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THE ECONOMIC ASPECTS OF GEOLOGY
C. K. LEITH
UNIVERSITY OF WISCONSIN
NEW YORK HENRY HOLT AND COMPANY
COPYRIGHT, 1921
BY
HENRY HOLT AND COMPANY
August, 1923
PRINTED IN THE U. S. A.
CONTENTS
PAGE CHAPTER I. INTRODUCTION 1
SURVEY OF FIELD 1
ECONOMIC APPLICATIONS OF THE SEVERAL BRANCHES OF GEOLOGY AND OF OTHER SCIENCES 3 Mineralogy and petrology 3 Stratigraphy and paleontology 4 Structural geology 5 Physiography 6 Rock alterations or metamorphism 10 Application of other sciences 10
TREATMENT OF THE SUBJECT IN THIS VOLUME 11
CHAPTER II. THE COMMON ELEMENTS, MINERALS, AND ROCKS OF THE EARTH AND THEIR ORIGINS 13
RELATIVE ABUNDANCE OF THE PRINCIPAL ELEMENTS OF THE LITHOSPHERE 13
RELATIVE ABUNDANCE OF THE PRINCIPAL MINERALS OF THE LITHOSPHERE 14
RELATIVE ABUNDANCE OF THE PRINCIPAL ROCKS OF THE LITHOSPHERE 16
WATER (HYDROSPHERE) 18
SOILS AND CLAYS 18
COMPARISON OF LISTS OF MOST ABUNDANT ROCKS AND MINERALS WITH COMMERCIAL ROCKS AND MINERALS 18
THE ORIGIN OF COMMON ROCKS AND MINERALS 18 Igneous processes 19 Igneous after-effects 19 Weathering of igneous rocks and veins 20 Sedimentary processes 22 Weathering of sedimentary rocks 23 Consolidation, cementation, and other sub-surface alterations of rocks 24 Cementation 24 Dynamic and contact metamorphism 25
THE METAMORPHIC CYCLE AS AN AID IN STUDYING MINERAL DEPOSITS 27
CHAPTER III. SOME SALIENT FEATURES OF THE GEOLOGY AND CLASSIFICATION OF MINERAL DEPOSITS 29
VARIOUS METHODS OF CLASSIFICATION 29
NAMES 31
MINERAL DEPOSITS AS MAGMATIC SEGREGATIONS IN IGNEOUS ROCKS 34
MINERAL DEPOSITS WITHIN AND ADJACENT TO IGNEOUS ROCKS, WHICH WERE FORMED IMMEDIATELY AFTER THE COOLING AND CRYSTALLIZATION OF THE MAGMAS THROUGH THE AGENCY OF HOT MAGMATIC SOLUTIONS 36 Evidence of igneous source 37 Possible influence of meteoric waters in deposition of ores of this class 41 Zonal arrangement of minerals related to igneous rocks 42 The relation of contact metamorphism to ore bodies of the foregoing class 45
SECONDARY CONCENTRATION IN PLACE OF THE FOREGOING CLASSES OF MINERAL DEPOSITS THROUGH THE AGENCY OF SURFACE SOLUTIONS 46
RESIDUAL MINERAL DEPOSITS FORMED BY THE WEATHERING OF IGNEOUS ROCKS IN PLACE 50
MINERAL DEPOSITS FORMED DIRECTLY AS PLACERS AND SEDIMENTS 51 Mechanically deposited minerals 51 Chemically and organically deposited minerals 52
SEDIMENTARY MINERAL DEPOSITS WHICH HAVE REQUIRED FURTHER CONCENTRATION TO MAKE THEM COMMERCIALLY AVAILABLE 54
ANAMORPHISM OF MINERAL DEPOSITS 57
CONCLUSION 58
CHAPTER IV. MINERAL RESOURCES—SOME GENERAL QUANTITATIVE CONSIDERATIONS 60
WORLD ANNUAL PRODUCTION OF MINERALS IN SHORT TONS 60
WORLD ANNUAL PRODUCTION OF MINERALS IN TERMS OF VALUE 62
SIGNIFICANCE OF GEOGRAPHIC DISTRIBUTION OF MINERAL PRODUCTION 63
THE INCREASING RATE OF PRODUCTION 63
CAPITAL VALUE OF WORLD MINERAL RESERVES 64
POLITICAL AND COMMERCIAL CONTROL OF MINERAL RESOURCES 65
RESERVES OF MINERAL RESOURCES 65
CHAPTER V. WATER AS A MINERAL RESOURCE 67
GENERAL GEOLOGIC RELATIONS 67
DISTRIBUTION OF UNDERGROUND WATER 68
MOVEMENT OF UNDERGROUND WATER 71
WELLS AND SPRINGS 72
COMPOSITION OF UNDERGROUND WATERS 73
RELATION OF GEOLOGY TO UNDERGROUND WATER SUPPLY 75
SURFACE WATER SUPPLIES 76
UNDERGROUND AND SURFACE WATERS IN RELATION TO EXCAVATION AND CONSTRUCTION 78
CHAPTER VI. THE COMMON ROCKS AND SOILS AS MINERAL RESOURCES 80
ECONOMIC FEATURES OF THE COMMON ROCKS 80 Granite 82 Basalt and related types 82 Limestone, marl, chalk 82 Marble 83 Sand, sandstone, quartzite (and quartz) 84 "Sand and gravel" 84 Clay, shale, slate 85 The feldspars 86 Hydraulic cement (including Portland, natural, and Puzzolan cements) 86
GEOLOGIC FEATURES OF THE COMMON ROCKS 88 Building stone 88 Crushed stone 90 Stone for metallurgical purposes 91 Clay 91 Limitations of geologic field in commercial investigation of common rocks 92
SOILS AS A MINERAL RESOURCE 94 Origin of soils 94 Composition of soils and plant growth 96 Use of geology in soil study 97
CHAPTER VII. THE FERTILIZER GROUP OF MINERALS 99
GENERAL COMMENTS 99
NITRATES 101 Economic features 101 Geologic features 102
PHOSPHATES 104 Economic features 104 Geologic features 105
PYRITE 107 Economic features 107 Geologic features 108
SULPHUR 109 Economic features 109 Geologic features 110
POTASH 111 Economic features 111 Geologic features 112
CHAPTER VIII. THE ENERGY RESOURCES—COAL, OIL, GAS (AND ASPHALT) 115
COAL 115
ECONOMIC FEATURES 115 World production and trade 115 Production in the United States 117 Coke 118 Classification of coals 119
GEOLOGIC FEATURES 123
PETROLEUM 127
ECONOMIC FEATURES 127 Production and reserves 128 Methods of estimating reserves 134 Classes of oils 136 Conservation of oil 137
GEOLOGIC FEATURES 140 Organic theory of origin 140 Effect of differential pressures and folding on oil genesis and migration 142 Inorganic theory of origin 143 Oil exploration 144
OIL SHALES 150
NATURAL GAS 151 Economic features 151 Geologic features 151
ASPHALT AND BITUMEN 151 Economic features 151 Geologic features 153
CHAPTER IX. MINERALS USED IN THE PRODUCTION OF IRON AND STEEL (THE FERRO-ALLOY GROUP) 154
GENERAL FEATURES 154
IRON ORES 158
ECONOMIC FEATURES 158 Technical and commercial factors determining use of iron ore materials 158 Geographic distribution of iron ore production 160 World reserves and future production of iron ore 162
GEOLOGIC FEATURES 166 Sedimentary iron ores 166 Iron ores associated with igneous rocks 171 Iron ores due to weathering of igneous rocks 171 Iron ores due to weathering of sulphide ores 173
MANGANESE ORES 173 Economic features 173 Geologic features 176
CHROME (OR CHROMITE) ORES 178 Economic features 178 Geologic features 179
NICKEL ORES 180 Economic features 180 Geologic features 181
TUNGSTEN (WOLFRAM) ORES 182 Economic features 182 Geologic features 184
MOLYBDENUM ORES 185 Economic features 185 Geologic features 186
VANADIUM ORES 187 Economic features 187 Geologic features 188
ZIRCONIUM ORES 189 Economic features 189 Geologic features 189
TITANIUM ORES 190 Economic features 190 Geologic features 190
MAGNESITE 191 Economic features 191 Geologic features 192
FLUORSPAR 193 Economic features 193 Geologic features 194
SILICA 195 Economic features 195 Geologic features 196
CHAPTER X. COPPER, LEAD AND ZINC MINERALS 197
COPPER ORES 197 Economic features 197 Geologic features 199 Copper deposits associated with igneous flows 200 Copper veins in igneous rocks 201 "Porphyry coppers" 203 Copper in limestone near igneous contacts 204 Copper deposits in schists 204 Sedimentary copper deposits 205 General comments 206
LEAD ORES 209 Economic features 209 Geologic features 211
ZINC ORES 213 Economic features 213 Geologic features 216
CHAPTER XI. GOLD, SILVER, AND PLATINUM MINERALS 221
GOLD ORES 221 Economic features 221 Geologic features 226
SILVER ORES 231 Economic features 231 Geologic features 234
PLATINUM ORES 237 Economic features 237 Geologic features 239
CHAPTER XII. MISCELLANEOUS METALLIC MINERALS 241
ALUMINUM ORES 241 Economic features 241 Geologic features 243
ANTIMONY ORES 246 Economic features 246 Geologic features 248
ARSENIC ORES 249 Economic features 249 Geologic features 251
BISMUTH ORES 252 Economic features 252 Geologic features 252
CADMIUM ORES 253 Economic features 253 Geologic features 254
COBALT ORES 254 Economic features 254 Geologic features 255
MERCURY (QUICKSILVER) ORES 255 Economic features 255 Geologic features 258
TIN ORES 260 Economic features 260 Geologic features 261
URANIUM AND RADIUM ORES 263 Economic features 263 Geologic features 264
CHAPTER XIII. MISCELLANEOUS NON-METALLIC MINERALS 267
NATURAL ABRASIVES 267 Economic features 267 Geologic features 269
ASBESTOS 270 Economic features 270 Geologic features 271
BARITE (BARYTES) 272 Economic features 272 Geologic features 273
BORAX 274 Economic features 274 Geologic features 275
BROMINE 277 Economic features 277 Geologic features 278
FULLER'S EARTH 278 Economic features 278 Geologic features 279
GRAPHITE (PLUMBAGO) 279 Economic features 279 Geologic features 282
GYPSUM 283 Economic features 283 Geologic features 284
MICA 285 Economic features 285 Geologic features 287
MONAZITE (THORIUM AND CERIUM ORES) 288 Economic features 288 Geologic features 289
PRECIOUS STONES 289 Economic features 289 Geologic features 291
SALT 294 Economic features 294 Geologic features 295
TALC AND SOAPSTONE 299 Economic features 299 Geologic features 299
CHAPTER XIV. EXPLORATION AND DEVELOPMENT 301
THE GENERAL RELATIONS OF THE GEOLOGIST TO EXPLORATION AND DEVELOPMENT 301
PARTLY EXPLORED VERSUS VIRGIN TERRITORIES 303
THE USE OF ALL AVAILABLE INFORMATION 304
COOPERATION IN EXPLORATION 305
ECONOMIC FACTORS IN EXPLORATION 306
GEOLOGIC FACTORS IN EXPLORATION 307
MINERAL PROVINCES AND EPOCHS 308
CLASSIFICATION OF MINERAL LANDS 309
OUTCROPS OF MINERAL DEPOSITS 311 Some illustrative cases 312 Topography and climate as aids in searching for mineral outcrops 314 Size and depth of ore bodies as determined from outcrops 315 The use of placers in tracing mineral outcrops 316 The use of magnetic surveys in tracing mineral ledges 317
THE USE OF ELECTRICAL CONDUCTIVITY AND OTHER QUALITIES OF ROCKS IN EXPLORATION 319
THE USE OF STRUCTURE AND METAMORPHISM IN EXPLORATION 319
DRILLING IN EXPLORATION 320
QUANTITATIVE ASPECTS OF GEOLOGIC EXPLORATION 321
ORIGIN OF MINERAL DEPOSITS AS A FACTOR IN EXPLORATION 322
LAKE SUPERIOR IRON ORE EXPLORATION AS AN ILLUSTRATION 323
DEVELOPMENT AND EXPLOITATION OF MINERAL DEPOSITS 326
CHAPTER XV. VALUATION AND TAXATION OF MINERAL RESOURCES 328
POPULAR CONCEPTION OF MINERAL VALUATION 328
VALUATION AND TAXATION OF MINES 329 Intrinsic and extrinsic factors in valuation 329 Values of mineral deposits not often established by market transfers 331 The ad valorem method of valuation 331 Other methods of mineral valuation and taxation 335
GENERAL COMMENTS ON TAXATION OF MINERAL RESOURCES 338
CHAPTER XVI. LAWS RELATING TO MINERAL RESOURCES 342
I. LAWS RELATING TO OWNERSHIP AND CONTROL OF MINERAL RESOURCES 342 On alienated lands 343 On the public domain 344 Nationalization of mineral resources 345 Effect of ownership laws on exploration 347 Use of geology in relation to ownership laws 349
II. LAWS RELATING TO EXTRACTION OF MINERAL RESOURCES 355
III. LAWS RELATING TO DISTRIBUTION AND TRANSPORTATION OF MINERAL RESOURCES 355
IV. OTHER RELATIONS OF GEOLOGY TO LAW 356
CHAPTER XVII. CONSERVATION OF MINERAL RESOURCES 359
THE PROBLEM 359
DIFFERENCES BETWEEN PRIVATE AND PUBLIC EFFORTS IN CONSERVATION 363
THE INTEREST RATE AS A GUIDE IN CONSERVATION 364
ANTI-CONSERVATIONAL EFFECTS OF WAR 365
CONSERVATION OF COAL 366
MEASURES INTRODUCED OR PROPOSED TO CONSERVE COAL 367 (A) Mining and preparation of coal 368 Progress in above methods 370 (B) Improvement of labor and living conditions at the mines 372 (C) Introduction or modification of laws to regulate or to remove certain restrictions on the coal industry 373 (D) Distribution and transportation of coal 376 (E) Utilization of coal 377 (F) Substitutes for coal as a source of power 378
DIVISION OF RESPONSIBILITY BETWEEN GOVERNMENT AND PRIVATE INTERESTS IN THE CONSERVATION OF COAL 379
CONSERVATION OF MINERALS OTHER THAN COAL 382
CHAPTER XVIII. INTERNATIONAL ASPECTS OF MINERAL RESOURCES 383
WORLD MOVEMENT OF MINERALS 383 Movement of minerals under pre-war conditions of international trade 385 Changes during the war 385 Post-war condition of the mineral trade 387
TENDENCIES TOWARD INTERNATIONAL COOPERATION AND POSSIBILITY OF INTERNATIONAL CONTROL OF MINERALS 389 Methods of international cooperation 391
CONSERVATION IN ITS INTERNATIONAL RELATIONS 393
EXPLORATION IN ITS INTERNATIONAL RELATIONS 395
VALUATION IN ITS INTERNATIONAL RELATIONS 396
RELATIVE POSITION OF THE UNITED STATES IN REGARD TO SUPPLIES OF MINERALS 396
THE COAL AND IRON SITUATION OF WESTERN EUROPE UNDER THE TERMS OF THE PEACE 400
CONCLUSION 403
LITERATURE 403
CHAPTER XIX. GEOLOGY AND WAR 405
GEOLOGY BEHIND THE FRONT 405
GEOLOGY AT THE FRONT 408
EFFECT OF THE WAR ON THE SCIENCE OF ECONOMIC GEOLOGY 412
CHAPTER XX. GEOLOGY AND ENGINEERING CONSTRUCTION 413
FOUNDATIONS 413
SURFACE WATERS 414
TUNNELS 414
SLIDES 415
SUBSIDENCE 417
RAILWAY BUILDING 417
ROAD BUILDING 418
GEOLOGY IN ENGINEERING COURSES 419
CHAPTER XXI. THE TRAINING, OPPORTUNITIES AND ETHICS OF THE ECONOMIC GEOLOGIST 420
PURE VERSUS APPLIED SCIENCE 420
COURSE OF STUDY SUGGESTED 422 Field work 425 Specialization in studies 426 A degree of Economic Geology 427
THE OPPORTUNITIES OF THE ECONOMIC GEOLOGIST 428
ETHICS OF THE ECONOMIC GEOLOGIST 430
ILLUSTRATIONS
FIGURE PAGE
1. Graphic representation of volume change in weathering of a Georgia granite 21
2. Commercial (financial) control of the mineral resources of the world 64
3. Political (territorial) control of the mineral resources of the world 64
4. The fertilizer situation in the United States 100
5. Diagram showing the chemical composition and heat efficiency of the several ranks of coal 122
6. Origin and development of coal 123
7. Chart showing the present tendency of the United States in respect to its unmined reserve of petroleum 134
8. The annual output of the principal oil fields of the United States for the last twenty years 135
9. Curve showing the usual decline in oil field production after the period of maximum output is reached 136
10. Chart showing the relative values of the principal petroleum products manufactured in the United States from 1899 to 1914 138
11. Alteration of Lake Superior iron formation to iron ore by the leaching of silica 168
12. Representing in terms of weight the mineralogical changes in the katamorphism of serpentine rocks to iron ore, eastern Cuba 172
13. Diagram showing gradation from syenite to bauxite in terms of volume 245
CHAPTER I
INTRODUCTION
SURVEY OF FIELD
In adapting ourselves to physical environment it has been necessary to learn something about the earth. Mainly within the last century has this knowledge been organized into the science of geology, and only within the last few decades have the complex and increasing demands of modern civilization required the applications of geology to practical uses, resulting in the development of the science generally known as economic geology. This science is not sharply marked off from the science of geology proper; almost any phase of geology may at some time or some place take on its economic aspect.
The usefulness of economic geology was first recognized in relation to mineral resources,—and particularly in relation to metallic resources, their discovery and development,—but the science has been found to have much wider practical application. The practice of the economic geologist in recent years has taken on many new phases.
The geologist is called upon to study the geologic features of mineral deposits, their occurrence, structure, and origin. The basic information thus acquired is useful in estimating reserves and life of mineral deposits. This leads naturally to considerations of valuation. Because valuation plays such a large part in any tax program, the geologist is being used by tax boards of the federal and state governments.
Both in the formulation of laws relating to mineral resources, and in the litigation growing out of the infraction of these laws, the economic geologist plays a part.
One cannot go very far with the study of mineral resources without consideration of the question of conservation. Geologists are called on not only for broad surveys of the mineral reserves, but for the formulation of general principles of conservation and their application to specific mines and minerals.
The geologist's familiarity with the distribution and nature of mineral resources has given him a part in coping with broad questions of international use of natural resources. War conditions made it necessary to use new sources of supply, new channels of distribution, and new methods of utilization. The economic geologist came into touch with questions of international trade, tariffs, and shipping.
But economic geology is not solely confined to mineral resources. In relation to engineering enterprises of the greatest variety—canals, aqueducts, tunnels, dams, building excavations, foundations, etc.—geology now figures largely, both in war and in peace.
The nature, amount, and distribution of underground water supplies are so involved with geologic considerations that a considerable number of geologists give up their time wholly to this phase of the subject.
It might seem from this list of activities that geology is spreading too far into the fields of engineering and commerce, but there are equally rapid extensions of other fields of knowledge toward geology. The organization of these intermediate fields is required both in the interest of science and in the interest of better adaptation of the race to its environment. The geologist is required to do his part in these new fields, but not to abandon his traditional field.
It is proposed in this volume to discuss the economic aspects of geology without exhaustive discussion of the principles of geology which are involved. Practically the whole range of geologic science has some sort of economic application, and it would be futile to attempt in one volume even a survey of the science of geology as a whole. Our purpose is rather to indicate and illustrate, in some perspective, the general nature of the application of geology to practical affairs.
In professional preparation for the practice of economic geology there is no easy short-cut. Students sometimes think that a smattering of geological principles, combined with a little business and economic information, may be sufficient. Analysis of professional successes should make it clear that economic geologists are most effective and in most demand, not primarily because of business aptitude, though this helps, but because of their proficiency in the science of geology itself. In short, to enter successfully the field of economic geology one should first become a scientist, if only in a limited field.
The traditional conception of the geologist as a musty and stooped individual, with a bag, hammer, and magnifying glass, collecting specimens to deposit in a dusty museum, will doubtless survive as a caricature, but will hardly serve to identify the economic geologist in his present-day work. In writing this book, it is hoped in some measure to convey an impression of the breadth and variety in this field. Few other sciences offer so wide a range of opportunity, from the purely scientific to the practical and commercial, coupled with travel, exploration, and even adventure.
ECONOMIC APPLICATIONS OF THE SEVERAL BRANCHES OF GEOLOGY AND OF OTHER SCIENCES
There is no phase of geology which at some time or place does not have its economic application. Many references to these applications are made in other chapters. It is proposed here to indicate briefly some of the phases of geologic science which are most necessary to the practice of economic geology. The student in his preparation cannot afford to eliminate any of them on the ground that they are merely "scientific" or "academic" or "theoretical."
MINERALOGY AND PETROLOGY
Mineralogy, the study of minerals, and petrology, the study of rocks (aggregations of minerals), are of course elementary requisites in preparation. There must be familiarity with the principal minerals and rocks, and especially with the methods and processes of their identification, with their nature, and with their origin. This involves a study of their crystallography, chemical composition, physical qualities, and optical properties as studied with the microscope. In recent years the microscopical study of polished and etched surfaces of ores has proved a valuable tool.
STRATIGRAPHY AND PALEONTOLOGY
Stratigraphy and paleontology are concerned with the sedimentary and life history of the earth. The determination of the ages of the earth's strata and of the conditions of their deposition is required in the practice of economic geology. For example, a detailed knowledge of the succession of rocks and their ages, as determined by fossils and other stratigraphic evidence, is vital to the interpretation of conditions in an oil or coal field, and to the successful exploration and development of its deposits. The success of certain paleontologists and stratigraphic specialists in oil exploration is an evidence of this situation. Certain iron ores, phosphates, salts, potash, and other minerals, as well as many of the common rocks used for economic purposes, are found in sedimentary deposits, and require for their successful exploration and development the application of stratigraphic and paleontologic knowledge.
Closely related to stratigraphy (as well as to physiography, see pp. 6-10) is the study of sedimentation,—i. e., the study of the physical, chemical, climatic, and topographic conditions of the deposition of sediments. This is coming to play an increasingly large part in geologic work, and is essential to the interpretation of many mineral deposits, particularly those in which stratigraphic and physiographic questions are involved.
Still another aspect of the problem of stratigraphy and sedimentation is covered by the study of paleogeography, or the areal distribution of the faunas and sediments of geologic periods caused by the alternating submergence and emergence of land areas. In the search for the treasures of sedimentary deposits, a knowledge of ancient geographies and of ancient faunas makes it possible to eliminate certain regions from consideration. From a study of the faunas of eastern Kansas and Missouri, and of those along the eastern part of the Rocky Mountains, it has been inferred that a ridge must have extended across eastern Kansas during early Pennsylvanian time,—a conclusion which is of considerable economic importance in relation to oil exploration.
STRUCTURAL GEOLOGY
Structural geology is the study of the physical forms and relations of rocks which result mainly from deformation by earth forces. If rocks remained in their original forms the structural problem would be a comparatively easy one, but usually they do not. Often they are faulted and folded and mashed to such an extent that it is difficult to go behind the superposed structural features to the original conditions in order to work out the geologic history. Not only is structural study necessary for the interpretation of geologic history, but it is often more directly applicable to economic problems,—as when, for instance, ore deposits have been formed in the cracks and joints of rocks, and the ore deposits themselves have been faulted and folded. Water resources are often located in the cracks and other openings of rocks, and are limited in their distribution and flow because of the complex attitude of deformed rocks. Oil and gas deposits often bear a well-defined relation to structural features, the working out of which is almost essential to their discovery.
It is not desirable to stop with the merely descriptive aspects of structural geology, as is so often done; for much light can be thrown on the economic applications of this subject by consideration of the underlying principles of mechanics,—involving the relations of earth stresses to rock structures. The mere field mapping and description of faults and joints is useful, but in some cases it is necessary to go a step further and to ascertain the mechanical conditions of their origin in order to interpret them clearly. If, for illustration, there are successive groups of mineralized veins in a mining camp, the later ones cutting the earlier ones, these might be treated as separate structural units. But if it can be shown that the several sets of veins have formed from a single movement, that there is no sharp genetic separation between the different sets and that they are a part of a single system, this interpretation throws new light on exploration and development, and even on questions of ownership and extralateral rights (Chapter XVI).
PHYSIOGRAPHY
Physiography is a phase of geology which investigates the surface features of the earth. It has to do not only with the description and classification of surface forms, present and past (physical geography or geomorphology), but with the processes and history of their development. The subject is closely related to geography, climatology, sedimentation, and hydrology. As one of the latest phases of geology to be organized and taught, its economic applications have been comparatively recent and are not yet widely recognized. Because of this fact its economic applications may be summarized at somewhat greater length than those of the other branches of geology above mentioned, which are to be more or less taken for granted.
The central feature of physiography is the so-called erosion cycle or topographic cycle. Erosion, acting through the agencies of wind, water, and ice, is constantly at work on the earth's surface; the eroded materials are in large part carried off by streams, ultimately to be deposited in the ocean near the continental margins. The final result is the reduction of the land surface to an approximate plain, called a peneplain, somewhere near sea level. Geological history shows that such peneplains are often elevated again with reference to sea level, by earth forces or by subsidence of the sea, when erosion again begins its work,—first cutting narrow, steep gulches and valleys, and leaving broad intervening uplands, in which condition the erosion surface is described as that of topographic youth; then forming wider and more extensive valleys, leaving only points and ridges of the original peneplains, in which stage the surface is said to represent topographic maturity; then rounding off and reducing the elevations, leaving few or none of the original points on the peneplain, widening the valleys still further and tending to reduce the whole country to a nearly flat surface, resulting in the condition of topographic old age. The final stage is again the peneplain. This cycle of events is called the erosion cycle or topographic cycle. Uplift may begin again before the surface is reduced to base level; in fact, there is a constant oscillation and contest between erosion and relative uplift of the land surface.
The action of the erosion cycle on rocks of differing resistance to erosion and of diverse structure gives rise to the great variety of surface forms. The physiographer sees these forms, not as heterogeneous units, but as parts of a definite system and as stages in an orderly series of events. He is able to see into the topographic conditions beyond the range of immediate and direct observation. He is able to determine what these forms were in the past and to predict their condition in the future. He is able to read from the topography the underground structure which has determined that topography. A given structure may in different stages of topographic development give quite diverse topographic forms. In such a case it is important to realize that the diversity is only superficial. On the other hand, a slight local divergence from the usual topographic forms in a given region may reflect a similar local divergence in the underground structure. Thus it is that an appreciation of the physiographic details may suggest important variations in the underground structure which would otherwise pass undiscovered.
Many mineral deposits owe their origin or enrichment to weathering and other related processes which are preliminary to erosion. These processes vary in intensity, distribution, and depth, with the stage of erosion, or in relation to the phase of the erosion cycle. They vary with the climatic conditions which obtain on the erosion surface. Mineral deposits are therefore often closely related to the topographic features, present and past, in kind, shape, and distribution. A few illustrative cases follow.
Many of the great copper deposits of the western United States owe their values to a secondary enrichment through the agency of waters working down from the surface. When this fact of secondary enrichment was discovered, it was naturally assumed that the process was related to the present erosion surface and to present climatic and hydrologic conditions. Certain inferences were drawn, therefore, as to depth and distribution of the enriched ores. This conception, however, proved to be too narrow; for evidences were found in many cases that the copper deposits had been concentrated in previous erosion cycles, and therefore in relation to erosion surfaces, now partly buried, different from the present surface. The importance of this knowledge from an exploring and development standpoint is clear. It has made it possible to find and follow rich ores, far from the present erosion surface, which would otherwise have been disclosed solely by chance. Studies of this kind in the copper camps are yet so recent that much remains to be learned. The economic geologist advising exploration and development in copper ores who does not in the future take physiographic factors into account is likely to go wrong in essential ways, as he has done in some cases in the past.
Not only is it necessary to relate the secondary enrichment of copper deposits to the erosion surface, present or past, but by a study of the conditions it must be ascertained how closely erosion has followed after the processes of enrichment. In some cases erosion has followed so slowly as to leave large zones of secondary enrichment. In other cases erosion has followed up so closely after the processes of secondary enrichment as to remove from the surface important parts of the secondarily enriched deposits.
The iron ores of the Lake Superior region are the result of the action of waters from the surface on so-called iron formations or jaspers. Here again it was at first supposed that the enrichment was related to the present erosion surface; but upon further studies the fact was disclosed that the concentration of the ores took place in the period between the deposition of Keweenawan and Cambrian rocks, and thus a new light was thrown on the possibilities as to depth and distribution of the ores. The old pre-Cambrian surface, with reference to which the concentration took place, can be followed with some precision beneath the present surface. This makes it possible to forecast a quite different depth and distribution of the ores from that which might be inferred from present surface conditions. Present surface conditions, of low relief, considerable humidity, and with the water table usually not more than 100 feet from the surface, do not promise ore deposits at great depth. The erosion which formed the old pre-Cambrian surface, however, started on a country of great relief and semi-arid climate, conditions which favored deep penetration of the surface waters which concentrated the ores.
The iron ores of eastern Cuba are formed by the weathering of a serpentine rock on an elevated plateau of low relief, where the sluggish streams are unable rapidly to carry off the products of weathering. Where streams have cut into this plateau and where the plateau breaks down with sharp slopes to the ocean, erosion has removed the products of weathering, and therefore the iron ore. An important element, then, in iron ore exploration in this country is the location of regions of slight erosion in the serpentine area. One of the largest discoveries was made purely on a topographic basis. It was inferred merely from a study of topography that a certain large unexplored area ought to carry iron ore. Subsequent work in the thick and almost impenetrable jungle disclosed it.
Bauxite deposits in several parts of the world require somewhat similar conditions of concentration, and a study of the physiographic features is an important factor in their location and interpretation.
A physiographic problem of another sort is the determination of the conditions surrounding the origin of sedimentary ores. Certain mineral deposits, like the "Clinton" iron ores, the copper ores in the "Red Beds" of southwestern United States and in the Mansfield slates of Germany, many salt deposits, and almost the entire group of placer deposits of gold, tin, and other metals, are the result of sedimentation, from waters which derived their materials from the erosion of the land surface. It is sometimes possible from the study of these deposits to discover the position and configuration of the shore line, the depth of water, and the probable continuity and extent of the deposits. Similar questions are met in the study of coal and oil.
This general problem is one of the phases of geology which is now receiving a large amount of attention, not only from the standpoint of ore deposition, but from a broader geologic standpoint. In spite of the fact that sedimentary processes of great variety can be observed in operation today, it is yet extremely difficult to infer from a given sedimentary deposit the precise conditions which determined its deposition and limited its distribution. For instance, sedimentary iron formations furnish a large part of the world's iron ore. The surface distribution, the structure, the features of secondary enrichment, are all pretty well understood; likewise the general conditions of sedimentation are reasonably clear,—but the close interpretation of these conditions, to enable us to predict the extent of one of these deposits, or to explain its presence in one place and absence in another, is in an early and sketchy stage.
An understanding of the principles and methods of physiography is also vital to an intelligent application of geology to water resources, to soils, to dam and reservoir construction, and to a great variety of engineering undertakings, but as these subjects involve the application of many other phases of geology, they are considered in separate chapters. (Chapters V, VI, and XX.)
ROCK ALTERATIONS OR METAMORPHISM
This is one of the newer special phases of geology which for a long time was regarded as the plaything of the petrographer or student of rocks. With the systematic development of the subject, however, it was found that the extremely numerous and complex alterations of rocks and minerals may be definitely grouped, and that they are controlled by broad principles. It became apparent also that these principles apply both to the economic and non-economic minerals and rocks,—in other words, that the segregation of economic minerals is a mere incident in pervasive cycles of the alterations which affect all rocks. Metamorphic geology, therefore, for some geologists becomes a convenient approach to the subject of economic geology. It has the great advantage that it tends to keep all minerals and all processes of ore deposition in proper perspective with relation to rocks and rock processes in general. It is not argued that this is the only approach or that it is the best for all purposes. A brief account of this phase of geology is given in Chapter II.
APPLICATION OF OTHER SCIENCES
Geology is sometimes defined as the application of other sciences to the earth. Considered broadly, there is no phase of science which is not involved in economic geology. In other chapters in this book many references are made to applications of engineering, mathematics, physics, chemistry, metallurgy, biology, and economics.
At different times and places the requirements for earth materials are quite different. In the Stone Age there was little use for metals; in later ages the use of metals broadened. The multiplicity of demands of modern civilization, the increasing knowledge of processes of metallurgy, chemistry and physics, better transportation, better organization of commercial life, and many other factors, tend to bring new earth materials into use,—and, therefore, into the field of economic geology. A comparatively few years ago alumina, one of the most common and abundant substances of the earth's crust, was in no general demand except for very limited use as an ornament. Little attention was paid to it by economic geologists as a commercial product; now, however, aluminum is in great demand, and the raw materials which produce it have become the subjects of intensive study by economic geologists.
In short, economic geology includes the consideration of man in reaction to his physical environment. There are some earth materials and some conditions of the earth environment which do not yet come within the field of economic geology. But so large a proportion of them do, that the "complete economic geologist" should indeed be almost omniscient. When one considers what an insignificantly small portion of this field can be covered by any individual, it is apparent that the title of economic geologist implies no mastery of the entire field. There is yet no crowding.
TREATMENT OF THE SUBJECT IN THIS VOLUME
In scope and manner of treatment this volume follows somewhat the writer's presentation of the subject in university teaching. The purpose is to explain the nature of the economic demands for the science of geology, and to discuss something of the philosophy of the finding and use of raw materials.
Somewhat generalized statistics are used as a means of gaining perspective. No effort has been made for detailed accuracy or for completeness. So far as possible the quantitative features are expressed in general proportions, and where specific figures are given they are meant to indicate only such general proportions. The thought has been not to be so specific that the figures would soon be out of date. All standard statistical sources have been drawn on, but the principal sources have been the results of the various special investigations called out by the war, in which the writer had a part.
On the geologic side many sources have been drawn on outside of the writer's own experience. For the most part, no specific references or acknowledgments are made, on the ground that the book aims to present the general features which are now the more or less common knowledge of economic geologists. To make the references really adequate for exhaustive study would not only burden the text, but would require a specificity of treatment which it has been hoped to avoid.
The illustrative cases chosen for discussion are often taken from the writer's field of experience. This field has been principally the Lake Superior region, but has included also the principal mineral deposits of North America, Cuba, and limited areas in South America and Europe. Thus the Lake Superior iron and copper region might seem to be brought forward more than is warranted by its scientific or economic importance. For this, the writer offers no apology. An author's perspective is largely determined by his background of training and experience, and a frank recognition of this fact may aid in determining the weight to be given to his conclusions. It might even add to scientific efficiency if each writer were to confine his discussion almost solely to matters within his own range of observation and study.
The writer's indebtedness for information derived from the printed page and for personal discussion and advice is of wide range. He would express his warm appreciation of the friendly spirit of cooperation and advice with which this effort has been aided—a spirit which he likes to think is particularly characteristic of the profession of economic geology. In particular he would acknowledge the efficient aid of Mr. Julian D. Conover in preparation and revision of the manuscript.
CHAPTER II
THE COMMON ELEMENTS, MINERALS, AND ROCKS OF THE EARTH AND THEIR ORIGINS
A list of the solid substances of the earth making up the so-called lithosphere (or rock sphere) in order of their abundance, does not at all correspond to a list made in order of commercial importance. Some of the most valuable substances constitute such a small proportion of the total mass of the lithosphere that they hardly figure at all in a table of the common substances.
RELATIVE ABUNDANCE OF THE PRINCIPAL ELEMENTS OF THE LITHOSPHERE
When reduced to the simplest terms of elements the outer ten miles of the lithosphere consists of:[1]
PERCENTAGE OF PRINCIPAL ELEMENTS IN THE LITHOSPHERE
Oxygen 47.33 Silicon 27.74 Aluminum 7.85 Iron 4.50 Calcium 3.47 Magnesium 2.24 Sodium 2.46 Potassium 2.46 ——- 98.05
The remainder of the elements exist in quantities of less than 1 per cent. None of these principal elements occur separately in nature and none of them are mined as elements for economic purposes.
RELATIVE ABUNDANCE OF THE PRINCIPAL MINERALS OF THE LITHOSPHERE
Minerals exceptionally consist of single elements, but ordinarily are combinations of two or more elements; for instance, quartz consists of a chemical combination of silicon and oxygen. The proportions of the common minerals in the outer ten miles of the lithosphere are in round numbers as follows:
PERCENTAGE OF COMMON MINERALS IN LITHOSPHERE
Feldspar 49 Quartz 21 Augite, hornblende, and olivine 15 Mica 8 Magnetite 3 Titanite and ilmenite 1 Kaolin, limonite, hematite, dolomite, calcite, chlorite, etc. 3 —- 100
In making up this table it is assumed that the rocks to a depth of ten miles are about 95 per cent of igneous type, that is, crystallized from molten magma, and about 5 per cent of sedimentary type, that is, formed from the weathering and erosion of igneous rocks or preexisting sediments, and deposited in beds or layers, either by water or by air (see pp. 16-17).
More reliable figures for the relative abundance of the minerals are available for each of the two classes of rocks, igneous and sedimentary. The igneous rocks contain minerals in about the following proportions:
PERCENTAGE OF COMMON MINERALS IN IGNEOUS ROCKS
Feldspar 50 Quartz 21 Augite, hornblende, olivine, etc. 17 Mica 8 Magnetite 3 Titanite and ilmenite 1 —- 100
The sedimentary rocks contain minerals in about the following proportions:
PERCENTAGE OF COMMON MINERALS IN SEDIMENTARY ROCKS
Quartz 35 Feldspar 16 White mica 15 Kaolin (clay) 9 Dolomite 9 Chlorite 5 Calcite 4 Limonite 4 Gypsum, carbon, rutile, apatite, magnetite, etc. 3 —- 100
The sedimentary rocks comprise three main divisions: (1) The muds and clays, with their altered equivalents, shale, slate, etc.; (2) the sands, with their altered equivalents, sandstone, quartzite, quartz-schist, etc.; (3) the marls, limestones, and dolomites, with their altered equivalents, marble, talc-schist, etc. For brevity these groups are referred to respectively as shale, sandstone, and limestone. The proportions of minerals in each of these groups of rocks are as follows:
PERCENTAGE OF COMMON MINERALS IN SHALE, SANDSTONE, AND LIMESTONE
- - - Average Average Average shale sandstone limestone - - - Quartz 31.91 69.76 3.71 Kaolin 10.00 7.98 1.03 White mica 18.40 Chlorite 6.40 1.15 Limonite 4.75 .80 Dolomite 7.90 3.44 36.25[1] Calcite 7.21 56.56 Gypsum 1.17 .12 .10 Feldspar 17.60 8.41 2.20 Magnetite .58 Rutile .66 .12 .06 Ilmenite .25 Apatite .40 .18 .09 Carbon .81 - - - Total 100.00 100.00 100.00 - - - 1: Includes small amount of FeCO_{3}.
In comparing the mineral composition of igneous and sedimentary rocks, it will be noted that the most abundant single mineral of the igneous rocks, and the most abundant mineral of the lithosphere as a whole, is feldspar; that next in order is quartz; and that third comes a group of dark green minerals typified by augite and hornblende, commonly called ferro-magnesian silicates because they consist of iron and magnesia, with other bases, in combination with silica. The sedimentary rocks, which are ultimately derived from the destruction of the igneous rocks, contrast with the igneous rocks mainly in their smaller proportions of feldspars and ferro-magnesian minerals, their higher proportions of quartz and white mica (sericite or muscovite), and their content of kaolin, dolomite, calcite, chlorite, limonite, etc., which are nearly absent from the unaltered igneous rocks. Evidently the development of sediments from igneous rocks has involved the destruction of much of the feldspars and ferro-magnesian silicates, and the building from the elements of these destroyed minerals of more quartz, white mica, clay, dolomite, calcite, chlorite and limonite. The composition of the minerals of the sedimentary rocks is such as to indicate that the constituents of the air and water have been added in important amounts to accomplish this change of mineral character. For instance, carbon dioxide of the atmosphere has been added to lime and magnesia of the igneous rocks to make calcite and dolomite, water has been added to some of the alumina and silica of the igneous rocks to make kaolin or clay, and both oxygen and water have been added to the iron of the igneous rocks to make limonite.
RELATIVE ABUNDANCE OF THE PRINCIPAL ROCKS OF THE LITHOSPHERE
Just as elements combine chemically to form minerals, so do minerals combine mechanically, either loosely or compactly, to form rocks. For instance, quartz is a mineral. An aggregation of quartz particles forms sand or sandstone or quartzite. Most rocks contain more than one kind of mineral.
Sedimentary rocks occupy considerable areas of the earth's surface, but they are relatively superficial. It has been estimated that if spread evenly and continuously over the earth, which they are not, they would constitute a shell scarcely a half mile thick.[2] Igneous rocks are relatively more abundant deep below the surface. If the sediments be assumed to be limited to a volume equivalent to a half-mile shell, and the remainder of the rocks be assumed to be igneous, it is evident that to a depth of ten miles 95 per cent of the rocks are igneous. Our actual observation is confined to a shallow superficial zone in which sediments make up at least half of all the rocks.
Igneous rocks can be divided for convenience into two main types: (1) granite and allied rocks, containing a good deal of silica and therefore acid in a chemical sense, and (2) basalt and allied types, containing less silica and more lime, magnesia, iron, soda and potassa, and therefore basic in a chemical sense. The former are light-colored gray and pink rocks while the latter are dark-colored green and gray rocks. Granite and basalt as technically defined are very common igneous rocks,—so common that the names are sometimes used to classify igneous rocks in general into two great groups, the granitic and the basaltic. It has been estimated that about 65 per cent of the igneous rocks are of the granitic group and 35 per cent of the basaltic group.
Sedimentary rocks, as already indicated, consist principally of three groups, which for convenience are named shale, sandstone, and limestone. If we approximate the average composition of each group and the average composition of the igneous rocks from which they are ultimately derived, it can be calculated that sedimentary rocks must form in the proportions of 82 per cent shale, 12 per cent sandstone, and 6 per cent limestone. Only this combination of the three sediments will yield an average composition comparable with that of the parent igneous rocks. As actually observed in the field the sandstones and limestones are in relatively higher percentage than is here indicated, suggesting that part of the shales may have been deposited in deep seas where they cannot be observed, and that part may have been so changed or metamorphosed that they are no longer recognized as shales.
SOILS AND CLAYS
Weathered and disintegrated rocks at the surface form soils and clays. No estimate is made of abundance, but obviously the total volume of these products is small as compared with the major classes of earth materials above noted, and in large part they may be included with these major classes.
WATER (HYDROSPHERE)
It has been estimated that all the water of the earth, including the ocean, surface waters, and underground waters, constitutes about 7 per cent of the volume of the earth to a depth of 10 miles.[3]
COMPARISON OF LISTS OF MOST ABUNDANT ROCKS AND MINERALS WITH COMMERCIAL ROCKS AND MINERALS
Of the common rocks and minerals figuring as the more abundant materials of the earth's crust, only a few are prominently represented in the tables of mineral resources. Of these water and soils stand first. Others are the common igneous and sedimentary rocks used for building and road materials. Missing from the lists of the most abundant minerals and rocks, are the greater part of the commercially important mineral resources—including such as coal, oil, gas, iron ore, copper, gold, and silver,—implying that these mineral products, notwithstanding their great absolute bulk and commercial importance, occur in relatively insignificant amounts as compared with the common rock minerals of the earth.
THE ORIGIN OF COMMON ROCKS AND MINERALS
The common rocks and minerals develop in a general sequence, starting with igneous processes, and passing through stages of weathering, erosion, sedimentary processes, and alterations beneath the surface. The commercial minerals are incidental developments under the same processes.
IGNEOUS PROCESSES
The earliest known rocks are largely igneous. Sedimentary rocks are formed from the breaking down of igneous rocks, and the origin of rocks therefore starts with the formation of igneous rocks. Igneous rocks are formed by the cooling of molten rock material. The ultimate source of this molten material does not here concern us. It may come from deep within the earth or from comparatively few miles down. It may include preexisting rock of any kind which has been locally fused within the earth. Wherever and however formed, its tendency is to travel upward toward the surface. It may stop far below the surface and cool slowly, forming coarsely crystallized rocks of the granite and gabbro types. Igneous rocks so formed are called plutonic intrusive rocks. Or the molten mass may come well toward the surface and crystallize more rapidly into rocks of less coarse, and often porphyritic, textures. Such intrusive rocks are porphyries, diabases, etc. Or the molten mass may actually overflow at the surface or be thrown out from volcanoes with explosive force. It then cools quickly and forms finely crystalline rocks of the rhyolite and basalt types. These are called effusives or extrusives, or lavas or volcanics, to distinguish them from intrusives formed below the surface. The intrusive masses may take various forms, called stocks, batholiths, laccoliths, sills, sheets and dikes, definitions and illustrations of which are given in any geological textbook. The effusives or volcanics at the surface take the form of sheets, flows, tuffs, agglomerates, etc.
Some of the igneous rocks are themselves "mineral" products, as for instance building stones and road materials. Certain basic intrusive igneous rocks contain titaniferous magnetites or iron ores as original constituents. Others carry diamonds as original constituents. Certain special varieties of igneous rocks, known as pegmatites, carry coarsely crystallized mica and feldspar of commercial value, as well as a considerable variety of precious gems and other commercial minerals. Pegmatites are closely related to igneous after-effects, discussed under the next heading. As a whole, the mineral products formed directly in igneous rocks constitute a much less important class than mineral products formed in other ways, as described below.
Igneous after-effects. The later stages in the formation of igneous rocks are frequently accompanied by the expulsion of hot waters and gases which carry with them mineral substances. These become deposited in openings in adjacent rocks, or replace them, or are deposited in previously hardened portions of the parent igneous mass itself. They form "contact-metamorphic" and certain vein deposits. Pegmatites, referred to above, are in a broad sense in this class of "igneous after-effects," in that they are late developments in igneous intrusions and often grade into veins clearly formed by aqueous or gaseous solutions. Among the valuable minerals of the igneous after-effect class are ores of gold, silver, copper, iron, antimony, mercury, zinc, lead, and others. While mineral products of much value have this origin, most of them have needed enrichment by weathering to give them the value they now have.
WEATHERING OF IGNEOUS ROCKS AND VEINS
No sooner do igneous rocks appear at or near the earth's surface, either by extrusion or as a result of removal by erosion of the overlying cover, than they are attacked vigorously by the gases and waters of the atmosphere and hydrosphere as well as by various organisms,—with maximum effect at the surface, but with notable effects extending as far down as these agents penetrate. The effectiveness of these agents is also governed by the climatic and topographic conditions. Under conditions of extreme cold or extreme aridity, weathering takes the form mainly of mechanical disintegration, and chemical change is less conspicuous. Under ordinary conditions, however, processes of chemical decomposition are very apparent. The result is definitely known. The rocks become softened, loose, and incoherent. Voids and openings appear. The volume tends to increase, if all end products are taken into account. The original minerals, largely feldspar, ferro-magnesian minerals, and quartz, become changed to clay, mixed with quartz or sand, calcite or dolomite, and iron oxide, together with residual particles of the original feldspars and ferro-magnesian minerals which have only partly decomposed. In terms of elements or chemical composition, water, oxygen, and carbon dioxide, all common constituents of the atmosphere and hydrosphere, have been added; and certain substances such as soda, potassa, lime, magnesia, and silica have in part been carried away by circulating waters, to be redeposited elsewhere as sediments, vein fillings, and cements. Figure 1 illustrates the actual mineral and volume changes in the weathering of a granite—one of the most common rocks. The minerals anorthite, albite, and orthoclase named in this figure are all feldspars; sylvite and halite are chlorides of potash and soda. The weathering processes tend to destroy the original minerals, textures, and chemical composition. They are collectively known as katamorphic alterations, meaning destructive changes. The zone in which these changes are at a maximum is called the zone of weathering. This general zone is principally above the surface or level of the ground-waters, but for some rocks it extends well below this level. In some regions the ground-water level may be nearly at the surface, and in others, especially where arid, it may be two thousand or more feet down. Disintegrated weathered rocks form a blanket of variable thickness, which is sometimes spoken of as the residual mantle, or "mantle rock."
Mineral products formed by weathering from common igneous rocks include soils, clay, bauxite, and certain iron, chromite, and nickel ores. Again the commercial importance of this group is not large, as compared with products formed in other ways described below.
The same weathering processes described above for igneous rocks cause considerable changes of economic significance in deposits formed as igneous after-effects. In some cases they result in removing the less valuable minerals, thus concentrating the more valuable ones, as well as in softening the rock and making it easier to work; and in other cases they tend to remove the valuable constituents, which may then be redeposited directly below or may be carried completely out of the vicinity. The oxide zones of many ore bodies are formed by these processes.
SEDIMENTARY PROCESSES
Sedimentary rocks are formed by the removal and deposition of the weathered products of a land surface. Air, water, and ice, moving under the influence of gravity and other forces, all aid in this transfer. The broken or altered rock materials may be merely moved down slopes a little way and redeposited on the surface, forming one type of terrestrial or subaerial deposits, or they may be transferred and sorted by streams. When deposited in streams or near their mouths, they are known as river, alluvial, or delta deposits. When carried to lakes and deposited they form lake deposits. Ultimately the greater part of them are likely to be carried to the ocean and deposited as marine sediments.
Part of the weathered substances are carried mechanically as clay and sand, which go to make up the shale and sandstone sediments. Part are carried in solution, as for example lime carbonate and magnesium carbonate, which go to make up limestone and dolomite. Some of the dissolved substances are never redeposited, but remain in solution as salts in the sea, the most abundant of which is sodium chloride. Some of the dissolved substances of weathering, such as calcite, quartz, and iron oxide, are carried down and deposited in openings of the rocks, where they act as cements.
The sediments as a whole consist of three main types,—shales (kaolin, quartz, etc.), sandstones (quartz, feldspar, etc.), and limestones or dolomites (carbonates of lime and magnesia). Of these, the shale group is by far the most abundant. There are of course many sediments with composition intermediate between these types. There are also sediments made up of large undecomposed fragments of the original rocks, cemented to form conglomerates, or made up of small fragments of the original rocks cemented to form arkoses and graywackes. These, however, may be regarded as simply stages in the alteration, which in repeated cycles of weathering must ultimately result in producing the three main groups,—shales, sandstones, and limestones.
Mineral products formed by sedimentary processes include sandstones, limestones, and shales, used as building stone and road materials; certain sedimentary deposits of iron, like the Clinton ores of the southeastern United States and the Brazilian ores; important phosphate deposits; most deposits of salt, gypsum, potash, nitrates, etc.; comparatively few and unimportant copper deposits; and important placer deposits of gold, tin, and other metals, and precious stones. With the aid of organic agencies, sedimentary processes also account for the primary deposition of coal and oil.
WEATHERING OF SEDIMENTARY ROCKS
After sedimentary rocks are formed, and in many cases covered by later sediments, they may be brought again by earth movements and erosion to the surface, where they in turn are weathered. The weathering of sedimentary rocks proceeds along lines already indicated for the igneous rocks. Residual mantles of impure clay and sand are commonly formed. The mineral composition of sedimentary rocks being different from that of igneous rocks to start with, the resulting products are in slightly different proportions; but the changes are the same in kind and tend merely to carry the general process of alteration farther in the same direction,—that is, toward the production of a few substances like clay, quartz, iron oxide, and calcite, which are transported and redeposited to form clay, sand, and limestone. Cycles of this kind may be repeated indefinitely.
By weathering of sedimentary rocks are produced some soils, certain commercial clays, iron ores, lead and zinc ores, and other valuable mineral products.
CONSOLIDATION, CEMENTATION, AND OTHER SUBSURFACE ALTERATIONS OF ROCKS.
Cementation. No sooner are residual weathered mantles formed or sedimentary rocks deposited, whether under air or water, than processes of consolidation begin. Settling, infiltration of cementing materials, and new growths, or recrystallization, of the original minerals of the rock all play a part in the process. The mud or clay becomes a shale, the sand becomes sandstone or quartzite, the marl becomes limestone or marble. All the minute openings between the grains, as well as larger openings such as fissures and joints, may thus be filled. At the same time the cementing materials may replace some of the original minerals of the rock, the new minerals either preserving or destroying the original textures. This process is sometimes called metasomatic replacement. Igneous rocks as a rule are compact, and hence are not so much subject to the processes of cementation as sedimentary rocks; but certain of the more porous phases of the surface lavas, as well as any joints in igneous rocks, may become cemented. All of these changes may be grouped under the general term cementation.
A special phase of consolidation and cementation is produced near intrusive igneous rocks through the action of the heat and pressure and the expelled substances of the igneous rock. This is called contact metamorphism or thermal metamorphism. The processes are even more effective when acting in connection with the more intense metamorphism described under the next heading.
By cementation some of the common rocks, especially the sediments, become sufficiently compact and strong to be useful as commercial products, such as building stones and road materials.
More important as mineral products are the cementing materials themselves. These are commonly quartz, calcite, or iron oxide, of no especial value, but locally they include commercially valuable minerals containing gold, copper, silver, lead, zinc, and many other mineral products.
It is a matter of simple and direct observation, about which there is no controversy, that many minerals are deposited as cements in the openings in rocks or replacing rocks. As to the source of the solutions bringing in these minerals, on the other hand, there has been much disagreement. In general, the common cementing materials such as quartz and calcite, as well as some of the commercial minerals, are clearly formed as by-products of weathering, and are transported and redeposited by the waters penetrating downward from the surface. The so-called secondary enrichment of many valuable veins is merely one of the special phases of cementation from a superficial source. In other cases it is believed that deep circulation of ordinary ground-waters may pick up dispersed mineral substances through a considerable zone, and redeposit them in concentrated form in veins and other trunk channels. For still other cementing materials, it is suspected that the ultimate source is in igneous intrusions; in fact, deposits of this general character show all gradations from those clearly formed by surface waters, independently of igneous activity, to those of a contact-metamorphic nature and others belonging under the head of "igneous after-effects."
Hypothesis and inference play a considerable part in arriving at any conclusion as to the source of cementing materials,—with the result that there is often wide latitude for difference of opinion and of emphasis on the relative importance of the different sources of ore minerals.
Dynamic and contact metamorphism. Beneath the surface rocks are not only cemented, but may be deformed or mashed by dynamic movements caused by great earth stresses; the rocks may undergo rock flowage. The result is often a remarkable transformation of the character of the rocks, making it difficult to recognize their original nature. Also, igneous intrusions may crowd and mash the adjacent rocks, at the same time changing them by heat and contributions of new materials. This process may be called contact metamorphism, but in so far as it results in mashing of the rocks it is closely allied to dynamic metamorphism. The former term is also applied to less profound changes in connection with igneous intrusions, which result merely in cementation without mashing.
Dynamic and contact metamorphism may in some cases produce rocks identical in appearance with those produced by ordinary processes of cementation and recrystallization without movement. For instance, it is difficult to tell how much movement there has been in the production of a marble, because both kinds of processes seem to produce much the same result. Commonly, however, the effect of dynamic metamorphism is to produce a parallel arrangement of mineral particles and to segregate the mineral particles of like kind into bands, giving a foliated or schistose or gneissic structure, and the rocks then become known as slates, schists, or gneisses. Commonly they possess a capacity to part along parallel surfaces, called cleavage. The development of the schistose or gneissic structure is accompanied by the recrystallization of the rock materials, producing new minerals of a platy or columnar type adapted to this parallel arrangement. Even the composition of the rock may be substantially changed, though this is perhaps not the most common case. Whereas by weathering the rock is loosened up and disintegrated, substances like carbon dioxide, oxygen, and water are abundantly added, and light minerals of simple composition tend to develop,—by dynamic metamorphism on the other hand, carbon dioxide, oxygen, and water are usually expelled, the minerals are combined to make heavier and more complex minerals, pore space is eliminated, and altogether the rock becomes much more dense and crystalline. While segregation of materials is characteristic of the surficial products of weathering, the opposite tendency, of mixing and aggregation, is the rule under dynamic metamorphism, notwithstanding the minor segregation above noted.
Dynamic metamorphism is for the most part unfavorable to the development of mineral products. Ore bodies brought into a zone where these processes are active may be profoundly modified, but not ordinarily enriched. One of the exceptions to this general rule is the development of the cleavage of a slate, which enables it to be readily split and thereby gives it value. Contact metamorphism, on the other hand, may develop valuable mineral deposits (see pp. 20, 45-46).
THE METAMORPHIC CYCLE AS AN AID IN STUDYING MINERAL DEPOSITS
All of the chemical, mineralogical, and textural changes in rocks above described may be collectively referred to as metamorphism. The phase of metamorphism dealing with surficial weathering, similar changes below the surface, and the formation of sediments, is called katamorphism or destructive change. The phase of metamorphism dealing with the constructive changes in rocks, due to cementation, dynamic movements, and igneous influences, is called anamorphism. Some geologists confine the term metamorphism to the changes involved in contact and dynamic metamorphism, and call the resulting products metamorphic rocks.
The zone in which katamorphism is most active, usually near the surface, is called the zone of katamorphism. The deeper zone in which anamorphism is preponderant is called the zone of anamorphism. There are no definite limits of depth to these zones. A given rock may be undergoing katamorphism while rocks on either side at the same depth are suffering anamorphism.
By katamorphism rocks break down to produce the surficial rocks, and by anamorphism the surficial rocks are again consolidated and altered to produce highly crystalline rocks, which are not dissimilar in many of their characteristics to the igneous rocks from which all rocks trace their ultimate origin. In other words, anamorphism tends toward the reproduction of igneous rocks, though it seldom fully accomplishes this result. These two main groups of changes together constitute the metamorphic cycle. Some rocks go through all phases of the cycle, but others may pass directly from one phase to an advanced phase without going through the intermediate stages. For instance, an igneous rock may become a schist without going through the intermediate stage of sedimentation.
Rocks are not permanent in their condition, but at practically all times and places are undergoing some kind of metamorphism which tends to adapt them to their environment. The conception of rocks as representing phases or stages in a progressive series of changes called the metamorphic cycle aids greatly in correlating and holding in mind many details of rock nature and origin, and brings into some sort of perspective the conditions which have produced rocks. A schistose sediment comes to be regarded as an end product of a long series of alterations, beginning with igneous rocks and passing through the stages of weathering, sedimentation, cementation, etc., each of which stages has been responsible for certain mineralogical, chemical, and textural features now characterizing the rock. The alternation of constructive and destructive changes of the metamorphic cycle, and the repetitions of the cycle itself, periodically work over the earth materials into new forms. Usually the cycles are not complete, in the sense that they seldom bring the rock back to exactly the same condition from which it started. More sediments are formed than are changed to schists and gneisses, and more schists and gneisses are formed than are changed back to igneous rocks. Salts in the ocean continuously accumulate. The net result of the metamorphic cycle, is, therefore, the accumulation of materials of the same kinds. Incidental to these accumulations is the segregation of commercial mineral products.
The metamorphic cycle becomes a logical and convenient geologic basis for correlating, interpreting, and classifying mineral products. Because of the great variety of materials and conditions represented in mineral deposits, prodigious efforts are required to remember them as independent entities; but as incidents or stages in the well-known progress of the metamorphic cycle, their essential characteristics may be easily remembered and kept in some perspective.
Ores of certain metals, such as iron, occur in almost every phase of the metamorphic cycle,—as igneous after-effects, as weathered products, as sediments, and as schists. The ores of each of these several phases have group characteristics which serve to distinguish them in important particulars from ores belonging to other phases of the cycle. Having established the position of any particular ore in the metamorphic cycle, a number of safe inferences are possible as to mineralogical composition, shape, extent, and other conditions, knowledge of which is necessary for an estimate of commercial possibilities.
FOOTNOTES:
[1] Clarke, F. W., Data of geochemistry: Bull. 695, U. S. Geol. Survey, 1920, p. 35.
[2] Clarke, F. W., Data of geochemistry: Bull. 695, U. S. Geol. Survey, 1920, p. 33.
[3] Clarke, F. W., Data of geochemistry: Bull. 695, U. S. Geol. Survey, 1920, pp. 22-23.
CHAPTER III
SOME SALIENT FEATURES OF THE GEOLOGY AND CLASSIFICATION OF MINERAL DEPOSITS
VARIOUS METHODS OF CLASSIFICATION
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.
As illustrative of the above remarks reference may be made to a few terms commonly used in economic geology, such as primary, secondary, syngenetic, epigenetic, supergene, hypogene, protore, etc.
The most commonly used of these terms are primary and secondary. It is almost impossible to define them in a way which will cover all the conceptions for which they have been used, and yet in their context they have been very useful in conveying essential ideas. An ore formed by direct processes of sedimentation has sometimes been called primary, whereas an ore formed by later enrichment of these sediments has been called secondary. An ore formed directly by igneous processes has been called primary, while an ore formed by enrichment of such primary ore by later processes has been called secondary. It is clear, however, that these terms are merely relative, with application to a specific sequence, and that they do not fix the absolute position of the ore in a sequence applying to all ores. For instance, ores deposited directly as sediments or placers may be derived from the erosion of preexisting ore bodies,—in which case it may sometimes be convenient to refer to the sedimentary ores or placers as secondary and the earlier ores as primary. Or a sulphide deposit originating through igneous agencies may undergo two or three successive enrichments, each successive one secondary to the preceding, but primary to the one following. In spite of these obvious difficulties, the terms primary and secondary may be entirely intelligible as indicating relative order of development under a given set of conditions.
The term syngenetic has been used for mineral deposits formed by processes similar to those which have formed the enclosing rocks and in general simultaneously with them, and epigenetic for those introduced into preexisting rocks. In certain cases syngenetic may be roughly synonymous with primary, and epigenetic with secondary, and yet a primary ore may be epigenetic. For instance, zinc sulphides in the Mississippi valley limestones (pp. 54-55) are epigenetic, and yet are primary with reference to a later enrichment. The two sets of terms are meant to convey somewhat different ideas and are not interchangeable.
Ransome[4] has suggested, especially for vein and contact deposits, a series of names which has the considerable advantage of definiteness:—hypogene ores, formed in general by ascending non-oxidizing solutions, perhaps hot; supergene ores, formed in general by oxidizing and surface solutions, initially cold and downward moving; and protores, or metallized rock or vein substances which are too low in tenor to be classed as ores, but which would have been converted into ores had the enriching process been carried far enough. In this connection Ransome defines primary ore as unenriched material that can be profitably mined. In view of the general use of the terms primary and secondary as expressing a sequential relation of ore development, it is doubtful whether this more precise definition will supersede the older usage. Also it may be noted that commercial conditions might require, under these definitions, the designation of an ore as a protore at one time or place and as a primary ore at another. Hypogene ores are dominantly primary, and supergene ores are dominantly secondary, but either may include both primary and secondary ores.
The terms of these several classifications overlap, and seek to express different aspects of the same situation. While almost synonymous in certain applications they are not in others.
In this text the writer has certainly not escaped the difficulties in regard to names above referred to, nor in fact has he made any exceptional effort to do so. His chief purpose is to convey, in somewhat elementary terms, an understandable idea of the central features of economic geology. In the main, the most widely accepted terms are used. Almost at every turn it would be possible, in the interests of precision, to introduce qualifying discussions of names,—but at the expense of continuity and perspective in the presentation of the principal subject-matter. The writer does not wish to minimize the necessity for careful and precise nomenclature; but he regards it important that the student focus his attention on the central objective facts of the subject, and that he do not become misled by the sometimes over-strenuous advocacy of certain names or classifications in preference to others. If the facts are understood, he will ordinarily have no difficulty in judging the significance of the variety of names proposed to express these facts. If, on the other hand, the student approaches the subject with a ready-made set of names and definitions learned by rote, he is in danger of perceiving his facts from one angle only and through a distorted perspective. |
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