Veins

Veins, in geology, masses of rock which occupy fissures in other rocks. They may have originated in many different ways and present a great variety of forms and structures. We may classify them in three groups: (i.) veins of igneous rock, (ii.) of sedimentary, and (iii.) of minerals deposited by water or by gases.

Veins of igneous rock are practically the same as dikes; yet a distinction is sometimes made that dikes are narrow, often straight-walled and run for considerable distances, while veins are irregular, discontinuous and of limited extent. Where granite invades sedimentary or metamorphic rocks it very commonly emits vast numbers of dikes. The margin of the granite is full of blocks of all sizes, so that it is often impossible to say where the solid granite ends and the fringe of veins begins.

An intrusion plexus of this sort seldom extends for more than a few hundred yards; many granites, on the other hand, have sharp and well-defined margins and send few veins into the country rock.

In plutonic rock areas veining is also very common. Great intrusive masses have not as a rule been injected in one stage but have been slowly enlarged by gradual or repeated inflows, and often the earliest portions had consolidated before the last were introduced. Very frequently the older rocks are of a different character, being usually more basic than those which succeed them, and this makes the veining more obvious. For instance, it is common to find peridotite traversed by many veins of gabbro, or diorite injected with numerous veins of granite, though in either case the rocks are part of one plutonic boss or laccolite. The crystalline structure of the vein-rock and the surrounding mass is usually quite similar and there may be no fine-grained edges to the veins; these facts establish that the older mass though solid had not yet cooled down, so that the veining is directly connected with the injection process and the two rocks have been derived from the same source, but one is slightly later than the other.

Among the Laurentian or Lewisian gneisses, which resemble granites, diorites and gabbros in composition, but have a banded or foliated structure, veining of this type is almost universal. The veins are of all sizes and of very irregular shape. Frequently they run along the foliation of the gneiss, but often also they cross it obliquely or at right angles. Such gneisses were produced by the injection of a partly differentiated and consequently non-homogeneous magma, by successive stages, under a rock crust which was in movement or was subjected to intermittent pressures during consolidation.

In certain cases the new material introduced into the rock by these veins bulks almost as largely as the original substance. A shale, slate or phyllite is sometimes so filled with threads of granite that its composition and appearance are completely altered. Thin pale threads of quartz and felspar, not more than a tenth of an inch in thickness may be seen following the bedding planes, or the cleavage and sometimes also the slip-cleavage. The distance between the veins may be no greater than the breadth of the veins themselves, and thus a striped or banded rock is produced, resembling a gniess but of dual origin, a mixed rock which is described properly as a "composite" or "synthetic" gneiss. The French geologists who first insisted on the importance of this group of rocks have called the process lit par lit (bed-by-bed) injection. The best examples of this in Britain are to be found around the granites of Mull and northern Sutherlandshire. The rocks invaded by granite in this manner often show intense contact alteration and are to a large extent recrystallized.

The short irregular veins which commonly occur within areas of granite, diorite, gabbro and other plutonic rocks are often much more coarsely crystalline than the rock around them. This is no doubt partly due to the high temperature of the whole complex and to slow crystallization, but it may also be ascribed to the action of vapours dissolved in the magma and gradually released as it solidifies. Such coarse-grained igneous rocks are called pegnatites (q.v.). It is clear that they are not purely igneous but are partly pneumatolytic.

With the pegmatites we may class the fine-grained acid veins (aplites) which are found not only in granites but also in many diabases. They occur in irregular streaks or as long branching well-defined veins, and are usually more rich in quartz and felspar than the surrounding rock. Formerly they were often described as contemporaneous or as segregation veins; but no vein can be in strict accuracy contemporaneous with the rock which it intersects, and many of them give evidence of having been intruded into their present situation, since their minerals are so arranged as to show flexion structure. But they are always intimately connected, as their mineral composition indicates, with the rock mass in which they lie, and they represent merely the last part of the magma to consolidate. The fissures they occupy are presumably due to contraction, seeing that they are not accompanied by displacement, brecciation or faulting.

Veins of sedimentary rock are few and of little importance. They occur where sediment has gathered in cavities of other rocks. Lava streams, for example, when they cool become split up into irregular blocks, and in the crevices between these ashes, sand and clay will settle. Submarine lavas are often traversed by great numbers of thin veins of sandstone, and a similar phenomenon may also be noted in the tuff of submarine necks or other ash beds. Cracks in limestone and dolomite are widened by the solvent action of percolating waters and may be filled with gravel, soil, clay and sand. In the Carboniferous Limestone, for instance, veins of bedded sandstone sometimes pass down from overlying Triassic deposits. The upper surface of the chalk in the south of England has frequently many deep funnel-shaped pipes which are occupied by Tertiary or recent accumulations.

The third group of veins, namely, those which have been filled by deposits from solution in water or in vapours, is of the greatest importance as including a very large number of mineral veins and ore-bodies. They are also the source of the great majority of the finely crystallized specimens of minerals.

The deposition of minerals on the walls of fissures by a, process of sublimation may be observed at any active volcano. The cracks in the upper part of lava flows are often lined by crystals of salammoniac, sodium chloride, ferric chloride and other volatile substances. By oxidation of the iron chloride bright scales of haematite (ferric oxide) arise; sulphurous acid and sulphuretted hydrogen, given out as gases, react on one another, producing yellow encrustations of sulphur; and copper oxide (tenorite) and a great variety of other minerals (alum, iron sulphate, realgar, borates and fluoride) are found about fumaroles of Vesuvius and other volcanoes.

Most veins, however, are not of superficial origin but have been formed at some depth. The heat given out by masses of rocks which were injected in a molten state is no doubt sufficiently high to volatilize many minerals. The pressure, however, also must be taken into account, as it tends to retain these substances in a liquid condition. Water vapour is always the most abundant gas in a volcanic magma, and next to it are carbonic acid, sulphurous acid, sulphuretted hydrogen and hydrochloric acid. The physical condition of the substances passing outwards from an igneous mass through fissures in the superincumbent rocks will depend on the nature of the substances, on the temperature and the pressure. Near the granite the heat is so great, at first at any rate, that gaseous materials must greatly preponderate; but farther away many of them will be condensed and hot aqueous solutions of complex composition will fill the cracks.

Veins deposited by the action of gases and vapours are said to be of "pneumatolytic" origin; where hot aqueous solutions have been the principal agency in their formation they are "hydatogenetic." It is often very difficult to ascertain to which of these classes a mineral vein belongs, especially as we are in ignorance of the behaviour of many substances at high temperatures and under great pressures.

T he veins which yield tin-ores in Cornwall and in most other tinproducing countries are generally regarded as typical pneumatolytic deposits. Tin forms a volatile fluoride which may be decomposed by water, forming tin oxide, the fluorine passing into hydrofluoric acid which may act as a catalytic agent or carrier by again combining with tin. Around tin-bearing veins and in the material which fills them there are usually many minerals containing fluorine, such as topaz, fluor-spar and white mica. Some borates too are volatile at high temperatures, and minerals containing boron (especially tourmaline) are very common in tin veins. Also since ore deposits of this character are found nearly invariably in granite or in the rocks which have been invaded by granite there is good reason to hold that fluoric and boric gases were important agents in the production of tin veins. It is not necessary, however, to believe that all the materials which are found in these veins were introduced as vapours, for as the temperature sank currents of hot water would follow which would fill up any cavities.

The tin veins of Cornwall often contain copper ores in their upper parts and at greater distances from the granite, a fact which indicates that the copper salts were deposited from solution at lower temperatures than the tin ores. A very large number of important ore deposits have been laid down by hot waters emanating from deep-seated intrusive masses. Nearly all the principal goldfields (except gravels or placers) are in districts where igneous dikes, veins and sills abound, and it is often perfectly clear that the introduction of the gold ores is intimately connected with the intrusive masses. The Witwatersrand deposits, although by many considered to be old auriferous gravel, have been regarded as owing their value to gold deposited from vapours emanating from certain of the dikes which traverse the banket rock or conglomerate. The importance of these hot ascending currents of water, proceeding from eruptive magmas, has been fully recognized, and is now probably the most widely accepted theory of the genesis of mineral veins.

The water falling on the earth's surface will to a large extent percolate downwards into the rocks, and it will dissolve mineral matters, especially at the greater depths, owing to the increased temperature and pressure; conversely, as it ascends it will lay down deposits or veins. This is the theory of "lateral secretion," at one time in great favour, but now regarded as of less importance. Ferruginous waters on passing through limestone rocks may deposit their iron as haematite or siderite, removing a proportionate amount of lime, and in this way great bodies of ironstone have been formed, as in Cumberland and Yorkshire, partly along the bedding of the limestone but also in veins, pockets and irregular masses. Many lead and zinc veins probably belong also to this class. By analysis it has been proved that in nearly all the common rocks there exist very minute quantities of such metals as gold, silver, lead, copper, zinc. If these can be extracted in solution in water they might conceivably be deposited subsequently in fissures in the rocks.

Controversy has raged between opposing schools of geologists, one considering that most mineral veins owe their existence to currents of hot water ascending from deep-seated igneous rocks, and the other that the metals were derived from the country rocks of the veins and were extracted from them by cold descending currents of water. There are cases which can be explained on one of these hypotheses only, and sufficiently establish that both of them are valid; but the general opinion at the present time is in favour of the first of these explanations as the most general.

The fissures in which veins have been deposited owe their origin to a variety of causes. Many of them are lines of fault, the walls of which have been displaced before the introduction of the vein minerals. Others seem to be of the same nature as joints, and are due either to contraction of the rocks on solidification, to folding or to earthquake shocks. In the vicinity of intrusive masses many fissures have been produced by the contraction of rock masses which had been greatly heated and then slowly cooled. Veins often occur in groups or systems, which have a parallel trend and may sometimes be followed for many miles. The larger veins may branch and the branches sometimes unite after a time, enclosing masses of country rock or "horses." Cross-courses are fissures which intersect the lodes; they are often barren, and at other times carry an entirely different suite of minerals from those of the mineral veins. A peculiar group of veins has been described from the Bendigo district of Australia; they are saddle-shaped and in transverse section resemble an inverted U. The beds in which they occur are folded sharply into arches and troughs, and in folding they have separated at the crests of the arches, leaving hollows which were subsequently filled up with ore.

The minerals occurring in the veins are sometimes classified as "ores" and "gangue". the former being those which are of value while the others are unprofitable. The commonest of the gangue minerals are quartz, calcite, barytes and fluor-spar. Usually a large number of minerals occurs in each vein, and the natural association or "paragenesis" of certain minerals which frequently are found together is a practical guide of much value to the engineer and prospector. A definite sequence in the order of deposit of the constituent minerals can often be recognized, the earlier being situated on the walls of the fissures or enclosed and surrounded by the later, and the microscopic study of veinstone shows that they have often a complicated history.

Many types of structure are met with in veinstones and vein deposits. Some are structureless, homogeneous or massive, like the quartz veins which are often found in districts composed of slate or phyllite. Others are banded, with sheets of deposit, each consisting of one mineral, usually parallel to the walls of the lode. These veins are often symmetrical, with corresponding layers following one another inwards from the walls on each side.

The veinstones are frequently crushed either by faulting or by irregular movements of the walls, and in such cases the veinstones have a shattered or brecciated appearance. If the crushing took place while the ore deposits were still being introduced, the broken rock is often cemented together into a compact mass. Rounded masses of rock or of veinstone are often met with, looking exactly like pebbles, but they are analogous to crush-conglomerates, as the fragments have been shaped by the movements of the walls of the vein. Frequently these movements have reopened a fissure which had been filled up, and a new vein is subsequently formed alongside of the old one; this process may be repeated several times.

The mineral-bearing solutions may exert a powerful influence on the walls of the veins, removing certain constituents and depositing others; in this way the walls of the vein become ill defined. The commonest change of this kind is silicification, and rocks of many different kinds, such as slate, limestone, andesite and felsite, are often completely replaced by quartz in the vicinity of mineral veins which have a quartzose gangue. Tin veins in granite and slate may be surrounded by a zone of rock which has been impregnated with cassiterite and is worth working for the metal. These changes are of a "metasomatic" type, involving replacement of the original rock-substance by introduced materials. Many of the best examples of this are furnished by limestone, which is one of the rocks most easily affected by percolating solutions.

The distinction between mineral veins and other veins is to a large extent artificial. With improvement of methods of mining and extraction deposits formerly unprofitable become payable, and in all cases veins vary considerably in the amount of ore they carry. The rich parts are sometimes called shcots or bonanzas, while the barren portions are often left standing in the mine. Near the ground surface the veinstones become oxidized and the metallic minerals are represented by oxides, carbonates, hydrates, or in the case of gold and silver veins they may be rich in the metals themselves. Below the zone where oxidizing surface-waters percolate a different series of minerals occurs, such as sulphides, arsenides and tellurides. If the ores are insoluble they will tend to be concentrated in the upper part of the vein rock, which may be greatly enriched in this way. Pyritic veins are changed to rusty-looking masses, "gossans," owing to the oxidation of the iron at the surface. Though instances are known of veins which come to an end when followed downwards, it seems probable that the majority of veins descend to great depths, and there is little reason to believe that they become less rich in the heavy metals. (J. S. F.)