Volcanic stratigraphy and epithermal mineralization of the DeLamar Silver Mine, Owyhee County, Idaho Public Deposited

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  • The DeLamar Silver Mine is in the north-trending OwyheeMountains of southwest Idaho. As part of the Silver City regionit is included in the Basin and Range physiographic province.The lithologic units of the Silver City region, surroundingthe DeLamar Silver Mine, are composed mostly of Cretaceous graniticrocks and Miocene volcanic rocks. The volcanic rocks which varyin composition from basalt to rhyolite are predominant in aerialextent. Regional structure is dominated by a set of N10-20°Wtrending oblique-slip, high angle faults and a less pronounced settrending N75°W. These faults may be related to crustal extensionin the Basin and Range province and to rifting in the Snake RiverPlain. The mineral deposits of the Silver City region occur mostlyas epithermal vein fillings of fractures and faults. The predominanttrend of the veins is north-northwest.The DeLamar Silver Mine is in a complex Miocene volcanicsequence. Units of various compositions including basalt, latite,rhyolite, and andesite were emplaced as coalescing flows,exogeneous domes, and necks. A porphyritic rhyolite is the mostwidespread unit in the mine area.Epithermal silver and gold mineralization at DeLamar is mostcommonly concentrated in the well-fractured, silicified, upperpart of the porphyritic rhyolite. The most continuous mineralizationis below the clay-altered base of an overlying fine-grainedbanded rhyolite.Mineralogy of the deposit is dominated by sulfides andselenides. Naumannite is the dominant silver mineral commonlyoccurring as small disseminated grains in quartz. Other silverbearing minerals include; acanthite, argentopyrite, and pyrargyrite.Fine-grained free gold is highly disseminated in the gangue. Pyriteis the most dominant and widespread sulfide of the deposit followedin abundance by marcasite.The gangue consists almost entirely of quartz. Several varietiesoccur including lamellar quartz; white, gray, and black common veinquartz; and well formed crystalline quartz. Hydrothermal alterationminerals at DeLamar include sericite, secondary quartz, kaolinite,alunite, chlorite, and zeolites.Mineralogy of the veins and alteration at DeLamar suggeststhat hydrothermal solutions were probably at temperatures 100 to300°C. These solutions are believed to have been highly dilutedsodium-chloride waters closely related to the rhyolitic volcanism.
  • The DeLamar Silver Mine is in the north-trending OwyheeMountains of southwest Idaho. As part of the Silver City regionit is included in the Basin and Range physiographic province.The lithologic units of the Silver City region
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