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 globaldemand for such metals. Ore formation controls of these deposits in Asia, whichinclude sub-tropical to temperate climate, and mainly calc-alkaline granites withvariable mineralogy between HREE and LREE bearing host-rocks, are used to searchfor compatible exploration targets. However, the discovery of RH-REE mineralization inChile associated with temperate climate epochs and granitic rocks in a magmatic arcsetting, suggests that the poor understanding of parameters controlling the formation ofRH-REE might be hindering exploration efforts outside of the context of Chinesedeposits. This work addresses RH-REE mineralization in the Nahuelbuta Range, thelocal name of the Chilean Coastal Range between the Biobío and Araucanía regions.The studied prospect is associated with Late Carboniferous–Early Permian I-typebiotite tonalite and amphibole-biotite tonalite. Mineralogical analysis (TIMA-X) indicatesthat allanite (range from 0.02–0.1%) and monazite (0.01–0.02%) are the main REEbearingminerals in the host rocks. There are two distinct regolith types in the studyarea, which are up to 60 m deep and contain an exchangeable-REE concentration upto 2000 ppm, as determined by portable X-ray fluorescence (pXRF) and ICP-MS. 1)Preserved regoliths where mineralization is slightly HREE-dominant (56% of total REE,(La/Yb) N [[EQUATION]] 2) and is covered by a leached upper pedolith; and 2)partially eroded regoliths where mineralization is slightly LREE-dominant (58% of totalREE, (La/Yb) N [[EQUATION]] 4) and it is exposed. A morphometric analysis (SCMand swath profiles) and two 3D models (geomorphic and geochemical) indicate thecurrent preservation of pediments and ore bodies and permitted the modelling of bothregolith categories. Analyses of the exchangeable REE-fraction show that pedogenesisleading to the deposit is associated to REE-fractionation processes, most notably: apositive cerium anomaly (Ce/Ce*) in the upper pedolith and preferential adsorption ofLREE at shallower depths, revealed by decreasing (La/Yb) N ratio with depth. Wedefine a mineralization model where the advance of the pedolith/saprolite horizonduring progressive weathering and the breakdown of allanite and monazite wasresponsible for the neoformation of RH-REE clays at the base of the pedolith.Additionally, the landscape evolution of the Nahuelbuta Range controlled thepreservation of mineralization, which is either cropping out or hidden underneath theupper pedolith. A good understanding of the interplay between tonalite regolithdevelopment controls and regolith preservation during landscape evolution areimportant tools for REE exploration in the region. Moreover, the search for pediplainson suitable lithologies, the mapping of stratigraphic soil columns, and their analysis viapXRF in the field by quantifying yttrium, appears to be a suitable method for addressingthe problem of early exploration of RH-REE deposits in the Chilean Coastal Range.