Weathering as a control on the triple oxygen isotopes of groundwater-associated ferromanganese deposits: lessons from the Grimlock Ni–Co–Mn prospect, Northern Territory, Australia

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 $δ$¹⁸O and temperature data to calculate $δ$¹⁸O values of fully altered mineralogical endmembers, then, using mass balance, discuss scenarios to elucidate measured whole-rock $δ$¹⁸O values. The $δ$′¹⁸O (′ denotes linearized notation) and $Δ$′¹⁷O of near-surface samples (0–8 m) are generally lower (mean of 8.892‰) and higher (mean of −0.141‰), respectively, than the $δ$′¹⁸O (mean of 12.767‰) and $Δ$′¹⁷O (mean of −0.176‰) of samples from greater depths (19–22 m). At 16–17 m depth, $δ$′¹⁸O and $Δ$′¹⁷O are relatively high (means of 17.509 and −0.118‰, respectively). The measured whole-rock $δ$¹⁸O values are explainable by substituting lower $δ$′¹⁸O values for the Mn-oxide and Fe-oxyhydroxide fractions, and higher $δ$′¹⁸O 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 https://doi.org/10.6084/m9.figshare.c.7071907