Summary:
The rocks of the Carina Project area are characterized by A-type granitic plutons composed of two large families, which range in composition from peraluminous (high Al2O3 content) to alkaline, with high K/Na ratios. They are rich in Zr, Y, and REEs. The rocks date between 1.77 and 1.74 Ga.
The Pedra Branca Massif is divided by a fault zone system trending NE-SW/70NW. The faults are associated with dark-coloured veins, high cassiterite content, and hydrothermal alteration that affected host lithologies. The Faixa Placha comprises the central and eastern side of the fault zone. Here, rocks are albitized medium-grained equigranular, partially albitized coarse-grained inequigranular, inequigranular, and coarse-grained porphyritic pink biotite granite. The Faixa Placha covers an area of approximately 15 km2 , is well deformed, and is controlled by the fault zone system. Deformation enhanced host rock porosity, causing thicker regolith development and REE enrichment: most of the resources of the Carina Project are within the Faixa Placha.
Albitization commonly occurs when hydrothermal fluids convert K-feldspar into albite. The 1,770 and 1,740 Ma A-type peraluminous and alkaline granites in the Pedra Branca Massif have been albitized and bleached white: the primary mineralogy was changed to almost pure albite. Albitization was caused by the infiltration of an ascending hydrothermal fluid that possibly originated during reactivation of the fault zone system and probably acquired high Na/K and Na/Ca ratios during migration through the metamorphic rocks. It did not modify the original textures of the host lithologies; though they may remain well preserved, they are depleted in LREEs.
Over the Faixa Placha, the metasomatic process was intense, affecting the host lithologies and leading to high hydrothermal alteration. Sharp contact between the microcline-albite granite (pink) and albite granite (white) is evident (e.g., albitized medium-grained equigranular pink biotite granite and partially albitized coarse-grained inequigranular pink biotite granite), accompanied by brecciation, greisenization, and high REE content. This area is where the Carina Central pit outline is hosted. Moving eastward and farther from the fault zone system, the intensity of the hydrothermal alteration decreases. Host rocks are characterized by inequigranular and coarse-grained porphyritic pink biotite granite.
Intense weathering led to the development of regolith. In the Carina Project area, regolith columns range from 2 to >40 m thick. The regolith profile is divided into subhorizons based on core logging description, geochemistry, mineralogy, and the Chemical Index of Alteration. From bottom to top, these are: unaltered bedrock; saprock (i.e., partially altered bedrock); a transition and/or semi-weathered zone associated with saprolite; and two completely weathered zones, the lower and upper pedolith. The saprolite horizon overlying the Faixa Placha is thick and is the principal mineralized geological domain. Here, the saprolite is better defined and thicker and demonstrates greater continuity than was observed during the previous auger drilling campaign.
The Carina Project was formed by primary enrichment with REEs via A-type granitic plutons of the Pedra Branca Massif. Subsequent hydrothermal alteration caused leaching of REEs. Weathering of the primary minerals created the exchangeable REE fraction, which is weakly adsorbed onto clay minerals—primarily kaolinite and halloysite formed by tropical or subtropical weathering of intrusive rocks. Thus, the Carina Project is classified as a “regolith-hosted deposit rich in ion-adsorption clays.”
The Pedra Branca Massif hosting the Carina Project is the main mineralized magmatic system of the Goiás Tin Province. These granite suites have petrographic and geochemical signatures similar to A-type magmatism, with high alkali, tin (Sn), tungsten (W), fluorine (F), rubidium (Rb), thorium (Th), niobium (Nb), gallium (Ga), and REE content. They contain evidence of late- to post-magmatic transformations, such as microclinitization, albitization, and greisenization. Weathering of the granite massif has led to the development of laterization over the top, forming a plateau. Other mappable features include alluvium, colluvium, lateritic residuum, saprolite, and fresh rock.
The Project area has been subjected to intense chemical weathering over tens of millions of years, resulting in REE mobilization, fractionation, leaching, enrichment processes, and cation exchange under pH-controlled conditions. The preservation of the unit clays is related to the topographic relief: clays became concentrated in flatter areas. Depending on impurity levels, maturity of the regolith horizons, and chemical conditions, the clays can absorb REE-rich exchangeable fractions. This type of deposit is the major global source of the HREEs gadolinium-lutetium (Gd-Lu) and Y.