1999 GSA Annual Meeting -- Denver, Colorado
1999 GSA Annual Meeting -- Denver, Colorado
Author(s): BERGER, Byron R., SNEE, Lawrence W., US Geological Survey, Federal Center MS964, Denver, CO 80225. bberger@usgs.gov; TINGLEY, Joseph V., Nevada Bureau of Mines and Geology, Reno, NV 89557
Keywords: economic geology, structural geology, geochronology
The interdependence of stress, fluid flow, and ore formation is being studied in the Aurora, Nevada (15-11 Ma) and Bodie, California (10-8 Ma) volcanic fields. At Aurora, the Brawley Peaks andesitic stratovolcano was constructed during active NW-striking, right-lateral faulting, and the magmatism appears to have accommodated extensional strain along a NE-striking fault zone. Sector collapse occurred in the plane of the primary, regional NW right-lateral strike-slip faulting, facilitated by an advanced-argillically altered cone. Lavas from an 11.21±0.03 Ma crater dome flowed down the edifice along the zone of collapse. The collapse had no effect on hydraulic conductivity on NE extensional fracture sets as evidenced by quartz-adularia vein patterns in the 10.4-10.2 Aurora district. However, a change in the regional stress field inverted motion on the NE fault system from extensional to strike-slip during the later stages of hydrothermal activity, which had an important effect on ore formation within the fault mesh. The Bodie Mt. dacitic stratovolcanic complex was constructed after the approximately 10.3 Ma change in the stress regime and accommodated extensional strain along NW-striking normal faults. The Bodie mining district is localized along the same NE fault zone important in the Aurora district, with mineralization being localized across a small left-lateral, extensional stepover. Au-Ag-base metal mineralization occurred south of the stepover and earliest at 8.41±0.02 Ma. At 8.33±0.02 Ma the locus had shifted northeast to a releasing bend at Standard Hill and the ore-mineral suite changed to Au-Ag without a polymetallic character, and the waning stages of hydrothermal activity were farther northeast on Bodie Bluff at 8.23±0.04. NW-striking normal faults at this same time localized mercury deposits in hot-spring discharge vents in the Paramount district.
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