Summary:
Located within the Pedra Branca Granitic Massif, the Carina Project highlights key geological, geochemical, mineralogical, and metallurgical features that align with or outperform the traditional benchmarks set by Chinese ion-adsorption clay (IAC) systems. The Carina Deposit serves as a reference example of a regolith-hosted heavy rare earth element (HREE) IAC deposit formed over hydrothermally altered peraluminous granites. It features thick weathering profiles and highly desorbable HREE (Dy + Tb) concentrations. The highest concentrations are found in Upper and Lower Saprolite regolith horizons, especially along the Faixa Placha corridor, emphasizing its significant influence on REE mineralization.
Carina is classified as a regolith-hosted IAC deposit formed through tropical weathering of hydrothermally altered A-type granites, especially porphyritic biotite granite. These granites experienced extensive high-temperature hydrothermal alteration and albitization, which increased the release and mobility of REEs into the regolith. As the granites weathered, the REEs became loosely adsorbed onto clay minerals, resulting in the IAC mineralization.
Comparative analysis of the Carina Project versus typical Chinese ion-adsorption clay deposits:
• Dominant REE types: HREE (Dy, Tb);
• Regolith thickness: 10–15 m (up to 40 m at Faixa Placha);
• Desorption efficiency: 25–40%;
• Geological host: Altered granite (PBG);
• Clay mineralogy: Kaolinite, illite, chlorite;
• Dy+Tb grade (desorbable): Up to 62.5 ppm.
Carina’s mineralized profile reaches up to 40 m in thickness in favourable zones, with vertical and lateral variability influenced by primary lithology, alteration intensity, structural setting, and regolith preservation. The most enriched areas are found within the Faixa Placha Corridor, which features thick regolith layers and intense hydrothermal alteration. A comprehensive comparative analysis following Acosta et al. (2023) identified Carina as one of the major HREE-bearing IAC deposits outside China.
Maps of desorbable Dy and Tb concentrations across the deposit show clear spatial zonation, particularly in the upper saprolite of the Faixa Placha Corridor. Core leaching data support the correlation between alteration zones (e.g., hydrothermal and albitized) and REE adsorption efficiency. These findings validate Carina's classification as a benchmark regolith-hosted HREE IAC deposit. Its substantial Dy-Tb inventory, detailed characterization, and high recovery potential establish it as a reference model for exploration and development of similar systems worldwide.
Porphyritic Biotite Granite (PBG)
The PBG is the main lithological unit within the Pedra Branca Massif and forms the primary host rock of the Carina regolith-hosted REE deposit. This granite shows a mediumto coarse-grained porphyritic texture, with large subhedral to euhedral K-feldspar phenocrysts (up to 3 cm) set in a groundmass of quartz, plagioclase, and biotite. Biotite is the dominant mafic phase and often displays alteration to chlorite or muscovite. Accessory minerals include zircon, fluorite, apatite, thorite, allanite, and locally monazite. These accessory minerals are significant contributors to the primary REE budget of the granite.
Geochemically, the PBG shows elevated levels of high-field-strength elements and REEs, including Zr (200–600 ppm), Y (80–200 ppm), Nb (20–60 ppm), and TREEs surpassing 1,000 ppm in several areas. The rocks range from metaluminous to weakly peraluminous and display higher gallium/aluminum (Ga/Al) ratios, iron/magnesium (Fe/Mg) ratios, and F content—indicators of fractionated magmas derived from crustal anatexis in within-plate tectonic settings.
High Hydrothermal Alteration (HA)
One of the main mineralogical changes observed in the PBG of the Pedra Branca Massif is intense hydrothermal alteration, which has significantly influenced the distribution and movement of REEs. This alteration is characterized by widespread changes in texture and mineralogy, including the recrystallization of quartz and feldspar, the replacement of biotite with muscovite or Li-rich siderophyllite, and the development of veinlets and halos composed of muscovite and topaz. These processes led to the formation of greisen assemblages, typically comprising quartz, muscovite, topaz, and fluorite.
Albitization Hydrothermal Alteration (AB)
A characteristic and diagnostic feature of this alteration system is albitization, where original plagioclase and K-feldspar crystals are replaced by fine-grained albite. This change is visually recognizable through milky-white albite aggregates that often obscure the original twinning and zoning textures of feldspar. Albitization usually occurs as halos surrounding fractures or dispersed throughout the granite, especially in areas with structural disturbance. Mineralogically, it results in the widespread growth of albite and is commonly linked to sericitic or muscovitic alteration along grain boundaries. Geochemically, albitization is marked by an increase in Na2O, a decrease in K2O and CaO, and often a concurrent enrichment in Al2O3 and SiO2. In the context of the Carina Project, albitization plays a key role in enhancing host rock porosity and permeability, thereby aiding the movement of mineralizing fluids. This process sets the stage for subsequent REE concentration, either through hydrothermal precipitation or mobilization into the overlying weathering profile, where REEs are adsorbed onto kaolinite and halloysite clays.
Regolith Horizons Characterization
Intense tropical weathering over geological timescales has led to the development of a well-formed regolith profile over the PBG of the Pedra Branca Massif. However, this profile has been variably modified or obliterated in areas affected by intense hydrothermal alteration, especially in zones characterized by high-grade alteration and albitization. These alteration processes have disrupted the primary weathering profile through metasomatic overprinting and mechanical disruption. The Faixa Placha Corridor represents the most structurally and geochemically favourable zone for regolith preservation and REE accumulation. In this corridor, regolith thickness can locally exceed 40 m, particularly in fault-damaged and alteration-enhanced zones where fluid pathways promote deep weathering and REE remobilization. In contrast, the Basin Zone exhibits significantly thinner regolith profiles, typically approximately 2 m, reflecting either limited weathering depth or erosional truncation. The distribution and preservation of the regolith are therefore spatially controlled by a combination of lithology, structural architecture, and alteration intensity, making the Faixa Placha Corridor the prime target for regolith-hosted ion adsorption-type REE mineralization at Carina.
Regolith Development within the High Hydrothermal Alteration Zone
The weathering profile in the 13.45 m High Hydrothermal Alteration Zone shows distinct features throughout the regolith column, indicating different levels of chemical weathering, mineral preservation, and REE enrichment. These profiles are categorized into six regolith units, each defined by texture, mineral content, geochemical signatures, and degree of weathering.
Regolith Development within the Albitization Alteration Zone
The ~10.45 m regolith profile within the Albitization Alteration Zone is arranged into the same six horizons as the High Hydrothermal Alteration Zone, based on differences in mineralogical content, physical appearance, geochemical composition, and preservation of primary textures. The sequence shows a gradual shift from intense weathering in the upper horizons to less altered conditions in the lower horizons.