216-7 Post-mineralization Exhumation and Hydrothermal Reheating of the Pogo Au-Bi-Te Deposit, Eastern Alaska

Session: Geochronology of Critical Mineral Deposits with Special Reference to U-Th-Pb Dating of Common-Pb-Rich Minerals

Presenting Author:

Authors:

Abstract:

Exhumation and alteration of mineralized crust is a key component of comprehensive ore genesis models. The Goodpaster district in eastern Interior Alaska comprises the ca. 104 Ma, ~8 Moz Pogo Au deposit and several Au-Bi-Te(-As, -Sb) prospects distributed along an ~50 km-long east-west trend. Prior Re-Os and ⁴⁰Ar/³⁹Ar (Ar) geochronologic data of alteration suggest continued modification of the deposit following main-phase Au mineralization and motivates efforts to (1) resolve the associated temperatures and spatial footprint of these thermal pulses, and (2) determine post-mineralization exhumation.

Zircon and apatite (U-Th)/He (ZHe, AHe), apatite fission-track (AFT), and apatite U-Pb dates were acquired from seventeen Cretaceous granitoid and Paleozoic gneiss samples distributed along the trend of mineralization. Samples within ~10 km of Pogo yield mean ZHe (n=3), AHe (n=4), and AFT (n=2) central dates of ~60, ~35, and ~35 Ma, respectively. Samples from the remainder of the transect yield ZHe (n=7), AHe (n=12), and AFT (n=5) dates of ~90-75 Ma, ~65-43 Ma, and ~93-40 Ma. Apatite U-Pb dates (n=6) are ~90 Ma across the study area.

Thermochronometric dates post-date constraints on near-surface conditions from (sub)volcanic rocks, implying time-temperature (tT) histories characterized by reheating. Forward and inverse models test several hypothesized monotonic and reheating tT scenarios. Model results variably show either post-magmatic thermal relaxation of ~95 Ma plutonic rocks (or reheating by this same magmatic suite), reheating at ~60-55 Ma to temperatures >160 ℃, or reheating at ~40 Ma to temperatures >130 ℃. The thermal footprint of ~95 Ma magmatism is regionally extensive and overlaps a later phase of Au mineralization constrained by previously published sulfide Re-Os data. tT inversions from granitoids of this suite imply emplacement at ambient temperatures <100-80 ℃ (≤2 km depth assuming a 45 ℃/km geotherm typical of magmatic terranes). Near Pogo, tT histories require reheating at ~60 and ~40 Ma, the former overlapping regional magmatism and Ar dates from sericite decorating post-mineral faults. Both ~60 and ~40 Ma phases are typified by >100 ℃/Myr cooling rates, which in concert with their spatial distribution, are most compatible with localized episodes of hydrothermal alteration and reheating along mesoscale fault networks coeval with regional strike-slip deformation. These data collectively constrain slow (~0.02 mm/yr) landscape-scale exhumation since ~95 Ma, punctuated by multiple localized post-mineralization reheating and alteration events.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10464

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