A Conceptual Model for Nickel Laterite Evolution beneath Sedimentary Cover: Case from Southeast Sulawesi, Indonesia

Nickel laterite deposits in the Tinanggea Block, Southeast Sulawesi, developed from the weathering of ultramafic rocks and were later preserved beneath a sedimentary cover. This study examines three representative profiles with contrasting overburden conditions: Asingi (sandstone), Atari Jaya (uncovered), and Watumerembe (limestone). Mineralogical and geochemical analyses, including petrography, X-ray Powder Diffraction, XRF, ICP-AES, ICP-MS, and SEM-EDS, were conducted to characterize the bedrock, saprolite, limonite, ferruginous cap, and overburden horizons. The results reveal progressive alteration of primary olivine and orthopyroxene into serpentine, smectite, magnetite, goethite, and hematite, along with systematic redistribution of elements. Nickel is significantly enriched within secondary silicates in saprolite and is adsorbed onto Fe-(oxy) hydroxides in limonite. The presence of sedimentary cover affects weathering intensity, Ni mobility, and deposit preservation, with limestone notably causing Ca enrichment in the upper horizons. Based on mineralogical evolution, elemental behavior, and stratigraphic relationships, a five-stage conceptual model for laterite development beneath sedimentary cover is proposed. The model highlights the roles of early serpentinization–carbonation, saprolite enrichment, limonite formation, ferruginous cap development, and final burial by clastic and carbonate sediments. These findings provide new insights into the formation mechanisms, preservation processes, and exploration potential of sediment-covered nickel laterites in Indonesia.