講演情報
[2102]Identifying petrophysical controls on fluid pathways in the peridotite bedrock-saprolite transition zone for Ni mineralization
○Anna Irada - Laode Malim1, Koki Kashiwaya1, Katsuaki Koike1 (1. Kyoto University)
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キーワード：
ultramafic weathering、petrophysical properties、serpentinization、preferential fluid pathways
The bedrock–saprolite transition zone in peridotite laterite profiles is a critical hydrological interface where fluid circulation, secondary porosity, and geochemical mass transfer are controlling critical metal mobility and carbon sequestration in ophiolite terranes. We integrated petrophysical measurements of 20 ultramafic rock samples (low, medium, and high weathering grade) with XRD mineral quantification and surface water hydrogeochemistry from nine catchment stations in Pomalaa, Southeast Sulawesi, Indonesia. This study aims to elucidate the role of petrophysical properties in generating preferential fluid pathways and driving nickel mobility throughout the weathering profile.The degree of serpentinization (Dser = 34.1–96.4%) exerts a primary, systematicaly control on bulk density (1.42–3.02 g/cm³; r = −0.57, p < 0.05) and drives porosity from 3.4% in low-serpentinized bedrock to 58.20% in high serpentinized but low weathered bedrock. High weathering grade samples recorded a mean porosity of 38.0% and a density of 2.06 g/cm³; a modal abundance of secondary mineral assemblages, such as serpentine minerals, clay phases, and Ni-bearing hydrous silicates of 87.4%, suppressed bulk density below 2.0 g/cm³. All water samples uniformly exhibit Mg–HCO₃ facies (Mg/Ca molar mean = 23.6; HCO₃⁻ up to 186.9 mg/L), confirming peridotite-silicate weathering sourcing. We conclude that laterite geochemical flux is governed by serpentinization degree, which controls porosity (3.4–58.20%) and pore connectivity. The degree of serpentinization also provides primary control variations in permeability and advective fluid transport.