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."
[Illustration: FIG. 1. Graphic representation of volume change
in weathering of a Georgia granite.]
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.
MINERAL DEPOSITS AS MAGMATIC SEGREGATIONS IN IGNEOUS ROCKS
In this class are included deposits which crystallize within the body of
igneous rock, almost, if not quite, simultaneously with the adjacent
rock. These deposits form one of the main types of _syngenetic_
deposits.
The titaniferous magnetites constitute a widely distributed but at
present commercially unavailable class of iron ores. The magnetite
crystals of these deposits interpenetrate with the other constituents of
an igneous rock, commonly of a gabbro type, and the deposits themselves
are essentially igneous rocks. Their shapes are for the most part
irregular, their boundaries ill-defined, and their concentration
varying. While their magmatic origin is clear, there is little agreement
as to the precise conditions which determined their segregation in the
molten rock. There is often a tendency for the ores to follow certain
primary sheeted structures in the igneous mass, a fact for which the
reason is not obvious.
The Sudbury nickel ores, of Ontario, Canada, the principal source of the
world's nickel, lie mainly within and along the lower margin of a great
intrusive igneous mass of a basic type called _norite_, and locally the
ores project beyond the margin into adjacent rocks. Their textures and
their intercrystallization with the primary minerals of the igneous rock
have suggested that they are essentially a part of the norite mass, and
that they crystallized during some segregative processes which were
effective before the magma had solidified. Near the ores there are
likely to be granitic rocks, which, like the ores, seem to be
segregations from the norite magma. Locally both the ores and the
associated granitic rocks replace the main norite body in such a fashion
as to indicate their slightly later crystallization. However, the
intimate association of the ores with the primary minerals in the
magma, together with their absence from higher parts of the norite and
from the extraneous rocks far from the contact, indicate to other
investigators that they were not brought in from outside in vagrant
solutions which followed the intrusion of the main magma, but that they
were segregated within the magma essentially in place. The occurrence of
these heavy ores near the base of the norite naturally suggests that
they were segregated by sinking to the bottom of the molten magma, but
this conclusion implies certain physical conditions of the magma which
have not yet been proved. Again the precise nature of the process and
the part played in it by aqueous and gaseous solutions are subject to
some doubt and controversy. The settlement of this problem awaits the
solution of the more general problem of the origin and crystallization
of magmas.
In this general class of igneous deposits may be mentioned also
diamonds, platinum, chromite, corundum, and other mineral products,
although for the formation of commercial ores of many of these
substances further concentration by weathering and sedimentation has
been required.
Pegmatites are coarsely crystalline acid dike rocks which often
accompany a large igneous intrusion and which have obviously
crystallized somewhat later than the main igneous mass. They may
constitute either sharply delimited dikes or more irregular bodies which
grade into the surrounding igneous mass. They have a composition roughly
similar to the associated igneous rock, but usually a different
proportion of minerals. They are probably the result of the
differentiation of the parent magma. The pegmatites are of especial
interest to the economic geologist because of the frequency with which
they carry commercial minerals, such as the precious stones, mica,
feldspar, cassiterite (tin ore), and others. They show a complete
gradation from dikes of definitely igneous characteristics to veins
consisting largely of quartz in which evidence of igneous origin is not
so clear. The pegmatites thus afford a connecting link between ores of
direct igneous sources and ores formed as "igneous after-effects," which
are discussed in the next paragraph. Aplites are fine-grained acid
igneous rocks of somewhat the same composition as the pegmatites and
often show the same general relations to ores.
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.
These deposits are closely associated in place and age with igneous
rocks, either intrusive or extrusive, and are usually considered to have
come from approximately the same source; and yet they afford distinct
evidence of having been deposited after the adjacent igneous rocks were
completely crystallized and fractured. They are thus _epigenetic_
deposits. They are not themselves igneous rocks and they do not
constitute pegmatites, but they often grade into pegmatites and belong
to the same general stage in the sequence of events. They include
deposits formed by contact metamorphism. They are sometimes designated
by the general term "igneous after-effects"--a term also applied in some
cases to pegmatites. Some geologists discriminate between "deep vein"
deposits (p. 43) and "contact-metamorphic" deposits, but the two are so
closely related in place and origin that for our purposes they will be
considered together.
The ores of this class are clearly deposited from vagrant solutions
which wander through openings of all kinds in the igneous rock and
outward into the adjacent country rocks. They also replace the wall
rocks; limestone is especially susceptible. This is a phase of contact
metamorphism. Some of the most important metalliferous deposits belong
in this class, including most of the gold, silver, copper, iron, lead
and zinc ores of the western United States and the copper deposits of
Lake Superior.
In general, ores of this class are more abundant about intrusive igneous
rocks, that is about igneous rocks which have stopped and cooled before
reaching the surface,--than in association with extrusive igneous rocks
which have poured over the surface as lava flows--but the latter are by
no means insignificant, including as they do such deposits as the Lake
Superior copper ores, the Kennecott copper ores of Alaska, some of the
gold-silver deposits of Goldfield and other Nevada camps, and many
others.
There is general similarity in the succession of events shown by study
of ore bodies related to intrusives. First, the invasion of the magma,
resulting in contact metamorphism of the adjacent rocks, sometimes with,
and often without conspicuous crowding effects on the invaded rocks;
second, the cooling, crystallization, and cracking of both the igneous
rock and the adjacent rock; third, the introduction of ore-bearing
solutions into these cracks,--sometimes as a single episode, sometimes
as a long continued and complex process forming various types of
minerals at successive stages. This order may in some cases be repeated
in cycles, and overlapping of the successive events is a common feature.
One of the interesting facts is the way in which the igneous mass has
invaded and extensively altered the country rocks in some mineral
districts,--in some cases by crowding and crumpling them, and in others
without greatly disturbing their structural attitudes. The latter is
illustrated in the Bingham district of Utah and the Philipsburg district
of Montana. In such cases there is so little evidence of crowding of the
country rocks as to raise the question how such large masses of
intrusives could be introduced without greater disturbing structural
effect. This leads naturally to consideration of the general problem of
the manner of progress of magmas through adjacent rocks,--a subject
which is still largely in the realm of speculation, but which is not
thereby eliminated from the field of controversy. Facts of this kind
seem to favor the position of certain geologists that magmas may
assimilate the rocks they invade.
EVIDENCE OF IGNEOUS SOURCE
No one ever saw one of these deposits in the process of formation; the
conclusion, therefore, that they originated from hot solutions, either
aqueous or gaseous, or both, which were essentially "after-effects" of
igneous activity and came from the same primary source as the associated
igneous rocks, is an inference based on circumstantial evidence of the
kind below summarized:
(1) The close association both in place and age with igneous rocks. This
applies not only to individual deposits, but to certain groups of
deposits which have common characteristics, and which constitute a
metallogenic province; also to groups of the same geologic age, which
indicate a metallogenic epoch (pp. 308-309). The association with
igneous rocks in one place might be a coincidence but its frequent
repetition can hardly be so explained. A zonal arrangement of minerals
about intrusives is often noted. Geologic evidence often shows the
processes of ore deposition to have been complete before the next
succeeding geologic event,--as for instance in the Tonopah district of
Nevada (p. 236), where the ores have been formed in relation to certain
volcanic flows and have been covered by later flows not carrying ore,
without any considerable erosion interval between the two events.
(2) The general contrast in mineralogical and chemical composition,
texture, and mineral associations, between these ore minerals and the
minerals known to be formed by ordinary surficial agencies under
ordinary temperatures. The latter carry distinctive evidences of their
origin. When, therefore, a mineral group is found which shows
contrasting evidences, it is clear that some other agencies have been at
work; and the natural assumption is that the solutions were hot rather
than cold; that they came from below rather than above.
(3) The contrast between the character and composition of these ores
(and their associated gangue) and the character and composition of the
wall rocks, together with the absence of leaching of the wall rocks,
favor the conclusion that the ore minerals are foreign substances
introduced from extraneous sources. The source not being apparent above
and the processes there observed not being of a kind to produce these
results, it is concluded that the depositing solutions were hot and came
from below.
(4) The fact that many of the ore minerals are never known to develop
under ordinary temperatures at the surface. For some of them,
experimental work has also indicated high temperature as a requisite to
their formation.
Quartz, which is a common associate of the ores and often constitutes
the principal gangue, serves as a geologic thermometer in that it
possesses an inversion point or temperature above which it crystallizes
in a certain form, below which in another. In deposits of this class it
has often been found to crystallize at the higher temperatures.
The quartz sometimes shows bubbles containing liquid, gas, and small
heavy crystals, probably of ferric oxide, as in the Clifton-Morenci
district of Arizona. It is clear that the ore-bearing solutions in
these cavities, before the crystallization of the heavy mineral
inclusions, held dissolved not only much larger quantities of mineral
substances than can be taken up by water at ordinary temperatures, but
also a substance like ferric oxide which is entirely insoluble under
ordinary cool conditions.
(5) The association of the ores with minerals carrying fluorine and
boron, with many silicate minerals, such as garnet, amphiboles,
pyroxenes, mica (sericite) and others, and with other minerals which are
known to be characteristic developments within or near igneous masses
and which are not known to form under weathering agencies at the
surface. Various characteristic groupings of these associated minerals
are noted. In limestone much of the mass may be replaced by garnet and
other silicates in a matrix of quartz. In igneous rock the ore-bearing
solutions may have altered the wall rock to a dense mixture of quartz,
sericite, and chlorite. Where sericite is dominant, the alteration is
called sericitic alteration. Where chlorite is important, it is
sometimes called chloritic or "propylitic" alteration. The chloritic
phases are usually farther from the ore deposit than the sericitic
phases, indicating less intense and probably cooler conditions of
deposition. Locally other minerals are associated with the ores, as, for
instance, in the Goldfield district of Nevada (p. 230), where alunite
replaces the igneous rock. Alunite is a potassium-aluminum sulphate,
which differs from sericite in that sulphur takes the place of silicon.
In the quartzites of the lead-silver mines of the Coeur d'Alene district
of Idaho (p. 212), siderite or iron carbonate is a characteristic gangue
material replacing the wall rock.
Quartz in some cases, as noted above, gives evidence of high temperature
origin and therefore of igneous association. Jasperoid quartz, as well
illustrated in the Tintic district of Utah (p. 235), may show texture
and crystallization suggestive of deposition from colloidal solution,--a
process which can occur under both cold and hot conditions, but which is
believed to be accelerated by heat.
Certain minerals, such as magnetite, ilmenite, spinel, corundum, etc.,
are often found as primary segregations within the mass of igneous rock.
These and other minerals, including minerals of tin and tungsten,
monazite, tourmaline, rutile, and various precious stones, are
characteristically developed in pegmatites, which are known to be
igneous rocks, crystallized in the later stages of igneous intrusion.
When, therefore, such minerals are found in other ore deposits an
igneous source is a plausible inference. For instance, in the copper
veins of Butte, Montana (p. 201), are found cassiterite (tin oxide) and
tungsten minerals. Their presence, therefore, adds another item to the
evidence of a hot-water source from below.
(6) The occasional existence of hot springs in the vicinity of these ore
deposits. Where hot springs are of recent age they may suggest by their
heat, steady flow, and mineral content, that they are originating from
emanations from the still cooling magmas. In the Tonopah camp (p. 236),
cold and hot springs exist side by side, exhibiting such contrasts as to
suggest that some are due to ordinary circulation from the surface and
that others may have a deep source below in the cooling igneous rocks.
This evidence is not conclusive. Hot springs in general fail to show
evidence of ore deposition on any scale approximating that which must
have been involved in the formation of this class of ore bodies. Much
has been made of the slight amounts of metallic minerals found in a few
hot springs, but the mineral content is small and the conclusion by no
means certain that the waters are primary waters from the cooling of
igneous rocks below.
In this connection the mercury deposits of California (p. 259),
contribute a unique line of evidence. In areas of recent lavas, mercuric
sulphide (cinnabar) is actually being deposited from hot springs of
supposed magmatic origin, the waters of which carry sodium carbonate,
sodium sulphide, and hydrogen sulphide,--a chemical combination known
experimentally to dissolve mercury sulphide. The oxidation and
neutralization of these hot-spring solutions near the surface throws out
the mercury sulphide. At the same time the sulphuric acid thus formed
extensively leaches and bleaches the surrounding rocks. Such bleaching
is common about mercury deposits. When it is remembered that the mercury
deposits contain minor amounts of gold and silver and sulphides of other
metals; that they are closely associated with gold and silver deposits;
and further that such gold, silver, and other sulphide deposits often
contain minor amounts of mercury,--it is easy to assume the possibility
that these minerals may likewise have had their origin in hot solutions
from below. The presence of mercury in a deposit then becomes suggestive
of hot-water conditions.
(7) Ores sometimes occur in inverted troughs indicating lodgment by
solutions from below, as, for instance, in the saddle-reef gold ores of
Nova Scotia and Australia, and in certain copper ores of the Jerome camp
of Arizona (p. 204.) This occurrence does not indicate whether the
solutions were hot or cold, magmatic or meteoric, but in connection with
other evidences has sometimes been regarded as significant of a magmatic
source beneath.
Perhaps no one of these lines of evidence is conclusive; but together
they make a strong case for the conclusion that the solutions which
deposited the ores of this class were hot, came from deep sources, and
were probably primary solutions given off by cooling magmas.
The conclusion that some ores are derived from igneous sources, based on
evidence of the kind above outlined, does not mean necessarily that the
ore is derived from the immediately adjacent part of the cooling magma.
In fact the evidence is decisive, in perhaps the majority of cases, that
the source of the mineral solutions was somewhat below; that these
solutions may have originated in the same melting-pot with the magma,
but that they came up independently and a little later,--perhaps along
the same channels, perhaps along others.
POSSIBLE INFLUENCE OF METEORIC WATERS IN DEPOSITION OF ORES OF THIS CLASS
It is hardly safe, with existing knowledge, to apply the above
conclusion to all ore deposits with igneous associations, or in any case
to eliminate entirely another agency,--namely, ground-waters of surface
or meteoric origin, which are now present and may be presumed often to
have been present in the rocks into which the ores were introduced. Such
waters may have been heated and started in vigorous circulation by the
introduction of igneous masses, and thereby may have been enabled to
effectively search out and segregate minutely disseminated ore particles
from wide areas. This has been suggested as a probability for the
Kennecott copper ores of Alaska (p. 200) and for the copper ores of Ely,
Nevada. In the Goldfield camp (p. 230) the ores are closely associated
with alunite in such a manner as to suggest a common origin. It has been
found difficult to explain the presence of the alunite except through
the agency of surface oxidizing waters acting on hydrogen sulphide
coming from below.
In the early days of economic geology there was relatively more emphasis
on the possible effectiveness of ground-waters in concentrating ores of
this type. With the recognition of evidence of a deeper source related
to magmas, the emphasis has swung rapidly to the other extreme. While
the evidence is sound that the magmatic process has been an important
one, it is difficult to see how and to just what extent this process may
have been related to the action of ground-waters,--which were probably
present in a heated condition near the contact. It may never be possible
to discriminate closely between these two agencies. It seems likely that
at some stages the two were so intimately associated that the net result
of deposition cannot be specifically assigned either to one or to the
other.
ZONAL ARRANGEMENT OF MINERALS RELATED TO IGNEOUS ROCKS
Evidence is accumulating in many mining districts that ore deposits of
these igneous associations were deposited with a rough zonal arrangement
about the igneous rock. At Bingham, Tintic, and Butte (pp. 204, 208,
235), copper ores are on the whole closest to the igneous rock, and the
lead, zinc, and silver ores are farther away. Furthermore, the quartz
gangue near the igneous rock is likely to contain minerals
characteristic of hot solutions, while farther away such minerals as
dolomite and calcite appear in the gangue, suggestive of cooler
conditions. In Cornwall (p. 262), tin ores occur close to the
intrusives, and lead-silver ores farther away. The gradations are by no
means uniform; shoots of one class of ore may locally cut abruptly
across or through those of another class.
The existence of zones horizontally or areally arranged about intrusives
suggests also the possibility of a vertical zonal arrangement with
reference to the deep sources of the solutions. Of course when secondary
concentration from the surface, described later, is taken into account,
there may be a marked zonal distribution in a vertical direction, but
this is not primary zoning. A few veins and districts show evidence of
vertical zoning apparently related to primary deposition; for the most
part, however, in any one mine or camp there is yet little evidence of
primary vertical zoning. On the other hand, certain groups of minerals
are characteristic of intense conditions of heat and pressure, as
indicated by the coarse recrystallization and high degree of
metamorphism of the rocks with which they are associated; and other
groups have such associations as to indicate much less intense
conditions of temperature and pressure. Depth is only one factor
determining intensity of conditions, but it affords a convenient way to
indicate them; so mineral deposits associated with igneous rocks are
sometimes classified by economic geologists on the basis of deep,
intermediate, and shallow depths of formation.
There are a considerable number of minerals which are formed in all
three of these zones, although in differing proportions. There are
comparatively few which are uniformly characteristic of a single zone.
On the whole, it is possible to contrast satisfactorily mineral deposits
representing very intense metamorphic conditions, usually associated
with formation at great depth, with those formed at or near the surface;
but there are many deposits with intermediate characteristics which it
is difficult to place satisfactorily.
The accessible deposits of the deep zone are associated with plutonic
igneous rocks which have been deeply eroded, and not with surface lavas.
They are characterized by minerals of gold, tin, iron, titanium, zinc,
and copper, and sometimes of tungsten and molybdenum, in a gangue of
quartz, which contains also minerals such as garnet, corundum,
amphibole, pyroxene, tourmaline, spinel, and mica. The deep-zone
minerals are not unlike the pegmatite minerals in their grouping and
associations.
Deposits formed at shallow depths are related to extrusive rocks and to
intrusives near the surface. Erosion has not been deep. Mercury, silver
and gold (tellurides, native metals, and silver sulphides), antimony,
lead, and zinc minerals are characteristic, together with alunite,
adularia, and barite. Metallic copper also is not infrequent. Very often
the gangue material is more largely calcite than quartz, whereas calcite
is not present in the deep zone.[5]
The trend of evidence in recent years has favored the conclusion that
the principal ores associated with igneous rocks have not developed at
very great depths. Even within our narrow range of observation there is
a difference in favor of the shallower depths, and the greatest depths
we can observe are after all but trivial on the scale of the earth.
A survey of the ore deposits of Utah has suggested the generalization
that ores are more commonly related to intrusive stocks than to the
forms known as laccoliths, and that within and about intrusive stocks
the ores are much more abundant near the top or apex of a stock than
lower down.[6] In parts of the region where erosion has removed all but
the deeper portions of the stocks, ore bodies are less abundant. It will
be of interest to follow the testing of this generalization in other
parts of the world.
The scientist is constantly groping for underlying simple truth. Such
glimpses of order and symmetry in the distribution of ore around igneous
rocks as are afforded by the facts above stated, tempt the imagination
to a conception of a simple type or pattern of ore distribution around
intrusions. For this reason we should not lose sight of the fact that,
in the present state of knowledge, the common and obvious case is one of
irregular and heterogeneous distribution, and that there are many
variations and contradictions even to the simplest generalization that
can be made. The observer is repeatedly struck by the freakish
distribution of ores about igneous masses, as compared with their
regularity of arrangement under sedimentary processes to be discussed
later. It is yet unexplained how an intrusive like the Butte granite can
produce so many different types of ores at different places along its
periphery or within its mass, and yet all apparently under much the same
general conditions and range of time. It is difficult also to discern
the laws under which successive migrations of magma, from what seems to
be a single deep-seated source or melting-pot, may carry widely
contrasting mineral solutions. Far below the surface, beyond our range
of observation, it is clear that there is a wonderful laboratory for the
compounding and refinement of ores, but as to its precise location and
the nature of its processes we can only guess.
Other features of distribution of minerals associated with igneous rocks
are indicated by their grouping in metallogenic provinces and epochs
(see pp. 308-309).
THE RELATION OF CONTACT METAMORPHISM TO ORE BODIES OF THE FOREGOING CLASS.
The deposition of ores of igneous source in the country rock into which
the igneous rocks are intruded is a phase of contact metamorphism.
Ordinarily where this deposition occurs there are further extensive
replacements and alterations of the country rock, resulting in the
development of great masses of quartz, garnet, pyroxene, amphibole, and
other silicates, and in some cases of calcite, dolomite, siderite,
barite, alunite, and other minerals. Looked at broadly, the deposition
of ores at igneous contacts under contact metamorphism is a mere
incident in the much more widespread and extensive alterations of this
kind. Hence it is that the subject of contact metamorphism is of
interest to economic geologists. The minerals here formed which do not
constitute ores throw much light on the nature of the ore-bearing
solutions, the conditions of temperature and pressure, and the processes
which locally and incidentally develop the ore bodies. The subject,
however, is a complex one, the full discussion of which belongs in
treatises on metamorphism.[7] We may note only a few salient features.
For many hundreds of yards the rocks adjacent to the intrusions may be
metamorphosed almost beyond recognition. This is especially true of the
limestone, which may be changed completely to solid masses of quartz and
silicates. The shales and sandstones are ordinarily less vitally
affected. The shales become dense, highly crystalline rocks of a
"hornstone" type, with porphyritic developments of silicate minerals.
The sands and sandstones become highly crystalline quartzites, spotted
with porphyritic developments of silicates. Occasionally even these
rocks may be extensively replaced by other minerals, as in the Coeur
d'Alene district, where quartzites adjacent to the ore veins may be
completely replaced by iron carbonate.
A question of special interest to economic geologists is the source of
the materials for the new minerals in these extensively altered zones.
In some cases the minerals are known to be the result of
recrystallization of materials already in the rock, after the
elimination of certain substances such as carbon dioxide and lime, under
the pressures and temperatures of the contact conditions. In such cases
there has obviously been large reduction in volume to close the voids
created by the elimination of substances. In the majority of cases, the
new substances or minerals are clearly introduced from the igneous
source, replacing the wall rock volume for volume so precisely that such
original textures and structures as bedding are not destroyed. In many
cases the result is clearly due to a combination of recrystallization of
materials already present and introduction of minerals by magmatic
solutions from without. So obvious is the evidence of the introduction
of materials from without, that there has been a tendency in some
quarters to overlook the extensive recrystallization of substances
already present; and the varying emphasis placed on these two processes
by different observers has led to some controversy.
SECONDARY CONCENTRATION IN PLACE OF THE FOREGOING CLASSES OF MINERAL
DEPOSITS THROUGH THE AGENCY OF SURFACE SOLUTIONS
Mineral deposits of direct magmatic segregation are seldom much affected
by surficial alteration, perhaps because of their coarse crystallization
and their intermingling with resistant crystalline rocks. Mineral
deposits of the "igneous after-effect" type may be profoundly altered
through surficial agencies. The more soluble constituents are taken
away, leaving the less soluble. The parts that remain are likely to be
converted into oxides, carbonates, and hydrates, through reaction with
oxygen, carbon dioxide, and water, which are always present at the
surface and at shallow depths. These processes are most effective at the
surface and down to the level of permanent ground-water, though locally
they may extend deeper. This altered upper part of the ore bodies is
usually called the _oxide zone_. It may represent either an enrichment
or a depletion of ore values, depending on whether the ore minerals are
taken into solution less rapidly or more rapidly than the associated
minerals and rocks; all are removed to some extent. In certain deposits,
there is evidence that both zinc and copper have been taken out of the
upper zone in great quantity; but they happen to be associated with
limestone, which has dissolved still more rapidly, with the result that
there is a residual accumulation of copper and zinc values. Manganese,
iron, and quartz are usually more resistant than the other minerals and
tend to remain concentrated above. The same is true to some extent of
gold and silver. The abundance of iron oxide thus left explains the name
"iron cap" or "gossan" so often applied to the upper part of the oxide
zone. Not infrequently, and especially in copper ores, the upper part of
the oxide zone is nearly or entirely barren of values and is called the
_capping_.
The depth or thickness of the oxide zone depends on topography, depth of
water table, climatic conditions, and speed of erosion. A fortunate
combination of conditions may result in a deep oxide zone with important
accumulations of values. In other cases erosion may follow oxidation so
rapidly as to prevent the growth of a thick oxide zone.
It is clear from the study of many ore deposits that the process of
oxidation has not proceeded uniformly to the present, but has depended
upon a fortunate combination of factors which has not been often
repeated during geologic time. As illustrative of this, the principal
oxidation of the Bisbee copper ores of Arizona (p. 204) occurred before
Tertiary time, with reference to a place that has since been covered by
later sediments. The conditions in the Ray, Miami, and Jerome copper
camps of Arizona (pp. 203-205) likewise indicate maximum oxidation at an
early period. The Lake Superior iron ore deposits (pp. 167-170) were
mainly concentrated before Cambrian time, during the base-leveling of a
mountainous country in an arid or semi-arid climate. The oxide zone of
these deposits has no close relation to the present topography or to the
present ground-water level. In the Kennecott (Alaska) copper deposits
all oxidation has been stopped since glacial time by the freezing of the
aqueous solutions. At Butte and at Bingham the main concentration of the
ores is believed to have occurred in an earlier physiographic cycle than
the present one. The _cyclic_ nature of the formation of oxide zones is
of comparatively recent recognition, and much more will doubtless be
found out about it in the comparatively near future. Its practical
bearing on exploration is obvious (see p. 325).
It should be clearly recognized that oxidizing processes are not limited
to the zone above the ground-water level. Locally oxidizing solutions
may penetrate and do effective work to much greater depths, especially
where the rocks traversed at higher elevations are of such composition
or in such a stage of alteration as not to extract most of the oxygen.
Consequently the presence of oxide ores below the water table is not
necessarily proof that the water table has risen since their formation.
On the other hand, the facts of observation do indicate generally a
marked difference, in circulation and chemical effect, between waters
above and below this horizon, and show that oxidation is dominantly
accomplished above rather than below this datum surface.
During the formation of the oxide zone, erosion removes some of the ore
materials entirely from the area, both mechanically and in solution.
Part of the material in solution, however, is known to penetrate
downward and to be redeposited in parts of the ore body below the oxide
zone,--that is, usually below the water table. Evidence of this process
is decisive in regard to several minerals. Copper is known to be taken
into solution as copper sulphate at the surface, and to be redeposited
as chalcocite where these sulphate solutions come in contact with
chalcopyrite or pyrite below. Not only has the process been duplicated
in the laboratory, but the common coating of chalcocite around grains of
pyrite and chalcopyrite below the water level indicates that this
process has been really effective. Sulphides of zinc, lead, silver, and
other metals are similarly concentrated, in varying degrees. The zone of
deposition of secondary sulphides thus formed is called the zone of
_secondary sulphide enrichment_. Ores consisting mainly of secondary
sulphides are also called _supergene_ ores (p. 33). In some deposits, as
in the copper deposits of Ray and Miami, there is found, below the
secondary sulphide zone, a lean sulphide zone which is evidently of
primary nature. The mineralized material of this zone, where too lean to
mine, has been called a _protore_.
With the discovery of undoubted evidence of secondary sulphide
enrichment, there was a natural tendency to magnify its importance as a
cause of values. Continued study of sulphide deposits, while not
disproving its existence and local importance, has in some districts
shown clearly that the process has its limitations as a factor in ore
concentration, and that it is not safe to assume its effectiveness in
all camps or under all conditions. At Butte for instance, secondary
chalcocite is clearly to be recognized. The natural inference was that
as the veins were followed deeper the proportion of chalcocite would
rapidly diminish, and that a leaner primary zone of chalcopyrite,
enargite and other primary minerals would be met. However, the great
abundance of chalcocite in solid masses which have now been proved to a
depth of 3500 feet, far below the probable range of waters from the
surface in any geologic period, seems to indicate that much of the
chalcocite is primary. The present tendency at Butte is to consider as
secondary chalcocite only certain sooty phases to be found in upper
levels. The solid masses of chalcocite in the Kennecott copper mines
seem hardly explainable as the result of secondary sulphide enrichment.
No traces of other primary minerals are present and the chalcocite here
is regarded as probably primary.
The possible magnification of the process of secondary enrichment above
referred to has had for its logical consequence a tendency to
over-emphasize the persistence of primary ores in depth. The very use of
the terms "secondary" and "primary" has suggested antithesis between
surficial and deep ores. Progress in investigation, as indicated on
previous pages, seems to indicate that the primary ores are not
uniformly deep and that in many cases they are distinctly limited to a
given set of formations or conditions comparatively near the surface.
In general the processes of oxidation and secondary sulphide enrichment
have been studied mainly by qualitative methods with the aid of the
microscope and by considerations of possible chemical processes. These
methods have disclosed the nature but not the quantitative range and
relations of the different processes. Much remains to be done in the way
of large scale quantitative analysis of ores at different depths, as a
check to inferences drawn by other methods. One may know, for instance,
that a mineral is soluble and is actually removed from the oxide zone
and redeposited below. The natural inference, therefore, is that the
mineral will be found to be depleted above and enriched below. In many
cases its actual distribution is the reverse,--indicating that this
process has been only one of the factors in the net result, the more
rapid solution and deposition of other materials being another factor.
If one were to approach the study of the concentration of iron ores with
the fixed idea of insolubility of quartz from a chemical standpoint, and
were to draw conclusions accordingly, he would fail to present a true
picture of the situation. While quartz is insoluble as compared with
most minerals, it is nevertheless more soluble than iron oxide, and
therefore the net result of concentration at the surface is to
accumulate the iron rather than the silica. Descriptions of enrichment
processes as published in many reports are often misleading in this
regard. They may be correct in indicating the actual existence of a
process, but may lead the reader to assumptions as to net results which
are incorrect.
RESIDUAL MINERAL DEPOSITS FORMED BY THE WEATHERING OF IGNEOUS ROCKS IN
PLACE
Igneous rocks not containing mineral deposits may on weathering change
to mineral deposits. The lateritic iron ores such as those of Cuba (p.
172), many bauxite deposits, many residual clays, and certain chromite
and nickel deposits are conspicuous representatives of this class. The
chemical and mineralogical changes involved in the formation of these
deposits are pretty well understood. Certain constituents of the
original rock are leached out and carried away, leaving other
constituents, as oxides and hydrates, in sufficiently large percentage
in the mass to be commercially available. The accumulation of large
deposits depends on the existence of climatic and erosional conditions
which determine that the residual deposit shall remain in place rather
than be carried off by erosion as fast as made. In the glaciated parts
of the world, deposits of this nature have usually been removed and
dispersed in the glacial drift.
When the minerals of these deposits are eroded, transported, and
redeposited in concentrated form, they come under the class of placer or
sedimentary deposits described under the following heading. There are of
course many intermediate stages, where the residual deposit is only
locally moved and where the distinction between this class of deposits
and that next described is an arbitrary one.
MINERAL DEPOSITS FORMED DIRECTLY AS PLACERS AND SEDIMENTS
Mineral deposits of this class are of large value, including as they do
salt, gypsum, potash, sulphur, phosphates, nitrates, and important
fractions of the ores of iron, manganese, gold, tin, tungsten, platinum,
and precious stones; also many common rocks of commercial importance.
The minerals of these deposits are derived from the weathering and
erosion of land surfaces, either igneous or sedimentary. They are
deposited both under air and under water, both mechanically and
chemically (in part by the aid of organisms). These deposits form the
principal type of _syngenetic_ deposits (p. 32); the term _sedigenetic_
deposits has also been applied to them.
MECHANICALLY DEPOSITED MINERALS
Mechanical erosion of preexisting mineral deposits or rocks and their
transportation, sorting, and deposition are responsible for the placers
of gold, tin, tungsten, platinum, and various precious stones, and for
certain iron sands and conglomerates. Sands, sandstones, shales, and
certain clays and bauxites also belong in this group. These deposits may
be formed under air or under water, and under various climatic and
topographic conditions. During the process of formation the minerals of
differing density are more or less sorted out and tend to become
segregated in layers. The process is not unlike the artificial process
of mechanical concentration where ores are crushed, shaken up, and
treated with running water. The process is most effective for minerals
which are resistant to abrasion and to solution, and of such density as
to differentiate them from the other minerals of the parent rock.
The origin of deposits of this kind is fairly obvious where they are of
recent age and have not been subsequently altered or buried. A
considerable amount of experimental work has brought out clearly the
main elements of the processes. Physiographic and climatic conditions
play an important part, and cannot be safely overlooked by anyone
studying such deposits.
Extensive copper deposits exist as sediments (pp. 205-206). It is not
clear to what extent they are mechanically or to what extent chemically
deposited. For the most part the concentration of copper in this manner
has not been sufficient to yield deposits of large commercial value; the
mineral is too much dispersed. Relatively small amounts are mined in the
Mansfield shales of Germany and the Nonesuch shales and sandstones of
the Lake Superior country.
The Clinton and similar iron ores of the United States and Newfoundland,
the pre-Cambrian iron ores of Brazil, and the Jurassic iron ores of
England and western Europe (pp. 166-167) are now commonly agreed to be
direct sedimentary deposits in which mechanical agencies of sorting and
deposition played a considerable part. How far chemical and bacterial
agencies have also been effective is not clear. The climatic,
topographic, and other physiographic and sedimentary conditions which
cause the deposition of this great group of ores present one of the
great unsolved problems of economic geology. The study of present-day
conditions of deposition affords little clue as to the peculiar
combination of conditions which was necessary to accomplish such
remarkable results in the past.
On the whole, minerals of this mechanically deposited group are not
greatly affected by later surficial alteration and concentration,
because, having already been subjected to weathering, they are in a
condition to resist such influences.
CHEMICALLY AND ORGANICALLY DEPOSITED MINERALS
The products of surface weathering and erosion are in part carried away
in chemical solution and redeposited as sediments. Sediments thus formed
include limestone and dolomite, siderite, salt, gypsum, potash, sulphur,
phosphates, nitrates, and other minerals. Precipitation may be caused by
chemical reactions, by organic secretion, or by evaporation of the
solutions. The processes are qualitatively understood and it is usually
possible to ascertain with reasonable accuracy the conditions of depth
of water, relation to shore line, climate, nature of erosion, and other
similar factors; yet the vast scale of some of these deposits, and their
erratic areal and stratigraphic distribution, present unsolved problems
as to the precise combinations of factors which have made such results
possible.
Chemically and organically deposited minerals of this class are usually
susceptible to further alteration by surface weathering, and some of
them, for instance the phosphates and siderites, are thus secondarily
concentrated. These processes are discussed under the next heading.
In general the great unsolved problem of the origin of the entire group
of mineral deposits in placers and sediments relates to the scale of the
results. Observation of present-day processes and conditions of
deposition of these minerals affords satisfactory evidence of their
nature, but fails to give us a clear idea of the precise combinations of
agencies and conditions necessary to produce such vast results as are
represented by the mineral deposits. For example, solution of iron on a
land surface and redeposition in bogs and lagoons (as actually observed
to be taking place today) show how some iron-ore sediments may be
formed; but these processes are entirely inadequate to explain the
deposition of iron ores in thick masses over broad areas without
intermingling of other sediments--as represented by the Clinton iron
ores of North America, the Jurassic ores of Europe and England, and the
ancient iron ores of Brazil. The Paleozoic seas in northern and eastern
United States encroached over land areas to the north and east and
deposited ordinary sediments such as sandstone, shale, and limestone.
Suddenly, without, so far as known, tapping any new sources of supply on
the ancient land areas, and without any yet ascertainable change in
topographic or climatic conditions, they deposited enormous masses of
iron ore. There is clearly some cyclic factor in the situation which we
do not yet understand.
The various deposits of salt, gypsum, potash, sulphur, and other
minerals are known to be the result of evaporation, and the deposition
of each of these minerals is known to be related to the degree of
evaporation as well as to temperature, pressure, and factors such as
mass action and crystallization of double salts. The nature of the
processes is fairly well understood; but again, observation of the
present-day operation of these processes fails to give us much clue to
the enormous accumulations at certain times and places in the past. It
is difficult to say just what conditions of climate, in combination with
particular physiographic factors, could have preserved uniformity of
conditions for the long periods necessary to account for some of the
enormously thick salt deposits. Again some cyclic factor in the
situation remains to be worked out.
SEDIMENTARY MINERAL DEPOSITS WHICH HAVE REQUIRED FURTHER CONCENTRATION
TO MAKE THEM COMMERCIALLY AVAILABLE
The conditions for the direct deposition of sedimentary mineral deposits
of the foregoing class are also responsible for the deposition of
minerals in more dispersed or disseminated form, requiring further
concentration through surface agencies to render them commercially
available. Some of these deposits are discussed below.
The lead and zinc ores of the Mississippi Valley, Virginia, Tennessee,
Silesia, Belgium, and Germany (pp. 211-212, 216-219) are in sedimentary
rocks far removed from igneous sources. Lead and zinc were deposited in
more or less dispersed form with the enclosing sediments. It is supposed
that deposition was originally chemical and was favored by the presence
of organic material, which is a rather common accompaniment of the
sediments. It is supposed further that these organic participants were
originally localized during sedimentation in so-called estuarine
channels and shore-line embayments. When subsequently exposed to
weathering, the lead and zinc minerals were dissolved and redeposited in
more concentrated form in fissures and as replacements of limestone.
Agreement as to origin of these deposits, so far as it exists, does not
go beyond these broad generalizations. There is controversy as to
whether the original sources of the ore minerals were the sediments
directly above, from which the mineral solutions have been transferred
downward during weathering and erosion, or whether the original minerals
were below and have been transferred upward by artesian circulation, or
whether they were situated laterally and have been brought to their
present position by movement along the beds, or whether there has been
some combination of these processes. It is the writer's view that the
evidence thus far gathered favors on the whole the conclusion of direct
downward concentration from overlying sources which have been removed by
erosion, although this conclusion fails to explain why certain sulphide
deposits give so little evidence of important downward transfer from
their present position. This matter is further discussed on pages
216-219. The choice of the various alternatives has some practical
bearing on exploration.
Since these ores were brought into approximately their present
position, they have undergone considerable oxidation near the surface
and secondary sulphide enrichment below. The chemical and mineralogical
changes are pretty well understood, but the quantitative range of these
changes and their relative importance in determining the net result are
far from known. Undoubted evidence of secondary sulphide enrichment has
led in some quarters to an assumption of effectiveness in producing
values which is apparently not borne out by quantitative tests.
A group of mineral deposits in sandstones in Utah is regarded as due to
chemical concentration of material originally disseminated in the rock.
They include silver, copper, manganese, uranium, and radium deposits.
The Silver Reef deposits, including silver, copper, uranium, and
vanadium, are commercially the most important of this type.[8] The ore
minerals are commonly associated with carbonized material representing
plant remains, and have replaced the calcareous and cementing material
of the rock, and also some of the quartz grains. The deposits are
regarded as having been formed by circulating waters which collected the
minerals disseminated through the sedimentary rocks, and deposited them
on contact with carbonaceous matter, earlier sulphides, or other
precipitating agents. The circulation in some places is believed to have
been of artesian character and to have been controlled to a large extent
by structural features. The Silver Reef deposits are near the crest of a
prominent anticline. Most of the minerals have been later altered by
surface solutions.