Page images

to the quicksilver by a single point only, where the gold comes to the surface and finds contact with the quicksilver. Crushing and grinding the pyrites also liberates the contained gold, but has the disadvantage of breaking up the ragged gold at the same time, and the color of the pyrites is often so nearly like the gold that it is difficult to distinguish one from the other. Mr. H. G. Hanks * describes large crystals of pyr. ite from a mine in El Dorado County, California, on the surface of which small globular masses of gold may be seen. The opinion that gold is mechanically and not chemically disseminated in pyrites is defended by Adolph Ott, Henry Wurtz, and others. Ott t gives several good reasons in support of his opinion, and cites Bergmann, who as early as 1735 showed by digestion in nitric acid gold to exist in pyrites in small angular grains, thus proving the metal to exist in the state of simple mixture, and not of composition in the pyrites. I Joseph Black appears to have held the same opinion. §

There is also good reason to believe that the gold in pyrites is generally in a crystalline condition, not compactly aggregated, but spread in a net-work in parallelism with the crystalline planes of the pyrites, forming a rude crystalline skeleton or crystalline fragments. The late Prof. John Torrey, of the United States assay office, showed that after the treatment of pyrites with nitric acid gold appears under the microscope in lamina and in filiform and spongy particles.

An interesting example of spongy gold occurring in the mine at Angel's camp was brought to my notice by the late Dr. Hill. It has the appearance of precipitated gold and the form apparently of a crystal of iron pyrites, looking as if it had been aggregated in a cavity left by decomposing pyrite. It is very light and is easily impressed and burnished by the nail. It is a good example of spongy crystalline gold of natural origin.


There is a fine example of the occurrence of free metallic gold in porphyry at the Contention mine, Tombstone, Arizona. This gold is found chiefly in the thin cracks and cleavage surfaces of the partially decayed porphyry, and it appears to have been deposited there from solution and not mechanically. It occurs in thin subcrystalline flakes and scales, and may have been derived from the decomposition of the iron pyrites with which the adjoining sedimentary formations are charged. Small implanted cubical crystals of silver chloride are found in close association. The average value of the ores as worked in silver and gold was about seventy dollars per ton, and the gold constituted from 20 to 25 per cent. of the value of the product.


The size of the fragments of gold in streams depends, of course, primarily upon the size of the masses in the parent source. Wherever veins furnish large masses of gold of great weight and solidity, coarse and heavy nuggets are found in the adjacent detrital deposits. If the veins have yielded fine particles of gold only, the gold in the placers below is

* Report of the State Mineralogist, 1884, page 192.
† Adolph Ott, Jour. Franklin Institute, 1878, page 128.
1 Fourcroy's General System of Chemical Knowledge, vi, p. 489, 1804.
♡ Black, Lectures on Chemistry, III, 384.

in fine scales, or may have mostly disappeared by reason of the violent currents of water and the wear and tear to which all placer or wash gold is subjected. As a rule, gold in the beds of rivers becomes smaller and lighter the further it is carried down the valley by the force of the stream. It is subjected to constant wear by attrition and pounding, especially where the current of water is swift enough to move the bowl. ders and gravel down to the bed-rock. The size of the gold generally bears a direct relation to the size of the bowlders. But there may be frequent exceptions to this generalization respecting the size of gold in river beds, for side valleys may introduce gold which was not moved far from its original source, and the river may intersect some new source of gold, such as a group of veins or some ancient river channel carrying very coarse gold. This change in the nature of the gold in the beds of rivers is well known. Each river bar may be said to have its own peculiar type or condition of gold, and expert gold-merchants in the chief mining camps soon become so well informed respecting the form and appearance of gold dust that they can identify at a glance the gold from each bar or placer, from each valley or "diggings" in their neighborhood. Gold thus has, it is evident, characteristic forms, or a "likeness,” by wbich those accustomed to it can recognize it and name its source. The color and general condition of the surface of the gold grains are also important characters. When the gravel is small and well rounded by attrition, the gold is generally small and scaly. It is a well-known fact that in most of the California rivers in the gold region, such as the American, the Feather, the Yubas, the San Joaquin, the Merced, and others, the gold at the foot-hills of the Sierra Nevada, where the gravel is smooth and round, is in small scales, also well rounded and worn.

In a sample from the American River, near Coloma, mined in 1849, these scales measure on an average less than one-sixteenth of an inch = one millimeter in diameter.

[graphic][graphic][merged small][merged small]

The scale-gold from the San Joaquin River and from the Fresno River is still smaller and more even in size, and compares best with the fine scale gold from the ocean beach at Port Orford, Oregon.

Similar-scale gold is found in some of the extinct river channels, notably

in the deep channel at Red Dog and at You Bet, Placer County, California, where the gravel is in some places literally packed with small scalegold. The gravel is much worn and rounded, showing the continued action of running water. There are certain streams which afford gold

[graphic][graphic][merged small][merged small]

in smail rounded grains or pellets, rather than in scales. The Boisé gold dust is an example. The grains are much thicker and heavier than the grains of the same breadth from the California rivers.

[graphic][graphic][merged small][merged small]

In most of the drift-mines upon the ancient river channels where the bowlders are large and heavy, the gold is also heavy and coarse, although well rounded and worn. The sample from which the illustration (Fig. 51) is drawn was taken out of a drift-mine in Georgetown, Cal, 1854. The angular and semi-crystalline gold from veins and slates is in strong contrast with the well-rounded and water-worn samples, as shown in the two following illustrations, one from the angular gold of the Mameluke vein, Georgetown, Cal., and the other from the Loud deposit, Lumpkin County, Georgia.

The placer or wash gold of the San Francisco River, above Clifton, Ariz., is in irregular scales, rather thicker and heavier than the gold of the American River.

[graphic][graphic][merged small][merged small]

The placer gold of the Black Hills, Dakota Territory, is most ancient in its origin, being pre-Silurian in its formation and its distribution. It was broken out from its matrix in the veins and was distributed over the floor of upturned Archæan schists when the foundations of the Potsdam sandstone were laid. It occurs at Deadwood, according to Dev. ereux,* in smooth rounded grains slightly flattened. It is finer in composition than the Homestake vein gold, this being the vein from which the placer gold appears to have been derived. Such a difference of fineness is not unusual in placer gold, which is generally finer than the gold unbroken from the vein which supplied it. And that this greater fineness is due largely, if not wholly, to the loss of silver, its natural alloy, at and near the surface of the grains and masses of gold is shown by a series of assays made to determine this point, some of the assays being of the coarse gold and some of the smaller grains, the latter for equal weight exposing a greater surface than the coarse gold. The smaller grains were found to contain less silver than the coarse grains.

Another very interesting fact noted by Mr. Devereux in his paper is that where igneous dikes traverse the ancient gold deposits, and notably at the place known as Bald Mountain, the gold has largely disappeared near the dikes, although the original conditions for its occurrence were favorable. It appears to have been dissolved out near the dikes by some powerful solvent. That this is the true explanation of its absence is made more probable by the fact that some coarse pieces which, though similar in shape to others, had lost the characteristic smooth-worn surfaces of the wash gold and had acquired a corrugated roughened surface, such as metals acquire when undergoing solution. Such pieces did not, however, show traces of crystalline structure. It is stated also that gold in thin films has been found on the surfaces of the schists to a depth of several feet, with all the appearance of baving been deposited from solution, this gold being probably the same that was dissolved out of the deposits by the solvents in connection with the dikes. Schists

* Transactions American Institute Mining Engineers, X, 468.

so coated or gilded with gold have been found to pay for milling to a depth of 10 feet, where the gold generally gives out or is not present in sufficient quantity to pay.

Gold which occurs as a mere film, like gilding, on the surfaces of rocks, or limonite, rarely pays for working. It is difficult to save. It is often found in this condition upon pyrites, and has been seen also upon massive hematite.


Masses of gold of unusual size and weight, now generally known as “ nuggets,” are found in all gold regions. The gold fields of Victoria, Australia, have yielded the largest masses yet discovered. In 1858 thé celebrated Welcome nugget was found at Bakery Hill, Ballarat. This weighed 2,195 ounces troy, or over 182 troy pounds. The Blanche Barkley nugget is said to have weighed 146 pounds, and of this only 6 ounces was rock. The largest mass of which we have any record from California was taken out of the great vein at Carson Hill, in Calaveras

[graphic][merged small]

County, and weighed, according to Mr. T. Deer, one hundred and eight pounds, of which four pounds' weight was quartz. This is probably avoirdupois weight. Masses weighing five or ten pounds were numerous. The weight of the largest mass is variously stated. By some it is said that the mass weighed one bundred and sixty pounds, part of which was quartz. In the State Mineralogist's report the weight is given as one hundred and ninety-five pounds (=2,340 ounces) and the value as $43,534.

H. Ex. 268 -38

« PreviousContinue »