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The extent of fracturing in volcanic rocks is directly related to the degree of welding <br />if other factors are equal. Hence, the more welded Burns Formation tends to be more <br />fractured than the Henson Formation. <br />After the deposition of the Burns and Henson formations there was a broad resurgent <br />doming between the San Juan caldera and the Uncompahgre caldera. This resurgent <br />doming resulted in extensive distension fracturing in a northeast/southwest-trending <br />direction (Steven and Lipman, 1976, p. 13) as shown in Figure 3. Later collapse of <br />the resurgent doming along steeply dipping, northeast/southwest-trending fractures <br />formed the Eureka graben. Arcuate faults related to the collapse of the Silverton <br />caldera (such as the Bonita fault) appear to be contemporaneous with the bounding <br />faults of the Eureka graben. Although some later faulting exists, the Eureka graben <br />fracture system was the last major set of fractures imprinted on the area of the <br />Sunnyside Mine. During mineralization 13.0 to 16.6 MYBP (Casadevall and Ohmoto, <br />1977), the fractures of this system served as flow conduits and sites for ore <br />deposition. <br />The Sunnyside Mine is located within the Eureka graben at the junction of the Ross <br />Basin fault and the Sunnyside fault as shown in Figure 4. Figure 4 also illustrates the <br />dominant northeastlsouthwest fracture trend. In the vicinity of the mine, the dip of <br />originally horizontal strata now ranges from 10° to 14° to the southwest (Langston, <br />1978, p. 17). <br />eunnylwpltraci122911Mar1993.Rpt 1 1 <br />NCI SIIT1011HYDAO-SEARCH <br />