Radiolysis and Fe‐Biotite Alteration: Questioning the Origin of H 2 ‐Bearing Fluid Inclusions in the Yorke Peninsula Granites, South Australia

Abstract Multiple drillings in the Yorke Peninsula, South Australia, recently confirmed the presence of natural hydrogen (H 2 ) in shallow sedimentary units. While radiolysis and Fe‐oxidation in the basement are potential generation mechanisms, their respective contributions remain unconstrained. This study investigates the H 2 ‐generation potential of Fe‐rich Hiltaba Suite granites (∼1.5 Ga) through petrographic analyses of a basement drill core collected near H 2 occurrences. Fluid circulation, a prerequisite for both radiolysis and Fe‐oxidation, is evidenced by feldspar sericitization and fluid inclusions. Raman spectroscopy and sample crushing analyses reveal H 2 (0.46 mmol/kg rock ) and He (0.70–0.88 μmol/kg rock ), suggesting that the Hiltaba granites contribute to the gas budget observed in the overlying sedimentary units. He isotopic analyses from bulk granites yield 3 He/ 4 He ratios from 0.0003 to 0.0056 Ra, indicating a crustal He origin and a radiolytic H 2 generation. A combination of SEM, TEM, EPMA and STXM analyses showed that biotite chloritization did not involve Fe‐oxidation. As biotite is the only primary Fe‐bearing mineral affected by alteration, this rules out any significant contribution of Fe oxidation toH 2 generation. Geothermometric calculations indicate that the last fluid circulation event occurred at 300–400°C, implying that the system last opened dynamically at least 370 Ma ago. Consequently, we propose that diffusive migration ofH 2 and He from the basement accounts for the gas fluxes in the sedimentary cover. This study represents a first step toward understanding subsurfaceH 2 and He generation in the Yorke Peninsula and highlights the Hiltaba Suite's potential to source H 2 and He over geological timescales.