Hidden Mechanisms of Critical-Metal Enrichment in Deep Tropical Lateritic Weathering Profiles

This review brings together recent field based and laboratory studies (2020 to 2025) that investigate concealed concentrations of critical metals, including rare earth elements (REEs), scandium, nickel and cobalt, within deep tropical lateritic weathering profiles. Using a PRISMA framework, the authors reduced an initial pool of 1,240 records to fifty studies that satisfied strict inclusion criteria for geochemical, isotopic and mineralogical data quality. The aim is to integrate lines of evidence to explain how these metals are mobilized, transported and fixed beneath the visible laterite mantle and to highlight consequences for exploration and environmental management. The selected investigations deployed high sensitivity trace element analysis (ICP MS), neodymium and strontium isotopes measured by multi collector ICP MS, X ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, and sequential leaching experiments to partition operational metal fractions. By comparing depth resolved geochemical profiles, isotopic fingerprints and mineral host relationships, the synthesis distinguishes metals inherited from parent lithology from those redistributed by fluid flow and redox processes. Clear and consistent patterns emerge across diverse tropical settings. Parent rock composition exerts a primary control: Mafic protoliths favor nickel and cobalt enrichment typically hosted by iron oxyhydroxides and manganese phases, while felsic sources more often retain REE and scandium within resistant accessory minerals such as zircon, monazite and anatase. Isotopic gradients, together with sequential leach results, support a working model in which metals are leached from surficial horizons and reprecipitated at depth, forming cryptic enrichment zones commonly between about 10 and 20 m below ground. Seasonal water table shifts, oscillating redox conditions, organic complexation and microbial activity emerge as key modulators of metal mobility. The review demonstrates that reliance on shallow sampling can overlook blind deposits. Integrated exploration strategies that combine deep coring, isotopic mapping, targeted mineralogical screening and sequential leaching enhance discovery success. Equally important, responsible development requires life cycle assessment, circular economy measures and inclusive land use planning to limit deforestation, erosion and water contamination. By synthesizing multiproxy evidence, this study advances a refined conceptual model for hidden lateritic enrichment and offers practical guidance for more effective and environmentally responsible critical metal exploration in tropical regions.