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be explained by the lower degree of welding of the Henson Formation. <br /> After the deposition of the Burns and Henson formations there was a broad resurgent doming <br /> between the San Juan caldera and the Uncompahgre caldera. This resurgent doming resulted <br /> in extensive distension fracturing in a northeast/southwest-trending direction (Steven and <br /> Lipman, 1976, p. 13) as shown in Figure 3. Later collapse of the resurgent doming along <br /> steeply dipping, northeast/southwest-trending fractures formed the Eureka graben. Arcuate faults <br /> related to the collapse of the Silverton caldera (such as the Bonita fault) appear to be <br /> contemporaneous with the bounding faults of the Eureka graben. Although some later faulting <br /> exists, the Eureka graben fracture system was the last major set of fractures imprinted on the <br /> area of the Sunnyside Mine. During mineralization 13.0 to 16.6 MYBP (Casadevall and <br /> Ohmoto, 1977), the fractures of this system served as flow conduits and sites for ore deposition. <br /> The Sunnyside Mine is located within the Eureka graben at the junction of the Ross Basin fault <br /> and the Sunnyside fault as shown in Figure 4. Figure 4 also illustrates the dominant <br /> northeast/southwest fracture trend. In the vicinity of the mine, the dip of originally horizontal <br /> strata now ranges from 10° to 14° to the southwest (Langston, 1978, p. 17). <br /> Rock alteration and mineralization is widespread in the vicinity of the San Juan caldera. <br /> "Propylitic alteration has affected many cubic miles of volcanic rocks throughout and beyond <br /> the [Silverton] caldera" (Burbank, 1960). In the propylitized rocks "pyrite is ubiquitous and <br /> forms between 0.1 and 2.0 percent" of the rock volume (Casadevall and Ohmoto, 1977, p. <br /> sanjuan\sunny\110361\mt91.Rpt 9 <br /> a snon H7U cno= ncH <br />