RECALIBRATING ORE DEPOSIT GENESIS USING HEMATITE GEOCHRONOLOGY (Invited Presentation)

Quantifying when and for how long ore and accessory minerals precipitate to form economic mineral deposits is key to enhancing orebody knowledge and exploration targeting. The common iron oxide hematite (α-Fe₂O₃) has been shown to incorporate wt.% concentrations of U into its crystal structure and retain radiogenic Pb and He over geological time. Therefore, within ore deposits where iron oxides are the dominant components (e.g., BIF hosted iron ore, iron-oxide-Cu-Au), hematite is an essential geochronometer for constraining genesis from the Archean to recent.

The development of hematite geochronology (e.g., analytical protocols, micro-sampling, reference materials) in studies utilizing (U-Th)/He and more recently U-Pb, allow these isotope systems to be merged into “double–dating” and define the timing of low (< 200 °C) to high (> 400 °C) temperature processes effecting single ore samples.

A major unknown in BIF-hosted iron ore deposits worldwide is their age and thermal history at deposit to province scales. We present a case study from the Pilbara region (Western Australia) and provide the first direct U-Pb dates for iron ore across several major deposits. LA-ICPMS U–Pb data were collected using a 193 nm RESOlution–LR excimer laser coupled to an Agilent 8900–QQQ ICPMS and calibrated using a hematite reference material (MR-HFO).

Hematite dating reveals that Proterozoic iron deposits formed several hundred million years later than was previously determined via accessory mineral geochronology (zircon, xenotime). New U-Pb ages allow correlation with plate reorganization follwing supercontinent breakup, shedding first light on possible tectonic triggers for ore formation at the province scale. (U-Th)/He dating of the same samples (Alphachron I) delineates the timing of cratonic denudation, which influenced later ore weathering and upgrade.

The robustness of dated hematite is tested through a combination of nanoscale studies (FIB-HAADF STEM) and He diffusion experiments, establishing that U-Th-Pb is lattice bound and that He is retained within single diffusion domains over geologic time. Combined U–Pb and (U–Th)/He dating of hematite is a new tool that can define the timing and thermal history of ore deposits to enhance genesis and weathering modelling.