T4 Advances and Applications of Thermochronology to Tectonic, Magmatic, Basin, and Geomorphic Problems

Sunday, 22 September 2024
8:00 AM - 12:00 PM
212B

DECONVOLVING THERMAL SIGNATURES OF EXHUMATION, MAGMATISM, AND FLUID FLOW AT THE POGO AU DEPOSIT, EASTERN ALASKA

Paper number: T4-12
Program: Technical Sessions
Time: 10:55 AM – 11:10 AM

Authors

Abstract

Exhuming mineralized crust is key to generating economic ore deposits. The Goodpaster district in eastern interior Alaska hosts the mid-Cretaceous, ~8 Moz Pogo Au deposit and several Au-Bi-Te(-As, Sb) prospects that define a ~50 km-long, east-west trend. Prior and new deposit models highlight enigmatic mineralization depth and thermal events, requiring new constraints on post-mineralization exhumation.

New zircon and apatite (U-Th)/He (ZHe, AHe) dates from seventeen samples collected along the mineralized trend show two groups based on location, mean date, and AHe date-effective uranium (eU) patterns. Samples across the study area yield mean ZHe and AHe dates of ~90–75 Ma and ~65–43 Ma. AHe dates from individual aliquots form a positive date-eU trend from ~35 Ma at low eU (~5 ppm) to ~65 Ma at high eU (>100 ppm). Samples within 10 km of Pogo have mean ZHe dates of ~60 Ma and ~40 Ma AHe dates that are invariant to >400 ppm eU. Thermal history inversions show rapid cooling from >160–<60 ℃ at ~75-65 Ma regionally and at ~40 Ma near Pogo.

Post-mineralization exhumation is likely <2 km based on evidence for boiling in syn-mineralization veins. Cooling at ~75-65 Ma is coeval with regional deformation and magmatism and partially attributable to the latter. At Pogo, ~40 Ma cooling conflicts with paleosurface constraints from ~55 Ma volcanics. AHe dates from low-eU grains constrain the regional thermal history to <40 ℃, which we combine with topographic swath profiles, models of topographically-influenced isotherms, and paleosurface markers. We infer <1 km of Cenozoic exhumation by relief reduction, and no more than 1.5 km during Cretaceous cooling. These constraints, with high cooling rates (>100 ℃/Ma, inferred from a flat apatite He date-eU relation) and a lack of large structures to accommodate substantial differential exhumation, suggests 40 Ma cooling at Pogo is most likely from reheating during hydrothermal fluid flow. ZHe dates from these samples overlap published 40Ar/39Ar plateau dates from sericite alteration, suggesting other, earlier fluid events that perturbed thermochronometric data. Results highlight a nonmonotonic thermal history at the Pogo Au deposit with only limited exhumation. Our dataset further provides an example of integrating radiation damage systematics, geologic data, and sample spatial relations to deconvolve thermal histories of mineralized rocks.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-404470

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