LINKS BETWEEN DEVONIAN BASIN ARCHITECTURE AND GOLD MINERALIZATION, NORTH-CENTRAL NEVADA: AN UNDERVALUED TECTONIC CONTROL?

Recognition of the relationship between Devonian stratigraphy in Nevada and Carlin-type gold (Au) deposits, which comprise the world's third largest gold district, highlights the importance of understanding the architecture of the Devonian basin and the evolution of the carbonate-platform margin in Nevada. Where reconstructed from outcrop and cores, the Early and Middle Devonian platform-to-slope transition in north-central Nevada is characterized by abrupt, and possibly truncated, facies boundaries that do not readily fit a simple rimmed-shelf margin or ramp depositional model.

Common 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 may have controlled basin topography and sedimentation on the outer carbonate-platform margin, and therefore may have played a key role in controlling the facies relationships across this transition. This model suggests that extensional foundering of the carbonate platform established small, fault-controlled, restricted basins on the outer continental margin. Our present work on the northern Carlin trend has shown that such faults were the loci of major venting of Au-rich reduced basinal brines in Middle Devonian time that formed sedex Au mineralization.

The alignment of Silurian to Middle Devonian facies changes and isopach thicknesses with (a) Early to Middle Devonian basin-margin faulting, (b) underlying Precambrian basement structure, (c) onset of proto-Antler tectonism, (d) sedex Au mineralization, and (e) Carlin-type gold deposits in north-central Nevada suggest strongly that these features reflect a geologically linked process. Thus, understanding the evolution of the Devonian carbonate-platform margin might be central in understanding the complicated genesis of Carlin-type deposits.