Abstract

The Escalante silver district, Iron County, Utah, exists within a group of low hills extending into the Escalante Desert from the Black Hills to the south, approximately 45 mi west of Cedar City, Utah. Tertiary volcanics are host rocks for silver-bearing quartz veins. Five map units, with a total of 12 members are recognized. X-ray fluorescence and field mapping enable differentiation of the units, while thin-section analysis characterized the members. The map units formed in three episodes that could possible be related to caldera activity. The oldest is extrusive doming of distinctive older lava and breccia flows. The middle group is made up of tectonic breccias, altered, poorly lithified lava flows, and volcaniclastic sediments that fill a pre-existing moat. The youngest group consists of lava and breccia flows, along with hot-spring deposits related to regional eruptions and local hydrothermal activity. Four sets of linear features cut the area: East-west-trending, northeast-trending, northwest-trending, and arcuate-shaped features. Most are related to normal faults associated with basin-and-range faulting. Arcuate features may be related to tensional stress brought on by caldera activity.

A model for the vein mineralization within the district may involve depositional environments and tectonics related to caldera activity. Key variables to this mineralization includes the following: 1) Permeable volcanics and volcaniclastic sediments localized in the setting; 2) a fracturing framework provided by caldera activity conducive to ore deposition; and 3) hydrothermal circulation set in motion by intrusions or rhyolitic domes.