FAULT CONTROLLED FLUID CIRCULATION IN ACTIVE HOT SPRING AND EOCENE CARLIN-TYPE SYSTEMS IN THE GREAT BASIN

Fault-controlled fluid circulation plays a critical role in both the formation of present-day hot springs and Eocene Carlin-type gold deposits. Both modern and fossil geothermal systems are characterized by remarkably high surface heat flow (1000 mW/m²<) with varying degrees of fluid-rock isotope exchange. The absence of a clear magmatic source adjacent to present-day hot springs suggests fluid circulation must extend down to depths greater than 5 km. A series of idealized hydrothermal models were constructed for Beowawe hot spring and Carlin-type gold systems to elucidate their similarities and differences. A novel aspect of our approach is that we simultaneously track fluid-rock isotope exchange, silica precipitation, apatite fission track annealing, and groundwater residence time using a parallel computational scheme. Model results suggest that both modern and fossil geothermal systems had deep seated (~6 to 7 km), fault-controlled flow systems driven by natural-convection. The Beowawe flow system is characterized by a single-pass hydrothermal flow cell with relatively little fluid-rock isotope exchange. In order to match modern heat flow and isotopic data, we found that the Beowawe flow system must include lateral circulation through thin permeable Paleozoic karst units at depth. The fossil Carlin geothermal system was characterized by a loop convection cell with higher amounts of fluid-rock isotope exchange. In the Carlin flow system, lateral flow through coarse-grained neoProterozoic siliciclastic rocks facilitated higher degrees of mixing between meteoric fluids and older, more highly exchanged groundwater. In order to match fluid inclusion homogenization temperatures, a permeability contrast on the order of 10³ to 10⁴ was required between lithostratigraphic units and faults. Faults cutting fine-grained siliciclastics had to be assigned lower vertical permeability in order to allow for lateral fluid circulation through underlying carbonate rocks which host the gold mineralization. Results also indicate that these flow systems must have been short-lived (10⁵ years for Eocene and 10³ years for present day). Our results fit models for Carlin-type gold deposits involving deep convection of meteoric water.