RECORD OF MULTI-SCALE HYDROTHERMAL FLUID PATHWAYS CONTROLLED BY A TRANSTENSIONAL FAULT NETWORK: INSIGHT FROM THE NORTHERN NEVADA RIFT

The fossilized northern Nevada rift (NNR) presents a unique opportunity to investigate multi-scale fault network controls on hydrothermal fluid pathways of low-sulfidation epithermal systems. Evidence suggests that structural controls at all scales directly influenced when and where a series of genetically related Au-Ag epithermal deposits formed within the rift. Field observations indicate the southern half of the NNR formed a series of five half graben basins, aligned on a 340° trending axis. All basins have ~340° striking, E-dipping, boundary fault systems on their western margins. E-dipping boundary faults work in concert with incipient, parallel, W-dipping boundary faults on the eastern basin margins. W- and E-dipping dip-slip boundary faults commonly display apparent dextral offset, are linked by ~50°-70° striking dip-slip faults that frequently display sinistral offset. Collectively, the faults accommodated asymmetrically subsiding blocks to form volcanic depocenters and the basin architecture suggest extension occurred with a slight dextral motion across the ~40 km-wide NNR, consistent with the expected motions of coevally forming structures in a Riedel shear model. The Mule Canyon and Fire Creek deposits are located in similar structural settings in horse blocks bound by the primary boundary fault in the hanging wall and its conjugate on the footwall. Mechanically the conjugate normal faults form subvertical Riedel shear zones that focused hydrothermal fluid flow. Geometries of dike swarms and veins at both deposits suggest that linkages between conjugate normal faults, manifested as surficial relay-ramps, serve as high flux fluid conduits. Steeply plunging ore shoots and kinematic indicators at Fire Creek suggest dip-slip motion on faults formed plunging Riedel shear fabric that channelled fluid flow along individual vein systems. Observations of coseismic deposition of Au-Ag-bearing bands within veins, suggest that fault kinematics and resulting shear fabric were essential in channelizing the flow of Au-Ag-bearing fluids to the epithermal environment at all scales. Results from this study suggest that the fractal nature of shear fabrics may play a key role in understanding the dynamic interaction of structural development, seismicity and fluid flow in extensional environments.