Summary:
The Carina Project is part of the Paranã Fold Belt, situated in the eastern part of the Tocantins Structural Province, which has unique geological features. The area is characterized by A-type granites, such as the Pedra Branca Massif, which form the bedrock of the region's geological landscape and play an important role in the local geological setting. The rocks situated on the Carina Project area are characterized by A-type granitic plutons composed of two large families. The region also forms part of the Goiás Tin Province, presenting REE-enriched A-type granites with potential for ion-adsorption-type mineralization. The Pedra Branca Massif also shows significant REE enrichment in parent granite, greisens, and accompanying regolith.
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.
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.
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.
The PBG contacts several other granite facies of the Pedra Branca Suite, including the following:
• Fine-Grained Biotite Granite: This unit usually appears at the edges of the PBG and in shallow apophyses. It is fine-grained, less altered, and somewhat more evolved, with concentrated REEs, especially in monazite and xenotime. Its geochemical profile remains aligned with the Atype affinity but shows less fluid interaction;
• Leucogranites: Leucocratic granites rich in quartz and albite with minor mafic phases are the latest-stage differentiates of the magma chamber and are often associated with miarolitic cavities and quartz veins. These bodies are generally lower in REEs but can localize hydrothermal fluids along their contacts;
• Microgranite and Aplite Dykes: These fine-grained, muscovite-bearing dykes cut through all granite facies and are common near structural zones. Although they are generally not mineralized, they can act as pathways for fluid flow and serve as structural controls on alteration.
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.
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.