Deposit types are also quite varied and include coherent flows, volcaniclastic tuffs, volcanic mudflows, and volcanogenic sediments. Of the middle member mapped to date, there are five separate units which also range in composition from mafic to silicic. Ash flow tuffs of the magnitude seen in the EMV are believed to result from fissure eruptions associated with caldera formation. During the fall of 1996, the welded ash flow tuff was studied in more detail. On the basis of this analysis, the unit has been subdivided into three different zones which may or may not represent three separate eruption events.
Samples decrease in porosity up-section due to an increase in welding.
The rhyodacitic tuffs of the upper member are densely welded to partially welded. The 1997 field season will complete the mapping of the middle and upper members on the eastern flanks of Bull Mountain and will perform a detailed stratigraphic measurement of the welded ash-flow tuff unit. To date, no evidence has been found to support the hypothesis that a caldera existed within the immediate area. Although two breccia pipes have been mapped within the study area, no economically significant mineralization has been observed in either pipe. Secondary copper has been noted in a unit that is at least spatially related to one of the breccia pipes.