Epithermal Au-Ag Ores of War Eagle and Florida Mountains, Silver City District, Owyhee County, Idaho

摘要

The Silver City district, located 80 km SSW of Boise, Idaho, is best known for the low-sulfidation DeLamar Ag-Au deposit at DeLamar Mountain. However, similar ores from War Eagle and Florida Mountains were just as important as DeLamar in the early mining history of the district prior to 1920. From 1977-1999, modern open-pit bulk-mining and heap-leaching operations exploited disseminated ores at the crest of both DeLamar and Florida Mountains. Waldemar Lindgren did the most extensive study of the epithermal ores in the district in the 1890s, and it is clear that this research helped him formulate his ideas about epithermal deposits. Lindgren documented the selenium-rich nature of the epithermal ores in the Silver City district, and described for the first time the origin and significance of hydrothermal potassium feldspar (adularia) in these ores. The basic geological setting of the Silver City area is simple. The basement is Silver City granite, a Cretaceous granitoid correlative with the Idaho batholith. This granitoid is unconformably overlain by Miocene mafic lavas, which in turn are overlain by Miocene rhyolite lavas and tuffs. Lindgren recognized that adularia was almost absent in vein ores from DeLamar at the western end of the district. The amount of adularia increases dramatically toward the east in the Ag-Au ores on Florida and War Eagle Mountains. We propose here that this relationship is a function of the amount of granitoid present in the wall rocks hosting vein mineralization at War Eagle and Florida Mountains. Veins exploited on Florida and War Eagle Mountains are typically steeply dipping quartz-adularia veins up to 2 m wide with strike lengths up to 2 km. Principal veins mined were the Orofino and Poorman veins on War Eagle Mountain, and the Black Jack-Trade Dollar vein on Florida Mountain. Ore minerals are similar in the veins on both mountains. Naumannite, aguilarite, Se-rich acanthite, electrum, and ruby silvers are the main Ag-Au minerals, with associated pyrite, chalcopyrite, galena and sphalerite in lesser abundances. Fluid inclusion analyses indicate veins formed from dilute (<3 wt % NaCl equiv.) solutions at~180-280°C. The paleosurface in the district is preserved in a down-faulted block north of DeLamar Mountain and consists of hot spring sinter. Adularia crystals found in close association with Ag-Au minerals were used to conduct high-precision ~(40)Ar/~(39)Ar geochronology on the principal veins from War Eagle and Florida Mountains. Results indicate that veins formed at approximately the same time as host Miocene volcanic rocks. On War Eagle Mountain, ages for the Orofino vein range from 15.6 to 16.3 Ma, whereas the Poorman vein ages range from 15.6-15.9 Ma. Similarly, ages for the Black Jack-Trade Dollar vein on Florida Mountain range from 15.4 to 16.0 Ma. These new mineralization ages in the eastern part of the district compare favorably with the single published mineralization age on DeLamar Mountain of 15.7±0.5 Ma. Reconnaissance Pb-, Cu- and S-isotope analyses conducted on ore minerals suggest that metals and sulfur both had a mag-matic source.