The useful minerals occur in a variety of forms and conditions, and the deposits which are composed of or include them may be classified into superficial, stratified, and unstratified deposits.
In these, the materials are yet unconsolidated, and have been washed down from cliffs and mountain slopes composed of rocks that contain metals, ores, and gems, either in veins or irregularly disseminated. This category includes the gold of the surface deposits associated with the gold-bearing rocks of California, Colorado, Australia, the Ural mountains, etc.; the platinum of Oregon and Siberia; the "stream tin" of Cornwall, Banca, Australia, and Durango; the diamonds of Golconda, Brazil, and South Africa; and the rubies and sapphires of Ceylon. From such deposits all the platinum, all the diamonds, and probably nine tenths of all the gold in use have been procured. The working of these deposits constitutes the simplest form of mining, viz., washing with water. The minerals obtained from them are usually distributed very sparsely through the rocks in which they primarily occur; and as in the course of ages, by frost, sun, rain, and ice, these rocks have been comminuted and washed away, the metals and precious stones they contain have been sorted and concentrated by the action of water, so that in many instances they are gathered with little labor.
By the Spaniards, the superficial deposits containing gold are called placers, and the working of these deposits has come to be gradually termed placer mining.
The useful minerals sometimes form entire strata, such as beds of coal and iron ore; and they are sometimes disseminated through sedimentary rocks of which they form a larger or smaller part. In this latter category are included the clay-ironstone of the coal measures, which forms nodules thickly set in beds of shale, and the copper in the copper schists of Mansfeld, in the triassic sandstones of New Mexico, and in the sandstones and conglomerates of Lake Superior.
Formerly most of the deposits of crystallized iron ore (magnetite and specular iron) were supposed to be eruptive. We have learned now that they are for the most part only stratified deposits very much disturbed and metamorphosed. Such is certainly the character of the iron ores of Missouri, Lake Superior, and the Alleghany belt, all of which may be shown to be simply changed and disturbed beds of iron ore, once deposited in nearly horizontal sheets. The famous deposits of iron ore of the island of Elba and of Nizhni Tagilsk in Russia are still believed to be eruptive, but it is probable that they will hereafter be shown to belong to the same category with our American crystalline iron ores. Iron is a common ingredient of volcanic rocks, and in some instances the quantity contained in them is so great that they may be termed low-grade iron ores; but such cases are extremely rare, and it is very doubtful whether there are any deposits of metals or ores of economic value which can properly be regarded as eruptive masses.
It is scarcely necessary to say that the great deposits of metallic copper found on Lake Superior, and supposed at one time to be the result of subterranean fusion, are now clearly shown to be the products of chemical precipitation, and to have been deposited much as copper is precipitated in the electrotype process.
Of this class of deposits, magnetic iron contained in volcanic rocks and the copper in the amgydaloids of Lake Superior may be taken as examples. In Japan the iron derived from decomposing volcanic rocks is collected and used, and large quantities of copper are obtained from the so-called melaphyres of the Portage Lake district on Lake Superior; but as a general rule igneous rocks are very barren in useful minerals.
The plane of junction between two rocks of different kinds, such as igneous and sedimentary rocks, is frequently the place where metals or ores have accumulated and form concretions, strings, or sheets. Examples of this may be seen in the deposits of copper at the junction of the trap and ash bed and of the trap and sandstone in the Lake Superior series.
In certain cases metalliferous minerals are found diffused irregularly through rocky masses, the deposits of ore having no definite boundaries or any regularity of structure, and appearing as though the rock had soaked up or absorbed the minerals, as water saturates a sponge. Such accumulations of ore are called impregnations. The deposits of mercury exhibit this character in a marked degree.
This name has been given to a peculiar kind of deposit where the ore is sparingly diffused through certain layers, which are prone to disintegrate and are more fahl (i. e., foul or rotten) than the associated strata. Typical examples of this kind of deposit may be seen in the silver mines of Kongsberg, Norway, but they are not common elsewhere. Usually the fahlbands are only rich enough for working where cut by veins.
"Where the masses of metalliferous rock are penetrated in every direction by threads or strings of ore, so that the whole must be taken out together, it is called a stockwork. Such deposits occur locally in many mines, but rarely to such a degree as to give character to the mining operations. The copper mines of Lake Superior, and the silver mines of Norway, Freiberg (Saxony), and Nevada, all furnish examples of stockwork.
These are usually sheets of mineral matter, of greater or less lateral and vertical extent. They have been divided into three principal varieties, which are generally well marked, but which sometimes blend in such a way as not to be easily separated. These varieties of mineral veins are known as gash veins, segregated veins, and fissure veins. a. Gash Veins. These are such as are confined to a single stratum or formation, and hence are of limited extent both laterally and verti-cally. The best examples of gash veins are seen in the lead mines of the upper Mississippi. Here the ore is found in a single formation, the Galena limestone, a member of the Trenton group of the lower Silurian. It usually occurs in vertical fissures at no great depth, sometimes very narrow, sometimes opening into caves or chambers lined with " mineral." These gash veins have apparently been formed by the shrinkage of the Galena limestone after its deposition. Subsequently the shrinkage cracks were enlarged by the dissolving away of their walls, and were lined with galena, deposited from a solution which exuded from the adjacent rocks.
Similar veins also containing galena occur in Missouri, but in a formation somewhat more ancient, the magnesian limestone, supposed to be the equivalent of the calciferous sand rock of New York. The fact that these gash veins are limited to a single formation has been amply proved by numerous shafts sunk in the hope of finding the ore at a greater depth, all of which have been failures, b. Segregated Veins.
Fig. 1. - Gash Viens filled with Lead Ore Galena Lime stone. a.Crevice opening.b, c. Crevices with pocket openings.
Fig. 2. - Horizontal Gash Veins or Floors of Lead in Galena Limestone, a. Crevice with pocket opening, b. Crevice opening.
Fig. 3. - Segregated Veins of Auriferous Quartz in Gneiss.
These are usually lenticular sheets of ore-bearing mineral, which are conformable to the bedding of the associated rocks, that is, are interposed between the layers of such rocks. Segregated veins always occur in metamorphic rocks, and are usually inclined at a high angle with the horizon. They are called segregated veins because they are supposed to have been formed in the process of metamorphism by the separation or withdrawal of the materials which compose them from the adjacent strata, and their concentration along certain lines. Segregated veins are limited both laterally and vertically. They rarely exhibit anything of the banded structure which characterizes fissure veins, are chiefly composed of quartz, and form the great repositories of gold. All the quartz veins, which are so common in the granitoid rocks of the Alleghanies, belong to this class and carry more or less gold. Iron pyrites is an almost constant associate of gold in segregated veins, sometimes being present in great quantities. Copper is also frequently contained in them, and in less quantity nickel. Though segregated veins have generally no great lateral or vertical extent, they sometimes attain a thickness of 20 or 30 ft., and have a length on the surface of a mile or more.
The Pine Tree and Josephine lodes on the Mariposa estate in California have a thickness of 4 to 12 ft., and are said to be traceable for some miles. These are reported to be segregated veins lying in sheets of slate rock of Jurassic age. Segregated veins are generally of much more modest dimensions, and are seen to lie alternately or en echelon along the outcrops of the containing rocks. In Australia the gold-bearing segregated veins are commonly termed " quartz reefs," having doubtless derived their name from the fact that, being harder than the associated rocks and yielding less readily to atmospheric erosion, they are left in relief, sometimes projecting in ridges above the surface. c. Fissure Veins. These are of indefinite extent, laterally and vertically. They have been formed by volcanic or earthquake action, by which the rocks have been fractured and displaced. In all cases where an important crack or fissure is made by subterranean upheaval, either by the slipping in of wedges of rock or by the shifting of the sides of the fissure so that their irregularities fail to match, the walls are prevented from returning to their original positions, and an irregular open crevice is produced.
When subsequently filled by foreign matter containing metals or ores, such a fissure becomes a fissure vein. In some instances the fracture of the rocks has considerable regularity, and the fissure may be of uniform width for several hundred feet in either direction. More generally, and especially where a fracture is attended with displacement, the fissure is of very unequal width; the vein matter has in places a thickness of many feet, while at other points, where the projecting walls approach or come in contact, the vein becomes very thin and may be quite pinched out. From their mode of formation fissure veins are without definite limits horizontally or vertically. They may frequently be traced for miles upon the surface, and their limits in depth are rarely reached. Hence they hold more extensive and continuous deposits of ore than any other kind of mineral veins, and constitute the most trustworthy bases for mining operations. Fissure veins frequently exhibit a banded structure in the materials which compose them, and this forms one of their most striking characteristics. This feature is produced by the deposition on their walls of successive layers of different minerals, such as quartz, fluor spar, calc spar, copper or iron pyrites, blende, galena, and baryta.
These layers often correspond on either side of the central line, showing that the deposition of the different sheets took place simultaneously on both walls. Sometimes a fissure vein exhibits a double or triple series of bands, showing that after being filled with ores it was again opened and a new fissure formed, and then this was filled in the same way as the first. The quartz which constitutes a large part of the material composing fissure veins frequently shows a "comby" structure, due to the formation of crystals which shoot out from the walls and interlock where they meet. Another common feature in fissure veins is the "fluccan " or "selvege," a sheet of clay which lines either wall and causes the vein matter to cleave off readily. This fluccan seems to be due partly to the attrition of the sides when moved with immense force upon each other, and partly to the action on the walls of chemical solutions filling the fissure. The sides and sometimes the interior of fissure veins generally show polished and vertically striated surfaces ("slickensides"). These are undoubtedly produced by the friction of the walls on each other or on the material composing the vein.
As will be inferred from what has been said of their mode of formation, fissure veins cut indiscriminately through all kinds of rock. They frequently traverse stratified rocks across their lines of deposition and outcrop, and are then called "cross-cut" veins, to distinguish them from those that are more or less accordant with the tratification. The materials composing fissure veins are very varied; indeed, it may be said that nearly all the minerals known are found in them. Quartz is a conspicuous ingredient in fissure veins, but sulphate of baryta, calc spar, and fluor spar sometimes form almost the entire mass of the veinstone. - The ores which are contained in fissure veins, like the earthy minerals are widely varied. Silver, copper, lead, tin, zinc, antimony, iron, and more rarely many other metals are found in them. Gold is less common in fissure than in segregated veins, and it is almost never the sole object of search in their exploitation; but it is a recognized constituent in the veins worked in Cornwall, and in the silver ores obtained from some of the mines in Idaho, Nevada, Mexico, and South America. Silver may be regarded as the most valuable constituent of fissure veins, and all the great silver mines of the world are worked in veins of this character.
The Comstock lode, near Virginia City inNevada, is a true fissure vein, and perhaps the largest and richest known. It has been traced on the surface for several miles, and has been worked to the depth of 1,600 ft. Its normal width is perhaps 200 ft,, but in places it expands to 800 ft, though here divided into seveal veins by great wedges or "horses," split off from the walls. It cuts through syenite and propylite, and evidently marks the line of a great fissure opened by volcanic action. All the silver mines of Nevada belong to the same class with the Comstock, though the other veins are generally of much more mod.-rate dimensions They are worked in fissure vems which traverse all the varieties of rock found in that region, such as granite, porphyry, trachyte, slate, and limestone. The number of these veins, the disturbed and broken condition of the strata, and the abundance of volcanic rocks in the district, all prove that it has been long the theatre of intense volcanic, and earthquake action. In Nevada and Utah some very rich mines have been worked in deposits of ore, of which the true character has been imperfectly understood and very much misjudged.
These are the chambers or pockets of ore so characteristic of the White Pine district, Nevada, and that of Little Cottonwood canon in Utah, the latter including the famous Emma mine. These districts have been the centre of intense mining excitement and scenes of the wildest speculation; of the most unparalleled successes and sudden and complete failures. A large part of this history has been consequent on the peculiar nature of these mineral deposits. In both these districts the ore occurs in limestone, and often in chambers frequently of considerable size. These when first opened were supposed to hold incalculable wealth, but they proved to be of limited extent and were soon worked out. The relations of the ore chambers of White Pine and Little Cottonwood to the silver-bearing fissure veins of Nevada and Utah are not at first sight apparent, and yet they are unquestionably products of the same general cause. The true theory of their deposit is probably this. The "country rock," i. e., the rock enclosing the deposits of ore, unlike that of most of the mining districts of the west, is limestone. Many limestones are soluble in atmospheric water which holds carbonic -acid in solution.
In some limestone countries the underlying rock is honeycombed by caves and subterranean galleries, forming a system of underground drainage. The table land of Kentucky affords a typical example of such a region, and the Mammoth cave is only one of an immense system of natural sewers, by which the drainage of the surface is effected. If now the Kentucky table land were much disturbed by an earthquake and fissures were opened through the limestone, and these fissures were filled to form mineral veins, then, wherever these fissures communicated with the subterranean chambers and galleries, these would also be filled with vein matter and ore, and a condition of things would be produced similar to what we now find in Utah and Nevada, though on a much grander scale. With this explanation the western pockets and chambers of silver ore are seen to be natural offshoots and appendages of the fissure veins so common in the region where they occur. - The Filling of Mineral Veins. The manner in which the materials composing mineral veins have been deposited has been a matter of much discussion among geologists, and one about which there has been and still is considerable diversity of opinion. The theories which have been advanced to account for the phenomena are briefly as follows, a.
The Theory of Injection. This was proposed by the Plutonists, who were prone to ascribe all the great changes on the earth's surface to the action of heat. There are few mineral veins, however, composed of materials which can be regarded as even the possible product of fusion, and most of them contain minerals which could never have been formed in the presence of great heat. The veins containing great masses of copper on the south shore of Lake Superior, when first described, were considered as shining examples of the truth of the igneous theory; but the frequent occurrence of masses of native silver in the copper, both metals being distinct and almost chemically pure, prove that these metallic masses could never have been fused together, as in that case they would have formed an alloy. Other evidence has been cited by Prof. Pumpelly which demonstrates that none of the copper veins have been filled by igneous action, but that the materials they contain have been deposited from solution. Trap dikes, which are fissures filled by injected volcanic material, have doubtless suggested the igneous theory of mineral veins; but when they are carefully examined the materials which compose them are found to be quite different in their nature and arrangement from those which form mineral veins.
In fact, dikes and veins have only this in common, that they fill similar fissures produced by subterranean violence.
Fig. 4. - Section of a Fissure Vein, showing banded structure, a a. Country rock, b b. Calc spar, c c. Galena. d d. Heavy spar - sulphate of baryta, e e. Comby quartz.
Fig. 5. - Double Fissure Vein, a a. Country rock, b b. Calc spar, c c, e e. Comby quartz, d. Heavy spar. A B. First and second fissures.
Fig. 6. - Fissure Vein with Cavity of "Tug" at Centre. aa Country rock, b b. Heavy spar, c c. Calc spar. dd. Blende, e e. Coinby quartz. f. Vug.
This theory apparently emanated from the Wernerian school, who regarded water as the great, if not the sole cause of geological phenomena. The advocates of this theory have suggested that fissures have been opened up into seas or other water basins, and that the vein material has been deposited from water, as limestone and other sedimentary rocks are laid down. A fatal objection to this theory is that we never find the materials composing true fissure veins horizontally stratified as aqueous sediments are, but on the contrary these materials are often deposited vertically against the walls of the fissures. Again, if the vein materials were deposited from reservoirs into which they opened, the bottoms of these reservoirs ought to show similar sheets of matter, whereas nothing of the kind has ever been found, c. Lateral Secretion. According to this theory, the materials of mineral veins have been derived from the adjacent rocks by percolation through the vein walls. If this were true, we should find the contents of veins changing with every stratum through which they pass, whereas in fact the composition of a mineral vein is often nearly identical in all parts of its course, notwithstanding it may pass through a variety of strata.
Again, where two systems of veins cut through the same stratum, according to this theory, in that stratum their contents should be similar, whereas we often find them totally diverse. Where two veins cross each other, they are often seen to be of different ages, and to be composed of materials so different that they must have been derived from different sources. The banded structure of fissure veins seems also quite incompatible with this theory, for it is scarcely possible to conceive of the formation of the different layers which compose these veins on the supposition that they have been deposited by exudation from the walls of the fissure, and that the totally distinct minerals composing the inner and newer layers have been transmitted through those first formed. As has been mentioned, in gash veins the cavities are filled or lined with materials derived from the adjacent rocks, but these cases afford us no satisfactory explanation of the filling of fissure veins, the only ones about which there is any question, d. Sublimation. Most of the minerals, and perhaps all of the metals, can be sublimed at a very high temperature; and some of them, as zinc, arsenic, and mercury, are vaporized at a comparatively low temperature.
The fissures about a volcanic crater are frequently lined, sometimes filled, with minerals, some of them ores, which have plainly been driven out from the volcano in a state of vapor. Such cases have led some theorists to suppose that sublimation played an important part in the filling of mineral veins. As has been said, the deposits of mercury have often the character of impregnations, and in some instances at least we have good evidence that mercury is diffused in the form of vapor; but these deposits have certainly very little in common with the distinctly limited, often banded and crystallized matter filling mineral veins, properly so called. Hence this theory is in the main rejected by modern mineralogists, e. Chemical Precipitation. This theory attributes the deposition of mineral matter in veins mainly to preeipitation from solution, and this is the view now generally taken by those best informed on the subject. According to this theory, fissures destined to become fissure veins are first filled with water, usually flowing from sources deep in the earth, where, highly heated and under great pressure, it becomes charged with mineral substances.
As it approaches the surface and the temperature and pressure are reduced, its powers of solution are diminished, and a large part of the materials it has carried are precipitated on the sides of the channel through which it flows. The abundant and varied deposits made by thermal springs illustrate the sufficiency of this cause. In this view, the banded structure which is exhibited by mineral veins is attributed to changes during the lapse of ages in the nature of the solution, dependent upon some deep-seateel cause, such as successive convulsions opening new sources for the supply of material. Sulphur we know is one of the most common constituents of volcanic emanations, and the normal condition of most ores found in veins is that of the sulphide; and wo have reason to believe that they are mainly deposited from a hot solution in which sulphur was the most conspicuous ingredient. Highly heated water or steam, containing sulphur, fluorine, and chlorine, would be capable of dissolving most of the minerals with which it came in contact.
It would certainly be charged with silica, and if flowing or driven through rocks containing even minute quantities of silver, gold, load, iron, copper, or other metal, would gather these materials, and coming toward the surface would precipitate them in the form of sulphides. The replacement of animal and vegetable tissues by mineral matter which often occurs in their fossilization, affords proof that chemical solution is entirely adequate to produce all the phenomena exhibited in the tilling of mineral veins. In petrified wood the vegetable tissue is replaced, particle by particle, by silica, evidently deposited from solution. The sulphides of copper and iron often replace wood in the same way, and this could only take place on a great scale, as it often does, when the rocks were saturated with a solution containing these metals. The formation of geodes, the filling of the cavities of amygdaloids with agate, chalcedony, and zeolites, the sheets and stalactites of iron and lead in the Galena mines, and the stalactites of lime in caves, prove that such solutions are constantly flowing through the rocks beneath us.
In gome formations and localities molluscous fossils are found completely replaced by galena, pyrites, and blende, the lime of their shells having been carried away and the different ores deposited in its place. In the cavities left by some shells, successive layers of sulphides of lead, iron, and zinc are deposited, showing in miniature almost precisely the phenomena observed in mineral veins. - A few general features in mineral veins remain to be noticed. Oftener than otherwise, the mineral veins of any district are seen to belong to one or more systems in which the individual veins have nearly a common bearing and a general similarity of composition. In the mining district of Cornwall, England, there are two principal systems of veins, one running nearly north and south, the other approximately east and west. The latter carry copper and tin, the former chiefly lead and iron. In the lead region of the upper Mississippi there are also two principal systems of veins, which vary somewhat in their bearing, but are generally known as the north and south and east and west courses.
In the mining district of Frei-.berg. Saxony, nine systems of veins are said to have been identified; and in the silver belt extending parallel with the Pacific coast, from Idaho and Nevada to Chili, which has been almost constantly shaken and shattered by earthquakes, the systems of veins are almost innumerable. In less disturbed regions, like the Mississippi lead district, the courses of the veins coincide with the jointing of the rocks, and thus in many instances exhibit a kind of polarity; that is, one set of joints coincides with or approaches in direction the meridian, while the other is nearly at right angles to this. There is little doubt that the system in the jointing of rocks, and hence in the bearings of mineral veins, often determined by the jointing has been considerably affected by terrestrial magnetism. It is also probable that this cause has operated to control or influence the deposition of mineral matter in veins. Mr. Fox of England found that the water in the copper mines was a weak solution of salts of copper, and that the galleries filled with this solution were in fact cells of galvanic batteries, from which well marked currents were produced.
Solutions similar to these found in old copper mines, but hotter and stronger, have undoubtedly filled most of the fissures now occupied by metalliferous veins. It is easy to see that such cells might generate powerful magneto-electric currents, by which the metals, especially silver and copper, might be precipitated in great quantity, just as they are now precipitated in the electro-plating process. - Gossans. Nearly all mineral veins are found to be very much weathered and decomposed along their line of outcrop. The decomposition generally extends down to the permanent water level, below which the ore is in its normal state, and this for the most part is sulphide. When exposed to the action of atmospheric water and air, the sulphides are oxidized, and the whole mass of the veinstone is frequently rendered soft and spongy, and highly colored in various ways. When the vein contains much iron pyrites, this is converted into the hydrated sesquioxide of iron, coloring all the decomposed mass brownish red. From this fact the changed portion of the vein is called in Germany the Eisenhut, iron hat. In Cornwall the decomposed portion of a mineral vein is called a gossan; and this term has been universally adopted in all mining districts where English is spoken.
In the gossans of veins we usually find the sulphides of silver converted into chloride, bromide, iodide, etc, with many sprigs and masses of native silver. Copper ore, generally the sulphide of copper and iron in its normal state, is converted first into red or black oxide, and then into malachite, azu-rite, and chrysocolla, the carbonates and silicate of copper. Locally, when effected by saline solutions, as in South America, ataca-mite, the chloride, is produced. All these secondary forms of ores are more easily treated than the sulphide, and the gossan which contains them is usually loose and easily excavated. This portion of a mineral vein is therefore much more easily and cheaply worked than that which lies below the permanent water level. Hence the first workings of mineral veins are frequently highly remunerative, while the cost of deeper excavations in harder rock, and the expense of treating the more intractable sulphides, cause subsequent operations below the water level to result in disappointment.
In many mining districts, like those of the southern Alleghanies in the United States and of Sonora in Mexico, the first comers, by working the gossans, were able practically to skim the cream of the mineral veins, carrying off great profits, and leaving to the second generation an inheritance of which the value is often worse than doubtful.