Controls on the enrichment of the Serra Verde rare earth deposit, Brazil

Abstract

Ion-adsorption rare earth element deposits (IADs) are weathered granites that contain economically exploitable REE-bearing clays, hosting 90 % of world HREE resources, and thus a vital supply of critical metals. The mechanisms that lead to the enrichment of REE, and the controls on the development of the host weathered profile are poorly understood. The Serra Verde deposit in Brazil is one of the few IADs discovered outside China, and therefore provides a valuable case study in understanding the factors that lead to their formation. Samples from the weathered profile and bedrock were analysed by SEM, EMP, and XRD to characterise the deposit and assess the controls on the enrichment. The Serra Verde IAD developed over a strongly REE-enriched, phosphate-poor, and weakly peraluminous A-type granite. The REE-mineral assemblage of the granite is largely hydrothermal, formed of REE-fluorcarbonates, altered zircons, metamict niobiates, F-rich xenotime-Y, and monazite-La. Breakdown of the granite under wet, tropical conditions, produced a strongly REE-enriched saprolite. Mass balance analysis indicates that enrichment occurred through efficient mass transfer of REE from the upper profile. Lateral variations in ore grade, REE mineralogy, and saprolite thickness are attributed to variable drainage across the profile. LREE-fluorcarbonates are the principle source of mobilised REE, and show complex dissolution and re-precipitation patterns across the saprolite. The formation of supergene fluorcarbonates reduces the amount of REE adsorption on clays, and are distinguished by their negative Ce anomalies. Leaching experiments were carried out to quantify the percentage of ion-exchangeable REE (PER), the REE residence in ore minerals, and the sensitivity to both leaching time and pH. The PER at Serra Verde is controlled by REE mineralogy, clay content and composition. Leaching times over 10 minutes result in a significant decrease in extracted REE, while lower pH increased the PER at the expense of introducing additional impurities from clay breakdown.