Regolith-hosted rare earth exploration in the Chilean Coastal Range of the Central Andes

Abstract

Regolith-hosted rare earth element deposits (RH-REE) are well-known in Asia, especially China, and are becoming increasingly important with the growing global demand for such metals. Ore formation controls of these deposits in Asia, which include sub-tropical to temperate climate, and mainly calc-alkaline granites with variable HREE and LREE bearing-minerals, are used to search for compatible exploration targets. However, the poor understanding of parameters controlling the formation of RH-REE mineralization outside China, e.g., Chile, might be hindering exploration efforts world-wide. This work addresses RH-REE mineralization in the Nahuelbuta Range, the local name of the Chilean Coastal Range between the Biobío and Araucanía regions. The studied prospect is associated with late Carboniferous–early Permian I-type biotite tonalite and amphibole-biotite tonalite. Mineralogical analysis (TIMA-X) indicates that allanite and monazite are the main REE-bearing minerals in the host rocks. The regolith in the study area is up to 60 m deep and contains an exchangeable-REE concentration up to 2000 ppm, as determined by portable X-ray fluorescence (pXRF) and ICP-MS. The mineralization is divided into two types: 1) Preserved regoliths (e.g. drillhole RLS-09) where mineralization is HREE-dominant (56% of total REE, (La/Yb)N ~2) and is covered by a leached upper pedolith; and 2) partially eroded regoliths (e.g. drillhole RLS-07) where mineralization is LREE-dominant (58% of total REE, (La/Yb)N ~4) and exposed. A morphometric analysis (SCM and swath profiles) and two 3D models (geomorphic and geochemical) indicate the current preservation of pediments and ore. Analyses of the exchangeable REE-fraction show that pedogenesis, leading to the deposit, is associated with REE-fractionation processes, most notably: a positive cerium anomaly (Ce/Ce*) in the upper pedolith and preferential adsorption of LREE at shallower depths, revealed by decreasing (La/Yb)N ratio with depth. We propose a mineralization model where the advance of the pedolith/saprolith horizon during progressive weathering and the breakdown of allanite and monazite was responsible for the neoformation of RH-REE clays at the base of the pedolith. Additionally, the landscape evolution of the Nahuelbuta Range controlled the preservation of ore bodies, which are either cropping out or hidden underneath the upper pedolith. A good understanding of the interplay between tonalite regolith development controls and regolith preservation during landscape evolution provide a strategy for REE exploration in the region. Moreover, the search for pediplains on suitable lithologies, mapping of regolith profiles, and their analysis via pXRF in the field by quantifying yttrium, appears to be a suitable strategy for early exploration of RH-REE deposits in the Chilean Coastal Range.