Three-Dimensional Geological Modeling and Hydrothermal Titanite Geochronology of the Antas Norte ISCG Deposit: Implications for the Archean to Paleoproterozoic Fluid Evolution in the Carajás Province

1. Introduction

The Carajás Mineral Province (CMP), located in the southeastern Amazonian Craton, is the Brazil’s most significant mineral province and one of the most important globally. It is renowned for its high-grade iron, manganese, platinum group elements (PGE), nickel, and, notably, its Iron Oxide Copper-Gold (IOCG) deposits. Since the discovery of the Salobo and Sossego deposits, Carajás has been a key area for understanding IOCG systems and their diverse geological expressions.

Despite considerable advances, many aspects regarding the diversity of mineralization types within the province remain poorly understood. In particular, the Iron Sulfide Copper-Gold (ISCG) subtype, characterized by abundance of iron sulfides rather than oxides, has been little explored. The Antas Norte deposit presents a rare opportunity to study this less common mineralization expression.

This study integrates fieldwork, petrographic analyses, structural interpretation, and 3D geological modeling to characterize the Antas Norte deposit. By delineating hydrothermal alteration patterns, mineralization styles, and structural controls, the study aims to better understand the processes controlling mineralization in reduced environments within the Carajás Province.

2. Regional Geological Context

The Carajás Mineral Province (CMP) is part of the southeastern Amazonian Craton and comprises Archean basement rocks, Neoarchean supracrustal sequences, and Paleoproterozoic intrusions. The province is subdivided into two major domains—the Rio Maria and Carajás domains—separated by an east-west trending shear zone (Santos, 2003).

The Carajás Domain hosts the Xingu and Pium Complexes, the Rio Novo Group, and the Itacaiúnas Supergroup, as well as granitoid magmatism events spanning from 2.76 Ga to 1.88 Ga (Hirata et al., 1982; Machado et al., 1991). These geological units present a complex evolutionary history, marked by extensive hydrothermal activity and multiple mineralization events (Figure 1)

The Carajás Domain is characterized by the Mesoarchaean base.

3. Iron Oxide Copper-Gold (IOCG) Deposits in the Carajás Province

The IOCG deposit class emerged as a major exploration target following the discovery of the Olympic Dam deposit in Australia (Roberts & Hudson, 1983). IOCG systems typically exhibit significant copper reserves and grades, along with enrichment in elements such as Au, Ag, U, REE, Co, and Ni. However, there are no universally accepted genetic models , given the broad diversity of host rocks, structural settings, alteration styles, and fluid sources (Hitzman, 2000; Williams et al., 2005).

In the Carajás Province, IOCG deposits show a wide range of hydrothermal alterations and ore mineral assemblages, reflecting diverse crustal levels of formation and fluid evolutions (Xavier et al., 2010, 2012).

Key characteristics of Carajás IOCG deposits include:

• Metavolcano-sedimentary host rocks from the Itacaiúnas Supergroup;
• Strong structural control, often associated with shear zones;
• Proximity to diverse intrusive suites (granite, diorite, gabbro);
• Abundant hydrothermal breccias;
• Intense sodic, potassic, and magnetite alterations;
• Polymetallic enrichment (REE, P, U, Ni, W, Sn, Co, Pd);
• Wide variation in formation temperatures (100–570°C) and salinities (0–69 wt.% NaCl eq.).

Deposits in the northern sector (e.g., Salobo, Grota Funda) are associated with the Cinzento Shear Zone and exhibit high-temperature potassic alteration, whereas southern deposits (e.g., Sossego, Cristalino) are controlled by the Canaã Shear Zone and present sodic-calcic to potassic alteration sequences (Monteiro et al., 2008a; Moreto, 2013).

4. Materials and Methods

4.1. Fieldwork, Petrography, and SEM Analyses

Field investigations at the Antas Norte mine included the description of 10 drill cores and 9 outcrops. Observations focused on deposit geometry, lithology, hydrothermal alteration, and mineralization styles.

Petrographic analyses of 34 samples were conducted at the Federal University of Ceará (UFC) to characterize hydrothermal alteration assemblages and ore mineral associations. Additionally, Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-Ray Spectroscopy (EDS) was used to determine the chemical composition of accessory and ore minerals.

4.2. 3D Geological Modeling and Structural Analysis

Three-dimensional modeling was performed using the Leapfrog Geo 2022 software, integrating:

• Geological surface maps,
• Drill hole datasets (totaling 30,657.55 meters),
• Detailed topographic surveys.

Structural measurements (n=57) of faults, fractures, and stretching lineations were analyzed using the Stereonet software to determine the kinematic evolution and structural mineralization controls .

5. Local Geology

5.1. Host Rocks

The Antas Norte deposit is located near the Estrela Granite Complex and is hosted by mafic and felsic metavolcanic rocks belonging to the Grão Pará Group, part of the Itacaiúnas Supergroup (Figure 2). Gabbro dikes crosscut the host sequence and the mineralized zones. Regional structures have WNW-ESE trend, linked to the Carajás Fault, whereas the ore body is aligned along a NE–SW secondary shear zone.

Mafic volcanic rocks (Figure 3) exhibit fine to medium grain size and dark gray to greenish color. These rocks present varying shearing degrees and hydrothermal alteration, which have obliterated up to 90% of their original mineralogy. In the distal parts of the deposit, penetrative mylonitic foliation (Figure 3A) is observed, while brecciation (Figure 3B) is common in proximal zones. Despite the extensive alteration, the presence of relic minerals, such as clinopyroxene (Figure 3F), k-feldspar (Figure 3E), and titanite (or ilmenite) (Figure 3D), provides clues about their original composition. Hydrothermal minerals, including amphiboles (hornblende and actinolite) (Figure 3) and biotite (Figure 3F), replaced most primary minerals. Other alteration minerals include apatite (Figure 3B and 3E), scapolite (Figure 3F), chlorite (Figure 3D), ilmenite (Figure 3B and 3D), and epidote (Figure 3E). Albite (Figure 3A and 3C), quartz (Figure 3A and 3C), and chalcopyrite (Figure 3B) are also present, especially near felsic volcanic rocks and mineralization zones.

Primary minerals, such as clinopyroxene, K-feldspar, and titanite (or ilmenite), are preserved as relics within a matrix dominated by hydrothermal minerals, including hornblende, actinolite, biotite, scapolite, chlorite, and apatite. Albite and quartz are present, especially near felsic volcanic rocks and mineralized zones.

5.2. Felsic Volcanic Rocks

Felsic volcanic rocks are fine-grained, presenting light gray to reddish coloration, moderately brecciated, and intensely altered. Phenocrysts of feldspar and quartz occur in a fine-grained albite-quartz matrix, suggesting dacitic composition. Sodic alteration is dominant, characterized by pervasive albite replacement. Hornblende and calcite are common near mafic-felsic contacts.

Felsic volcanic rocks (Figure 4A, 4C, and 4E) are fine-grained with light gray to reddish coloration. Brecciation is moderate, and hydrothermal alteration is prominent, with feldspar (Figure 4E) and quartz phenocrysts (Figure 4C and 4E) set in a fine-grained albite (Figure 4E) and quartz matrix, indicating dacitic composition. Sodium alteration is dominant, forming albite as the main hydrothermal product. Amphiboles (Figure 4A and 4C) and calcite (Figure 4C, 4E and F) are present near contacts with mafic rocks.

5.3. Gabbro Dikes

Gabbro dikes are subparallel to the mineralized shear zone and show medium- to fine-grained phaneritic texture. Less altered gabbros exhibit ophitic to subophitic textures composed of plagioclase and pyroxene. Increased alteration results in mineral assemblage dominated by amphiboles, albite, and chlorite.

6. Hydrothermal Alterations

The alteration sequence recognized at Antas Norte comprises:

• Sodic alteration: Dominated by albite replacement, pervasive in host rocks, particularly in felsic volcanics.
• Potassic alteration: Characterized by biotite and scapolite replacing amphiboles and feldspars, centered around mineralized zones.
• Calcic alteration: Marked by amphibole (hornblende, actinolite) and apatite formation, closely associated with copper mineralization.
• Silicification: Pervasive quartz veining and replacement, overprinting previous alteration stages.
• Propylitic alteration: Development of chlorite, epidote, and calcite, particularly at the periphery of ore zones.

The progressive evolution from sodic to calcic assemblages suggests cooling and decreasing salinity of hydrothermal fluids during the mineralization process.

Distal zones primarily exhibit sodic and potassic alterations. Sodic alteration forms albite (Figure 5A), with minor quartz and ilmenite (Figure 5A). Potassic alteration forms biotite (Figure 5B) and scapolite (Figure 5B). Proximal zones are dominated by calcic alteration, marked by amphiboles (Figure 5C) and apatite (Figure 5C). Mineralized breccia bodies (Figure 5E and 5F) contain altered rock clasts surrounded by a sulfide matrix, predominantly chalcopyrite (Figure 5D).

Figure 6 shows hydrothermal Alterations in the Antas Norte Area, Focusing on Sodic and Potassic Alterations (Figs A, B, C, D, E, F, G, H and I).

7. Mineralization

Copper mineralization at the Antas Norte deposit is hosted within the mafic and felsic metavolcanic rocks of the Grão Pará Group and is spatially associated with calcic hydrothermal alteration (hornblende–apatite–allanite–titanite assemblage). The mineralized body exhibits a subvertical dip towards the southeast and shows NE–SW trend, paralleling the shear zone controlling the deposit.

Mineralization occurs in three principal styles:

• Massive sulfide ore: Dominant in the central portions of the orebody, primarily composed of chalcopyrite with subordinate pyrrhotite and pyrite.
• Brecciated sulfide ore: Present along the margins of massive zones, associated with brecciated hydrothermalized host rocks, where sulfides infill breccia matrices.
• Veins and veinlets: Less volumetrically significant, predominantly composed of chalcopyrite, pyrrhotite, and minor ilmenite, crosscutting earlier alteration zones.

The massive sulfide portions are dominated by chalcopyrite, with pyrrhotite and pyrite inclusions. Preserved cores of hydrothermal minerals, including hornblende, actinolite, and apatite, are observed within the sulfide mass, indicating overprinting of earlier alteration assemblages by mineralization.

Breccia zones exhibit matrix composed of chalcopyrite + pyrrhotite + pyrite ± ilmenite ± sphalerite ± pentlandite ± hornblende ± actinolite ± quartz ± apatite ± chlorite ± epidote ± albite. Chalcopyrite dominates the matrix, while hydrothermal clasts of hornblende, actinolite, and quartz occur as angular to rounded fragments.

Veins and veinlets, predominantly composed of chalcopyrite with minor pyrrhotite and ilmenite, are observed crosscutting both calcareous and sodic alteration zones. Gabbro dikes that intersect the ore zone show minimal sulfide mineralization, suggesting post-mineralization emplacement or weak mineralizing fluid interaction.

Massive ore at the body core (Figure 7A). 2. Brecciated ore at the edges, interacting with hydrothermal alterations (Figure 7B). 3. Veins and veinlets, which are less expressive (Figure 7C).

The mineralization is associated with a ductile-brittle regime, where the central zones remain undeformed, while brecciation affects the edges. The presence of gold is inferred from chemical analyses, as it was not identified in petrographic slides.

In massive sulfide portions, chalcopyrite dominates (Figure 7D, &E), with inclusions of pyrrhotite (Figure 7E), pyrite (Figure 7D), and sphalerite (Figure 7E). These zones envelop areas of prior alteration, where hornblende, actinolite, and apatite cores are preserved (Figure 7F).

Breccias show mineral association of chalcopyrite + pyrrhotite + pyrite ± ilmenite + sphalerite ± pentlandite + hornblende + actinolite ± quartz + apatite ± chlorite ± epidote ± albite. Chalcopyrite dominates the breccia matrix (Figure 7G, 7H, 7I,7J)), with accessory phases including pyrrhotite (Figure 7K), pyrite, ilmenite (Figure 7J), sphalerite, and pentlandite. Hydrothermal minerals such as hornblende (Figure 7H, 7I), actinolite (Figure 7H, 7I), quartz (Figure 7I), and apatite form angular to rounded clasts. Chlorite (Figure 7H) and epidote (Figure 7H) appear in areas of propylitic alteration, while albite (Figure 8I) occurs at contacts between calcic and sodic alteration zones.

8. Geocronology

The U-Pb isotopic dating of hydrothermal titanite from the Antas Norte deposit yielded two significant concordant age populations. Analyses show ages around 2.50 Ga and 2.20 Ga, corresponding to two distinct tectonothermal events affecting the region. The older population (~2.50 Ga) likely shows an early hydrothermal alteration event during late Archean crustal processes, while the younger age (~2.20 Ga) corresponds to a major Paleoproterozoic tectonothermal reactivation. These results confirm that the copper mineralization was associated with multiple fluid flow episodes linked to the regional deformation history. The high degree of concordance between 206Pb/238U and 207Pb/235U systems in titanite grains supports the robustness of these ages for constraining the timing of mineralization and associated hydrothermal events at Antas Norte.

9. Structural Geology

The Carajás Mineral Province has undergone multiple tectonic events from the Archean to the Paleoproterozoic, imprinting a complex network of ductile to brittle structures. Regional tectonics are dominated by the Carajás Transcurrent System, characterized by NW–SE trending shear zones that exert strong control over ore localization.

At Antas Norte, the mineralized body is hosted within a secondary NE–SW-oriented shear zone, distinct from the major Carajás Fault (WNW–ESE orientation). The shear zone exhibits features typical of a dextral transpressional regime, with deformation patterns transitioning from ductile to brittle conditions.

Structural analysis reveals two foliation generations :

• Sn foliation: A pervasive mylonitic foliation defined by the preferred orientation of amphiboles, plagioclase, and biotite, which trends NNE–SSW and dips ESE.
• Sn+1 foliation: A later foliation characterized by the alignment of hydrothermal minerals, particularly oriented mafic minerals surrounding quartz and feldspar porphyroclasts, which trends ENE–WSW and dips SSE.

The orebody is located along a sigmoidal SC (Shear–C) structure, where mineralized zones are concentrated in dilational spaces between Sn and Sn+1 foliations. Stretching lineation measurements indicate a plunge of 54° towards 225° (SW), which is consistent with the orientation of high-grade mineralization zones (Figure 9).

Hydrothermal fluids exploited these deformation zones, resulting in intense alteration and mineral precipitation along the structural conduits.

10. 3D Geological Modeling

The 3D geological modeling of the Antas Norte deposit was developed using an implicit modeling approach using the Leapfrog Geo 2022 software. The model was constructed by integrating geological surface maps, drill hole data (totaling 30,657.55 meters), detailed topographic surveys, and structural measurements.

The results highlight that the mineralized body is spatially controlled by a NE–SW-trending shear zone. Secondary NNW–SSE-oriented faults were identified, offsetting the mineralized zones laterally. These structures were crucial for understanding the orebody geometry and its internal segmentation.

The mineralization is associated with a major subvertical tabular body that trends NE–SW, dipping steeply southeast, and is aligned with the primary shear zone. High-grade massive sulfides (>3% Cu) occur in the body core , surrounded by lower-grade disseminated mineralization (<3% Cu).

Hydrothermal alterations are spatially zoned:

• Sodic alteration (albite) dominates at regional scale;
• Potassic alteration (biotite + scapolite) surrounds the mineralized core;
• Calcic alteration (amphibole + apatite) is closely associated with copper mineralization.

Silicification and propylitic alteration were pervasive but diffuse and, therefore, were not modeled as discrete solids.

The shear zone acted as the primary conduit for hydrothermal fluids, controlling both the localization and morphology of mineralized zones. The 3D model provides critical insights into the hydrothermal fluid flow pathways and the spatial distribution of metal precipitation within the structural framework (Figure 10).

11. Discussion

The Antas Norte deposit exhibits a distinctive mineralogical and structural signature compared to most Iron Oxide Copper-Gold (IOCG) deposits in the Carajás Mineral Province. Unlike classic IOCG deposits such as Salobo and Sossego, which are typically associated with extensive iron oxide (magnetite and/or hematite) mineralization, Antas Norte is characterized by the dominance of iron sulfides (pyrrhotite and pyrite) and ilmenite.

The spatial mineralization association with secondary NE–SW-oriented shear zone highlights the critical role of structural controls in focusing hydrothermal fluids. The evolution of hydrothermal alteration, progressing from early sodic (albite-rich) to later potassic (biotite + scapolite) and calcic (amphibole + apatite) assemblages, reflects a dynamic fluid regime with decreasing temperature, salinity, and pH over time. The presence of widespread silicification and propylitic alteration suggests the late influx of meteoric fluids during shear zone reactivation.

The mineralogical assemblages at Antas Norte imply hydrothermal conditions significantly different from those prevailing in the more oxidized IOCG systems of Carajás. Specifically, the stability of ilmenite rather than magnetite indicates formation under lower oxygen fugacity (fO₂) conditions. Ilmenite stability at deeper crustal levels has been reported in other iron oxide–apatite (IOA) systems worldwide, where reduced magmatic conditions promote titanium retention and iron sulfide over iron oxide mineralization.

These features suggest that Antas Norte represents a deeper, more reduced end-member within the IOCG mineralization spectrum of Carajás. Such an interpretation is consistent with recent models proposing that IOCG and ISCG deposits form a continuum related to variations in redox state, fluid composition, and crustal depth.

Therefore, the Antas Norte deposit provides important insights into the diversity of mineral systems in the Carajás Province, highlighting the influence of structural reactivation, fluid evolution, and redox controls on deposit genesis.

12. Conclusions

The Antas Norte deposit represents a distinct copper-gold mineralization style within the Carajás Mineral Province, characterized by the predominance of iron sulfides (pyrrhotite, pyrite) and ilmenite rather than iron oxides.

Three main mineralization styles were identified:

• Massive sulfide bodies,
• Breccia matrix sulfide infill,
• Sulfide veins and veinlets.

These mineralization styles are spatially and genetically associated with calcic hydrothermal alteration and localized within a NE–SW-trending shear zone, evidencing the critical role of structural controls on fluid flow and ore deposition.

The hydrothermal evolution of Antas Norte, from sodic to potassic to calcic alteration assemblages, reflects a dynamic fluid system with progressive changes in temperature, salinity, and pH. The stability of ilmenite and dominance of iron sulfides over iron oxides indicate a reduced hydrothermal environment, contrasting with the more oxidized conditions typical of classic IOCG systems in Carajás.

The integration of field observations, petrography, structural analysis, and 3D geological modeling supports the interpretation of Antas Norte as a deeper, more reduced end-member of the IOCG mineralization spectrum. This study contributes to a better understanding of the diversity of mineralization styles in Carajás and emphasizes the need for refined exploration models considering structural and redox variations across the province.