At the Grimlock laterite deposit (Northern Territory, Australia), Co and Ni mineralization occurs mainly in the Mn-oxide rich layers of the ferromanganese (Fe-Mn) crust overlying ultramafic bedrock. Groundwater-associated Fe-Mn crusts consist of mineral (e.g. Mn-oxide, Fe-oxyhydroxide and silicate) groups suitable for studying triple oxygen isotopes and present unique interpretative challenges (e.g. small Mn-oxide fractions relative to Fe-Mn precipitates from other formation environments and extensive weathering). We evaluate triple oxygen isotopes within the context of changes to properties (i.e. mineralogy, major and trace element geochemistry, and degree of weathering) of a lateritic profile. We use pre-existing mineral-water fractionation factors, meteoric water δ18O and temperature data to calculate δ18O values of fully altered mineralogical endmembers, then, using mass balance, discuss scenarios to elucidate measured whole-rock δ18O values. The δ18O (′ denotes linearized notation) and Δ17O of near-surface samples (0–8 m) are generally lower (mean of 8.892‰) and higher (mean of −0.141‰), respectively, than the δ18O (mean of 12.767‰) and Δ17O (mean of −0.176‰) of samples from greater depths (19–22 m). At 16–17 m depth, δ18O and Δ17O are relatively high (means of 17.509 and −0.118‰, respectively). The measured whole-rock δ18O values are explainable by substituting lower δ18O values for the Mn-oxide and Fe-oxyhydroxide fractions, and higher δ18O values for the aluminosilicate fraction, changes coinciding with greater alteration. These results suggest that mineral weathering is primarily responsible for observed variations in the triple oxygen isotopes of groundwater-associated Fe-Mn crusts, rather than variation in the initial source of oxygen incorporated into Mn-oxide.

Supplementary material: Grimlock sample photographs, XRD patterns and supplementary figures and tables are available at

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