See also Custer County Idaho Gold Production
Alder Creek District
Commodities: copper, zinc
This district, sometimes called the White Knob or Mackay, was discovered about 1884, and produced somewhat over $2,000,000 prior to 1912, mainly in copper ore from the Empire mine, and about $13,000,000 since then. This mine appears to have been called the White Knob by some, but a much less extensively developed property now bears this name.
Nearly 50 lodes are known, but nearly all the production has come from the Empire and Copper Basin copper mines and the White Knob and Horseshoe lead mines, all clustered close to Mackay, the terminus of a branch of the Union Pacific Railroad. The last two also contain bodies of copper and zinc ores. This district is notable because it contains one of the few really productive copper mines in Idaho. Little has been done for several years because of poor market conditions, but reserves of both oxidized and sulphide ore remain.
The exhaustion of the principal developed bodies of oxidized copper ore and the lack of a suitable market for the zinc sulphide ore which is locally plentiful presented problems that hampered development here even before the general decline in the price of base metals caused operations to halt. In 1939, there was a revival of interest in this district.
The district is underlain largely by dolomitic limestone of the Brazer and other Paleozoic sedimentary beds, emerging from a cover of Challis volcanics. Near the Empire mine, granite and related porphyries of probable Miocene age cut both Paleozoic and Tertiary strata.
The Empire and nearby deposits lie along the border of the main intrusive mass, the ore being in part in the limestone and in part in the igneous rock. In and close to the ore bodies the usual contact-metamorphio lime silicates are plentiful. Ore deposition was in part controlled by zones of shearing that are arranged radially with respect to the intrusive mass. In most deposits in this part of the district, the principal valuable hypogene (primary) mineral is chalcopyrite, although much of the ore so far mined has been more or less thoroughly oxidized. In the Horseshoe mine, however, most shipments to date have been lead carbonate ore. Much of the ore remaining in the Horseshoe contains abundant marmatite (iron-rich sphalerite) and pyrite, chalcopyrite, and galena.
The Reed and Davidson or Copper Basin and nearby properties in the western part of the district differ mainly in that the predominant country rock is quartzite. All ore mined is oxidized and most of it is in replacements along the bedding of the quartzite or of intercalated limestone. Individual ore bodies are smaller and lime-silicates less abundant than in the Empire and neighboring mines.
Some lead veins are reported to be present in outlying parts of this district, but little is known about them.
Alto District
Commodities: lead, zinc, copper
Mining has been intermittently carried on in this district (about 15 miles by highway from Ketchum, the terminus of a branch of the Union Pacific Railroad) since 1896 or before, but the production so far is very small. Lead and copper are the principal metals sought.
The district is underlain by argillaceous and locally calcareous quartzites of Ordovician and Pennsylvanian age cut by granitic stocks and locally covered by the Challis volcanics. There are several large faults, both normal and reverse.
Some lead veins similar to those of the adjoining Warm Springs district are reported, but most of the prospects are on lodes of contact metamorphic type. The more calcareous beds have been more or less thoroughly replaced by diopside, grossularite, epidote, augite, prehnite, wollastonite, fluorite, quartz, calcite, and similar minerals. In restricted and highly irregular parts of beds thus metamorphosed there has been sporadic and rarely abundant deposition of such minerals as galena, sphalerite, pyrite, chalcopyrite, and arsenopyrite.
Bayhorse District
Commodities: silver, lead, copper
This district (about 75 miles by highway from Mackay, the terminus of a branch of the Union Pacific Railroad) was discovered about 1877 and by 1898, when its most productive period closed, had reached a total production of roughly $10,000,000, of which about $6,900,000 was from silver, $2,700,000 from lead, and $650,000 from copper. The principal subsequent periods of revival were in 1912 and in 1920 to 1925. In recent years, the Clayton silver mine has been the principal producer. The production since the boom days has probably totaled a little over $2,500,0000.
The district is underlain by a thick sequence of Paleozoic strata, which have been deformed into a peculiar anticline with flat top and bulging sides. There are normal and thrust faults, the largest of which parallel the axes of folding. Ore deposition is in part controlled by fractures related to the bulging of the flanks of the anticline, A small .boss of granodiorite and sills of hydrothermally altered gabbro of uncertain age cut the Paleozoic beds and the Idaho batholith lies just west of the district, The Challis volcanics mask the older rocks over large areas.
Many of the lodes, such as the Beardsley, Excelsior, and Red Bird are localized mainly because of the superior replaceability of dolomitic rocks. The Bayhorse dolomite (Cambrian (?)) is so favorable to mineralization of this character that scarcely an exposure of it exists that does not somewhere show its effects, although in many places the amount is insufficient to be of commercial interest. Other dolomitic rocks were also locally affected by mineralization. The known hypogene metallic minerals include galena, sphalerite, pyrite, tetrahedrite, and chalcopyrite, and the gangue minerals are quartz, barite, calcite, and fluorite. Oxidation, while commonly incomplete, is more prevalent in these lodes than in many of those in Idaho, and much of the early success in both mining and local smelting is due to this fact.
In the Bayhorse dolomite, the deposits are highly irregular in shape. In the Red Bird and other properties in different dolomitic rocks, the influence of shear zones in localizing ore deposition is somewhat more evident and in otherwise similar replacements in more argillaceous beds, shearing locally was a dominant feature, although ore shoots are irregular and discontinuous. The last mentioned variety, typified by the Twin Apex and Dryden, has as yet failed to yield large production.
A variety of ore deposit distinctly different from any of those above mentioned is typified by the Ramshorn and Skylark mines. In this variety, pre deposition took place largely by replacement along well defined shear zones in argillaceous and quartzitic rock and siderite, largely absent in the replacements in dolomite, is an essential constituent of the gangue. These lodes resemble the principal lodes of the Wood River region except that silver-bearing tetrahedrite is locally abundant.
Much of the high-grade silver ore that has been mined in such lodes comes from the tetrahedrite-rich portions, which, so far as present data show, are not systematically arranged. Some of the ore in narrow bands is reported to have contained as much as 1,000 ounces of silver to the ton. While the best ore in known shoots has been mined, some high-grade and considerable low-grade sulphide ore remains in present workings. Here, as in the Wood River region, the discovery of new ore shoots remains an attractive possibility.
Most of the siderite lodes tend to conform to regional structural trends, but there is much local variation. Some lodes approximately accord with the bedding; others parallel joint systems; still others appear to be transverse to local structures or to be irregular and discontinuous. In the principal mines, the lodes form linked systems. The shearing is commonly both wider and more continuous than the ore deposition. Stope widths are rarely as much as 10 feet.
Boulder Creek District
Commodities: silver, lead, zinc
This district (about 60 miles by road from Mackay, the terminus of a branch of the Union Pacific Railroad) has been known since about 1882, and some ore was shipped in the early days, but by far the greater part of the production of the district came from the Livingston mine after the discovery of a new lead-silver ore body there in 1925 until mining operations ceased in 1930. Some work has been done here at intervals since 1930.
Most of the known lodes are in the argillaceous Milligen formation close to its contact with the Idaho batholith and related stocks, but a few are in metamorphosed patches of the more quartzitic Wood River formation similarly situated and some mineralization apparently without economic value has also occurred in the Challis volcanics.
The Livingston and others in the Milligen formation are in shear zones and are characterized by jamesonite with sphalerite and other sulphides. Calcite is the principal introduced gangue mineral. Aplite dikes are associated with some of the lodes. At the Livingston mine, the best lead-silver ore, consisting mainly of nearly massive jamesonite, in a shoot with a pitch length of 1,550 feet, has been worked out leaving sulphides with relatively abundant sphalerite remaining in the walls.
No other ore shoot of comparable grade or size is known in either this mine or the prospects on similar deposits, but it is to be expected that such shoots exist. Some of the ore in the limited prospect workings of Strawberry Basin and elsewhere contains massive jamesonite.
Copper Basin District
Commodities: lead, silver
This district (over 50 miles by road from Mackay, the terminus of a branch of the Union Pacific Railroad) is of somewhat indefinite extent. Some include in it the western part of the Alder Creek district, as that area is mapped on Plate 1 which follows the usage of the State Inspector of Mines. The principal mine in the Copper Basin district, as that term is here used, is the Star Hope. It was discovered in 1880 and produced about $50,000 from rich silver-lead ore prior to 1890, since which date it has been mostly inactive.
The district is underlain by deformed and locally much metamorphosed Paleozoic sedimentary rocks cut by a granitic stock probably related to the Idaho batholith and by dikes of several kinds. The Challis volcanics cover extensive areas. The Star Hope lode appears to be a quartz vein containing partly oxidized galena and probably other sulphides in quartzite, but little information regarding this or other deposits in the district is available.
East Fork District
Commodities: lead, silver
This is a poorly defined area, which contains a number of prospects and some mines, mainly in and near Germania Basin. It is about 50 miles from Ketchum, the terminus of a branch of the Union Pacific Railroad, the last few miles being over a road scarcely passable for automobiles. The total production, largely from oxidized silver-lead ore, may exceed $500,000, mostly shipped prior to 1890. Recently gold prospects in the nearby Washington Basin have received some attention.
The district is underlain by highly deformed and metamorphosed Carboniferous strata, in which much of the original calcareous material is replaced by silicates, intruded by granitic rocks related to the Idaho batholith and by dikes. These rocks are locally covered by the Challis volcanics.
The lead-silver deposits, such as the Tyrolese and Biblebaok, are replacements more or less closely related to shearing. Individual ore bodies appear to have been small and irregular. Most of the ore mined was oxidized and has a gangue of quartz and calcite. Certain prospects, such as those on Fourth of July Creek, are in part on deposits in which sulphides are disseminated through irregular bodies containing quartz and lime-silicates.
In Washington Basin, there are five quartz veins reported to contain 0.1 to 1 ounce a ton in gold. They are replacements on shear zones and contain sparsely disseminated pyrite, pyrrhotite, stibnite, arsenopyrite, and other sulphides, including a little tetradymite.
Loon Creek District
Commodities: gold, copper, lead
The placers of this district (125 miles by road from Mackay, the terminus of a branch of the Union Pacific Railroad) were discovered about 1869, and were actively worked during the following decade with brief revivals of interest since then. Their production is variously estimated from $500,000 to $2,000,000, The Lost Packer, the only lode that has yet received much development, was located in 1902, The production from lodes totals about $600,000, nearly all in copper and gold from the Lost Packer, although a number of copper end lead prospects are known.
Most of the lodes are in pre-Cambrian schist and Ordovician (?) calcareous and quartzitic rocks, which are in close proximity to the Idaho batholith. These rocks are out by aplite and other dikes related to the batholith and by the more numerous Tertiary dikes that are younger than the main lodes but which are genetically related to the contact metamorphism of Permian (?) and Tertiary volcanics which crop out in the vicinity.
The Lost Packer lode consists of a series of shoots with step-like arrangement in contorted schist. The schist is cut by altered, pre-mineral dikes of aplite, lamprophyre, and pegmatite, and by fresher porphyritic dikes of Tertiary age that cut both ore and schist. The steps in the lode are supposed by local people to result from post-mineral faulting masked by later intrusion of the porphyry dikes, but a more probable explanation is that they result from original distribution of the fissures that guided the ore-forming solutions. Some post-mineral faulting has occurred, but there is no proof that it has anywhere materially altered the relations of the ore bodies.
The principal hypogene minerals in the ore are chalcopyrite, quartz, and siderite, with minor amounts of tetrahedrite, pyrite, pyrrhotite, calcite, and barite. Some oxidation and secondary enrichment have taken place, but not enough to be of much commercial importance. Little ore such as that mined in the past, reported to contain $80 to $90 a ton, remains in known shoots, but considerable material of lower grade is said to have been left on the borders of the stopes.
The prospects in the vicinity are in part closely similar, but, in some, argentiferous galena, instead of chalcopyrite, is the principal sulphide. The ore in the contact metamorphic deposits has been utilized only as flux for the Lost Packer smelter, now abandoned. The gold content helped to pay the cost of handling.
The placer diggings are confined to an accumulation of gravel mainly of probable Pleistocene age in the valley of Loon Creek near the abandoned town of Casto, although placer gold is known in other parts of this valley and its tributaries. Although the deposits that have been worked include terraces fully 100 feet above the present channel, most of the production has come from a strip about 75 feet wide and a mile long, comprising part of an ancient channel not far above the modern stream. This strip may be worked out, but there is considerable gravel that has received little attention as yet. Some of this contains boulders, but these, on the whole, are neither as large nor as numerous as in some other deposits in mountain valleys in this region.
Robinson Bar Placer District
Commodity: gold
Placer districts have been worked at several places on the Salmon River between Stanley and Challis. Robinson Bar is one of the best known of these. Except for recent small-scale operations, almost no placer mining has been done here for a long time.
Seafoam (Greyhound) and Sheep Mountain Districts
Commodities: gold, silver, lead
Most of the properties in the Seafoam (Greyhound) district are within 10 miles of the Rapid River Ranger Station, 102 miles by road from Ketchum, the terminus of a branch of the Union Pacific Railroad, Some properties in the Seafoam end all in the Sheep Mountain district can be reached only by trail. Both districts were known in the eighties. The principal development at that time was in the placers, especially at the original settlement of Seafoam, where the narrow canyon of Rapid River locally widens, but small shipments of ore containing precious metals and lead were sent to Ketchum from the Mountain King in the Sheep Mountain district and other prospects in the general area.
Since then, several of the lodes have been intermittently developed, the last period of activity being in 1927 and 1928. The production of the two districts is not known, but they were not among the larger placer camps, and, although the Greyhound and other lode mines have shipped, the aggregate obtained from lodes has been relatively small.
The districts are underlain by the Idaho batholith close to its eastern border. In this marginal part of the batholith, there is considerable variation in detail in the character of the rock. Engulfed blocks of the roof rocks, some partly resorbed, are present in places, and dikes of aplite, lamprophyre, and rarely pegmatite are locally plentiful.
The Seafoam (Greyhound) and many of the other lodes consist of roughly lenticular masses of quartz and altered rock, generally arranged en echelon in shear zones. The products of silicification and sericitization of the granitic rocks are characteristic in and near the lodes. In many places, the sulphides are so fine-grained and sparsely disseminated that little or no evidence of metallization is visible even where a good tenor in precious metals is reported to have been shown by assays, Pyrite is the most commonly visible metallic mineral, but chalcopyrite, galena, sphalerite, and arsenopyrite are also present. Locally, galena is sufficiently abundant to constitute a lead ore.
A different kind of lode is represented by the Mountain King where irregular replacement deposits in blocks of dolomitic limestone enclosed in granitic rook contain galena, sphalerite, pyrite, and chalcopyrite in quartz, Locally, the sulphides are so abundant as to constitute a high-grade lead ore, A variety, represented by the Mystery prospect, appears to have formed at higher temperatures for hudsonite and other contact-metamorphic silicates are present in the galena ore and there are irregular veins of glistening quartz and pyrrhotite.
Stanley District
Commodity: gold
This is a large area containing widely scattered, little-developed, gold lode prospects and placer deposits that have been intermittently worked on a small scale. Most properties are connected by short trains or roads with the Sawtooth highway. The distance by this highway from Stanley to Ketchum, on a branch of the Union Pacific Railroad, is about 65 miles. A dredge was installed in one valley, but appears never to have been operated to any extent.
The area is underlain by the Idaho batholith with masses of Carboniferous, argillaceous, and quartzitic rocks on its border and extensive covers of alluvial and glacial deposits in Stanley Basin and tributary valleys. In the southern part of the area, there are patches of Challis volcanics.
Most of the lodes are narrow quartz veins and silicified shear zones in the granitic rook. Some, like the Aztec which has recently been reopened, are in the sedimentary beds. Locally, there are rhyolite dikes containing pyrite and free gold, mainly in cracks.
Yankee Fork District
Commodities: gold, silver, lead, copper
This district is about 85 miles from Ketchum and 115 miles from Mackay by highway, both being at the ends of branches of the Union Pacific Railroad. A number of branch roads in, the district are now almost or quite impassable to vehicles.
Placers in this district are reported to have been discovered in 1862, but never attained much development. In recent years, the possibility of dredging- in the alluvial flats has been considered and some preparatory work has been done. Lode mining started in 1875 and was actively carried on for over 20 years with several later revivals. The total production is probably over $12,000,000, nearly all in silver and gold. Most of this came from the General Custer mine which was the mainstay of the district until it closed in 1905. Less than $500,000 appears to have been produced since 1900 and a total of only about $50,000 is credited to placer mines.
The area containing the mines is underlain by gently flexed flows and tuffs of the Challis volcanics. On the margins of the district, the Idaho batholith, Permian (?) volcanics and Carboniferous sedimentary rocks are exposed but show little mineralization.
Most of the lodes are fillings of shear zones and fractures with subordinate replacement of the wall rocks. Hydrothermal alteration of the wall rocks is widespread and near the veins is intense. In a few of the zones of brecciation, for example in the Montana and Sunbeam mines, some of the ore shoots have formed largely by replacement. Some of the lodes are long, individual veins with average thicknesses of about 4 feet, but many are aggregates of small gash veins.
The hypogene metallic minerals include selenides, pyrite, galena, chalcopyrite, enargite, tetrahedrite, and probably others. In most of the ore now exposed, the sulphides are so sparse or fine-grained as to be discovered only on careful inspection, but some lead ore has been mined and some of those familiar with the district think valuable bodies of base metal sulphides remain. The predominant gangue is banded, cryptocrystalline quartz with minor amounts of opal, adularia, calcite, and albite in places.
Some,or most,of the ore mined in the early days was oxidized and doubtless enriched by supergene agencies, but so much unoxidized pyrite remains in surface exposures that it is obvious that oxidation was incomplete. This raises the question whether the high-grade pockets, some of which yielded $7,000 a ton in gold and silver, may not have been in part of hypogene origin. If so, other such pockets doubtless remain as yet undiscovered in the deeper parts of the lodes. Most of the ore, still unmined, however, contains $20 or less in gold and silver, much of which is not amenable to amalgamation.
