COUPLED Mg-Ni ISOTOPE SYSTEMATICS OF WEATHERING INTENSITY AND GARNIERITE FORMATION IN TROPICAL Ni LATERITE SYSTEMS, SULAWESI, INDONESIA

Abstract Magnesium and nickel isotopes provide insights into weathering processes and lateritic Ni enrichment; however, their coupled behavior during garnierite formation remains poorly constrained. We present Mg-Ni isotope compositions for serpentinized bedrock, garnierite, and laterite from the Morowali Ni laterite deposit, Sulawesi, Indonesia, to elucidate weathering intensity, solid-solution interaction, and mechanisms of Ni-bearing phyllosilicate precipitation. Weathering from bedrock (δ26Mg = –0.33 to –0.11‰) through saprolite (–0.20 to 0.14‰) to laterite (0.47‰) indicates preferential leaching of light Mg and retention of heavy Mg. Conversely, δ60Ni decreases from bedrock (–0.30 to 0.14‰) through saprolite (–0.78 to 0.03‰) to laterite (–1.34‰), reflecting mobilization of heavy 60Ni and adsorption of light 58Ni onto Fe oxyhydroxides. Garnierite occurs as two mineralogically and isotopically distinct groups: serpentine-like (δ26Mg = –0.10 to 0.12‰; δ60Ni = 0.98–1.46‰) and talc-like types (δ26Mg = 0.54–1.12‰; δ60Ni = 1.08–1.67‰), comparable to Ni isotope ratios in seawater (δ60Ni ≈ 1.44‰). Covariation among Mg-Ni isotopes and Ni/Mg ratios along the weathering profile suggests that both isotopes record weathering intensity and dissolution-precipitation processes. Infiltration of 60Ni-enriched fluids into the saprolite promoted garnierite precipitation and produced 60Ni-enriched ores. Petrography and Fe contents indicate that serpentine-like garnierite formed first, followed by talc-like garnierite during downward groundwater percolation. Color zoning within talc-like garnierite, from emerald rims to white cores, reflects progressive Ni and 26Mg depletion from evolving fluids. Elevated δ26Mg and depleted δ60Ni values in thick laterite horizons mark advanced weathering and may serve as geochemical indicators of underlying Ni-rich zones in Sulawesi.