The US has produced silver from a number of areas in the mountain west. The mining of these deposits were all important in the development of the early US economy, and they are described below. The most important silver mining regions in the US form into five distinct districts: (1) The Comstock Lode; (2) Tonopah; (3) the Eureka district in Nevada; (4) Leadville in Colorado, and (5) the Coeur d'Alene region in Idaho. In each of these regions, there are multiple mines that have been important silver producers. The silver ores are found in many geologic formations and the deposits have been formed in many different ways. Here is a description of each of them:
Comstock Lode: This great fissure vein, four miles long, is forked into two branches above, along a line of faulting in eruptive rocks of the Tertiary age, and chiefly andesites. In the central part of the vein the displacement has been about 3,000 feet, shading out, however, at the ends. The ores are high-grade silver and gold ores in quartz, and occur in great bodies, called "bonanzas," along the east vein. Over $325,000,000 in gold and silver has been extracted, in the ratio of two of the former to three of the latter. The vein lies on the easterly slope of a northeasterly spur of the Sierras. West of it is Mount Davidson. The outcroppings lie on the flank of the latter, about 6,500 feet above the sea and 1,500 below the summit. The general strike of the vein is east of south, and it dips east.
The Comstock Lode occupies a line of faulting in rock of the Tertiary age. The ore and gangue are thought to have been brought up from below by solfataric action, in which fluorine, chlorine, and sulphur were the principal dissolving agents. The vein filled a fissure between the syenite of which Mount Davidson consists, and the late Tertiary eruptive rocks poured out against its flanks. One of the peculiar features of the deposit is the extreme irregularity of the ore, which occurs in great "bonanzas," some of which carried several thousand dollars to the ton. The high temperatures encountered in the lower levels of the mines indicate that there is probably a partially cooled mass of igneous rock at a shallow depth. The ores consist of quartz and some calcite, in places banded with pyrite, galena, chalcopyrite, sphalerite, and finely distributed rich silver minerals. The valuable minerals are mainly native gold, acanthite, stephanite, and polybasite.
Tonopah: The Tonopah district is situated in a region of Tertiary volcanics, and the study of the general geology has chiefly to do with the nature, period, and effects of the different volcanic eruptions. The volcanic rocks comprise andesites, rhyolites, dacites (latites), and basalt. There are also present Tertiary lake-beds, mostly stratified tuffs derived from the volcanic outbursts. The most important veins of the Tonopah district are in the early andesite and do not extend into the overlying rocks. The rocks are complexly faulted, and the movement has occurred at different periods. The primary ore consists of finely divided native gold, argentite, and polybasite in a gangue of quartz and adularia. In the oxidized ore, which may extend to over 700 ft., cerargyrite, embolite and iodyrite are found.
In the primary sulfide ores, lying below the oxidized zone, the principal gangue-minerals are quartz, adularia, and some sericite, with occasional carbonates of lime, magnesia, iron, and manganese. The ore-minerals consist of sulfides of silver, antimony, copper, iron, lead, and zinc, in the form of stephanite, polybasite, argentite, chalcopyrite, pyrite, galena, and sphalerite. A considerable quantity of silver selenide is also present, and gold in a yet-undetermined form. The remarkable thing about the metallic contents is the relative scarcity of the common elements and the abundance of the rarer ones. The depth of oxidation in these veins is very irregular, dependent upon the relative porosity of the overlying rocks. In the oxidized zone, horn-silver is abundant, with some bromides and iodides. Free gold has been deposited. The presence of limonite and black oxide of manganese is characteristic. Pyrargyrite or ruby-silver and argentite frequently occur, coating crevices in primary ore, and in such cases are evidently of secondary deposition.
Eureka District: The Eureka district of eastern Nevada, discovered in 1868, is chiefly of historic importance. The ores are oxidized lead-silver ores, carrying some gold. They occur in Cambrian limestone which is much faulted and crushed, and is part of a Paleozoic section 30,000 feet thick. The ore is associated with a great fault, and is oxidized to a depth of 1000 feet. There are two mining districts, Prospect Hill and Ruby Hill. Near the mines are great porphyry masses which are supposed to have afforded heat which led to the formation of the ores. Up to 1882 the output was not far from $60,000,000 of precious metals and 225,000 tons of lead, but the production from the district now almost all from disseminated gold ore deposits.
Leadville, Colorado: There are seven distinct formations in the district which may be outlined as follows: first, the lower-most, consists of Archaean granite and carries no ore; second, a Cambrian quartzite which contains seams and stringers of ore, but in small quantities; third, Silurian limestone, which at the contact with the quartzite below bears ore-bodies of fair size; fourth, a sheet of "parting quartzite" overlying the ore-bodies in the limestone; fifth, a blue Carboniferous limestone, designated as the blue, which is the principal ore-bearing formation of the district; sixth, an overflow of porphyry, which comes in contact with the ore-bodies below; and seventh, a deposit of glacial wash also containing ore. The ore-bodies occurring in the blue-gray dolomitic limestone of the Lower Carboniferous and lying near or at the contact with the porphyry above constitutes a sort of contact sheet, the upper surface of which is fairly regular and well defined, the lower surface being exceedingly irregular. The thickness of the deposit in this limestone varies from a few feet to the whole depth of the enclosing stratum, the transition between the limestone and ore-body being gradual.
The ore-bodies are often separated from one another by rolls of barren rock and dikes of intervening porphyry, the ore being found at the contact plane of the blue limestone and porphyry, in the lower limestone horizons and in the dikes. At a depth of 600 feet, below the oxidation zone the ores are largely siliceous in character, being stained with oxides and often carrying silver and lead. Considerable of the gold occurs free, especially in the porphyry as at the Antioch mine, but the greater portion of it is in highly siliceous ore carrying silver fully 80 per cent of the value is gold. The normal condition of the ore is sulfide, consisting of sulfides of iron, copper, zinc, and silver, with some native gold and native silver. However, the most common and important ore is argentiferous galena with cerussite, carbonate of lead with cerargyrite. Gold occurs in the galena in the filiform state and in the ores ordinarily in small flakes and leaflets. The gangues are quartz, hydrous silicates of alumina, barite, pyrite, siderite and gypsum.
The district is usually considered as a silver producer. The silver-lead ores were first discovered in 1879 on Fryer, Carbonate and Iron hills. The principal mines are the R. E. Lee, Little Pittsburgh, Iron, Crysolite, New Discovery, Little Chief, A. Y. and Minnie, Crown Point, Maid of Erin, Adams, Henrietta, Morning and Evening Star, Matchless and Catalpa. A body of porphyry lying between the Pilot and Mike faults on the northwestern slope of Printer Boy Hill is unlike the other porphyry of the district. It is of considerable importance in that it contains the Printer Boy and Five-Twenty gold-bearing lodes. The lodes, especially the Printer Boy, were discovered before the carbonate ore was known of, and yielded considerable gold. It consists of a deposit along a jointing or fracture plane in the porphyry having a strike of east of north. The gangue is decomposed porphyry, the gold and other metallic contents being mostly invisible. Formerly, free-gold occurred in noticeable quantities with chalcopyrite, galena and tennantite. The "Down Town" mines first produced carbonate of lead near the surface, which changed, however, to sulfides in depth. When the depression, due to financial conditions following the depreciation of silver, was felt the mines adapted themselves to the new conditions by becoming gold producers. It was found that at a certain depth practically coincident with the silver belt, gold and auriferous sulfides occurred in a zone of decomposed porphyry, which was first discovered in the "Little Johnny" or Ibex. Mines similar to those at Leadville are found at: Carbonate Camp (Iron Hill and Wilkinson mines), Dakota; Aspen, Colorado; Eureka Hill, Nevada; Little Cottonwood and the Bingham and Dry Canon mines, Utah; the Sierra Mojada, Mexico; North Park mines, Colorado, etc.
Coeur d'Alene, Idaho: The Coeur d'Alene district (which is really made up of several local mining districts) lies in Shoshone County, mostly on the western slope of the Coeur d'Alene Mountains. Wallace is the principal town, but there are several smaller ones, as Wardner, Mullan, Burke, Mace, Gem, and Murray. The prevailing rocks here are a thick (10,000 feet), apparently conformable series of shales, sandstones, and some limestones of Algonkian age, which on the west are faulted down against granitic and gneissic rocks, but extend some distance to the eastward. The igneous rocks includes some small intrusive stocks of monzonite, and a few dikes of diabase and lamprophyre-like rocks, but the age of all is uncertain. The rocks show a series of complex, sometimes overturned, folds as well as extensive faults, and slaty cleavage has been developed in all except the quartzite. The largest ore bodies, although wonderfully persistent, are likely to become poor at depths ranging from 1000 to 2000 feet. Three types of ore bodies are recognized, and of these, which are described below, the first is the most important.
Lead-silver deposits, consisting essentially of metasomatic fissure veins, formed in greater part by replacement of siliceous sedimentary rocks, along zones of fissuring, and carrying mainly galena and siderite. The galena may first replace the quartzite, or siderite may replace quartzite first and then be replaced by galena. Pyrite and sphalerite are always present, and tetrahedrite, if found, indicates high silver values, but chalcopyrite is rare, oxdized ores occur above. Origin of lead-silver ores. It is believed that the association of the ore with fissures and the absence of irregular deposits indicate that it has been deposited by ascending solutions, moreover the mineralogical composition of the ore suggests its precipitation from hot solutions. These solutions are thought to have been given off by the monzonite in vaporous form, producing contact metamorphism and depositing ores rich in sphalerite and pyrrhotite associated with garnet and biotite, found in some parts of the district. Farther away from the intrusive the lead-silver ores were deposited. It is probable that the solutions entered the stratified rocks carrying ferrous carbonate and lead sulfide, and not only filled the open spaces but replaced the quartzite.
The first prospecting occurred in this district about 1878, and subsequent discoveries in 1879 started a rush to this region, but this centered round the placers, which commanded the most attention even up to 1885; but in the following year the miners awoke to an appreciation of the lead - silver deposits, and the building of a railroad into the district in 1887 gave a great impetus to the lode-mining industry. Since then the Coeur d'Alene has been an important producer, in spite of severe though temporary setbacks due to labor troubles in 1892 and 1899. The lead-silver veins, which He mainly in that portion drained by the south forks of the Coeur d'Alene River and its tributaries, occur mostly in the Burke formation, while a few are found in the Revett, Wallace, St. Regis, and Prichard. The average contents of ore in silver is a little over half an ounce to each per cent of lead per ton. In 1914 the Bunker Hill and Sullivan milling ore assayed 10.35 per cent of lead and 3.796 ounces silver, while concentrates from the same mine averaged 62.91 per cent lead and 20.68 ounces silver. The tailings assayed 2.675 per cent lead and 1.331 ounces of silver. The bulk of the ore ranges from 3 to 14 per cent lead and 2.5 to 6 ounces silver per ton. Most of the ore in the district is concentrated to 50 or 60 per cent lead.
