MONAZITE PETROCHRONOLOGY OF INTERIOR ALASKA'S POGO AU DEPOSIT

The relationship between magmatism, metamorphism, deformation, and Au mineralization at Interior

Alaska’s Pogo deposit remains enigmatic. The deposit is hosted in meter scale quartz veins subparallel to and

cutting the foliation of greenschist to amphibolite grade gneisses. Pogo is <2km from a coeval granite

batholith and postdates peak metamorphism at ~115 Ma by ~10 My. Kinematic indicators and mineral

barometers indicate that this region was undergoing extension and rapid exhumation during ~105 Ma

mineralization. However, no petrochronology deconvoluting deformation, metamorphism, and fluid flow has

been performed. Vein textures vary from plastically deformed, granular and brecciated to recently

recognized late bladed quartz-carbonate suggestive of boiling and shallow emplacement. Although Pogo has

previously been classified as both orogenic and intrusion related, these models do not consider the tectonic

exhumation or account for textures indicative of shallow emplacement. We integrate outcrop-scale structure

with microstructural analysis and monazite petrochronology to test a model where continuous exhumation

of the host terrane caused progressive localization of deformation. Exhumation-induced localization should

focus fluid flow, causing polybaric mineralization of Au-Bi-Te quartz veins. Monazite’s utility as a

petrochronometer and propensity to recrystallize through coupled fluid-mediated dissolution reprecipitation

reactions allow the dating of both deformation and fluid flow, where the age progression of those two events

constrain how the structural evolution relates to ore-forming processes. Multi-scale WDS compositional

mapping of oriented thin sections aids the determination of whether ore fluids were introduced in a static or

dynamic setting. Progressive localization of deformation is tested using laser ablation split stream analysis of

U/Pb and trace elements. Initial microstructural analysis from wall rock samples shows a nearly continuous

transition from plastic to brittle deformation with increasing proximity to veins, consistent with progressive

localization during retrogression. Results will allow us to constrain the timing and extent of deformation

relative to mineralizing fluid flow.