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Page PLATE 1. Map of central Nevada, showing location of places referred to in the text___________-___-__-_-_-___ In packet 2. Stratigraphic sections of Lower and Middle Paleozoic rocks in the vicinity of Eureka, Nev______________ In packet FIGURE 1. Sections of Carboniferous sedimentary rocks at Tyrone Gap, Eureka,' and Diamond Peak.._______________ 4 2. Sketch of geology between Ruby Hill and Adams Hill_:___..

45 45 45 46 46 46 GEOLOGY OF THE CERRO GORDO MINING DISTRICT The Union ore channel was bottomed near the northwestward-trending San Felipe siliceous vein where the vein lies against a dacite porphyry dike. The very steep Jefferson chimney extended to a much greater depth, but was cut off below the 900 level by northwest-trending normal faults. Ore in the Despreciada section of the mine may represent faulted deeper parts of the Jefferson chimney. South of Cerro Gordo, the Morning Star mine, the Charles Lease tunnel, and the 8,100-foot low-level Estelle tunnel were opened to explore the Castle Rock siliceous vein and ground beneath gossans in the Tin Mountain limestone. The Estelle also provided means of searching for inferred deep continuations of the rich Cerro Gordo ore channels. Morning Star and Estelle production was small. That of the Morning Star came principally from the Gold stope in Lost Burro marble. Estelle ore was mined near the tunnel level from upper Hidden Valley dolomite east of the Cerro Gordo anticline axis. • Among lesser mines the Ella, the Perseverance, and mine~ in Belmont Canyon yielded siliceous silver-bearing ores used as fluxing material in the Cerro Gordo furnaces. These mines are in a wide zone of northwest shearing which includes northwestward-trending quartz veins of the tetrahedritegalena-barite type characteristics of the region. West of Cerro Gordo the now inaccessible Ignacio mine lies in altered and intruded Chainman shale near the boundary with overlying silicated limestone of the Keeler Canyon formation. Principal Ignacio silver production seems to have come from a fissure zone along the northeastward-trending Ignacio fault. Westernmost mine of the Cerro Gordo area is the Sunset, which lies in partly silicated limestone of the Keeler Canyon. A small amount of lead and silver came from two narrow intersecting veins.