1911 Encyclopædia Britannica/Mineral Deposits
MINERAL DEPOSITS. The subject of mining (q.v.) can only be properly understood after the general features of mineral deposits have been elucidated. In this article deposits of all kinds of useful minerals are included, whether they are metalliferous or earthy. In general practice it is customary to treat the former under the name “ore-deposits” and the latter as the “non-metallics.” This is warranted because in a large degree different geological problems are presented and different methods of mining are pursued. Nevertheless there are other important similar or common features and they may be classed together without great disadvantage.
The word “ore” is used in several meanings, each of which depends for its special significance upon the connexion. In Ore. purely scientific applications “ore” implies simply a metalliferous mineral, and in this sense it appears in works on mineralogy and petrology. In former years and in connexion with practical mining an ore was defined as a compound of metal or of metals with one or more non-metallic elements, called mineralizers, of which oxygen and sulphur were the chief. The ore must, in addition, be sufficiently rich to be mined at a profit. Native metals not being compounds were not considered ores. The product of the copper mines on Keweenaw Point, Lake Superior, was, and to a great extent is still, called copper rock rather than copper ore, and native gold in quartz is often described as gold quartz rather than gold ore, but these restrictions are gradually disappearing. An ore may therefore be defined as a metalliferous mineral or aggregate of metalliferous minerals mingled with a greater or less amount of barren materials called the “gangue,” and yet rich enough to be mined at a profit. When not proved to be sufficiently rich to be remunerative, the aggregate is called “mineral.” The “mineral” of to-day may be changed by the advent of a railway or the rise in the price of metal into the “ore” of to-morrow. The question has repeatedly appeared in litigation involving contracts or property rights.
Since the greater number of the ores are believed to have been precipitated from aqueous solution, or to have been otherwise formed through the agency of water, the term “ore-deposit” has resulted; and inasmuch as nearly all the other useful minerals owe their origin to the same agent, the term “mineral deposit” is equally well justified. A few, however, have been produced in a different way, such as certain iron ores of igneous origin; certain igneous rocks used for building stone, as in the case of granite; and the accumulations of vegetable material in coal beds. These latter, the igneous masses and the vegetable, accumulations, being placed in two divisions by themselves, we may group the larger number into two main classes, depending on their precipitation from solution or from suspension. In the case of solution we will further subdivide on the place, and therefore in large part on the cause, of precipitation, since these are the questions chiefly involved in actual development.
Especially in connexion with ore-deposits widening experience has modified the older conceptions of relative values in the several types. In the early days of geology, Cornwall and Saxony were the two regions where the most active and influential students of ore-deposits were trained and where the principal books relating to mining originated. The pronounced and characteristic fissure veins of England and Germany became the standards to which the phenomena met elsewhere were referred, and by means of which they were described. This particular form, the fissure vein along a fault, assumed a predominating importance, both in the thought and in the literature of the day. Widening experience, however, especially in the Cordilleran region of North America, in the Andes of South America, in Australia and in South Africa, has brought other types into equally great and deserved prominence. Comprehensive treatment to-day therefore departs somewhat from earlier standards.
As far as analyses and estimates permit, the common useful metals occur in the earth's crust in approximately Occurrence. the following percentages:—
By the letter x is meant some undetermined digit in the corresponding place of decimals. Apart from aluminium, iron, manganese and nickel, the figures show how small is the contribution made by even the commoner metals to that portion of the mass of the globe which is open to observation and investigation.
As compared with the earth's crust at large certain of the metals are known to be locally present in favourable, usually igneous, rocks in richer amounts, according to the following determinations which have been made upon large, samples of carefully selected materials. Copper, 0.009%; lead, 0.0011-0.008; zinc, 0.0048-0.009; silver, 0.00007-0.00016; gold, 0.00002-0.00004. Iron and aluminium seldom fail, and vary from 1 to 2% as a minimum, up to 25% as a maximum.
In order that the several metals may constitute ores, their percentages must be the following—the percentages of each vary with favourable or unfavourable conditions at the mine, and can therefore be expressed only in a general way; ores favourable to milling and concentration may go below these limits, and the mingling of two metals of which one facilitates the extraction of the other may also reduce the percentages:—
Cobalt is a by-product in the metallurgy of nickel and is usually in much inferior amount to the latter.
When we compare the first and second tabulations with the third it is at once apparent that with the possible although only occasional exception of iron the production of an ore-body from the normal rocks which constitute the outer mass of the earth requires the local concentration of each of the metals by one or several geological processes, and to a degree that is only occasionally developed in the ordinary course of nature. It is, therefore, an instance of somewhat exceptionally good fortune when one is discovered, and it is only the part of ordinary prudence to develop and utilize it as one would treat a resource which is limited and subject to exhaustion.
The minerals which constitute ore-bodies are divided into two Classes of Mineral. great classes: the ores proper, which contain the metals; and the barren minerals or gangue, which reduce the yield.
The ores are generally and naturally subdivided into two groups: first, the sulphides and related compounds containing arsenic, antimony, tellurium and selenium; and, second, the oxidized compounds embracing oxides, carbonates, sulphates, silicates, phosphates, arsenates, chromates, &c. With the oxides are placed, because of related geological occurrence, a few rare compounds with chlorine, bromine and iodine into which silver more than any other metal enters, and to the same group we may add a few metals which occur in the native state. Iron, manganese, aluminium and tin differ from the rest of the metals in their original occurrence in the oxidized form, whereas the others with the exception of gold, platinum, and possibly copper, in their first precipitation in ore-bodies are in the form of sulphides or related compounds. Only by subsequent changes, characteristic of the upper parts of the deposits, do they pass by oxidation into the minerals of the second group.
With regard to the nature and source of the water which serves to gather up the widely disseminated metals and concentrate them in ore-bodies two contrasted views are now current, not necessarily antagonistic but applied in different degrees by different observers. The older view attributes the water primarily to the rainfall, and therefore it is called meteoric water. After falling upon the surface the meteoric water divides into three parts. The first, and smallest, evaporates; the second, the largest portion, joins the surface drainage and is called the run-off; while the third, intermediate in amount, sinks into the ground and mingles with the ground-waters. The ground-waters rise in springs, usually fed from no great depth, and themselves pass into the surface drainage after a small subterranean journey. While as a rule the ground-water level is fairly definite, yet it sometimes displays even in the same mining district great irregularity.
The section of active circulation and work of the descending meteoric waters between the surface and the ground-water level was called by Franz Posepny (1836-1895) the vadose or shallow region (“Genesis of Ore-deposits,” Trans. Amer. Inst. Min. Eng., xxiii., xxiv., 1893; reprinted as a book, 2nd ed., 1902). It has been long recognized by miners as the home of the oxidized ores, and the place of the work of the descending waters. The deep-waters are relatively motionless and their movements as far as visible are comparatively slow. But the really important feature of the ground-water as regards the filling of veins is the depth to which it extends. This remained a somewhat indefinite matter until L. M. Hoskins showed mathematically that cavities in the firmest rocks became impossibilities at about 10,000 metres. Down to some such limiting depth as an extreme the groundwater was believed by many to descend; to migrate laterally; to experience the normal increase of temperature with depth; the effect of pressure; the increased efficiency as a solvent peculiar to the conditions; and finally with a burden of dissolved gangue and ore to rise again, urged on by the “head” of the descending column. In its ascent it was supposed to fill the veins. Mining experience has, however, indicated that the known ground waters are comparatively shallow and seldom extend lower than 500-600 metres. It is conceivable that during faulting and the formation of great dislocations this upper reservoir might be tapped into greater depths and set in limited circulations through deeper-seated rocks. But so far as these objections have weight they have greatly restricted the vertical range of the meteoric ground-waters as they were formerly believed to exist.
In contrast with the meteoric waters outlined above, other waters are believed by many geologists to be given off by the deep-seated intrusive rocks, and are generally called magmatic. We are led to this conclusion by observing the vast quantities of steam and minor associated vapours which are emitted by volcanoes; by the difficulty of accounting in any other way for the amount and composition of certain hot springs; and by the marked and characteristic association of almost all ore-deposits in the form of veins with eruptive rocks. That igneous masses have been connected with the formation of veins is further brought out by the following general consideration, which has hitherto received too little attention. Aside from pegmatites, veins rich enough to be mined and even large veins of the barren gangue-minerals are exceptional phenomena when we compare the regions containing them with the vast areas of the earth which have been carefully searched for them and which have failed to reveal them. As components of the earth's crust the useful metals except iron and aluminium are extremely rare. Some sharply localized, exceptional, and briefly operative cause must have brought the veins into being. The universal circulation of the ground-water of meteoric origin fails to meet this test, since if it is effective we ought at least to find veins of quartz and calcite fairly universal in older rocks. In North America, moreover, by far the greater number of veins which have been studied date from the Mesozoic and Tertiary times. The ore deposits of older date are chiefly of iron and manganese and can be satisfactorily explained in many cases by the reactions of the vadose region, or by crystallization from molten masses.
In summary it may be stated that the meteoric waters are of great importance and of unquestioned efficiency in the shallow vadose region, or, as named by C. R. van Hise, “the zone of weathering.” In it the disintegration of rocks exposes them to the searching action of solutions, and the portions of ore-bodies already deposited undergo great modifications. The deeper and far more immovable ground-water probably extends to but moderate depth and is chiefly affected as regards movement by the head of waters entering heights of land and by local intrusions of igneous rocks. It is very doubtful if the normal increase of temperature with depth produces much effect. The meteoric waters are of altogether predominant importance in all surface concentrations of a mechanical character. The magmatic waters, on the other hand, seem to be of paramount importance and of great efficiency in producing the deposits of ores in the contact zones next eruptives, and in the formation of veins which are reasonably to be attributed to uprising heated waters in regions of expiring vulcanism. They start with their burden of dissolved metals and minerals under great heat and pressure, amid conditions favouring solution, and migrate to the upper world into cooling and greatly contrasted conditions which favour precipitation. Undoubtedly they are responsible for many low-grade deposits which have later been enriched by the action of descending meteoric waters. They are more copiously yielded, so far as we may judge, by acidic magmas than by basic ones.
The natural waterways are furnished by the cavities in rocks. They vary in size from very minute pores, where movement is slow because of friction, but where solution takes place, through others of all dimensions up to great fault-zones. The smallest cavities are the natural pores of minerals; cleavage cracks; the voids along the contacts of different minerals; cracks from crushing during dislocation; cellular lavas; volcanic necks; voids among the grains, pebbles, or boulders of fragmental rocks; joints; caves, and faults. So far as waters have deposited ores and yielded ore-bodies by subterranean circulations the latter are guided by some such controlling influence as these in all cases, and they will be selected as the governing principle in a large part of the scheme of classification. The types will be reviewed in the following order:—
|I.—Of Igneous Origin.|
|A.||Eruptive masses of non-metalliferous rock.|
|B.||Basic segregations from fused and cooling magmas.|
|C.||Deposits produced in contact metamorphism, most commonly by the action of intrusive masses on limestones.|
|II.—Precipitated from Solution.|
|B.||Impregnations in naturally open-textured rocks.|
|C.||Impregnations and replacements of naturally soluble rocks.|
|D.||Deposits along broken anticlinal summits and in synclinal troughs.|
|E.||Deposits in shear zones.|
|F.||Deposits in faults.|
|G.||Deposits in volcanic necks.|
|III.—Deposited from Suspension.|
|IV.—Carbonaceous Deposits from Vegetation.|
I. Of Igneous Origin.—A. Eruptive Masses of Non-metalliferous Rock.—Among the non-metallic objects of mining and quarrying which are of igneous nature, building stone is the chief. Granites, syenites, and other light-coloured rocks are the most important. These rocks occur as intrusive masses called bosses when of limited extent and diameter, and bathyliths when of vast, irregular area. The main point of importance is the jointing and cleavage, which should in each case yield blocks as nearly rectangular as possible so as to save tool treatment. Dark, basic igneous rocks in dikes, sills and surface flows are employed for macadam, and are often of excellent quality for this purpose.
B. Basic Segregations from Fused and Cooling Magmas.—A few ore-bodies, of which the best-known involve iron, are believed to result directly in the igneous processes by which molten rock cools and crystallizes. Thus magnetite, one of the common iron ores, is a widely distributed component in the eruptive rocks, rarely if ever failing in any variety. It is one of the first minerals to crystallize, and it possesses a much higher specific gravity than the other constituents. There is reason, therefore, to believe that, forming in some molten magmas in relatively large quantity, it sinks to or toward the bottom of the mass until the latter is at least greatly enriched with it, if not actually changed to iron ore. If the molten rock, after passing through a stage of partial crystallization, moves toward the surface of the earth, the body of ore may occupy almost any position in it other than the bottom. The flowing of the magma in original movements or from pressure sustained in subsequent metamorphic processes, or both, may give the ore the lenticular shape which is quite characteristic of magnetite bodies the world over. Almost all iron ores of recognized eruptive origin contain titanium oxide in amounts from a few units to over 40%. They are most frequently found in dark basic rocks. These ores are not at present of much commercial value because of the difficulties of treating titaniferous varieties in the modern blast furnace practice, but, there is little doubt that in the near future they will be extensively mined.
Non-titaniferous magnetite's, which often form lenses in gneissoid rocks of more acidic character than those with which the titaniferous are associated, are likewise believed by some observers to be of igneous origin, but there are equally positive believers in sedimentary deposition followed by metamorphism.
Besides magnetite, chromite is a characteristically igneous mineral and is always found in the richly magnesian rocks. Whether the relatively large masses which appear in serpentine are direct crystallizations from fusion, or whether they have segregated from a finely disseminated condition during the change of the original eruptive to serpentine, is a matter of dispute, but the general trend of later opinion is toward an original igneous origin. Although not strictly an ore, corundum is another mineral which is the direct product of igneous action.
A form of ore-body which marks a connecting and transitional member between those just treated and those of the next group is furnished by the sulphides of iron, nickel and copper which are found in the outer borders of basic igneous intrusions. Observers differ somewhat as to the relative importance to be attributed to reactions purely of the nature of crystallization from fusion or those brought about by the agency of gases or other highly heated solvents in the cooling stages. The most important example is afforded by the mingled ores of nickel and copper which are developed in their largest form in the region of Sudbury, Ontario, Canada, and are now the principal source of nickel for the world. The ores are chalcopyrite and pyrrhotite, the latter containing throughout its mass at Sudbury the mineral pentlandite, a rich nickel-iron sulphide and the real source of the nickel. With the base metal there are also found minute traces of the metals of the platinum group. Wherever these ore-bodies have been observed they invariably occur in the borders of intrusive masses. The sulphides constitute an integral part of the rocky mass, which shows almost no signs of alteration or vein production in the ordinary sense. Only some slight rearrangements have subsequently taken place through the agency of water, but all this is a small factor in the total.
C. Ore-Bodies produced by Contact Metamorphism.—Great bodies of igneous rock have often been forced in a molten and highly heated condition through other rocks when at a distance below the surface of the earth. After coming to rest they have remained during the cooling stages for long periods in contact with the surrounding walls. All molten igneous magmas are more or less richly charged with aqueous vapour, doubtless in a dissociated state; with carbonic acid and probably with other gases, especially those involving sulphur. During the cooling stages the gases are emitted and carry with them silica, iron, alumina and metallic elements in less amount, of which copper is the commonest, but among which are also numbered lead, zinc, gold and silver. If the rock standing next the intrusive mass is limestone, the silica and iron, and to a less degree the alumina, combine with the lime to the elimination of the carbonic acid and produce extensive zones of lime silicates, of which garnet is the most abundant. Disseminated throughout these garnet-zones are large and small masses of pyrite and chalcopyrite, oftentimes in amounts sufficient to yield large ore-bodies. Again in the limestone outside the garnet-zones, but none the less closely associated with them, are bodies of sulphides containing copper. The copper ores of Bisbee and Morenci, Arizona, of Aranzazu near Concepcion del Oro, Mexico, and of many other parts of the world not yet studied in detail are of this type. The eruptive which most frequently produces contact zones is of a marked acidic or siliceous character, since among eruptive these are the ones most richly charged with gases. When the copper ores are of low-grade in their original deposition it often happens that processes of secondary enrichment, which are later described, are required to bring them up to a richness which warrants mining. Less often than copper appear lead, zinc or gold ores in the same relations.
D. Pegmatites.—One other phase of eruptive activity needs also to be briefly mentioned before passing to the discussion of the ore bodies, which have hitherto chiefly occupied students of the subject. In the regions surrounding intrusive masses of granite we almost always see dikes or veins of coarsely crystalline quartz, felspar and mica radiating outward, it may be, for very long distances. They are believed to be produced by emissions from the eruptive similar to those which yield the garnet-zones just mentioned. The veins are technically called pegmatites. They are characteristic carriers of tin and of minerals containing the rare earths, and less commonly are known to yield gold or copper.
II. Precipitated from Solution.—A. Surface Deposits.—The chief ore-body under this type is furnished by iron. The peculiar chemical property possessed by this metal of having two oxides, a ferrous, which is relatively soluble, and a ferric, which is insoluble, leads to its frequent precipitation from bodies of standing or comparatively quiet waters. Ferruginous minerals of all sorts, but more particularly pyrite and siderite, pass into solution in the descending oxidizing or carbonated surface waters, either as ferrous sulphate, or as salts of organic acids, or ferrous carbonate, the last-named dissolved in an excess of carbonic acid. On being exposed to the atmosphere when the solutions come to rest, or to the breaking up of organic acids, or to alkaline reagents, or sometimes to fresh-water algae, the hydrated sesquioxide 2Fe2O3, 3H2O is precipitated as the familiar beds of bog ore. The ore usually forms earthy aggregates or crusts and cakes, but may also, as in the interesting case of the Swedish lake deposits, yield small concretions. Bog ores are not very rich in iron and are apt to have much sand and clay intermingled. If subsequently buried under later sediments they may become dehydrated and changed to red hematite, as in the case of some of the Clinton iron ores of the eastern United States. These widely extended beds in the lower strata of the Upper Silurian are often oolitic red hematite's, consisting of concentric shells of iron oxide and
chalcedonic silica, deposited around grains of sand. The most extensive of all ore-beds of this type and the mainstay of the German and Belgian smelting industry, are the Jurassic ores, locally called minette, of Luxemburg and the neighbouring territories. Three principal and several subordinate beds are distinguished, which furnish a product ranging from 30 to 40% of iron and between 1 and 2% of phosphoric oxide (P2O5). They are generally believed to have been deposited on the bottoms of embayments of the Jurassic sea. The iron was furnished by the drainage of the land and was precipitated, according to Van Werweke, as silicate, carbonate, sulphide and as several forms of oxide. More than two billions of tons are believed to be available. Very similar deposits occur in the Cleveland district, England, in the Middle Lias.
In the presence of much organic matter which creates reducing conditions, concretions and even beds of spathic ore or black-band may result and afford the ores of this type extensively utilized in the Scottish iron industry and formerly of some importance in the eastern United States.
The brown hematites often have more or less manganese, and manganese ores themselves may result by closely related reactions, since manganese is very similar to iron in its chemical properties. Aluminium is yielded by deposits of bauxite, the hydrated oxide, which in the states of Georgia and Alabama, of the United States, are the result of surface precipitations. In the depths it is believed that pyritous shales exist. The oxidation of the pyrite supplies sulphuric acid which takes into solution the alumina of the shales. Rising to the surface along a marked series of faults, the aluminium sulphate meets calcium carbonate in an overlying limestone, and the aluminium hydrate is precipitated as concretions at the vents of the springs.
Of scientific importance but as yet not of commercial value are the siliceous sinters deposited around the vents of hot springs which yield appreciable amounts of both the precious and the base metals. While surface precipitations in every particular, they are yet chiefly important in casting light on the processes of vein formation in the depths.
Non-metallic minerals which are deposited from solution on the surface of the earth are the salines, rock-salt, related potassium salts, gypsum and the rarer nitrates. The alkaline chlorides and gypsum are derived, in nearly all cases, from impounded bodies of sea-water, which, exposed to evaporation with or without constant renewal, finally yield beds of rock-salt and related minerals. Shallow estuaries cut off from the sea, it may be by the sudden rising of a bar during a heavy storm or brines impounded in deep bays with a shallow connexion as in the “bar theory” of Ochsenius, have given rise to the great stores of these minerals which are so extensively mined. The potassium compounds have only been found as yet in large quantities in the Stassfurt region of Germany, and seem to be due to the fact that in this locality the mother-liquors of the rock-salt deposits failed to escape, and were evaporated to dryness. The nitrates are chiefly obtained in northern Chile and are the result of the reaction of nitrogenous organic matter, upon alkaline minerals and under conditions where there is enough but not too much water.
Another very important mineral found in surface deposits formed from solution is asphalt. It has happened in various parts of the world, but especially in the island of Trinidad, in the Carribean Sea, that petroleum with an asphalt base has reached the surface, has evaporated, and has become oxidized so as to leave a residuum of asphalt suitable for street-paving or other purposes. So-called pitch-lakes are afforded which may be of great commercial value.
Again, if large sheets, crusts, stalactites and stalagmites are deposited from calcareous water by the escape of the solvent carbonic acid, beautiful ornamental stones are afforded, generally known as Mexican onyx.
B. Impregnations in Open-textured Rocks.—In a number of instances in various parts of the world naturally open-textured rocks have been discovered so richly impregnated with the metalliferous minerals as to be ores. The enriching minerals have been introduced in solution, and the solvent has found its way through the rock because of its natural character, and not because geological movements have opened it. Porous sandstones are one of the most common cases. Deposits of silver ores have been extensively mined at St George in southern Utah, consisting of films of argentite and cerargyrite, which have been precipitated upon fossil leaves, sticks, and in the sandstone itself. Over wide areas in the northern United States, copper in various minerals has been discovered in sandstones of Permian or Triassic age. At Silver Cliff, Colorado, silver ores have impregnated a volcanic tuff, while at the Boleo mines in Lower California tuffs yield copper ores. In at least two of the great copper mines on Lake Superior the native metal impregnates a conglomerate, and in a number of others it has enriched a cellular basalt, filling the blow-holes with shots and pellets. In the Commern district between Bonn and Aachen, sandstones of the Triassic Buntersandstein contain knots of galena, distributed over wide areas as impregnations. Organic matter is believed to have precipitated the galena by a reducing action upon percolating solutions of lead.
All these porous rocks have been fed by solutions which have entered along waterways, clearly due to faults or some extensive breaks which have provided introductory conduits. The solutions have then been tapped off from the main passages by the porous rock. They are, therefore, closely connected with faults.
Non-metallic minerals in the form of petroleum and asphalt may also impregnate sedimentary beds or other rocks of open texture. Many oil wells derive their supplies from lenticular beds of sandstone in the midst of impervious shales, and others, as those in the Mexican fields near Tampico, from volcanic tuffs. Asphalt may saturate both sandstones and limestones in such richness as to furnish a natural paving material when crushed, heated and laid. Brines are also yielded by porous strata and supply much of the salt of the world.
C. Impregnations and Replacements of Naturally Soluble Rocks.—Ore-deposits of great importance appear in different regions which can only be interpreted as having been formed by the replacement of some or all of a rock with the metallic minerals. The most common rock to yield in this way is limestone, because of its soluble nature, but important cases occur of others composed of silicates. Replacement implies the precipitation of the ore and gangue, molecule by molecule, in the position of the original minerals but without, as in pseudo morphs, the necessary reproduction of crystalline forms. Some waterway must of course introduce the ore bearing solutions, but it may be slight compared with the great size of the resulting ore-bodies. Lead and zinc ores, often carrying some silver, are those most widely distributed, as they were also the earliest recognized in deposits of this character. More than any other metals their association with limestone is pronounced. The replacements may be found near the supply fissure as in the great zinc deposits near Aachen, or the supply fissures may be obscure as at Leadville, Colorado. While ores occur in the limestone, they are often close along its contact with some relatively impervious stratum, which seems partly to have directed the circulations, partly to have checked or stagnated them, while precipitation took place. With the lead and zinc sulphides, pyrites and chalcopyrite are commonly associated in greater or less degree, the copper increasing locally. All the sulphides are exposed to oxidation above the ground-waters and mining in the upper levels has been often directed against the carbonate and sulphate of lead, or the mingled carbonate and hydrated silicate of zinc.
A non-metallic deposit formed by replacement and of much scientific interest is furnished by sulphur when derived from gypsum, as in the Sicilian and other localities of Europe.
D. Deposits along Anticlinal Summits and in Synclinal Troughs.—When strata experience folding they are violently strained at the bends, and, if stiff or brittle like limestone, often crack in limited fissures, which in anticlines open upward and in sync lines downward. They thus yield joints in relatively great numbers. Softer rocks, such as shales, are moulded by the strains without fracturing. Very gentle folds seem to have yielded such abundance of cracks in the lead and zinc district of the Upper Mississippi Valley as to cause the so-called “gash veins” which have been worked for many years. The crevices are not all vertical, but often run horizontally and are due to the parting and buckling of individual beds. The resulting ore-bodies are chiefly limited to a single great stratum, and are believed to have been formed by the infiltration of galena, blende and pyrite from overlying formations. When strata are stiff enough to buckle under violent folding and part so as to produce openings of a crescentic cross-section which afterwards become filled, there result the “saddle-reefs” so remarkably illustrated in the gold veins of Victoria, Australia, and in pitching anticlines of a much larger character in Nova Scotia.
Of far the greatest importance of all the ore-bodies in troughs are the iron ores of the Lake Superior region, now the most productive of all the iron-mining districts. In a series of sedimentary formations, generally of Huronian age, and with associated eruptives, there occur strata consisting of a cherty iron carbonate, which were probably originally marine deposits akin to glauconite. They rest upon relatively impervious rocks, and are often penetrated by basaltic dikes. The entire series has been folded, so that the cherty carbonates, shattered by the strains, have come to rest in troughs of relatively tight, impervious rocks. The descending surface waters have next altered them, have taken the iron into solution, and have redeposited it in the troughs as a slightly hydrated red hematite. The silica has usually been precipitated elsewhere.
The most important of the non-metallic which occur along anticlinal summits are petroleum and natural gas, but it is true only in a very limited sense that they are introduced in solution. The general cause of the accumulation is, however, the same as that of the metallic minerals, i.e. that storage cavities are afforded. In the most productive oil-fields it is the general experience to find the oil and gas impounded in porous rocks, either sandstones or limestones, at the crests of anticlines and beneath impervious shales which do not shatter or crack with gentle folding.
E. Deposits in Shear-Zones.—It sometimes happens both in massive rocks and in sediments that strains of compression have been eased by local crushing along comparatively narrow belts without appreciable or measurable displacement of the sides such as would be required by a pronounced fault. The word shear-zone has become quite widely used in recent years as a descriptive term applicable to these cases.
The gold-bearing reefs of the Transvaal present a good illustration. Beds of conglomerate consisting chiefly of quartz and quartzite pebbles have experienced crushing and shattering, and have had their natural porosity, much enhanced by these after-effects. Solutions of gold, coming through, have encountered pyrites and have had the gold precipitated upon the pyrites, which is itself often broken and granulated. In other regions shearing has led to sheeting and opening of the rocks by many parallel cracks but almost always with such marked displacement that the next type most correctly describes them. From any point of view the shear-zone is a natural transition to the fault and closely related to it.
F. Deposits in Faults.—This type of ore-body was one of the earliest established, and has always figured very prominently in the minds of students of the subject since the first systematic formulations of our knowledge. The dislocation of the earth's crust by faults has furnished either clean-cut fissure or else lines of closely set parallel fractures, whose combined displacement has been comparatively great. The faults go to relatively profound depths and they furnish therefore waterways of extended character, which may reach from regions of heat and pressure in depth to regions of cold and diminishing pressure above; thus from conditions favourable to solution below to conditions favouring precipitation toward the surface. Faults often occur, moreover, in connexion with eruptive outbreaks, and therefore in circumstances especially favourable to ore deposition. From all these reasons it is not surprising that the “true fissure vein” based on a profound fault has been the ideal of the prospector's search in many parts of the world, and has often been his reward. The historic veins of Cornwall and of Saxony are of this type, also the great silver veins of Mexico, the gold veins of California, the great silver-gold deposits of the Comstock lode, and many in South America.
Faulting often leads to great shattering of the country rock, and instead of being a clean-cut open cavity, there results a brecciated belt which may then be cemented by infiltrating ore and gangue. In the midst of this the richer ore occurs as bonanzes or chutes, which are succeeded by leaner stretches. The movement of the walls produces the polished surfaces specifically called “slicken-sides,” parallel to which the ore-chutes often run. The change in the character of the entering solutions from time to time gives a banded character to the deposit, so that from both walls toward the centre corresponding layers succeed one another. At the centre the last layers may meet as interlocking crystals in the familiar comb-in-comb structure or they may leave cavities called “vugs” into which beautiful and perfectly formed crystals project (see fig.). Fault fissures swell and pinch affording wide and narrow places in the resulting ore-body. They often intersect each other and one may throw or heave another, according to the mechanics of faulting as set forth under the article on Geology.
While fault-fissures have in no way failed in later years to be appreciated by mining geologists, yet they do not hold that predominant place which in the days of more limited experience was theirs. On the contrary, other types such as contact zones, replacements and impregnations are found to be of scarcely inferior importance. Nevertheless the last two, at least, must usually owe to the fault-fissure the waterway which has brought in the solutions.
A very peculiar non-metallic deposit found in fault-fissures and imitating the ordinary veins in all essentials is furnished by the asphaltic minerals, often described as asphaltic coals and known in mineralogy as “grahamite,” “albertite,” “uintaite,” “gilsonite,” &c. Petroleums with asphaltic bases have percolated into fault-fissures and have there deposited on evaporation and oxidation their dissolved burdens. The black coaly mineral presents all the geological relations of a fissure vein and is mined like so much ore.
G. Volcanic Necks.—A very unusual ore-body is furnished by this type, which is only known in a few instances. In two mines, however, in Colorado, the Bassick and the Bull-Domingo, there occur chimneys of elliptical cross-section filled with rounded boulders, and believed with much reason to be the tubes of small explosive volcanoes. After brief periods of activity they became waterways for uprising heated solutions which filled the interstices with ore.
III. Deposited from Suspension.—The ores which result from this process are all formed upon the surface of the earth and through the action of water. They are primarily the result of the weathering of rocks and of the removal of the loose products thus afforded in the ordinary processes of erosion.
A. Placers.—Many useful minerals, including some of a metallic character, are very resistant to the agents of decomposition which cause the disintegration of the common rocks. Thus magnetite is a mineral present in a minor capacity in all eruptives and in fairly large percentage in many of the basic types. It is proof against protracted exposure to natural reagents, and it is heavy. Becoming freed by the disintegration of the containing rock it is mingled with the transported materials of running streams, and settles with other heavy minerals wherever the current slackens to a sufficient degree. Concentration may thus ensue and beds of black sand result. If again deposits of loose sand containing more or less magnetite are exposed to the surf of the ocean, or even to the waves of lakes, a similar sorting action takes place on the beach. The magnetite remains behind while the undertow removes the lighter materials. Iron sands of either of these varieties are usually too rich in titanium to be of commercial value, but with the magnetite may be gold or platinum in sufficient amount to be of value.
While magnetite is the commonest of the ores to be found in placers, gold is the metal which usually gives them value. Wherever systems of drainage have eroded gold-bearing rocks, the gold has passed into the streams with the other detrital materials, and, even though in very fine flakes, being yet very heavy has sunk to the bottom in the slackened water and has there enriched the gravel. The gold tends to work its way through the gravels even to the bed-rock, or to some bed of interstratified and impervious clay, and there to be relatively rich. It favours also the insides of bends and the heads of quiet reaches. When a small tributary stream joins a larger one and is both checked itself and checks the current of the large one, the gold, as in the Klondike, tends to settle in relatively great abundance.
Pot-holes, strangely enough, or related rock-cavities, often fail to yield the nuggets, apparently because the swirl of the water and grit has ground them to impalpable powder. The particles have then been washed elsewhere.
When the gold-bearing gravels are panned down a small residue is obtained of all the heavy minerals in the gravel. Magnetite is the commonest and gives the technical name of “black sand” to the concentrate. With it, however, there are almost always found garnet and other less familiar minerals. If the stream valley has been hunted over by sportsmen with shot-guns or rifles, the lost shot and bullets are commonly caught in the pan. Even diamonds have been rarely noted and they may, indeed, be specially sought in gravels.
Along sea-beaches where great beds of auriferous gravel have been attacked by the surf, concentrated bars carrying nuggets and flakes of gold in workable quantity have not infrequently resulted. Cape Nome, Alaska, is perhaps the most productive of all. The gold in the beach-placers is usually worn by the constant attrition into extremely fine particles, and the flakes or colours are more difficult to save than in the case of stream-placers.
In some regions of gold-bearing rocks, as in the south-eastern United States, the products of superficial decay of rocks may remain in situ and be sufficiently charged with gold to be washed for the yellow metal. They are different from the usual placer deposit although hydraulicked in the same way. They might be properly considered residual deposits under the next head.
Auriferous stream-gravels of ancient and long-abandoned systems of drainage may remain beneath lava flows or later sedimentary accumulations and be the objects of underground mining. Both in Australia, where they are called “deep leads,” and in California, where they are called “buried channels” or “deep gravels,” they have been for many years the objects of mining. In California the bed-rock is usually slate or schist and a series of technical terms have resulted descriptive of the rich streaks. The bed-rock is called the rim-rock; the pay-streaks which appear on its sides, bench-gravels, and the lowest one the channel-gravel. Tunnels are often very skilfully driven through the rim-rock to strike the channel-gravel and at the same time preserve the proper slope for drainage and extraction. The buried channels in California have proved of much scientific interest from the remains of prehistoric man, skulls, mortars and pestles which they have yielded.
Among the non-metallic minerals sought from placers, phosphates for fertilizers hold a position of great importance.
B. Residual Deposits.—As contrasted with the placers whose materials are derived by transport from a distance, we sometimes find heavy and resistant minerals, once contained in the rock but freed by the process of decay and disintegration. The lighter loose materials are washed away and deposited elsewhere. The heavy remain behind in a concentrated condition. Iron ores of this character are known, and chromite is set free in the same way by the decomposition of serpentine.
In the decay of ferruginous rocks like limestones the iron may be changed to the insoluble ferric hydrate, brown hematite, and remain as veinlets and crusts throughout a mantle of clay. The brown hematite may be freed by artificial washing and used as an iron ore.
IV. Carbonaceous Deposits from Vegetation.—Far the most important of the non-metallic minerals are those composing the coal series. They yield entire strata analogous to other sedimentary rocks, but in most cases from vegetation which has grown in situ. They are found in all stages from nearly carbonized leaves and woody tissue in peat, through much more altered materials in lignite and bituminous coal to extremes in anthracite and graphite. The prime necessity for their preservation from decay is furnished by water, in or near which they must grow, and beneath which they must be deposited, so that oxidation may be retarded. In instances they have been heaped together by rivers, especially when at flood. The method of origin is fully discussed under Coal and under
Mining, but it may be remarked here that once formed thgiy undergo all the foldings, faulting and disturbances which have a ected the sedimentary rocks of other kinds.
Bibliography.—The following are general works on the deposits of the useful minerals, in addition to Posepny's volume already mentioned: In English—J. A. Phillips, revised by Henry Louis, Treatise on Ore-Deposits (London, 1896); J. F. Kemp, Ore-Deposits of the United States and Canada (New York, 1900); Prime's translation of Von Cotta's Ore-Deposits (New York, 1870); H. Ries, Economic Geology of the United States (New York, 1906); W. H. Weed's translation of Beck's The Nature of Ore-Deposits (New York, 1905); Genesis of Ore-Deposits (American Institute of Mining Engineers, 1901); G. P. Merrill, The Non-Metallic Minerals (New York, 1904). In German—B. von Cotta, Die Lehre von den Erzlagerstätten (Freiberg, 1859); A. von Groddeck, Die Lehre von den Lagerstätten der Erze (Leipzig, 1879); R. Beck, Lehre von den Erzlagerstätten (Berlin, 1904); A. W. Stelzner and A. Bergeat, Die Erzlagerstätten (Leipzig, 1905-1906). In French—E. Fuchs and L. de Launay, Traité des gîtes minéraux et métallifères (Paris, 1893); G. Moreau, Étude industrielle des gîtes métallifères (Paris, 1894). (J. F. K.)
- A. H. Barlow, “On the Sudbury Deposits,” Geol. Survey of Canada Ann. Rept., vol. xiv., part H; A. P. Coleman, Ann. Report of the Ontario Bureau of Mines, vol. xiv., part iii. (1905).