See also Owyhee County Idaho Gold Production
Bruneau District
In an unorganized area on Little Valley Creek near Bruneau, volcanic glass sand at a depth of 250 feet supposed to contain platinum has been explored by drilling and a shaft has been begun. In 1932, a sample obtained from a churn drill hole sunk under the supervision of the State Inspector of Mines was reported by the U. S. Bureau of Mines to contain neither gold nor platinum.
Castle Creek District
Commodities: lead, silver, copper
A number of prospects showing lead, silver, and copper were reported in this district in the Inspector of Mines' annual report for 1906, but further details are lacking.
Rough Mountain District
Commodity: silver
One idle silver mine is reported in the 1932 report of the State Inspector of Mines in this small district near Grandview on the Snake River.
Silver City Region
Commodities: gold, silver
The area of Tertiary precious metal deposits centering around Silver City about 25 miles by unimproved road from Murphy, the terminus of a branch railroad that leaves the main line of the Union Pacific at Nampa, is most conveniently described as a unit, although composed of four mining districts. The Carson district, which includes Silver City, is also termed the Florida Mountain, Silver City, and War Eagle district, and part of it is often called the De Lamar district. The French district adjoins it on the southeast and includes the Oro Fino and other mines. The Flint district lies to the southwest and the Steele district to the south. The latter apparently includes the Mammoth district.
Placer deposits were found in the vicinity of De Lamar in 1863, and the first lode locations followed promptly. In 1865, the interest in placers had largely subsided, but the discovery of very rich ore at the Poorman and Oro Fino led to active lode mining attended by much excitement. In 1876, a combination of exhaustion of some of the known ore bodies and of the failure of the Bank of California terminated the first period of activity after nearly $12,500,000 in precious metals had been produced.
The second and greater period began in 1889 with discovery of the Black Jack and De Lamar ore bodies, followed in later years by others. Decline in production with exhaustion of known ore bodies, began about 1910 and since 1914 much of the development has ceased, although desultory mining has continued to the present time with several attempts at revival. The Silver City region has produced a total of over 1,000,000 ounces of gold and well over 20,000,000 ounces of silver, and has a record of more sustained active production than any other region in the state except the Coeur d’Alene, Apparently much of the recent work has been in the placers.
The region contains a stock of granodiorite, petrographically somewhat similar to and presumably of about the same age as the Idaho batholith. Small blocks of schistose sedimentary rocks remain locally on the margin of the stock. There are aplite dikes related to the granodiorite and also dikes of diorite and dacite porphyry that are supposed likewise to be so related. Their resemblance in petrography and structural relations to certain of the dikes in the "porphyry belt" of the Boise Basin suggests, however, the possibility that they are of similar age and hence much later than the granodiorite. All the rocks above mentioned are regarded by those who have studied the region as older than the Tertiary basalt and rhyolite flows, with associated dikes that at one time appear to have completely covered them
Pronounced sheeting or closely spaced jointing, earlier than the extrusion of the lava, is conspicuous in the granodiorite, and is locally followed by certain of the veins, particularly in the Flint district. Subsequent to the volcanism, a complicated fault system was developed and other veins were deposited above certain of the fractures thus formed. Still later another set of fractures broke and offset the veins. Sericite and pyrite developed in the crushed rock of the last set, but precious metals were not deposited.
Four varieties of lodes are recognized in the region; (1) those with abundant white quartz such as those of the Flint district; (2) those with lamellar, pseudo-morphic quartz as in the vicinity of De Lamar; (3) silicified and mineralized shear zones such as the Poorman; and (4) cemented breccias such as the Oro Fino. The veins tend to be exceptionally persistent along both strike and dip. Some are several thousand feet long and the Oro Fino-Golden Chariot is still strong at a depth of 2,500 feet below the outcrop.
The principal hypogene metallic minerals of the region include argentite, electrum, jamesonite, ruby silver, and related minerals; naumannite, owyheeite, stibnite and tetrahedrite, arsenopyrite, galena, pyrite, marcasite, and others occur in minor amount. The gangue minerals include barite, calcite (in mineralized gouge), quartz (by far the most abundant gangue mineral), chalcedony, sericite, siderite, and valencianite. Argentite, naumannite, cerargyrite, silver and ruby silver minerals are believed also to have been formed by supergene processes.
It appears that the very rich ore mined close to the surface resulted from secondary supergene enrichment, but that a large part of the ore that has been mined was of hypogene origin. The latter ranged in value from 0.7 to perhaps 2.5 ounces per ton. It appears from the data available that conditions governing the localization and continuity of ore shoots were complex and not entirely understood by the miners. It is entirely possible that ore of good grade remains in this old and once prosperous region.
South Mountain District
Commodities: lead, silver
This lead-silver district, about 20 miles southwest of Silver City and 82 miles by road from Homedale, Oregon, its most convenient railroad shipping point, was discovered about 1868 and its principal activity was from 1871 to 1875 when the failure of the Bank of California forced a shut-down. A smelter, built in 1874, is reported to have produced $150,000 in bullion. Since then there have been brief attempts to revive the mines.
The district is at the southern tip of a small mountain range composed of close-jointed granodiorite (Mesozoic ?) and a roof pendant of steeply folded and overthrust metamorphosed sedimentary rocks (possibly Carboniferous) surrounded and in part covered by basalt flows. There are dikes of pegmatite, aplite, and dacite, all regarded as related to the granodiorite. Dikes and replacement veins in part accord with vertical fractures complimentary to overthrusts in the sedimentary rocks and many of the dikes follow the joint sheeting in the granodiorite.
Most of the past exploration has been on replacement veins, in which the ore is largely confined to segments with calcareous wall rocks. These veins, commonly 3 to 8 feet wide, contain abundant galena and sphalerite and lesser amounts of chalcopyrite, pyrite, arsenopyrite, pyrrhotite, and tetrahedrite in a gangue of quartz and calcite. As early operations were largely directed at recovery of the silver in the oxidized ore, which gave way to sulphides at slight depth, it would appear that considerable amounts of hypogene ore remain. Early accounts indicate that in places massive sulphide fairly rich in silver was exposed in the workings.
The limestone which makes up a large part of the sedimentary sequence is locally intensely contact metamorphosed with the development of abundant hedenbergite, ilvaite, and other silicates. In such rook there are in places deposits containing abundant chalcopyrite and sphalerite, and lesser amounts of galena, pyrite, and arsenopyrite. Such ore could evidently not be profitably handled in the early days and received little attention.
In addition, quartz veins without visible metallic minerals but containing a little gold are locally present in the sedimentary rocks. These have not yet proved valuable.
