Lateritic Contribution to Enhancing the Grade of Iron Ore from Serra Leste Deposit in Carajás Mineral Province, Brazil

Abstract

The Carajás Province, located in the southeastern Amazon, hosts some of the world’s largest high-grade iron deposits. Despite their economic importance, the processes linking lateritic weathering and iron enrichment remain incompletely understood. This study investigates the role of lateritic weathering in the evolution of the Serra Leste iron deposit through the characterization of a weathering profile and its parent rocks using drill-core samples. Analytical methods included X-ray diffraction (XRD), optical microscopy, scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), whole-rock geochemistry, and Mössbauer spectroscopy. Jaspilites weathered into ferruginous saprolite while preserving relic banding and mineral textures. Magnetite alteration produced pseudomorphic hematite with dissolution cavities progressively infilled by goethite, indicating iron remobilization during weathering. Weathering of chloritites generated clayey saprolite enriched in kaolinite and iron oxyhydroxides, with gibbsite occurring in more advanced stages. The uppermost horizon consists of a ferroaluminous duricrust composed of massive, spherulitic, and brecciated iron oxyhydroxides associated with gibbsite. Up-profile geochemical trends are marked by decreasing SiO₂ and increasing Fe₂O₃. The mineralogical, textural, and geochemical relationships indicate that the ferroaluminous duricrust was developed through contributions from both ferruginous and clayey saprolitic systems, particularly from the latter. These results support the interpretation that lateritic weathering played an important role in iron redistribution and supergene enrichment within the Serra Leste deposit, consistent with mature Amazonian lateritic systems.

1. Introduction

High-grade hematite deposits are mostly associated with banded iron formations (BIFs) that have undergone geological processes, normally deep tropical weathering (lateritization), leading to iron enrichment [1]. Major regions hosting these deposits include Carajás and the Iron Quadrangle in Brazil, Hamersley in Western Australia, and significant occurrences in Africa and India [2,3]. In Carajás, these deposits are specifically found in three main areas—Serra Norte, Serra Sul, and Serra Leste—all of which are currently operational.

The transformation of the proto-ore, BIF, into high-grade hematite ore is driven by both weathering [4,5,6,7,8] and hydrothermal processes [1,9,10,11,12] depending on the specific section of the deposit under examination. Lateritic weathering has been prevalent in the Amazon since the end of the Mesozoic era, with one of its most notable occurrences in the Carajás region. This area is extensively covered by lateritic formations, typical of tropical and paleotropical continental regions [13,14,15]. The process of BIF enrichment to form iron ore through lateritization has been documented worldwide [14,16,17,18,19]. In the Hamersley Province (Western Australia), weathering has driven martite–goethite and martite microplate hematite transitions. This has resulted in a weathering profile that includes, from bottom to top: an iron ore retaining the original BIF texture, a goethite-dominated horizon, and an upper hematite-rich duricrust [20]. In India, BIF weathering resulted in the formation of friable hematite ore, followed by extensive goethitization [21]. In Guinea, saprolites 10 to 30 m thick are overlain by a 1 m thick lateritic duricrust [22]. In the Mbalam iron ore district, southern Cameroon (Congo Craton), weathering profiles developed over itabirite comprise a saprolitic horizon preserving relict BIF structures, overlain by friable hematite ore and goethite-bearing zones, with locally indurated ferruginous horizons occurring toward the top of the profile [23]. In the Iron Quadrangle (Brazil), BIF lateritization forms thick profiles with numerous horizons, culminating in lateritic duricrusts [24,25].

The Carajás iron deposits are associated with jaspilites along the Carajás mountain range (Serra Norte, Serra Sul, and Serra Leste). The first local investigations into the formation of iron ore from BIF enrichment were carried out by [3], who indicated the leaching of silica and modification of the iron content from 36–45% in the proto-ore to 64–68% in the ore, due to lateritization. Recent studies in Serra Norte and the S11D iron deposit (Serra Sul) show that saprolite extends down to 450 m, with lateritic duricrusts extensively covering the landscape [26,27].

The Serra Leste deposit and mine are located in the municipality of Curionópolis, state of Pará. Numerous studies have been dedicated to the origin of Carajás iron deposits [4,5,6,7,8,9,10,11,12], particularly in Serra Norte and Serra Sul, where deep lateritic weathering profiles and supergene enrichment processes have been extensively documented [26,27]. However, Serra Leste remains comparatively less studied, especially regarding the mineralogical and geochemical evolution of its weathering profile and its relationship with lateritic iron enrichment processes. In addition, the contribution of ferruginous and clay-rich weathering systems to the development of ferroaluminous duricrusts in Serra Leste is still poorly constrained. This research aims to characterize these mineralogical and geochemical transformations, integrating X-ray diffraction (XRD), optical microscopy, scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), whole-rock geochemistry, and Mössbauer spectroscopy, in order to clarify how Serra Leste fits into the broader Cenozoic evolution of Carajás. This study also evaluates the role of lateritic weathering in iron redistribution and supergene enrichment within the deposit. The results contribute to the understanding of lateritic iron enrichment processes in Carajás and provide geological information that may support mineral exploration and decision making related to ore quality variability and deleterious components in mining operations.

2. Geological Setting

The Carajás Mineral Province, in the southeastern part of the Amazon Craton, includes two distinct tectonic domains. The Mesoarchean Rio Maria Domain is located to the south, and the Neoarchean Carajás Domain is to the north [28,29]. The investigated profiles are situated within the Carajás Domain, which comprises tonalite–trondhjemite–granodiorite terrains of the Xingu Complex, locally migmatized, and granites of the Pium Complex (~2.8 Ga). Also found are layers of mafic to intermediate volcanic rocks from the Itacaiúnas Supergroup (2.73–2.84 Ga) and sedimentary sequences from the Águas Claras Formation (2.64–2.68 Ga) [30], as shown in Figure 1A.

The Itacaiúnas Supergroup comprises mafic to intermediate metavolcanic sequences hosting significant iron deposits, in addition to copper, gold, and manganese mineralizations [26,30,31,32]. The Grão-Pará Group (~2.76 Ga) represents the principal volcano-sedimentary sequence of the Carajás Basin and is distinguished by extensive mafic-dominated volcanism [30,31]. It consists of two distinct paleovolcanic sequences separated by the BIF of the Carajás Formation. The lower sequence (Parauapebas Formation) includes mafic volcanic rocks interspersed with felsic volcanics and shows evidence of possible hydrothermal alterations. The upper sequence (Igarapé Cigarra Formation) comprises basic volcanic rocks along with intercalations of tuffs, clastic, and chemical sediments [9,33,34,35].

The Carajás Formation consists of jaspilites defined by one-centimeter-thick bands of iron oxide alternating with chert. These jaspilites typically have an average thickness of 200 to 250 m in the Serra Norte and exceed 300 m in the Serra Sul, intersected by mafic dikes and sills [35]. The BIFs of Carajás were likely deposited on shallow, stable platforms under wave influence, with a minimum deposition age of 2740 ± 8 Ma [9,31]. Chloritites are commonly associated with hydrothermally altered mafic metavolcanic rocks, where intense chloritization produced chlorite-rich lithologies. Metavolcanic mafic sequences and BIFs host large volumes of soft hematite with lenses of hard hematite exploited in the Carajás Mineral Province [8,11,27,36,37].

Figure 1. (A) Geological map of the Carajás Mineral Province (modified from [38,39]), showing the locations of the main iron ore deposits and mines. (B) A digital elevation model of Serra dos Carajás, based on data from the Shuttle Radar Topography Mission (SRTM) provided by the USGS Earth Explorer (https://earthexplorer.usgs.gov/, accessed on 16 March 2020).

Figure 1. (A) Geological map of the Carajás Mineral Province (modified from [38,39]), showing the locations of the main iron ore deposits and mines. (B) A digital elevation model of Serra dos Carajás, based on data from the Shuttle Radar Topography Mission (SRTM) provided by the USGS Earth Explorer (https://earthexplorer.usgs.gov/, accessed on 16 March 2020).

The Carajás landscape comprises large plateaus, 600 to 900 m above sea level, surrounded by gently rolling plains at 100 to 250 m (ref. [26], Figure 1B). These plateaus are capped by ferruginous duricrusts, whose oldest profiles developed around 70 Ma, according to isotopic and cosmogenic dating reported by [6,26].

3. Materials and Methods

This study was conducted using 36 samples obtained from two boreholes drilled at the Serra Leste deposit, provided by Vale S.A, the company holding mining rights in the area. Borehole SL-110, which is 309 m deep and oriented at 86° north, was drilled at the current pit location. On the other hand, borehole SL-132, with a depth of 351 m and a vertical orientation, is situated 600 m northwest of SL-110 (Figure 2). Samples were selected to represent the main lithological and weathering horizons identified along the profiles, including jaspilite, ferruginous saprolite, clayey saprolite, and ferroaluminous duricrust. The selection aimed to capture the mineralogical and geochemical variability associated with lateritic weathering and iron enrichment processes.

Textural analyses were performed at the Mineralogy, Geochemistry, and Applications Laboratory (LAMIGA) using a LEICA DM 2700 P optical microscope equipped for both transmitted and reflected light, coupled with a LEICA MC 170 HD camera (Leica Microsystems, Wetzlar, Germany). Furthermore, images were acquired at the Microanalysis Laboratory using scanning electron microscopy (SEM) with a Zeiss SIGMA-VP scanning electron microscope (SEM) (ZEISS Group, Oberkochen, Germany), through secondary and backscattered electron imaging, assisted by an IXRF Sedona-SD energy-dispersive spectrometer (EDS) (IXRF Systems, Inc., Austin, TX, USA). The operating conditions were: electron beam current = 80 μA, constant accelerating voltage = 20 kV, working distance = 8.5 mm, and counting time for elemental analysis = 30 s. These procedures were carried out on fragments up to 1 cm long extracted from raw samples, and on polished thin sections, both mounted on double-sided adhesive tape attached to a glass slide and coated with gold for 1.5 min in an automatic sputter coater (Emitech K550X model) (Quorum Technologies, East Sussex, UK) in order to make them electrically conductive.

The mineralogical composition was determined at the Mineral Characterization Laboratory (LCM) using X-ray diffraction (XRD) employing the powder method. XRD analysis was performed on a Panalytical Empyrean diffractometer equipped with a ceramic X-ray tube and Co anode (Kα1 = 1.78901 Å), featuring a long fine focus, Kβ Fe filter, and a PIXcel3D-Medpix3 1 × 1 detector in scanning mode (Malvern Panalytical, Worcestershire, UK). Operating parameters included a voltage of 40 kV, a current of 35 mA, a step size of 0.0260° 2θ, a sweep from 3° to 94° 2θ, and a time per step of 30.6 s. The instrument was set up with a 1/4° divergent slit, a 1/2° anti-scatter slit, and a 10 mm mask. Data acquisition utilized X’Pert 3.0 Data Collector Software 3.0, followed by processing with X’Pert HighScore Plus 3.0.

Mössbauer spectroscopy analyses were conducted at the LAMIGA for detailed characterization of the iron oxyhydroxides, using the WissEl equipment from Wissenschaftliche Elektronik GmbH (Wissenschaftliche Elektronik GmbH, Deutschland, Germany). Measurements were taken at room temperature in constant acceleration mode, utilizing a nominal 50 mCi ⁵⁷Co source embedded in a Rh matrix. Spectra were acquired within a velocity range of about ±10 mm/s, employing a 512-channel analyzer. Velocity calibration was based on α-Fe at room temperature. The obtained spectra were processed and refined using WinNormos 7 running on IgorPro 7 software. The fitting procedure was based on a least-squares method, using a combination of Lorentzian doublets and sextets to resolve the spectral components. Hyperfine parameters (isomer shift, quadrupole splitting, and magnetic hyperfine field) were allowed to vary within physically constrained ranges consistent with iron oxides and oxyhydroxides reported in the literature. Phase assignments were made based on these hyperfine parameters by comparison with reference values for hematite and goethite.

Chemical analyses of 20 selected samples, including major, minor, trace, and rare earth elements (REEs), were carried out at ALS laboratories in Peru. Major elements were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) following four-acid digestion (ME-4ACD81 and ME-ICP06). Base metals (Ag, Cd, Co, Cu, Li, Mo, Ni, Pb, Sc, and Zn) were determined using the same ICP-AES analytical procedure after four-acid digestion (ME-4ACD81). Trace and rare earth elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) following lithium borate fusion (ME-MS81 and ME-MS42). Total concentrations were determined using the ALS TOT-ICP06 analytical procedure. Loss on ignition (LOI) was determined gravimetrically after calcination at 1000 °C (OA-GRA05). Total carbon and sulfur contents were quantified using a LECO analyzer (C-IR07 and S-IR08; LECO Corporation, St. Joseph, MC, USA).

Analytical quality control included the use of certified reference materials and analytical blanks, following standard ALS protocols for geological materials. Detection limits are reported in the analytical tables and values below detection limits are indicated by the symbol “<”. Cluster analysis was performed using OriginPro 9.0. The analysis was based on the geochemical data presented in the Section 4. All variables were standardized using z-score normalization. Hierarchical agglomerative clustering was applied using Euclidean distance.

4. Results

4.1. The Lateritic Succession

The investigated profiles comprise the parent rocks (chloritites and jaspilites), as well as a series of horizons that include ferruginous saprolite, clayey saprolite, and ferroaluminous duricrust. The weathering horizons are interlayered with chloritites and jaspilites (Figure 2).

• Mineralogical Succession and Texture Evolution

Chloritites are typically foliated, partially mylonitic, and crenulated. In addition to chlorite, they contain quartz and accessory opaque minerals (Figure 2A-I,B-I). Jaspilites alternate with chloritites. They consist of bands of quartz and iron minerals. They have a compact structure, and some areas look brecciated because of the quartz filling up microfractures (Figure 2A-II,B-II).

In the ferruginous saprolite, a mass primarily composed of hematite (with subordinate goethite) prevails. This saprolite can appear either friable (soft ferruginous saprolite) or more consolidated (hard ferruginous saprolite), with these variations most clearly observed in profile SL-110. Soft ferruginous saprolite (SFS) is the most common type of ferruginous saprolite, characterized by a gray color and a powdery appearance, consisting of an aggregate of microporous, centimetric hematite fragments that are internally massive or platy. Quartz and traces of magnetite are present as subordinate phases (Figure 2A-III).

The hard ferruginous saprolite (HFS) exhibits a microporous appearance and high tenacity. In profile SL-132, it displays centimetric brown bands formed of hematite and goethite, alternating with dark gray laminae composed only of hematite without goethite (Figure 2A-IV). In profile SL-110, transitional textures can be observed between jaspilite and HFS at depths of 155 to 180 m. Transitional textures between HFS and SFS are observed in profile SL-132 from 270 to 305 m deep. Relic blocks of jaspilite occur in both types of ferruginous saprolite (HFS and SFS).

The clayey saprolite (CLS) consists of a reddish-brown mass composed of kaolinite, muscovite, quartz, and accessory hematite (Figure 2B-III). Relic chloritite blocks can be found in this horizon, particularly prominent in profile SL-132 (Figure 2B). At the bottom of the CLS, there is a clay matrix made up of kaolinite, gibbsite, hematite, and muscovite, with hematite and kaolinite veins (Figure 2A-V). At the top, the clayey matrix locally embeds gibbsite nodules. The CLS locally includes ferruginous zones equivalent to the soft ferruginous saprolite and hard ferruginous saprolite.

The ferroaluminous Duricrust (FAD) at the top of the profiles consists of hematite, gibbsite, goethite, and accessory anatase. The FAD mostly exhibits a reddish-brown color with ochre patches and a cavernous appearance. In profile SL-132, the FAD is massive at the bottom (Figure 2A-VI), transitioning to spherulitic upwards (Figure 2A-VII). In profile SL-110, the FAD comprises a brecciated aggregate of elongated hematite fragments coated with goethite (Figure 2B-IV). The mineralogical composition of all horizons (HFS, SFS, CLS, and FAD) is presented in Figure 3A–E.

4.2. Micromorphology

Chloritites exhibit a smooth crenulation cleavage (Figure 4A), based on the classification by [40]. In addition to the predominant crenulation, chloritites show localized mylonitization, with plastic deformation evident in quartz porphyroclasts and anastomosing chlorite (Figure 4B). Quartz veinlets are both discordant and concordant with the foliation planes. In jaspilites, hematite appears as a cryptocrystalline mass or as pseudomorphic euhedral crystals (Figure 4C). Magnetite is found as a relic within pseudomorphic hematite (Figure 4D). Goethite is observed as a coating or infilling of cavities and fissures (Figure 4E,F).

The ferruginous saprolite shows soft and hard hematite texturally similar to those in jaspilites. This similarity can be seen in the continuous association between massive hematite and pseudomorphic hematite. Additionally, microplaty hematite is present in ferruginous saprolite. In the hard ferruginous saprolite, hematite appears as euhedral platy crystals, up to 0.15 mm in diameter, arranged without a preferential direction (Figure 5A,B). In the same sections, some cavities are partially infilled with colloform goethite (Figure 5C). In the soft ferruginous saprolite, goethite often partially replaces hematite crystals or infills pore spaces (Figure 5D). This matrix, composed of hematite and goethite, also contains magnetite relics (Figure 5E,F).

In the clayey saprolite, microplaty hematite often invades the matrix composed of massive and octahedral hematite crystals (Figure 5G). In some areas, microplaty hematite is found alongside well-developed gibbsite crystals within microfractures (Figure 5H). Goethite frequently cements the interstitial spaces between octahedral hematite crystals (Figure 5I) and coats the walls of cavities in the form of fibroradial crystals.

4.3. Relative Abundance of Iron-Bearing Minerals

Room-temperature Mössbauer spectra were used to determine the relative abundance of iron minerals in the hard ferruginous saprolite, soft ferruginous saprolite, clayey saprolite, and ferroaluminous duricrust. Quantification was based on the relative areas of the subspectra, which represent the individual contributions of each mineral phase. Overall, three patterns were observed: one sextet, two sextets, and a sextet plus doublet (Figure 6A–E). The more pronounced sextets (with higher intensity hyperfine fields) were identified as hematite, while the more compact sextets and the doublet were indicative of goethite.

In the hard ferruginous saprolite, the relative spectral areas indicate hematite as the dominant phase (61.8%), with goethite accounting for 38.7%. This dominance is further enhanced in the soft ferruginous saprolite, where hematite reaches 79.7% and goethite decreases to 20.2%. In the clayey saprolite, only the hematite sextet was identified. A similar pattern is observed in the ferroaluminous duricrust samples, although minor goethite contributions occur locally. Overall, the combined XRD and Mössbauer data indicate that hematite is the prevailing iron phase throughout the profile, while goethite, when present, consistently occurs in subordinate proportions (

Table 1. Parameters obtained from Mössbauer spectroscopy in selected samples from the Serra Leste profile. Is (isomeric shift); Qs (quadrupole splitting); B_hf (hyperfine field); RSA (relative subspectrum area). HFS (hard ferruginous saprolite), SFS (soft ferruginous saprolite), CLS (clayey saprolite), FAD (ferroaluminous duricrust).

Sample	Mineralogy	Is (mm/s−1)	Qs (mm/s−1)	Bhf (T)	RSA (%)
HFS (sample SL132-97 m)	Hematite	−0.32	0.21	51.81	61.8
	Goethite	−0.33	0.28	37.60	38.7
SFS (sample SL132-132.5 m)	Hematite	−0.32	0.20	51.8	79.7
	Goethite	−0.33	0.28	38.49	20.2
CLS (sample SL110-114.4 m)	Hematite	−0.32	0.21	52.02	100
FAD (sample SL110-2.73 m)	Hematite	−0.32	0.20	51.86	100
FAD (sample SL132-1.3 m)	Hematite	−0.32	0.22	50.9	61.1
	Goethite	−0.32	0.57	-	38.8

).

4.4. Chemical Composition Changing Along the Profile

4.4.1. Major Elements

The average chemical composition of the Serra Leste profiles demonstrates that the contents of major elements (Fe₂O₃, SiO₂, Al₂O₃, TiO₂, MnO, and P₂O₅) are consistent with the mineralogical composition previously outlined. Variations delineate the bedrock and each overlying horizon (

Table 2. Chemical composition of selected samples in each horizon of profile SL-110. CHL (chloritite); FES (ferruginous saprolite); CLS (clayey saprolite); FAD (ferroaluminous duricrust); UCC (chemical composition of the Earth’s upper continental crust according to [41]).

	JAS	CHL	FES				CLS					FAD	UCC
Depth (m)	236.7	189.9	114.4	2.7	10.8	11.5	139.5	96.5	54.8	50.0	44.0	0.3
(Wt%)
SiO2	35.8	47.1	6.53	0.79	0.67	0.37	61.6	37.5	9.89	20.6	1.98	0.57	66.6
TiO2	0.01	0.89	0.02	0.04	0.11	0.17	1.08	1.06	0.57	1.83	2.27	1.62	0.64
Al2O3	0.16	14.25	0.56	0.12	1.53	1.76	12.15	27.9	13.45	21.5	39	11.9	15.4
Fe2O3	60.3	22.5	75.4	80.5	87.6	73.9	17.8	22.9	69.7	40	33.2	73.3	5.04
MnO	0.02	0.29	1.11	0.09	0.3	0.3	0.02	0.01	0.19	0.16	0.25	0.12	0.1
MgO	0.03	9.62	0.02	0.01	0.02	0.01	0.03	0.16	0.2	0.02	0.15	0.01	2.48
CaO	0.01	0.04	0.04	0.02	0.03	0.02	0.04	0.01	0.02	0.02	0.37	0.02	3.59
Na2O	<0.01	<0.01	<0.01	<0.01	<0.01	0.01	<0.01	0.01	0.01	<0.01	0.01	<0.01	3.27
K2O	0.01	0.04	<0.01	<0.01	<0.01	0.01	0.02	0.83	0.74	0.08	0.09	<0.01	2.8
P2O5	0.12	0.01	0.03	0.05	0.11	0.31	0.07	0.05	0.07	0.25	0.16	0.23	0.15
SrO	<0.01	<0.01	<0.01	<0.01	<0.01	0.01	<0.01	<0.01	<0.01	0.01	<0.01	<0.01	-
LOI	1.36	6.31	0.88	0.41	1.4	2.99	6.26	11.65	5.52	9.38	21.4	8.27	-
Total	97.82	101.06	84.85	82.04	91.79	79.89	99.24	>102.00	100.42	93.95	98.92	96.07	-
C	0.03	0.01	0.06	0.01	0.01	0.06	0.06	0.07	0.02	0.04	0.16	0.13	-
S	0.01	0.01	0.01	0.01	0.01	0.02	0.01	0.02	0.01	0.02	0.07	0.05	0.062
(ppm)
Li	<10	40	<10	<10	<10	<10	<10	10	<10	<10	<10	<10	24
Sc	3	32	8	1	3	6	31	23	17	59	27	14	14
V	40	100	43	20	58	48	1240	330	190	320	67	151	97
Cr	51	250	20	30	40	70	244	231	76	66	220	80	92
Co	<1	45	100	<1	<1	<1	19	<1	<1	<1	<1	<1	17.3
Ni	62	171	<1	<1	<1	<1	95	253	16	68	13	<1	47
Cu	3	234	79	57	42	71	229	550	50	29	12	22	28
Zn	13	437	<2	12	19	12	21	17	145	140	48	17	67
Ga	3.2	20.3	4.5	2.4	3.3	6.4	16.6	26	14.6	27.9	35.6	13.6	17.5
Ge	<5	<5	10	12	7	11	<5	<5	<5	<5	<5	<5	1.4
As	0.8	0.3	1.1	0.9	1.9	1.2	10.4	0.5	1	0.9	0.5	1.2	4.8
Se	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	0.6	<0.2	<0.2	<0.2	<0.2	<0.2	0.09
Rb	<0.2	1.4	0.7	0.3	0.4	0.3	0.7	32.9	25.7	3.9	3.4	0.5	84
Sr	0.3	5.5	5.7	1.7	34.1	76.1	1.4	1.8	18.7	129.5	13.6	16.1	320
Y	5.4	34.2	22.2	1.9	13.6	14.8	36.4	33.5	52	61.8	31.1	20.4	21
Zr	5	101	8	10	20	51	120	132	228	121	155	138	193
Nb	0.3	4.5	0.4	0.5	1.7	3.1	6.2	5.6	11.1	4.8	7.5	8.3	12
Mo	<1	<1	1	<1	1	1	<1	<1	<1	<1	1	1	1.1
Ag	<0.5	<0.5	0.5	<0.5	0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	0.05
Cd	1.5	0.5	1.2	1.9	1.8	1.9	<0.5	<0.5	0.7	0.7	<0.5	1.3	0.09
Sn	1	3	7	3	5	2	2	7	5	2	2	3	2.1
Sb	<0.05	<0.05	0.06	0.15	0.16	0.15	<0.05	<0.05	<0.05	0.05	0.16	0.12	0.4
Te	0.01	0.02	0.01	0.03	0.03	0.05	0.05	0.07	0.06	0.02	<0.2	0.03	-
Cs	0.02	0.04	0.06	0.02	0.06	0.05	0.05	0.33	0.2	0.06	0.09	0.12	4.9
Ba	8.2	24	1995	10.5	62.3	131.5	21.3	248	211	609	82	54	628
Hf	0.2	2.8	0.2	0.2	0.5	1.5	3.1	3.6	6.5	3.6	4.4	3.8	5.3
Ta	<0.1	0.2	<0.1	<0.1	<0.1	0.1	0.3	0.3	1	0.3	0.4	0.4	0.9
W	1	2	16	2	4	2	2	5	2	3	2	3	1.9
Re	<0.001	0.001	<0.001	<0.001	<0.001	<0.001	<0.001	0.001	0.001	<0.001	<0.001	0.001	<0.001
Hg	0.012	0.024	0.055	0.02	0.055	0.083	0.026	0.021	0.03	0.091	0.106	0.095	0.05
Tl	<0.02	<0.02	0.08	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	0.03	0.03	<0.02	0.9
Pb	<2	<2	5	<2	20	25	<2	<2	39	36	18	84	17
Bi	0.08	0.02	0.3	0.2	1.89	1.01	0.23	0.01	1.04	0.74	0.52	1.14	0.16
Th	0.1	4.61	0.26	0.4	0.79	3.45	3.59	5.91	14.85	1.03	1.63	2.05	10.5
U	0.1	1.22	0.51	1.05	8.9	4.93	1.58	2.08	6.8	7.07	6.46	8.13	2.7
La	2	32.1	11.9	1.7	13.7	26.2	25.3	6.8	22.5	45.4	10.5	39.9	31
Ce	5.6	44	19.2	4	27.7	65.3	36.9	10.8	59	63.4	25.3	84.6	63
Pr	0.51	4.88	2.53	0.25	3.34	7.66	6.71	1.57	6.07	7.25	3.39	6.71	7.1
Nd	2.2	18	9.7	0.9	15.2	34.1	27.4	6.4	25.9	29.9	15.6	23.9	27
Sm	0.71	4.19	2.62	0.22	3.36	7.84	5.95	2.17	5.76	5.19	4.37	5.39	4.7
Eu	0.25	1.08	1.07	0.06	1.1	2.47	1.9	0.66	1.5	1.83	1.53	1.61	1
Gd	0.96	4.52	3.05	0.23	3.67	10.1	5.47	3.03	6.68	5.48	6.31	4.48	4
Tb	0.15	0.7	0.51	0.01	0.63	1.57	0.91	0.55	1.13	1	1.05	0.65	0.7
Dy	1	5.02	3.63	0.21	4.25	8.06	6.36	4.13	6.81	7.79	7.24	4.16	3.9
Ho	0.22	1.13	0.77	0.04	0.63	0.93	1.28	1.04	1.81	2.69	1.33	0.73	0.83
Er	0.55	3.04	1.92	0.23	1	1.1	3.8	3.68	4.91	5.63	3.16	1.83	2.3
Tm	0.08	0.48	0.37	0.01	0.12	0.11	0.54	0.63	0.67	0.64	0.51	0.31	0.3
Yb	0.42	3.05	1.61	0.2	0.58	0.49	3.07	4.08	4.51	3.39	3.28	2.21	1.96
Lu	0.05	0.44	0.26	0.01	0.08	0.09	0.42	0.81	0.63	0.55	0.47	0.34	0.31
∑LREE	11.27	104.25	47.02	7.13	64.4	143.57	104.16	28.4	120.73	152.97	60.69	162.11	133.8
∑HREE	3.43	18.38	12.12	0.94	10.96	22.45	21.85	17.95	27.15	27.17	23.35	14.71	14.3
∑REE	14.7	122.63	59.14	8.07	75.36	166.02	126.01	46.35	147.88	180.14	84.04	176.86	148.1
Eu/Eu*	0.93	0.76	1.16	0.82	0.97	0.86	1.01	0.79	0.74	1.05	0.9	0.98	-
Ce/Ce*	1.22	0.72	0.75	1.24	0.90	1.02	0.62	0.72	1.11	0.72	0.94	1.08	-

and

Table 3. Chemical composition of selected samples in each horizon of profile SL-132. CHL (chloritite); FES (ferruginous saprolite); CLS (clayey saprolite); FAD (ferroaluminous duricrust); UCC (chemical composition of the Earth’s upper continental crust according to [41]).

	JAS	CHL	FES			CLS	FAD		UCC
Depth (m)	238.7	337.5	97.0	202.7	289.2	48.5	1.3	9.8
(Wt%)
SiO2	51.3	53.1	0.79	1.34	2.88	22	1.21	0.53	66.6
TiO2	<0.01	0.67	<0.01	0.02	0.02	1.17	2.24	1.96	0.64
Al2O3	0.07	14.25	0.35	0.43	0.31	27.9	24.7	11	15.4
Fe2O3	48.9	14.3	77.1	81.3	72.8	35.5	54.5	74.1	5.04
MnO	0.01	0.19	0.15	0.05	0.13	0.45	0.02	0.07	0.1
MgO	<0.01	10.25	0.01	0.02	0.02	0.03	0.03	0.02	2.48
CaO	<0.01	0.06	0.02	<0.01	0.08	0.01	0.05	0.01	3.59
Na2O	<0.01	0.02	<0.01	<0.01	<0.01	0.03	0.01	0.03	3.27
K2O	0.01	0.32	0.01	0.02	0.02	0.02	0.02	0.02	2.8
P2O5	<0.01	0.01	0.13	0.01	0.02	0.13	0.3	0.18	0.15
SrO	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	-
LOI	0.23	6.65	4.35	2.07	2.19	13.55	16.4	10.5	-
Total	100.52	99.87	82.91	85.26	78.5	100.81	99.54	98.45	-
C	0.03	0.02	0.02	0.05	0.06	0.04	0.22	0.21	-
S	<0.01	0.05	0.01	<0.01	0.02	<0.01	0.07	0.09	0.062
(ppm)
Li	<10	50	<10	<10	<10	<10	<10	<10	24
Sc	<1	32	2	2	1	11	28	24	14
V	<5	225	22	8	11	351	874	416	97
Cr	<10	380	10	10	20	80	460	240	92
Co	<1	62	<1	<1	<1	<1	<1	<1	17.3
Ni	<1	94	52	<1	<1	26	<1	2	47
Cu	3	379	50	42	84	120	33	72	28
Zn	<2	584	150	<2	<2	50	<2	13	67
Ga	2	15	1.7	1.7	1.5	29.2	45.2	32.3	17.5
Ge	<5	<5	6	6	<5	<5	<5	<5	1.4
As	0.7	0.3	3.6	1.4	0.9	0.8	2.7	1.8	4.8
Se	<0.2	<0.2	0.2	<0.2	<0.2	<0.2	<0.2	0.5	0.09
Rb	0.2	26	0.3	0.4	0.6	0.7	0.3	0.2	84
Sr	3.5	1.9	3.6	0.5	1.7	3.1	9.5	4.5	320
Y	2.2	16.6	11.1	4.9	4.1	23.5	18.3	12	21
Zr	5	96	8	10	8	139	361	243	193
Nb	5.5	4.1	3.7	0.5	0.3	5.5	22	12.7	12
Mo	1	2	3	2	3	1	8	2	1.1
Ag	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	0.05
Cd	<0.5	0.6	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	0.09
Sn	2	1	1	2	1	2	7	4	2.1
Sb	0.07	<0.05	0.11	0.11	0.43	0.19	0.57	0.1	0.4
Te	<0.2	<0.2	0.2	<0.2	<0.2	<0.2	<0.2	0.5	-
Cs	0.01	0.68	0.01	0.01	0.02	0.04	<0.01	<0.01	4.9
Ba	13.3	26.2	22.5	10.5	61.8	66.2	9.6	8.7	628
Hf	<0.2	2.6	0.2	0.4	0.3	4.5	10	6.5	5.3
Ta	0.2	0.2	0.2	<0.1	<0.1	0.4	1.4	1	0.9
W	2	2	3	2	10	5	16	5	1.9
Re	<0.001	0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Hg	<0.005	0.025	0.006	0.01	0.041	<0.005	0.184	0.046	0.05
Tl	<0.02	0.04	0.05	<0.02	0.02	0.09	<0.02	<0.02	0.9
Pb	<2	155	<2	<2	<2	35	2	<2	17
Bi	0.04	0.04	0.26	0.14	0.14	0.16	0.53	0.36	0.16
Th	0.21	3.05	0.37	0.37	0.41	7.6	17.6	10.9	10.5
U	3.6	0.83	4.51	0.34	0.18	2.95	3.13	1.95	2.7
La	4.7	6.9	24.6	2	1.6	8.6	12	7.1	31
Ce	8.6	14.9	21.8	3.3	3.5	205	17.1	9.6	63
Pr	1.02	1.75	6.21	0.37	0.32	2.03	2.12	1.16	7.1
Nd	3.7	6.9	26.1	1.3	1.1	7.5	7.5	3.9	27
Sm	0.67	1.71	6.18	0.34	0.24	2.24	1.72	1.01	4.7
Eu	0.32	0.47	2.42	0.32	0.21	0.92	0.42	0.34	1
Gd	0.47	2.28	5.06	0.42	0.35	3.66	1.73	1.42	4
Tb	0.07	0.39	0.62	0.07	0.05	0.72	0.35	0.31	0.7
Dy	0.29	2.99	3.04	0.33	0.28	4.62	2.71	1.97	3.9
Ho	0.06	0.61	0.43	0.08	0.08	0.95	0.63	0.46	0.83
Er	0.16	1.82	0.97	0.27	0.36	2.62	2.09	1.44	2.3
Tm	0.08	0.26	0.15	0.08	0.05	0.44	0.31	0.25	0.3
Yb	0.03	1.66	0.76	0.16	0.26	2.57	2.07	1.68	1.96
Lu	0.02	0.26	0.11	0.03	0.06	0.35	0.33	0.27	0.31
∑LREE	19.01	32.63	87.31	7.63	6.97	226.29	40.86	23.11	133.8
∑HREE	1.18	10.27	11.14	1.44	1.49	15.93	10.22	7.8	14.3
∑REE	20.19	42.9	98.45	9.07	8.46	242.22	51.08	30.91	148.1
Eu/Eu*	1.67	0.73	1.33	2.61	2.23	0.99	0.74	0.88	-
Ce/Ce*	0.85	0.94	0.39	0.81	1.05	10.71	0.71	0.69	-

).

The bedrock exhibits the highest SiO₂ content across the entire profile, with values ranging from 47.1% to 53.1% in chloritites and from 35.8% to 51.3% in jaspilites. In the ferruginous saprolite, SiO₂ contents are significantly lower than in the bedrock, ranging from 0.67% to 6.53% in profile SL-110 and from 0.79% to 2.88% in profile SL-132 (Figure 7A). The clayey saprolite shows greater heterogeneity in SiO₂ distribution, with contents generally much lower than the substrate across most samples in both profiles (ranging from 0.37% to 37.5%), with one sample notably containing 61.6% SiO₂. In the ferroaluminous duricrusts of both profiles, SiO₂ contents range from 0.57% to 9.8% considering both profiles.

The Fe₂O₃ content varies significantly between the two major bedrocks. In chloritites, the Fe₂O₃ content ranges from 14.3% to 22.5%, while in jaspilite, it ranges from 48.9% to 60.3% (Figure 7A). Contrasting iron contents are also observed in the two types of saprolite. The ferruginous saprolite exhibits Fe₂O₃ contents consistently above 75.4% in profile SL-110 and above 72.8% in profile SL-132. In contrast, the clayey saprolite shows lower Fe₂O₃ content, varying from 17.8% to 73.9%.

Al₂O₃ contents are 14.25% in the chloritites of both holes. These values are somewhat similar to those found in the clayey saprolite, which ranges from 12.15% to 39% in hole 110 and shows a value of 35.5% in the only sample analyzed from this horizon in hole 132 (Figure 7A). Conversely, the Al₂O₃ contents are 0.16% and 0.07% in the jaspilites of profiles SL-110 and SL-132, respectively, closely aligning with the levels found in ferruginous saprolite (0.12% to 0.56% in profile SL-110 and 0.31% to 0.43% in profile SL-132).

The relative distributions of SiO₂, Al₂O_3, and Fe₂O₃ contents allowed the samples to be grouped in three fields matching previously described horizons (Figure 7B). In field I, the samples primarily consist of chloritite and clayey saprolite. Their distribution corresponds to variations in relative SiO₂ content, with intermediate levels of Fe₂O₃ and Al₂O₃. One sample from this group exhibits very low SiO₂ values but high Al₂O₃ levels, indicating local bauxite formation within the clayey saprolite. This observation also supports the genetic relationship between chloritite and clayey saprolite. Field II includes samples of jaspilite and ferruginous saprolite, characterized by low Al₂O₃ content. Field III is represented by the ferroaluminous duricrust.

The distribution of TiO₂ is similar to that of Fe₂O₃ and Al₂O₃, as it also highlights the resemblance between jaspilites or chloritites and specific horizons. The TiO₂ contents are comparable in chloritite (0.67% and 0.89%) and clayey saprolite (1.08% to 2.27%) and are also similar between jaspilite (<0.01% to 0.01%) and ferruginous saprolite (<0.01% to 0.11%). In the ferroaluminous duricrust, TiO₂ content ranges from 1.62% to 2.24%, closely resembling the levels found in clayey saprolite.

The contents of MnO, MgO, CaO, K₂O, and P₂O₅ are below the UCC contents in both chloritite and jaspilite. These levels are even lower in ferruginous and clayey saprolite and in the ferroaluminous duricrust, often falling below the detection limit. Conversely, the loss on ignition is significantly higher in clayey saprolite (5.52–21.4%) and the ferroaluminous duricrust (8.27–16.4%) compared to chloritite (6.31–6.65%), jaspilite (0.23–1.36%), and ferruginous saprolite (0.41–4.35%).

The similarity in the vertical distribution of Al₂O₃ and LOI values, along with high SiO₂ contents, reflects the formation of kaolinite in the clayey horizon (Figure 7B). However, in certain sections, such as at 9.8 m depth in profile SL-132 and at 44 m in SL-110, Al₂O₃ and LOI are much higher than SiO₂ due to the domain of gibbsite. This underscores the contribution of aluminum-rich parent rocks, particularly chloritites, to the development of the studied profiles. Minor variations in analytical totals, including values slightly below or above 100%, may be related to the combination of different analytical procedures and digestion methods applied to major, trace, and volatile components.

4.4.2. Trace Elements

The trace elements that presented concentrations above the detection limit in most of the analyzed samples include Sc, V, Cr, Ni, Cu, Zn, Ga, Zr, Y, Cd, Sn, In, Ba, Bi, Th, U, and REE, whose concentrations ranged from below to above the UCC values [41] (Table 2 and Table 3). Their distribution will be described using normalization to the chemical composition of the UCC and, for REE, to the average chemical composition of the chondrite meteorite (Figure 8A,B). The purpose is not to establish enrichment or depletion relative to the reference compositions, but rather to evaluate similarities and differences in the normalized distribution patterns among the investigated horizons and between these horizons and their parent rocks.

Ferruginous saprolite and clayey saprolite display similar trace element distribution patterns relative to the UCC (Figure 8A,B). In both horizons, Cu, Y, Cd, Sn, In, Hg, Pb, Bi, and U occur at concentrations above the UCC values. Some differences are also observed: Mo presents concentrations above the UCC only in ferruginous saprolite, whereas Sc, V, Cr, and Ni occur above the UCC only in clayey saprolite. In addition, both saprolitic horizons exhibit similar distribution patterns to jaspilite. Conversely, they may show values either above or below those observed in chloritite, depending on the element considered.

Overall, the trace element distribution pattern of the ferroaluminous duricrust relative to the UCC more closely resembles that of the clayey saprolite than that of the ferruginous saprolite, particularly for Sc, V, Cr, and Ni. However, the distribution patterns are very contrasting between profiles. In SL-110, most elements occur at concentrations above the UCC, including Sc, V, Cr, Cu, Cd, Ga, Zr, Nb, Mo, Sn, In, Sb, Hf, Ta, Bi, Th, and U. In contrast, in SL-132, only Sc, V, Cd, Sn, In, Hg, Pb, Bi, and U present concentrations above the UCC.

The REE distribution patterns normalized to the chondrite of [42] reveal a strong similarity between the horizons in both profiles, with a few exceptions (Figure 9A,B). In general, REE concentrations are higher than those found in chondrite. Bedrock samples display an enrichment of light REE compared to the heavy ones, which is more pronounced in jaspilites (La/Lu N average = 14.27) than in chloritites (La/Lu N average = 5.16).

The enrichment of light REE relative to heavy REE is observed in the overlying horizons, more pronounced in the ferruginous saprolite (La/Lu_n average = 16.99 in SL-110 and 10.97 in SL-132) than in the clayey saprolite (La/Lu_n = 4.85 in SL-110 and 2.55 in SL-132). The ferroaluminous duricrust displays significant variation between the two profiles (La/Lu_n average = 12.18 in SL-110 and 3.25 in SL-132). Thus, REE data exhibit minimal fractionation between the investigated horizons, resulting in an approximately horizontal pattern. Nonetheless, a subtle similarity is evident between jaspilite and ferruginous saprolite, both showing greater enrichment of light REE relative to heavy REE, as well as between chloritite and clayey saprolite, where this enrichment pattern is less pronounced.

The jaspilite sample from profile SL-132 shows a notable positive europium anomaly (Eu/Eu* = 1.67), as do three samples of ferruginous saprolite (Eu/Eu* = 1.33, 2.23, and 2.61), which contrasts with the overall pattern observed in the profile. Additionally, one of the clayey saprolite samples displays a positive cerium anomaly (Ce/Ce* = 10.71), as shown in Figure 9B.

5. Discussion

5.1. Mineralogical and Textural Transformations in Iron-Bearing Minerals

The mineral paragenesis in the Serra Leste profiles enabled the interpretation of the succession of iron-bearing phases. The jaspilites represent the parent rock, where iron occurs as magnetite and both massive, pseudomorphic, and platy hematite. The octahedral forms observed within the jaspilites are relict, representing magnetite crystals preserved from the BIF protolith, containing magnetite cores that gradually transform into pseudomorphic hematite at the edges. This conversion indicates an initial stage of weathering, where only magnetite was affected, and the quartz grains remained undissolved. The platy form reflects a more accelerated stage of magnetite replacement by hematite (Figure 10A), where the octahedral parting planes facilitate oxygen diffusion along preferred directions, causing iron to migrate and form hematite lamellae (Figure 10A, Equation (1)) [16,43,44,45].

The ferruginous saprolite inherits massive, pseudomorphic and platy hematite from the jaspilites, in turn with no magnetite relics. Cavities left by quartz dissolution become progressively abundant, partially infilled by colloform goethite (Figure 10B). Microcavities found in pseudomorphic hematite within the ferruginous saprolite (Figure 10C) indicate it was also partially dissolved, creating a microporosity also infilled by massive goethite (Figure 10D). Thus, as the dissolution process advances, goethite replaces hematite from the edges inward (Figure 10E). This transformation typically occurs in two stages. First, hematite is reductively dissolved, releasing Fe²⁺ that becomes soluble in organic complexes (Equation (2)). Next, Fe²⁺ is re-oxidized, leading to the formation of goethite (Equation (3)) [45].

$$2{\mathrm{Fe}}_3{\mathrm{O}}_{4 \left(\mathrm{mag}\right)} +\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right. {\mathrm{O}}_{2 \left(\mathrm{dis}\right)} \leftrightarrow 3{\mathrm{Fe}}_2{\mathrm{O}}_{3 \left(\mathrm{hem}\right)}$$

(1)

$$4{\mathrm{Fe}}_2{\mathrm{O}}_{3 \left(\mathrm{hem}\right)}+{\mathrm{CH}}_3{\mathrm{COOH}}_{ \left(\mathrm{aq}\right)}+16{{\mathrm{H}}^{+}}_{\left(\mathrm{aq}\right)}\leftrightarrow 8{{\mathrm{Fe}}^{+2}}_{ \left(\mathrm{aq}\right)} + 2{\mathrm{CO}}_{2 \left(\mathrm{dis}\right)} + 10{\mathrm{H}}_2{\mathrm{O}}_{ \left(\mathrm{liq}\right)}$$

(2)

$$2{{\mathrm{Fe}}^{2+}}_{ \left(\mathrm{aq}\right)}+0.5{\mathrm{O}}_{2 \left(\mathrm{dis}\right)}+4{{\mathrm{OH}}^{-}}_{ \left(\mathrm{aq}\right)} \leftrightarrow 2{\mathrm{FeOOH}}_{ \left(\mathrm{gt}\right)} + {\mathrm{H}}_2\mathrm{O}$$

(3)

5.2. Geochemical Fractionation

These samples follow a pattern of decreasing SiO₂ and increasing Fe₂O₃, reflecting the formation of ferruginous saprolite from jaspilite through a classic weathering process (e.g., [13,14,15]). A similar pattern was verified in other lateritic profiles of Carajás at least partially derived from chloritites or related rocks, referred to as mafic rocks in S11D deposit or substratum relicts in Serra Norte deposit [27,37]. Considering vertical variation, the significant decrease in SiO₂ content upwards indicates the dissolution of quartz and leaching of silica in the jaspilites, attributed to hydrolysis and silica mobilization during lateritic weathering processes (e.g., [46]). In contrast, chloritites exhibit a less intense decrease in SiO₂ levels since kaolinite, one of their final products, retains some silica. Despite the overall decrease in both cases, marked compositional variations occur due to profile heterogeneity, including preserved portions of chloritite within the ferruginous saprolite and relict chloritite in the clayey saprolite. The ferroaluminous duricrust exhibits an intermediate composition between the two groups, suggesting contributions from both weathering systems.

Iron, originally derived from magnetite and hematite in jaspilite, remains within the system due to the high stability of hematite in the supergene environment (e.g., [47,48]). Consequently, iron concentrates in the forms of hematite and goethite while silica is leached, leading to an increase in Fe₂O₃ concentrations from the bottom to the top as jaspilite transforms into ferruginous saprolite. For chloritites, iron mainly originates from chlorite, which is associated with quartz, and its concentration also increases towards the clayey saprolite due to its retention as iron oxyhydroxides.

The SiO₂ and Fe₂O₃ contents exhibit the negative correlation typically found in lateritic profiles (Figure 11A), in response to the leaching of SiO₂ and the relative concentration of Fe₂O₃ (e.g., [49]). Conversely, TiO₂ and Al₂O₃ show a positive correlation (Figure 11B), as both are considered immobile in lateritic environment. Aluminum is present as kaolinite and gibbsite, and TiO₂ as anatase. TiO₂ and Zr exhibit an even stronger positive correlation (Figure 11C), due to their low mobility, with TiO₂ in the form of newly formed anatase and Zr as resistate zircon [50,51,52]. While the possible presence of rutile as a resistate could also explain the immobility of TiO₂, XRD analysis identified only anatase.

REE shows no correlation with TiO₂ (Figure 11D), contrasting with the behavior reported for most lateritic profiles, in which zircon tends to be the primary carrier of REE and concentrates upward along with anatase (e.g., [53,54]). This suggests that in the Serra Leste profile, there are REE-bearing minerals other than zircon.

The cluster analysis of samples from the Serra Leste profile, based on their whole multi-element chemical composition, including major and trace elements (Table 2 and Table 3), underscores the genetic affinity between horizons (Figure 12). The analysis identifies three main groups: Group I predominantly includes samples of chloritite, clayey saprolite, and ferroaluminous duricrust, while Groups II and III comprise jaspilites and ferruginous saprolite. This pattern supports the mineralogical and textural data indicating an association between the ferruginous saprolite and jaspilites. In contrast, the clayey saprolite and ferroaluminous duricrust show a stronger association with chloritites, resembling the bauxite profiles formed from mafic rocks in the N5 and N4 areas of Carajás [37,55].

5.3. Evidence for the Lateritic Origin of Serra Leste Iron Ore

Under humid tropical conditions, interlayered jaspilite and chloritite underwent simultaneous lateritization (Figure 13). In the jaspilites, silica was leached out following quartz dissolution within the chert bands, as indicated by dissolution cavities (Figure 4C–F) and corroborated by reduction in SiO₂ content from the bottom (jaspilite) toward the top of the profile. Magnetite from jaspilites was converted into hematite, as demonstrated by the presence of octahedral (pseudomorphic) hematite in the soft ferruginous saprolite associated with platy hematite (Figure 13A).

The continuity of lateritization led to hematite dissolution, forming cavities, followed by reprecipitation of iron as oxyhydroxides later converted to goethite, resulting in the infilling of cavities in the hematite bands and in chert bands of the jaspilites by goethite (e.g., [16]). The detection of quartz in the soft ferruginous saprolite by XRD demonstrates that its dissolution within chert bands was incomplete.

The described lateritic process resulted in the formation of a ferruginous saprolite horizons that is economically important for mining and jaspilites served as proto-ore, which, upon weathering, produced the high-grade iron ore, as previously observed in the Serra Norte and Serra Sul iron deposits [27,37]. This saprolitic iron ore is not the same massive, compact hypogene iron ore described in other Carajás deposits [8,11,36]. Supergene and hypogene interpretations for iron ore in Carajás are not contradictory, but rather refer to different ore types in distinct geological contexts.

Conversely, chloritites underwent hydrolysis reactions, decomposing the dominant chlorite plates and producing intermediate clay minerals and kaolinite as a more evolved product (Figure 13B). Those intermediate phases were not identified, but their participation is interpreted because chlorite is not able to form kaolinite directly, but the formation of intermediate phases, as vermiculite and interstratified chlorite/vermiculite, can make this transition possible [56]. Kaolinite as the only clay mineral suggests the high intensity of weathering even in the saprolite. The iron released from chlorite weathering and intermediate phases accumulated to form goethite, as demonstrated by the iron-rich film covering and cementing kaolinite platy crystals (e.g., [57]).

As demonstrated, the primary product of chlorite weathering is clayey saprolite. The alternation between ferruginous saprolite and clayey saprolite along the investigated profiles suggests intercalation between chloritite and jaspilite as parent rocks.

The ferroaluminous duricrusts of the Serra Leste profile were crucial for preserving the underlying laterite/supergene ore, as they acted as a protective layer against erosion while remaining sufficiently permeable to meteoric water, which leached the underlying jaspilite, leading to residual enrichment of iron oxides. The composition and texture of the ferroaluminous duricrusts indicate formation through repeated dissolution and reprecipitation of iron-bearing minerals. This process is evidenced by the alternation of goethite and hematite layers forming spherulites, as well as by the cementation of duricrust fragments by goethite. Thus, the formation of goethite infillings and cements in the saprolite and ferroaluminous duricrusts, respectively, is an additional evidence of supergene mineralogical evolution, in addition to gibbsite (e.g., [23]).

Goethite evolves relics of microplaty hematite and poorly preserved relics of octahedron faces. The presence of aluminum into the system is linked to its retention as kaolinite, locally converted into gibbsite, as demonstrated by gibbsite infilling cavities of the ferroaluminous duricrust. Exceeding aluminum was incorporated by goethite (e.g., [24]). This supports the contribution of the clayey saprolite as precursor of the ferroaluminous duricrust. But the participation of the ferruginous saprolite is also considerable, mainly in the brecciated ferroaluminous duricrust, in which hematite fragments are cemented by iron oxyhydroxides (Figure 13).

Regarding the precursor of the ferroaluminous duricrust, depending on the level of erosional truncation of the profile, there will be a more intense contribution from the jaspilite–ferroaluminous saprolite system or from the chloritite–clayey saprolite system. The first case was found in the SL-132 and SL-110 profiles, exhibiting Al-goethite-bearing spherulites and gibbsite infilling in cavities. The second one is found in the SL-110 profile, with brecciated fragments of compact hematite cemented by iron oxyhydroxides (Figure 13C).

Trace element distribution and multivariate chemical analysis shows more similarity between the ferroaluminous duricrust and the chloritites, confirming the predominance of chloritite–clayey saprolite contribution to its formation. When more related to the clayey saprolite, the ferroaluminous duricrust tends to be enriched in a wider group of trace elements (Figure 13), compared to those derived from the ferruginous saprolite, because chloritites comprise a more diversified chemical composition compared to jaspilites. The contrast between the ferroaluminous duricrust, as well as the association of jaspilite–ferruginous saprolite and chloritite–clayey saprolite, is confirmed by the REE patterns.

The Serra Leste profiles exhibit mineralogical and geochemical compatibility with other laterite profiles in Carajás, particularly those of Serra Norte and Serra Sul [27,38]. This suggests that they were formed from the same lateritization events, which began in the Paleocene–Eocene and continued intermittently during the Cenozoic [6,13]. These profiles constitute a set of deep weathering profiles of the Carajás Surface, which have been subjected to extremely low erosion rates for tens of millions of years [26], making them relict profiles comparable to weathering profiles in other regions, such as the Iron Quadrangle and the Hamersley Province in Western Australia [17].

6. Conclusions

This study documents the contribution of lateritic processes to the formation of the Serra Leste iron ore deposit in Carajás. Interlayered jaspilites and chloritites were subjected to weathering. In jaspilites, dissolution of chert bands promoted silica leaching and resulted in the formation of ferruginous saprolite, which is currently exploited as iron ore, whereas chloritites were transformed into clayey saprolite.

The ferruginous saprolite and the clayey saprolite are capped by a ferroaluminous duricrust derived from both the jaspilite–ferruginous saprolite and chloritite–clayey saprolite systems, predominantly from the latter.

In the jaspilites and ferruginous saprolite, lateritic features include pseudomorphic replacement of magnetite by hematite and the development of dissolution cavities. In the ferroaluminous duricrust, these features include cavity infilling by goethite, gibbsite formation, and spherulitic textures. In the ferruginous saprolite, clayey saprolite, and ferroaluminous duricrust, primary parent rock structures are locally preserved.

The Serra Leste profiles represent an important record of lateritization in the Carajás Mineral Province. The iron ore (ferruginous saprolite) exhibits mineralogical, textural, and geochemical characteristics comparable to those reported for lateritic iron deposits worldwide, including examples from the Iron Quadrangle, Australia, India, and South Africa, particularly with respect to supergene iron enrichment and the development of ferruginous saprolite under tropical weathering conditions.