ECONOMIC FEATURES
The principal and most essential use of gold is as a standard of value
and a medium of exchange. Gold has been prized since the earliest times
because of its luster, color, malleability, and indestructibility, and
has long been used as a trading medium. At present little of the metal
is actually circulated from hand to hand. Stocks of gold, however,
accumulated by governments and banking interests, form the essential
foundation of paper currency and of the vast modern system of credit
relations. In the settlement of international trade balances
considerable quantities of gold frequently move from debtor to creditor
nations. Although the amounts thus shipped are frequently great in
value, they are very small in volume. It is interesting to note that the
entire accumulated gold stocks of the world's governments--about nine
billion dollars--cast in a solid block, with the horizontal dimensions
of the Washington monument, would be only about 12 feet high.
Other uses of gold are in dentistry, and in the arts for jewelry,
gilding, and other forms of ornamentation. Consumption for these
purposes has been increasing of late years and now takes a third or more
of the world's annual production. In the United States, before war-time
restrictions were adopted, the consumption for jewelry and similar uses
exceeded the consumption in coinage. Since the war it has exceeded the
total domestic production of gold. An interesting problem for the future
is how an adequate supply of gold is to be distributed between monetary
uses and the arts. The curve of increase in the requirements of the arts
indicates that, unless there is greatly increased production, all the
world's gold will be necessary for the arts in a comparatively few
years. To retain it for monetary purposes would require government
restrictions.
Of all the mineral commodities, gold has played perhaps the most
important and certainly the most romantic part in the world's history.
The "lure of gold" has taken men to the remotest corners of the globe.
It has been the moving force in the settlement and colonization of new
countries, in numerous wars, and in many other strenuous activities of
the human race.
About two-thirds of the annual gold production of the world comes from
the British Empire--from South and West Africa, Australasia, Canada, and
India. A single colony, the Transvaal, produces about 40 per cent of the
world's total. British capital, which seems to have a particular
affinity for investments in gold mines, controls not only the larger
part of the output from the colonies, but also important mines in
Siberia, Mexico, South America, and the United States.
Russia, Mexico, and Japan have small gold production. The chief deposits
of Russia are those of Siberia, which have had an important output and
have apparent great possibilities of increase. Other foreign districts
are numerous and widely scattered, but, with the exception of Colombia
and Korea, no one of them yields 1 per cent of the world's gold.
French interests control about a tenth of the production of the
Transvaal, and minor supplies in Mexico and South America--in all about
6 per cent of the world's production. Germany and Austria control less
than 1 per cent of the total gold production. German interests formerly
had extensive holdings in South Africa and Australia, but during the war
this control was eliminated.
The United States, the second largest gold-producing country, supplies
about 20 per cent of the world's total. Commercially it controls
production of another 5 per cent in foreign countries, chiefly in
Canada, Mexico, South America, and Korea. About one-fourth of the United
States production comes from California. Other producing states in order
of importance are Colorado, Alaska, South Dakota, Nevada, Arizona,
Montana, and Utah. These eight states supply 95 per cent of the
country's output, and most of the remainder is obtained from other
western states.
International movements of gold depend chiefly upon its use in the
settlement of trade balances, and are not governed by the considerations
which control ordinary mineral commodities. Imports and exports vary
with changing foreign trade balances. Large amounts of gold normally go
to London, because Great Britain requires all gold produced in the
colonies to be sent to England; but since England ordinarily has an
unfavorable balance of trade, much of this gold is reexported. The
United States up to a few years ago was also a debtor nation, and more
gold was exported than was imported. During the war, however, this
country became the greatest of the creditor nations and imports of gold,
chiefly from Europe, were several times the exports.
The total world's gold production up to 1920 has been upwards of 19
billions of dollars, of which about 10 billions have gone into the arts
or been hidden and lost, leaving 9 billions in monetary reserve.
At the present writing the United States government holds an unusually
large fraction of the world's gold reserve, about 28 per cent or 2
billion dollars,--an amount equal to two-thirds of the aggregate
production of the United States to date. Other large stocks of gold are
held, in order, by Great Britain, France, and Russia, these three with
the United States holding over a half of the world's total gold reserve.
Germany has about 1-1/2 per cent of the total reserve, and, with its
tremendous debt and no sources of new production, is of course in a
particularly unfavorable position.
The total amount of gold now (1920) accounted for by governments as
money is not more than 10 per cent of the value of the notes and
currency issued against this gold. Before the war it was 60 per cent. In
the United States the pre-war percentage was 99-1/2 per cent. Since the
war it has been 45 per cent. The ratio of gold to currency is now so
small that the gold standard is hardly a physical fact, but is to be
regarded rather more as a profession of faith. Notwithstanding the
recent falling off in gold production, an increment of approximately 350
million dollars is potentially available each year to be added to the
gold reserves. Whether this increment, or a larger increment which may
come from new discoveries, is sufficient to maintain a reasonable
proportion between gold stocks and the necessary normal increase in
paper currency, has been, and doubtless will continue to be, a subject
of vigorous discussion and speculation.
During and immediately following the war, the gold production of the
world showed rather an alarming progressive decrease. About 1915 the
group of three greatest producers--South Africa, United States, and
Australia,--reached the acme of its production, and output then fell
off. Simultaneously there was a marked decrease of production in many of
the less important districts. This general decline was due in
considerable part to the fact that during the war the price of gold was
fixed and its use restricted to monetary purposes. The price of gold,
which is itself the standard of value, could not rise to offset growing
mining costs and to maintain profits, as was the case with iron, copper,
and the other metals,--with the result that the margin of profit in gold
mining became so small as materially to affect exploration and
production. Another important cause of decreased production was the
actual exhaustion of certain mines, and the lowering of the grades of
ore available in many others. New discoveries did not supply these
deficiencies. In the United States, for instance, physical conditions of
one kind or another were responsible for lessening of production from
Alaska, Cripple Creek, and California. Minor causes included conflicts
in California between agricultural and mining interests over water
rights, and a succession of dry seasons which did not afford enough
water for the working of placers; and in Alaska difficulties due to
litigation over the oil-flotation process of recovering gold from its
ores. As a result of all these conditions, many of the smaller mines
were closed down, others continued operations only by curtailing
exploration and by mining solely the richest and most accessible ore
bodies, and there was a general discouragement and lack of inducement to
engage in gold mining.
The gold situation has become a matter of great concern to the various
governments, since national financial stability and the confidence of
the public in the national credit are based largely upon the acquirement
of an adequate gold reserve. Both in England and in the United States,
committees of experts have been appointed to make exhaustive
investigations and present recommendations for measures to stimulate
production. The report of the joint committee from the United States
Bureau of Mines and Geological Survey gives a comprehensive review of
the conditions in the gold-mining industry.[34]
During the war there was vigorous demand by gold miners both in the
United States and South Africa for a bonus on gold. These demands
received serious consideration on the part of the governments, but were
denied on the general ground of the doubtful adequacy of such a measure
to meet the situation, and the danger of upsetting the gold standard of
value. In the United States, for instance, a bonus of $10 per ounce was
asked for. It did not appear likely that this could increase the annual
production from the United States by more than 10 per cent, in face of
the physical conditions being met in gold mining. The bonus would have
had to be paid on all the gold mined, which would make the increment of
production very expensive; to secure an added production of ten million
dollars would have cost in the neighborhood of forty millions. Ten
millions is only one-third of 1 per cent of the gold reserve already
held by this country, and it would obviously have taken a long time for
this small increase in annual production to make itself felt in the size
of the gold reserves.
Since the war gold has gone to a considerable premium in England, due to
the action of the British government in establishing a "free"
market,--that is, abandoning the restriction that gold marketed in
London should be offered to the government or the Bank of England at the
fixed statutory price for monetary purposes. With the pound sterling at
a considerable discount outside of England, other countries could afford
to bid, in terms of British currency, far above the British mint price.
The result is that the South African miner of gold receives a premium
due to depreciation of sterling exchange, while the American miner still
receives the regular mint price. The agitation for a bonus therefore
continues in the United States. However, with the removal of war-time
restrictions gold has been allowed to go to the arts, the demand from
which is already equal to one-third of the world's gold production, is
rapidly increasing, and is temporarily acute due to the accumulation of
requirements resulting from war restrictions. This situation has a
general tendency to improve the position of the gold miner, though the
outlook is still far from bright.
It is an interesting fact that India is absorbing a good half of the
free gold. India, in regard to its demand for precious metals and
stones, has been described as "an abyss from which there is no return."
This is an important contributing cause of the shortage of gold in the
rest of the world.
Looking forward to the future, it seems that increased exploration,
which is resulting from the present premium on gold, is likely to bring
in new reserves to increase production. Because of lack of important
discoveries in recent years, there is pessimism in some quarters as to
the possibilities of large increase of production; but, considering the
history of gold discoveries, and the amount of ground still to be
explored both areally and vertically, this pessimism does not seem to be
wholly justified from the geologic standpoint. Curves representing the
world's gold production in past years show periods of increasing annual
production as new fields are discovered, followed by periods of
decreasing production when no new ore bodies are coming in to replace
dwindling reserves. It is entirely possible that in recent years the
gold-mining industry has been merely in one of these temporary stagnant
periods. There are many regions, both in the vicinity of worked-out
lodes and in unsettled and poorly explored countries, where gold may
still be discovered; there may be far greater resources of this metal
still covered up than all those which man has thus far uncovered. A
single new deposit or district may make a great difference in the
world's production, as suggested by the experience of the past. Regions
which are especially attractive for exploration and the discovery of new
deposits are in Siberia and South America, which in the opinion of many
engineers may eventually rival South Africa. Mexico, with the
establishment of a stable government, should also have a greatly
increased production.
GEOLOGIC FEATURES
The principal gold mineral is native or metallic gold. This occurs in
nature in small scales, crystals, and irregular masses, and also in
microscopic particles mechanically mixed with pyrite and other
sulphides. Chemically, gold is very inactive and combines with but few
other elements. A small part of the world's supply is obtained from the
gold-silver tellurides--calaverite, sylvanite, krennerite, and petzite.
Gold deposits are of two general classes--placers, and veins or lodes.
Placers, which are in general the more easily discovered and more easily
worked deposits, have in the past been the chief source of the world's
gold supply. It is estimated that in the first twenty-seven years of the
modern era of gold-mining, beginning with the discovery of gold in
California in 1848, 87 per cent of the world's production was obtained
from placers. At present the placers of recent geologic age supply a
tenth to a fifth of the gold, and ancient or fossil placers in the
Transvaal supply another two-fifths. In the United States about a fourth
of the gold production comes from placers, mainly from California and
Alaska.
Placers are detrital or fragmental sediments containing the ore in
mechanical fragments, which are derived from the erosion and
transportation of solid-rock veins or lodes, sometimes called the
"mother lode." During the process of transportation and deposition there
is more or less sorting, because of differing density of the mineral
fragments, resulting in the segregation or concentration of the ore
minerals in certain layers or channels. Gold, because of its weight,
tends to work down toward bedrock, or into scoured or excavated portions
of stream channels. In a few cases it is carried in some quantity to the
sea and concentrated in beach sands. The processes are not unlike the
mechanical concentration of ores by crushing and water sorting. Seldom,
however, do the processes go far enough in nature to produce an ore
which can be used directly without some further mechanical sorting. Ore
minerals concentrated in placers are those which resist abrasion and
chemical solution during the processes of weathering and transportation,
and which have a density sufficiently high so that they are partially
sorted out and concentrated from the accompanying quartz and other
minerals. To warrant their recovery they must also be of such high
intrinsic value that it pays to mine small quantities. The most
important of such minerals are gold, tin, platinum, and the precious
stones. Iron, copper, lead, and zinc minerals are often somewhat
concentrated as placers, but their intrinsic value is not high enough to
warrant the attempt to recover them in the large amounts necessary to
make them commercially available.
Placers are forming now and have formed at all stages of the earth's
history. Early placers may be reworked and further concentrated by
renewal of the proper erosional and transportational conditions. Old
placers may be buried beneath younger rocks, cemented, and more or less
recrystallized. "Fossil" placers of this kind are best represented by
deposits in the Black Hills of South Dakota and probably by the South
African gold deposits.
In the Witwatersrand deposits of South Africa, the gold is concentrated
in the lower parts of large conglomerate and quartz sand layers of great
areal extent. Pebbles of the conglomerate are mainly quartz and
quartzite. The gold, in particles hardly visible to the eye, is in a
sandy matrix and is associated with chloritoid, sericite, calcite,
graphite, and other minerals. The origin of the gold deposits of this
district is not entirely agreed on, but the evidence seems on the whole
to favor their placer origin. Some investigators of these ores believe
them to have been introduced into the conglomerate and sand by later
solutions, possibly by hot solutions related to certain diabase
intrusions that cut the beds.
In the vein or lode or hard-rock deposits, the gold is mainly metallic
gold, and to a minor extent is in the form of gold tellurides. It is
usually closely associated with iron pyrite in a matrix or gangue of
quartz. Seldom is a gold deposit free from important values in other
minerals. About 84 per cent of the gold mined in vein or lode deposits
of the United States is associated with silver minerals, the combined
value averaging about $6 per ton; about 13 per cent comes from copper
ores which have an average yield of gold and silver of 50c. per ton; and
3 per cent comes from zinc and lead ores, with an average gold and
silver yield ranging from $1 to $6 per ton. The geologic occurrence of
gold in the copper, lead, and zinc ores has already been referred to in
the discussions of these ores.
Reference will be made here only to the vein deposits in which gold,
with silver, constitutes the principal values. Because of their common
gangue of quartz these are often called "dry" or "siliceous" ores. Their
principal occurrence is in distinct fissure veins in igneous rocks, with
more or less replacement of the wall rock. The igneous rocks are
commonly acid intrusives of a granite or porphyry type, less commonly
intrusives of gabbro and diabase and surface lavas of rhyolite and
basalt. In a few cases the ores are contact-metamorphic deposits of the
type described under copper ores. In still rarer cases they are in
pegmatites. Gold is commonly associated with minerals and wall-rock
alterations indicating deposition by hot solutions, which are inferred
to have come from the igneous rocks.
Because of the resistant nature both of ore minerals and gangue,
weathering and secondary concentration have had little effect in
enriching gold deposits. So far as there has been any noticeable effect
on the gold content of the ores, it has been due to the leaching out of
other constituents, principally pyrite and other sulphides, leaving the
gold present in slightly larger proportions. Locally there is evidence
of solution of gold in weathered zones and its deposition in the
sulphide zones below. Solution is believed to be accomplished by
chloride solutions, and is favored by the presence of manganese which
delays precipitation. The precipitating agent below may be ferrous
sulphate, various sulphides, native metals, or organic matter.
Of the vein or lode gold ores in the United States some of the most
productive and best known have the following geologic features:
The California gold belt extends north and south along the west slope of
the Sierra Nevada Mountains. The ore is in a series of parallel and
overlapping veins striking with the trend of the range, associated with
granodiorite intrusives in schist and slate. There is no pronounced
secondary concentration. These deposits are the source of most of the
great placer deposits of California, hence the name "Mother Lode"
applied to a part of them. The principal ore deposits are somewhat
removed from the main mass of intrusive which forms the crest of the
Sierra Nevada range, and are more closely related to the smaller similar
intrusive masses farther down the slope. The gangue is mainly quartz.
At Juneau, Alaska, great dikes of albite-diorite intrude greenstones and
schists, and low-grade gold ores occur in shattered portions of the
diorite. These ores were mined on a great scale at the Treadwell Mine.
Another famous low-grade deposit is the Homestake Mine in the Black
Hills of South Dakota, where pre-Cambrian slates and schists of
sedimentary origin are impregnated with gold, associated with quartz,
dolomite, calcite, pyrite, and other minerals. The origin is supposed to
have some connection with intrusives into the schists; but the relations
of the ores to intrusives, both in age and in place, present many
puzzling questions which make conclusion as to origin very difficult.
In the Cripple Creek district of Colorado, a volcanic neck two or three
miles in diameter breaks through pre-Cambrian granites, gneisses, and
schists. The volcanic rocks consist mainly of tuffs and breccias cut by
basic dikes. The ore bodies are in fissures and sheeted zones,
principally in the granitic rocks, but associated with these dikes. The
ore is mainly gold telluride, in a gangue of quartz together with pyrite
and a variety of minerals characteristic of hot-water solutions. Also
the wall rocks have the characteristic hot-water alterations. There is
slight enrichment near the surface.
At Goldfield, Nevada, native gold is found in surface igneous flows of a
dacite type, which have undergone extensive hydrothermal alterations
characterized by the development of alunite (a potassium-aluminum
sulphate), quartz, and pyrite. The ore fills fissures to some extent,
but is mainly a replacement of the wall rock. Association with typical
hot-water minerals and hydrothermal alterations of the wall rock are
again believed to indicate the origin of the ores through ascending hot
solutions from a deep source.
One of the interesting features of this occurrence is the abundance of
alunite. Sulphate minerals are commonly formed by oxidizing solutions.
The abundant presence, therefore, of a sulphate mineral with minerals of
a primary deep-seated source has led to much discussion of origin. The
hypothesis was developed that these minerals result from the interaction
of deep-seated sulphide-bearing solutions with surface oxidizing
solutions.[35] It may be noted that in recent years other sulphate
minerals have been occasionally regarded as primary, including gypsum,
anhydrite, barite, and others. It has been suggested that if igneous
emanations contain free oxygen and sulphur, or sulphur dioxide, it would
be expected that as they become cool sulphur trioxide would be formed
which would result in the sulphate at suitable temperature.[36]
Other deposits containing gold are discussed in connection with silver
on following pages.
SILVER ORES
ECONOMIC FEATURES
Silver has two important uses--in money and in the arts. As money, it is
used in the United States and Europe for subsidiary coinage,--silver
coins normally circulating at more than their intrinsic value,--but its
greatest monetary use is in India and China, where it has been the basis
for the settlement of foreign exchange balances. In China also it is the
money standard of the country. In the arts, silver is employed chiefly
in the making of articles of luxury, such as jewelry and tableware. In
the Orient this use is closely related to its use as money, since the
natives invest their savings both in silver jewelry and silver coins.
There is some consumption of silver by certain chemical industries, and
quantities of increasing importance are used in the form of silver salts
by the photographic and moving picture industries. It has been estimated
that before 1914 about two-thirds of the new silver produced went into
the arts and one-third into money. During the war, however, increasing
amounts were used in coinage, and less than one-fifth of the output was
used in the arts. Demands for silver for monetary purposes will probably
continue to take the larger part of the world's production for some
time. In this connection it may be noted that India has adopted a gold
standard, but that the conservative habits of the population will
doubtless continue to call for large amounts of silver.
About half of the silver production of the world comes from the dry or
siliceous silver ores, which are mined solely for that metal and the
associated gold; and about half of the production is obtained as a
by-product in the mining of other metals, principally copper and lead.
The average grades of these ores, in combined values of gold and silver,
were referred to on p. 228. While the aggregate amount of silver
obtained as a by-product of other ores is large, the percentage of
silver in the copper or lead in any mine is ordinarily very small.
Consequently the world output of silver depends to a considerable extent
upon conditions in the copper- and lead-mining industries.
Of the total world output of silver, normally about 75 per cent comes
from North America. Of this the United States and Mexico each produce
about two-fifths and Canada one-fifth, and minor amounts are produced in
Central America. In late years, political disturbances in Mexico reduced
that country's production to less than half the normal figure, and the
United States took the place which Mexico had held for many years as the
leading silver producer. The United States and Mexican supply is
obtained from the Rocky Mountain belt, and the Canadian production comes
chiefly from the Cobalt, Ontario, district. Outside of North America the
principal producing areas are Australia, South America (Peru and to a
less extent Bolivia and Chile), Europe (chiefly from Spain, Germany, and
Austria-Hungary, but with smaller amounts from all the other countries),
and Japan. Thus, while there are sources of silver in many places, the
great bulk of the world's output comes from North America. In the
financial ownership of mines, including ownership in other countries,
the United States controls over half the world's silver, Great Britain
about a third, and Germany about a tenth (principally in Mexico).
All the silver mined in the United States is smelted and refined by
domestic plants; and in addition much of the Canadian, Mexican, and
South and Central American silver is exported to the United States as
ore and base bullion, to be treated in this country. The United States
is therefore the great silver-selling country of the world.
The great silver-consuming countries are India and China, and normally
about a half of the world's output goes to these two countries. This
major movement of silver, from America to the Far East, takes place
through the London market, since England has been the chief nation
trading in the Orient. The balance of the world's silver consumption is
widely distributed among the countries of Europe and South America and
the United States (which consumes about one-tenth of the total). For the
European trade most of the silver also goes through London, which is the
great clearing-house and the market where prices are fixed.
In the later years of the war and immediately after, the demands for
silver were probably twice the world's output. The resulting rise in
price was unprecedented. Silver actually became worth more as bullion
than as currency, and in Europe much trouble was experienced because of
its withdrawal from currency to be melted up. This condition was later
followed by an equally striking drop in price as supply caught up with
demand.
In the United States, as in many other countries, it was desired during
the war to accumulate large stocks of gold as a basis of credit for the
flotation of government loans, and the export of gold was prohibited.
Consequently in the settlement of foreign trade balances, particularly
with the nations of the Orient, very large amounts of silver bullion had
to be used. Current production proved inadequate, and it was necessary
to utilize the stocks of silver dollars in the United States Treasury.
To this end the Pittman Silver Act, passed in April, 1918, authorized
the melting down and conversion into bullion of 350,000,000 dollars out
of the Treasury stock, and the retirement of a corresponding number of
silver certificates and the issue of Federal Reserve bank notes. In this
manner old stocks of silver, Manila dollars, etc., were called into
service--though the stage was not reached, as it was in Germany, where
it became necessary to melt down silver plate and ornaments. The silver
used for exchange and export was to be replaced by the purchase of
bullion from American producers at $1 per ounce, and its coining into
new dollars. A minimum price of $1 per ounce was thus established for
silver bullion.
The immediate result was to increase the price of silver at the mine;
but with the continued rise in demands for silver, the price in the open
market went far above this figure, the maximum being reached in 1920
when the price of silver went to $1.39 per ounce. Naturally, but little
silver was then offered to the government at the fixed price of $1 under
the Pittman Act. With the more recent slump in the general market for
silver to a price below $1, offers to the government under the Pittman
Act have been renewed.
That part of the silver production which is a by-product of copper
production has been low since the war, because of the stagnation in the
copper industry. The production from lead ores, on the other hand, was
not handicapped by lack of demand for lead. With the restoration of
order in Mexico, a presumption of large silver production in that
country may be expected. Increases may probably be expected also from
new mines in Burma and from Bolivia. On the whole, no large increase in
world production can be assumed from present known resources. New
discoveries will be necessary to make any considerable change.
Of the mine production of silver in the United States, about two-thirds
of the total comes from the states of Montana, Utah, Idaho, and Nevada.
Other considerable producers are Colorado, Arizona, California, Alaska,
and New Mexico. All the other states together produce less than 5 per
cent of the total. The most important single districts are the Butte
district of Montana, the Coeur d'Alene district of Idaho, and the
Tonopah district of Nevada, supplying respectively about one-fifth,
one-eighth, and one-tenth of the country's total silver output.
GEOLOGIC FEATURES
The most important mineral of silver is the sulphide, argentite or
"silver glance." Other minerals which yield a minor percentage of the
total silver produced are the silver-antimony sulphides, pyrargyrite or
"ruby silver," stephanite or "black silver," and polybasite; the
silver-arsenic sulphides, proustite or "light ruby silver" and pearcite;
and the silver antimonide, dyscrasite. In the oxide zone the most
abundant minerals are cerargyrite (silver chloride) and native or "horn"
silver. In addition to these definite mineral forms, silver is present
in many ores in an undetermined form in other sulphides, notably in
galena, sphalerite, and pyrite. Silver differs from gold in that it is
chemically active and forms many stable compounds, of which only the
more important have been mentioned.
The fact that half the world's silver is obtained as a by-product in the
mining of other metals has been referred to. In the United States about
a third of the production comes from dry or siliceous ores, over a third
from lead and zinc ores, and a fourth to a third from copper ores. A
fraction of 1 per cent of the total is obtained as a by-product of gold
placers, and all the remainder is won from lode or hard-rock deposits.
The general geologic features of the silver-bearing copper and lead
ores, and of the dry or siliceous gold and silver ores, have been
described on previous pages. The Philipsburg district has been referred
to in connection with manganese ores, and the Bolivian tin-silver ores
will be described in connection with tin. We shall consider here only a
few of the more prominent districts which have been primarily silver
producers.
The Cobalt district of northern Ontario is the most productive silver
district in North America. The ores are found in numerous short, narrow
veins, principally in pre-Cambrian sediments near a thick quartz-diabase
sill. Locally they penetrate the sill. Native silver and various silver
sulphides, arsenides, and antimonides are associated with minerals of
cobalt, nickel, bismuth, lead, and zinc, in a gangue of calcite and some
quartz. The ore is of very high grade. The ore minerals are believed to
have been deposited by hot solutions emanating from deep magmatic
sources after the intrusion of the diabase. The present oxidized zone is
very shallow, but may have been deeper before being stripped off by
glaciation; it is characterized by native silver and arsenates of nickel
and cobalt in the form of the green "nickel bloom" and the pink "cobalt
bloom." The silver minerals are distinctly later in origin than the
cobalt and nickel in the unoxidized zone, as evidenced by the relations
of the mineral individuals when seen under the microscope. This fact,
together with the abundance of native silver in the oxide zone, has
suggested downward concentration of the silver by surface waters; but
recent studies have indicated the probability that some of the silver at
least was deposited by the later ascending solutions of magmatic origin.
In the Tintic district of central Utah, Paleozoic limestones have been
intruded by monzonite (an acid granitic or porphyritic igneous rock),
and covered by surface flows, the flows for the most part having been
removed by subsequent erosion. The sediments have been much folded and
faulted, and the ore bodies occur as fissure veins which locally widen
into chimneys or pipes in fracture zones, accompanied by much
replacement of limestone. There is a rough zonal arrangement of the ore
minerals around the intrusive, gold and copper minerals (chiefly
enargite and chalcopyrite) being more prominent near the intrusive, and
argentiferous galena and zinc blende richer at greater distances. Silver
constitutes the principal value. The gangue is mainly fine-grained
quartz or jasperoid, and barite. The water table is at unusually great
depths (2,400 feet) and there is a correspondingly deep oxidized zone,
which is characterized by lead and zinc oxide minerals much as at
Leadville (p. 219).
The Comstock Lode at Virginia City, Nevada, on the east slope of the
Sierra Nevadas, was one of the most famous bonanza deposits of gold and
silver in the world. While the richer ore has all been extracted,
lower-grade material is still being mined and the fissure is still being
followed, in the hope of some day striking another fabulously rich ore
body. The lode occupies a fault fissure parallel to the trend of the
range and dipping about 40 degrees to the east, which can be traced
about two and a half miles along the strike, with igneous rocks forming
both hanging and foot walls. There are no sedimentary rocks in the
district. The high-grade part of the vein is several hundred feet in
thickness, with many irregular branches; the great thickness has been
thought to be at least in part due to the tremendous pressure exerted by
growing quartz crystals. The wall rocks have undergone a "propylitic"
alteration, with development of chlorite, epidote, and probably
sericite, much as at Butte. The ore contains rich silver sulphide
minerals and native gold, in a gangue composed almost entirely of
quartz. The ore was doubtless formed by hot solutions, but the exact
nature of these solutions, whether magmatic or meteoric, has not been
proven. The hypothesis was early developed that the ores were deposited
by surface waters,--which are supposed to have fallen on the summits of
the Sierra Nevadas, to have sunk to great depths where they were heated,
enabling them to pick up metallic constituents from the diabase forming
one wall of the ore body, and to have risen under artesian pressure
along the fault plane, where loss of heat and pressure resulted in
deposition. Later studies have emphasized the similarity of the
ore-depositing conditions with those in other districts where the ores
are believed to have come directly from magmatic sources, and this
origin is now generally favored for the Comstock Lode. However, the
earlier theory has not been disproved.
The Tonopah, Nevada, district is very similar to the Goldfield district
(p. 230). Silver and gold are found in veins and replacements in a
series of Tertiary volcanic flows and tuffs, all of which have been
complexly faulted. Silver is the dominant constituent of value. The
formation of fissures and faults accompanying and caused by the
intrusion and cooling of lavas was first clearly shown in this district.
Evidences of origin through the work of hot solutions, probably
magmatic, are the close association of the ores in place and in time
with the igneous rocks--ore deposition in most of the flows having taken
place before the next overlying flows were put down,--the presence of
fluorine, the nature of the wall-rock alterations, the fact that both
hot and cold springs are found close together underground (indicating
unusual sources for the hot springs), the contrast in composition
between the ores and the country rock, and the general relation of these
ores to a large number of similar occurrences in Tertiary lavas in the
same general area.
Under weathering conditions, the silver sulphide minerals in general are
oxidized to form native silver and cerargyrite, which are relatively
insoluble and remain for the most part in the oxide zone. Silver is less
soluble than copper and zinc, but more soluble than gold; and to some
extent it is removed in solution, particularly where the oxidation of
pyrite forms ferric sulphate. Farther down it may be reprecipitated as
native silver, argentite, and the sulpho-salts, by organic matter or by
various sulphides. The secondarily enriched ores are in a few districts,
as at Philipsburg, Montana, the most valuable portions of the deposits.
In other cases, sulphide enrichment does not appear to have contributed
greatly to the values. The zones of oxide ores, secondary sulphide ores,
and primary or protores are in most silver deposits much less regular
and much less definitely marked than in the case of copper ores.
PLATINUM ORES
ECONOMIC FEATURES
The principal uses of platinum are: as a catalytic agent in the contact
process for the manufacture of sulphuric acid, and in the making of
nitric acid from ammonia; for chemical laboratory utensils that must be
resistant to heat and acids; for electrical contacts for certain
telephone, telegraph, and electrical control instruments, and for
internal combustion engines; in dental work; and for jewelry. In normal
times before the war, it is estimated that in the United States the
jewelry and dental industries used 75 per cent of the platinum metals
consumed, the electrical industry 20 per cent, and the chemical industry
5 per cent. During the war, with the extraordinary expansion of
sulphuric and nitric acid plants, these proportions were reversed and
the chemical and electrical industries consumed about two-thirds of the
platinum. Substitutes have been developed, particularly for the
electrical uses, and the demand from this quarter may be expected to
decrease.
About 90 per cent of the world's crude platinum produced annually comes
from the Ural Mountains in Russia. The deposits next in importance are
those of Colombia. Small amounts are produced in New South Wales,
Tasmania, New Zealand, Borneo, British Columbia, United States, India,
and Spain; and as a by-product in the electrolytic refining of the
Sudbury, Canada, nickel ores. The extension of this method of refining
to all of the Sudbury ores would create an important supply of platinum.
The Colombian output has been increasing rapidly since 1911. Meanwhile
the Russian production has declined; and from the best information
available, it is not likely that Russia will be able to maintain
production for many more years. Estimates of the life of the Russian
fields are from 12 to 20 years at the pre-war rate of production.
The platinum situation is commercially controlled by buying and
mine-operating agencies,--the French having, before the war, practically
dominated the Russian industry, while American interests controlled in
Colombia. The situation is further influenced by four large refineries,
in England, Germany, United States, and France.
Before the war the United States produced less than 1 per cent of the
new platinum it consumed annually. Production comes principally from
California, with smaller amounts from Oregon, Alaska, and Nevada. The
many efforts which have been made to develop an adequate domestic supply
of this metal do not indicate that the United States can ever hope to
become independent of foreign sources for its future supplies of
platinum.
There is little reason to doubt that the Colombia field, commercially
dominated by the United States, holds great promise for the future. The
output has come largely from native hand labor, and with the
installation of dredges can probably be greatly increased.
During the war, the need for platinum for war manufactures was so urgent
and the production so reduced, that restrictions against its use in
jewelry were put into force in all the allied countries. The United
States government secured quantities of platinum which would have been
sufficient for several years' use if war had continued. With the
cessation of hostilities restrictions on the use of platinum were
removed, and the accumulated metal was released by the government from
time to time in small quantities; but the demands for platinum in the
arts were so great that prices for a time tended to even higher levels
than during the war. More recently supply is again approaching demand.
GEOLOGIC FEATURES
Platinum, like gold, occurs chiefly as the native metal. This is usually
found alloyed with iron and with other metals of the platinum group,
especially iridium, rhodium, and palladium. Most of the platinum as used
in jewelry and for electrical purposes contains iridium, which serves to
harden it. Paladium-gold alloys are a substitute for platinum, chiefly
in dental uses.
The original home of platinum is in basic igneous rocks, such as
peridotites, pyroxenites, and dunites, where it has been found in small,
scattered crystals intergrown with olivine, pyroxene, and chromite.
Platinum is very dense and highly resistant to oxidation and solution.
In the breaking up and washing away of the rocks, therefore, it is
concentrated in small grains and scales in stream and beach placers. Of
the world production of platinum over 99 per cent has been derived from
placers.
The Ural Mountain deposits of Russia are gold- and platinum-bearing
placers, in streams which drain areas of dunite rock containing minute
quantities of native platinum. The deposits of Colombia and Australasia
are placers of a similar character. In the United States small
quantities of platinum are recovered from the gold-bearing gravels of
California and Oregon, where the streams have come from areas of
serpentine and peridotite.
A platinum arsenide, called sperrylite, is sometimes found associated
with sulphide minerals in basic igneous rocks. At Sudbury, Ontario, this
mineral, together with palladium arsenide, is found in the nickel ores,
especially in the weathered zone where it is concentrated by removal of
more soluble materials. It has also been found in the copper mines of
Rambler, Wyoming. In the Yellow Pine district of southern Nevada,
metallic gold-platinum-palladium ore shoots are found in association
with copper and lead ores, in a fine-grained quartz mass which replaces
beds of limestone near a granitic dike. No basic intrusives are known
in the district. The deposit is unusual in that it has a comparatively
high content of platinum (nearly an ounce to the ton), and is probably
genetically related to acid intrusives. From all these deposits, only
small quantities of platinum are mined.
FOOTNOTES:
[34] Report of a joint committee appointed from the Bureau of Mines and
the United States Geological Survey by the Secretary of the Interior to
study the gold situation: _Bull. 144, U. S. Bureau of Mines_, 1919. See
also Report of Special Gold Committee to Secretary of the Treasury,
February 11, 1919.
[35] Ransome, F. L., The geology and ore deposits of Goldfield, Nevada:
_Prof. Paper 66, U.S. Geol. Survey_, 1909, p. 193.
[36] Butler, B. S., Loughlin, G. F., Heikes, V. C., and others, The ore
deposits of Utah: _Prof. Paper 111, U.S. Geol. Survey_, 1920, p. 195.
CHAPTER XII
MISCELLANEOUS METALLIC MINERALS
ALUMINUM ORES
ECONOMIC FEATURES
Bauxite (hydrated aluminum oxide) is the principal ore of aluminum. Over
three-fourths of the world's bauxite production and 65 per cent of the
United States production is used for the manufacture of aluminum. On an
average six tons of bauxite are required to make one ton of metallic
aluminum. Other important uses of bauxite are in the manufacture of
artificial abrasives in the electric furnace, and in the preparation of
alum, aluminum sulphate, and other chemicals which are used for
water-purification, tanning, and dyeing. Relatively small but
increasingly important quantities are used in making bauxite brick or
high alumina refractories for furnace-linings.
Aluminum is used principally in castings and drawn and pressed ware, for
purposes in which lightness, malleability, and unalterability under
ordinary chemical reagents are desired. Thus it is used in parts of
airplane and automobile engines, in household utensils, and recently in
the framework of airplanes. Aluminum wire has been used as a substitute
for copper wire as an electrical conductor. Aluminum is used in
metallurgy to remove oxygen from iron and steel, and also in the
manufacture of alloys. Powdered aluminum is used for the production of
high temperatures in the Thermite process, and is a constituent of the
explosive, ammonal, and of aluminum paints.
Deposits of bauxite usually contain as impurities silica (in the form of
kaolin or hydrous aluminum silicate), iron oxide, and titanium minerals,
in varying proportions. Bauxites to be of commercial grade should carry
at least 50 per cent alumina, and for the making of aluminum should be
low in silica though the content of iron may be fairly high. For
aluminum chemicals materials low in iron and titanium are preferred; and
for refractories which must withstand high temperatures, low iron
content seems to be necessary. The abrasive trade in general uses
low-silica high-iron bauxites.
The only large producers of bauxite are the United States and France,
which supplied in normal times before the war over 95 per cent of the
world's total. Small amounts are produced in Ireland, Italy, India, and
British Guiana. During the war a great deal of low-grade bauxite was
mined in Austria-Hungary and possibly in Germany; but on account of the
large reserves of high-grade material in other parts of the world, it is
doubtful whether these deposits will be utilized in the future. Bauxites
of good grade have been reported from Africa, Australia, and many
localities in India. From geologic considerations it is practically
certain that there are very large quantities available for the future in
some of these regions.
The international movements and the consumption of bauxite are largely
determined by the manufacture of aluminum, and to a lesser extent by the
manufacture of abrasives and chemicals. The principal foreign producers
of aluminum are France, Switzerland (works partly German-owned), Norway
(works controlled by English and French capital), England, Canada,
Italy, Germany, and Austria. French bauxite has normally supplied the
entire European demands,--with the exceptions that Italy procures part
of her requirements at home, and that the Irish deposits furnish a small
fraction of the English demand.
The deposits of southern France, controlled largely by French but in
part by British capital, have large reserves and will probably continue
to meet the bulk of European requirements. France also has important
reserves of bauxite in French Guiana.
The United States produces about half of the aluminum of the world, and
is the largest manufacturer of artificial abrasives and probably of
aluminum chemicals. Most of these are made from domestic bauxite. Prior
to the war, the United States imported about 10 per cent of the bauxite
consumed, but these imports were mainly high-grade French bauxite which
certain makers of chemicals preferred to the domestic material. The
small production of Guiana is also imported into the United States.
Bauxite is exported to Canadian makers of aluminum and abrasives.
During the war period domestic deposits were entirely capable of
supplying all the domestic as well as Canadian demands for bauxite,
although these demands increased to two and one-half times their
previous figure. At the same time considerable amounts of manufactured
aluminum products were exported to Europe, whereas aluminum had
previously been imported from several European countries.
The United States production of bauxite comes mainly from Arkansas, with
smaller amounts from Tennessee, Alabama, and Georgia. The reserves are
large but are not inexhaustible. Most of the important deposits are
controlled by the large consumers of bauxite, principally the Aluminum
Company of America and its subsidiaries, though certain chemical and
abrasive companies own some deposits. The Aluminum Company of America
also controls immense deposits of high-grade bauxite in Dutch and
British Guiana, and further exploration by American interests is under
way.
With the return to normal conditions since the war, some of the domestic
bauxite deposits probably can not be worked at a profit, a situation
which is likely to require the development of the tropical American
deposits.
GEOLOGIC FEATURES
Aluminum is the third most abundant element in the common rocks and is
an important constituent of most rock minerals; but in its usual
occurrence it is so closely locked up in chemical combinations that the
metal cannot be extracted on a commercial scale. In the crystalline form
aluminum oxide constitutes some of the most valuable gem stones. Many
ordinary clays and shales contain 25 to 35 per cent alumina
(Al_{2}O_{3}), and the perfection of a process for their utilization
would make available almost unlimited aluminum supplies. The principal
minerals from which aluminum is recovered today are hydrous aluminum
oxides, the most prominent of which are bauxite, gibbsite, and
diaspore--the aggregate of all these minerals going commercially under
the name of bauxite.
Prior to the discovery of bauxite ores, cryolite, a sodium-aluminum
fluoride obtained from pegmatites in Greenland, was the chief source of
aluminum. It is only within about the last thirty-five years that
bauxite has been used and that aluminum has become an important material
of modern industry. Cryolite is used today to form a molten bath in
which the bauxite is electrolytically reduced to aluminum.
Bauxite deposits in general are formed by the ordinary katamorphic
processes of surface weathering, when acting on the right kind of rocks
and carried to an extreme. In the weathering of ordinary rocks the bases
are leached out and carried away, leaving a porous mass of clay (hydrous
aluminum silicates), quartz, and iron oxide. In the weathering of rocks
high in alumina, and low in iron minerals and quartz, deposits of
residual clay or kaolin nearly free from iron oxide and quartz are
formed. Under ordinary weathering conditions the kaolin is stable; but
under favorable conditions, such as obtain in the weathered zones of
tropical climates, it is broken up, the silica is taken into solution
and carried away, and hydrous aluminum oxides remain as bauxite ores.
This extreme type of weathering is sometimes called lateritic alteration
(see pp. 172-173). Impurities of the bauxite ores are the small
quantities of iron and titanium present in the original rocks, together
with the kaolin which has not been broken up. The deposits usually form
shallow blankets over considerable areas, with irregular lower surfaces
determined by the action of surface waters--which work most effectively
where joints or other conditions favor the maximum circulation and
alteration. A certain degree of porosity in the original rock is also
known to favor the alteration. A complete gradation from the unaltered
rock through clay to the high-grade bauxite, with progressive decrease
in bases and silica, concentration of alumina and iron oxide, and
increase of moisture and pore space, is frequently evident (see Fig.
13). The bauxite is earthy, and usually shows a concretionary or
pisolitic structure similar to that observed in residual iron ores (p.
172). Near the surface there may be an increase in silica,--probably due
to a reversal of the usual conditions by a slight leaching of alumina,
thus concentrating the denser masses of kaolin which have not been
decomposed.
The Arkansas bauxite deposits, the most important in the United States,
are surface deposits overlying nepheline-syenite, an igneous rock with
a high ratio of alumina to iron content. The most valuable deposits are
residual, and some parts have preserved the texture of the original
rock, though with great increase in pore space; most of the ore,
however, has the typical pisolitic structure. Near the surface the
pisolites are sometimes loosened by weathering, yielding a gravel ore,
and some of the material has been transported a short distance to form
detrital ores interstratified with sands and gravels. The complete
gradation from syenite to bauxite has been shown.
[Illustration: FIG. 13. Diagram showing gradation from syenite
to bauxite in terms of volume. The columns represent a series of samples
from a single locality in Arkansas. After Mead.]
In the Appalachian region of Tennessee, Alabama, and Georgia, bauxite
occurs as pockets in residual clays above sedimentary rocks, chiefly
above shales and dolomites. Its origin has probably been similar to that
described.
The bauxite deposits of southern France occur in folded limestones, and
have been ascribed by French writers to the work of ascending hot waters
carrying aluminum sulphate. They present some unusual features, and
evidence as to their origin is not conclusive.
At the present time bauxite is doubtless forming in tropical climates,
where conditions are favorable for deep and extreme weathering of the
lateritic type. The breaking up of kaolin accompanied by the removal of
silica is not characteristic of temperate climates, though many clays in
these climates show some bauxite. It is possible that, at the time when
the bauxite deposits of Arkansas and other temperate regions were
formed, the climate of these places was warmer than it is today.
In studying the origin of bauxites, it should not be overlooked that
they have much in common with clays, certain iron ores, and many other
deposits formed by weathering.
ANTIMONY ORES
ECONOMIC FEATURES
Antimony is used mainly for alloying with other metals. Over one-third
of the antimony consumed in the United States is alloyed with tin and
copper in the manufacture of babbitt or bearing-metal. Other important
alloys include type-metal (lead, antimony, and tin), which has the
property of expanding on solidification; "hard lead," a lead-antimony
alloy used in making acid-resisting valves; Britannia or white metal
(antimony, tin, copper, zinc), utilized for cheap domestic tableware;
and some brasses and bronzes, solders, aluminum alloys, pattern metals,
and materials for battery plates and cable coverings. Antimony finds a
very large use in war times in the making of shrapnel bullets from
antimonial lead. Antimony oxides are used in white enameling of metal
surfaces, as coloring agents in the manufacture of glass, and as paint
pigments; the red sulphides are used in vulcanizing and coloring rubber,
as paint pigments, in percussion caps, and in safety matches; and other
salts find a wide variety of minor uses in chemical industries and in
medicine.
Antimony ores vary greatly in grade, the Chinese ores carrying from 20
to 64 per cent of the metal. The presence of arsenic and copper in the
ores is undesirable. Several of the more important antimony districts
owe their economical production of that metal to the presence of
recoverable values in gold. Some lead-silver ores contain small
quantities of antimony, and "antimonial lead," containing 12 to 18 per
cent antimony, is recovered in their smelting.
China is by far the most important antimony-producing country in the
world, and normally supplies over half the world's total. Chinese
antimony is exported in part as antimony crude (lumps of needle-like
antimony sulphide), and in part as antimony regulus, which is about 99
per cent pure metal. France was the only other important source of
antimony before the war (25 to 30 per cent of the world production), and
Mexico and Hungary produced small amounts. The large demand for antimony
occasioned by the war, besides stimulating production in these
countries, brought forth important amounts of antimony ore from Algeria
(French control) and from Bolivia and Australia (British control), as
well as smaller quantities from several other countries. Of the
war-developed sources, only Algeria and perhaps Australia are expected
to continue production under normal conditions.
Before the war, antimony was smelted chiefly in China, England, and
France, and to a lesser extent in Germany. British and French commercial
and smelting interests dominated to a considerable extent the world
situation, and London was the principal antimony market of the world.
During the war Chinese antimony interests were greatly strengthened, and
facilities for treating the ore in that country were increased. Japan
also became important as a smelter and marketer of Chinese ore, and
increasing quantities of antimony were exported from China and Japan
directly to the United States. English exports ceased entirely and were
replaced in this country by Chinese and Japanese brands.
The United States normally consumes about one-third of the world's
antimony. Before the war the entire amount was secured by importation,
two-thirds from Great Britain and the rest from the Orient, France, and
other European countries. Domestic production of ore and smelting of
foreign ores were negligible. (These statements refer only to the purer
forms of antimony; the United States normally produces considerable
amounts of antimonial lead, equivalent to somewhat less than 5 per cent
of the country's total lead production, but this material cannot be
substituted for antimony regulus in most of its uses.)
During the war, under the stimulus of rising prices, mining of antimony
was undertaken in the United States and several thousand tons of metal
were produced--principally from Nevada, with smaller amounts from
Alaska, California, and other western states. The great demands for
antimony, however, were met chiefly by increased importation. Imports
were mainly of regulus from Chinese and Japanese smelters of Chinese
antimony; but about a third was contained in ores, including most of the
production of Mexico which had formerly gone to England, and about 15
per cent of the Bolivian output. Antimony smelters were developed in the
United States to handle these ores.
At the close of hostilities there had accumulated in the United States
large surplus stocks of antimony and antimonial materials. With a very
dull market and low prices, domestic mines and smelters were obliged to
close down. The dependence of the United States on foreign sources of
antimony and the importance of the metal for war purposes led to some
agitation for a protective tariff--in addition to the present import
duty of 10 per cent on antimony metal--in order to encourage home
production (see pp. 365-366, 393-394).
In summary, the United States is almost entirely dependent upon outside
sources for its antimony, although there are inadequately known reserves
in this country which might be exploited if prices were maintained at a
high level. The future of United States smelters is problematical.
China, the world's chief source of antimony, at present dominates the
market in this country, largely due to the low cost of production and
favorable Japanese freight rates.
GEOLOGIC FEATURES
The antimony sulphide, stibnite, is the source of most of the world's
production of this metal. Antimony oxides, including senarmontite,
cervantite, and others, are formed near the surface, and in some of the
deposits of Mexico and Algeria they supply a large part of the values
recovered. Jamesonite, bournonite, and tetrahedrite (sulphantimonides of
lead and copper), when found in lead-silver deposits, are to some extent
a source of antimony in the form of antimonial lead.
Stibnite is found in a variety of associations and is present in small
quantities in many types of deposits. In the commercial antimony
deposits, it is in most cases accompanied by minor quantities of other
metallic sulphides--pyrite, cinnabar, sphalerite, galena, arsenopyrite,
etc.--in a gangue of quartz and sometimes calcite. Many of the deposits
contain recoverable amounts of gold and silver.
The deposits of the Hunan Province of southern China occur as seams,
pockets, and bunches of stibnite ore in gently undulating beds of
faulted and fissured dolomitic limestone. In the vicinity of the most
important mines no igneous rocks have been observed, and the origin of
the ores has not been worked out.
In the Central Plateau of France the numerous antimony deposits are
stibnite veins cutting granites and the surrounding schists and
sediments. An origin related in some way to hot ascending solutions
seems probable.
The deposits of the National district of western Nevada, the most
important war-developed antimony deposits of the United States, consist
of stibnite veins with a gangue of fine-grained drusy quartz, cutting
through flows of rhyolite and basalt. They are intimately related to
certain gold- and silver-bearing veins, and all are closely associated
with dikes of rhyolite, which were the feeders to the latest extrusion
in the district. The wall rocks have undergone alteration of the
propylitic type. These relations, and the presence of the mercury
sulphide, cinnabar, in some of the ores (see pp. 258-259), suggest an
origin through the work of ascending hot waters or hot springs. These
waters probably derived their dissolved matter from a magmatic source,
and worked up along vents near the rhyolite dikes soon after the
eruption of this rock.
In the weathering of antimony deposits, the stibnite usually alters to
form insoluble white or yellowish oxides, which are sometimes called
"antimony ocher." These tend to accumulate in the oxide zone through the
removal of the more soluble accompanying minerals. Secondary sulphide
enrichment of antimony deposits, if it occurs at all, is negligible.
ARSENIC ORES
ECONOMIC FEATURES
About two-thirds of the arsenic consumed in recent years has been used
in agriculture, where various arsenic compounds--arsenic trioxide or
"white arsenic," Paris green, lead arsenate, etc.--are used as
insecticides and weed killers. Arsenic compounds are also used in
"cattle-dips" for killing vermin. The only other large use of arsenic is
in the glass industry, arsenic trioxide being added to the molten glass
to purify and decolorize the product. Small quantities of arsenic
compounds are used in the preparation of drugs and dyeing materials, and
metallic arsenic is used for hardening lead in shot-making.
The principal arsenic-producing countries are the United States,
Germany, France, Great Britain, Canada, and Mexico. Spain, Portugal,
Japan, and China are also producers, and recent trouble with the
"prickly-pear" pest in Queensland, Australia, has led to local
development of arsenic mining in that country. For the most part,
European production has been used in Europe and American production in
the United States.
Arsenic is recovered almost wholly as a by-product of smelting ores for
the metals. The potential supply is ample in most countries where
smelting is conducted, but owing to the elaborate plant required to
recover the arsenic, apparatus is not usually installed much in advance
of the demand for production. Rapid expansion is not possible.
Before the war the arsenic needs of the United States (chiefly
agricultural) were supplied by a few recovery plants in the United
States, Mexico, and Canada. Several large smelters had not found it
profitable to install recovery plants, as the market might have been
oversupplied and prices were low. During the war, with the extensive
demand for insecticides for gardening, there was a considerable
deficiency of arsenic supplies. With rising prices production was
stimulated, but was still unable to meet the increased demand. This
situation resulted in regulation of the prices of white arsenic by the
Food Administration.
Production of arsenic in the United States comes chiefly from smelters
in Colorado, Washington, Utah, Montana, and New Jersey. Small amounts
are produced by arsenic mines in Virginia and New York. A Mexican plant
at Mapimi has been shipping important quantities to the United States.
The plant at Anaconda, Montana, is expected to produce an ample supply
in the future.
The United States is entirely independent in arsenic supplies and will
probably soon have an exportable surplus. Export trade, after the
reconstruction period, will probably meet competition from France and
Germany where production was formerly large.
GEOLOGIC FEATURES
Arsenic-bearing minerals are numerous and rather widely distributed, but
only a few of them are mined primarily for their content of arsenic.
Arsenopyrite or "mispickle" (iron-arsenic sulphide) has been used
intermittently as a source of white arsenic in various places,--notably
at Brinton, Virginia, and near Carmel, New York. The former deposits
contain arsenopyrite and copper-bearing pyrite impregnating a
mica-quartz-schist, adjacent to and in apparent genetic relation with
aplite or pegmatite intrusives. In the latter locality arsenopyrite is
found associated with pyrite in a gangue of quartz, forming a series of
parallel stringers in gneiss close to a basic dike.
The orange-red sulphides of arsenic, orpiment and realgar, are formed
both as primary minerals of igneous source and as secondary products of
weathering. They are rather characteristic of the oxide zones of certain
arsenical metallic ores, and are believed in many cases to have formed
from arsenopyrite. They are mined on a commercial scale in China.
The great bulk of the world's arsenic, as previously stated, is obtained
as a by-product of smelting operations. The enargite of the Butte copper
ores (pp. 201-203) contains a considerable amount of arsenic, a large
part of which will be recovered from the smelter fumes by new processes
which are being installed. The gold-silver ores of the Tintic district
(pp. 235) also yield important amounts, the arsenic-bearing minerals
being enargite and tennantite (copper-arsenic sulphides) and others. The
silver ores of the Cobalt district of Ontario (pp. 234-235), containing
nickel and cobalt arsenides, produce considerable arsenic. Many other
metallic ores contain notable amounts of arsenic, which are at present
allowed to escape through smelter flues, but which could be recovered
under market conditions which would repay the cost of installing the
necessary apparatus.
BISMUTH ORES
ECONOMIC FEATURES
Bismuth metal is used in alloys, to which it gives low fusibility
combined with hardness and sharp definition. Bismuth alloys are employed
in automatic fire sprinklers, in safety plugs for boilers, in electric
fuses, in solders and dental amalgams, and in some type and bearing
metals. Bismuth salts find a considerable application for pharmaceutical
purposes, especially in connection with intestinal disorders, and the
best grades of bismuth materials are used for this purpose. The salts
are also used in porcelain painting and enameling and in staining glass.
Bolivia is the most important producer of bismuth ore. The output is
controlled entirely by British smelting interests. An important deposit
exists in Peru, the output of which is limited by the same British
syndicate. Considerable bismuth is produced in Australia, Tasmania, and
New Zealand, all of which likewise goes to England. Germany before the
war had three smelters which produced bismuth from native ores in
Saxony; bismuth was one of the few metals of which Germany had an
adequate domestic supply. Recently southern China is reported to be
mining increasing amounts of bismuth.
The United States produces the larger part of its bismuth requirements,
chiefly from plants installed at two lead refineries. A further
installation would make this country entirely independent of foreign
supplies if occasion required. Imports, from England and South America,
have been steadily declining, but during the war were somewhat
increased. The United States does not export bismuth so far as known.
GEOLOGIC FEATURES
The principal minerals of bismuth are bismuthinite (bismuth sulphide),
bismutite (hydrated carbonate), bismite or bismuth ocher (hydrated
oxide), and native bismuth.
The native metal and the sulphide are believed to be formed mainly as
primary minerals of igneous origin. In the deposits of New South Wales
they are found associated with molybdenite in quartz gangue, in
pipe-like deposits in granite. The oxide and the carbonate are probably
products of surface weathering. The Bolivian deposits contain the native
metal, the oxide, and the carbonate, associated with gold, silver, and
tin minerals, in one locality in slates and in another locality in
porphyry. The origin is not well known.
In the United States, the sulphide, bismuthinite, is found in the
siliceous ores of Goldfield, Nevada (p. 230), and in minor amounts in a
great number of the sulphide ores of the Cordilleran region. The ores of
the Leadville and Tintic districts (pp. 219 and 235) yield the larger
part of the United States production, the bismuth being recovered as
by-product from the electrolytic refining of the lead bullion. Large
amounts of bismuth pass out of the stacks of smelters treating other
western ores, and while it would not be cheap nor easy to save the
bismuth thus lost, it could probably be done in case of necessity.
CADMIUM ORES
ECONOMIC FEATURES
Cadmium is used in low melting-point alloys--as, for example, those
employed in automatic fire-extinguishers and electric fuses,--in the
manufacture of silverware, and in dental amalgams. During the war the
critical scarcity of tin led to experiments in the substitution of
cadmium for tin in solders and anti-friction metals. Results of some of
these experiments were promising, but the war ceased and demands for tin
decreased before the cadmium materials became widely used. Future
developments in this direction seem not unlikely. Cadmium compounds are
used as pigments, particularly as the sulphide "cadmium yellow," and to
give color and luster to glass and porcelain. Cadmium salts are also
variously used in the arts, in medicine, and in electroplating.
Practically the entire cadmium output of the world comes from Germany
and the United States. In addition, England produces a very small
quantity. Before the war Germany produced about two-thirds of the
world's total, and supplied the European as well as a considerable part
of the United States consumption. During the war the United States
production increased three to four fold, imports ceased, and
considerable quantities were exported to the allied nations in Europe
and to Japan. At present the United States is entirely independent as
regards cadmium supplies. Production is sufficient to supply all the
home demand and to permit exports of one-third of the total output. A
considerable number of possible cadmium sources are not being used, and
the production is capable of extension should the need arise.
GEOLOGIC FEATURES
Nearly the only cadmium mineral known is the sulphide, greenockite, but
no deposits of this mineral have been found of sufficient volume to be
called cadmium ores. Sphalerite almost always contains a little cadmium,
probably as the sulphide; and in zinc deposits crystals of sphalerite in
cavities are frequently covered with a greenish-yellow film or coating
of greenockite. These coatings have probably been formed by the
decomposition of cadmium-bearing zinc sulphide in the oxide zone, the
carrying down of the cadmium in solution, and its precipitation as
secondary cadmium sulphide. The zinc oxide minerals in the surficial
zone also are sometimes colored yellow by small amounts of greenockite.
In the zinc ores of the Joplin district of Missouri, cadmium is present
in amounts ranging from a trace to 1 per cent and averaging 0.3 per
cent.
Germany's cadmium is produced by fractional distillation of the Silesian
zinc ores, which contain at most 0.3 per cent cadmium. In the United
States there are large potential sources in the zinc ores of the
Mississippi valley, and considerable cadmium is recovered in roasting
them. Much of the American cadmium is also obtained from bag-house dusts
at lead smelters.
The general geologic conditions of the cadmium-bearing ores are
indicated in the discussion of lead and zinc deposits in an earlier
chapter.
COBALT ORES
ECONOMIC FEATURES
Cobalt finds its largest use in the form of cobalt salts, employed in
coloring pottery and glass and in insect poisons. Cobalt is also used in
some of the best high-speed tool steels. "Stellite," which is used to a
limited extent in non-rusting tools of various sorts, and in
considerable quantity to replace high-speed tool steels, is an alloy of
cobalt, chromium, and small quantities of other metals. Considerable
experimental work has been done on the properties and uses of cobalt
alloys, and their consumption is rapidly on the increase.
Cobalt is an item of commerce of insignificant tonnage. There are only
two countries, Canada (Ontario) and the Belgian Congo, which produce
noteworthy amounts. The Katanga district in the Congo produces blister
copper that contains as much as 4 per cent of cobalt, though usually
less than 2 per cent. This product formerly went to Germany, and now
goes entirely to Great Britain. Just how much cobalt is saved is
unknown, but probably several hundred tons annually. It is probable that
most of the cobalt in these ores will be lost on the installation of a
leaching process for recovery of the copper. Canada exports most of its
product to the United States, though the amount is small. Domestic
production in this country has been too small to record. The United
States has been dependent on imports from Canada.
GEOLOGIC FEATURES
The principal cobalt minerals are smaltite (cobalt arsenide), cobaltite
(cobalt-arsenic sulphide), and linnaeite (cobalt-nickel sulphide). Under
weathering conditions these minerals oxidize readily to form asbolite, a
mixture of cobalt and manganese oxides, and the pink arsenate, erythrite
or "cobalt bloom."
Cobalt minerals are found principally in small quantities disseminated
through ores of silver, nickel, and copper. The production of Canada is
obtained mainly as a by-product of the silver ores of the Cobalt
district (described on pp. 234-235), and smaller amounts are recovered
from the Sudbury nickel ores (pp. 180-182). The cobalt of Belgian Congo
is obtained from rich oxidized copper ores which impregnate folded
sediments (p. 205).
MERCURY (QUICKSILVER) ORES
ECONOMIC FEATURES
Uses of mercury are characterized by their wide variety and their
application to very many different phases of modern industry; they will
be named here in general order of decreasing importance. About one-third
of the mercury consumed in this country goes into the manufacture of
drugs and chemicals, such as corrosive sublimate, calomel, and glacial
acetic acid. Mercury fulminate is used as a detonator for high
explosives and to some extent for small-arms ammunition--a use which was
exceedingly important during the war, but is probably of minor
consequence in normal times. Mercuric sulphide forms the brilliant red
pigment, vermilion, and mercuric oxide is becoming increasingly
important in anti-fouling marine paint for ship-bottoms. Either as the
metal or the oxide, mercury is employed in the manufacture of electrical
apparatus (batteries, electrolyzers, rectifiers, etc.), and in the
making of thermostats, gas governors, automatic sprinklers, and other
mechanical appliances. Mercuric nitrate is used in the fabrication of
felt hats from rabbits' fur. In the extraction of gold and silver from
their ores by amalgamation, large amounts of metallic mercury have been
utilized, but of late years the wide application of the cyanide process
has decreased this use. Minor uses include the making of certain
compounds for preventing boiler-scale, of cosmetics, and of dental
amalgam.
The ores of mercury vary greatly in grade. Spanish ores yield an average
in the neighborhood of 7 per cent, Italian ores 0.9 per cent, and
Austrian ores 0.65 per cent of metallic mercury. In the United States
the ores of California yield about 0.4 per cent and those of Texas range
from about 0.5 to 4 per cent. In almost all cases the ores are treated
in the immediate vicinity of the mines, and fairly pure metal is
obtained by a process of sublimation and condensation. This is usually
marketed in iron bottles or flasks containing 75 pounds each.
The large producers of mercury are, in order of normal importance,
Spain, Italy, Austria, and United States. Mexico, Russia, and all other
countries produce somewhat less than 5 per cent of the world's total.
The largest quicksilver mines of the world are those of Almaden in
central Spain, which are owned and operated by the Spanish government.
This government, after reserving a small amount for domestic use, sells
all the balance of the production through the Rothschilds of London. In
addition British capital controls some smaller mines in northern Spain.
England thus largely controls the European commercial situation in this
commodity, and London is the world's great quicksilver market, where
prices are fixed and whence supplies go to all corners of the globe.
Reserves of the Almaden ore bodies are very large. Sufficient ore is
reported to have been developed to insure a future production of at
least 40,000 metric tons--an amount equivalent to the entire world
requirements at pre-war rates of consumption for 100 years.
The mercury deposits of the Monte Amiata district of central Italy were
in large part dominated by German capital, but during the war were
seized by the Italian government. The mines of Idria, Austria-Hungary,
were owned by the Austrian government and their ultimate control is at
present uncertain. Reserves are very large, being estimated at about
one-half those of Almaden. Although England has had a considerable
control over the prices and the market for mercury, the Italian and
Austrian deposits have provided a sufficient amount to prevent any
absolute monopoly. English interests have now secured control of the
Italian production, and it is expected that they will also control the
Austrian production--thus giving England control of something over
three-fourths of the world's mercury.
In the United States about two-thirds of the mercury is produced in the
Coast Range district of California, and most of the remainder in the
Terlingua district of Texas. Smaller quantities come from Nevada,
Oregon, and a few other states. The output before the war was normally
slightly in excess of domestic demand and some mercury was exported to
various countries. Due to the exhaustion of the richer and more easily
worked deposits, however, production was declining. During the war, with
increased demands and higher prices, production was stimulated, the
United States became the largest mercury-producing country in the world,
and large quantities were exported to help meet the military needs of
England and France.
With the end of war prices and with high costs of labor and supplies,
production in the United States has again declined. Many of the mines
have passed their greatest yield, and though discovery of new ore bodies
might revive the industry, production is probably on the down grade.
Future needs of this country will probably in some part be met by
imports from Spain, Italy, and Austria, where the deposits are richer
and labor is cheaper. This situation has caused much agitation for a
tariff on imports. The present tariff of 10 per cent is not sufficient
to keep out foreign mercury.
Outside of the United States large changes in distribution of production
of quicksilver are not expected for some time. The reserves of the
European producers are all large and are ample to sustain present output
for a considerable number of years. It is reported that there will be a
resumption of mining in the once very productive Huancavelica District
of Peru and in Asia Minor, and with restoration of political order there
may be an increase in output from Mexico and Russia,--but these
districts will be subordinate factors in the world situation. On
geologic grounds, new areas of mercury ores may be looked for in regions
of recent volcanic activity, such as the east coast of Asia, some
islands of Oceania, the shores of the Mediterranean, and the Cordilleras
of North and South America,--but no such areas which are likely to be
producers on a large scale are now known.
GEOLOGIC FEATURES
The chief mineral of mercury, from which probably over 95 per cent of
the world's mercury comes, is the brilliant red sulphide, cinnabar.
Minor sources include the black or gray sulphide, metacinnabar, the
native metal, and the white mercurous chloride, calomel. The ores are
commonly associated with more or less iron sulphide, and frequently with
the sulphides of antimony and arsenic, in a gangue consisting largely of
quartz and carbonates (of calcium, magnesium, and iron). The precious
metals and the sulphides of the base metals are rare.
Mercury deposits are in general related to igneous rocks, and have
associations which indicate a particular type of igneous activity. They
are not found in magmatic segregations, in pegmatites, nor in veins
which have been formed at great depths and under very high temperatures.
On the contrary, the occurrence of many deposits in recent flows which
have not been eroded, their general shallow depth (large numbers
extending down only a few hundred feet), and the association of some
deposits with active hot springs now carrying mercury in solution,
suggest an origin through the work of ascending hot waters near the
surface. The mercury minerals are believed to have been carried in
alkaline sulphide solutions. Precipitation from such solutions may be
effected by oxidation, by dilution, by cooling, or by the presence of
organic matter. Being near the surface, it is a natural assumption that
the waters doing the work were not intensely hot. At Sulphur Bank
Springs, in the California quicksilver belt, deposition of cinnabar by
moderately hot waters is actually taking place at present; also these
waters are bleaching the rock in a manner often observed about mercury
deposits.
The Coast Ranges of California contain a great number of mercury
deposits extending over a belt about 400 miles long. The ore bodies are
in fissured zones in serpentine and Jurassic sediments, and are related
in general to recent volcanic flows. A considerable amount of bituminous
matter is found in the ores, and is believed to have been an agent in
their precipitation.
The Terlingua ores of Texas are found in similar fractured zones in
Cretaceous shales and limestones associated with surface igneous flows.
The occurrence of a few ore bodies in vertical shoots in limestone,
apparently terminating upward at the base of an impervious shale,
furnishes an additional argument for their formation by ascending
waters.
In the few deposits (_e. g._, those of Almaden, Spain, and of the deep
mines of New Almaden and New Idria, California,) where there is no such
clear relation to volcanic rocks as generally observed, but where the
ores contain the same characteristic set of minerals, it is concluded
that practically the same processes outlined above have been active in
their formation; and that the volcanic source of the hot solutions
either failed to reach the surface or has been removed by erosion. The
same line of reasoning is carried a step further, and in many
gold-quartz veins in volcanic rocks, where cinnabar and its associated
minerals are present, it is believed that waters of a hot-spring nature
have again been effective. Thus cinnabar, when taken with its customary
associations, is regarded as a sort of geologic thermometer.
In the weathering of mercury deposits, cinnabar behaves somewhat like
the corresponding silver sulphide, argentite. In the oxide zone, native
mercury and the chloride, calomel, are formed. In the Texas deposits a
red oxide and a number of oxychlorides are also present. The carrying
down of the mercury and its precipitation as secondary sulphide may
have taken place in some deposits, but this process is unimportant in
forming values.
TIN ORES
ECONOMIC FEATURES
The largest use of tin is in the manufacture of tin-plate, which is
employed in containers for food, oil, and other materials. Next in
importance is its use in the making of solder and of babbitt or bearing
metal. Tin is also a constituent of certain kinds of brass, bronze, and
other alloys, such as white metal and type metal. Minor uses include the
making of tinfoil, collapsible tubes, wire, rubber, and various
chemicals. Tin oxide is used to some extent in white enameling of metal
surfaces. Roughly a third of the tin consumed within the United States
goes into tin-plate, a third into solder and babbitt metal, and a third
into miscellaneous uses.
The ores of tin in general contain only small quantities of the metal.
Tin has sufficient value to warrant the working of certain placers
containing only a half-pound to the cubic yard, although the usual run
is somewhat higher. The tin content of the vein deposits ranges from
about 1 per cent to 40 per cent, and the average grade is much closer to
the lower figure.
Great Britain has long controlled the world's tin ores, producing about
half of the total and controlling additional supplies in other
countries. The production is in small part in Cornwall, but largely in
several British colonies--the Malay States, central and south Africa,
Australia, and others. The Malay States furnish about a third of the
world's total. Another third is produced in immediately adjacent
districts of the Dutch East Indies, Siam (British control), and China,
and some of the concentrates of these countries are handled by British
smelters, especially at Singapore.
Tin is easily reduced from its ores and most of the tin is smelted close
to the sources of production. Considerable quantities, however, have
gone to England for treatment. London has been the chief tin market of
the world, and before the war the larger portion of the tin entering
international trade went through this port. During the war a good deal
of the export tin from Straits Settlements was shipped direct to
consumers rather than via London, but it is not certain how future
shipments may be made.
Significant features of the tin situation in recent years have been a
decline of production in the Malay States, and a large and growing
production in Bolivia. Malayan output has decreased because of the
exhaustion of some of the richer and more accessible deposits; certain
governmental measures have also had a restrictive effect. Bolivian
production now amounts to over a fifth of the world's total and bids
fair to increase. About half the output is controlled by Chilean, and
small amounts by American, French, and German interests. A large portion
of the Bolivian concentrates formerly went to Germany for smelting, but
during the war American smelters were developed to handle part of this
material; large quantities are also smelted in England.
The United States produces a small fraction of 1 per cent of the world's
tin, and consumes a third to a half of the total. The production is
mainly from the Seward Peninsula of northwestern Alaska. For American
tin smelters, Bolivia is about the only available source of supplies;
metallic tin can be obtained from British possessions, but no ore,
except by paying a 33-1/3 per cent export tax. The United States exports
tin-plate in large amounts, and in this trade has met strong competition
from English and German tin-plate makers.
A world shortage of tin during the war required a division of available
supplies through a central international committee. Somewhat later, with
the removal of certain restrictions on the distribution of tin,
considerable quantities which had accumulated in the Orient found their
way into Europe and precipitated a sensational slump in the tin market.
GEOLOGIC FEATURES
The principal mineral of tin is cassiterite (tin oxide). Stannite, a
sulphide of copper, iron, and tin, is found in some of the Bolivian
deposits but is rare elsewhere.
About two-thirds of the world's tin is obtained from placers and
one-third from vein or "lode" deposits. Over 90 per cent of the tin of
southeastern Asia and Oceania is obtained from placers. Tin placers,
like placers of gold, platinum, and tungsten, represent concentrations
in stream beds and ocean beaches of heavy, insoluble minerals--in this
case chiefly cassiterite--which were present in the parent rocks in
much smaller quantities, but which have been sorted out by the
classifying action of running water.
The original home of cassiterite is in veins closely related to granitic
rocks. It is occasionally found in pegmatites, as in certain small
deposits of the Southern Appalachians and the Black Hills of South
Dakota, or is present in a typical contact-metamorphic silicated zone in
limestone, as in some of the deposits of the Seward Peninsula of Alaska.
In general, however, it is found in well-defined fissure veins in the
outer parts of granitic intrusions and extending out into the
surrounding rocks. With the cassiterite are often found minerals of
tungsten, molybdenum, and bismuth, as well as sulphides of iron, copper,
lead, and zinc, and in some cases there is evidence of a rough zonal
arrangement. The deposits of Cornwall and of Saxony show transitions
from cassiterite veins close to the intrusions into lead-silver veins at
a greater distance. The gangue is usually quartz, containing smaller
amounts of a number of less common minerals--including lithium mica,
fluorite, topaz, tourmaline, and apatite. The wall rocks are usually
strongly altered and in part are replaced by some of the above minerals,
forming coarse-grained rocks which are called "greisen."
The origin of cassiterite veins, in view of their universal association
with granitic rocks, is evidently related to igneous intrusions. The
occurrence of the veins in distinct fissures in the granite and in the
surrounding contact-metamorphic zone indicates that the granite had
consolidated before their formation, and that they represent a late
stage in the cooling. The association with minerals containing fluorine
and boron, and the intense alteration of the wall rocks, indicate that
the temperature must have been very high. It is probable that the
temperature was so high as to cause the solutions to be gaseous rather
than liquid, and that what have been called "pneumatolytic" conditions
prevailed; but evidence to decide this question is not at present
available.
The most important deposits of tin in veins are those of Bolivia, some
of which are exceptionally rich. These are found in granitic rocks
forming the core of the high Cordillera Real and in the adjacent
intruded sediments, in narrow fissure veins and broader brecciated zones
containing the typical ore and gangue minerals described above, and
also, in many cases, silver-bearing sulphides (chiefly tetrahedrite).
There appear to be all gradations in type from silver-free tin ores to
tin-free silver ores, although the extremes are now believed to be rare.
In the main the tin ores, with abundant tourmaline, appear to be more
closely related to the coarse-grained granites, and to indicate intense
conditions of heat and pressure, while the more argentiferous ores, with
very little or no tourmaline, are found in relation to finer-grained
quartz porphyries and even rhyolites, and seem to indicate less intense
conditions at the time of deposition. The ores of the whole area, which
is a few hundred miles long, have been supposed to represent a single
genetic unit, and the sundry variations are believed to be local facies
of a general mineralization. Processes of secondary enrichment have in
places yielded large quantities of oxidized silver minerals and wood tin
near the surface, with accumulations of ruby silver ores at greater
depths.
The only other vein deposits which are at present of consequence are
those of Cornwall. Here batholiths of granite have been intruded into
Paleozoic slates and sandstones, and tin ores occur in fissures and
stockworks in the marginal zones. With the exhaustion of the more easily
mined placers, the lode deposits will doubtless be of increasing
importance.
Cassiterite is practically insoluble and is very resistant to
decomposition by weathering. Oxide zones of tin deposits are therefore
enriched by removal of the more soluble minerals. Stannite probably
alters to "wood tin," a fibrous variety of cassiterite. Secondary
enrichment of tin deposits by redeposition of tin minerals is
negligible.
URANIUM AND RADIUM ORES
ECONOMIC FEATURES
Radium salts are used in various medical treatments--especially for
cancer, internal tumors, lupus, and birth marks--and in luminous paints.
During the latter part of the war it is estimated that over nine-tenths
of the radium produced was used in luminous paints for the dials of
watches and other instruments. In addition part of the material owned by
physicians was devoted to this purpose, and it is probable that the
accumulated stocks held by the medical profession were in this way
reduced by one-half. The greatly extended use of radium, together with
the distinctly limited character of the world's known radium supplies,
has led to some concern; and considerable investigation has been made of
the possibilities of mesothorium as a substitute for radium in luminous
paints. Low-grade radium residues are used to some extent as
fertilizers.
Uranium has been used as a steel alloy, but has not as yet gained wide
favor. Uranium salts have a limited use as yellow coloring agents in
pottery and glass. The principal use of uranium, however, is as a source
of radium, with which it is always associated.
European countries first developed the processes of reduction of radium
salts from their ores. Most of the European ores are obtained from
Austria, where the mines are owned and operated by the Austrian
government, and small quantities are mined in Cornwall, England, and in
Germany. Production is decreasing. The European hospitals and
municipalities have acquired nearly all of the production.
The United States has the largest reserves of radium ore in the world,
and the American market has in recent years been supplied from domestic
plants. Before the war, radium ores were shipped to Europe for treatment
in Germany, France, and England, and radium salts were imported from
these countries. There are now radium plants in the United States
capable of producing annually from domestic ores an amount several times
as large as the entire production of the rest of the world. Practically
all the production has come from Colorado and Utah. Known reserves are
not believed to be sufficient for more than a comparatively few years'
production, but it is not unlikely that additional deposits will be
found in the same area.
GEOLOGIC FEATURES
Uranium is one of the rarer metals. Radium is found only in uranium ores
and only in exceedingly small quantities. The maximum amount which can
be present in a state of equilibrium is about one part of radium in
3,000,000 parts of uranium. The principal sources of uranium and radium
are the minerals carnotite (hydrous potassium-uranium vanadate) and
pitchblende or uraninite (uranium oxide).
The deposits of Joachimsthal, Bohemia, contain pitchblende, along with
silver, nickel, and cobalt minerals and other metallic sulphides, in
veins associated with igneous intrusions.
The important commercial deposits of Colorado and Utah contain
carnotite, together with roscoelite (a vanadium mica) and small amounts
of chromium, copper, and molybdenum minerals, as impregnations of
flat-lying Jurassic sandstones. The ores carry up to 35 per cent uranium
oxide (though largely below 2 per cent), and from one-third as much to
an equal amount of vanadium oxide. The ore minerals are supposed to have
been derived from a thick series of clays and impure sandstones a few
hundred feet above, containing uranium and vanadium minerals widely
disseminated, and to have been carried downward by surface waters
containing sulphates. The ore bodies vary from very small pockets to
deposits yielding a thousand tons or so, and are found irregularly
throughout certain particular beds without any special relation to
present topography or to faults. The association of many of the deposits
with fossil wood and other carbonaceous material suggests that organic
matter was an agent in their precipitation, but the exact nature of the
process is not clear. In a few places in Utah the beds dip at steep
angles, and the carnotite appears in spots along the outcrops and
generally disappears as the outcrops are followed into the hillsides;
this suggests that the carnotite may be locally redissolved and carried
to the surface by capillary action, forming rich efflorescences. Because
of the nature of the deposits no large amount of ore is developed in
advance of actual mining; but estimates based on past experience
indicate great potentialities of this region for future production.
In eastern Wyoming is a unique deposit of uranium ore in a quartzite
which lies between mica-schist and granite. The principal ore mineral is
uranophane, a hydrated calcium-uranium silicate, which is believed to be
an oxidation product of pitchblende. Some of the ore runs as high as 4
per cent uranium oxide, and the ore carries appreciable amounts of
copper but very little vanadium.
Very recently radium ores have been discovered in the White Signal
mining district of New Mexico, which was formerly worked for gold,
silver, copper, and lead. The radium-bearing minerals are torbernite and
autunite (hydrous copper-uranium and calcium-uranium phosphates), and
are found in dark felsite dikes near their intersections with east-west
gold-silver-quartz veins. The possibilities of this district have not
yet been determined.
Pitchblende has been found in gold-bearing veins in Gilpin County,
eastern Colorado, and in pegmatite dikes in the Appalachians, but these
deposits are of no commercial importance. Pitchblende is grayish-black,
opaque, and so lacking in distinctive characteristics that it may
readily be overlooked; hence future discoveries in various regions would
not be surprising.
CHAPTER XIII
MISCELLANEOUS NON-METALLIC MINERALS
NATURAL ABRASIVES
ECONOMIC FEATURES
Natural abrasives are less important commercially in the United States
than artificial abrasives, but a considerable industry is based on the
natural abrasives.
Silica or quartz in its various crystalline forms constitutes over
three-fourths of the tonnage of natural abrasives used in the United
States. It is the chief ingredient of sand, sandstone, quartzite, chert,
diatomaceous earth, and tripoli. From the sand and sandstone are made
millstones, buhrstones, grindstones, pulpstones, hones, oilstones, and
whetstones. Sand, sandstone, and quartzite are also ground up and used
in sand-blasts, sandpaper, and for other abrasive purposes. Chert or
flint constitutes grinding pebbles and tube-mill linings, and is also
ground up for abrasives. Diatomaceous (infusorial) earth is used as a
polishing agent and also as a filtering medium, an absorbent, and for
heat insulation. Tripoli (and rottenstone) are used in polishing powders
and scouring soaps as well as for filter blocks and many other purposes.
Other important abrasives are emery and corundum, garnet, pumice,
diamond dust and bort, and feldspar.
Imports of abrasive materials into the United States have about
one-third of the value of those locally produced. While all of the
various abrasives are represented in these imports, the United States is
dependent on foreign sources for important parts of its needs only of
emery and corundum, garnet, pumice, diamond dust and bort, and grinding
pebbles.
Emery and corundum are used in various forms for the grinding and
polishing of hard materials--steel, glass, stone, etc. The principal
foreign sources of emery have been Turkey (Smyrna) and Greece (Naxos)
where reserves are large and production cheap. Production of corundum
has come from Canada, South Africa, Madagascar, and India. The domestic
production of emery is mainly from New York and Virginia, and corundum
comes from North Carolina. Domestic supplies are insufficient to meet
requirements, and cannot be substituted for the foreign material for the
polishing of fine glass and other special purposes. Curtailment of
imports during the war greatly stimulated the development of artificial
abrasives and their substitution for emery and corundum.
Garnet is used chiefly in the form of garnet paper for working leather,
wood, and brass. Garnet is produced mainly in the United States and
Spain. The United States is the only country using large amounts of this
mineral and imports most of the Spanish output. The domestic supply
comes mainly from New York, New Hampshire, and North Carolina.
Pumice is used in fine finishing and polishing of varnished and enameled
surfaces, and in cleaning powders. The world's principal source for
pumice is the Lipari Islands, Italy. There is a large domestic supply of
somewhat lower-grade material (volcanic ash) in the Great Plains region,
and there are high-grade materials in California and Arizona. Under war
conditions these supplies were drawn on, but normally the high-quality
Italian pumice can be placed in American markets more cheaply.
Diamond dust is used for cutting gem stones and other very hard
materials, and borts or carbonadoes (black diamonds) for
diamond-drilling in exploration. Most of the black diamonds come from
Brazil, and diamond dust comes from South Africa, Brazil, Borneo, and
India.
Chert or flint pebbles for tube-mills are supplied mainly from the
extensive deposits on the French and Danish coasts. The domestic
production has been small, consisting principally of flint pebbles from
the California beaches, and artificial pebbles made from rhyolite in
Nevada and quartzite in Iowa. War experience demonstrated the
possibility of using the domestic supply in larger proportion, but the
grade is such that in normal times this supply will not compete with
importations.
Feldspar as an abrasive is used mainly in scouring soaps and
window-wash. Domestic supplies are ample. The principal use of feldspar
is in the ceramic industry and the mineral is discussed at greater
length in the chapter on common rocks (p. 86).
For the large number of abrasives produced from silica, outside of flint
pebbles, domestic sources of production are ample. Siliceous rocks are
available almost everywhere. For particular purposes, however, rocks
possessing the exact combinations of qualities which make them most
suitable are in many cases distinctly localized. _Millstones and
buhrstones_, used for grinding cereals, paint ores, cement rock,
fertilizers, etc., are produced chiefly in New York and Virginia; partly
because of trade prejudice and tradition, about a third of the American
requirements are imported from France, Belgium, and Germany.
_Grindstones and pulpstones_, used for sharpening tools, grinding
wood-pulp, etc., come mainly from Ohio and to a lesser extent from
Michigan and West Virginia; about 5 per cent of the consumption is
imported from Canada and Great Britain. _Hones_, _oilstones_, and
_whetstones_ are produced largely from a rock called "novaculite" in
Arkansas, and also in Indiana, Ohio, and New England; imports are
negligible. _Flint linings_ for tube-mills were formerly imported from
Belgium, but American products, developed during the war in
Pennsylvania, Tennessee, and Iowa, appear to be wholly satisfactory
substitutes. _Diatomaceous earth_ is produced in California, Nevada,
Connecticut, and Maryland, and _tripoli and rottenstone_ in Illinois,
Missouri, and Oklahoma; domestic sources are sufficient for all needs,
but due to questions of back-haul and cost of rail transportation there
has been some importation from England and Germany.
GEOLOGIC FEATURES
The geologic features of silica (quartz), feldspar, and diamonds are
sufficiently indicated elsewhere (Chapter II; pp. 84, 196, 86, 291-292).
Diatomaceous earth is made up of remains of minute aquatic plants. It
may be loose and powdery, or coherent like chalk. It is of sedimentary
origin, accumulated originally at the bottoms of ponds, lakes, and in
the sea.
Tripoli and rottenstone are light, porous, siliceous rocks which have
resulted from the leaching of calcareous materials from various
siliceous limestones or calcareous cherts in the process of weathering.
Grinding pebbles are derived from the erosion of limestone or chalk
formations which contain concretions of extremely fine-grained and dense
chert. Under stream and wave action they are rounded and polished. The
principal sources are ocean beaches.
Corundum as an abrasive is the mineral of this name--made up of
anhydrous aluminum oxide. Emery is an intimate mechanical mixture of
corundum, magnetite, and sometimes spinel. Corundum is a product of
contact metamorphism and also a result of direct crystallization from
molten magma. Canadian corundum occurs as a constituent of syenite and
nepheline-syenite in Lower Ontario. In North Carolina and Georgia, the
corundum occurs in vein-like bodies at the contact of peridotite with
gneisses and schists, and also in part in the peridotite itself. In New
York the emery deposits are segregations of aluminum and iron oxides in
norite (a basic igneous rock). The emery of Greece and Turkey occurs as
lenses or pockets in crystalline limestones, and is the result of
contact metamorphism by intrusive granites.
Garnets result mainly from contact metamorphism, and commonly occur
either in schists and gneisses or in marble. The principal American
occurrences are of this type. Being heavy and resistant to weathering,
they are also concentrated in placers. The Spanish garnets are reported
to be obtained by washing the sands of certain streams.
Pumice is solidified rock froth formed by escape of gases from molten
igneous rocks at the surface. It is often closely associated with
volcanic ash, which is also used for abrasive purposes.
In general, the geologic processes entering into the formation of
abrasives cover almost the full range from primary igneous processes to
surface alterations and sedimentation.
ASBESTOS
ECONOMIC FEATURES
The principal uses of asbestos are in high-pressure packing in heat
engines, in thermal and electrical insulation, in fire-proofing, and in
brake-band linings.
The largest producers of asbestos are Canada (Quebec) and, to a
considerably less extent, Russia. United States interests have financial
control of about a fourth of the Canadian production, and practically
the entire export trade of Canada goes to the United States. Russia
exports nearly all her product to Germany, Austria, United Kingdom,
Belgium, and the Netherlands. Previous to the war the output was largely
controlled by a German syndicate. There is a considerable recent
production in South Africa, which is taken by England and the United
States, and small amounts are produced in Italy, Cyprus, and Australia.
The United States has been a large importer of asbestos, from Canada and
some other sources. Domestic production is relatively insignificant, and
exports depend chiefly on an excess of import. Georgia is the principal
local source. Arizona and California are also producers, their product
being of a higher grade. The United States is the largest manufacturer
of asbestos goods, and exports go to nearly all parts of the world.
So long as the abundant Canadian material is accessible on reasonable
conditions, the United States is about as well situated as if
independent. Some Canadian proposals of restriction during the war led
to a study of other supplies and showed that several deposits, such as
those in Russia and Africa, might compete with the Canadian asbestos.
GEOLOGIC FEATURES
Asbestos consists mostly of magnesium silicate minerals--chrysotile,
anthophyllite, and crocidolite. The term asbestos covers all fibrous
minerals with some tensile strength which are poor conductors and can be
used for heat-protection. Like talc, they are derived principally from
the alteration of olivine, pyroxene, and amphibole,--or more commonly
from serpentine, which itself results from the alteration of these
minerals. Chrysotile is the most common, and because of the length,
fineness, and flexibility of its fibers, enabling it to be spun into
asbestos ropes and fabrics, it is the most valuable. Anthophyllite
fibers, on the other hand, are short, coarse, and brittle, and can be
used only for lower-grade purposes. Crocidolite or blue asbestos is
similar to chrysotile but somewhat inferior in fire-resisting qualities.
Asbestos deposits occur chiefly as veinlets in serpentine rock, which is
itself the alteration of some earlier rock like peridotite. They are
clearly formed in cracks and fissures through the agency of water, but
whether the waters are hot or cold is not apparent. The veinlets have
sometimes been interpreted as fillings of contraction cracks, but more
probably are due to recrystallization of the serpentine, proceeding
inward from the cracks. In Quebec the chrysotile asbestos (which is
partly of spinning and partly of non-spinning grade) forms irregular
veins of this nature in serpentine, the fiber making up 2 to 6 per cent
of the rock.
In Georgia the asbestos, which is anthophyllite, occurs in lenticular
masses in peridotite associated with gneiss. It is supposed to have
formed by the alteration of olivine and pyroxene in the igneous rocks.
In Arizona chrysotile is found in veins in cherty limestone, associated
with diabase intrusives. Here it is believed to be an alteration product
of diopside (lime-magnesia pyroxene) in a contact-metamorphic silicated
zone.
Crocidolite is mined on a commercial scale only in Cape Colony, South
Africa. The deposits occur in thin sedimentary layers interbedded with
jaspers and ironstones. Their origin has not been worked out in detail.
The deposits of Russia, the Transvaal, Rhodesia, and Australia are of
high-grade chrysotile, probably similar in origin to the Quebec
deposits. The asbestos of Italy and Cyprus is anthophyllite, more like
the Georgia material.
BARITE (BARYTES)
ECONOMIC FEATURES
Barite is used chiefly as a material for paints. For this purpose it is
employed both in the ground form and in the manufacture of lithopone, a
widely used white paint consisting of barium sulphate and zinc sulphide.
Ground barite is also used in certain kinds of rubber goods and in the
making of heavy glazed paper. Lesser amounts go into the manufacture of
barium chemicals, which are used in the preparation of hydrogen
peroxide, in softening water, in tanning leather, and in a wide variety
of other applications.
Germany is the world's principal producer of barite and has large
reserves of high grade. Great Britain also has extensive deposits and
produces perhaps one-fourth as much as Germany. France, Italy, Belgium,
Austria-Hungary, and Spain produce smaller but significant amounts.
Before the war the United States imported from Germany nearly half the
barite consumed in this country, and produced the remainder. Under the
necessities of war times, adequate domestic supplies were developed and
took care of nearly all the greatly increased demands. Production has
come from fourteen states, the large producers being Georgia, Missouri,
and Tennessee. During the war, also, an important movement of
barite-consuming industries to the middle west took place, in order to
utilize more readily and cheaply the domestic product. For this reason
it is not expected that German barite will play as important a part as
formerly in American markets,--although it can undoubtedly be put down
on the Atlantic seaboard much more cheaply than domestic barite, which
requires long rail hauls from southern and middle-western states.
GEOLOGIC FEATURES
The mineral barite is a heavy white sulphate of barium, frequently
called "barytes" or "heavy spar." Witherite, the barium carbonate, is a
much rarer mineral but is found with barite in some veins.
All igneous rocks contain at least a trace of barium, which is probably
present in the silicates, and these small quantities are the ultimate
source of the more concentrated deposits. Barite itself is not found as
an original constituent of igneous rocks or pegmatites, but is
apparently always formed by deposition from aqueous solutions. It is a
common gangue mineral in many deposits of metallic sulphides, both those
formed in relation to igneous activity and those which are independent
of such activity, but in these occurrences it is of little or no
commercial importance.
The principal deposits of barite are found in sedimentary rocks, and
especially in limestones and dolomites. In these rocks it occurs in
veins and lenses very similar in nature to the lead and zinc deposits of
the Mississippi valley (p. 211 _et seq._), and, like them, probably
deposited by cold solutions which gathered together small quantities of
material from the overlying or surrounding rocks. The Missouri deposits
are found in limestones in a region not far from the great southeastern
Missouri lead district, and vary from the lead deposits in relative
proportions rather than in kind of minerals; the veins consist chiefly
of barite, with minor quantities of silica, iron sulphide, galena, and
sphalerite. The deposits of the southern Appalachians occur as lenses in
limestones and schists.
Barite is little affected by surface weathering, and tends to remain
behind while the more soluble minerals of the associated rock are
dissolved out and carried away. A limited amount of solution and
redeposition of the barite takes place, however, resulting in its
segregation into nodules in the residual clays. Most of the barite
actually mined comes from these residual deposits, which owe their
present positions and values to katamorphic processes. The accompanying
clay and iron oxide are removed by washing and mechanical concentration.
Certain investigators of the deposits of the Mississippi valley are
extremely reluctant to accept the idea that the ores are formed by
surface waters of ordinary temperatures, and are inclined to appeal to
heated waters from a hypothetical underlying magmatic source. The fact
that barite is a characteristic mineral of many igneous veins, and the
fact that in this same general region it is found in the
Kentucky-Illinois fluorspar deposits,--where a magmatic source is
generally accepted,--together with doubts as to the theoretical efficacy
of meteoric waters to transport the minerals found in the barite
deposits, have led certain writers to ascribe to these barite deposits a
magmatic origin. The magmatic theory has not been disproved; but on the
whole the balance of evidence seems strongly to indicate that the barite
deposits as well as the lead and zinc ores, which are essentially the
same in nature though differing in mineral proportions, have been
concentrated from the adjacent sediments by ordinary surface waters.
BORAX
ECONOMIC FEATURES
Borax-bearing minerals are used almost entirely in the manufacture of
borax and boric acid. Fully a third of the borax consumed in the United
States is used in the manufacture of enamels or porcelain-like coatings
for such objects as bathtubs, kitchen sinks, and cooking utensils. Other
uses of borax or of boric acid are as a flux in the melting and
purification of the precious metals, in decomposing chromite, in making
glass, as a preservative, as an antiseptic, and as a cleansing agent.
Recent developments indicate that the metal, boron, may play an
important part in the metallurgy of various metals. It has been used in
making very pure copper castings for electrical purposes, in aluminum
bronzes, and in hardening aluminum castings; and an alloy, ferroboron,
has been shown experimentally to act on steel somewhat like
ferrovanadium.
The bulk of the world's borax comes from the Western Hemisphere, the
United States and Chile being the two principal producers. There are
additional large deposits in northern Argentina, southern Peru, and
southern Bolivia, which have thus far been little drawn on because of
their inaccessibility. English financial interests control most of these
South American deposits.
The only large European producer of borax is Turkey. Italy and Germany
produce small amounts. There has also been small production of borax in
Thibet, brought out from the mountains on sheep-back.
The United States supplies of borax are sufficient for all domestic
requirements and probably for export. Small quantities of boric acid are
imported, but no borax in recent years. The domestic production comes
entirely from California, though in the past deposits in Nevada and
Oregon have also been worked.
GEOLOGIC FEATURES
The element boron is present in various complex boro-silicates, such as
datolite and tourmaline, the latter of which is used as a precious stone
(pp. 290, 293). None of these are commercial sources of borax. The
principal boron minerals are borax or "tincal" (hydrated sodium borate),
colemanite (hydrated calcium borate), ulexite (hydrated calcium-sodium
borate), and boracite (magnesium chloro-borate). Commercially the term
borax is sometimes applied to all these materials. These minerals appear
in nature under rather widely differing modes of origin.
The borax production of Italy is obtained from the famous "soffioni" or
"fumaroles" of Tuscany. These are volcanic exhalations, in which jets of
steam carrying boric acid and various borates, together with ammonium
compounds, emerge from vents in the ground. The boric acid material is
recovered by a process of condensation.
Borates, principally in the form of borax, occur in hot springs and in
lakes of volcanic regions. The Thibet deposits, and those formerly
worked at Borax Lake, California, are of this type. Certain of the
hot-spring waters of the California coast ranges and of Nevada carry
considerable quantities of boron, together with ammoniacal salts, and in
some places they deposit borax along with sulphur and cinnabar. It seems
probable (see p. 40) that these waters may come from an igneous source
not far beneath.
Most of the borax deposits of California, Nevada, and Oregon, though not
at present the largely producing ones, and probably most of the Chilean
and adjacent South American deposits, are formed by the evaporation of
desert lakes. They are products of desiccation, and in Chile are
associated with the great nitrate deposits (pp. 102-104), which are of
similar origin. The salts contained in these deposits are mainly borax,
ulexite, and colemanite. The sources of these materials are perhaps
deposits of the type mentioned in the last paragraph, or, in California,
certain Tertiary borate deposits described below. Whatever their source,
the borates are carried in solution by the waters of occasional rains to
shallow basins, which become covered with temporary thin sheets of water
or "playa lakes." Evaporation of these lakes leaves broad flats covered
with the white salts. These may subsequently be covered with drifting
sands and capillary action may cause the borates to work up through the
sands, becoming mixed with them and efflorescing at the surface. One of
the largest of the California deposits of this general class is that at
Searles Lake, from which it has been proposed to recover borax along
with the potash (pp. 113-114).