2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 5
Presentation Time: 2:45 PM


NUTT, C.J., Mineral Resources Team, U.S. Geological Survey, Box 25046, M.S. 973, Denver, CO 80225 and THORMAN, C.H., CTGS Intl., Inc, 12464 W. 2nd Drive, Lakewood, CO 80228, cthorman@comcast.net

The importance of low-angle to bedding-parallel faults in Carlin-type deposits in northern Nevada is under-appreciated. Lately, controversy over the involvement of igneous rocks, circulating meteroic water, and metamorphic water in the mineralizing process has overshadowed the importance of specific deposit models. Regardless of the source of metals and transporting fluids, structures control the localization of Carlin-type deposits. High-angle faults are the upward and/or lateral pathways for the fluids, but without the development of a plumbing system more or less parallel to bedding, large-scale Carlin-type deposits would not exist. Low-angle faults cut by high-angle structures are a major aspect of the plumbing system that allowed mineralization ‘parallel' to bedding. These low-angle faults commonly are bedding parallel and both repeat and cut out section, forming at contrasting rheologic boundaries (i.e. shale/carbonate, limestone/dolomite, within thin-bedded limestone). Fluids moved along the high-angle faults and commonly spread laterally when they encountered a rock section disrupted by the low-angle faults. The age of the low-angle faulting may range from Devono-Mississippian to Cretaceous, the only requisite being that it predates middle Eocene time.

The low-angle structures typically occur in selected intervals in the Roberts Mountain Formation, Hanson Creek Dolomite and at and just below the base of the Roberts Mountain allochothon. Some names commonly applied to these structures are attenuation faults, duplex structures, and decollement faults. Examples can be seen at the Pipeline, Cortez, Carlin, Post-Betze, and Jerritt Canyon deposits. Transport direction of the hanging wall is almost uniformly eastward based on drag folds. This is consistent with the general compressional history of the region from Late Devonian to early Tertiary time. Displacement generally is impossible to determine, but appears to be relatively small (~1m to <1 km). We propose that these low-angle faults formed during Paleozoic to early Tertiary compressional orogenies that affected the area, but may have been reactivated by extensional events.