Are epicontinental rift sedimentary basins fertile tectonic environments for gold? reflections on the north-central Nevada gold province

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Emsbo, Poul۱; Scott, Robert۲; Bull, Stuart۲
Holding Date 27 September 2008

Recognition of the relationship between Devonian stratigraphy/basin architecture and gold deposits in north-central Nevada, the world’s third largest gold province, highlights the importance of understanding the evolution of the associated carbonate-platform margin. Work in north-central Nevada suggests the Early and Middle Devonian platform-to-slope transition is characterized by abrupt, and possibly truncated, facies boundaries that do not readily fit a simple rimmed-shelf margin or ramp depositional model. Abrupt lateral and vertical facies changes, thick debris flows, and synsedimentary slump structures suggest that Early to Middle Devonian syndepositional extensional faulting along the platform margin controlled basin topography and sedimentation on the outer carbonate-platform margin. Extensional foundering of the carbonate platform established small, fault-controlled, restricted basins on the outer continental margin that are regionally expressed by changes in Silurian to Middle Devonian facies relations and isopach thicknesses. Spatial coincidence of basins with underlying Precambrian basement structure suggests that deep basement faults localized Devonian synsedimentary extension and normal faulting and controlled the Devonian rift basin geometry.
Devonian synsedimentary faults focused hydrothermal venting of Au-rich reduced basinal brine that formed significant SEDEX (synsedimentary exhalative) Au deposits and enriched the Devonian stratigraphy in Au along basin margins. These rift-generated extensional basins, characterized by relatively low degrees of amagmatic extension, along a "passive" margin have been long recognized as classic environments for the formation of Zn-Pb-Ag SEDEX deposits from oxidized brines. Alignment of Devonian rift basins with regional geochemical patterns and major gold deposit "trends" in north-central Nevada suggest strongly that these features reflect a geologically linked process. While most of the economic gold deposits along the trends are Eocene Carlin-type deposits, the gold trends also host a striking superposition of different gold deposit types, both prior to (Devonian, Mississippian, Jurassic, Cretaceous) and after (Miocene and Pliocene) Carlin-type deposit formation. Occurrence of spatially superposed gold events, related to a variety of ore fluids with distinctive chemical and alteration signatures, in distinct tectonic settings with different structural orientations likely reflects gold remobilization from a pre-enriched Devonian stratigraphy rather than the operation of distinct magmatic or fluid-related processes. As such, the remarkable Au endowment of north-central Nevada may reflect a resonance effect ultimately controlled by the Devonian tectonic and metallogenic evolution of the carbonate-platform margin of western North America.