over the globe, that they will supply demands upon them to the remote
future. Reserves become available and valuable only by the expenditure
of effort and money. Ores are the multiplicand and man the multiplier in
the product which represents value or availability. Iron ore can be made
available, when needed, almost to any extent, but at highly varying cost
and degree of effort. The highest grade ores, requiring minimum
expenditure to make them available, are distinctly limited as compared
to total reserves. Any waste in their utilization will lead more quickly
to the use of less available ores at higher cost. One of the significant
consequences of the exhaustion of the highest grade reserves will be an
increased draft upon fuel resources for the smelting of the lower grade
ores. Availability of iron ores is limited, not by total reserves, but
by economic conditions.
GEOLOGIC FEATURES
Iron rarely exists in nature as a separate element. It occurs mainly in
minerals which represent combinations of iron, oxygen, and water, the
substances which make up iron rust. Very broadly, most of the iron ores
might be crudely classified as iron rust. In detail this group is
represented by several mineral varieties, principal among which are
hematite (Fe_{2}O_{3}), magnetite (Fe_{3}O_{4}), and limonite (hydrated
ferric oxide). Iron likewise combines with a considerable variety of
substances other than oxygen; and some of these compounds, as for
instance iron carbonate (siderite), iron silicate (chamosite,
glauconite, etc.), and iron sulphide (pyrite), are locally mined as iron
ores. While an ore of iron may consist dominantly of some one of the
iron minerals, in few cases does it consist exclusively of one mineral.
Most ores are mixtures of iron minerals.
Fully nine-tenths of the iron production of the world comes from the
so-called hematite ores, meaning ores in which hematite is the dominant
mineral, though most of them contain other iron minerals in smaller
quantities. About 5 per cent of the world's iron ores are magnetites,
and the remainder are limonites and iron carbonates.
Iron ores are represented in nearly all phases of the metamorphic cycle,
but the principal commercial values have been produced by processes of
weathering and sedimentation at and near the surface.
=Sedimentary iron ores.= Over 90 per cent of the world's production of
iron ore is from sedimentary rocks. The deposits consist in the main
either of beds of iron ore which were originally deposited as such and
have undergone little subsequent alteration, or of those altered
portions of lean ferruginous beds which since their deposition have been
enriched or concentrated sufficiently to form ores. A minor class of
iron ores in sediments consists of deposits formed by secondary
replacement of limestones by surface waters carrying iron in solution.
1. Deposits of the first class,--originally laid down in much their
present form,--are usually either oolitic, _i. e._, containing great
numbers of flat rounded grains of iron minerals like flaxseeds, or
consist in large part of fossil fragments of sea shells, replaced by
iron minerals. The Clinton ores of the Birmingham district, the Wabana
ores of Newfoundland, the minette ores of the Lorraine district in
central Europe, and the oolitic ores of northern England are all of
these types. Their principal iron mineral is hematite, although the
English ores also contain considerable iron carbonate or siderite. The
cementing or gangue materials are chiefly calcite and quartz, in
variable proportions.
The large reserves of high-grade hematite in the Minas Geraes district
of Brazil are also original sediments, but lack the oolitic texture.
An insignificant proportion of the world's iron is obtained from "bog
ores," which are sedimentary deposits of hydrated iron oxide in swamps
and lakes. These ores have been used only on a small scale and chiefly
in relatively undeveloped countries. They are of particular interest
from a genetic standpoint in that they show the nature of some of the
processes of iron ore deposition as it is actually going on today.
None of the ores of this class, with the exception of the iron
carbonates, have undergone any considerable surface enrichment since
their primary deposition. Neither, with the exception of the Brazilian
ores, have they undergone any deep-seated metamorphism. The shapes,
sizes, and distribution of the deposits may be traced back to the
conditions of original deposition. In England and western Europe the
principal deposits have been only slightly tilted by folding. In the
United States the Clinton ores have partaken in the Appalachian folding.
In Brazil, the ores have undergone close folding and anamorphism.
2. Deposits of the second class, which owe much of their value to
further enrichment since deposition, are represented by the hematite
ores of the Lake Superior district. These may be thought of as the
locally rusted and leached portions of extensive "iron formations," in
which oxidation of the iron, and the leaching of silica and other
substances by circulating waters, have left the less soluble iron
minerals concentrated as ores. The Lake Superior iron formations now
consist near the surface mainly of interbanded quartz (or chert) and
hematite, called _jasper_ or _ferruginous chert_ or _taconite_. These
are similar in composition to the leaner iron ores of Brazil, called
_itabirite_, but differ in that the silica is in the form of chemically
deposited chert, rather than fragmental quartz grains.
[Illustration: FIG. 11. Alteration of Lake Superior iron
formation to iron ore by the leaching of silica.]
When originally deposited the iron was partly hematite (perhaps some
magnetite) and largely in the form of iron carbonate (siderite) and
iron silicate (greenalite), interbanded with chert. The original
condition is indicated by the facts that deep below the surface, in
zones protected from weathering solutions, siderite and greenalite are
abundant, and that they show complete gradation to hematite in
approaching the surface. The ore has been concentrated in the iron
formation almost solely by the process of leaching of silica by surface
or meteoric waters, leaving the hematite in a porous mass. Figure 11
illustrates this change as calculated from analyses and measurements of
pore space. During this process a very minor amount of iron has been
transported and redeposited. In short, the Lake Superior iron ores are
residual deposits formed by exactly the same weathering processes as
cause the accumulation of clays, bauxites, and the oxide zones of
sulphide deposits. The development of an iron ore rather than of other
materials as an end-product is due merely to the peculiar composition of
the parent rock. The solution of silica on such an immense scale as is
indicated by these deposits has sometimes been questioned on the general
ground that silica minerals are insoluble. However, there is plenty of
evidence that such minerals _are_ soluble in nature; and the assumption
of insolubility, so often made in geologic discussions, is based on the
fact that most other minerals are _more_ soluble than silica minerals,
and that in the end-products of weathering silica minerals therefore
usually remain as important constituents. Iron oxide, on the other hand,
is _less_ soluble even than silica,--with the result that when the two
occur together, the evidence of leaching of silica from the mixture
becomes conspicuous.
The fact that these deposits are almost exclusively residual deposits
formed by the leaching of silica has an important bearing on
exploration. If they have been formed by the transportation and
deposition of iron from the surrounding rocks, there is no reason why
they should not occasionally be found in veins and dikes outside of the
iron formation. As a matter of fact they do not transgress a foot beyond
the limits of the iron formation. Failure to recognize the true nature
of the concentration of these ores has sometimes led to their erroneous
classification as ores derived from the leaching and redeposition of
iron from the surrounding rocks.
The distribution and shapes of ore deposits of this class are far more
irregular and capricious than those of the primary sediments, as would
be expected from the fact that their concentration has taken place
through the agency of percolating waters from the surface, which worked
along devious channels determined by a vast variety of structural and
lithological conditions. The working out of the structural conditions
for the different mines and districts constitutes one of the principal
geologic problems in exploration. These conditions have been fully
discussed in the United States Geological Survey reports, and are so
various that no attempt will be made to summarize them here.
One of the interesting features of the concentration of Lake Superior
iron ores is the fact that it took place long ago in the Keweenawan
period, preceding the deposition of the flat-lying Cambrian formations,
at a time when the topography was mountainous and the climate was arid
or semi-arid. These conditions made it possible for the oxidizing and
leaching solutions to penetrate very deeply, how deeply is not yet
known, but certainly to a depth below the present surface of 2,500 feet.
At present the water level is ordinarily within 100 feet of the surface,
and oxidizing solutions are not going much below this depth. This
region, therefore, furnishes a good illustration of the intermittent and
cyclic character of ore concentration which is now coming to be
recognized in many ore deposits.
Subsequent changes far beneath the surface have folded, faulted, and
metamorphosed some of the Lake Superior iron ores but have not enriched
them. The same processes have recrystallized and locked together the
minerals of some of the lean iron formations, making them hard and
resistant, so that subsequent exposure and weathering have had little
effect in enriching them to form commercial ores.
The weathering of limestones containing minor percentages of iron
minerals originally deposited with the limestones may result in the
residual concentration of bodies of limonite or "brown ores" associated
with clays near the surface. This process is similar in all essential
respects to the concentration of the Lake Superior ores. Such limonitic
ores are found rather widely distributed through the Appalachian region
and in many other parts of the world. Because of the ease with which
they can be mined and smelted on a small scale they have been used since
early times, but have furnished only a very small fraction of the
world's iron.
3. In a third class of sedimentary ores, the iron minerals are supposed
to have been introduced as replacements of limestones subsequent to
sedimentation. Such ores are not always easy to discriminate from ores
resulting primarily from sedimentation. This class is represented by the
high-grade deposits of Bilbao, Spain, Austrian deposits, and by smaller
deposits in other countries. The Bilbao ores consist mainly of siderite,
which near the surface has altered to large bodies of oxide minerals.
They occur in limestones and shales and are not associated with igneous
rocks. The deposits are believed to have been formed by ordinary surface
waters carrying iron in solution, and depositing it in the form of iron
carbonate as replacements of the limestones. The original source of the
iron is believed to have been small quantities of iron minerals
disseminated through the ordinary country rocks of the district. The
action of surface waters, in thus concentrating the iron in certain
localities which are favorable for precipitation, is similar to the
formation of the lead and zinc ores of the Mississippi valley, referred
to in the next chapter. Deposits formed in this manner may be roughly
tabular and resemble bedded deposits, or they may be of very irregular
shapes.
The sedimentary iron ores in general evidently represent an advanced
stage of katamorphism, and illustrate the tendency of this phase of the
metamorphic cycle toward simplification and segregation of certain
materials. The exact conditions of original sedimentation present one
of the great unsolved problems of geology, referred to in Chapter III.
=Iron ores associated with igneous rocks.= About five per cent of the
world's production of iron ore is from bodies of magnetite formed in
association with igneous rocks. These are dense, highly crystalline
ores, in which the iron minerals are tightly locked up with silicates,
quartz, and other minerals, suggestive of high temperature origin. The
largest of these deposits is at Kiruna in northern Sweden; in fact this
is the largest single deposit of high-grade ore of any kind yet known in
the world. Here the magnetite forms a great tabular vertical body lying
between porphyry and syenite. In the Adirondack Mountains of New York
and in the highlands of New Jersey, magnetites are interbedded and
infolded with gneisses, granites, and metamorphic limestones. In the
western United States there are many magnetite deposits, not yet mined,
at contacts between igneous intrusives and sedimentary rocks,
particularly limestones (so-called "contact-metamorphic" deposits). The
ores of the Cornwall district of Pennsylvania and some of the Chilean,
Chinese, and Japanese ores are of the same type.
Magnetites containing titanium, which prevents their use at the present
time, are known in many parts of the world as segregations in basic
igneous rocks. They are actually parts of the igneous rock itself (p.
34). Among the large deposits of this nature are certain titaniferous
ores of the Adirondacks, of Wyoming, and of the Scandinavian peninsula.
In all of these cases, it is clear that the origin of the ores is in
some way related to igneous processes, and presumably most of the ores
are deposited from the primary hot solutions accompanying and following
the intrusion of the igneous rocks; but thus far it has been difficult
to find definite and positive evidence as to the precise processes
involved. None of these deposits have undergone any important secondary
enrichment at the surface. Their sizes, shapes, and distribution are
governed by conditions of igneous intrusion, more or less modified, as
in the Adirondacks, by later deformation.
=Iron ores due to weathering of igneous rocks.= A small part of the
world's iron ores, less than 1 per cent of the total production, are the
result of surface alteration of serpentine rocks. These ores are mined
principally in Cuba (Fig. 12). Here they have been developed on a
plateau-like area on which erosion is sluggish. The process of formation
has been one of oxidation of the iron minerals and leaching of most of
the other constituents, leaving the iron concentrated near the surface
in blanket-like deposits. The minerals of the original rock contained
alumina, which, like the iron, is insoluble under weathering conditions,
and hence the Cuban iron ores are high in alumina. They also contain
small quantities of nickel and chromium which have been concentrated
with the iron. A large part of the iron minerals, especially where close
to the surface, have been gathered into small shot-like nodules called
_pisolites_. It is thought that the solution and redeposition of the
iron by organic acids from plant roots may be at least a contributing
cause in the formation of this pisolitic texture.
[Illustration: FIG. 12. Representing in terms of weight the
mineralogical changes in the katamorphism of serpentine rock to iron
ore, on the assumption that alumina has remained constant, eastern
Cuba.]
The Cuban iron ores are similar in their origin to _laterites_, which
are surface accumulations of clay, bauxite, and iron oxide minerals,
resulting from the weathering of iron-bearing, commonly igneous, rocks.
The typical laterites carry more clay and bauxite than the Cuban iron
ores, but this is due merely to the fact that the original rocks
commonly carry more materials which weather to clay. In fact the Cuban
iron ores are themselves, broadly speaking, laterites.
=Iron ores due to weathering of sulphide ores.= A relatively minute
portion of the world's iron ore comes from the "gossans" or "iron caps"
over deposits of iron sulphides. The gossans are formed by oxidation and
leaching of other minerals from the deposits, leaving limonite or
hematite in concentrated masses (see pp. 46-47).
MANGANESE ORES
ECONOMIC FEATURES
Manganese ores are used mainly in the manufacture of steel, the alloys
spiegeleisen and ferromanganese being added to the molten steel after
treatment in the Bessemer converter and open-hearth furnace in order to
recarburize and purify the metal. The alloy ferromanganese is also used
in the production of special manganese steels. Manganese ore is used in
relatively small amounts in dry batteries, in the manufacture of
manganese chemicals, in glass making, and in pigments. Steel uses 95 per
cent of the total manganese consumed, batteries and chemicals 5 per
cent. On an average each ton of steel in the United States requires 14
pounds of metallic manganese, equivalent to 40 pounds of manganese ore.
With manganese ores, as with iron ores, the percentage of minor
constituents,--phosphorus, silica, sulphur, etc.,--determines to a large
extent the manner of use. Low-grade manganese ores, ranging from 10 to
35 per cent in manganese, 20 to 35 per cent in iron, and containing less
than 20 per cent of silica, are used mainly in the production of the
low-grade iron-manganese alloy called _spiegeleisen_ or _spiegel_ (16 to
32 per cent manganese). The higher-grade ores, ranging from 35 to 55 per
cent in manganese, are used mainly in the production of the high-grade
alloy called _ferromanganese_ or _ferro_, in which the manganese
constitutes 65 to 80 per cent of the total. To a very limited extent
manganese is smelted directly with iron ores, thus lessening the amount
to be introduced in the form of alloys; this, however, is regarded as
wasteful use of manganese, since its effectiveness as so used is not
very great. Steel makers usually prefer to introduce manganese in the
form of ferromanganese rather than as spiegel. On the other hand, the
ores of the United States as a whole are better adapted to the
manufacture of spiegel. With the shutting off of foreign high-grade
supplies during the war, resulting in the increased use of local ores,
it became necessary to use larger amounts of the spiegel which could be
made from these ores. Metallurgists stated that it was theoretically
possible to substitute spiegel for the higher grade alloy up to 70 per
cent of the total manganese requirement, but in actual practice this
substitution did not get much beyond 18 per cent.
The principal manganese ore-producing countries in normal times are
Russia, India, and Brazil. Relatively little ore is used in these
countries, most of it being sent to the consuming countries of Europe
and to the United States. The Indian ore has been used largely by
British steel plants, but much of it also has gone to the United States,
Belgium, France, and Germany. The Russian ore has been used by all five
of these countries, Germany having a considerable degree of commercial
control and receiving the largest part; a small quantity is also used in
Russia. Brazilian ore has gone mainly to the United States, and in part
to France, Germany, and England.
Smaller amounts of manganese ore have been produced in Germany,
Austria-Hungary, Spain, and Japan. This production has had little effect
on the world situation. That produced in Austria-Hungary and Germany is
used in the domestic industry. That from Spain and Japan is in large
part exported.
The highest grade of manganese ore comes from the Russian mines,
especially those in the Caucasus region. Most of the ore used for the
manufacture of dry batteries and in the chemical industry, where
high-grade ores are required, has come from Russia. By far the larger
part of the Russian production, however, has gone into steel
manufacture. Indian and Brazilian ores have likewise been used mainly in
the steel industry. Some Japanese ore also is of high grade and is used
for chemical and battery purposes.
Nature has not endowed the United States very abundantly with manganese
ores, and such as are known are widely scattered, of relatively small
tonnage, and of a wide range of grade. The principal producing districts
are the Philipsburg district of Montana and the Cuyuna Range of
Minnesota; there are also scattering supplies in Virginia, Arizona,
California, and many other states. The use of domestic ores has
sometimes been unsatisfactory, because of frequent failure of domestic
producers to deliver amounts and grades contracted for. It has been, on
the whole, cheaper, easier, and more satisfactory for the large
consumers to purchase the imported ore, which is delivered in any
desired amount and in uniform grades, rather than to try to assemble
usable mixtures from various parts of the country.
Before the European War, the United States produced only 1 to 2 per cent
of its needed supply of manganese, the rest being imported mainly from
India, Russia, and Brazil, in the form of ore, and from England in the
form of ferromanganese (about half of the total requirement). The
partial closing of the first two and the fourth of these sources of
supply under war conditions made it necessary to turn for ore to Brazil
and also to Cuba, where American interests developed a considerable
industry in medium-grade ores. At the same time steps were taken to
develop domestic resources; and with the high prices imposed by war
conditions, the domestic production, both of high- and low-grade ore,
was increased largely, but still was able to supply only 35 per cent of
the total requirements of manganese.
At the close of the war sufficient progress had been made--in the
discovery of many new deposits in the United States, in the use of
low-grade domestic ores, which before had not been able to compete with
imported ores, and in the increased use of spiegel, allowing wider use
of low-grade ores,--to demonstrate that, if absolutely necessary, and at
high cost, the United States in another year or two could have been
nearly self-sufficing in regard to its manganese requirements. The
release of shipping from war demands resulted immediately in larger
offerings of foreign manganese ore and of ferromanganese from England,
at prices which would not allow of competition from much of the domestic
or Cuban ore production or from the domestic manufacture of alloys. The
result was a rather dramatic closing down of the manganese industry,
with much financial loss, the passage of a bill for reimbursement of
producers, and a demand on the part of the producers, though not of
consumers, for a protective tariff. In the questions thus raised it is
desirable that geologists and engineers professionally connected with
the industry thoroughly understand the basic facts; for they are liable
to be called upon for advice, not only on questions relating to domestic
supplies affected by possible future foreign policies, but on the
formulation of the policies themselves. Conservation, cheaper steel, and
future trade relations of the United States all require consideration,
before action is taken to protect this one of several similarly situated
mineral industries, in the effort to make the country self-supporting.
These questions are further dealt with in Chapters XVII and XVIII.
Manganese production was also developed during the war in the Gold Coast
of West Africa, in Costa Rica, in Panama, in Java, and elsewhere; but
with the possible exception of Java and Chile, none of these sources are
likely to be factors in the world situation. The war-developed manganese
production of Italy, France, Sweden, and United Kingdom is also unlikely
to continue on any important scale.
GEOLOGIC FEATURES
Like iron ores, manganese ores consist principally of the oxides of
manganese (pyrolusite, psilomelane, manganite, wad, and others), and
rarely the carbonate of manganese (rhodochrosite). They are similar in
their geologic occurrence to many of the iron ores and are often mixed
with iron ores as manganiferous iron ores and ferruginous manganese
ores.
The higher grade manganese ores are of two general types. Those of the
Caucasus district in Russia are sedimentary beds, oolitic in texture,
which were originally deposited as rather pure manganese oxides, and
which have undergone little secondary concentration. They are mined in
many places in much the same manner as coal. Those of India and Brazil
are chiefly surface concentrations of the manganese oxides, formed by
the weathering of underlying rocks which contain manganese carbonates
and silicates. The origin of the primary manganese minerals in the
Indian and in some of the Brazilian deposits is obscure. In others of
the Brazilian ores, the manganese was deposited in sedimentary layers
interbedded with siliceous "iron formations," and the whole series has
subsequently been altered and recrystallized.
The manganese ores of Philipsburg, Montana, the principal large
high-grade deposits mined in the United States, were derived by surface
weathering from manganese carbonates which form replacements in
limestone near the contact with a great batholith of granodiorite. The
primary manganese minerals probably owe their origin to hot magmatic
solutions, as suggested by the close association of the ores with the
igneous rock, the presence of minerals containing chlorine, fluorine,
and boron, and the development in the limestone of dense silicates and
mineral associations characteristic of hot-water alteration. The
manganese ores are mined principally in the oxidized zone. Rich silver
ores are found below the water table, but mainly in veins independent of
the manganese deposits.
At Butte, Montana, a little high-grade manganese material has been
obtained from the unoxidized pink manganese carbonate, which is a common
mineral in some of the veins. It is associated with quartz and metallic
sulphides and is similar in origin to the copper ores of the same
district (pp. 201-202).
The lower-grade and the more ferruginous manganese ores are of a
somewhat similar origin to the principal high-grade ores, in that they
represent surface concentrations of the oxides from smaller percentages
of the carbonates and silicates in the rocks below. Deposits of this
nature have been derived from a wide variety of parent rocks--from
contact zones around igneous intrusions, from fissure veins of various
origins, from calcareous and clayey sediments, and from slates and
schists. The manganese and manganiferous iron ores of the Cuyuna
district of Minnesota, the largest source of low-grade ores in this
country, were formed by the action of weathering processes on
sedimentary beds of manganese and iron carbonates constituting "iron
formations." The process is the same as the concentration of Lake
Superior iron ores described elsewhere.
Manganese, like iron, is less soluble than most of the rock
constituents, and tends to remain in the outcrop under weathering
conditions. To some extent also it is dissolved and reprecipitated, and
is thus gathered into concretions and irregular nodular deposits in the
residual clays. In some cases it is closely associated with iron
minerals; in others, due to its slightly greater solubility, it has been
separated from the iron and segregated into relatively pure masses. With
manganese, as with iron, katamorphic processes are responsible for the
concentration of most of the ores. The ores are in general surface
products, and rarely extend to depths of over a hundred feet.
CHROME (OR CHROMITE) ORES
ECONOMIC FEATURES
The principal use of chrome ores is in the making of the alloy
ferrochrome (60 to 70 per cent chromium), used for the manufacture of
chrome, chrome-nickel, and other steels. These steels have great
toughness and hardness, and are used for armor-plate, projectiles,
high-speed cutting tools, automobile frames, safe-deposit vaults, and
other purposes. Chrome ore is used also both in the crude form and in
the form of bricks for refractory linings in furnaces, chiefly
open-hearth steel furnaces; and as the raw material for bichromates and
other chemicals, which are used in paints and in tanning of leather. In
the United States in normal times about 35 per cent of the total
chromite consumed is used in the manufacture of ferrochrome, and about
35 per cent for bichromate manufacture, leaving 30 per cent for
refractory and other purposes.
In the higher commercial grades of chrome ore the percentage of chromic
oxide is 45 to 55 per cent, but under war conditions ore as low as 30
per cent in Cr_{2}O_{3} was mined. Recovery of chrome from slags
resulting from the smelting of chromiferous iron ores was one of the
war-time developments.
The principal chromite-producing countries in normal times are New
Caledonia, and Rhodesia (controlled by French and British interests),
and to a somewhat lesser extent Russia and Turkey (Asia Minor). Small
amounts of chromite are mined in Greece, India, Japan, and other
countries. The Indian deposits in particular are large and high-grade
but have been handicapped by inadequate transportation. The production
of chrome ore in New Caledonia, Rhodesia, Russia, and Turkey has usually
amounted to more than 90 per cent of the total world's production. The
ore from New Caledonia has been used by France, Germany, England, and to
some extent by the United States. Rhodesian ore has been used by the
United States and the principal European consumers. Latterly more
Rhodesian ore has gone to Europe and more Caledonian ore to the United
States. The Russian ore has been in part used in Russia and in part
exported, probably going mainly to France and Germany. The Turkish ore
has been exported to the United States, England, and Germany; it
probably supplied most of Germany's chromite requirements during the
war.
During the war the United States was temporarily an important producer,
as were also Canada, Brazil, Cuba, and to a minor degree Guatemala.
The richest chrome ore mined at present comes from Guatemala, but the
mines are relatively inaccessible. The New Caledonian, Rhodesian,
Russian, Turkish, and Indian ores are also of high grade. The ores mined
in the United States, Canada, Brazil, Cuba, Greece, and Japan are of
lower grade.
The use of domestic chromite supplies in the United States presents much
the same problem as does manganese. The ore bodies are small, scattered,
and of a generally law grade. War-time experience showed that they could
be made to meet a large part of the United States requirements, but at
high cost and at the risk of early exhaustion of reserves. California
and Oregon are the principal sources, and incidental amounts have been
produced in Washington, Wyoming, and some of the Atlantic states. With
the resumption of competition from foreign high-grade ores at the close
of the war, the domestic mining industry was practically wiped out; the
consequences being financial distress, partial direct relief from
Congress, and consideration of the possibilities of a protective
tariff,--which in this case would have to be a large one to accomplish
the desired results (see Chapters XVII and XVIII).
GEOLOGIC FEATURES
The principal chrome mineral is chromite, an oxide of chromium and iron.
Chromite is a common minor constituent of basic igneous rocks of the
peridotite and pyroxenite type. In these rocks it occurs both as
disseminated grains, and as stringers, and large irregular masses which
probably represent magmatic segregations. Alteration, and weathering of
the parent rock, forming first serpentine and then residual clays, make
the chromite bodies progressively richer and more available, by leaching
out the soluble constituents of the rock leaving the chromite as
residual concentrates. All the important chromite deposits of the world
are associated in somewhat this manner with serpentine or related
rocks. They are formed in the same way as the lateritic iron ores of
Cuba, and from the same sort of rocks (pp. 171-173). Chromite is very
insoluble, and the mechanical breaking down of deposits and
transportation by streams frequently forms placers of chrome sands and
gravels. Such placers have not been worked to any extent.
Katamorphic processes give the important values to chromite deposits.
NICKEL ORES
ECONOMIC FEATURES
The principal use of nickel is in the manufacture of nickel steel, the
most important of all alloy steels. Ordinary nickel steels carry about
3-1/2 per cent nickel. Nickel is used in all gun and armor-plate steels,
and in practically all other good steels except tool steels. It is also
extensively alloyed with other metals, particularly with copper to form
the strong non-corrosive metal (monel metal) used for ship propellers
and like purposes. Nickel is also used for electroplating, for nickel
coins, for chemicals, etc. Of the total production about 60 per cent is
used in steels, 20 per cent in non-ferrous alloys and 20 per cent in
miscellaneous uses. The ores mined range from 2 to 6 per cent in
metallic nickel.
Canada (Sudbury, Ontario) produces over three-fourths of the world's
nickel and is likely to have an even greater share of the future
production. The French supply from New Caledonia is second in
importance, and minor amounts are produced in Norway and in several
other countries. The control and movement of the Canadian and New
Caledonian supplies are the salient features of the world nickel
situation. Nickel leaves the producing countries mostly as matte.
Canadian matte has been refined mainly in the United States, but the
tendency is toward refining a larger proportion in Canada. In Europe
there are refineries in France, England, Belgium, Germany, and Norway,
which normally treat the bulk of the New Caledonian and some of the
Canadian production. Small quantities of New Caledonian matte or ore are
also refined in Japan, and during the war considerable amounts came to
the United States.
The United States now produces perhaps 10 per cent of its normal
requirements of nickel from domestic sources, principally as a
by-product of copper refining. However, the United States has a large
financial interest in the Canadian deposits, and refines most of the
matte produced from Sudbury ores in a New Jersey refinery. Shipments to
Europe of Canadian nickel refined in the United States have been a
feature of the world's trade in the past.
The nickel-bearing iron ores of Cuba, consumed in the United States,
constitute a potential nickel supply of some importance, if processes of
preparation become commercially perfected.
Known supplies of nickel in Canada and New Caledonia are ample for a
considerable future, and geologic conditions promise additional
discoveries at least in the former field. The probable reserves of the
Sudbury district have been estimated to be fully 100,000,000 tons, which
would supply the world's normal pre-war requirements for about a hundred
years.
In recent years the British and Canadian governments have taken an
active interest in the nickel industry. They organized a joint
commission for its investigation, the report[31] of which furnishes the
most comprehensive view of the world nickel situation yet available. The
British government has directly invested in shares of the
British-American Nickel Company, and has negotiated European contracts
for sale of nickel for this company. The Canadian government has exerted
some pressure toward larger refining of nickel matte in Canada.
GEOLOGIC FEATURES
The principal ore minerals are the nickel sulphides and arsenides
(particularly pentlandite, but also millerite, niccolite, and others),
which are found at Sudbury intergrown with the iron and copper
sulphides, pyrrhotite and chalcopyrite; and the hydrated
nickel-magnesium silicates (garnierite and genthite), which are products
of weathering. The richer ores of Canada contain about 5 or 6 per cent
of nickel, the New Caledonian ores less than 2 per cent. The Sudbury
ores carry also an average of about 1.5 per cent of copper.
Nickel, while present in the average igneous rock in greater amounts
than copper, lead, or zinc, is apparently not so readily concentrated in
nature as the other metals and is rarely found in workable deposits. The
few ore bodies known have been formed as the result of unusual
segregation of the nickel in highly magnesian igneous rock of the norite
or gabbro type, at the time of its solidification or soon after; and in
some cases, in order to produce the nickel ore, still further
concentration by the agency of weathering has been necessary. Thus there
are two main types of deposits.
The first, the sulphide type, is represented by the great ore bodies of
the Sudbury district. These are situated in the basal portions of a
great norite intrusive, and are ascribed to segregation of the sulphides
as the rock solidified. To some extent the segregation was aided by
mineralizing solutions following the crystallization of the magma, but
in general there is little evidence that the ores were deposited from
vagrant solutions of this kind (see pp. 34-35). These ores owe their
value to primary concentration; secondary transportation and
reprecipitation by surface waters has not been important. A small amount
of the green arsenate, annabergite or "nickel bloom," has been developed
by oxidation at the surface.
The second, the garnierite or "lateritic" type of nickel ores, is
somewhat more common and is represented by the deposits of New
Caledonia. In this locality the original rock is a peridotite,
relatively low in nickel, which has been altered to serpentine.
Weathering has concentrated the more resistant nickel at the expense of
the more soluble minerals, and has produced extensive blanket deposits
of clay, which in their lower portions contain nickel in profitable
amounts. Similar processes, working on material of a somewhat different
original composition, have produced the nickel-bearing and
chrome-bearing iron ores of Cuba (pp. 171-173).
TUNGSTEN (WOLFRAM) ORES
ECONOMIC FEATURES
The principal use of tungsten is in the making of high speed tool
steels. It is added either as the powdered metal or in the form of
ferrotungsten, an alloy containing 70 to 90 per cent of tungsten.
Tungsten is also used for filaments in incandescent lamps, and in
contacts for internal combustion engines, being a substitute for
platinum in the latter use. Of late years tungsten alloys have also been
used in valves of airplane and automobile engines.
The average grade of tungsten ores mined in the United States is less
than 3 per cent of the metal; before smelting they are concentrated to
an average grade of 60 per cent tungsten oxide.
Germany through its smelting interests controlled the foreign tungsten
situation prior to the war; two-thirds of its excess output of
ferrotungsten was consumed by England and the balance principally by the
United States and France. Other consumers in the main satisfied their
requirements by imports of tool steel from these four countries.
The bulk of the tungsten ore consumed in Europe prior to 1914 came from
British possessions; these were principally the Federated Malay States,
Burma, Australia, and New Zealand. The United States, Portugal, Bolivia,
Japan, Siam, Argentina, and Peru were also producers. The great demand
for tungsten created by the war added China to the list of important
producers and greatly increased the production from Burma and Bolivia.
Smelting works were established in England and those of the United
States and France were greatly enlarged. England is at present in a
position to dominate the world tungsten situation. The question of
control of the ores obtainable in China, Korea, Siam, Portugal, and
western South America is likely to be an important one for the future.
Of the annual pre-war world production, the United States used about
one-fifth. Three-fourths of this requirement was met by domestic
production. The balance was obtained by importation, chiefly from
Germany, from Portugal and Spain, and from England, both of concentrates
and of ferrotungsten.
To the considerable demand for high speed tool steels occasioned by
munitions manufacture, production in the United States responded
quickly. Supplies of tungsten came chiefly from California, Colorado,
Arizona, Nevada, and South Dakota. At the same time importation largely
increased, chiefly from the west coast of South America and the Orient.
Consumption reached a half of the world's total. Considerable amounts of
ferrotungsten were exported to the Allies.
The end of the war created a possible tungsten shortage in this country
into a tungsten surplus. In so far as actual domestic consumption is
concerned there has been a return to something like pre-war conditions,
as the only known new use to which tungsten may be put--the manufacture
of die steel--does not involve the use of any large amount of
ferrotungsten. The richer mines of the two chief tungsten-producing
districts in the United States have shown impoverishment and at present
no important new deposits are known. The grade of the producing deposits
is on an average low. The domestic production of tungsten ore will
doubtless decrease, owing to the importation of cheaper foreign ores,
unless a high tariff wall is erected. Importation from the Orient and
the west coast of South America should continue in reduced amounts,
depending upon the ability of domestic manufacturers to obtain and hold
foreign markets for ferrotungsten and high speed tool steel. In the
commercial control of tungsten ores the United States has at present a
strong position, second only to that of England.
GEOLOGIC FEATURES
Tungsten ores contain tungsten principally in the form of the minerals
scheelite (calcium tungstate), ferberite (iron tungstate), huebnerite
(manganese tungstate), and wolframite (iron-manganese tungstate). All
these minerals are relatively insoluble and have high specific gravity,
and as a consequence they are frequently accumulated in placers, along
with cassiterite and other stable, heavy minerals. A large part of the
world's tungsten production in the past has been won from such deposits.
Placers are still important producers in China, Siam, and Bolivia,
although in these countries vein deposits are also worked.
With the exhaustion of the more easily worked placer deposits,
increasing amounts of tungsten are being obtained from the primary or
fixed deposits. These are found almost exclusively in association with
granitic rocks, and have a variety of forms. The most productive
deposits are in the form of veins, cutting the granites and the
surrounding rocks into which the granites were intruded, and containing
quartz, metallic sulphides, and in some cases minerals of tin, gold, and
silver. The deposits of the two most important districts in the United
States, in Boulder County, Colorado, and at Atolia, California, are of
this general nature. The close association of such deposits with
plutonic igneous rocks, and the characteristic mineral associations (see
pp. 37-41) suggest strongly that the deposits were formed by hot
solutions deriving their material from a magmatic source.
Other tungsten deposits, which only recently became of importance, are
of the contact-metamorphic type--in limestones which have been invaded
by hot aqueous and gaseous solutions near the borders of granitic
intrusions. In these occurrences the tungsten mineral is almost
invariably scheelite, and is associated with calcite, garnet, pyroxene,
and other silicates. A magmatic origin of the tungsten is probable. Some
of the deposits of the Great Basin area and of Japan are of this nature,
and it is believed that important deposits of this type may be
discovered in many other countries.
Tungsten is likewise found in original segregations in igneous rocks and
in pegmatite dikes, but these deposits are of comparatively small
commercial importance.
In some tungsten deposits a hydrated oxide called tungstite has been
formed as a canary-yellow coating at the surface. On the whole, however,
tungsten minerals are very resistant to weathering, and in all their
deposits secondary concentration by chemical action at the surface has
not played any appreciable part. The disappearance of tungsten minerals
from alluvial materials which are undergoing laterization, which has
been described in Burma,[32] seems to indicate that the tungsten is
dissolved in surface waters to some extent; but in the main it is
probably carried completely out of the vicinity and not reprecipitated
below.
MOLYBDENUM ORES
ECONOMIC FEATURES
The main use of molybdenum is in the manufacture of high-speed tool
steels, in which it has been used as a partial or complete substitute
for tungsten. Its steel-hardening qualities are more effective than
those of tungsten, but it is more difficult to control metallurgically.
It has been used in piston rods and crank shafts for American
airplanes. Its use in tool steel is mainly confined to Europe, where its
metallurgical application is in a more advanced stage than in the United
States. Molybdenum is added to steel either as powdered molybdenum or in
the form of ferromolybdenum, an alloy containing 60 to 70 per cent of
the metal. Molybdenum chemicals are essential reagents in iron and steel
analysis and other analytical work; they are also used as pigments.
Molybdenum metal has been used to a small extent in incandescent lamps
and as a substitute for platinum in electric contacts and resistances.
Molybdenum ores range from considerably less than 1 per cent to about 5
per cent in molybdenum.
The world's principal sources of molybdenum ores in approximate order of
importance are the United States, Canada, Norway, Australia, Korea,
Austria, Peru, and Mexico.
About half of the world's supply is produced in the United States.
Production of molybdenum in this country practically began in 1914. Most
of the production has come from Colorado and Arizona. It is believed
that the United States contains reserves more than sufficient to meet
any possible future demand. Thus far the demand has not kept up with
capacity for production. The principal consuming countries are England,
France, and Germany.
GEOLOGIC FEATURES
The chief ore minerals are molybdenite (molybdenum sulphide) and
wulfenite (lead molybdate). The larger part of the world's production is
from the molybdenite ores. Molybdenite occurs principally in association
with granitic rocks,--in pegmatite dikes, in veins, and in
contact-metamorphic deposits,--in all of which associations its origin
is traced to hot solutions from the magma. It is frequently present as
an accessory mineral in sulphide deposits containing ores of gold,
copper, silver, lead, and zinc. At Empire, Colorado, one of the
principal producing localities, it is found in veins, associated with
pyrite, and filling the interstices between brecciated fragments of a
wall rock composed of alaskite (an acid igneous rock). In molybdenite
deposits secondary concentration has not been important.
Wulfenite is rather common in the upper oxidized zone of deposits which
contain lead minerals and molybdenite. It is probably always secondary.
Deposits of wulfenite have been worked on a small scale in Arizona.
VANADIUM ORES
ECONOMIC FEATURES
Vanadium is used mainly in steel, to which it gives great toughness and
torsional strength. Vanadium steels are used in locomotive tires,
frames, and springs, in those parts of automobiles that must withstand
special bending strains, in transmission shafts, and in general in
forgings which must stand heavy wear and tear. Vanadium is also used in
high-speed tool steels, its use materially reducing the amount of
tungsten necessary. It is added in the form of ferrovanadium, carrying
35 to 40 per cent of vanadium. Another use of vanadium is in
chrome-vanadium steels for armor-plate and automobiles. Minor amounts
are used in making bronzes, in medicine, and in dyeing.
The low-grade ores of the United States range from 1 to 8 per cent of
vanadium oxide, the general mean being nearer the lower figure. The
high-grade ores of Peru contain from about 10 to as high as 50 per cent
of the oxide; the roasted ore as shipped averages about 35 to 40 per
cent.
Two-thirds of the world's supply of vanadium comes from Peru, where the
mines are under American control. The concentrates are all shipped to
the United States and some of the ferrovanadium is exported from this
country to Europe. The Germans during the war supplied their needs for
vanadium from the minette iron ores in the Briey district in France, and
presumably the French will in the future utilize this source. An
unrecorded but small quantity is obtained by the English from
lead-vanadate mines in South Africa. There are some fairly large
deposits of vanadium minerals in Asiatic Russia, which may ultimately
become an important source.
The United States supplies less than one-half of its normal needs of
vanadium, from southwestern Colorado and southeastern Utah. The grade of
these deposits is low and the quantity in sight does not seem to promise
a long future. Through its commercial control of the Peruvian deposits,
the United States dominates the world's vanadium situation.
GEOLOGIC FEATURES
The Minasragra vanadium deposit of Peru contains patronite (vanadium
sulphide) associated with a peculiar nickel-bearing sulphide and a black
carbonaceous mineral called "quisqueite," in a lens-shaped body of
unknown depth, enclosed by red shales and porphyry dikes. The origin is
unknown. The patronite has altered at the surface to red and brown
hydrated vanadium oxides.
The deposits of Colorado and Utah are large lens-shaped bodies
containing roscoelite (a vanadium-bearing mica) in fissures and
brecciated zones and replacing the cementing materials of flat-lying
sandstones. Locally the sandstones contain as much as 20 per cent of the
roscoelite. The deposits contain small amounts of fossil wood which may
have been an agent in the precipitation of the vanadium. There is
considerable doubt as to their origin, but it is generally supposed that
they represent concentrations by surface waters of minute quantities of
material originally scattered through the surrounding sediments; it has
also been suggested that certain igneous dikes in this region may have
had some connection with the mineralization. Deposits of carnotite, a
potassium-uranium vanadate, which have been worked for their content of
uranium and radium and from which vanadium has been obtained as a
by-product, are found as impregnations of the sandstone in these same
localities (p. 265).
There are other deposits containing small amounts of vanadium which are
not at present available as ores. Vanadinite, a lead-vanadate, and
descloizite, a vanadate of copper or lead, are found in the oxide zones
of a number of lead and copper deposits in the southwestern United
States and Mexico. Titaniferous iron ores, extensive deposits of which
are known in many places, usually contain a small percentage of
vanadium.
Outside of the Peruvian deposit, the affiliations of which are doubtful,
the vanadium deposits of economic importance owe their positions and
values mainly to the action of surface processes, rather than to igneous
activity.
ZIRCONIUM ORES
ECONOMIC FEATURES
The oxides of zirconium have high refractory properties which make them
useful for refractory bricks and shapes for furnace linings, for
chemical ware, and for other heat, acid, and alkali resisting articles.
For these purposes they find a limited market. Experimental work seems
to show possibilities of a very considerable use of zirconium as a steel
alloy; indeed, results are so suggestive that during the war the
government conducted an active campaign of investigation with a view to
using it in ordnance and armor steel. For such purposes the alloy
ferrozirconium is used, which carries 25 to 35 per cent zirconium metal.
The principal known deposits of zirconium ores, in order of commercial
importance, are in Brazil, in India, and in the United States (Pablo
Beach, Florida). The Brazilian and Indian deposits are also the
principal sources of monazite (pp. 288-289). The United States controls
one of the important Brazilian deposits. Germany before the war
controlled the Indian deposits, and is reported to have taken much
interest in the development of zirconium steels. During the war German
influence in India was effectively broken up. The use of zirconium has
been in an experimental state, and known sources of supply have been
ample for all requirements.
GEOLOGIC FEATURES
The zirconium silicate, zircon, is a fairly common accessory constituent
of granitic rocks and pegmatite veins. From these rocks it is separated
by weathering, disintegration, and stream transportation, and, having a
high specific gravity, it becomes concentrated in placers. The deposits
of southern India, of the coast of Brazil, and of Pablo Beach, Florida,
all contain zircon along with ilmenite, garnet, rutile, monazite, and
other insoluble, heavy minerals, in the sands of the ocean beaches.
Smaller deposits of zircon-bearing sands exist in rivers and beaches in
other parts of the United States and in other countries, but none of
these deposits has thus far proved to be of commercial importance.
The largest and most important zirconium deposits are on a mountainous
plateau in eastern Brazil and are of a unique type, entirely different
from those just described. They contain the natural zirconium oxide,
baddeleyite or brazilite, mixed with the silicate, the ore as produced
carrying about 80 per cent zirconia (ZrO_{2}). The ores consist both of
alluvial pebbles and of extensive deposits in place. The latter are
associated with phonolite (igneous) rocks, and seem to owe their origin
to the agency of hot mineralizing solutions from the igneous rocks.
TITANIUM ORES
ECONOMIC FEATURES
Titanium is sometimes used in steel manufacture to take out occluded
gases and thus to increase the strength and wearing qualities. Its
effect is to cure certain evils in the hardening of the molten steel,
and it is not ordinarily added in amounts sufficient to form a definite
steel alloy. Aluminum is frequently used in place of titanium. Titanium
is added in the form of ferrotitanium, containing either about 15 per
cent titanium and 6 to 8 per cent carbon, or about 25 per cent titanium
and no carbon. Titanium compounds are also used in pigments, as
electrodes for arc-lights, and by the army and navy for making
smoke-clouds.
The United States has domestic supplies of titanium sufficient for all
requirements. Production has come chiefly from Virginia. Additional
quantities have been imported from Canada and Norway. The recently
developed deposits of Pablo Beach, Florida, may produce important
amounts of titanium minerals along with the output of zircon and
monazite.
GEOLOGIC FEATURES
The principal titanium minerals are rutile (titanium oxide) and ilmenite
(iron titanate). These minerals are formed mainly under high
temperatures, either during the original solidification of igneous
rocks, or as constituents of the pegmatites which follow the
crystallization of the main igneous masses. The Virginia production
comes from pegmatite dikes cutting through gabbros, syenites, and
gneisses. The deposits contain rutile in amounts as high as 30 per cent
of the mass, but averaging 4 or 5 per cent, in addition to varying
amounts of ilmenite. Titaniferous magnetites, formed in many basic
igneous rocks by the segregation of certain iron-bearing materials into
irregular masses, contain large quantities of ilmenite which are not
commercially available under present metallurgical processes.
Rutile and ilmenite both have high specific gravity and are little
affected by weathering. Consequently they are not decomposed at the
surface, but when carried away and subjected to the sorting action of
streams and waves, they form placer deposits. Both of these minerals are
recovered from the sands at Pablo Beach, Florida.
MAGNESITE
ECONOMIC FEATURES
The most important use of magnesite is as a refractory material for
lining furnaces and converters. It is also used in the manufacture of
Sorel cement for stucco and flooring, in making paper, in fire-resisting
paint, in heat insulation, and as a source for carbon dioxide. Small
amounts are used in Epsom salts and other chemicals.
As taken from the ground the ore consists principally of the mineral
magnesite or magnesium carbonate, with minor impurities (1 to 12 per
cent) of lime, iron, silica, and alumina. In making magnesite bricks, it
is calcined or "dead-burned" to drive out the carbon dioxide.
Austria-Hungary and Greece are the large European producers of magnesite
and Scotland supplies a little. Most of the European production is
consumed in England and the Central European countries, but part has
been sent to America. Outside the United States there are American
supplies in Canada, and recent developments in Venezuela and Mexico
(Lower California).
Magnesite is produced in considerable quantities in the United States,
in California and Washington. Some material is imported from Canada, and
a small amount comes from Scotland as return cargo for ballast purposes.
Before the war only about 5 per cent of the United States requirements
of magnesite were met by domestic production. The country was
practically dependent on imports from various European countries;
chiefly from Austria-Hungary and Greece The Austrian magnesite
(controlled in large part by American capital) was considered especially
desirable for lining open-hearth steel furnaces, because of the presence
of a small percentage of iron which made the material slightly more
fusible than the pure mineral. When the shipments from this source were
discontinued during the war and prices rose to a high figure,
experiments were made with American magnesite, and the deposits on the
Pacific Coast were developed on a large scale. A process of treatment
was perfected by which the Washington magnesite was made as desirable
for lining furnaces as the Austrian material. At the same time large
amounts were imported from Canada and Venezuela and lesser amounts from
Lower California.
Under the high prices which prevailed during the war, dolomite was to
some extent substituted for magnesite. Dolomite, which may be thought of
as a magnesite rock high in lime, occurs in large quantities close to
many points of consumption. It is cheaper but less satisfactory than
magnesite, and is not likely to be used on any large scale.
While the United States has undoubtedly sufficient reserves of magnesite
to supply the domestic demands for many years, the mines are far from
the centers of consumption and it is expensive to transport the
material. Since the war, magnesite shipped from Canada and overseas has
again replaced the American product in the eastern market to some
extent. The Canadian magnesite is of lower grade than the domestic and
European magnesite and is consequently less desirable. Deposits in
Venezuela are also expected to furnish some material for the eastern
furnaces, in competition with those of Austria and Greece. Austrian
magnesite, however, will be likely to dominate the market in the future
if delivered at anything like pre-war prices. This situation has led to
agitation for a protective tariff on magnesite.
GEOLOGIC FEATURES
Magnesite, as noted above, is the name of a mineral, the composition of
which is magnesium carbonate. The principal magnesite deposits are of
two types, of different modes of origin and of somewhat different
physical characteristics.
The large magnesite deposits of Austria and of Washington, as well as
those of Quebec, occur as lenses in beds of dolomite (calcium-magnesium
carbonate). They are in fairly close proximity to igneous rocks, and
magnesia-bearing solutions issuing from these rocks are believed to have
dissolved out the calcium carbonate of the dolomite and replaced it with
magnesium carbonate. In these deposits the material is coarsely
crystalline and forms fairly large, continuous bodies, which are worked
by quarrying. The Washington deposits closely resemble marble, and had
sometimes been mistaken for that rock until war-time needs resulted in
their more thorough investigation.
The commoner type of magnesite deposits is represented by those of
Greece, California, Venezuela, and many other countries. These consist
of veins and replacements in serpentine. The original rock was a highly
magnesian igneous rock of the peridotite type, which is very unstable
under weathering conditions, and rapidly alters to serpentine. Magnesite
is formed both by this process and by the further breaking down of the
serpentine itself. The processes are those of katamorphism. Under these
circumstances the magnesite is characteristically fine-grained or
massive, and occurs in veins, lenses, and irregular bodies in cavities
and fractured zones. It is usually worked by open cuts.
Magnesite is also reported to occur in sedimentary beds in which it was
primarily deposited in its present form and has not undergone later
alteration. Such deposits are not important commercially.
FLUORSPAR
ECONOMIC FEATURES
The chief use of fluorspar is as a flux in the manufacture of
open-hearth steel. Minor uses are in chemical and enameling industries,
in the smelting of copper, lead, and iron, and in the manufacture of the
ferro-alloys in the electric furnace.
In order to be used in steel-making, the fluorspar after being
concentrated should contain at least 85 per cent calcium fluoride and
less than 4 per cent silica. Chemical and enameling industries require
material with 95 to 98 per cent calcium fluoride and less than 1 per
cent silica.
The chief foreign producer of fluorspar is Great Britain, and much of
this product comes to the United States. Canada produces a small
amount, some of which also comes to the United States. Several thousand
tons are produced yearly in Germany and France, and are largely consumed
there.
The production of fluorspar in the United States is several times that
of any other country. The ore mined comes principally from the southern
Illinois and western Kentucky field, and is used largely for fluxing
purposes in open-hearth steel furnaces. Minor amounts are produced in
Colorado, New Mexico, and other states.
The United States has sufficient supplies of fluorspar to meet all its
own demands for this material. Small amounts, however, are imported for
use in eastern furnaces because the material can be brought over from
England very cheaply. The domestic fluorspar is suitable for practically
all purposes for which fluorspar is used except for lenses in optical
instruments. For this use very small quantities of material imported
from Japan have been used, but recently fluorspar of a grade suitable
for optical purposes has been found in Illinois, Kentucky, New
Hampshire, and other states. For fluxing purposes domestic fluorspar is
superior to the foreign product.
GEOLOGIC FEATURES
Fluorspar is the trade name for the mineral fluorite, which is composed
of calcium fluoride. This is a common mineral in veins and replacements
which carry ores of zinc, lead, silver, gold, copper, and tin. It is
formed under a variety of conditions, but is always ascribed to
solutions coming from nearby igneous rocks.
The large fluorspar deposits of Illinois and Kentucky contain fluorite
with calcite, barite, and metallic sulphides, in wide veins filling
fissures in limestones and sandstones and replacing the fissure walls.
Into these sediments there are intruded certain peridotite dikes. The
fluorite and associated minerals were probably deposited by hot
solutions bringing the material from some large underlying igneous mass
of which the dikes are off-shoots.
In the western United States many metalliferous deposits carry large
amounts of fluorite, which is treated as a gangue or waste mineral, but
which could be profitably extracted if there were local markets. In
England, fluorite is obtained in this manner as a by-product from lead
and zinc mines.
SILICA
ECONOMIC FEATURES
Silicon and its oxide, silica, find important applications in the
manufacture of iron and steel. Silicon, like manganese, is an important
constituent of many steels, the alloy ferrosilicon being added to
deoxidize and purify the metal and thus to increase its tensile
strength. Like titanium, it is added chiefly for its curative effect
rather than as a useful ingredient. On an average 4 pounds of 50 to 55
per cent ferrosilicon are used in the United States for each ton of
steel produced. A higher grade of ferrosilicon (80 to 85 per cent) is
used for certain special steels, and during the war considerable
quantities were used in making hydrogen gas for balloons. Lower grades
(10 to 15 per cent silicon) are practically a high silicon pig iron.
Silica has an important use in the form of silica brick or "ganister"
for lining furnaces and converters in which acid slags are formed. For
this purpose siliceous rocks, chiefly quartzites and sandstones, are
ground up, mixed with lime as a binder, and fused and pressed into
bricks and shapes. For the most satisfactory results the rock should
contain 96 per cent or more of silica, and very little of the alkali
materials, which increase the fusibility.
In addition to its applications to the iron and steel industry, silica
finds an almost universal use in a wide variety of structural and
manufacturing operations. The extensive use of sand and gravel--composed
chiefly of silica--for road materials and railway ballast is well known.
In construction work silica is used in the form of stone, sand-lime
brick, cement, mortar, concrete, etc. Large quantities of sand, or
silica, are used for molds in foundries, for abrasives, for the
manufacture of glass, for filters, and for a great variety of other
purposes which readily suggest themselves (see pp. 84, 267).
For most uses of silica there are local supplies available. For certain
purposes requiring material of a particular chemical composition or
texture, however, satisfactory deposits are known in only a few places.
For example, the material for silica refractories is obtained in the
United States chiefly from certain regions in Pennsylvania, Missouri,
and Wisconsin. The United States has ample domestic supplies of silica
for practically all requirements.
Ferrosilicon of the higher grades is manufactured principally in
electric furnaces at Niagara Falls. The capacity is ample to meet all
demands, but cheap ferrosilicon from Canada also enters United States
markets.
GEOLOGIC FEATURES
Silicon and oxygen, making up the compound silica, are the two most
abundant elements in the earth's crust, and quartz (SiO_2) is a very
abundant mineral. The processes of weathering and transportation
everywhere operative on the surface of the earth tend to separate quartz
from other materials, and to concentrate it into deposits of sand.
Katamorphism is primarily responsible for most of the deposits of silica
which are commercially used. Anamorphism--cementing and hardening the
sands into sandstones and quartzites--has created additional value for
certain uses, as in refractories, building stones, and abrasives (see
pp. 84, 267).
FOOTNOTES:
[31] Report of the Royal Ontario Nickel Commission. Printed by order of
the Legislative Assembly of Ontario, Toronto, 1917.
[32] Campbell, J. Morrow, Tungsten deposits of Burma and their origin,
_Econ. Geol._, vol. 15, 1920, p. 511.
CHAPTER X
COPPER, LEAD, AND ZINC MINERALS
COPPER ORES
ECONOMIC FEATURES
The electrical industry is the largest consumer of copper. The
manufacture of brass, bronze, and other copper alloys constitutes
another chief use for the metal. Considerable quantities of copper
sheets, tubes, and other wares are used outside of the electrical
industry, as for instance in roofing, plumbing, and ship bottoms. Copper
is also used in coinage, particularly in China, where it is the money
standard of the working population.
The average grade of all copper ores mined in the United States in
recent years has been about 1.7 per cent metallic copper. Ores
containing as low as 0.6 per cent have been mined in the Lake Superior
country, and bonanza deposits containing 20 to 60 per cent have been
found and worked in some places, notably in Alaska and Wyoming. The
lower-grade ores, carrying 1 to 3 per cent copper, are usually
concentrated before smelting, while the richer ores, carrying 3 to 5 per
cent or more, are generally smelted direct. Many of the ores contain
values in gold and silver, and also in lead and zinc. An average of
about 40c. worth of gold and silver per ton is obtained from all the
copper ores of the United States.
In other countries the average grade of copper ores mined is somewhat
higher than in the United States,--where large scale operations,
particularly the use of steam-shovel methods on extensive bodies of
disseminated or "porphyry" copper ores, as well as improvements in
concentrating and metallurgical processes, have made possible the use of
low-grade ore.
The principal sources of copper are the North American continent, Chile
and Peru, Japan, south and central Africa, Australia, and Spain and
Portugal. Smaller quantities are produced in Russia, Germany, Norway,
Cuba, Serbia, and a number of other countries.
The United States normally produces nearly two-thirds of the world's
copper and consumes only about one-third. In addition the great bulk of
the South American, Mexican, and Canadian crude copper comes to the
United States for refining. Through financial interests abroad and by
means of refining facilities, the United States controls a quantity of
foreign production which, together with the domestic production, gives
it control of about 70 per cent of the world's copper. No other country
produces one-sixth as much copper as the United States.
England, because of production in the British Empire (mainly Africa and
Australia) and British financial control of production in various
foreign countries, is not dependent upon the United States for supplies
of raw copper. Japan, Spain, Portugal, and Norway are able to produce
from local mines enough copper for their own needs and for export. But
France, Italy, Russia, Germany, and the rest of Europe normally are
dependent upon foreign sources, chiefly the United States. South
America, Mexico, Canada, Africa, and Australia are exporters of copper.
The control of these countries over their production in each case is
political and not financial, except in the case of Canada, where about
half the financial control is also Canadian. It is in these countries
and in Spain that the United States and England have financial control
of a large copper supply.
Before the war German interests had a considerable control over the
American copper industry through close working arrangements with
electrolytic refineries. Germany was the largest foreign consumer of
copper, and German companies bought large quantities of the raw copper
in the United States, Canada, Mexico, and South America, had it refined,
and sold the finished material in both the American and foreign markets.
During the war this control was broken up.
In view of the importance of copper metal as a raw material,
particularly in the electrical industry, the strength of the United
States in copper as a key resource ranks even above its control of
petroleum.
In the United States in recent years about 40 per cent of the annual
production of copper has come from Arizona, chiefly from the Bisbee,
Globe, Ray-Miami, Jerome, and Morenci-Metcalf districts; about 18 per
cent has come from the Butte district of Montana; about 12 to 15 per
cent from Keweenaw Point, Michigan; and about 12 per cent from Bingham,
Utah. From 3 to 5 per cent of the country's output comes from each of
the states of New Mexico, Nevada, Alaska, and California. All other
states together produce only a little over 2 per cent of the total.
The so-called "porphyry" coppers in Utah, Arizona, Nevada, and New
Mexico, described below, are the source of about 35 per cent of the
present production of the United States. The deep mines of Butte and
Michigan are responsible for about 30 per cent of the production, and
the ore bodies of Arizona (other than porphyry) and of Alaska produce
about 25 per cent.
Reserves of copper ore are such as to give no immediate concern about
shortage, nor to indicate any large shift in the distribution of
production in the near future. Development is on the whole considerably
in advance of present demands. The principal measured reserves are in
the so-called porphyry coppers of the United States and Chile. In the
United States the life of these reserves now estimated is approximately
25 years. The reserves of the Chile Copper Company are the largest of
any known copper deposit in the world, and the Braden copper reserve
(also in Chile) is among the largest. For the deep mines of the United
States, the developed reserves have a life of perhaps only five years,
but for most of these mines the life will be greatly extended by further
and deeper development. The porphyry coppers, because of their
occurrence near the surface and the ease with which they may be explored
by drilling, disclose their reserves far in advance. The deep mines are
ordinarily developed for only a few years in advance of production.
GEOLOGIC FEATURES
The principal copper minerals may be classified into the sulphide group,
the oxide group, and native copper. Native copper, mined in the Lake
Superior region, is the source of 8 to 10 per cent of the world's copper
supply. The oxide group of minerals--including the copper carbonates,
azurite and malachite; the silicate, chrysocolla; the oxide, cuprite;
the sulphates, chalcanthite and brochantite; and some native copper
associated with these minerals--probably supplies another 5 per cent.
The remaining 85 per cent is derived from the sulphide group. Of the
sulphide group by far the most important mineral is chalcocite (cuprous
sulphide), which supplies the bulk of the values in the majority of the
mining camps of the western hemisphere. Locally, as at Butte, enargite
(copper-arsenic sulphide) is of great value. Other minerals of
considerable importance in some districts are chalcopyrite and bornite
(copper-iron sulphides), tetrahedrite (copper-antimony sulphide), and
covellite (cupric sulphide). Very commonly the copper sulphides are
associated with large quantities of the iron sulphide, pyrite, as well
as with varying amounts of lead and zinc sulphides and gold and silver
minerals.
The principal copper ores originate in the earlier stages of the
metamorphic cycle, in close association with igneous activity.
Katamorphism or weathering, in place, has played an important part in
enriching them. The processes of transportation and sedimentary
deposition, which have done so much toward making valuable iron ore
deposits, have contributed little to the formation of copper ores.
=Copper deposits associated with igneous flows.= The copper ores of the
Lake Superior district, and of a few small deposits in the eastern
United States, contain small percentages of native copper in
pre-Cambrian volcanic flows or in sediments between the flows. The ore
bodies have the form of long sheets parallel to the bedding, the copper
and associated minerals filling amygdaloidal openings and small fissures
in the flows, and replacing conglomeratic sediments which lie between
the flows. The copper was probably deposited by hot solutions related to
the igneous rocks, either issuing from the magmas or deriving heat and
dissolved material from them. Secondary concentration has not been
important. There is practically none of it near the present erosion
surface; but it appears in one part of the district near an older
erosion surface covered by Cambrian sediments, suggesting a different
climatic condition at that time.
The Kennecott copper deposits of Alaska have a number of resemblances to
the Lake Superior copper deposits, suggesting similarity in origin. The
Kennecott deposits occur exclusively in limestone, which rests
conformably on a tilted surface of igneous flows ("greenstones") not
unlike those of Lake Superior. The flows carry native copper and copper
sulphides in minutely disseminated form and in amygdules, but apparently
not in quantities sufficiently concentrated to mine. The flows are
supposed to be the original source of the copper now in the limestone.
The primary copper mineral in the limestone is chalcocite, in
exceptionally rich and solid masses, showing no evidence of having
replaced earlier sulphides. It is regarded as a product of primary
deposition, under the influence of hot solutions related in some way to
the igneous flows; but whether the solutions were magmatic, originating
in the lavas or below, or whether they were meteoric waters rendered hot
by contact with the extrusives, and thereby made effective in leaching
copper from them, is not clear. The oxidation of the Kennecott copper
ores is not extensive. It presents an interesting feature, in that since
glacial time the ground has been frozen and the moisture is now present
in the form of ice. The oxidation clearly took place before glacial
time. Abundant fragments of both the oxide and the sulphide ores are
mined from the lateral moraine of a nearby glacier. This is a good
illustration of the cyclic nature of secondary concentration which is
coming to be recognized in so many camps.
The Boleo copper deposits of Lower California occur in volcanic tuffs
and associated conglomerates of Tertiary age. They have certain peculiar
mineralogic associations--the ores containing large quantities of all
the common copper oxide minerals, and a number of rare oxide minerals of
copper, lead, silver, and cobalt, together with gypsum, sulphur, and
much iron and manganese oxide. The copper oxides and carbonates are in
places gathered into rounded concretions called "boleos" (balls).
Sulphides are present in the lowest beds and may represent the form in
which the copper was originally deposited. The copper-bearing beds have
been much silicified, and it has been suggested that mineralization was
accomplished by hot-spring waters, probably of igneous origin. These
deposits have a few marked similarities to the Lake Superior copper
ores.
=Copper veins in igneous rocks.= A second group of copper ores in
igneous rocks is made up of deposits in distinct fissure veins and as
replacements along such veins. The chief deposits of this type are at
Butte, Montana--which is, from the standpoint of both past and present
production, the greatest single copper district in the world. Here a
large batholith of Tertiary granite was intruded by porphyry dikes; and
faulting, accompanying and following the intrusions of the dikes,
developed numerous fissures. The fissures were mineralized with copper
sulphides and arsenides, iron sulphides, and locally with zinc sulphide
and manganese carbonate,--all in a matrix of quartz. At the same time
the wall rocks were extensively mineralized and altered; the fissure
veins grade off into the wall rock, and in fact the larger part of the
ore is simply altered granite with disseminated sulphides. The solutions
which deposited the ores are inferred to have been hot from the nature
of the wall-rock alterations, from the presence of hot-water minerals
like fluorite, cassiterite, and others, and from the general association
of the ores in time and place with the porphyry intrusions. The
solutions are believed to have originated from the porphyry and possibly
from other intrusives.
In the Butte district, and in the great majority of copper sulphide vein
ores throughout the world, secondary concentration by surface waters has
played a considerable part in developing ores of commercial value. Near
the surface the copper is leached out and carried down by waters
containing various solvents, particularly sulphuric acid from the
oxidation of pyrite. A leached zone is formed containing the ordinary
products of rock weathering,--rusty quartz and clay, sometimes black
with manganese oxides. A small part of the copper remains in this zone
as oxides, carbonates, and silicates. Below the oxidized and leached
zone there is evidence of deposition of a large amount of secondary
copper sulphide in the form of chalcocite. This is supposed to have been
formed by the leaching of copper from above as soluble copper sulphate,
and its precipitation below by iron and other sulphide minerals which
the solutions meet on their downward course--a reaction which has been
demonstrated experimentally. It was formerly supposed that most of the
chalcocite was of this origin; but as chalcocite is found in important
amounts with enargite and chalcopyrite to great depths (now 3,500 feet),
where the veins are still rich and strong, it begins to appear that much
of the chalcocite is of primary origin.
The fissures along which the Butte ores occur are in three main sets,
which in order of age strike roughly east-west, northwest-southeast, and
northeast-southwest. Two-thirds of the ore is in the first set, about 30
per cent in the second, and the remainder in the third. The
mineralization of the several vein systems cannot be discriminated, and
it is thought that it was accomplished as a more or less continuous and
progressive process. There is some evidence, also, that the fracturing
in the several fracture systems was likewise a nearly continuous
progressive process, contemporaneous with the ore deposition, and
perhaps developing under a single great shear which caused more or less
simultaneous and overlapping systems of fractures in the various
directions.
="Porphyry coppers."= Another type of copper deposits in igneous rocks
is the disseminated or "porphyry" deposits. The term "porphyry" as
commonly used includes true porphyries, monzonites, granites, and other
igneous rocks. Ores of this type are represented by the great deposits
of Bingham, Utah; Ray, Miami, and the New Cornelia mine of Arizona; Ely,
Nevada; Santa Rita, New Mexico; Cananea, Sonora, Mexico; northern Chile;
and many other districts of importance. They form the greatest known
reserves of copper ore. These deposits contain copper minerals, usually
in the marginal portions of acid porphyries, in many irregular, closely
spaced veins, and in minute seams and spots disseminated through the
mass of the rock. In the Ray and Miami and other districts the
mineralization has spread largely through adjacent schists, but these
deposits are included with the porphyry copper deposits in commercial
parlance. The porphyry deposits are of an undulating blanket form of
considerable areal extent and shallow depth. At the surface is a leached
and weathered zone, often containing more or less of the oxides,
carbonates, and silicates of copper, ranging in thickness up to 1,000
feet, but averaging 200 feet or less. Below this is a zone carrying
copper in the form of chalcopyrite, enriched by chalcocite deposition
from above, ranging in thickness up to 400 feet. The ore in this zone
varies from one-half of 1 per cent to 6 per cent of copper and
ordinarily averages between 1 and 2 per cent. The use of ore of this
grade is made possible by the large quantities and by the cheap and
efficient mining and metallurgical practices. The ore body grades below
into a zone characterized by lean chalcopyrite, which is supposed to
represent original or primary deposition from hot waters associated with
the porphyry intrusion. This primary ore, or protore, was clearly formed
after the solidification of the igneous rocks, though soon after, by
solutions from igneous sources which followed fractured and shattered
zones.
=Copper in limestone near igneous contacts.= Another great group of
copper deposits occurs as replacements of limestone adjacent to porphyry
or granitic intrusives. This type is illustrated by some of the deposits
at Bingham, Utah, and at Bisbee, Arizona. The primary deposition was of
chalcopyrite and other copper sulphides, together with garnet, diopside,
and other minerals known to have required high temperature in their
formation. The ore fills fissures and replaces extensive masses of the
limestone. It is likely to show a fairly sharp contact on the side
toward the intrusive, and to grade off into the country rock on the
other side with numerous embayments and irregularities. These deposits
have been enriched by weathering in the same manner as indicated above
for the porphyry coppers, but to highly varying degrees. In the Bisbee
deposits large values were found in the weathered zone, and secondary
sulphide enrichment below this zone is also important. In the Bingham
camp, on the other hand, the weathered zone is insignificant and most of
the ore beneath is primary. The weathering of the silicated limestone
gangue results in great masses of clay which are characteristic features
of the oxide zones of these deposits.
=Copper deposits in schists.= Other copper deposits, as at Jerome,
Arizona, in the Foothill and Shasta County districts of California, at
Ducktown, Tennessee, etc., are irregular lenticular bodies in schists
and other rocks, but all show relationship to igneous rocks. The Rio
Tinto ores of Spain and Portugal, which belong in this group, have been
referred to on page 108.
In the Jerome or Verde district of central Arizona, folded pre-Cambrian
greenstones and sediments were invaded by masses of quartz-porphyry, and
after further deformation, rendering many of the rocks schistose, were
intruded by an augite-diorite. Contact metamorphism along both the
quartz-porphyry and the diorite contacts was practically lacking. The
ore bodies were formed as irregular pipe-like replacements of the
schists, being localized in one case by a steeply pitching inverted
trough of impervious diorite, and in other cases by shear zones which
favored vigorous circulation. A later series of small diorite or
andesite dikes cut the ore bodies. The primary ores consist of pyrite,
chalcopyrite, and other sulphides, with large amounts of jaspery quartz
and some calcite and dolomite. They were clearly formed by replacement
of the schists particle by particle, as shown by the frequent
preservation of the schist structure in a banding of the sulphide
minerals, the residual shreds of unreplaced schist material in the ores,
and the usual gradual transition from unreplaced schists to those
completely replaced by massive sulphides. The localization of the most
important mineralization in an inverted trough is good evidence that the
solutions came from below, and the nature of the mineral associations
suggests an origin through the work of hot waters associated with
igneous intrusives. The diorite, being most closely related in time and
space with the ore bodies, seems the most logical source of the ore
materials.
Secondary concentration of the Jerome ores has proceeded along the
general lines previously outlined (pp. 46-50, 202). Here again the
evidence is clear that the ores were concentrated in an earlier period,
in this case in pre-Cambrian times, probably during the long interval
required for the base-leveling of the pre-Cambrian mountains. Since
Cambrian times the deposits have been for the most part buried by later
sediments. Some of the deposits are still protected by this overlying
blanket and mining has not yet reached the zone of altogether primary
sulphides. Others have been faulted up and again exposed by erosion; but
since being uncovered, steep slopes and rapid erosion have apparently
favored the scattering of the copper rather than its concentration and
enrichment. In the United Verde Mine, oxidizing conditions at present
prevail to the bottom of the chalcocite zone.
The very large reserves of the Katanga copper belt of the Belgian Congo
are in the form of tabular masses in schistose and highly metamorphosed
Paleozoic sediments. The ore bodies are roughly parallel to the bedding,
but in instances follow the schistosity which cuts across the bedding.
They consist dominantly of the oxide minerals, though in several ore
bodies sulphides have been shown by diamond-drilling. The ores have a
high content of cobalt and also carry precious metals. The origin of the
deposits is not known, but has been ascribed to granitic masses
intrusive into the schists.
=Sedimentary copper deposits.= In the later phases of the metamorphic
cycle, the agencies of transportation (in solution) and sedimentary
deposition have resulted in some low-grade deposits of copper sulphides
in sedimentary rocks. Deposits of this type are found in the Rocky
Mountain region, where they are referred to as the "Red Beds" coppers,
but are of no commercial importance. Similar deposits in Germany, the
Mansfield copper-bearing shales, have been worked for some time, and
during the war were Germany's main source of copper. On Keweenaw Point,
Michigan, deposits of native copper formed in this manner in the
"Nonesuch" beds have been worked on a commercial scale. Other copper
ores on Keweenaw Point are replacements of conglomerate beds between
igneous flows, and are of a different origin already described (p. 200).
While much of the copper of sedimentary beds gives evidence that it was
deposited from solution in cracks and as replacements of the wall rocks,
often through the agency of abundant organic material in the beds, and
while also comparatively little of this copper can be identified as
having been deposited in detrital flakes or fragments along with the
other mineral fragments, there is, nevertheless, considerable evidence
that some of these deposits were formed essentially during the
sedimentation of the enclosing beds and as incidents to this process.
Such evidence consists of a close limitation of the copper to certain
beds, its wide and uniform distribution within these beds, its absence
in similar beds near at hand, the absence of evidence of feeding and
escape channels of the kind which would be necessary in case the
solutions were introduced long afterward, and often a minute
participation of the copper minerals in the minor structures of bedding,
false-bedding, and ripple-marks, which would be difficult to explain as
due to secondary concentration.
The Corocoro copper deposits of Bolivia occur in beds of sandstone with
no igneous rocks in the vicinity. However, they are all closely
associated with a fault plane, igneous rocks occur at distances of a few
miles, and the general mineralization is coextensive with the belt of
igneous rocks; the deposits are therefore ascribed to a magmatic source
rather than to sedimentary processes. Toward the surface the copper is
in part in the form of sulphides, somewhat altered to oxide minerals,
and farther down it is entirely native copper, associated with gypsum.
This is the only district outside of Lake Superior where native copper
has been mined on an important scale.
=General comments.= In general, the commercially prominent copper
deposits show a close relationship to igneous rocks in place, time, and
origin. Seldom do the ores extend more than 1,000 feet away from the
igneous rock.
The common downward order in sulphide deposits is: first, a weathered
zone, originally formed mainly above the water table, consisting above
of a leached portion and below of oxides and carbonates of copper in a
gangue of quartz or clay; second, a zone of secondary sulphide
enrichment, characterized by chalcocite coatings, chalcopyrite, and
pyrite, with a gangue of quartz and igneous rock or limestone; and
third, a zone of primary deposition with similar gangue, characterized
by chalcopyrite, and at Butte by enargite and chalcocite. The oxide zone
as a whole may be rich or lean in values, depending on the nature of the
associated gangue material and country rock. When these are more soluble
than the copper--as is commonly the case in limestone--the copper may be
residually concentrated, notwithstanding the fact that much copper
originally present has been carried off in solution. When the associated
gangue and country rock are less soluble than the copper--as is common
with quartz and igneous rocks--the oxide zone is likely to be depleted
of values.
The zones formed by weathering and secondary enrichment are extremely
irregular, both in distribution and depth, in any one deposit, and they
overlap and grade into one another in a very complex fashion. In many
places the primary zone is too lean to be mined to commercial advantage;
but in other places, as at Butte, and in the limestone deposits of
Bingham, the primary ores are of considerable importance.
When evidence of secondary sulphide enrichment was first recognized
there was a tendency to magnify its effectiveness, and to assume that in
most cases the values were due to this process; that the primary zones
would be found to be valueless. In recent years the emphasis is being
somewhat changed because of the recognizing in many camps of rich
primary zones. While some chalcocite is clearly the result of secondary
enrichment from above, other chalcocite seems to have been related
closely to the primary deposition. The quantitative discrimination of
the two is a matter of great difficulty.
It has come to be recognized that the zonal arrangement caused by
enrichment from the surface has been imposed usually on a zonal
arrangement caused by the primary hot solutions and not related to the
surface but to the source of the solutions. In some districts, as
illustrated by Butte and Bingham, the copper-bearing minerals seem to
have been deposited nearest the igneous source, while the lead, zinc,
gold, and silver minerals have been deposited farther away,--suggesting
the cooling of the solutions with increasing distance from the igneous
source. The further investigation of this primary zonal arrangement
promises interesting results with a practical bearing on exploration and
development.
One of the newer features of the investigation of copper deposits has
been the recognition of the cyclic nature of the secondary
concentration. This process has been related not only to the present
erosion surface, but to older surfaces now partly buried under later
rocks. Ransome's[33] summary of conditions at the Ray-Miami camp has a
somewhat general application.
Supergene enrichment has generally been treated as a
continuously progressive process. There is considerable
probability, however, that it is essentially cyclic, although
the cyclic character may not be patent in all deposits. A full
development of the cycle can take place only under a certain
equilibrium of a number of factors, including climate,
erosion, topography, and character of rock. The essential fact
appears to be that as enrichment progresses and chalcocite
increases the process of enrichment becomes slower in action,
and erosion may, in some circumstances, overtake it. With the
removal of some of the protecting zone of chalcocite the
protore is again exposed to oxidation and a second cycle of
enrichment begins.
Although much of the enriched ore is now below ground-water
level, it probably was once above that level, and enrichment
is believed to have taken place mainly in the zone of rock
above any general water table.
Where the old erosion surface roughly coincides with the present erosion
surface, the deposits follow more or less the topography. Where the old
erosion surface pitches below later sediments, the ores pitch with it,
and therefore do not follow the present topography. The recognition of
the cyclic nature of secondary concentration is obviously of great
significance in exploration and development.
Although a vast amount of study has been devoted to the origin and
enrichment of copper deposits, and although the general conditions and
processes are pretty well understood, the results thus far have been
largely qualitative rather than quantitative.
LEAD ORES
ECONOMIC FEATURES
The most prominent uses of lead are in the manufacture of alloys, such
as type-metal, bearing metal, shot, solder, and casting metal; as the
oxide, red lead, and the basic carbonate, white lead, in paints; for
lead pipe, cable coverings, and containers of acid active material; and
in lead compounds for various chemical and medical uses. Of the lead
consumed in the United States before the war about 38 per cent was
utilized in pigments, 30 per cent in alloys other than shot, 15 per cent
in pipe, 10 per cent in shot, and 7 per cent in all other uses. During
the war much larger quantities were used in munitions, such as shot and
shrapnel.
The lead content of commercial ores varies widely. It ranges from as low
as .25 per cent in the Joplin district of Missouri, to about 15 per cent
in the Broken Hill deposits of Australia, and over 20 per cent in the
Bawdwin mines of Burma. In the Coeur d'Alene district of Idaho and the
southeastern district of Missouri, the two greatest lead producers in
the United States, the average grades are about 10 per cent and about
3-1/2 per cent respectively. The grade of ore which may be profitably
worked depends not only upon the economic factors,--such as nearness to
consuming centers, and the price of lead,--but also upon the amenability
of the ore to concentration, the content of other valuable metals, and
the fact that lead is very useful in smelting as a collector of gold and
silver.
Most lead ores contain more or less zinc, and lead is obtained as a
by-product of most zinc ores. Argentiferous lead ores form one of the
principal sources of silver, and also yield some gold. Lead and copper
are produced together from certain ores. Thus the separation of many
ores into hard and fast classes, as lead, or zinc, or copper, or silver,
or gold ores, cannot be made; in some of the mineral resource reports of
the United States Geological Survey the statistics of these five metals
are published together.
The main sources of lead ore, named in order of their importance, are
the United States, Australia, Spain, Germany, and Mexico, which account
for over 80 per cent of the world's production. Most of the countries of
Europe outside of Spain and Germany produce small amounts of lead, but
are largely dependent on imports. Spain exports argentiferous lead and
pig lead mainly to England and France, with minor quantities to other
countries of Europe and to Argentina. Before the war Germany, which was
the largest European consumer, utilized all its own production of lead
ores and imported an additional 10 per cent of the world's ores for
smelting, as well as considerable amounts of pig lead. Its principal
deposits were those of Silesia; under the Peace Treaty they may possibly
be lost to Poland, leaving German smelters largely dependent on imports.
Australia before the war normally shipped lead concentrates and pig lead
to England and also to Belgium, Germany, and Japan. England, the second
largest European consumer, before the war had insufficient smelting
capacity within the British Empire and was partly dependent on
foreign-smelted lead. During the war, however, England contracted for
the entire Australian output, and enlarged its smelting capacity
accordingly. This may mean permanent loss to Belgium, which had depended
mainly on the Australian ores for its smelting industry before the war.
In North Africa there is a small but steady production of lead, most of
which goes to France. Recent developments in Burma have shown large
reserves of high-grade lead-zinc-silver-copper ores, and this region may
be expected to become an important producer. There are also large
reserves of lead in the Altai Mountains of southwestern Siberia and in
the Andes Mountains of South America.
England, through control of Australian and Burman lead mines and
smelters, domestic smelting facilities, and some financial control in
Spain, Mexico, and elsewhere, and France, through financial control of
Spanish and North African mines and Spanish, Belgian, and domestic
smelters, have adequate supplies of lead.
The United States produces about a third of the world's lead and twice
as much as any other country. Normally the domestic production is almost
entirely consumed in this country. Mexico sends large quantities of
bullion and ore to the United States to be smelted and refined in bond.
Mexican lead refined and exported by the United States equals in amount
one-sixth of the domestic production. Small quantities of ore or bullion
from Canada, Africa, and South America are also brought into the United
States for treatment.
Through domestic production, smelting facilities for Mexican ore, and
commercial ownership in Mexico and elsewhere, the United States controls
over 45 per cent of the world's lead production. Before the war Germany,
through the "Lead Convention" or International Sales Association, and
through smelting and selling contracts with large producing mines,
practically controlled the European lead market as well as exports from
Mexico and the United States and from Australia. During the war German
foreign influence was practically destroyed.
In the United States about a third of the production of lead comes from
southeastern Missouri and about a fourth from the Coeur d'Alene
district of Idaho. The five states, Missouri, Idaho, Utah, Colorado and
Oklahoma, produce about nine-tenths of the country's total output.
Reserves of lead ore are not large in proportion to demand, contrasting
in this regard with zinc ore.
GEOLOGIC FEATURES
The principal lead mineral is the sulphide, galena, from which the great
bulk of the world's lead is derived. Cerussite (lead carbonate) and
anglesite (lead sulphate) are mined in some places in the upper part of
sulphide deposits, and supply a small fraction of the world's output.
The ores of lead are of two general classes:
The first class, the so-called "soft" lead ores, nearly free from copper
and precious metals, and commonly associated with zinc ores, are found
in sedimentary beds independent of igneous intrusion. They are of
world-wide distribution, were the first to be extensively exploited,
were at one time the dominant factor in world production of lead, and at
present produce about 30 per cent of the world's total. They are
represented by the deposits of the Mississippi Valley, of Silesia, and
some of the Spanish deposits. The general description of the origin of
the zinc ores of the Mississippi Valley on pp. 216-218 applies to this
class of lead ores. It should be noted, however, that in the principal
United States lead-producing district, that of southeastern Missouri,
the lead ores occur almost to the exclusion of the zinc ores, and are
more disseminated through the limestone than is characteristic of the
zinc ores. Ores of this type have been found extending only to shallow
depths (not over a few hundred feet), and because of the absence of
precious metals their treatment is comparatively simple.
The second class consists of ores more complex in nature, which are
found in association with igneous rocks, and which usually contain some
or all of the metals, zinc, silver, gold, copper, iron, manganese,
antimony, bismuth, and rare metals, with various gangue minerals among
which quartz, siderite, and silicates are important. Today these ores
are the source of about 70 per cent of the world's lead. They are
represented by the lead deposits of the Rocky Mountain region (Coeur
d'Alene, Idaho; Leadville, Colorado; Bingham, Utah; etc.); of Broken
Hill, New South Wales; of Burma; and of many other places. They are all
related to the earlier stages of the metamorphic cycle and occur in
close genetic association with igneous activity. They include deposits
in the body of igneous rocks,--in the form of well-defined veins,
replacements along zones of fissuring and shearing, and disseminated
masses,--as well as veins and replacements in the rocks, particularly in
limestones, adjoining igneous intrusions. The deposits present a wide
variety of shapes depending on the courses of the solutions by which
they were formed. The materials of the ore minerals are believed to have
been derived from the igneous rocks and to have been deposited by hot
solutions. The source of the solutions--whether magmatic or
meteoric--presents the same problems which have been discussed elsewhere
(pp. 41-42). The ores are frequently mined to great depths. Because of
their complexity they require involved processes of treatment to
separate out the values.
Ores of this nature have already been referred to in the discussion of
the copper ores of Bingham and Butte, and will be referred to in
connection with the zinc-lead-silver ores of Leadville, Colorado.
Special reference may be made here to the Coeur d'Alene district of
Idaho, which is the second largest producer of lead in the United
States.
The Coeur d'Alene deposits are almost unique in that they contain
galena as vein-fillings and replacements in quartzite, with a gangue of
siderite (iron carbonate). Quartzite (instead of limestone) is an
unusual locus of replacement ores, and siderite is an unusual gangue.
These ores are believed to owe their origin to acid igneous intrusives,
because of the close association of the ores with some of these
intrusives, and because of the content of high-temperature minerals.
Some of the ore bodies are found far from intrusives, but it is supposed
that in such cases further underground development may disclose the
intrusives below the surface. Secondary concentration has been
insignificant.
In general, weathering of lead ores at the surface and secondary
sulphide enrichment below are not so extensive as in the case of copper
and zinc. Galena is fairly stable in the oxide zone, and even in moist
climates it is found in the outcrop of many veins. Weathering removes
the more soluble materials and concentrates the lead sulphide with the
residual clay and other gangue. In some districts cerussite and a little
anglesite are also found in the oxide zone. The carrying down of lead in
solution and its deposition below the water table as a secondary
sulphide is not proved on any extensive scale. In this respect it
contrasts with zinc; and when the two minerals occur together, lead is
likely to be more abundant in the oxide zone, and zinc in the sulphide
zone below. Such a change in composition with depth is also found in
some cases as the result of primary vertical variations in the
mineralization.
ZINC ORES
ECONOMIC FEATURES
Zinc metal has commonly gone under the name of "spelter." Brass and
galvanized iron contain zinc as an essential ingredient. Of the total
United States zinc consumption in normal times, about 60 per cent is
used in galvanizing iron and steel objects to protect them from rust, 20
per cent is used in the manufacture of brass and other alloys, 11 per
cent goes into the form of rolled sheets for roofing, plumbing, etc., 1
per cent is employed in desilverizing lead bullion, and the remaining 8
per cent is used for pigments, electrodes, and other miscellaneous
purposes. During the war the use in brass-making was greatly increased.
The zinc content of the ores mined today ranges from a little over
1-1/2 per cent in the Joplin district of Missouri, to 25 per cent and
higher in some of the deposits of the Coeur d'Alene and other western
camps, and over 40 per cent in certain bonanzas in British Columbia and
Russia. The ores usually contain both zinc and lead in varying
proportions, and sometimes gold, silver, and copper are present. Of the
zinc produced in the United States, about 73 per cent is obtained from
ores containing zinc as the principal element of value, about 25 per
cent from zinc-lead ores, and 2 per cent from copper-zinc and other
ores. The average grade of the straight zinc ores is about 2-1/2 per
cent.
Of the world's zinc ore, the United States produces in normal times
about one-third, Germany about one-fifth, Australia about 15 per cent,
Italy, North Africa and Spain each about 5 per cent. The remaining 15 to
20 per cent comes from a large number of scattered sources, including
Japan, East Asia, Norway and Sweden, Canada, Mexico, Austria, France,
Greece, Siberia, and Russia. In the near future the Bawdwin mines of
Burma will probably be increasingly important producers. Large reserves
of zinc also exist in the Altai Mountains of southwestern Siberia, and
in the Cordilleran region of South America. In short, zinc is one of the
most widely distributed of metallic resources; there is consequently
less necessity for great international movements than in the case of
many other commodities.
The smelting of zinc concentrates is in general carried on close to the
points of consumption and where skilled labor is available, rather than
at the mines,--although smelters to handle part of the output have
recently been built in Australia and in Burma. In Europe the great
smelting countries have been Germany and Belgium, and to a lesser extent
England and France. Before the war these four countries with the United
States produced over nine-tenths of the world's spelter. Belgium did
principally a custom business, and a large part of its exports went to
England. Australian and Tasmanian zinc ores were the basis of the
Belgian and English smelting industries, and also supplied about
one-third of the German requirements. Since the war England has
contracted to take practically the entire Australian output. This fact,
in connection with war-time destruction of Belgian smelters, leaves the
future of the Belgian zinc industry in some doubt. Germany may possibly
lose to Poland its richest zinc mines, those of Silesia. German
activity in the rich deposits of Mexico is to be expected. France
controls the deposits of North Africa and satisfies a considerable part
of its requirements from that source. Smaller movements of zinc include
exports from Italy to England, and a complex interchange among the
lesser producers of Europe. English and French zinc-smelting capacity
was expanded during the war, and the industry in these countries is in a
strong position. Japan also developed a considerable smelting industry
during the war, importing ores from eastern Asia and Australia.
The United States normally smelts and consumes all its large production
of zinc ores and does not enter foreign markets to any extent. Small
amounts of zinc concentrates are brought in from Mexico and Canada to be
smelted in bond. During the war,--when the Allies were cut off by enemy
operations from the customary Belgian and German supplies of spelter,
and by shortage of ships from Australian zinc ores,--Australian,
Spanish, Italian, and other ores were imported into the United States,
and large quantities of spelter were exported from this country to
Europe. Mine and smelter capacities were greatly increased,
over-production ensued, and with the cessation of hostilities many
plants were obliged to curtail or cease operations. The United States
has now about 40 per cent of the zinc-smelting capacity of the world.
For the present at least the capacity is far in excess of the domestic
requirements.
Before the war German control of the international zinc market was even
stronger than in the case of lead. The German Zinc Syndicate, through
its affiliations, joint share-holdings, ownership of mines and smelters,
and especially through smelting and selling contracts, controlled
directly one-half of the world's output of zinc and three-fourths of the
European production. It regulated the Australian exports by means of
long-term contracts, and had considerable influence in the United
States. To some extent it was able to so manipulate the market that zinc
outside the syndicate was also indirectly controlled. During the war
political jurisdiction was used by the Allied countries to destroy this
German influence.
In the United States the principal zinc-producing regions are the Joplin
and adjacent districts of Missouri, Oklahoma, Kansas, and Arkansas,
furnishing about one-third of the country's output; the Franklin Furnace
district of New Jersey, and the Butte district of Montana, each
yielding about one-sixth of the total supply; and the Upper Mississippi
Valley district of Wisconsin, Iowa, and Illinois, the Leadville district
of Colorado, and the Coeur d'Alene district of Idaho, each producing
between one-tenth and one-twentieth of the total. Smaller quantities are
produced in Tennessee, New Mexico, Nevada, and several other states.
Reserves of zinc are ample for the future. They are now developed
considerably in advance of probable requirements, a fact which causes
keen competition for markets and renders zinc-mining more or less
sensitive to market changes.
GEOLOGIC FEATURES
The most important mineral of zinc is the sulphide, sphalerite or "zinc
blende." The minerals of the oxide zone are smithsonite (zinc carbonate)
and calamine (hydrous zinc silicate), which yield minor amounts of zinc
and are especially productive at Leadville, Colorado. Zincite (zinc
oxide) and willemite (zinc silicate) are the important minerals in the
deposits of Franklin Furnace, New Jersey. The association of most
deposits of zinc with more or less lead has been noted.
The ores of zinc are of two general classes, corresponding to the two
classes of lead ores (pp. 211-212). Zinc ores of the first type are in
veins and replacements in sedimentary rocks at shallow depths,
independent of igneous association, and are supposed to have been formed
by cold solutions. They are found in the Mississippi Valley, in Silesia,
and in many of the smaller European deposits. They were formerly the
leading zinc-producers, and now produce about 45 per cent of the world's
total. Zinc ores of the second type consist of veins and replacements
related to intrusive rocks, sometimes extending to considerable depths,
and of more complex composition. They include most of the deposits of
the American Cordilleran region (Butte, Coeur d'Alene, Leadville,
etc.), of Franklin Furnace, of Australia, of Burma, and of many other
places.
The zinc-lead ores of the type found in the Mississippi Valley are of
special interest, in that they are sulphide ores of an origin apparently
independent of igneous agencies. These ores occur as fissure-fillings
and replacements, mainly in nearly flat-lying Paleozoic limestones and
dolomites--the Bonne Terre dolomitic limestone of southeastern Missouri,
the Boone formation of southwestern Missouri and Oklahoma, the Galena
dolomite of Wisconsin and Illinois. They are variously associated with a
gangue of dolomite, calcite, quartz, iron pyrite, barite, and chert. Not
infrequently they are spread out both in sheets and in disseminated form
along carbonaceous layers within or at the base of the limestone.
The source of the primary sulphides has been a subject of much
discussion. All are agreed that they were first deposited with the
sediments in minutely dispersed form, through the agency of the organic
contents of the sediments, and that such deposition was somewhat
generally localized by estuarine conditions which favored the
accumulation of organic remains. Many years ago, before the evidence of
estuarine deposition was recognized, Chamberlin suggested an ingenious
hypothesis for the northern Mississippi Valley,--that the organic
material had been localized by ocean's currents forming something in the
nature of a Sargasso sea. Differences of opinion become acute, however,
when the attempt is made to name the precise sedimentary horizon, out of
several available horizons, in which for the most part this primary
concentration occurred. Judging from the organic contents of the several
beds, the primary source may have been below, within, or above the
present ore-bearing horizons. If the ore came from the lower horizons,
it was introduced into its present situation by an artesian circulation,
for which the structural conditions are favorable. If the ore was
derived from overlying horizons, downward moving solutions accompanying
erosion did the work. If the primary source was within the horizon of
present occurrence of the ores, both upward and downward moving waters
may have modified and transported them locally. For each of these
hypotheses a plausible case can be made; but much of the evidence can be
used interchangeably for any one of them. In spite of the wealth of data
available, it is astonishingly difficult to arrive at a conclusion which
is exclusive of other possibilities. Without attempting to argue the
matter in detail the writer merely records his view, based on some
familiarity with these districts, that, on the whole, the evidence
favors the accumulation of these deposits by downward moving meteoric
solutions during the weathering of overlying strata; but that it is by
no means certain that a part of the ores has not been derived from lower
horizons. The great area of the producing districts in comparison with
their depth, the uniform association of the ore-bearing zone with the
surface regardless of geologic horizon uncovered by erosion, the failure
of the ores to extend in quantity under cappings of later formations,
and the known efficacy of oxidizing waters in local downward transfers
of zinc and lead, seem to suggest concentrating agencies which are
clearly related to surface conditions.
It is of interest to note that in many places in the limestones of
Missouri and Virginia, and elsewhere in the Paleozoic rocks, there are
sinks of limonite and clay near the surface, which are likewise believed
to have originated through downward movement of waters deriving their
mineral contents from the erosion and stripping of overlying sediments.
Still further, the primary deposition of Clinton iron ores in many parts
of the Mississippi Valley and eastward to the Appalachians took place in
stratigraphic horizons not far removed from the horizons of lead and
zinc deposition. When the peculiar conditions controlling the deposition
of the Clinton ores are understood (see pp. 52-53) it is entirely
possible that they may throw some light on the genesis of the lead and
zinc ores.
Since the ores were introduced into essentially their present locations,
secondary concentration has produced an oxide zone of clay, chert, and
iron oxide, with varying amounts of zinc carbonate, zinc silicate, lead
sulphide, and rarely lead carbonate. This zone is obviously developed
above water level, and is seldom as much as 100 feet thick. Zinc, and to
a less extent lead, have been taken into solution as sulphates, with the
aid of sulphuric acid resulting from the oxidation of the associated
pyrite. Zinc has been carried away from the weathered zone in solution
faster than lead, leaving the lead more or less concentrated near the
surface. Some of the zinc carried down has been redeposited secondarily
as zinc sulphide. Evidences of this secondary sulphide enrichment can be
seen in many places; yet certain broad quantitative considerations raise
a doubt as to whether this process has been responsible for the main
portion of the values of the sulphide zone. If downward secondary
enrichment had been a dominant process, it might be expected that the
ores would be richer in places where erosion had cut away more than
half the limestone formation carrying the ore, than in places where it
had barely cut into the formation. This is not the fact,--which suggests
that erosion in its downward progress has carried a large part of the
zinc completely out of the vicinity.
Zinc ores of this same general character are also found in Paleozoic
rocks (Knox dolomite) in Virginia and Tennessee. Their manner of
occurrence suggests the same problem of origin as in Missouri and
Wisconsin, but no decisive evidence of their source has been discovered.
Of the zinc ores associated with igneous intrusions, those of the Butte
and Coeur d'Alene districts are described in connection with copper
and lead ores on pp. 201-203, 208, and 212-213.
Zinc constitutes about 75 per cent by weight of the recoverable metals
of the Leadville district of Colorado. About two-thirds of the zinc
occurs as the sulphide and about one-third as the carbonate resulting
from weathering of the sulphide. The zinc sulphide is associated with
lead, iron, and copper sulphides and gold and silver minerals. In the
oxide zone the zinc carbonate is associated with oxides and carbonates
of various metals, including those of lead, copper, iron, and manganese.
The iron and manganese oxides are mined in considerable tonnage as a
flux. It is an interesting fact that, although mining has been carried
on in this district for upwards of forty years, only within the last
decade has the existence of zinc ores in the oxide zone been recognized.
This has been due largely to the fact that the iron and manganese oxides
effectively stain and mask the zinc carbonate.
The Leadville ores occur as replacements and vein-fillings in a gently
faulted and folded Carboniferous limestone, in deposits of a general
tabular shape, parallel to the bedding but with very irregular lower
surfaces. The limestone is intruded by numerous sheets of porphyry,
mainly parallel to the bedding but sometimes cutting across it, against
the under sides of which most of the ore occurs. The primary sulphides
are believed to be genetically related in some fashion to these
porphyries. The older view was that the agents of deposition were
aqueous solutions from the surface above, which derived their mineral
content chiefly from the porphyries. Later views favor solutions coming
directly from the porphyries or deeper igneous sources. While in form
and association these ores are characteristic igneous contact deposits,
they lack the high-temperature silicates which are so distinctive of
many ores of this type.
The zinc ores of Franklin Furnace, New Jersey, belong in the group
associated with igneous agencies, but have certain unique features. They
consist of willemite and zincite, together with large amounts of
franklinite (an iron-manganese oxide) and silicates, in a pre-Cambrian
white crystalline limestone near its contact with a coarse-grained
granite-gneiss. The origin of the ores is obscured by later shearing and
metamorphism, but it seems best explained by replacement of the
limestone by heated solutions coming from the granitic mass. The view
has also been advanced that the ores originated in the limestone before
the advent of the igneous rocks. Secondary concentration is not
apparent.
FOOTNOTES:
[33] Ransome, Frederick Leslie, The copper deposits of Ray and Miami,
Arizona: _Prof. Paper 115, U.S. Geol. Survey_, 1919, pp. 12-13.
CHAPTER XI
GOLD, SILVER, AND PLATINUM MINERALS