Veins and Vein Minerals
Most of the important minerals deposits, especially those that furnish the valuable metals, are found in what are known as veins. A brief discussion of veins and vein minerals follows.
The rocks of the earth’s crust have many opening existing within them. These opening vary in size from microscopic cracks to cavities of considerable extent. The opening may be irregular and discontinuous or they may be in the form of fissures which are continuos for greater or less distances. Below a certain inconsiderable depth, these opening are largely filled by water. This underground water, as it is termed, slowly circulates throughout the rocks by means of the opening in them. Though a large part of its circulation, the water must exist at a high temperature and pressure, and under these circumstances becomes a strong solvent and active chemical agent. Underground water in general descends slowly through the smaller openings in the rocks, and then gradually finding its way into the bigger openings will at last enter some larger fissure and changing its course will begin to ascend. On its passage through the rocks, it will have dissolved their more soluble constituents, and when it ultimately enters the larger fissures and commences to rise will be carrying considerable amount of dissolved minerals material. The igneous rocks in particular are important factors in furnishing underground waters with mineral constituents partly because of the effect of their heat upon its activity, and patly because they give off in form of vapors a large amount of mineral material which ultimately gets into the underground circulation. When these mineral laden waters commence to rise in the larger fissures, they slowly come into regions of lower pressure and temperature. Under these changing conditions, the water will not be able to retain all its mineral constituents in solution, and their points of saturation being reached various minerals will begin to crystalline out and be deposited on the walls of the fissure. It time, if the process continues, the fissure may be completely filled from wall to with minerals deposited in this way. Such a filled fissure is known as a mineral
Evidence that the minerals of a vein have been deposited from solution is given by the flowing facts. Often a mineral vein shows distinctly banded or ribboned structure. That is, the different minerals occur in more or less regular layers which lie parallel to the walls. This shows that the various minerals have not been deposited simultaneously, but in a definite order of such cession. Again, frequently it will be observed that the vein material has not completely filled the fissure, but that there are openings left along its central line. These openings are termed vugs and are often lined whit crystallized minerals. These conditions cannot be easily explained except on the assumption that the contents of a mineral vein have been deposited from solution.
They shape and general physical character of a vein depends upon the type of fissure its minerals have been deposited in, and the type of fissure in turn depends upon the character of rock in which it lies and kind of fore which originally caused its formation . in a firm homogeneous rocks, like a granite, a fissure will be fairly regular and clean cut in character. It is liable to be comparatively narrow in respect to its horizontal and vertical extent and reasonably straight in its course. On the other hand, if a rock that is easily fractured and splintered, like a slate or a schist, is subjected to a breaking strain, we are more liable to have formed a zone of narrow and interlacing fissures, rather than one straight crack. In an easily soluble rock like a limestones, a fissure will often be extremely irregular in its shape and size due more or less to a solution of its walls y the waters that have flowed though it.
A typical vein consists of a mineral deposit which has filled a fissure solidly from wall, and shows sharply defined boundaries. There are, however, many variation from this type.
Frequently, as observed above, irregular opening termed vugs may occur among the vein minerals. It is from these vugs that we obtain many of our crystallized mineral specimens. Again, the walls of a vein may not be sharply defined. The mineralizing waters that filled the fissure may have acted upon the wall rocks and partially dissolving them may have replaced them with the veins minerals. Consequently we may have almost a complete gradation from the unaltered rock to the pure veins filling, and with no sharp line of division between. Some deposits have been largely formed by the deposition of vein minerals in the wall rocks. Such deposits are known as replacement deposits. They are more liable to be found in the soluble rocks like limestones. There is every gradation possible from a true vein with shapely defined walls to a replacement deposit with indefinite boundaries.
The mineral contents of a vein depend chiefly upon the chemical composition of the waters from which its minerals have crystallized. There are many different sorts of veins, and many different mineral associations are observed in them. There are, however, certain minerals and associations that are more frequent in their occurrence to which attention should be drawn. The sulfides form perhaps the most characteristic chemical group of minerals to be found in veins. The following minerals are very common vein minerals, pyrite, FeS2, chalcopyrite, CuFeS2, galena, Pbs, sphalerite, ZnS, chalcocite, Cu2S, bornite, Cu5FeS7, etc. In addition to these, which in large part comprise our ore minerals, certain nonmetallic minerals are also commonly to be observed.
These being of no particular commercial value are called gangue minerals (gangue is from gang, a vein). They include the following: quartz, SiO2, calcite, CaCO3, dolomite, CaMg(CO3)2, siderite, FeCO3, barite, BaSO4, fluorite, CaF2, hodochrosite, MnCO3, etc.
While comparatively few positive statements concerning the associations of vein minerals can be made, the following points are of interest.
Gold-bearing Quartz Veins: Native gold is most commonly found in quartz veins. It may occur alone in the quartz either in nests or in finely disseminated particles, or it may occur in connection with certain sulfides in the veins. The most common sulfides found in such connections are pyrite, chalcopyrite and arsenopyrite.
Gold- and Silver- bearing Copper Veins: The gold and silver content of these veins is associated with the various copper sulfides. Frequently the amount of the precious metals is quite small. The chief minerals are chalcopyrite, tetrahedrite, bornite, chalcocite, pyrite and various rarer silver minerals.
Silver- bearing Lead Veins: Silver and lead minerals are very commonly associated with each other. These veins contain such minerals as galena, argentite, tetrahedrite, pyrite, calcite, dolomite, rhodochrosite, etc.
Lead-zinc Veins: Lead and zinc minerals often occur together particularly in deposits that lie in limestones. The chief minerals of such deposits are galena, sphalerite, marcasit, chalcopyrite, smithsonite, calamine, cerussite, calcite, dolomite.
Copper-iron Veins: Copper and iron sulfides are quite commonly associated with each other, the prominent minerals of such veins being pyrite, chalcocite, bornite, tetrahedrite, enargite, etc.