THE BREVITY OF HYDROTHERMAL FLUID FLOW REVEALED BY THERMAL HALOES AROUND GIANT AU-DEPOSITS

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 in a Jurassic intrusion that acted as an impermeable side to an Eocene hydrothermal system that formed the Betze-Post Carlin-type gold deposit in Nevada. The hydrothermal fluid responsible for Au mineralization conducted heat into the intrusion over the time that it flowed. To derive first-order estimates of the duration of hydrothermal fluid 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 patterns of AFT age and track length data observed across the intrusion.

Our results constrain the Carlin hydrothermal system to a minimum of one conductive heat pulse that most likely lasted between ~12 - 40 k.y.. It is also possible to explain observed levels of AFT annealing by a longer history of multiple shorter-lived heat pulses (as little as 1 - 15 k.y.). The greater the number of pulses, the shorter-lived and more temporally isolated they need to have been. It is unlikely that any history of multi-pulsed hydrothermal fluid flow involved more than 100 - 1000 individual pulses of near constant temperatures and duration. Rather, a smaller number of longer and/or higher temperature pulses probably dominated the conductive resetting of apatite fission-tracks. Such short pulses of hydrothermal activity were likely driven by seismicity or magmatic activity, rather than free convection. We suggest that, if the Carlin deposits did form from multiple pulses rather than a single pulse of hydrothermal flow, only a small number of such pulses carried significant Au. This possibly reflects times when a larger or deeper focused fault rupture pierced a deeper, and perhaps hotter, auriferous fluid reservoir, and/or times when an auriferous hydrothermal fluid was exsolved from an intrusion. Furthermore, results indicate that very large ore deposits can form over geologically short time periods.