Extensive Olivine Depletion in Archean Greenstone Belts: Evidence from Drill Core and Implications for Natural Hydrogen Generation

Jiejie Li , Peter Haines, Andrew Feitz, Trevor David Waite, Ryan Armstrong

Natural hydrogen models commonly assume that ultramafic rocks in Archean greenstone belts retain olivine for continued serpentinization, but the regional extent of preservation is poorly constrained. We analyze thermal-infrared mineral logs from 18,692 m of ultramafic drill core in 218 Western Australian boreholes. Olivine exceeds a defined spectral threshold in 97% of intrusion-hosted core but only 24% of greenstone-hosted core, a contrast robust to threshold selection and borehole or cluster weighting. Talc–carbonate assemblages dominate 56% of greenstone-hosted core. Fewer than 1% of these intervals meet the olivine criterion. Mineral assemblages are spatially clustered even within tectonic units (permutation p ≤ 0.007), and serpentine declines relative to olivine below 100 m. These results reveal extensive but spatially uneven olivine depletion in the sampled Archean greenstones, challenging the premise of broadly retained olivine in greenstone-based natural hydrogen source models. Assessing remaining olivine therefore requires mineralogical evidence rather than lithological extent alone.