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MINERAL DEPOSITS
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See also:MINERAL DEPOSITS . The subject of See also:mining (q.v.) can only be properly understood after the See also:general features of mineral deposits have been elucidated. In this See also: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 See also:geological problems are presented and different methods of mining are pursued. Nevertheless there are other important similar or See also:common features and they may be classed together without See also:great disadvantage. The word " ore " is used in several meanings, each of which depends for its See also:special significance upon the connexion. In Ole. purely scientific applications " ore " implies simply a metalliferous mineral, and in this sense it appears in See also:works on See also:mineralogy and See also:petrology. In former years and in connexion with See also:practical mining an ore was defined as a See also:compound of See also:metal or of metals with one or more non-metallic elements, called mineralizers, of which See also:oxygen and See also:sulphur were the See also:chief. The ore must, in addition, be sufficiently See also:rich to be See also:mined at a profit. Native metals not being compounds were not considered ores. The product of the See also:copper mines on Keweenaw Point, See also:Lake See also:Superior, was, and to a great extent is still, called copper See also:rock rather than copper ore, and native See also:gold in See also: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 " See also: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-See also:day may be changed by the See also:advent of a railway or the rise in the See also:price of metal into the " ore " of to-morrow. The question has repeatedly appeared in litigation involving contracts or See also:property rights.
Since the greater number of the ores are believed to have been precipitated from aqueous See also:solution, or to have been otherwise formed through the agency of See also:water, the See also:term " ore-See also:deposit " has resulted; and inasmuch as nearly all the other useful minerals owe their origin to the same See also:agent, the term " mineral deposit " is equally well justified. A few, however, have been produced in a different way, such as certain See also:iron ores of igneous origin; certain igneous rocks used for See also:building See also: Comprehensive treatment to-day there-fore departs somewhat from earlier standards. As far as analyses and estimates permit, the common useful metals occur in the See also:earth's crust in approximately the following percentages: 1. See also:Aluminium 8.13 7. Copper . o•0000x 2. Iron . . 4.71 8. See also:Lead . o•0000x 3. See also:Manganese 0.07 9. See also:Zinc . o•0000x 4. See also:Nickel . o•ot to. See also:Silver . o•000000x 5. See also:Cobalt o.0005 tt. Gold . . O.0000000x 6. See also:Tin . . o•000x-•0000x 12. See also:Platinum o•00000000x By the See also:letter x is meant some undetermined See also: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 See also: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 See also: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, Occurrence. o•oorr-o•oo8; zinc, o•oo48-o•009; silver, o•000c7-0.00016; gold, 0.00002-0.00004. Iron and aluminium .seldom fail, and vary from r to 2% as a minimum, up to 25% as a maximum. In See also: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: Aluminium . 30 Nickel . . 2-5 Copper . . 2-10 Platinum . 0.00005 Gold . . . o•oo3-•00016 Silver . . o•03-0.16 Iron . . . 35-65 Tin . . . 1.5-3 Lead. . . 2-25 Zinc. . . 5-25 Manganese . 40-50 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 See also:production of an ore-See also:body from the normal rocks which constitute the See also:outer mass of the earth requires the See also:local concentration of each of the metals by one or several geological processes, and to a degree that is only occasion-ally See also:developed in the See also:ordinary course of nature. It is, therefore, an instance of somewhat exceptionally See also:good See also: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 great classes: the ores proper, which contain the mineral. metals; and the barren minerals or gangue, which reduce the yield. The ores are generally and naturally subdivided into two See also:groups: first, the sulphides and related compounds containing See also:arsenic, See also:antimony, See also:tellurium and See also:selenium; and, second, the oxidized compounds embracing oxides, See also:carbonates, sulphates, silicates, See also:phosphates, arsenates, chromates, &c. With the oxides are placed, because of related geological occurrence, a few rare compounds with See also:chlorine, See also:bromine and See also: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 See also:state. Iron, manganese, aluminium and tin differ from the See also:rest of the metals in their See also: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 See also: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-See also:waters. The ground-waters rise in springs, usually fed from no great See also:depth, and themselves pass into the surface drainage after a small subterranean See also:journey. While as a See also:rule the ground-water level is fairly definite, yet it sometimes displays even in the same mining See also:district great irregularity. The See also:section of active circulation and See also:work of the descending meteoric waters between the surface and the ground-water level was called by See also:Franz Posepny (1836-1895) the vadose or shallow region (" See also:Genesis of Ore-deposits," Trans. Amer. Inst. See also:Min. Eng., See also:xxiii., See also:xxiv., 1893; reprinted as a See also:book, 2nd ed., 1902). It has been See also:long recognized by miners as the See also:home of the oxidized ores, and the, place of the work of the descending waters. The505 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 See also:matter until L. M. See also:Hoskins showed mathematically that cavities in the firmest rocks became impossibilities at about ro,000 metres. Down to some such limiting depth as an extreme the ground-water 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 See also:peculiar to the conditions; and finally with a See also:burden of dissolved gangue and ore to rise again, urged on by the " See also:head " of the descending See also: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 See also:lower than 500-600 metres. It is conceivable that during faulting and the formation of great dislocations this upper See also:reservoir might be tapped into greater depths and set in limited circulations through deeper-seated rocks. But so far as these objections have See also:weight they have greatly restricted the See also: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 See also:steam and See also:minor associated vapours which are emitted by volcanoes; by the difficulty of accounting in any other way for the amount and See also: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 See also:consideration, which has hitherto received too little See also: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 See also: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 See also: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 See also:crystallization from molten masses. In See also: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. See also:van Hise, " the See also:zone of weathering." In it the disintegration of rocks exposes them to the searching See also: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 See also:movement by the head of waters entering heights of See also: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 See also:mechanical See also:character. The magmatic waters, on the other See also:hand, seem to be of See also: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 See also:heat and pressure, amid conditions favouring solution, and migrate to the upper See also:world into cooling and greatly contrasted conditions which favour precipitation. Undoubtedly they are responsible for many See also: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 See also:judge, by acidic magmas than by basic ones. The natural waterways are furnished by the cavities in rocks. They vary in See also:size from very See also:minute pores, where movement is slow because of See also: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; See also: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 See also:influence as these in all cases, and they will be selected as the governing principle in a large part of the See also:scheme of See also: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 See also:metamorphism, most commonly by the action of intrusive masses on limestones. D. Pegmatites. II.—PRECIPITATED FROM SOLUTION. A. Surface deposits. 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. A. Placers. B. Residual deposits. IV.—CARBONACEOUS DEPOSITS FROM VEGETATION. I. OF IGNEOUS ORIGIN.—A. Eruptive Masses of Non-metalliferous Rock.—Among the non-metallic See also:objects of mining and See also:quarrying which are of igneous nature, building stone is the chief. Granites, yenites, and other See also:light-coloured rocks are the most important. 1 hese rocks occur as intrusive masses called bosses when of limited extent and See also:diameter, and bathyliths when of vast, irregular See also: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 See also:save See also: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 See also: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 See also: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 See also: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 See also:titanium See also:oxide in amounts from a few See also: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 See also:modern blast See also:furnace practice, but there is little doubt that in the near future they will be extensively mined. Non-titaniferous magnetites, 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 See also:positive believers in sedimentary deposition followed by metamorphism. Besides magnetite, See also:chromite is a characteristically igneous mineral and is always found in the richly magnesian rocks. Whether the relatively large masses which appear in See also:serpentine are See also:direct crystallizations from See also:fusion, or whether they have segregated from a finely disseminated See also:condition during the See also:change of the original eruptive to serpentine, is a matter of dispute, but the general trend of later See also:opinion is toward an original igneous origin. Although not strictlyan ore, See also: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 See also: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 See also:Sudbury, See also:Ontario, See also:Canada, and are now the principal source of nickel for the world.' The ores are chalcopyrite and See also: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 See also: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 See also:factor in the See also: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 See also:acid and probably with other gases, especially those involving sulphur. During the cooling stages the gases are emitted and carry with them See also: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 See also:standing next the intrusive mass is See also:limestone, the silica and iron, and to a less degree the alumina, combine with the See also:lime to the elimination of the carbonic acid and produce extensive zones of lime silicates, of which See also: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 lime-stone 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, See also:Arizona, of Aranzazu near See also:Concepcion del Oro, See also: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 eruptives 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, See also:felspar and See also: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 See also: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 See also:atmosphere when the solutions come to rest, or to the breaking up of organic acids, or to alkaline reagents, or sometimes to fresh-water See also:algae, the hydrated sesquioxide 2Fe2O3, 3H2O is precipitated as the See also:familiar beds of See also:bog ore. The ore usually forms earthy aggregates or crusts and cakes, but may also, as in the interesting case of the See also:Swedish lake deposits, yield small concretions. Bog ores are not very rich in iron and are See also:apt to have much See also:sand and See also: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 See also:Clinton iron ores of the eastern See also:United States. These widely extended beds in the lower strata of the Upper See also:Silurian are often oolitic red hematites, consisting of concentric shells of iron oxide and _ma ' A. See also:Barlow, " On the Sudbury Deposits," Geol. Survey of Canada See also:Ann. Rept., vol. xiv., part H; A. P. Coleman, Ann. See also:Report of the Ontario See also:Bureau of Mines, vol. xiv., part iii. (1905).
chalcedonic silica, deposited around grains of sand. The most extensive of all ore-beds of this type and the mainstay of the See also:German and Belgian smelting See also:industry, are the See also:Jurassic ores, locally called minette, of See also: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 I and 2 % of phosphoric oxide (P205). They are generally believed to have been deposited on the bottoms of embayments of the Jurassic See also: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 See also:Cleveland district, England, in the See also:Middle See also:Lias.
In the presence of much organic matter which creates reducing conditions, concretions and even beds of spathic ore or See also:black-See also: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 See also: 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 See also:series of faults, the aluminium sulphate meets See also:calcium carbonate in an overlying limestone, and the aluminium See also: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 See also: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-See also:salt, related See also:potassium salts, See also: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 See also: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 See also:bar during a heavy See also: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 See also:Stassfurt region of Germany, and seem to be due to the fact that in this locality the See also:mother-liquors of the rock-salt deposits failed to See also:escape, and were evaporated to dryness. The nitrates are chiefly obtained in See also:northern See also: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 See also:asphalt. It has happened in various parts of the world, but especially in the See also:island of See also:Trinidad, in the Carribean Sea, that See also: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 See also:street-paving or other purposes. So-called See also:pitch-lakes are afforded which may be of great commercial value. Again, if large sheets, crusts, See also: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 See also: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 See also:George in See also:southern See also:Utah, consisting of films of See also:argentite and See also:cerargyrite, which have been precipitated upon fossil leaves, sticks, and in the See also:sandstone itself. Over wide areas in the northern United States, copper in various minerals has been discovered in sandstones of See also:Permian or Triassic See also:age. At Silver Cliff, See also:Colorado, silver ores have impregnated a volcanic See also:tuff, while at the Boleo mines in Lower See also:California tuffs yield copper ores. In at least two of the great copper mines on Lake Superior the native metal impregnates a See also:conglomerate, and in a number of others it has enriched a cellular See also:basalt, filling the See also:blow-holes with shots and pellets. In the Commern district between See also:Bonn and Aachen, sandstones of the Triassic Buntersandstein contain knots of See also: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 extensivebreaks which have provided See also: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 See also:wells derive their supplies from lenticular beds of. sand-stone in the midst of impervious shales, and others, as those in the Mexican See also:fields near See also: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 See also: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, See also:molecule by molecule, in the position of the original minerals but without, as in pseudomorphs, the necessary See also:reproduction of crystal-See also:line 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 See also:Leadville, Colorado. While ores occur in the lime-stone, they are often See also: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, See also: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 See also:interest is furnished by sulphur when derived from gypsum, as in the Sicilian and other localities of See also: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 synclines downward. They thus yield joints in relatively great See also:numbers. Softer rocks, such as shales, are moulded by the strains without fracturing. Very See also:gentle folds seem to have yielded such abundance of cracks in the lead and zinc district of the Upper See also: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, See also:blende and pyrite from overlying formations. When strata are stiff enough to See also:buckle under violent folding and part so as to produce openings of a crescentic See also:cross-section which afterwards become filled, there result the " See also:saddle-reefs " so remarkably illustrated in the gold veins of See also:Victoria, Australia, and in pitching anticlines of a much larger character in Nova See also: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 See also: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-metallics which occur along anticlinal summits are petroleum and natural See also:gas, but it is true only in a very limited sense that they are introduced in solution. The general cause of the See also: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 See also: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 See also:recent years as a descriptive term applicable to these cases. The gold-bearing reefs of the See also:Transvaal present a good See also:illustration. Beds of conglomerate consisting chiefly of quartz and See also: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 forrnulations 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 See also:cold and diminishing pressure above; thus from conditions favour-able 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 See also:search in many parts of the world, and has often been his See also:reward. The historic veins of See also:Corn-See also:wall 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 See also:country rock, and instead of being a clean-cut open cavity, there results a brecciated See also: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 " slickensides," parallel to which the ore-chutes often run. The change in the character of the entering solutions from See also: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 See also:comb-incomb 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 See also: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 pre-dominant place which in the days of more limited experience was theirs. On the contrary, other types such as contact zones, re-placements 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," " See also:albertite," " uintaite," " See also: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 See also: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. 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 See also:proof against protracted exposure to natural reagents, and it is heavy. Becoming freed by the disintegration of the containing rock it is mingled withthe ,transported materials of See also: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 See also: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 See also:fine flakes, being yet very heavy has sunk to the bottom in the slackened water and has there enriched the See also:gravel. The gold tends to work its way through the gravels even to the See also: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 See also:Klondike, tends to See also: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 See also:powder. The particles have then been washed elsewhere. When the gold-bearing gravels are panned down a small See also: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 See also: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 See also:Nome, See also:Alaska, is perhaps the most productive ofall. The gold in the beach-placers is usually worn by the constant See also:attrition into extremely fine particles, and the flakes or See also: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 See also:ancient and long-abandoned systems of drainage may remain beneath See also:lava flows or later sedimentary accumulations and he 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 See also: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, See also: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 See also: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 See also: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 See also: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 See also: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 See also:tissue in See also:peat, through much more altered materials in See also:lignite and bituminous coal to extremes in See also:anthracite and See also:graphite. The See also:prime See also: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 See also:rivers, especially when at See also:flood. Additional information and CommentsThere are no comments yet for this article.
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