« PreviousContinue »
Gold associated with tale-schists. The magnesian rocks of the Sierra Nevada consist chiefly of serpentine and talc and chlorite schists. All of these rocks together with some others of similar origin are frequently found in the same area, the different varieties alternating rapidly in a perplexing manner. There are, however, especially in the area of the Bidwell Bar atlas sheet (Butte and Plumas counties) very considerable streaks of talc and chlorite-schists with little or no serpentine. It has been noted by the writer that while quartz veins are very common in the talc-schist belts, they are very rare in the serpentine. Veins containing gold and forming pocket mines do exist in the serpentine areas, but in the two examples which the writer has himself seen, there is talc-schist directly asso. ciated with the vein, forming one or both walls.
One of the veins here referred to occurs on the Downieville sheet in Sierra County, on the spur north of Rock Creek and one and a half miles east of Goodyear's Bar. Here is a small quartz vein in serpentine with tale-schist forming one wall. This vein had evidently been worked for gold, and the writer was informed that a gold pocket was found in it.
The other mine is in Mariposa County on the Mariposa Estate, and is in charge of Mr. Ludwig, who kindly showed me the deposit. There is here a streak of talc-schist in serpentine near the west border of the large belt of that rock that extends from near Princeton to Mariposa forming the high ridge just west of the latter town. The exact locality is one and three-fourths miles a little south of east from Princeton. The deposit consists besides the talc, of white dolomite looking precisely like that associated with mariposite at the Josephine Mine near Bear Valley, pyrite, and a black mineral, the latter occurring in plates with metallic surfaces in the dolo. mite. This black mineral was determined by Dr. W. F. Hillebrand to be titanic iron ore (ilmenite). The gold occurs native in the talc-schist, and the pyrite and ilmenite are also saved for reduction. The writer's notes make no mention of quartz in this vein.
As stated above, the talc, chlorite, and other associated schists form considerable belts in the area of the Bidwell Bar atlas sheet, and contain frequent quartz veins, as may well be seen at Quartz Hill north of Lumpkin. The writer knows of no case, however, where one of these veins has proved to be large enough and to contain enough gold to warrant the erection of a quartz-mill.
The rare occurrence of quartz veins in serpentine, a very basic magnesian rock, and their comparative abundance in talc rocks, which are much more acid, would seem to indicate a connection between quartz veins and the rock in which they form.
But as both these rocks are altered forms of deep-seated igneous rocks, it does not follow that the silica of any particular quartz vein was leached out of the wall rock and re-deposited nearly in place. These igneous masses may extend to a great depth and the ascending hot waters and gases may have been in contact with rock like the wall-rock for a long distance and for a considerable time.
As a matter of fact, quartz-veins are more common in California in sedimentary rocks which are not presumed to extend deep into the earth's crust, than in igneous masses. The cause of this is more probably a physical than a chemical one, for fissures form more readily in sedimentary than in massive igneous rocks. It is extremely likely that the sedimentary series of the Gold Belt of California is underlain throughout by granite, and that this rock is the chief source of the silica of the quartz veins in the clay slates, and other associated rocks.
Serpentine being a rock in which fissures may be supposed to form with difficulty, it is by no means improbable that there is a physical as well as a chemical reason for the lack of quartz veins in that rock.
Mariposite.—The green micaceous mineral called mariposite by Silliman occurs abundantly at the Josephine Mine near Bear Valley. Several specimens of this were obtained in 1893, and submitted to Prof. F. W. Clarke for analysis. Thin sections of the material were made and these show that the mineral is micaceous, nearly colorless or slightly greenish with brilliant polarizing colors, resembling talc. There appears to be no perceptible pleochroism. The mineral is in the form of fibers and minute irregular foils with ragged edges, and extinguishes nearly or quite parallel to the longer axis of the fibers. Macroscopically it is not all green, some of it being nearly white. Two analyses are appended by Dr. Hillebrand, one of the green, and the other of the white mineral. Analyses of Mariposite. (438 Sierra Nevada Coll.)
8.92 (Li Na),0*
•171 H,Of ....
100.84 * Very strong lithium reaction.
No water given off below 300 C.
Containing some K,0.
that nium in the be 2.787817 at 20. the spe
The thin sections show that there is carbonate, probably chiefly dolomite mixed with the mariposite. This with some carbonate of iron was extracted with acetic acid followed by warm dilute HCI, the mariposite substance remaining unattacked. Dr. Hillebrand calls attention to the resemblance of the mineral in composition to pinite, and states that no definite formula is deducible. He determined the specific gravity of the green mineral to be 2.817 at 29.5° C. and that of the white mineral to be 2.787 at 28.5° C. The occurrence of chromium in the green variety and not in the white suggest that to be the cause of the green color. While resembling talc optically it will be noted that the chemical composition is very different.
Gold quartz veins in Tertiary Rocks. — Precious metal deposits in rocks of the Tertiary period are not uncommon in the western United States. As notable examples of this may be mentioned the Comstock lode in Nevada in part at least in Tertiary lavas, and the gold and silver veins of the Bodie district in hornblende-andesite. * Silver deposits also occur in rhyolite in Southern California. But in the Sierra Nevada gold quartz veins in any but the Paleozoic or Jura-Trias rocks are rare. The occurrence of quartz with native gold in a rhyolite dike of Tertiary age in Plumas County has already been described. The gold in the Silver Mountain district in Alpine County (Markleeville atlas sheet) is in chalcedonic quartz in Tertiary andesitic tuffs and the deposits of the Monitor district are likewise in Tertiary volcanic rocks. One of the ore specimens given the writer by Judge Arnot as coming from the last district is chalcedonic quartz containing gold. In both these districts the rocks containing the deposits are much decomposed by solfataric action, and both are on the east slope of the range in the Great Basin drainage.
About one and a half miles south of La Grange in Stanislaus County (Sonora atlas sheet) in a flat-topped hill there are abundant veins of white quartz in clay which appears at first glance to be the basal portion of the Tertiary clastic series that caps the hill. Overlying the clay is a sandstone containing pebbles of white quartz and pearly scales of a hydrous silicate of alumina, which is very abundant. in the Ione sandstone.s The age of the sandstone is thought to be Miocene. Portions of the underlying clay are white in color, other portions stained pinkish in streaks and patches. When first visited, some years ago, the clay appeared to the writer to represent the lower clay of the Ione formation, which is well exposed at Ione and elsewhere, and as the quartz veins are unquestionably in the clay it was then thought that the quartz veins were of Tertiary age. The quartz is the white, compact kind that occurs in the majority of the gold quartz veins, and not the chalcedonic quartz known to exist in veins in Tertiary rocks.
* This was first noted by Mr. W. Lindgren. + W. Lindgren, Trans. Am. Inst. Mng. Eng., February, 1887. | This Journal, vol. xlvii, p. 472. $ American Geologist, vol. xii, p. 240.
On a second visit to the locality in 1894, good evidence was found that the clay is but the decomposed bed rock, which is here a quartz-porphyrite. Pebbles of the hardened clay were found in the lower part of the sandstone and along some sharp contacts of the clay and overlying sandstone it was noted that the quartz veins stopped short at this contact. No quartz veins were found with certainty in the sandstone itself. Moreover some cracks in the clay extending down from its upper surface were filled with the material of the sandstone, showing that these cracks were in existence when the sandstone was being deposited and were filled in from above. At the head of a little gulch on the west side of the hill is a good exposure of the clay with numerous quartz veins. The latter have a varying course dipping mostly north at angles from 10° upward, some veins curving very noticeably in a vertical direction. In some of this much stained and discolored clay, porphyritic quartzcs are to be seen, and as lower down in the gulch there is little altered quartz-porphyrite in place, there seems little question that the clay is a decomposed form of the same rock. At other points, notably on the east side of the hill the white clay shows no evidence of its derivation from the bed rock, being of even texture throughout and without discolora tion. Slickensided surfaces were noted in the clay at several points, along seams that intersect at varying angles.
Tetrahedrite.—This sulphide of copper and antimony has not often been noted by the writer in the gold ores of the Sierra Nevada. What appears to be this mineral, however, occurs very abundantly in the quartz veins of Mono Pass, east of the Yosemite Valley. The specimens (No. 455 S. N. collection) collected there by the writer from the Golden Crown ledge were examined by Prof. R. L. Packard, who reported that the sulphide is tetrahedrite or an allied mineral giving blowpipe reactions for sulphur, antimony, copper, lead and iron. The ore is presumed to contain silver and perhaps gold, but neither of these were determined.
Mr. W. Lindgren informs me that he has detected tetrahedrite at the following mines : The Boulder, Hathaway, Golden Stag, and Pine Tree mines in the Ophir district in Places County ; the Osborne Hill mine at Grass Valley, Nevada County; and the Miller & Holmes, Knox & Boyle, and Whiskey Hill in Tuolumne County, azurite being associated with the tetrahedrite in the last three mines.
Tioga mining district. This is situated to the northwest of Mono Pass in the same body of schists that occurs in the pass. Some specimens obtained here in 1886 by the writer from the Isbell claim on Lee Vining Creek. These were assayed by Dr. W. H. Melville with the following results : No. 876 Sierra Nevada Collection
a: chiefly made up of zinc blende; contains 5 oz. gold
and 7 oz. silver to the ton. 6: largely iron and copper pyrites; contains a trace of
gold, and nearly 16 oz. silver to the ton. c: contains a large amount of arsenical pyrite, 51 oz. gold
and 32 oz. silver to the ton. The above samples probably do not represent an average of the ore and are merely given to show the association of minerals in the vein.
Washington, D. C.
ART. XXX.—On Some Relations between Temperature,
Pressure, and Latent Heat of Vaporization; by C. E. LINEBARGER.
The well-known equation
dp p or dp – A_L. (1)
AT="T(0–0') or IT = ATdv in which p is the pressure; T, the temperature; p, the latent heat of vaporization ; v, the volume of the saturated vapor; and v', that of the liquid, may be considered to resume most of the relations between temperature, pressure, and latent heat of vaporization; it expresses fundamental relationships between heat, -and volume-energy, as is at once seen, when it is thrown into the form :
dp dv = APT, an equation of which the left-hand member contains only the factors of volume-energy, and the right-hand member only those of heat-energy. But certain relationships between these factors of energy were found out quite independently of the fundamental equation ; guided by no theoretical considerations, their discoverers, by scrutinizing experimental data, saw some regularities which, when generalized, became laws, although approximate and containing inexplicable anomalies. Also, the