The Brevity of Hydrothermal Fluid Flow Revealed by Thermal Halos around Giant Gold Deposits: Implications for Carlin-Type Gold Systems

Abstract

The duration of hydrothermal activity required to form ore deposits is poorly constrained. We demonstrate that thermochronology data, coupled with thermal modeling, can be used to constrain the duration of hydrothermal fluid flow. Apatite fission-track (AFT) thermochronology data define a conductive halo around an Eocene hydrothermal system that formed the Betze-Post gold deposit on the northern Carlin trend in Nevada. The premineralization Goldstrike stock acted as an essentially impermeable side to the auriferous Carlin hydrothermal system. The hydrothermal fluid conductively heated the intrusion over the time that it flowed past it. To derive first-order estimates for the maximum duration of this flow we numerically modeled one-dimensional conductive heat flow into the intrusion and used the results to forward model ensuing AFT annealing. Modeled levels of annealing were compared to AFT dates and track length data measured across the intrusion. Our results indicate that the episode of main ore-stage hydrothermal fluid flow (mean temperature of 200°C) that formed the ~1,150 metric ton (t) Betze-Post gold deposit had a maximum duration of <15 to 45 ka. The average gold flux over this period was ~80 to 30 kg yr−1, comparable to that measured in the deep reservoirs of several modern geothermal fields. Conservative estimates of gold concentration in the main ore-stage fluids imply that fluid upflow rates and total advective heat flow were also comparable to modern geothermal systems. This suggests that the most important factors for generating the large gold deposits of the northern Carlin trend were a large and/or continuous source of gold, and a very efficient means of removing it from the fluid, rather than the hydrologic system itself. The short duration of main ore-stage fluid flow is unlikely to represent a steady-state convective system. Instead, it most likely reflects a transient period of flow following slip and permeability generation along the steeply dipping Post-Genesis fault system that hosts many of the deposits along the northern Carlin trend. A sudden increase in the permeability of a fault may have led to a transitory period of peak fluid temperature as the fault initially tapped meteoric fluid that had resided at depth and had thermally equilibrated with the host rocks. With continued convection the flow drew cooler, less rock-buffered meteoric water down from higher in the system.