Abstract

Mapping in the northern Keg Mountain area, in conjunction with the USGS Delta CUSMAP project, focused on the geologic setting of several little-explored occurrences of silver, lead, zinc, and copper mineralization. Paleozoic rocks, probably deformed during the Sevier orogeny, consist of Lower Cambrian Prospect Mountain Quartzite thrust over Middle Cambrian limestone. These rocks are unconformably overlain by Eocene andesite and latite. Subsidence of a newly recognized cauldron (approximately 5 km in diameter), between 39.4 and 36.2 m.y. ago, resulted in the formation of megabreccia. Eruption of a lithic-crystal ash-flow tuff may have accompanied subsidence and an inferred vent may mark the structural wall of the cauldron. Intrusion of several granodiorite stocks and plugs, about 36 m. y. ago, caused widespread alteration and mineralization. Later intrusions formed pebble dikes and granite plugs.

Jasperoid bodies occur along high-angle faults, lie within 1 km of granodiorite stocks, and show enrichment in silver, lead, and molybdenum. Replacement mineralization produced nearly massive pods of argentiferous galena replacing limestone adjacent to a granodiorite dike. Bulk sampling of the most promising occurrence, quartz stockwork and disseminated mineralization hosted by the Prospect Mountain Quartzite, yielded silver grades as high as 27 oz/ton and associated values of lead, copper, and bismuth. Mineral occurrences may be genetically similar to polymetallic replacement (Tintic-type) deposits.