be remembered that not all applications of geology are made by geologists. It is hard to find a prospector or explorer who has not absorbed empirically some of the elements of geology, and locally this may be enough. Very often men who take pride in the title of "practical prospectors" are the ones with the largest stock of self-made geological theories.

During a prospecting boom it is not uncommon for speculators and promoters to attempt to discount geologic considerations where these run counter to their plans. The catching phrase "bet against the geologist" has a broad appeal to an instinctive preference for the practical as opposed to the theoretical. If the public would stop to note the character of the support behind the geologist, including as it does the larger and more successful operators, it would not be so ready to accept this implication.

Another aspect of this question might be mentioned. There is scarcely an oil field or mining camp in the world without a cherished tradition to the effect that, prior to discovery, the mineral possibilities had been reported on unfavorably by the geologists,—again implying that success has been due to the hard common sense of the horny-handed prospector. These traditions persist in the face of favorable geological reports published before discovery; they are natural expressions of the instinctive distrust of any knowledge which is beyond the field of empirical experience. In many cases the discoveries were made long before geologists appeared on the scene. In others, possibly one or two geologic reports were unfavorable, while many were favorable. In the aggregate, there can be no question that, in proportion to the scale of its use, geological advice has had more than its proportion of success.

Even under the most favorable conditions, the chances against the success of an individual drill hole or underground development are likely to be greater than the chances for it. The geologist may not change this major balance; but if he can reduce the adverse chances by only a few per cent, his employment is justified on purely commercial grounds.

The above comments refer to sound geological work by competent scientists. The geologic profession, like many others, is handicapped by numbers of ill-trained men and by many who have assumed the title of geologist without any real claim whatever,—who may do much to discredit the profession. The very newness of the field makes it difficult to draw a sharp line between qualified and unqualified men. With the further development of the profession this condition is likely to be improved (see pp. 427-428).

So new is the large-scale application of geology to exploration and development, and so diverse are the scientific methods of approach, that it is difficult to lay out a specific course for a student which will prepare him for all the opportunities he may have later. In the writer's experience, both in teaching and practice, the only safe course for the student is to prepare broadly on purely scientific lines. With this background he will be able later to adapt himself to most of the special conditions met in field practice.

Partly Explored Versus Virgin Territories

In selecting an area to work, the geologic explorer will naturally consider various factors mentioned in succeeding paragraphs; but the natural first impulse is to start for some place where no one else has been, and to keep away from the older principal mining camps,—on the assumption that such grounds have been thoroughly explored and that their geological conditions bearing on exploration are fully understood. It is safe to say that very few mineral districts are thoroughly understood and explored. Numerous important discoveries of recent years have been in the extensions of old mines and old districts; and when one considers the scale of even the most extensive mine openings in comparison with the vast body of rock available for exploration, it is clear that this will continue to be the situation far into the future. It is the writer's belief that the economic geologist stands at least as good a chance of success in exploration in the older districts as he does in new fields. Nature is exceedingly erratic and economical in providing places favorable for mineral production; in a producing district the geologic conditions have been proved to be right, and the explorer starts here with this general pragmatic advantage. The explorer here has another great advantage, that much essential information has been gathered which can be built into his plan of operations. He can start, scientifically and practically, where the other man left off. One of the best-known economic geologists has maintained that the more previous work done, the better, because it furnished him more tools to work with. There is no such thing as "skimming the cream" from a geologic problem; there is no end in sight in the search for more knowledge.

This attitude toward the problem of exploration has also proved advantageous on the business or financial side. A successful backer of mineral enterprises once remarked that his best prospecting was done from the rear platform of a private car,—meaning that this mode of transportation had carried him to the center of important mining activities, where the chances for large financial success showed a better percentage than in more general and miscellaneous exploration.

The Use of All Available Information

Effective scientific exploration requires the use of all available information applying to the specific area. This might seem to be too obvious to require mention, yet observance of the methods of explorers seems to call for warning against the rather common tendency to go into a field unprepared with a thorough knowledge of preceding work. It is easy to forget or overlook some investigation made many years previously; or to assume that such work is out of date, and of no special consequence in the application of new thought and method which is the basis of the faith and confidence of each new geologic explorer. A study of the reports on an old camp shows how often the younger generations have ignored the results of the older. Many of the same elementary truths are rediscovered by successive generations, after large efforts which could have been saved by means of proper care and investigation of the previous literature and mapping.

In outlying parts of the world, the existing information bearing on exploration may be at a minimum. In many of the older mining camps and throughout most civilized countries, however, careful investigation will usually disclose a considerable range of useful information bearing on the territory to be explored. In the United States the natural course to be pursued is to hunt carefully through the reports of the U. S. Geological Survey, the Bureau of Mines, various state surveys, universities, and private organizations (so far as these reports are available), and through the technical journals and the reports of technical societies, for something bearing on the district to be explored. Even if no specific report or map is to be found, it is usually possible to locate general maps or accounts which are likely to be of use.

CoÖperation in Exploration

Competition in exploration often develops an atmosphere of suspicion and furtiveness which is highly unfavorable to coÖperative efforts. Individuals and companies may handicap themselves greatly by a desire to play a lone hand, and by failure to take advantage of an exchange of information. This action may be based, particularly on the part of strong mining companies, on the assumption that they know all that is necessary about the problem, and that an outsider has nothing to contribute. Financial and other conditions may require this attitude; but in large part it is a result of temperament, as clearly indicated by the difference in methods followed by different groups and in different mining districts. From the scientific point of view this attitude can hardly be justified, in view of the extremely narrow limits of human knowledge as compared with the scientific field to be explored. The sum total of knowledge from all sources is only a small fraction of that necessary for the most effective results. The mutual exchange of information and discussion is usually justified on the basis of self-interest alone, to say nothing of the larger interest to the mineral district, to the country, or to science.

National and state survey organizations exercise considerable effort to secure records of drilling. In some cases they have the legal power to command this information, particularly in relation to appraisals for taxation and "blue sky" laws. In a larger number of cases drill records are secured through voluntary coÖperation with explorers. A considerable number of records are nevertheless not filed with public agencies and some of these are permanently lost. Even where the records are turned in to a public organization, they are in most cases not directly available to explorers.

Public registration of all drilling records is a highly desirable procedure in the interests of the development of the mineral industry as a whole. A vast amount of unnecessary duplication can thus be avoided. The record of a drill hole, even though barren, may be of vast significance in the interpretation of future developments and should be recorded as carefully as an abstract of land title. The property right of the explorer in such information can be and usually is protected by withholding the record from public inspection until sufficient time has elapsed to give him full opportunity to use the information to his own best advantage.

The opportunities for coÖperation with specialists of public organizations are almost unlimited. These organizations are likely to have an accumulation of data and experience extending through long periods and over large areas, which the private explorer ordinarily cannot hope to duplicate. With proper restrictions this information may be available for public use. A good illustration of current coÖperative effort of this kind is in the deep exploration for oil in the Trenton limestone of Illinois. Outcrops and other specific indications are not sufficient to localize this drilling; but the information along broad geologic and structural lines which has been collected previously by the Illinois Survey is sufficient so that, with a comparatively small amount of shallow drilling, the locus of the more favorable structural conditions may be determined. In this case the Survey is directing the initial exploration, which is financed by private capital.

Economic Factors in Exploration

The approach to the problem of exploration is very often determined by local requirements and conditions; but if one were to come at the problem from a distance and to keep matters in broad perspective, the first step would be a consideration of what might be called the economic factors. Let us suppose that the geologist is free to choose his field of exploration. An obvious preliminary step is to eliminate from consideration mineral commodities which are not in steady or large demand and are much at the mercy of market conditions, or which are otherwise not well situated commercially. The underlying factors are many and complex. They include the present nature and future possibilities of foreign competition, the domestic competition, the grades necessary to meet competition, the cost of transportation, the cost of mining under local conditions—including considerations of labor and climatic and topographic conditions,—the probability of increase or decrease in demand for the product, the possible changes in metallurgical or concentrating practice (such as those which made possible the mining of low-grade porphyry copper ores), the size of already available reserves, and the mining laws in relation to ownership and regulation. Most of these factors are discussed at some length on other pages. After looking into the economic conditions limiting the chromite, nickel, or tin developments in the United States, the explorer might hesitate to proceed in these directions,—for he would find that past experience shows little promise of quantities and grades equivalent to those available in other countries, and that there is little likelihood of tariffs or other artificial measures to improve the domestic situation. Before and during the war, commercial conditions might have shown the desirability of hunting for pyrite, but more recent developments in the situation cast some doubt on this procedure. To go ahead blindly in such a case, on the assumption that the pyrite market would in some fashion readjust itself, would not be reasoned exploration. Again, in considering exploration for copper, account should be taken in this country of the already large reserves developed far in advance of probable demand, which require that any new discoveries be very favorably situated for competition. In oil, on the other hand, a very brief survey of the economic factors of the situation indicates the desirability of exploration. The comparative shortage of lead supplies at the present time suggests another favorable field for exploration.

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.

The classification of the public mineral lands by government agencies is fully discussed by George Otis Smith and others in a bulletin of the United States Geological Survey.[37] The purposes, methods, and results of this classification should be familiar to every explorer. Nowhere else is there available such a vast body of information of practical value. Quoting from this report:

For this purpose the Geological Survey has made extensive classification of coal lands, oil and gas lands, phosphate lands, lands bearing potash and related salines, metalliferous mineral lands, miscellaneous non-metalliferous mineral lands, and water resources. The scope of the work may be indicated by the factors considered. For instance coal is investigated in relation to its character and heat-giving qualities (whence comes its value), quantity, thickness, depth, and other conditions that effect the cost of its extraction. Metalliferous mineral lands are considered in relation to general geology, country rock, intrusions and metamorphism, structure, outcrops and float of lodes, prospects and mines, samples, and history of the region.

Classifications of this kind have often proved useful to large holders of land as a basis for intelligent handling of problems of sale, taxation, and the granting of rights to explorers. Because of the lack of this elementary information, there has been in some quarters timidity about dealing with large holdings, for fear of parting with possible future mineral wealth,—with the result that such tracts are carried at large expense and practically removed from the field of exploration. To the same cause may be attributed some of the long delays on the part of the government in opening lands for mineral entry or in issuing patents on land grants.

Outcrops of Mineral Deposits

Many mineral deposits have been found because they outcrop at the surface; the discoveries may have been by accident or they may have been aided by consideration of geologic factors. There are still vast unexplored areas in which mineral deposits are likely to be found standing out at the surface. For much of the world, however, the surface has been so thoroughly examined that the easy surface discoveries have been made, and the future is likely to see a larger application of scientific methods to ground where the outcrops do not tell an obvious story. Mineral deposits may fail to outcrop because of covering by weathered rock or soil, by glacial deposits, or by younger formations (surface igneous flows or sediments), or the outcrop of a deposit may be so altered by weathering as to give little clue to the uninitiated as to what is beneath. Mineral deposits formed in older geologic periods have in most cases been deeply covered by later sediments and igneous rocks. Such deposits are in reach of exploration from the surface only in places where erosion has partly or wholly removed the later covering. An illustration of this condition is furnished in the Great Basin district of Nevada, where ore bodies have been covered by later lava flows. The ore-bearing districts are merely islands exposed by erosion in a vast sea of lava and surface sediments. Beyond reasonable doubt many more deposits are so covered than are exposed, and it is no exaggeration to say that by far the greater part of the mineral wealth of the earth may never be found. Where a mineral-bearing horizon is exposed by erosion at the surface, underground operations may follow this horizon a long way below the capping rocks; but, after all, such operations are geographically small as compared with the vast areas over which the covering rocks give no clue as to what is beneath. One of the principal problems of economic geology for the future is to develop means for exploration in territories of this sort. A beginning has been made in various districts by the use of reconnaissance drilling, combined with interpretation of all the geologic and structural features. The discovery of one of the largest nickel deposits in the Sudbury district of Canada was made by reconnaissance drilling to ascertain the general geologic features, in an area so deeply covered as to give little suggestion as to the proper location for attack.

Some Illustrative Cases

The use of outcrops in oil exploration has been noted on other pages (pp. 146-147).

Outcrops of coal seams may be found in folded or deeply eroded areas. For the most part, however, and especially in areas of flat-lying rocks, the presence of coal is inferred from stratigraphic evidence and from the general nature of the geologic section—which has been determined by outcrops of associated rocks or by information available at some distant point. The structural mapping of coal beds on the basis of outcrops and drill holes has been referred to (pp. 126-127).

Iron ores are very resistant to solution. Where hard and compact they tend to form conspicuous outcrops, and where soft they may be pretty well covered by clay and soil. In glaciated areas, like the Lake Superior region, outcrops of iron ore are much less numerous because of the drift covering. Certain of the harder iron ores of the Marquette, Gogebic and Menominee districts of Michigan and of the Vermilion district of Minnesota project in places through the glacial drift, and these ores were the first and most easily found. Much the greater number of iron ore deposits of Lake Superior, including the great soft deposits of the Mesabi range of Minnesota, fail to outcrop. On the other hand the iron formation, or mother rock of the ore, is hard and resistant and outcrops are numerous. The hematite ores of Brazil have many features in common with the Lake Superior ores in age and occurrence, but they have not been covered with glacial deposits. Outcrops of the iron ore are large and conspicuous, and the surface in this territory gives one some idea of what the Lake Superior region may have looked like before the glaciers came along. Certain of the soft iron ores of the lateritic type, as in Cuba, outcrop over great areas where their topographic situation is such that erosion has not swept them off. On erosion slopes they are seldom found. The Clinton iron ores of the southeastern United States outcrop freely.

Some of the lead and zinc deposits of the Mississippi Valley outcrop at the grass roots as varying mixtures of iron oxide, galena, chert, and clay, though they seldom project above the general surface. The old lead ranges of Wisconsin and Illinois, found at the surface a century ago by the early explorers and traders, have served as starting points for deeper exploration which has located the zinc deposits. Erosion channels have freely exposed these ore bodies, and in the Wisconsin-Illinois deposits most of the ores thus far found are confined to the vicinity of these channels. The greater number of the lead and zinc deposits of the Mississippi Valley, however, are covered with weathered material or with outliers of overlying sediments, with the result that underground exploration is necessary to locate them.

Sulphide deposits in general, including those carrying gold, silver, copper, lead, zinc, and other metals, have many common features of outcrop. The iron sulphide commonly present in these ore bodies is oxidized to limonite at the surface, with the result that prospectors look for iron-stained rocks. These iron-stained rocks are variously called the "gossan," the "iron capping," the "colorado," or the "eiserner Hut" (iron hat). The gossan is likely to resist erosion and to be conspicuous at the surface,—though this depends largely on the relative resistance of the wall rocks, and on whether the gangue is a hard material like quartz, or some material which weathers more rapidly like limestone or igneous rock. The gossan does not often carry much value, though it may show traces of minerals which suggest what may be found below. Gold, silver, and lead are not easily leached out of the surface outcrops. Copper and zinc are much more readily leached, and in the outcrop may disclose their existence only by traces of staining. It happens not infrequently, therefore, that copper and zinc deposits are found through the downward exploitation of oxidized gold, silver, and lead ores. The veins at Butte were first worked for silver, and the ore bodies at Bingham, Utah, and Jerome, Arizona, were first mined for gold. Exceptionally, copper ore in enriched, oxidized form outcrops, as at Bisbee, Arizona.

It is not always true that valuable sulphide deposits have an iron-stained outcrop, for in some of them iron sulphide or pyrite is so scarce that the surface outcrops may be light-colored clayey and siliceous rocks.

Silver is often represented in the outcrop by silver chloride or cerargyrite, which may be easily identified. The prospecting for such surface ores is sometimes called "chloriding."

The presence in the outcrop of dark manganese oxides associated with vein quartz sometimes indicates the presence below of copper and zinc and other minerals, as at Butte.

Extensive alterations of the country rock in the way of silicification and sericitization, and the presence of minerals like garnet, tourmaline, diopside, and others, known to be commonly deposited by the same hot solutions which make many ore deposits, may furnish a clue for exploration below. These characteristics of the country rock, however, are likely to be masked at the outcrop by later weathering, which superposes a kaolinic or clayey alteration.

Topography and Climate As Aids in Searching for Mineral Outcrops

The topographic expression of a mineral deposit depends upon its hardness and resistance to erosion as compared with the adjacent rocks. If more resistant it will stand out at the surface; if less resistant, it will form a depression. The conditions determining resistance are exceedingly variable, and no broad generalization can be made; but within a local province a given group of mineral deposits may characteristically form depressions or ridges, and thus topographic criteria may be very useful in exploration. Even with such limitations, the variations of the topographic factor may be so great as to require much care in its use. Sulphide ores in quartzites are likely to develop depressions under erosion. In limestones they are more likely to stand out in relief, because of the softer character of the limestone, though this does not always work out. Crystalline magnetite and hematite are more resistant to erosion than almost any other type of rock, and stand out at the surface with proportional frequency.

Climatic conditions may determine the locus of search for certain surface minerals. Bauxite and lateritic iron ores, for instance, are known to favor tropical climates. In exploration for these minerals, the climatic factor must be applied in connection with the topographic considerations already mentioned, and both, in turn, in connection with the character of the country rock as determined by general geologic surveys. A combination of climatic, topographic, and other physiographic conditions may be used also in exploration for certain types of residual clays.

Size and Depth of Ore Bodies As Determined From Outcrop

Where the ore body is harder than the surrounding rock, it stands out in conspicuous outcrops and is likely to show a narrowing below. Where it is softer than the surrounding rocks, and outcrops in a topographic depression, it is perhaps more likely to show widening below. These features are due to the general facts that, where the ore body is hard and resistant, the downward progress of erosion is likely to be arrested where the adjacent rocks occupy the larger part of the surface, that is, where the ore body is narrower. This principle is often vaguely recognized in the assumption that an exceptionally large outcrop of an ore vein may be "too good to last." Again, such a generalization must be applied to a specific case with much caution.

Attempts to forecast the depth of veins from their extent at the surface meet with only partial success. In a very general way great persistence horizontally suggests persistence in depth, on the ground that the section exposed on the surface is as likely to be a section of average dimensions as one along vertical lines.

Faith is the first article of the prospector's creed, and it is hard to shake his conviction that every ore outcrop must widen and improve below. As expressed by the French-Canadian prospector in the Cobalt district, the "vein calcite can't go up, she must go down." While the scientist may have grounds to doubt this reasoning, he is not often in a position to offer definite negative evidence.

The Use of Placers in Tracing Mineral Outcrops

Outcrops of ore-bearing rock may occasionally be located by tracing a placer deposit back to its source, or by following up ore fragments in the "wash" on mountain sides to the place of origin, or by noting ore fragments in glacial deposits. The presence of an ore mineral in a placer naturally raises the question as to whence it came. If it is a recent placer, it may be comparatively easy to follow up the stream channels to the head-water territory which is delivering the main mass of sediment, and there to locate a vein in place. The problem is complicated by multiplicity of tributaries and by large size of the drainage areas. In such cases careful panning and testing of the gravels at frequent intervals may show which of several tributaries are contributing most of the values, and thus may further localize the area of search. Many important mining districts, including Butte, Bisbee, the Mother Lode region of southern California, the diamond fields of Africa, and others, have been found by tracing up placers in this manner. In the case of an older placer deposit, where the topography and drainage have been much altered since its formation, or where the deposit has been covered by later sediments, the problem is of course much more difficult.

Much less than a commercially valuable placer deposit in unconsolidated surface rocks may start a search for the mother lode. A single fragment of ore in the "wash" naturally directs attention up the slope, and the repetition of fragments in a certain direction may lead unerringly to the source. The fragments may not even in themselves carry value, but may consist of detrital material from the leached outcrop—such as iron or manganese oxides, which, because of their red or black color, stand out conspicuously in the rock dÉbris.

In the Lake Superior region large angular fragments of iron ore or iron formation in the glacial drift immediately raise question as to source. If the fragments are rounded and small, they usually indicate a very distant source. The general direction of glacial movement is known in most places, and by tracing up the fragments in this direction the outcrop may be found; or the chain of fragments may be traced to a point where they stop, which point may serve to locate the parent bedrock carrying the ore body, even though it does not outcrop.

An interesting suggestion was made some years ago with reference to the diamonds found sporadically in the terminal moraines in Wisconsin and other mid-west states. The diamonds are of such size and quality as to indicate surely the existence of a real diamond field somewhere to the north. The locations of these diamond finds were platted on a glacial map, and lines were projected in a general northerly direction along the known lines of the glacial movement. It was found that these lines converged at a point near Hudson's Bay. The data were too meager and the base line too short for this long projection, and the indicated source of the diamonds can be regarded as the merest speculation. However, with the finding of additional diamonds in the drift, as seems very likely, the refinement of this method might conceivably bring results in time.

The Use of Magnetic Surveys in Tracing Mineral Ledges

Magnetic surveys are often useful in tracing iron-bearing rocks beneath the surface, in the discovery of outcrops of such rocks, and in working out their lines of connection. This method is in general use for the crystalline iron ores in the Lake Superior region, Canada, the Adirondacks, and elsewhere in the glaciated portions of the United States. It is not so useful for the brown ores and the Clinton ores of the southeastern United States, which are only slightly magnetic and can be commonly located by other methods.

Where the ore is strongly magnetic, and is associated with other rocks which are non-magnetic, the nature of the magnetic field determined by a surface survey with vertical and horizontal needles may tell something about the shape and size of the ore body. Commonly, however, magnetic ores are associated with leaner magnetic rocks,—with the result that the magnetic survey, unless it happens to lead to an outcrop of ore, indicates only the general area through which underground exploration might be warranted. In the hematitic iron ores of Lake Superior, magnetism is less pronounced than in the magnetites; and in the soft hydrous hematites, like those of the Mesabi district, it may cause only slight disturbance of the magnetic needle. This disturbance is usually sufficient to locate the position of the iron-bearing formation, though not the position of the ore.

Where the iron formation has been highly metamorphosed, and rendered resistant to weathering and erosion so that it will not concentrate into ore, it is likely to have higher magnetic attraction than the richer ores. For this reason an area of strong magnetic attraction is ordinarily regarded as not particularly favorable to the finding of important hematite deposits. However, this attraction may be very useful in tracing out the formation to a place where it is less metamorphic, less resistant to erosion, less likely to outcrop, and yet more promising for the discovery of iron ore. For instance, on the east end of the Mesabi and on the east and west ends of the Gogebic district, magnetic surveys trace the iron formation with great ease to points where the attraction is low and the conditions for exploration more favorable.

The magnetic needle has also been used in the search for nickel ore in the Sudbury district of Ontario, but without great success, because of the variety of rocks other than nickel which are more or less magnetic, and because of the slight magnetic properties of the nickel ore itself. In a large-scale exploration of this type, conducted some years ago, a favorable magnetic belt was discovered, and a pit was sunk to water level but not to bedrock. Years later, the extension of this pit by only a few feet disclosed one of the great ore bodies of the district.

Experimental work on the use of the magnetic needle on copper deposits has yielded some interesting and suggestive results, but this investigation is still under way and the results have not been published.

The Use of Electrical Conductivity and Other Qualities Of Rocks in Exploration

In addition to magnetism, rocks and ores have other properties susceptible to observations made at a distance, such as electrical conductivity, transparency to X-rays, specific induction, elasticity, and density. All these qualities have been of interest to geologists in some connection or another, but none of them have yet been used effectively in exploration for mineral resources. The only one of these properties that has thus far seemed to promise practical results is electrical conductivity. The results yet obtained are slight, and this kind of investigation has rested under something of a cloud, due to extravagant claims of inventors. Nevertheless, there has been a considerable amount of scientific work by physicists, geologists, and engineers, supplemented by special war-time investigations of rock and earth conductivity in connection with ground telephones and the tapping of enemy conversations, which seems to indicate a distinct possibility of practical results in the future,—perhaps not so much in locating specific ore bodies as in locating general types of formation and structures,—which may serve to supplement other methods of search.[38]

The transmission and reflection of sound waves in rocks have also been more or less investigated with reference to their possible military use. It seems not impossible that these phenomena may be of some geologic aid in the future, but experimental work is yet in a very early stage.

The Use of Structure and Metamorphism in Exploration

The necessity for careful use of structural data in exploration scarcely requires discussion. References have been made to structural features in connection with coal, oil, iron ore, and other minerals. This phase of study can scarcely be too intensively followed. The tracing of a folded or faulted vein, in a particularly complex system of veins, requires application of all of the methods and principles of structural geology.

Similarly, the importance of applying the principles of metamorphism, embodied in the metamorphic cycle (pp. 27-28) is almost self-evident. Certain kinds of metamorphism are suggestive of the nature of the mineral deposits with which they are associated. One would not look for minerals known to be caused mainly by surficial processes in rocks which have been altered mainly by deep-seated processes. The presence of metamorphism indicating high temperatures and pressures to some extent limits the kinds of minerals which one may expect to find. On the other hand, minerals known to be primarily formed at great depths, providing they are resistant to surface weathering, may be found in deposits which are the result of surficial alterations or katamorphic processes; that is, they may become concentrated as residual materials in weathered zones or as placers.

Drilling in Exploration

In the absence of distinctive outcrops, as well as when outcrops are found, drilling is a widely used method of underground exploration in advance of the sinking of shafts or the driving of tunnels. Drilling is more useful in the locating and proving of mineral deposits of large bulk, like deposits of coal, iron, and oil, than mineral deposits of small bulk and high value, like gold and silver deposits. However, it is not always used in the exploration of the first class of deposits and is not always eliminated in the exploration of the second class. With the development of better mechanical devices, better methods of controlling and ascertaining the direction of the drill hole, and more skillful interpretation of drill samples, the use of drilling is rapidly extending into mineral fields where it was formerly thought not applicable.

The geologist takes an active part in drilling operations by locating the drill holes, by determining the angle of the holes, by identifying and interpreting the samples, by studying bedding, cleavage, and other structures as shown in the samples, and determining the attitude of these structures in the ground, by determining when the horizon is reached which is most promising for mineral, and by determining when the hole shall be stopped. With a given set of surface conditions, the problem of locating and directing a drill hole to secure the maximum possible results for the amount expended requires the careful consideration of many geologic factors,—and, what is more important, their arrangement in proper perspective and relationship. Faulty reasoning from any one of the principal factors, or over-emphasis on any one of them, or failure to develop an accurate three-dimensional conception of the underground structural conditions, may lead to failure or extra expense. Success or failure is swiftly and definitely determined. The geologist is usually employed by the company financing the drilling; but in recognition of the importance of his work, some of the large contracting drill companies now employ their own geologists. The technique of the geologic interpretation and direction of drilling has become rather complicated and formidable, and has resulted in the introduction of special college courses in these subjects.

The desirability of public registration of drilling records is discussed on another page (pp. 305-306).

Quantitative Aspects of Geologic Exploration

In recent years there has been a tendency to reduce the geologic factors in exploration to some kind of a quantitative basis. While these factors may be very variable and very complex, their net effect frequently may be expressed in terms of quantitative averages. In various mines and mining districts where operations are of wide extent, local quantitative factors have been worked out which are useful in predicting results from proposed explorations in undeveloped portions. Figures of this sort may be useful and practical guides in planning any given exploration, its cost, and its probable outcome.

Quantitative methods are illustrated in the general account of Lake Superior iron ore exploration in a later section.

Curves of production from oil wells and from oil districts have been found to have certain characteristic features in common which are often used in predicting the future output and life of a given well, property, or district. Where associated with coal, the percentage of fixed carbon in the coal may be a guide to the presence and nature of the oil (see Chapter VIII).The geological staff of the Netherlands East Indies estimated the tin reserves of one of these islands by the use of a factor or coefficient, based on the experience of another island.

In the Cobalt district of Canada a factor for future discoveries and output, based on past experience, was similarly developed.

Hoover[39] made a statistical study of several hundred metal mines in various parts of the world, and found that not 6 per cent of the mines that yielded profits ever made them from ore mined below 2,000 feet; and that of the mines that paid dividends, 80 per cent did not yield profit below 1,500 feet, and most of them died above 500 feet.

Attempt has been made by a Swedish geologist to estimate the iron ore resources of continents by the use of an iron coefficient. This coefficient was obtained by dividing the known iron ore resources of the comparatively well-investigated portions of the world by the number of square miles in which they occurred, and was then multiplied into the area of the continents whose resources were to be determined.

The application of quantitative methods of this kind has not yet become very general, nor is it possible to use them in some cases; where applied many of them have been very crude and others have been partly disproved by experience. With increasing knowledge and experience, such methods are becoming more accurate and useful, and are likely to have wider use in the future.

Origin of Mineral Deposits As a Factor in Exploration

In exploration, the geologist is keen to ascertain the origin of the mineral deposit. This is often a source of wonder to the layman or "practical" man, and the geologist may be charged with having let his fondness for theory run away with him. A widespread fatalistic conception is expressed in the Cornishman's dictum on ore, "Where it is, there it is." Yet an understanding of the origin of any particular ore, the "why" of it, is coming to be recognized as the most effective means of reaching sound practical conclusions. By ascertaining the approximate origin of the ore, it may be possible at once to infer a whole group of practical considerations based on experience with ores of like origin in other localities. The origin of the ore is the geologist's primary interest, and it is this which gives him his most effective and distinctive tool in exploration. Many other phases of exploration work may be picked up empirically by any one familiar with the local conditions; but when the man without sound geologic training attempts to go into this particular field, his lack of background and perspective often leads to fantastic hypotheses which may vitiate the inferences on which he plans his exploration.

The scientific investigator, while not accepting the fanciful theories of the local observer, will make a mistake if he fails to recognize the residuum of solid fact on which they are built. Many practical explorers are shrewd observers of empirical facts, even though their explanations may show a lack of comprehension of the processes involved. Any assumption of superiority, intolerance, or lack of sympathy, on the part of the geologist, toward the inadequate explanations and descriptions given him by the practical man, is likely to indicate a weakness or limitation in his own mental processes. The geologist's business is to sift out the fact from the inference, and not to throw over the whole structure because some of the inferences are faulty.

Lake Superior Iron Ore Exploration As an Illustration

To illustrate the application of some of the methods of exploration of the kinds described in this chapter, the writer selects an example from his own experience in the Lake Superior iron fields.[40]

In this region, consideration of the economic aspects of the problem may eliminate from the best explorable field certain Canadian portions which are far from water transportation, because the conditions in these sections would prevent the use of anything but an exceptionally large and rich deposit. Economic conditions determine in advance also that it is not worth while looking for ores of certain grades, either because they are not usable on account of deleterious constituents or low content of iron, or because these particular grades have already been developed in excess of requirements. Having determined what ore is desired, whether Bessemer or non-Bessemer, whether open-hearth or foundry, further elimination of area is possible on the basis of past experience.

Coming to the geologic phases of the problem, the first step is to eliminate great areas of rock which are known never to contain iron ore, like the granite areas and the quartzite and limestone areas. Within the remaining areas, by examination of the surface outcrops and with the aid of magnetic surveys, iron formations are found which are the mother rock of the ores. In Michigan, it has been possible to use certain percentage expectations in the areal location of iron formations within certain series of rocks extending over wide areas. Such percentage coefficients have been useful, not only in exploration, but also in the valuation of lands which are so covered with drift that no one knows whether they carry an iron formation or not.

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.

FOOTNOTES:

[37] Smith, George Otis, and others, The classification of the public lands: Bull. 537, U.S. Geol. Survey, 1913.