Timing Constraints of Gold Mineralization along the Carlin Trend Utilizing Apatite Fission-Track, 40Ar/39Ar, and Apatite (U-Th)/He Methods

Apatite fission-track analysis is used to demonstrate that the timing of gold mineralization at the Carlin East and Betze-Post deposits is 37.3 ± 1.5 Ma, based on the weighted mean of measured fission-track ages, and 41.6 ± 1.6 Ma, based on the weighted mean of the ages of the oldest fission tracks retained. Regional measured apatite fission-track ages cluster between 28 and 41 Ma to the south of the Carlin East deposit whereas measured ages are greater than ~56 Ma to the north. The regional trend of the apatite fission-track ages and the pattern of gravity and magnetic anomalies suggest that the thermal center for gold mineralization along the Carlin trend was south of the Carlin East deposit, near the 37 Ma Welches Canyon stock. Modeling of apatite fission-track-age data on both sides of the Post fault indicates that the Little Boulder stock was approximately 35°C cooler at 40 Ma relative to similarly positioned samples in the Goldstrike stock. It is unclear whether this thermal history difference across the Post fault is due to post-40 Ma differential exhumation or differential heating across the fault at the time of gold mineralization. Comparison of apatite fission-track and whole-rock 40Ar/39Ar ages from altered dikes in the ore zone at Carlin East demonstrates that 40Ar/39Ar ages do not reflect the age of gold mineralization, whereas the apatite fission-track ages were completely reset at the time of hydrother-mal activity. (U-Th)/He ages are younger than the inferred mineralization age and may reflect weak hy-drothermal activity associated with Miocene volcanism.