Extensional Deformation and Ore Deposits along a Transect through Northern Nevada

Abstract

In the northern Basin and Range province of southwestern North America, multiple generations of Eocene through modern, active normal faults are superimposed upon the well-mineralized northern Nevada gold province and geologic architecture of the former western margin of North America. This study examines variably tilted domains containing features related to hydrothermal systems and structural markers, such as Eocene volcanic and lacustrine rocks, that allow the pre-extensional geologic architecture of northern Nevada to be reconstructed. These reconstructions allow examination of the geology at the time of ore formation and more direct tests of hypotheses involving the genesis of ore deposits. Normal faults under common stress conditions initiate at ~60 and rotate until the faults lock up and new faults initiates. Using structural restorations, the coefficient of rock surface friction for fault systems can be calculated to better examine potential controls. While no single factor controls friction, the wide natural variability of friction and related factors highlight the diverse conditions extensional faulting occurs in the Earth’s crust. The Hilltop, Lewis, and Bullion districts of the northern Shoshone Range are examined in this study. In the study area, previous interpretations largely dismissed the involvement of normal faulting in the range. However, new mapping and reinterpretation of previously published observations demonstrate the range is significantly extended (50-120%) and the continuation of the same normal fault system that extended the Caetano caldera to the south. The variation in depth to the lower plate of the Roberts Mountains thrust (and potential Carlin-type gold mineralization) is a function of an old Mesozoic fold belt that was subsequently faulted and tilted by this mid-Miocene normal fault system. Reconstructions of mid-Cenozoic extension across the northern Great Basin reveal a Mesozoic fold belt from Buffalo Mountain to the Piñon Range, and ~40% extension since ~40 Ma between Winnemucca and Wendover. Most Carlin-type mineralization formed adjacent to Eocene extensional faults, proximal to overlying Eocene lakes, and in the anticlinal hinges of the Mesozoic fold belt. These results help further the debate surrounding competing models for the genesis of Carlin-type mineralization and combine with previous work to provide a more complete understanding of the deformation history of the northern Great Basin.