The Economic Aspect of Geology Part 27

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 quant.i.ties 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 quant.i.ties 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 pa.s.sed 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 sh.o.r.es 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 a.s.sociated with more or less iron sulphide, and frequently with the sulphides of antimony and a.r.s.enic, 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 a.s.sociations which indicate a particular type of igneous activity. They are not found in magmatic segregations, in pegmat.i.tes, 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 a.s.sociation 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 a.s.sumption 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 Jura.s.sic 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 a.s.sociated 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 a.s.sociated minerals are present, it is believed that waters of a hot-spring nature have again been effective. Thus cinnabar, when taken with its customary a.s.sociations, is regarded as a sort of geologic thermometer.

In the weathering of mercury deposits, cinnabar behaves somewhat like the corresponding silver sulphide, argent.i.te. 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 const.i.tuent of certain kinds of bra.s.s, 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 quant.i.ties 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 quant.i.ties, 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 s.h.i.+pped direct to consumers rather than via London, but it is not certain how future s.h.i.+pments 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 quant.i.ties 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 compet.i.tion 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 quant.i.ties which had acc.u.mulated in the Orient found their way into Europe and precipitated a sensational slump in the tin market.

GEOLOGIC FEATURES

The princ.i.p.al mineral of tin is ca.s.siterite (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 ca.s.siterite--which were present in the parent rocks in much smaller quant.i.ties, but which have been sorted out by the cla.s.sifying action of running water.

The original home of ca.s.siterite is in veins closely related to granitic rocks. It is occasionally found in pegmat.i.tes, 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 ca.s.siterite are often found minerals of tungsten, molybdenum, and bis.m.u.th, 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 ca.s.siterite 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 apat.i.te. The wall rocks are usually strongly altered and in part are replaced by some of the above minerals, forming coa.r.s.e-grained rocks which are called "greisen."

The origin of ca.s.siterite veins, in view of their universal a.s.sociation 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 a.s.sociation 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 coa.r.s.e-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 quant.i.ties of oxidized silver minerals and wood tin near the surface, with acc.u.mulations 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.

Ca.s.siterite 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 ca.s.siterite. 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 acc.u.mulated 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 subst.i.tute 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 gla.s.s. The princ.i.p.al use of uranium, however, is as a source of radium, with which it is always a.s.sociated.

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 quant.i.ties are mined in Cornwall, England, and in Germany. Production is decreasing. The European hospitals and munic.i.p.alities 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 s.h.i.+pped 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 quant.i.ties. 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 princ.i.p.al sources of uranium and radium are the minerals carnot.i.te (hydrous pota.s.sium-uranium vanadate) and pitchblende or uraninite (uranium oxide).