A recent paper by Prof. George P. Merrill, Curator of the Department of Geology of the U. S. National Museum, Washington, upon ” An Occurrence of Free Gold in Granite,” describes an interesting instance of the dissemination of this noble metal in the substance of granite of normal composition believed to be from Sonora, Mexico. He found the gold in small scales, rarely exceeding a millimeter in diameter, distributed through the scales of mica and apparently enclosed in both the feldspar and quartz granules. A number of thin sections of the rock submitted to examination with the aid of the microscope gave confirmatory evidence. Sulphides were not detected, nor any indication of a secondary impregnation, though the granite had undergone some alteration, apparently by weathering. Mr. Merrill concludes that ” there is apparently no way of accounting for the gold other than by considering it an original constituent of the rock, a product of cooling and crystallization from the original magma.”
We thus have another link in the chain of evidence showing that gold is a constituent of granite and of plutonic rocks, and that such crystalline rocks may be the primal source of the gold, which is concentrated in veins.
We do not, however, overlook the fact that the oceans may have contributed a portion of their dissolved gold to the sedimentary rocks, such as the slates, magnesian or otherwise, of different geological epochs; such slates being generally known to us as the country, or wall-rocks, of most of the auriferous quartz-veins, especially of the central gold-region of California.
As investigation progresses, and our knowledge is increased, it becomes more and more evident that such pyritous sediments derived their metallic contents from the waters of the ocean at the time of their sedimentation, through the reducing agency of organic matters, or the exhalation of sulphuretted or carburetted gases, as, for example, from the petroleum shales.
We encounter here the difficulty that gold is not uniformly present in pyritous deposits, as we might expect considering the universality of the oceanic source, and the general distribution of gold in this menstruum. But before we can generalize satisfactorily we require much more evidence regarding the dissemination of gold in the mass of the various kinds of rocks, independently of veins. Examples of the nature cited by Prof. Merrill are thus of great importance, not only in themselves, as hearing upon the question of the origin and distribution of gold, but as tending to stimulate observation, inquiry and discussion. Prof. Merrill refers to the description by J. B. Jaquet of an occurrence of free-gold in microcline in a rock consisting essentially of microcline and quartz impregnated with hematite, and, also, to the occurrence of free gold in glass-like and crystalline varieties of a quartz-trachyte, in Chili, S. A., as described by Moricke.
It is only within a few years that the instances of the occurrence of gold in the crystalline rocks have multiplied until we cannot longer regard them as exceptional, or hold that gold, as a rule, is confined to the magnesian and argillaceous slates.
One of the earliest cited and best examples of the occurrence of coarse free gold in the midst of granite, seemingly without any extraneous origin by impregnation accompanied by quartz, was at the Amargosa gold-mines at the sink of the Mojave river, San Bernardino county, California.
The native gold in visible masses was there disseminated in the midst of the aggregation of soda-feldspar (albite) and the quartz, as if an original constituent of the mass and indigenous to the rock, which it may have been. Pyrites were not present in the specimens obtained; but some fragments of white arsenic in granite from the same locality may be regarded as an indication of the former presence of an arsenide in association, and as possibly an impregnation subsequent to the formation of the rock. The specimens, unfortunately, have been packed for years and are not now accessible. In all cases of this kind it is important to study the rock in situ and in mass, and to note carefully all the surrounding conditions. It is not safe to make deductions from the phenomena exhibited by isolated specimens.
The influence of dikes of plutonic rock upon the mineralization of veins with the precious metals, has long been known and recognized by miners.
If we accept the theory of lateral secretion for the formation of mineral veins, such veins bearing the precious metals are perhaps as good evidence as we need of the diffusion of gold and silver in the mass of the adjoining country-rock, or, at least, that solutions bearing these metals may traverse the rocks by osmosis or otherwise. If such evidence may be admitted, the range of the phenomena and of the evidence is widely extended.
Some of the most notable districts in California where gold-bearing veins traverse crystalline rocks of the granite family are at Grass Valley and Nevada, Nevada county; also at Forbestown, Butte county; Ophir, Placer county; and West Point, Calaveras county. In these localities the veins are numerous, and appear to have been formed from the substance of the country-rock. In Mariposa county, at the southern end of the Mariposas estate, where the slates of the Jura-Trias age, with their accompanying large “Mother-Vein,” give place to granite, we find a vein of auriferous quartz which may not have any relation in origin to the vein in the slates, with which the vein in the granite contrasts strongly in its formation and in the distribution of the gold. It is known as a ” pocket-vein,” yielding the gold in isolated but rich bunches, and of a higher grade of fineness and less crystalline than the gold of the Princeton vein, which traverses the secondary slates. It would be instructive to determine the relative ages of these adjacent veins and the influence of the two different kinds of country-rock upon the deposition of the gold. It would be extremely instructive if we could find an instance of a gold-bearing vein passing from one formation into another, as, for example, from slates into compact granite, so as to exhibit the effects of change of country-rock, or of the walls, upon the mineralization of the vein—effects so strikingly shown in the lodes of Cornwall, which are copper-bearing in the ” killas ” and tin-bearing in the granite. It should be mentioned that the tin-lodes are not without some gold, being thus indicative of its presence in the granite.
The well-defined granite rock of Butte and Walkerville, Mont., affords conspicuous examples of gold-bearing and silver-bearing veins, originating apparently by lateral secretion from the body of the granite on each side. The Rainbow and Blue Bird lodes especially appear to have derived their mineral and metallic contents from the granite in much the same way as the tin-bearing lodes are formed in Cornwall, according to the investigations of Le Neve Foster.
The occurrence of gold in thin flakes upon the surfaces and in crevices of the porphyry at the famous Contention mine at Tombstone, Arizona, has been shown ; but it is not yet known with certainty whether this gold, either in its free state or combined with disseminated pyrite, was an original constituent of the dike, or whether it was derived from the diffused pyrites of the stratified beds traversed by the dike. So far as regards the plainly visible gold, it appears to be confined to the partly decomposed portions of the porphyry dike, at or near the contact with the other rocks, and to be a secondary or late deposition and not indigenous.
The mines of the Homestake group in the Black Hills of Dakota afford a good example of the occurrence of gold in ancient crystalline gneiss or granitic schists of preCambrian age. These schists are much plicated and are traversed by felsitic dikes, distinctly intrusive. Whether these are auriferous or not remains to be ascertained, and without a careful examination it is not possible to state the source of the gold, whether indigenous to the schists or introduced from the dikes or with quartz-veins from remote sources. The rock for the stamp-mills is quarried, rather than mined in the manner usual with veins, and the rock is milled, together with any veins traversing it.
The ore of the Treadwell mine, Alaska, is described by Adams and Dawson as a hornblende-granite, ” much crushed, altered, and impregnated with secondary quartz, calcite, and pyrite.” The pyrite contains the gold.
In the gold-bearing region of Northern Sonora, Mexico, the gold-veins are chiefly in or closely associated with granitic and plutonic rocks. The veins of El Grupo concession, about 100 miles southeast of Tucson, Arizona, traverse a fine-grained granite, and hear both gold and silver. A dioritic rock at El Plomo, in the same State, appears to be especially favorable to the occurrence of gold. At the San Francisco mines, 100 miles southwest of Gila Bend, Arizona, gold-bearing quartz-veins traverse Huronian or pre-Cambrian gneiss near or at the contact with a great dike of diorite on one side and of porphyritic granite on the other. El Campana, a large quartz-vein, also in Sonora, is at the contact between an obscurely defined felsitic rock, like granite, on one side, and a heavily-bedded quartzite (probably Cambrian) on the other.
In Central and Northern Arizona gold-bearing veins are found in granite. The famous Congress vein now worked to a depth of some hundreds of feet, traverses granitic rock in close association with a dike of plutonic rock. North of Indio, on the Colorado desert, California, in the second range of mountains, a ridge of granite contains irregularly spread patches or bunches of pyrite which, by decomposition, liberate a small amount of gold. This pyrite does not appear to be connected with any vein, but seems to be one of the original constituents of the rock. Somewhat similar pyritic impregnations in granite were worked a few years since in Arizona, near Peeples Valley and Rich Hill, north of Stanton, and yielded free gold.
The close association of gold and granite at Cripple Creek must not be overlooked. Some specimens of porphyritic granite with cavernous spaces partially filled with purple or amethystine fluorite are very rich in gold.
Penrose, in summarizing Chapter IV. of his monograph of the mining geology of the Cripple Creek district, says :
” The gold and associated vein-minerals were probably derived from the volcanic rocks and, to a less extent, from the adjacent granite, at greater or less but not extremely profound depths.”
Again (p. 150), ” The most profitable mines yet discovered in the Cripple Creek district are in the eruptive rocks or in the granite immediately adjacent to the main volcanic vent or vents.” On Battle mountain, Cripple Creek, the veins are described as in some cases well-defined bodies of quartz, ” in other cases they are impregnations and partial replacements of the country-rock with mineral matter along fissures. The prominent veins are both in the breccia and in the granite immediately adjoining the breccia area.” This breccia, as found at the Independence No. 4 mine, is described as containing, besides volcanic rocks, large quantities of granite fragments, some of them several feet in diameter.
The ore of the Independence mine consists of ” granite from which the mica and quartz have been partly or wholly removed, leaving a honey-combed, vesicular mass of partially kaolinized feldspar ” (p. 201).
The phenomena of the occurrence of the precious metals in the celebrated Mercur mining district, Utah, furnish evidence which may be used in support of either view of the primal source of these metals. Both sedimentary and plutonic rocks are there found in conformable layers, and gold has been found in both, but the commercially available deposits are along and near the plane of contact of porphyry and limestone, but below the porphyry, the deposits thus being what is ordinarily described by miners as a contact deposit. According to the researches of Mr. J. Edward Spurr, there is a slight mineralization which is pretty generally distributed throughout the rocks of the basin. ” Four assays were made of comparatively fresh Eagle Hill porphyry. Two assays showed 0.01 ounce of gold to the ton; one showed a trace of gold, and the fourth was entirely barren. Of two assays of the birdseye porphyry, one showed 0.01 ounce of gold to the ton, the other showed a trace of gold.”
Nine assays were made of the altered limestone, seven of which showed very small quantities of gold, and two did not yield a trace. Of twelve assays of rock at the contact where there was no evidence of mineralization, nine yielded small quantities of gold, and the other three did not show the presence of the metal.
The phenomena indicate to me that the porphyries and possibly the shales above them are the sources of the gold, but Mr. Spurr concludes that the thickness of the porphyry-sheet is so slight that it is not possible that the ores could have been derived from it by leaching, and so carried downward into the limestone, and his explanation, or theory, is that ” the mineralizing agents rose from below till they met the sheet of altered porphyry, when they spread out along the under contact and so produced the mineralization.”
He also found a series of nearly vertical fissures or fractured zones through which he believes communication was established with a body of uncooled igneous rock at an uncertain depth below, permitting the ascent of moist volcanic vapors (p. 453).
The researches of Emmons upon the dissemination of gold in the rocks at Leadville have added largely to our knowledge of this subject.
In the representative series of gold specimens and ores sent by the Minister of Mines and Agriculture from New South Wales to the Columbian Exposition in Chicago in 1893, there were numerous specimens from granitic and feldspathic lode-stuff. Nos. 10, 11 and 12 of the collection were from ” binary granite.” No. 10 contained gold associated with copper pyrites and iron pyrites from the Challenger mine, Adelong. No. 11 was from the 978-foot level of the Great Victoria mine, and No. 12 from the 770-foot level of the same mine.
A pyritous granite was shown from Dargue’s Reef, Major’s Creek, Braidwood. This auriferous stuff is described as 25 feet in width at the 225-foot level. Nos. 23, 24, 25 and 27 of the same collection consisted of auriferous feldspathic lode-stuff from different mines at Yalwal, some specimens showing free gold. Feldspathic lode-stuff with mispickel and oxidized pyrite is found also at the Junction Reef, Mandurama. Auriferous quartz and feldspathic vein-stone rich in gold occur in the Hill End district and at Delaney’s Dike, near Molong.
In the collection from Sydney there were gold-bearing specimens with feldspathic gangue from Saw Pit Gulley, Fairfield. Three mines at Timbarra were represented by masses of auriferous granite. The feldspathic gangue occurs in some places in a brecciated condition. At Wann’s lode, Drake, New England (No. 140) the gold is obtained from a siliceous feldspathic breccia. At the Mount Graham gold-mine, Pambula, the occurrence of the gold is described as ” unlike anything hitherto discovered in any of the Colonies. The lodes are in the main conglomerates and felsitic breccias, in many instances only to be distinguished from the country-rock by irregular walls. The gold is extremely fine and difficult to follow; frequently there is nothing to distinguish the gold-bearing from the barren portions ; the drillings and the mortar are the only guides.”
Specimens of vesicular and amygdaloidal basalt, claimed to be auriferous, were shown from Black Rock, Bullina.
A careful chemical investigation for gold of the basalt of Ovifak, Greenland, which contains the large and small masses of metallic iron, and which is believed to come from great depths in the earth’s crust, would be exceedingly interesting.
Examples of the presence of gold in granitic and plutonic rocks might be multiplied, but those given are sufficient to show that we must recognize such rocks as truly gold-bearing.