Supergene enrichment has generally been treated as a continuously progressive process. There is considerable probability, however, that it is essentially cyclic, although the cyclic character may not be patent in all deposits. A full development of the cycle can take place only under a certain equilibrium of a number of factors, including climate, erosion, topography, and character of rock. The essential fact appears to be that as enrichment progresses and chalcocite increases the process of enrichment becomes slower in action, and erosion may, in some circumstances, overtake it. With the removal of some of the protecting zone of chalcocite the protore is again exposed to oxidation and a second cycle of enrichment begins.Although much of the enriched ore is now below ground-water level, it probably was once above that level, and enrichment is believed to have taken place mainly in the zone of rock above any general water table.
Supergene enrichment has generally been treated as a continuously progressive process. There is considerable probability, however, that it is essentially cyclic, although the cyclic character may not be patent in all deposits. A full development of the cycle can take place only under a certain equilibrium of a number of factors, including climate, erosion, topography, and character of rock. The essential fact appears to be that as enrichment progresses and chalcocite increases the process of enrichment becomes slower in action, and erosion may, in some circumstances, overtake it. With the removal of some of the protecting zone of chalcocite the protore is again exposed to oxidation and a second cycle of enrichment begins.
Although much of the enriched ore is now below ground-water level, it probably was once above that level, and enrichment is believed to have taken place mainly in the zone of rock above any general water table.
Where the old erosion surface roughly coincides with the present erosion surface, the deposits follow more or less the topography. Where the old erosion surface pitches below later sediments, the ores pitch with it, and therefore do not follow the present topography. The recognition of the cyclic nature of secondary concentration is obviously of great significance in exploration and development.
Although a vast amount of study has been devoted to the origin and enrichment of copper deposits, and although the general conditions and processes are pretty well understood, the results thus far have been largely qualitative rather than quantitative.
The most prominent uses of lead are in the manufacture of alloys, such as type-metal, bearing metal, shot, solder, and casting metal; as the oxide, red lead, and the basic carbonate, white lead, in paints; for lead pipe, cable coverings, and containers of acid active material; and in lead compounds for various chemical and medical uses. Of the lead consumed in the United States before the war about 38 per cent was utilized in pigments, 30 per cent in alloys other than shot, 15 per cent in pipe, 10 per cent in shot, and 7 per cent in all other uses. During the war much larger quantities were used in munitions, such as shot and shrapnel.
The lead content of commercial ores varies widely. It ranges from as low as .25 per cent in the Joplin district of Missouri, to about 15 per cent in the Broken Hill deposits of Australia, and over 20 per cent in the Bawdwin mines of Burma. In the Cœur d'Alene district of Idaho and the southeastern district of Missouri, the two greatest lead producers in the United States, the average grades are about 10 per cent and about 3-½ per cent respectively. The grade of ore which may be profitably worked depends not only upon the economic factors,—such as nearness to consuming centers, and the price of lead,—but also upon the amenability of the ore to concentration, the content of other valuable metals, and the fact that lead is very useful in smelting as a collector of gold and silver.
Most lead ores contain more or less zinc, and lead is obtained as a by-product of most zinc ores. Argentiferous lead ores form one of the principal sources of silver, and also yield some gold. Lead and copper are produced together from certain ores. Thus the separation of many ores into hard and fast classes, as lead, or zinc, or copper, or silver, or gold ores, cannot be made; in some of the mineral resource reports of the United States Geological Survey the statistics of these five metals are published together.
The main sources of lead ore, named in order of their importance, are the United States, Australia, Spain, Germany, and Mexico, which account for over 80 per cent of the world's production. Most of the countries of Europe outside of Spain and Germany produce small amounts of lead, but are largely dependent on imports. Spain exports argentiferous lead and pig lead mainly to England and France, with minor quantities to other countries of Europe and to Argentina. Before the war Germany, which was the largest European consumer, utilized all its own production of lead ores and imported an additional 10 per cent of the world's ores for smelting, as well as considerable amounts of pig lead. Its principal deposits were those of Silesia; under the Peace Treaty they may possibly be lost to Poland, leaving German smelters largely dependent on imports. Australia before the war normally shipped lead concentrates and pig lead to England and also to Belgium, Germany, and Japan. England, the second largest European consumer, before the war had insufficient smelting capacity within the British Empire and was partly dependent on foreign-smelted lead. During the war, however, England contracted for the entire Australian output, and enlarged its smelting capacity accordingly. This may mean permanent loss to Belgium, which had depended mainly on the Australian ores for its smelting industry before the war.
In North Africa there is a small but steady production of lead, most of which goes to France. Recent developments in Burma have shown large reserves of high-grade lead-zinc-silver-copper ores, and this region may be expected to become an important producer. There are also large reserves of lead in the Altai Mountains of southwestern Siberia and in the Andes Mountains of South America.
England, through control of Australian and Burman lead mines and smelters, domestic smelting facilities, and some financial control in Spain, Mexico, and elsewhere, and France, through financial control of Spanish and North African mines and Spanish, Belgian, and domestic smelters, have adequate supplies of lead.
The United States produces about a third of the world's lead and twice as much as any other country. Normally the domestic production is almost entirely consumed in this country. Mexico sends large quantities of bullion and ore to the United States to besmelted and refined in bond. Mexican lead refined and exported by the United States equals in amount one-sixth of the domestic production. Small quantities of ore or bullion from Canada, Africa, and South America are also brought into the United States for treatment.
Through domestic production, smelting facilities for Mexican ore, and commercial ownership in Mexico and elsewhere, the United States controls over 45 per cent of the world's lead production. Before the war Germany, through the "Lead Convention" or International Sales Association, and through smelting and selling contracts with large producing mines, practically controlled the European lead market as well as exports from Mexico and the United States and from Australia. During the war German foreign influence was practically destroyed.
In the United States about a third of the production of lead comes from southeastern Missouri and about a fourth from the Cœur d'Alene district of Idaho. The five states, Missouri, Idaho, Utah, Colorado and Oklahoma, produce about nine-tenths of the country's total output. Reserves of lead ore are not large in proportion to demand, contrasting in this regard with zinc ore.
The principal lead mineral is the sulphide, galena, from which the great bulk of the world's lead is derived. Cerussite (lead carbonate) and anglesite (lead sulphate) are mined in some places in the upper part of sulphide deposits, and supply a small fraction of the world's output.
The ores of lead are of two general classes:
The first class, the so-called "soft" lead ores, nearly free from copper and precious metals, and commonly associated with zinc ores, are found in sedimentary beds independent of igneous intrusion. They are of world-wide distribution, were the first to be extensively exploited, were at one time the dominant factor in world production of lead, and at present produce about 30 per cent of the world's total. They are represented by the deposits of the Mississippi Valley, of Silesia, and some of the Spanish deposits. The general description of the origin of the zinc ores of the Mississippi Valley on pp. 216-218 applies to this class of lead ores. Itshould be noted, however, that in the principal United States lead-producing district, that of southeastern Missouri, the lead ores occur almost to the exclusion of the zinc ores, and are more disseminated through the limestone than is characteristic of the zinc ores. Ores of this type have been found extending only to shallow depths (not over a few hundred feet), and because of the absence of precious metals their treatment is comparatively simple.
The second class consists of ores more complex in nature, which are found in association with igneous rocks, and which usually contain some or all of the metals, zinc, silver, gold, copper, iron, manganese, antimony, bismuth, and rare metals, with various gangue minerals among which quartz, siderite, and silicates are important. Today these ores are the source of about 70 per cent of the world's lead. They are represented by the lead deposits of the Rocky Mountain region (Cœur d'Alene, Idaho; Leadville, Colorado; Bingham, Utah; etc.); of Broken Hill, New South Wales; of Burma; and of many other places. They are all related to the earlier stages of the metamorphic cycle and occur in close genetic association with igneous activity. They include deposits in the body of igneous rocks,—in the form of well-defined veins, replacements along zones of fissuring and shearing, and disseminated masses,—as well as veins and replacements in the rocks, particularly in limestones, adjoining igneous intrusions. The deposits present a wide variety of shapes depending on the courses of the solutions by which they were formed. The materials of the ore minerals are believed to have been derived from the igneous rocks and to have been deposited by hot solutions. The source of the solutions—whether magmatic or meteoric—presents the same problems which have been discussed elsewhere (pp. 41-42). The ores are frequently mined to great depths. Because of their complexity they require involved processes of treatment to separate out the values.
Ores of this nature have already been referred to in the discussion of the copper ores of Bingham and Butte, and will be referred to in connection with the zinc-lead-silver ores of Leadville, Colorado. Special reference may be made here to the Cœur d'Alene district of Idaho, which is the second largest producer of lead in the United States.
The Cœur d'Alene deposits are almost unique in that theycontain galena as vein-fillings and replacements in quartzite, with a gangue of siderite (iron carbonate). Quartzite (instead of limestone) is an unusual locus of replacement ores, and siderite is an unusual gangue. These ores are believed to owe their origin to acid igneous intrusives, because of the close association of the ores with some of these intrusives, and because of the content of high-temperature minerals. Some of the ore bodies are found far from intrusives, but it is supposed that in such cases further underground development may disclose the intrusives below the surface. Secondary concentration has been insignificant.
In general, weathering of lead ores at the surface and secondary sulphide enrichment below are not so extensive as in the case of copper and zinc. Galena is fairly stable in the oxide zone, and even in moist climates it is found in the outcrop of many veins. Weathering removes the more soluble materials and concentrates the lead sulphide with the residual clay and other gangue. In some districts cerussite and a little anglesite are also found in the oxide zone. The carrying down of lead in solution and its deposition below the water table as a secondary sulphide is not proved on any extensive scale. In this respect it contrasts with zinc; and when the two minerals occur together, lead is likely to be more abundant in the oxide zone, and zinc in the sulphide zone below. Such a change in composition with depth is also found in some cases as the result of primary vertical variations in the mineralization.
Zinc metal has commonly gone under the name of "spelter." Brass and galvanized iron contain zinc as an essential ingredient. Of the total United States zinc consumption in normal times, about 60 per cent is used in galvanizing iron and steel objects to protect them from rust, 20 per cent is used in the manufacture of brass and other alloys, 11 per cent goes into the form of rolled sheets for roofing, plumbing, etc., 1 per cent is employed in desilverizing lead bullion, and the remaining 8 per cent is used for pigments, electrodes, and other miscellaneous purposes. During the war the use in brass-making was greatly increased.
The zinc content of the ores mined today ranges from a littleover 1-½ per cent in the Joplin district of Missouri, to 25 per cent and higher in some of the deposits of the Cœur d'Alene and other western camps, and over 40 per cent in certain bonanzas in British Columbia and Russia. The ores usually contain both zinc and lead in varying proportions, and sometimes gold, silver, and copper are present. Of the zinc produced in the United States, about 73 per cent is obtained from ores containing zinc as the principal element of value, about 25 per cent from zinc-lead ores, and 2 per cent from copper-zinc and other ores. The average grade of the straight zinc ores is about 2-½ per cent.
Of the world's zinc ore, the United States produces in normal times about one-third, Germany about one-fifth, Australia about 15 per cent, Italy, North Africa and Spain each about 5 per cent. The remaining 15 to 20 per cent comes from a large number of scattered sources, including Japan, East Asia, Norway and Sweden, Canada, Mexico, Austria, France, Greece, Siberia, and Russia. In the near future the Bawdwin mines of Burma will probably be increasingly important producers. Large reserves of zinc also exist in the Altai Mountains of southwestern Siberia, and in the Cordilleran region of South America. In short, zinc is one of the most widely distributed of metallic resources; there is consequently less necessity for great international movements than in the case of many other commodities.
The smelting of zinc concentrates is in general carried on close to the points of consumption and where skilled labor is available, rather than at the mines,—although smelters to handle part of the output have recently been built in Australia and in Burma. In Europe the great smelting countries have been Germany and Belgium, and to a lesser extent England and France. Before the war these four countries with the United States produced over nine-tenths of the world's spelter. Belgium did principally a custom business, and a large part of its exports went to England. Australian and Tasmanian zinc ores were the basis of the Belgian and English smelting industries, and also supplied about one-third of the German requirements. Since the war England has contracted to take practically the entire Australian output. This fact, in connection with war-time destruction of Belgian smelters, leaves the future of the Belgian zinc industry in some doubt. Germany may possibly lose to Poland its richest zinc mines,those of Silesia. German activity in the rich deposits of Mexico is to be expected. France controls the deposits of North Africa and satisfies a considerable part of its requirements from that source. Smaller movements of zinc include exports from Italy to England, and a complex interchange among the lesser producers of Europe. English and French zinc-smelting capacity was expanded during the war, and the industry in these countries is in a strong position. Japan also developed a considerable smelting industry during the war, importing ores from eastern Asia and Australia.
The United States normally smelts and consumes all its large production of zinc ores and does not enter foreign markets to any extent. Small amounts of zinc concentrates are brought in from Mexico and Canada to be smelted in bond. During the war,—when the Allies were cut off by enemy operations from the customary Belgian and German supplies of spelter, and by shortage of ships from Australian zinc ores,—Australian, Spanish, Italian, and other ores were imported into the United States, and large quantities of spelter were exported from this country to Europe. Mine and smelter capacities were greatly increased, over-production ensued, and with the cessation of hostilities many plants were obliged to curtail or cease operations. The United States has now about 40 per cent of the zinc-smelting capacity of the world. For the present at least the capacity is far in excess of the domestic requirements.
Before the war German control of the international zinc market was even stronger than in the case of lead. The German Zinc Syndicate, through its affiliations, joint share-holdings, ownership of mines and smelters, and especially through smelting and selling contracts, controlled directly one-half of the world's output of zinc and three-fourths of the European production. It regulated the Australian exports by means of long-term contracts, and had considerable influence in the United States. To some extent it was able to so manipulate the market that zinc outside the syndicate was also indirectly controlled. During the war political jurisdiction was used by the Allied countries to destroy this German influence.
In the United States the principal zinc-producing regions are the Joplin and adjacent districts of Missouri, Oklahoma, Kansas, and Arkansas, furnishing about one-third of the country's output; the Franklin Furnace district of New Jersey, and the Buttedistrict of Montana, each yielding about one-sixth of the total supply; and the Upper Mississippi Valley district of Wisconsin, Iowa, and Illinois, the Leadville district of Colorado, and the Cœur d'Alene district of Idaho, each producing between one-tenth and one-twentieth of the total. Smaller quantities are produced in Tennessee, New Mexico, Nevada, and several other states.
Reserves of zinc are ample for the future. They are now developed considerably in advance of probable requirements, a fact which causes keen competition for markets and renders zinc-mining more or less sensitive to market changes.
The most important mineral of zinc is the sulphide, sphalerite or "zinc blende." The minerals of the oxide zone are smithsonite (zinc carbonate) and calamine (hydrous zinc silicate), which yield minor amounts of zinc and are especially productive at Leadville, Colorado. Zincite (zinc oxide) and willemite (zinc silicate) are the important minerals in the deposits of Franklin Furnace, New Jersey. The association of most deposits of zinc with more or less lead has been noted.
The ores of zinc are of two general classes, corresponding to the two classes of lead ores (pp. 211-212). Zinc ores of the first type are in veins and replacements in sedimentary rocks at shallow depths, independent of igneous association, and are supposed to have been formed by cold solutions. They are found in the Mississippi Valley, in Silesia, and in many of the smaller European deposits. They were formerly the leading zinc-producers, and now produce about 45 per cent of the world's total. Zinc ores of the second type consist of veins and replacements related to intrusive rocks, sometimes extending to considerable depths, and of more complex composition. They include most of the deposits of the American Cordilleran region (Butte, Cœur d'Alene, Leadville, etc.), of Franklin Furnace, of Australia, of Burma, and of many other places.
The zinc-lead ores of the type found in the Mississippi Valley are of special interest, in that they are sulphide ores of an origin apparently independent of igneous agencies. These ores occur as fissure-fillings and replacements, mainly in nearly flat-lyingPaleozoic limestones and dolomites—the Bonne Terre dolomitic limestone of southeastern Missouri, the Boone formation of southwestern Missouri and Oklahoma, the Galena dolomite of Wisconsin and Illinois. They are variously associated with a gangue of dolomite, calcite, quartz, iron pyrite, barite, and chert. Not infrequently they are spread out both in sheets and in disseminated form along carbonaceous layers within or at the base of the limestone.
The source of the primary sulphides has been a subject of much discussion. All are agreed that they were first deposited with the sediments in minutely dispersed form, through the agency of the organic contents of the sediments, and that such deposition was somewhat generally localized by estuarine conditions which favored the accumulation of organic remains. Many years ago, before the evidence of estuarine deposition was recognized, Chamberlin suggested an ingenious hypothesis for the northern Mississippi Valley,—that the organic material had been localized by ocean's currents forming something in the nature of a Sargasso sea. Differences of opinion become acute, however, when the attempt is made to name the precise sedimentary horizon, out of several available horizons, in which for the most part this primary concentration occurred. Judging from the organic contents of the several beds, the primary source may have been below, within, or above the present ore-bearing horizons. If the ore came from the lower horizons, it was introduced into its present situation by an artesian circulation, for which the structural conditions are favorable. If the ore was derived from overlying horizons, downward moving solutions accompanying erosion did the work. If the primary source was within the horizon of present occurrence of the ores, both upward and downward moving waters may have modified and transported them locally. For each of these hypotheses a plausible case can be made; but much of the evidence can be used interchangeably for any one of them. In spite of the wealth of data available, it is astonishingly difficult to arrive at a conclusion which is exclusive of other possibilities. Without attempting to argue the matter in detail the writer merely records his view, based on some familiarity with these districts, that, on the whole, the evidence favors the accumulation of these deposits by downward moving meteoric solutions during the weatheringof overlying strata; but that it is by no means certain that a part of the ores has not been derived from lower horizons. The great area of the producing districts in comparison with their depth, the uniform association of the ore-bearing zone with the surface regardless of geologic horizon uncovered by erosion, the failure of the ores to extend in quantity under cappings of later formations, and the known efficacy of oxidizing waters in local downward transfers of zinc and lead, seem to suggest concentrating agencies which are clearly related to surface conditions.
It is of interest to note that in many places in the limestones of Missouri and Virginia, and elsewhere in the Paleozoic rocks, there are sinks of limonite and clay near the surface, which are likewise believed to have originated through downward movement of waters deriving their mineral contents from the erosion and stripping of overlying sediments. Still further, the primary deposition of Clinton iron ores in many parts of the Mississippi Valley and eastward to the Appalachians took place in stratigraphic horizons not far removed from the horizons of lead and zinc deposition. When the peculiar conditions controlling the deposition of the Clinton ores are understood (see pp. 52-53) it is entirely possible that they may throw some light on the genesis of the lead and zinc ores.
Since the ores were introduced into essentially their present locations, secondary concentration has produced an oxide zone of clay, chert, and iron oxide, with varying amounts of zinc carbonate, zinc silicate, lead sulphide, and rarely lead carbonate. This zone is obviously developed above water level, and is seldom as much as 100 feet thick. Zinc, and to a less extent lead, have been taken into solution as sulphates, with the aid of sulphuric acid resulting from the oxidation of the associated pyrite. Zinc has been carried away from the weathered zone in solution faster than lead, leaving the lead more or less concentrated near the surface. Some of the zinc carried down has been redeposited secondarily as zinc sulphide. Evidences of this secondary sulphide enrichment can be seen in many places; yet certain broad quantitative considerations raise a doubt as to whether this process has been responsible for the main portion of the values of the sulphide zone. If downward secondary enrichment had been a dominant process, it might be expected that the ores would be richer in places whereerosion had cut away more than half the limestone formation carrying the ore, than in places where it had barely cut into the formation. This is not the fact,—which suggests that erosion in its downward progress has carried a large part of the zinc completely out of the vicinity.
Zinc ores of this same general character are also found in Paleozoic rocks (Knox dolomite) in Virginia and Tennessee. Their manner of occurrence suggests the same problem of origin as in Missouri and Wisconsin, but no decisive evidence of their source has been discovered.
Of the zinc ores associated with igneous intrusions, those of the Butte and Cœur d'Alene districts are described in connection with copper and lead ores on pp. 201-203, 208, and 212-213.
Zinc constitutes about 75 per cent by weight of the recoverable metals of the Leadville district of Colorado. About two-thirds of the zinc occurs as the sulphide and about one-third as the carbonate resulting from weathering of the sulphide. The zinc sulphide is associated with lead, iron, and copper sulphides and gold and silver minerals. In the oxide zone the zinc carbonate is associated with oxides and carbonates of various metals, including those of lead, copper, iron, and manganese. The iron and manganese oxides are mined in considerable tonnage as a flux. It is an interesting fact that, although mining has been carried on in this district for upwards of forty years, only within the last decade has the existence of zinc ores in the oxide zone been recognized. This has been due largely to the fact that the iron and manganese oxides effectively stain and mask the zinc carbonate.
The Leadville ores occur as replacements and vein-fillings in a gently faulted and folded Carboniferous limestone, in deposits of a general tabular shape, parallel to the bedding but with very irregular lower surfaces. The limestone is intruded by numerous sheets of porphyry, mainly parallel to the bedding but sometimes cutting across it, against the under sides of which most of the ore occurs. The primary sulphides are believed to be genetically related in some fashion to these porphyries. The older view was that the agents of deposition were aqueous solutions from the surface above, which derived their mineral content chiefly from the porphyries. Later views favor solutions coming directly from the porphyries or deeper igneous sources. While in form andassociation these ores are characteristic igneous contact deposits, they lack the high-temperature silicates which are so distinctive of many ores of this type.
The zinc ores of Franklin Furnace, New Jersey, belong in the group associated with igneous agencies, but have certain unique features. They consist of willemite and zincite, together with large amounts of franklinite (an iron-manganese oxide) and silicates, in a pre-Cambrian white crystalline limestone near its contact with a coarse-grained granite-gneiss. The origin of the ores is obscured by later shearing and metamorphism, but it seems best explained by replacement of the limestone by heated solutions coming from the granitic mass. The view has also been advanced that the ores originated in the limestone before the advent of the igneous rocks. Secondary concentration is not apparent.
[33]Ransome, Frederick Leslie, The copper deposits of Ray and Miami, Arizona:Prof. Paper 115, U.S. Geol. Survey, 1919, pp. 12-13.
[33]Ransome, Frederick Leslie, The copper deposits of Ray and Miami, Arizona:Prof. Paper 115, U.S. Geol. Survey, 1919, pp. 12-13.
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 reëxported. 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 1-½ 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 99-½ 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 noreturn." 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.
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 transportationalconditions. 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 alterationsindicating 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.