Disentangling intense weathering profile evolution in Western Australia: Insights from REE geochemistry and neodymium stable isotopes

Abstract

Weathering processes can facilitate rare earth element (REE) enrichment and ore formation; however, exploration models are often not representative of deeply and intensely weathered landscapes (e.g., Australia, India and Brazil). Mobility of REE has traditionally been traced through REE geochemistry and radiogenic Sm-Nd isotopes, but long-term intense weathering compromises the effectiveness of these tools. Therefore, this study, applies novel stable Nd isotopes (δ 146 Nd) as a weathering and REE mobility tracer, alongside traditional approaches, to reconstruct weathering profile evolution at Lady Ada Au open pit, Yilgarn Craton, Western Australia. At Lady Ada, REE geochemistry indicates two relict groundwater tables within the profile. A precursor shallow, ‘fresh’ and circumneutral-alkaline groundwater table prior to climatic changes in the middle Neogene-Quaternary in the upper profile, causing appreciable REE fractionation. Subsequent climatic changes facilitated groundwater table lowering, acidification and salinisation, and REE were remobilised to the lower profile with negligible REE fractionation. The saline groundwater, alongside cation leaching and alkaline pore water from pedogenic carbonate dissolution, promoted inner-sphere REE-clay complexation as demonstrated by light δ 146 Nd fractionation in the upper profile. The δ 146 Nd composition of the weathering profile is well correlated to traditional weathering and REE mobility tracers (e.g., clay content, Y/Ho ratios), while being unaffected by pH and redox changes. Whereas radiogenic Nd isotopes were ineffective in constraining pedogenesis and REE mobility, as a result of its time- and lithology-dependence. Consequently, utilising stable Nd isotopes alongside traditional weathering and REE mobility tracers broadens our understanding of weathering processes across diverse climatic regimes.