LINKS BETWEEN THE DEVELOPMENT OF EXTENSIONAL BASINS AND THE FORMATION OF LOW-SULFIDATION EPITHERMAL AU-AG DEPOSITS: INSIGHT FROM THE NORTHERN NEVADA RIFT

Low-sulfidation epithermal deposits are important sources of Au and Ag globally. A series of deposits coincides in space and time with the formation of the northern Nevada rift (NNR), a fossilized rift system composed of extensional basins. The mechanisms by which the NNR’s development influenced when and where deposits formed have not been determined. Ongoing research is documenting the development of five segmented basins at the south end of the NNR. Structural relations and supporting ⁴⁰Ar/³⁹Ar dates in the basins hosting the Mule Canyon and Fire Creek deposits suggest both formed during similar ~150 ky periods during ~1.5 my lifespan of the basins. Evidence suggests initial development of half graben architecture is a critical stage of rift development in channelling upwelling Au-Ag-bearing hydrothermal fluids. Field observations suggest boundary faults exceeded 30 km in length and <1 km displacement. Moment magnitude calculations suggest such faults were capable of producing 7 Mw earthquakes that were capable of rupturing the full depth of the brittle crust. Implications are subsequent seismic events were capable of triggering the migration of deep crustal fluids only after such boundary faults formed. Kinematic links between basin development and deposit formation are evident from veins at Mule Canyon and Fire Creek, which are hosted within small displacement faults antithetic to primary boundary faults, both of which display growth geometries. Further support comes from veins at Fire Creek where veins, within which faults display slip events that correspond to deposition of individual Au-Ag bearing bands. Both Mule Canyon and Fire Creek occur within a horse block in the immediate footwall of boundary faults (340° strike), suggesting such boundary faults were critical pathways for Au-Ag bearing hydrothermal fluids. At Mule Canyon, 310° oriented growth faults bound the deposit to the north and south, and field evidence suggests they served as relay-ramp linkages between the two primary conjugate boundary faults of the basins. The architecture set up by relay ramps linking boundary fault networks were critical in the formation of Mule Canyon and Fire Creek at all scales. The structural and kinematic relationships related to rift development can assist in targeting from the deposit to individual stope scales.