Associated Minerals: Placer gold invariably is accompanied by other heavy minerals, which comprise the black, white, or yellow concentrates found in the sluice box when cleaning up. Table 3 lists these minerals roughly in the order of their commonness. Some of the characteristics noted under Remarks apply chiefly to the minerals as they are found in sluice-box concentrates.
Magnetite.- Magnetite is by far the commonest mineral constituent of the heavy sands and often is a serious nuisance to the miner because it tends to pack in the riffles. It is almost always present and frequently is as much as 1 or 2 percent of the entire weight of gravel. In some beach and river-bar placers where the mining practice is to “skim” the rich streak, magnetite often amounts to several percent of the material washed. Although a valuable iron ore under certain conditions, magnetite is worthless as a byproduct of placer mines. The impression held by some miners that it contains gold is erroneous. No instance of physical or chemical natural combination of gold and magnetite is known to the authors. The gold content of placer concentrates consisting of magnetite and other heavy minerals is practically all in the form of loose particles of gold. Some of the gold, however, may be attached to quartz particles or other gangue material, particularly if near its source.
Titanium Minerals.- Ilmenite and rutile are the chief ore minerals of titanium but do not have commercial value as gold placer-mine products. It was formerly believed that the black sands of the Pacific coast, which in places contain considerable of these minerals, constituted economically valuable reserves of iron, platinum, and titanium. Recently titanium has been produced commercially from the beach sands of California. Probably no gold is recovered in this operation. Rutile is worth only a few cents per pound in the form of 94-percent concentrate; the demand is supplied amply by some 200 tons produced annually from lode deposits in Norway and Virginia.
Garnet.- Garnet is an abundant accessory mineral in many rocks, particularly if they are metamorphic. It has a higher specific gravity than the usual rock-forming minerals and is present in the concentrates from sluice boxes in many localities. The gem varieties of garnet are the commonest and least valuable of the semiprecious stones; they are used much in cheap jewelry, but the production of gem garnet in the United States, which was never large, has declined to such an extent that production figures are no longer kept. Garnet is one of the most useful abrasive materials, and a considerable production is so utilized, However, this production is from lode deposits exclusively, and no instance is known of the commercial production of either gem or abrasive garnet from gold placers in this country.
Zircon.- Zircon has the highest specific gravity of all gem stones; in certain characteristics of hardness, high refractive index, and color are slowly making it more popular as a gem. Its frequent occurrence in sluice-box concentrates is therefore of interest. It is said to be abundant in the gold-bearing gravels of Henderson County, N,C., and crystal of it are common in most auriferous sands; in some workings it comprises several percent of the concentrates. The crystals are seldom of gem quality, and the usual material is worthless as a zirconium ore because of the low price of the latter – about 10 cents per pound for a semirefined product. Even as a gem uncut zircon brings only a few dollars per carat and seldom over $15 when cut.
Hematite.- Hematite often is found in placer gravels, particularly in the dry placers of New Mexico and Arizona. Sometimes hematite cobbles and small pebbles are said to indicate rich ground, but this idea is likely to be misleading.
Chromite.- Chromite occurs very commonly in black sands, in some deposits greatly exceeding the magnetite in quantity, but has no commercial value as found in gold placers.
Olivine and Epidote.- Olivine and epidote are common dark, heavy, rock-forming minerals and consequently are often present in the concentrates. They are of no value.
Pyrite.- The occurrence of pyrite sometimes has mineralogical interest because it is believed to have formed in place in gold placers. It is often found in the Tertiary gravels in California and sometimes is thought to have resulted from the reducing action of organic matter on iron sulphates in the meteoric waters. Pyrite is also found as detrital grains or masses derived from lode deposits or from the country rock.
Monazite.- Monazite, a phosphate of the rare-earth metals, valuable because of its varying small content of thorium, is characteristic of the Boise Basin placer sands of Idaho but is found in many other districts in Idaho, Colorado, Montana, and Oregon, It is notably lacking in the California deposits, with rare exceptions. A serious attempt was made about 1909 to utilize the monazite contained in the tremendous quantities of hydraulic tailings lying in the Boise Basin fields. A 1,000-ton plant was built, comprising tables, drier, screens, and several magnetic concentrators. A 95-percent monazite sand could be made, containing about 5 percent thoria. None was ever marketed, probably because the cost would not permit competition with the much richer Brazilian and Indian beach deposits.
Limonite.- Limonite is an iron-ore mineral found occasionally in placer concentrates and has no economic importance in this connection.
Platinum and Osmiridium.- Platinum and osmiridium are found in many districts and are almost the only consistently valuable byproducts of American gold-placer mines. They are typical of the beach-placer sands of California and Oregon, where they occur in proportions as high as a tenth or more of the gold. Practically all the Sierra Nevada placers contain platinum-group metals, as do the placers of the Klamath Mountains, particularly those on the Trinity River in the Hay Fork and Junction City districts. None of these deposits, however, have contained enough platinum to be worked for that metal alone. Indeed, the dredge operations are the only ones to produce platinum consistently, even as a byproduct of gold mining, the total annual output seldom being over 300 or 400 ounces. In a recent year one dredging company, which recovered about 50,000 ounces of gold from 10,000,000 cubic yards of gravel, sold about 110 ounces of crude platinum metals; these contained 70 ounces of platinum and 6 or 8 ounces each of iridium and osmium. Very few hydraulic or other placer operators find it worth while to separate the platinum from the gold because of the small quantity involved and the difficulty of finding a purchaser, Although platinum sold for as much as $154 per ounce in 1920 its average price in 1932 was only $36.46 per troy ounce of refined metal, and its average price in January 1933 was $26.48. In January 1934 the price was $38.00 per ounce.
The prices of iridium and osmium fluctuate widely, iridium having sold during recent years for $50 to $300 per ounce and osmium for $25 to $115.
Cinnabar.- Cinnabar is found in sluice-box cleanups in a few localities. It is unmistakable because of its brilliant red color, Edman states that minute quantities of cinnabar, in grains and crystals, are found in the platinum-bearing sands of Plumas County, Calif, It was seen recently by the junior author of this paper in pan concentrates from the gravels of Copper Basin Wash near Skull Valley, Ariz., in the form of round pellets the size of bird shot. Cinnabar is known to occur in lode deposits in that locality.
Tungsten Minerals.– Wolframite and scheelite, the most important tungsten-ore minerals; occasionally are found in placer deposits. In Alaska they have been mined from placers in which they were associated with gold. In the States, particularly in the Atolia-Randsburg district of California and in the Boulder County tungsten district, Colo., considerable tonnages of tungsten minerals have been recovered from eluvial or residual placer deposits. At Atolia scheelite was mined and milled on a commercial scale until about 1930 when operations ceased due to the low price of tungsten. In Colorado the mineral was ferberite, the high-iron member of the wolframite series. Huebnerite, the high-manganese end member of the series, has been mined by dry-washing during periods of high prices from shallow alluvial or residual deposits at Round Mountain, Nev.
Cassiterite.– Cassiterite, the principal ore of tin, has been recovered from placer deposits in Alaska and has been noted in a few placer deposits in the States; small quantities have been mined in the Appalachian gold-tin district. None of the occurrences in the Western States has proved to have commercial importance.
Corundum.- Sapphire and ruby are varieties of the mineral corundum; they are translucent or transparent, of a fine blue or red color, and otherwise of gem quality. When the mineral is an opaque light blue, brown, or gray it is known as corundum, and if granular and mixed with impurities such as magnetite it is called emery. Neither corundum nor emery can be produced by placer mining to compete with richer and better-situated deposits or with the increasing production of artificial aluminum oxide abrasives; but sapphires and rubies, especially if of gem quality, are valuable,
It is not known that sapphires or rubies have aver been of much value in this country as byproducts of gold-placer operations. However, in several districts in Montana, notably along the Missouri River near Helena, sapphires were discovered in the course of gold mining, and several attempts were made in 1891 and later years to wash the bars for the gems. These ventures were financial failures, partly because the investment cost was high and partly because the sapphires were not of good color. In 1899-1900 about 25,000 carats of gems, including a few small rubies suitable for cutting, were selected from about 400,000 carats hydraulicked from the gravels of Rock Creek, Granite County, Mont. Beginning about 1905 these mines were again worked, the chief product being sapphires of a quality suitable for watch jewels and other bearings. Occasional gem stones also were found, and a small quantity of gold was saved as a byproduct. The average selling price of the bearing sapphire was about $1 per ounce. Since 1917 the mines have been operated intermittently on a reduced scale.
From about 1905 to 1915 a bucket dredge was operated intermittently in another Montana gold-sapphire district, on Cottonwood Creek, Deerlodge County. The gulch gravels here were 10 to 14 feet thick and were composed of porphyry cobbles and subangular boulders with 3 or 4 feet of black muck overburden. Both gold and sapphires were concentrated near bedrock. The gold was fine and was said to pay operating expenses. Most of the sapphires were of a quality suitable only for mechanical purposes, although some gem stones were found.
Montana has continued to produce sapphires from the Judith Basin district lode mines. One Montana lode-mining company in 1924, 1925, and 1926 produced an average of about 50,000 carats of sapphires suitable for cutting into gems less than 1 carat in weight. This output was valued at about 50 cents per carat, and the same company’s much greater production of industrial stones was valued at a small fraction of a cent per carat. (One carat equals 200 mg.) Large gem sapphires, on the contrary, have sold for as much as $1,500 per carat.
Diamonds. — Diamonds have been reported from gold-placer gravels in California and other districts. Lindgren states that the principal localities in California are Cherokee Flat, Butte County; where 56 specimens, ranging up to 1½ carats in size, were reported, and Placerville, Eldorado County. Tyler gives the placer mines of the Volcano and Fiddletown districts, Amador County, as the chief producers. The gold-bearing gravels of south central Indiana occasionally produce diamonds; one weighing 1½ carats has been described.
It is believed that diamonds originate in the same rocks as platinum — namely, the serpentines resulting from alteration of peridotites. The diamonds found in gold placers in this country have not been of commercial importance, being small, yellowish, and far from abundant. Doubtless many pass over the riffles because of their relatively low specific gravity (less than 40-percent greater than quartz), and probably many more pass unrecognized into the rejects from the treatment of concentrates. In Arkansas, where diamonds are mined by hydraulic methods, the principal recovery is made on grease-covered vibrating tables.
Quicksilver.- The natural origin of quicksilver in gold placers seldom can be established definitely, because very little strictly virgin ground is now being mined in the United States. Dana’s Textbook of Mineralogy states, however, that it does exist in some alluvial deposits. In view of the frequent occurrence in lode deposits of native quicksilver associated commonly with one of the mercury sulphides and the finding of cinnabar in placer deposits, it is not unreasonable to assume that some of the metallic mercury recovered is of natural origin. It is certain, however, that as a rule such quicksilver has been introduced in earlier mining activities. Dredges and hydraulic mines frequently recover considerable mercury in their riffles. In the Skull Valley district of Yavapai County, Ariz., the authors were informed that one plant excavating and treating the shallow gravels of certain tributaries of Copper Basin Wash had recovered enough mercury to increase considerably its original stock and to furnish a neighboring plant and a number of “snipers” with several pounds of mercury. This district has been the scene of intermittent 1- or 2-man operations for several decades, yet the occurrence of cinnabar in the gravels makes the natural origin of metallic mercury at least plausible.
Amalgam.- Amalgam is recognized as a mineral only when it is a crystalline combination of mercury and silver having fairly definite characteristics and is thus described in mineralogical textbooks. Clarke however, gives two analyses of native gold combined with mercury. One from Colombia contained approximately 84 percent of gold. 8 percent of silver, and 7 percent of mercury; the other specimen, described as amalgam from Mariposa County, Calif., contained 39.02 percent of gold and 60.98 percent of mercury (approximating the formula AuHg4, and had a specific gravity of 15.47. Most of the amalgam recovered in sluice boxes, however, undoubtedly is of artificial origin.
Galena.– Galena is an uncommon constituent of placer concentrates, presumably because it is both soft and friable and because it oxidizes readily, It was observed in sluice boxes by the authors in the Cedar Creek district of Montana; in some specimens it still adhered to part of the original matrix of gangue minerals.
Other Minerals.– Although all placer gold is alloyed with silver, no record has been found of the occurrence of native silver, as a separate metal, in placer deposits in the United States, and silver is included in the list only for comparison with gold. It is known, however, that many silver nuggets were found in the Nizina district of Alaska; one nugget, with attached quartz, weighed over 7 pounds.
Copper nuggets have been found in placer gravels in many districts. Like the reported occurrence of bismuth in the gravel of French Creek, Summit County, Colo., this fact is of mineralogical interest only, at least so far as any district, in the Western States is concerned. The placers of Chititu Creek in the Nizina district, Alaska,contain enough copper to be a nuisance (several hundred pounds at each clean-up). It is not known that any of this was ever marketed.
In California Gulch, above Leadville. Colo., where placer mining preceded the discovery of valuable lead deposits by several years, cerussite (lead carbonate) was known to the miners simply as a variety of rock that was so heavy as to obstruct sluicing.
The mineral columbite-tantalite occurs in some pegmatite veins. The Black Hills region of South Dakota furnishes fine specimens of this mineral, and it is reported, together with cassiterite and scheelite, in the placer gravels of Bear Creek, in the northern Black Hills.
Quartz and feldspar, which constitute the bulk of placer sands, are included at the end of table 3 for comparative purposes. It will be observed that their specific gravities are one fourth lower than those of olivine and epidote, a third lower than that of common garnet, half those of magnetite and hematite, and only a sixth or seventh that of gold.
Other minerals besides those listed in table 3 are found in sluice-box concentrates, some because of their heavy weight and some merely because of imperfect concentration. Many artificial objects likewise find their way into the concentrates, the typical ones being bird shot and nails—quite often the hobnails of the “oldtimers.”