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