(U-Th)/He-dating of ferruginous duricrust: Insight into laterite formation at Boddington, WA

Geochronological characterization of pisolites and nodules in ferruginous duricrust from laterite profiles exposed near the Boddington gold mine (BGM) in Western Australia was undertaken by application of (UTh)/He dating. Optical microscopy, Raman spectroscopy, X-ray diffraction (XRD) analysis and TESCAN Integrated Mineral Analyser (TIMA) mineral mapping identified areas in pisolites and nodules in pisolitic and fragmental duricrust suitable for (UTh)/He-dating. (UTh)/He dates of 5.8–1.3 Ma for pisolites and fragmental duricrust nodules indicate late-Miocene/Pliocene to possible early-Pleistocene ages as a minimum age for the formation of pisolites and nodules and a maximum age for formation of the BGM duricrust. Averaged (UTh)/He ages suggest at least two main episodes in the evolution of pisolite formation, the oldest occurring at 5.8–5.2 Ma and a younger episode at 3.9–3.2 Ma, with indication of an even later episode of pisolite formation at 2.3–2.0 Ma. These results are comparable with (UTh)/He data for pisolitic duricrust approximately 50 km to the east of Perth (Toodyay), and are consistent with other age estimates for lateritization in Western Australia, suggesting that processes of regolith formation and/or modification in the Darling Range, to the east and south-west of Perth, were regionally synchronous. The significant difference in age of the rim (2.0 ± 0.2 Ma) compared to the core (3.9 ± 0.5 Ma) of a single pisolite, in conjunction with a difference in mineralogy and textural evidence, suggest that ancient, episodic bushfires may have affected or reset the (UTh)/He system of iron oxides. Despite this, geochronological characterization can provide quantitative constraints on the age and relative timing of regolith formation.