Table of Contents
The exploration of deposits of the metals will never become an exact science. There will always be an element of uncertainty in prospecting and developing mines. In countries where the surface has been closely scrutinized, most of the deposits whose outcrops contain valuable metals have probably been discovered. Many metalliferous deposits, however, are normally so much altered at the surface that the true significance of their outcrops is likely to be obscured.
If a deposit is not exposed, its presence may become known by some one of its characteristics that is different in kind or in degree from a similar feature of the associated rock. Thus, a deposit that is not exposed may be discovered because of its magnetic property. So few minerals are strongly magnetic that the value of magnetic surveys is limited, but the dip needle has nevertheless proved of great aid in locating magnetic belts, and these in some regions are associated with workable iron ores. The Cuyuna Iron Range of Minnesota, which does not exhibit any outcrop of iron ore, was discovered by drilling “areas of attraction” that are due to belts of magnetic rocks associated with the iron ores. In some regions the use of the dip needle is justified, even if it does no more than reveal the strike of the rocks, since a knowledge of the latter enables the driller to locate and point his holes to advantage.
The measures of gravity by the use of the pendulum are so accurate, or the limit of error is so small, that one might detect the presence of a concealed tabular deposit approximately at the surface, 34 ft. (10.36 m.) thick, having a density double that of the surrounding rocks. However, not many deposits have a density twice that of ordinary rocks and a thickness as great as 34 ft.; if they lie a short distance below the surface the density or the thickness must be even greater. Consequently, there is not much hope of sufficiently developing the gravity method of detecting deposits of heavy ores to make it of practical value. If the limit of error could be reduced to about one-tenth the present factor, and if the rather tedious and painstaking methods of determining gravity that are now in use could be simplified, the method would promise some degree of usefulness in the exploration of certain types of heavy ores. This, however, seems unlikely, since great refinement of method is necessary to determine gravity with the degree of error that now exists. In the future, as in the past, the explorer will probably have to rely upon what he sees rather than upon some peculiar characteristic of concealed deposits.
Certain relations of the distribution of lode ores to geologic structures have long been recognized. These ores are deposited principally in regions of complex faulting and fracturing and in areas of igneous activity. The earlier prospector found that it was profitable to scrutinize the mountainous regions, for these in general are most complexly faulted and fractured, and are more likely to have been the centers of great igneous activities. In some regions, it is true, flat-lying rocks far removed from igneous centers yield deposits of the valuable metals; these, however, are rarely of the lode type.
Certain features of a region other than its outcrops may lead one to suspect the presence of ore lodes. Thus, the presence of placers will lead to a search for the sources of the metals they contain. In some regions, waters containing iron sulphate suggest the presence of deposits containing iron sulphides. At some places, many feet above the present streams, gravels are cemented by iron oxides that were evidently derived from the weathering of deposits containing iron sulphides. At Cananea, Mex., at Bingham, Utah, at Lead, S. D., below the Homestake lode, and in many other regions, iron-cemented gravels are conspicuously developed. These gravels were consolidated, long before the mines above them were opened, doubtless by iron oxide that was deposited by hydrolysis of iron sulphate formed by oxidation of pyrite during the weathering of the pyritic deposits.
Lode ores, it is believed, are deposited principally by ascending hot waters. This inference is justified since they are almost universally associated with intrusive igneous rocks. The solutions that course through fractures and shattered zones soak into the country rock. In some districts these solutions have wandered far from the master fractures, profoundly altering great areas of country rock. Where the prevailing hydrothermal alterations are sericitic, as at Butte, Mont., and in many other regions, the country rock is bleached. Where the alterations are propylitic, as in many of the precious-metal deposits of Nevada, igneous rocks by development of chlorites from dark minerals, become pale green. Prospectors, even those without academic training, know the significance of hydrothermal metamorphism. Some designate the result of such alteration as the “kindly look” of the rock and contrast it with the “hungry look” of the fresh unaltered rock which they have often found to be barren of ores.
Primary ore deposits fall into a few well-defined groups, each with characteristic features. These in different districts have definite relations to the geologic structure depending on their genesis. The mapping of the structures and the investigation of the relations of the deposits to structures are essential features of rational exploration.
Of the discoveries made in the United States within the past 10 years, perhaps as many have resulted from a knowledge of the geologic conditions of the districts containing the deposits as from the exploration of outcrops that were supposed to be the altered cappings of valuable deposits. The exploration made with an adequate knowledge of the structure of an area and also with an understanding of the characteristic superficial alteration of its deposits is the most likely to succeed.
Some outcrops themselves contain the valuable metals; others have had the valuable metals leached from them, but may carry the alteration products of associated minerals and these frequently furnish the clue leading to the discovery of orebodies. Pyritic copper sulphide deposits and pyritic zinc sulphide deposits are generally leached of copper and zinc near the surface. In the earlier days of mining many of the deposits were discovered incidental to mining associated metals less readily dissolved at the surface. In general these deposits were stained with iron oxide. The copper deposits and zinc deposits of Butte, Mont., were discovered by exploration of silver ores in the upper oxidized parts of the veins. The United Verde mine, Jerome, Ariz., the Highland Boy mine, Bingham, Utah, and the Mount Morgan mine, Queensland, were worked first as gold mines. All subsequently developed great bodies of copper ores carrying noteworthy amounts of gold.
Stimulated by examples of iron-stained rocks passing downward into workable copper ores, explorers investigated ferruginous outcrops systematically and many deposits were discovered below the valueless cappings. Many of the disseminated copper deposits are barren of copper at the surface. The exploration of these deposits soon showed, also, that some are not everywhere capped by gossans heavily stained with iron oxide. The cap rock in general is composed largely of sericite, kaolin, and chalcedonic silica. Some contain also sulphates. Such associations, though not so easily recognized as limonitic areas, are nevertheless significant and the recognition of the origin of such an outcrop is certain to play an increasingly important part in the explorations of the future. Because such an association of alteration products is less readily recognized, outcrops of this character are the most likely to have been overlooked in an area that has not been fully explored.
The problems connected with the superficial alteration of ore deposits, although complex, are susceptible of analysis and experimental study. The changes are accomplished by water, air, and the compounds resulting from the action of water and air upon the ore itself. One may take the ore, expose it to water and air, and ascertain what products are formed. One may expose the ore further to the action of water and the products of alteration in the absence of air, and ascertain the changes, that take place. By analysis one may ascertain the composition of the waters of mines. There are now available more than 50 analyses of waters from mines of sulphide ores. These are similar in composition and closely resemble solutions formed by placing sulphide ore in contact with pure water. It may be assumed with confidence that solutions that have accomplished superficial alteration of sulphide ores are systems of sulphates, carbonates, and chlorides, of heavy metals, of alkalies and of alkaline earths.
Near the surface solutions are acid and generally contain ferric sulphate. In depth, ferric sulphate is reduced to ferrous sulphate, acidity is decreased by reaction with minerals of the ore and wall rock, and at greater depths the solutions become neutral and ultimately alkaline. Subjecting for long periods various minerals and various combinations of minerals to solutions such as are known to accomplish superficial alteration, one may ascertain what changes take place and compare the reactivities of ores of the various metals and their reactivities in various mineral associations.
Some of the metals are easily dissolved near the surface where waters are acid and oxidizing. Of these some are precipitated in depth where acidity decreases and where solutions are reduced because oxygen of the air is excluded. Deposits of such metals are likely to be leached at the surface. If the metals are readily precipitated in depth the deposits become enriched below the surface. Other metals are very difficultly soluble and because associated materials are dissolved and removed deposits of such metals are likely to be enriched at the surface. Associated gangue minerals are important also because some metals dissolve readily in certain associations but not in others. One may separate the metals into groups—one of metals that dissolve readily, another of metals that dissolve slowly, and still another of metals that dissolve very slowly.
Copper, because of its chemical relations, is easily leached from the surface and precipitated in depth. As already observed, the outcrops of a copper sulphide deposit may be so thoroughly leached that practically all the copper is removed and carried downward.
In base-leveled countries or in countries where the surface has remained nearly stationary for a long time, the outcrops are generally depleted of copper. Even in mountainous countries, where erosion is comparatively rapid, not many large deposits of copper are workable at the surface. Ferric sulphate hydrolyzes, depositing limonite, so the deposits of many iron-copper sulphide ores are marked by a gossan or “iron hat.” As already noted, many copper deposits have been discovered by following downward a nearly barren gossan or by the downward exploitation of deposits of precious metals that are concentrated near the surface above deposits of copper ores in which the precious metals are present in small amounts.
In copper deposits that do not carry sulphides the downward transportation of copper is generally slow. The native copper deposits of Keweenaw Point, Mich., are workable at the surface, although the country has undergone erosion for a period so long that it has become nearly a peneplain.
Where the sulphides are present in subordinate quantities, copper carbonates and silicates may occur abundantly at and near the surface, as at Ajo, Ariz., where oxidized copper minerals are conspicuous in outcrops.
In limestone, copper will commonly segregate as carbonate at and near the surface, and many oxidized copper deposits in limestone have been worked by open pits. Limestones that have been altered by contact metamorphism are relatively impermeable, because their tough, heavy silicates, such as garnet, amphibole, and mica, are not readily fractured. Most deposits of this nature contain considerable calcite, and any copper-iron sulphide ore they carry will usually oxidize to carbonates, silicates, and oxides. The copper in such an ore is particularly stable and is likely to endure long weathering. Such deposits have stimulated deep prospecting in many districts where other types of deeper copper ores are present, and they have thus served as useful indicators of hidden wealth.
The copper lodes at Butte (in igneous rocks) are leached of copper, some of them to a depth of 400 ft. (121.9 m.). The disseminated ores in porphyry show great variation as to depth of leaching, but are commonly leached to depths of 100 to 300 ft. (30.5 to 91.4 m.) below the surface, and exceptionally to greater depths. Some of them show practically no copper at the surface. At Cananea, Sonora, and Morenci, Ariz., barren gossans that were explored to considerable depths have led to good deposits of chalcocite ore. Copper was only sparingly-present at most places in the outcrops of the great disseminated deposits at Miami and Ray,. Ariz. At Bingham it was locally somewhat conspicuous as carbonates and silicates.
In making explorations for copper the question frequently is raised whether drilling is justified in an area that shows but little iron oxide at the surface. Nearly all copper deposits mined in North America do show ferruginous outcrops, but some gossans that cap valuable disseminated ores in porphyry are not heavily stained with iron. At Cananea, Sonora, valuable chalcocite deposits occur below outcrops that show heavy iron stain only here and there. As stated above, however, the outcrops show much silicification and kaolinization, and generally some limonite is present.
As a rule, the disseminated deposits of chalcocite ore will show outcrops more highly stained with iron in the earlier stages of their chalcocitization. As the country is eroded and the chalcocite zone descends, more and more pyrite and chalcopyrite are replaced by copper sulphides. A point may be reached where the chalcocite ore contains very little of the original iron sulphide. Obviously the oxidation of such an ore would yield but little iron sulphate. It would yield even less limonite, if the solutions were actively descending. If the process goes still further, the proportion of iron may become insufficient to dissolve all the copper in the chalcocite zone. The gossan then will generally carry oxidized copper minerals such as cuprite, the carbonates, basic sulphates, and chrysocolla.
Zinc, like copper, is very readily dissolved from its sulphide deposits and in the presence of much pyrite or other iron sulphide, zinc near the surface is likely to be almost completely removed from its deposits. It forms the very soluble zinc sulphate. But zinc in depth is not so readily precipitated by sulphides as copper. It will remain dissolved in solutions from which copper may be precipitated. Thus secondary concentrations of zinc sulphide ores are rare compared, with the secondary ores of copper.
In limestone, a solution of zinc sulphate precipitates zinc carbonate ores. These ores have lately become prominent, particularly in several districts of western America, where superficial changes have produced notable concentration of zinc.
A common type of ore in limestone consists of pyrite, argentiferous galena, sphalerite, a little chalcopyrite, and other sulphides in a gangue of quartz. The orebodies, like many deposits in limestone, are commonly large irregular masses. In the oxidation of such a deposit the lead and much of the silver remain essentially in place, the galena being in part oxidized to anglesite and cerusite. The oxidation of pyrite and sphalerite yields acid, and zinc and iron sulphates, which are carried out of the deposits in great quantities. A part of the iron remains behind as oxide, but in some deposits practically all the zinc is removed. When the solution, which is doubtless acid and carries ferric, ferrous, and zinc sulphates, moving along a water channel, encounters the limestone that surrounds the orebody, it will precipitate iron and zinc.
Under some conditions a zinc-iron carbonate or sideritic smithsonite, monheimite, is formed. This reaction has recently been investigated by Wells. Dilute solutions of two metallic salts in equivalent (molar) quantities were precipitated with only enough sodium carbonate for one metal. With equivalent quantities of zinc and calcium nearly all the zinc and only a trace of calcium are precipitated. With equivalent quantities of iron and calcium, nearly all the iron and only a trace of calcium are precipitated. Sideritic smithsonite, or monheimite, contains iron carbonate in varying proportions. Some smithsonite is nearly pure and some contains as much as 20 per cent, of iron carbonate, or even more.
After it is formed the smithsonite, or monheimite, with the progress of the erosion of the country, is exposed to more highly oxygenated waters. The iron carbonate then oxidizes and stains the ore brown so that it may easily be mistaken for iron-stained limestone. Thus deposits of this character, though exposed in underground workings, have been overlooked for years.
Silver, like copper, is readily dissolved in ground waters. If the chloride forms in the gossan its solution is delayed. The chloride, however, is to be regarded as a temporary mineral except in arid countries where chlorine is commonly, present in considerable quantities in earth waters. In such countries more chloride forms and, moreover, its solution is prevented by chlorides present in ground waters. There may, therefore, be great enrichment of silver at and near the surface. In depth, silver is carried in acid solution. Air or ferric iron is necessary for its active solution in sulphate waters. However, the carbonate of silver is soluble and without much doubt some silver is carried downward in carbonated waters. Silver is precipitated from its sulphate and carbonate solutions in a sulphide environment. For this reason great bodies of secondary silver ore may be deposited below a leached or low-grade capping.
In acid waters that carry chloride in the presence of manganese oxides, gold is readily dissolved; by reduction of acidity, gold is precipitated. Frequently the manganese and gold are precipitated together; in deposits that contain manganese, gold may be earned downward in solution and accumulate below the outcrop. But gold is readily precipitated from its solutions by many minerals, and its migration is slow. In the presence of carbonates, or of other minerals that reduce acidity readily, gold in manganiferous deposits tends to remain near the surface and in manganiferous carbonate gangue it may even accumulate as placers. If the metal were not of great value, its secondary concentration would be of little economic importance because it is dissolved near the surface only in chloride solution and in solutions that reconcentrate sulphide ores and chlorides are much less abundant than sulphates. In deposits which do not contain manganese, gold dissolves very slowly, if at all and tends to accumulate at the surface. Such deposits are commonly enriched by removal of material other than gold.
Uranium and Vanadium
Uranium and vanadium are readily dissolved in sulphate waters and both are regarded as mobile metals. Both metals are precipitated from the soluble salts by organic matter. The carnotite deposits of Colorado are believed to be concentrations from cold ground waters that dissolved the metals from associated rocks. In many deposits vanadium minerals occur in relations that leave no doubt of their secondary origin.
Iron and Manganese
Iron and manganese are grouped with the metals that are readily dissolved in sulphide ores. In sulphide deposits containing iron and manganese these metals are generally present in ground waters. The iron and manganese solutions, however, are not very stable and the oxides of iron and manganese are readily precipitated by hydrolysis. Consequently deposits that contain appreciable amounts of iron and manganese will generally carry these metals at the surface. Secondary deposits of iron sulphide or manganese sulphide are almost unknown. At depth, iron and manganese are deposited along fractures as oxides and more rarely as carbonates, but these ores in lode deposits are of relatively small importance. Deposits of iron and manganese are commonly enriched by removal of other materials. In general, iron and manganese deposits carry rich ores at the surface.
Nickel is similar to iron in its chemical activities, but unlike iron it does not oxidize in bivalent to form trivalent salts and its sulphate does not hydrolize readily and deposit oxide. It is dissolved almost as readily as copper in its sulphide combinations and it is precipitated in depth as sulphide. It is not so easily precipitated as copper sulphide, however, and will not be thrown down in an acid environment. It therefore resembles zinc in its migrational reactivities more closely than it resembles copper. No large deposits of secondary nickel sulphide have been recognized. The best-known deposits of nickel sulphide ores have been glaciated and possibly secondary sulphide zones have been removed. At the Lancaster Gap nickel mine, Pennsylvania, secondary millerite has been present in bodies of economic value. The gossan of nickeliferous pyrrhotite is essentially limonite. In the weathering of nickeliferous basic rocks the nickel accumulates as silicates not far below the surface.
Cobalt salts are soluble and cobalt is dissolved readily from outcrops. No deposits of secondary sulphides are known. Its chemical behavior is closely similar to that of nickel and zinc and it should be grouped with the metals which are concentrated under favorable conditions. Like nickel, cobalt forms silicates and concentrates in the superficial zones of silicate deposits which are undergoing alteration. Cobalt, like nickel, forms a moderately insoluble arsenate in some outcrops. Cobalt is found in considerable quantities in the oxidized material which caps the altered peridotite of New Caledonia.
Mercury is dissolved and reprecipitated in underground waters. The reactions go on very readily with chloride waters but not in sulphate solutions. Mercuric chloride is soluble in water and does not give insoluble basic salts with water. Mercuric sulphate, on the other hand, is easily hydrolized and gives basic sulphate which is reduced to native metal. Experiments made over long periods by Broderick show that cinnabar is practically insoluble in sulphate waters although it is readily dissolved in hydrochloric acid, and more readily still in presence of manganese oxide. The secondary enrichment of mercury deposits is somewhat similar to that of gold deposits. Neither metal is appreciably dissolved except in presence of chlorides. Reconcentration of mercury is most marked in arid countries, where chlorides are present. Chlorine, however, is quite subordinate in underground waters compared with sulphates. Underground waters, therefore, can not move large masses of the metal and concentrate them as copper and silver are concentrated.. Gold is so valuable that even small concentrations are important economically. Mercury is less valuable a concentration of 2 oz. of gold per ton would be highly significant, but 2 oz. of mercury per ton would be unimportant. There is indisputable evidence that secondary mercury minerals result from processes of sulphide enrichment, but these are small in amount because chlorides in general are not abundant in waters of deposits of mercury ores. Mercury must be placed with metals that are not migratory.
Lead is one of the least soluble of the common metals, although the chloride is fairly soluble and the sulphide is dissolved in sulphuric acid in the presence of an oxidizing agent. The oxidation is attended by the formation of the relatively insoluble lead sulphate, anglesite. This coats the sulphide and tends to delay solution. Deposits with ores carrying galena will generally contain oxidized lead minerals near the surface. It is not uncommon to find galena partly altered to anglesite in the outcrops of deposits containing lead sulphide. Lead minerals, because of their low solubilities, are useful indicators of mineral deposits, and at many places have led to the discovery of silver, gold, and zinc deposits. Like deposits of other difficultly soluble metals; lead deposits are likely to be enriched by removal of valueless materials rather than by concentration in depth by the processes of solution and precipitation.
Antimony dissolves very slowly in the oxidizing zone. Its sulphide deposits form the oxides which are about as stable as the secondary lead minerals, cerusite and anglesite. In depth, however, antimony minerals dissolve readily in alkaline solution. The antimony salts unite with silver-bearing solutions and precipitate antimony sulphosalts of silver. Antimony is not to be classed as a readily migratory metal though it plays an important part in the precipitation of the secondary silver minerals.
Arsenic dissolves rather readily in acid solutions, but if it is present as a salt of H3AsO3 much water will hydrolize it, and if present as salts of a base-forming element these also are hydrolized. In an acid solution its salts are oxidized but do not migrate extensively. In alkaline solutions, however, they are easily dissolved and in depth the activities of arsenic in connection with the secondary enrichment of silver are important.
Bismuth is more or less closely allied to antimony and arsenic. Like them it forms oxides near the surface in its deposits. There is no evidence of its extensive migration. Reasoning from its chemical relations it is not likely that important secondary zones formed by the migration of bismuth will be found. Its sulphate hydrolizes readily in water and in weak acid, forming oxides, and its other salts also hydrolize so readily that they would not be transported.
Tin is one of the inert metals. The stannous salts, both sulphate and chloride, are easily soluble, but the stannic salts are readily hydrolized. Thus, in an oxidizing environment such as obtains in sulphide ores near the surface of the earth, the solution of tin is exceedingly difficult. The stannous salts when formed are oxidized to stannic salts and stannic salts will break down almost at once to the insoluble oxide, cassiterite.
Thus tin, although it may go in solution, will be almost immediately precipitated. Experiments by J. P. Goldsberry in the geological laboratory of the University of Minnesota have shown that cassiterite and stannite are both practically insoluble in tenth normal sulphuric acid and hydrochloric acid. Doelter observes that tin oxide is slightly soluble in water. This statement was found by Goldsberry to be erroneous. The method of determination would have detected one part in a million. At the end of one month, tenth normal sulphuric acid, in contact with stannite and cassiterite, showed only faint traces of tin dissolved. Tin is enriched but little by migration in its deposits, because of the hydrolysis of its stannic salts which, as above stated, are precipitated as the insoluble oxide.
Chromium, Molybdenum, Tungsten and Uranium
The metals—chromium, molybdenum, tungsten, and uranium—are closely affiliated chemically and all form acid trioxides like that of sulphur, SO3. Chromium forms also the very stable basic oxide Cr2O3 which resembles ferric oxide in its properties. It forms in silicate environment and is a constituent of igneous rocks. In sulphide deposits chromium is dissolved and reprecipitated, but so far as any evidence is available it is not very migratory. Molybdenum and tungsten oxidize slowly to secondary minerals which generally remain near the parent primary minerals.
Molybdenite is not dissolved in hydrochloric acid nor in sulphuric acid, nor, at the end of one month, in the presence of ferric salts.
Tungsten forms tungstic acid, H2WO4.H2O. This is somewhat soluble in water. In moist climates, it could be leached out of deposits which were exposed long enough, but the reaction is exceedingly slow. Tungsten minerals are so insoluble that they commonly form placers. Both molybdenum and tungsten are classed with the non-migratory metals.
Uranium, on the other hand, is carried in underground waters and doubtless forms secondary deposits of considerable importance. The deposits of southwestern Colorado, as already stated, are believed to be reconcentrations by ground water near the present surface.