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 Cœur 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.