Dome District
Commodities: lead, silver
This district was formerly called the Blackburn and then became part of the Hamilton district. The latter name is now restricted to the area north of the Dome district. Lead ore was discovered in the district about 1880, and about $75,000 was produced at that time. In 1900 to 1928, a total production of $1,952,216 is recorded mainly from the Wilbert mine, about 30 miles by road from a branch of the Union Pacific Railroad at Arco, which has long been the only property under extensive development. This mine has an enviable record for continuity of production, but has been shut down since 1930.
The district is underlain by intricately folded and faulted quartzite and dolomite (Ordovician). The only known intrusive rocks in or near it are some small, highly altered dikes (Miocene (?)) in the Wilbert mine.
The known ore deposits are all replacements in or immediately adjacent to dolomitic strata, in part along faults, in part along bedding. In the upper workings of the Wilbert, there is much oxidation. The hypogene metallic minerals include galena, pyrite, sphalerite, and locally some copper minerals and the gangue is silicified dolomite with introduced calcite and dolomite. The deposits appear to be of the type formed by solutions of magmatic origin far from their source.
Hamilton District
Commodities: lead, silver
This district has been known since the middle eighties, but production has been small. Its northern part, containing many of the known lodes, is sometimes, termed the Clyde district. The properties are about 30 miles from Mackay and about 40 miles by a better road from Arco. Both Mackay and Arco are on a branch of the Union Pacific Railroad. There are silver-lead ore bodies, possibly like those of the Dome district described above. At one place, oxidized copper ore in magnesian limestone has been prospected.
Lava Creek District
Commodities: silver, lead, zinc, tungsten
According to present usage, this district, which is about 25 miles by highway from Arco on a branch of the Union Pacific Railroad, includes the old Antelope, Era, and Lava Creek districts. The total production is roughly $400,000, of which $250,000 was from oxidized silver ore from the Hornsilver mine in 1886 and 1887.
Except for brief revivals in 1913 and 1928, there has been little development since 1893 when the shallow oxidized ore was largely exhausted.
The district contains Carboniferous sedimentary rocks overlain by the Challis volcanics, both intruded by granite and related dikes of Miocene age. There are small amounts of younger basaltic lava. The granitic intrusion domed and fractured both the volcanic and sedimentary rocks and developed lime silicate minerals in them.
The ore deposits are mainly in the Challis volcanics, but in part in nearby sedimentary rocks. Some are valuable mainly for silver, others for lead or zinc and a few for either antimony, copper, or tungsten. The lodes are in fissure veins and breccia zones, and formed mainly by replacement, but in part by fissure filling. Ore shoots are irregular and small, and in most exposures of hypogene (primary) ore the sulphides are sparce.
