Quartz vein

“ When a vein is composed of a single mineral, as quartz, it may rival a dike in its homogeneity. Most important veins, however, are composed of several or a large number of minerals, which may be sometimes more or less uniformly mixed with each other, but are usually distributed in the fissure in a very irregular manner. The great granite veins which are worked for mica, feldspar and quartz, are good illustrations, on a large scale, of the structure of veins in which several minerals have been deposited cotemporaneously. ”

“ Clay seams also separate the different veins. The foot wall diorite forms the country rock indefinitely eastward. The hanging wall is a fine, dark brown mica schist. The quartz is generally friable, and the granite vein matter much decomposed. The quartz at a depth of 300 feet carried one third of the gold in the sulphurets. A small amount of silver is also found. This vein is located for over a mile; direction a little east of north, dip 65° west. It is remarkable that there is no barren quartz; all the ore pays for working. In the lower workings the veins become more regular. ”

“ Most of the large veins are supposed to run for miles upon miles, though they can rarely be traced clearly on the surface for more than a furlong. The auriferous veins vary much in richness. No vein is wrought for more than a few hundred feet. Beyond that, it is either too poor to pay, or the vein is hidden. Some persons have supposed that there is one great gold-bearing quartz vein running along the side of the Sierra Nevada, from Mariposa to Plumas county, and that many of the richest claims are really in this one vein ”

“ But whatever may be the age of these different parts of the vein, they are all necessarily posterior to the rock of the mountain in which the vein occurs, and in the present case, to the granite and the grauwacke, the only two formations of any extent that are met with in Cornwall and Devonshire.
As the same vein-stones are found in different formations of veins, we must therefore admit, that the same rocks have been held in solution at different periods.
Massive quartz sometimes forms alone the greatest part of the vein-stone of certain veins. ”

“ A vein of quartz, from three or four inches to only half an inch or less thick, is exposed in the rock, either by the use of the pickaxe or by the aid of gunpowder. The workmen then examine the vein with the naked eye, and if any particles of gold are detected, they are removed, with the surrounding portion of matrix, by means of a chisel and hammer. ”

“ Quartz veins extend sometimes for miles, and it is necessary to suppose on the hydro-thermal theory that the fissures remained open sufficiently long for the gradual deposition of the veinstones, without the soft and shattered rocks at their sides falling in, nor yet fragments from above; although there are many lodes, fully twenty feet in width, filled entirely with quartz and mineral ores, without any included fragments of fallen rocks, and nowhere showing any trace of regular deposition on the sides. ”

“ The quartz veins now form the crests of many of the ranges, but are everywhere cut through by the lateral valleys. The beds of doleryte lie at low angles, through which the quartz veins cut nearly vertically. Excepting that they are very irregular in thickness, and often branch and send thin offshoots into the enclosing rocks, they resemble coal seams that have been turned up on edge, so as to be vertical instead of horizontal. They run for a great distance. ”

“ Other green-tinged veins were found bearing 205 degrees (mag.) from our camp. There is also a quartz-hill whose valley-drain, about a mile and a third long, leads down to the sea, about two minutes' walk south of the southern clump of "tabernacles" occupied by the Maknáwis. The dust is richest, as usual, at the walls where the vein is in immediate contact with the heat-altered granites, whose red variety, containing very little mica, becomes quasi-syenitic. ”

“ The business of vein or reef mining is a very much more elaborate and complicated affair. The lodes or veins of gold-bearing quartz that nature has distributed in certain localities in the rocky crust of the earth are from a few hundred to a few thousand feet in length, so far as their outcrop at the surface is concerned, and from a few inches to a dozen feet or more in width. ”

“ I believe that here the quartz is united to the granite without any saalbande intervening.
In the tin mines between St. Just and Cape Cornwall, quartz is united with black, massive, and radiated tourmaline. The quartz exists not only in the form of veins, but also in blocks. According to Bergman, there is in the mountain of Nasa, a block of quartz several hundred yards broad, and double that dimension in length. [133]
Dodecahedral hyaline quartz is frequently found among the vein atones of the lead mines at Beer-alston in Devonshire. ”

“ Nor is the quartz of this vein the only trustworthy source of yield. The underlying slate is filled with bunches of mispickel, not distributed in a sheet, or in any particular order, so far as yet observed, but developed throughout the slate, and varying in size from that of small nuts to many pounds in weight, masses of over fifty pounds having been frequently taken out. This peculiar mineral has always proved highly auriferous in this locality, and a careful search will rarely fail to detect "sights" of the precious metal imbedded in its folds, or lying hidden between its crystalline plates. ”

“ There is however little reason to doubt that the quartz forms a vein, but I suspect that the porphyry also is disposed in a similar manner, and that the appearance in question is the result of the interference of two veins. In either case the porphyry as a mineral specimen offers an aspect I believe as new as it is difficult to explain. Its basis is the usual indurated claystone, or compact felspar, or hornstone if that term be preferred, which is the most common basis of the porphyries that occur among these strata. ”

“ The principal interest attaching to the quartz veins is the fact that they frequently contain gold, though seldom in particles visible to the unaided eye. The quartz veins were worked for their gold by the ancients, the remains of whose dwellings and stone grinding pans are found at numerous places, as for instance in the Wadi Hangalia, at Kurdeman, near Gebel Sabahia, at Gebels Sukari and Allawi, in the Wadi Lewewi, and at Romit and Darahib. ”

“ Such cavities being formed the process of infiltration commences. The water existing in the rock percolates into the cavities, sometimes forming crystals, and sometimes filling the cavity with a solid mass of the matter which it held in solution. When silica exists in the rock, veins of quartz are thus formed; when lime, calcareous spar.
The solution of these earths in water is unquestionably more perfect in nature and the solutions more saturated than those which we can produce in our laboratories. ”

“ In connection with the durability of this supply from the quartz veins—besides the unsettled question as to the actual number and extent of such veins which further exploration will make out—there is the additional question as to how deep these veins will prove rich in gold. Our readers are probably aware that what are called veins are walls, more or less upright, which rise up from an unknown depth through the beds of rock which we have described as overlying each other like the leaves of a book. ”

“ Highly crystalline quartz veins render the rock surfaces even rougher than they would otherwise be, and in a few places the face of the rock is covered with egg-like projections, each containing a core of quartz. ”

“ In sinking a short shaft in a sunny country a large mirror, inclined at a suitable angle over the shaft, will give sufficient light.
Lodes or veins following the general trend of the auriferous quartz are much more likely to be rich than are those that cross it. Gold is never distributed evenly in veins, though it may be in great beds of low grade material; but more often rich areas alternate with barren portions.
Where quartz veins are small and the rich pockets separated by wide intervals of poor gangue the gravel of the district will usually be similar in character. ”

“ At times the felspar and quartz are so nearly equal in quantity, that the whole forms a pink coloured granular mass, often so loose as to crumble in the hand; and in this case the grains seldom bear any mark of crystallization, but are rounded as if they had undergone attrition previously to their aggregation: occasionally, but rarely, it contains imbedded fragments of quartz. It is not often traversed by quartz veins, but wherever these occur they are much confounded at their edges with the mass of the stone. ”

“ Incidentally, the substance has penetrated the calcareous strata, and has assumed the forms of fossils within them, thus bringing a new proof of aqueous precipitation.
Still more frequently than any other substance, is quartz distributed in large veins. The granitic plateau of France, Brittany, the Vosges, and the Pyrenees presents numerous examples of it. These veins can often be perceived from a considerable distance. Besides crystallized quartz, they frequently contain parcels of metalliferous minerals, and thus represent transitions toward metalliferous veins proper. ”

“ On different kinds of mineral veins.—Every geologist is familiarly acquainted with those veins of quartz which abound in hypogene strata, forming lenticular masses of limited extent. They are sometimes observed, also, in sandstones and shales. Veins of carbonate of lime are equally common in fossiliferous rocks, especially in limestones. Such veins appear to have once been chinks or small cavities, caused, like cracks in clay, by the shrinking of the mass, which has consolidated from a fluid state, or has simply contracted its dimensions in passing from a higher to a lower temperature. ”

“ In fact they are rather deposits than veins, and though often extremely rich are soon exhausted, while the lodes which run across the stratification, run far and deep, and have a regular and straight course and dip. Lodes lying between two different kinds of rock, are usually richer than those which have the same kind of rock on both sides. Thus it is said that the richest veins of auriferous quartz in California, have been discovered at the intersection of trap and serpentine, and the richest places in veins are where they cross from one kind of bed-rock into another. ”

“ In these, the crystals have not only been fractured in the same way, across their axes, but the fractures are filled by the quartz and felspar which constitute the body of the rock. The granite veins of Arran do not show more clearly the ramification of a central substance through the fractures of the neighbouring rock, than these specimens show the veins of quartz proceeding from the mass, and penetrating every fissure which had been formed in the crystal. ”

“ Many of the specimens found were transported stones, but in many other instances, which I have had occasion to notice in a paper on Glen Tilt, [1] the transition from quartz rock to granite is to be seen in situ. I have there also noticed that, which ought for the sake of uniformity to be repeated here, that the beds of quartz rock which are in immediate contact with the granite often pass into it by insensible degrees. It is well known that there is a gradual transition from mica slate into gneiss, and that this rock again by degrees equally evanescent passes into granite. ”

“ This cement may cause effervescence, being often formed of chemically deposited calcium carbonate.
When we consider the distribution of quartz in nature, we look to igneous and metamorphic rocks for the origin of the grains in sandstone. Quartz is one of the commonest minerals; but in granite and quartz-diorite it rarely forms more than half the bulk of the rock, felspar and mica and hornblende being its associates. Veins of quartz (quartz-rock) traverse many rocks, and become broken up into granular forms on weathering ”

“ It is glassy, waxy, greasy, or dull and is transparent or translucent. Pure quartz is colorless, but impurities make some varieties white, black, or a shade of red, yellow, blue, violet, or brown. Quartz is a hard mineral. It scratches window glass and cannot be scratched by a pocket knife or even by a steel file. It has a specific gravity of 2.65. The curved, conchoidal fracture shown by many specimens helps identify it.
Quartz is plentiful in Texas. It occurs in igneous rocks, such as granite, llanite, and pegmatite; in metamorphic rocks, such as quartzite, schist, and gneiss ”

“ It never predominates to such a degree as to constitute veins by itself. It is either disseminated, and as it were impasted in stony masses, or spread out in thin plates or grains on their surface, or, lastly, implanted in their cavities, under the shape of filaments or crystallized twigs. The minerals composing the veins are either quartz, calc-spar, or sulphate of baryta. The ores that accompany the gold in these veins are chiefly iron pyrites, copper pyrites, galena, blende, and mispickel (arsenical pyrites.) ”

“ If they are disjointed in position, or if they appear promiscuously scattered, they still retain their natural connection, while the identity of their mineral structure is every where consistent. In one place only some strata of a quartz rock are to be seen, which might lead us to hesitate did we not recollect that in other instances the same causes which have converted shale into siliceous schist have also been found to change sandstone into quartz. ”