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 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 delta (super 18) O and temperature data to calculate delta (super 18) O values of fully altered mineralogical endmembers, then, using mass balance, discuss scenarios to elucidate measured whole rock delta (super 18) O values. The delta ' (super 18) O and Delta ' (super 17) O of near-surface samples (0-8 m) are generally lower (mean of 8.892 per mille) and higher (mean of -0.14 per mille), respectively, than delta ' (super 18) O (mean of 12.767 per mille) and Delta ' (super 17) O (mean of -0.176 per mille) of samples from greater depth (19-22 m). At 16-17 m depth, delta ' (super 18) O and Delta ' (super 17) O are relatively high (means of 17.509 per mille and -0.118 per mille, respectively). The measured whole rock delta (super 18) O values are explainable by substituting lower delta ' (super 18) O values for the Mn-oxide and Fe-oxyhydroxide fractions, delta ' (super 18) O 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: https://doi.org/10.6084/m9.figshare.c.7071907