Summary:
The Penco Module deposits have been defined as “regolith-hosted REE deposits” or “ion-adsorption deposits” (IAD). These are traditionally formed by tropical or subtropical weathering and decomposition of intrusive rocks with a primary enrichment in either mid/heavy REEs (peralkaline igneous rocks) or light REEs (peraluminous igneous rocks or carbonatites), where REEs are readily liberated by ionic solutions and are hence ion exchangeable (Wang et al., 2015; Dostal, 2017; Borst et al., 2020). Exchangeable REEs are associated with kaolinite and halloysite, the dominant clay minerals in IADs due to their role in adsorbing and fractionating REEs (Wu et al., 1990; Bau, 1991; Jeong, 2000; Bao and Zhao 2008; Williams-Jones et al., 2012; Sanematsu and Watanabe, 2016; Li et al., 2019; Yang, 2019; Borst et al., 2020 and references within).
The regolith profiles in the Project were developed through subtropical weathering of a peraluminous garnet-bearing granitoid, hence richer in LREEs. The regolith ranges between 25 to 48 m in thickness and is more developed in the garnet-rich granitoid than the other granitoids. The primary REE source is hosted in monazite-allanite and lesser xenotime, garnet and ilmenite. A secondary paragenesis formed by a late propylitic hydrothermal alteration (chlorite, sericite, epidoteallanite, locally biotite) replaced monazite with allanite and torite, and was relevant for the subsequent REE fractioning (Dold, 2015).
The exchangeable REE fraction in the Penco Module orebodies was obtained after the destruction of allanite, xenotime, and garnet (not refractory minerals) by weathering. The exchangeable REE is inferred to be weakly adsorbed onto clay minerals, dominantly by kaolinite and less by illite and/or smectite. This primary adsorption is observed by the positive correlation between kaolinite abundance and REE recovery. Further confirming this observation, clay mineralogy from DRX characterization indicates kaolinite as the dominant clay mineral (after halloysite).
Geology and Mineralization
The Project covers an area of 6 km x 3 km, located in the Coastal Range in the Biobio Region in central-southern Chile and is hosted in a carboniferous granitoid batholith complex intruding the eastern metamorphic basement series. Four main rock complexes are recognized: Metapelites (Paleozoic basement), Eastern Concepcion Plutonic Complex (oldest intrusion, east of the project), Penco Granitoid Complex (host of REE-rich ore bodies) and the Quartz-Diorite (youngest intrusion). Locally, REE anomalies were detected through soil analysis, using a portable XRF in roadcut exposures. These findings were better defined by a radiometric flight, NanoTEM and LIDAR topography, confirming that the garnet-bearing granitoid (GG) is strongly correlated with the radiometric anomaly of thorium (Th).
These rocks have the development of an extensive and deeply weathered regolith (+/- 40 m). This regolith contains abundant clay minerals that were locally enriched with REE in the favorable horizons. Aclara carried out a geochemical program in the zone that found significant yttrium (Y), cerium (Ce) and thorium (Th) anomalies.
The regolith profile developed clay minerals with capacity for cation adsorption. Of them, the GG is the source of the REE mineralization and is the richest in exchangeable REE. Other lithologies such as the biotite-bearing diorite (DRT) and metapelites (MP) contain decreasing levels of exchangeable REE, based on proximity to the GG, due to secondary enrichment of REE-rich fluids sourced from the GG following lateral migration under specific geochemical conditions (pH, alteration). Thus, mineralization depends on GG weathering intensity and topography (flatter relief allows for thicker regolith profiles and preserves ore bodies).
The Penco regolith profile is up to 35 m thick and comprises, from the bottom up: Unaltered bedrock (Horizon D), transitional zone (Horizon C2), semi-weathered zone (Horizon C1), completely weathered zone (Horizon B), pedolith and topsoil (Horizon A).
The regolith profile is identified as the biotite-bearing diorite (DRT), metapelite (MP) and garnet-bearing granitoid (GG), the latter was used ahead as the model. Apart from core logging, geochemistry (major elements and total REE), mineralogy, pH, and exchangeable REE with ammonium sulfate were used to define the geologic units.