The Mineralogy, Geochemistry and Origin of the Supergene Manganese Occurrences in the Southern Minas Gerais, Brazil

Abstract

In equatorial and tropical regions, supergene mineral deposits created during water/rock interactions are found. Simply put, these supergene deposits are formed through the accumulation of low solubility ions or through the preservation of primary minerals. The supergene manganese (Mn) deposits are examples of the economic importance associated with the chemical weathering processes. In Brazil, the Southern Brasilia Orogen (SBO) was generated during the collision between the Paranapanema Craton and the passive margin of the São Francisco Craton. In the southern Minas Gerais (MG), several supergene Mn occurrences are hosted in the SBO, which were originated during the chemical weathering of gondites belonging to the Amparo Complex. Here, we studied the supergene Mn occurrences in the southern MG, more specifically in the municipalities of Ouro Fino and Careaçu. The MnO contents ranged from 25.50 to 28.40 wt% at Ouro Fino and from 16.80 to 21.20 wt% at Careaçu. These supergene Mn deposits have a diverse mineral assemblage, being composed of spessartine, quartz, Mn-oxides, goethite and kaolinite. The various Mn minerals formed due to spessartine incongruent dissolution were hollandites, cryptomelanes, romanechites, pyrolusites and lithiophorites. Both study areas are relevant for the possible opening of mines for the commercialization of Mn.

1. Introduction

Manganese (Mn) is an element that can occur in nature as oxides, hydroxides, silicates and carbonates, being found in various types of mineral classes [1,2]. Generally, Mn occurs in small proportions in soils as secondary minerals, originated during water/rock–soil interaction [3]. The Mn ore is one of the most essential raw materials for producing metallic alloys, especially combined with iron (Fe) for steel manufacturing. In addition, Mn can also be used in the production of other metal alloys associated with copper (Cu), zinc (Zn), aluminum (Al), titanium (Ti) and lead (Pb), in battery manufacturing and in the chemical industry [4,5].

Mn deposits are classified as hydrothermal, sedimentary or supergene deposits [6], which are the main Mn deposits found in tropical regions [7]. Supergene Mn deposits are associated with pedogenetic processes during the soil formation, reflecting the interactions between the bedrock and the local climate (mainly temperature and rainfall), as well as the vegetation and relief [8,9]. Mn oxides and hydroxides (here called MnOx) are the main secondary minerals forming during water/rock interactions, exhibiting a highly complex and diverse mineralogy [3]. MnOx are typical tunnel or layer structures resulting from the linkage of the MnO₆ octahedra, with high specific surface area and adsorption capacity, allowing the control of metal concentrations associated with soils and sediments in water [10,11,12,13].

In 2017, the largest global Mn reserves were described in Brazil (272 Mt—32.3%), South Africa (200 Mt—23.7%), Ukraine (140 Mt—16.6%), Australia (91 Mt—10.8%), India (52 Mt—6.2%), China (43 Mt—5.1%), Gabon (22 Mt—2.6%) and Ghana (12 Mt—1.4%), totaling 98.8% of the world’s total reserves [14]. Regarding Brazil, the average Mn content reaches 32.5%, with Minas Gerais (MG—181.2 Mt), Pará (PA—69.3 Mt), Amapá (AM—9.7 Mt) and Mato Grosso do Sul (MS—7.5 Mt) being the states with the largest national reserves [14]. The Brazilian Mn production in 2021 is concentrated in PA (720,182 t), followed by MG (346,475 t) and MS (271,011 t) [15]. The main companies producing Mn in Brazil are VALE SA (MG and PA), Buritirama Mining SA (PA) and Corumbaense Reunida Mining SA (MS) [16]. In addition, Mn ore is also produced in the states of Ceará (CE), Tocantins (TO) and Goiás (GO) [15].

The Southern Brasilia Orogen (SBO) hosts supergene Mn occurrences in the southeastern São Paulo (SP) and southern MG regions [17], formed during the water/rock interactions of gondites, which are metamorphic rocks composed for quartz and spessartine in similar proportions [18]. However, the mineral evolution, the geochemical constituents, and the origin of the supergene Mn occurrences formation are not well explained in the southern MG, more specifically in the municipalities of Ouro Fino and Careaçu. For this purpose, the characterization of supergene Mn occurrences was carried out through petrographic thin sections, X-ray diffraction and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). Additionally, chemical analyses and mineral chemistry were performed to classify these supergene Mn occurrences and to measure the Mn contents in primary and secondary minerals. Consequently, our results provide new insights into the mineralogy, geochemistry and origin of the supergene Mn occurrences in the SBO.

2. Geological Settings

Southeastern Brazil is characterized by the occurrence of several geotectonic units [19]. The Mantiqueira and Tocantins orogenic systems and a portion of the São Francisco Craton are the main geological structures in the area (Figure 1a). The Tocantins Orogenic Belt is formed by the following tectonic units: the Goiás Massif, Brasília Belt, Goiás Magmatic Arc and Paraguay-Araguaia Belt [20,21]. Its geodynamic evolution is associated with convergence events, called the Brasiliano Cycle, subdivided into Brasiliano I (900–700 Ma), Brasiliano II (670–530 Ma) and Brasiliano III (580–490 Ma) [22,23,24,25]. The Southern Brasilia Orogen (SBO) was generated during the collision between the Paranapanema Craton and the passive margin of the São Francisco Craton (630 Ma), giving rise to nappes exhibiting SW dipping and transport in an E–W direction [22,23,24,25].

In the study areas, the SBO is basically composed of Neoproterozoic rocks that belong to the Socorro–Guaxupé Nappe (SGN) and SBO basement (Itapira Group, Amparo/Serra Negra Complex and Pouso Alegre Complex) (Figure 1b). The SGN constitutes a series of nappes that represent a collisional process, representing a complex terrain of high-grade metamorphic rocks and a wide variety of granites [25,26,27,28,29]. Janasi et al. [30] subdivided the SGN into two segments (Guaxupé Nappe to the north and the Socorro Nappe to the south) by the NE-SW-trending Ouro Fino shear zone, and the area exhibits a dextral and transpessive nature. Rocha et al. 2017 [31] proposed a long-lived metamorphism event from 630 to 600 Ma, with a UHT peak at 630–625 Ma (12 kbar and ~1030 °C).

The SBO basement emerged due to the erosional processes that removed overlying rocks and was subsequently uplifted due to the activity of the Ouro Fino Shear Belt [30,32,33]. The infracrustal rocks of the Amparo Complex, along with the supracrustal rocks of the Itapira Group, constitute an elongated belt known as the Itapira/Amparo, exhibiting a NE-SW orientation, and located between the Guaxupé and Socorro complexes, which form the SGN [33]. The Itapira Group was defined by Ebert [34] as a group of rocks with synchronous ages that are accommodated within special synforms between antiforms composed of older lithotypes from the Amparo Complex. This group underwent regional metamorphism in the medium- to high-grade amphibolite facies, involving partial melting of less refractory rocks and the injection of acidic material [33]. The Amparo Complex, of Transamazonian age, is predominantly a metasedimentary unit, represented mainly by biotite and/or hornblende gneisses, with a predominance of plagioclase feldspar and subordinately garnet, diopside or sillimanite [30]. The Pouso Alegre Complex consists of rocks composed predominantly of layered orthogneisses, in amphibolite metamorphic facies and with tonalitic to granodiorite composition, where layers and boudins of mafic rocks occur [35].

3. Materials and Methods

3.1. Study Areas

The study areas are part of the Itapira Group (Figure 1b), composed of pelitic, psammo-pelitic, psammitic, grauwackean, arkosic, marl and calc-silicate sequences, with a regional metamorphism in the medium- to high-grade amphibolite facies [33]. The supergene Mn occurrences were generated during the chemical weathering of gondite rocks [36], which were stretched and fractured due to boudinage processes [17,37]. Gondites, which can be found as fresh or weathered rocks, are characterized by a persistent rhythmic structure, intercalating quartzite and Mn-garnet layers of varying thicknesses [38]. When these rocks undergo weathering, amorphous materials with a black coloration rich in Mn oxides and hydroxides are generated [39]. The southern MG lies in the Atlantic Plateau (AP) geomorphological provinces [40], more specifically in the Brazilian Atlantic Plateau (BAP), characterized by rolling hills [41].

Ultisols and Oxisols, in the USDA nomenclature, are the predominant soils in the SBO, with deeply chemically and mineralogically stratified weathering profiles [41]. The original vegetation was characterized by savanna vegetation [41]. Based on the Köppen classification [42], the climate in the study areas is classified as subtropical (Cwa), with annual average temperatures varying from 20 to 24 °C [43]. The annual average rainfall in the southern MG ranges from 1350 to 1550 mm, with the highest values of rainfall occurring in the summer period (October to March) compared with dry periods (April to September) [44]. In addition, the study areas are located in the upper course of the Mogi-Guaçu River basin, which has an area of 17,460 km² and an extension of 530 km, covering the northeast region of the state of São Paulo and the southeast of Minas Gerais [45].

3.2. Sampling and Analytical Procedures

Representative samples of supergene Mn occurrences were collected from outcrops in four sampling points near the municipalities of Ouro Fino (samples O3 and O10) and Careaçu (samples C1 and C2) (Figure 2). At the sampling points, the supergene Mn occurrences consist of tabular bodies interbedded in the soil derived from the basement rocks, and have a massive structure, with their color ranging from black to dark gray (Figure 2). Petrographic analyses were conducted on polished thin sections of samples using a Zeiss optical microscope, with digital camera Canon Power Shot G5 (model Axioskop 40, Zeiss, Jena, Germany). In order to confirm the minerals identified by optical microscopy, the samples were powdered and analyzed by X-ray diffractometry (XRD), operating at 40 kV and 40 mA, with CuKα radiation (model Empyrean, Malvern Panalytical, Malvern, United Kingdom). The mineralogical identification was performed by the software Highscore Plus (Malvern Panalytical, Malvern, United Kingdom). In addition, the polished thin sections were analyzed by scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS) (model JSM-6010LA, JEOL, Tokyo, Japan), operating using acceleration voltages of 15 kV and beam current of 15 nA.

The geochemical procedures were carried out at SGS Geosol Laboratory (Belo Horizonte, Brazil) and all details on the sample preparation and quality control are outlined by [46]. All samples were analyzed for major oxides (Na₂O, K₂O, MgO, CaO, SiO₂, TiO₂, Al₂O₃, Fe₂O₃, P₂O₅, MnO and BaO) using X-ray fluorescence (modelo Zetium, Malvern Panalytical, Malvern, United Kingdom), with detection limits of 0.10%. Loss on ignition (LOI) was measured by weight difference after combustion at 1000 °C. Selected spessartine and supergene Mn mineral grains underwent electron microprobe analysis (model JXA-8230 Superprobe, JEOL, Tokyo, Japan). The electron microprobe was operated using a beam diameter of 0.5 µm, acceleration voltages of 15 kV and beam current of 15 nA. Oxygen was measured as an unknown and the following standards were used: KAlSi₃O₈ (K), CaSiO₃ (Ca), NaAlSi₃O₈ (Na and Si), CaMgSi₂O₆ (Mg), Fe₂O₃ (Fe and O), FeTiO₃ (Ti), Al₂O₃ (Al), Ca₅(PO₄)₃(Cl) (P), MnO₂ (Mn) and BaSO₄ (Ba).

4. Results

4.1. Petrography

The petrographic analysis revealed a mineral assemblage with a short variation, being composed of primary minerals such as quartz and spessartine, representing ~ 40 and 33%, respectively, followed by Mn-oxides (25%), kaolinite (1%) and goethite (1%) (Figure 3). The XRD patterns confirm the main mineralogy (Figure 4). At Ouro Fino, the grains are equigranular, while at Careaçu, the grains are inequigranular. Ranging from 0.1 to 2.0 mm, the quartz and spessartines grains are anhedral with angular boundaries. Kaolinites and goethite exhibit a maximum dimension of 0.4 mm and occur between the spessartine and quartz.

The Mn-oxides observed during petrography share very similar physical characteristics and do not exhibit preferential orientation or significantly different properties among them, not allowing for Mn-oxides identification. Therefore, the Mn-oxides were identified through SEM-EDS analysis, as suggested by Ramdohr [47]. Lithiophorite, cryptomelane, romanechite, hollandite and pyrolusite were the Mn-oxides characterized, with a maximum size of 0.8 mm (Figure 5). These minerals can occur individually but are commonly associated with spessartine.

4.2. Chemical Composition

The chemical composition of the supergene Mn occurrences is displayed in

Table 1. Chemical composition (wt%) of the supergene manganese occurrences in the southern Minas Gerais.

Sample	Na2O	K2O	MgO	CaO	SiO2	TiO2	Al2O3	Fe2O3	P2O5	MnO	BaO	LOI 1	Total
Ouro Fino
O3a	<0.01	0.21	0.22	0.26	36.10	0.53	13.60	5.41	0.10	28.40	0.39	13.49	98.71
O3b	<0.01	0.67	0.16	0.76	35.70	0.47	14.90	6.14	0.19	27.40	0.12	12.03	98.54
O10a	<0.01	0.11	0.29	1.14	39.10	0.28	13.20	6.87	0.13	25.50	0.32	11.42	98.36
O10b	<0.01	0.25	0.45	1.61	38.10	0.61	12.20	7.53	0.15	26.90	0.17	10.89	98.86
Careaçu
C1a	<0.01	0.07	1.26	5.93	33.20	0.15	18.60	10.40	0.08	19.50	0.04	7.43	97.66
C1b	<0.01	0.19	1.50	3.24	32.50	0.32	19.90	10.21	0.08	21.20	0.13	8.68	97.95
C2a	<0.01	0.08	1.16	4.92	39.60	0.15	16.80	12.20	0.08	16.80	0.12	6.28	98.19
C2b	<0.01	0.06	1.14	4.99	38.30	0.30	16.30	13.19	0.18	17.30	0.07	6.81	98.64

¹ Loss on ignition.

.

Table 2. Electron microprobe analyses (wt%) for all grains of spessartine and Mn-oxides collected in the southern Minas Gerais.

Sample	Mineral	O	Al	Si	Na	Mg	Ca	Ba	Ti	K	P	Mn	Fe	Total
Ouro Fino
O3	Spessartine	40.85	10.95	14.58	<0.01	0.55	2.13	<0.01	0.09	0.01	0.01	26.74	3.35	99.26
O3	Spessartine	39.23	10.91	15.95	0.01	0.51	2.17	<0.01	0.11	<0.01	<0.01	27.65	3.30	99.83
O10	Spessartine	40.70	10.50	14.02	0.01	0.52	2.41	<0.01	0.14	<0.01	<0.01	27.63	3.32	99.24
O10	Spessartine	39.90	10.99	15.15	<0.01	0.51	2.13	<0.01	0.15	<0.01	<0.01	27.51	3.39	99.74
O3	Cryptomelane	36.29	0.02	0.58	<0.01	0.05	<0.01	<0.01	<0.01	1.61	0.08	58.98	0.81	98.41
O3	Cryptomelane	35.90	0.35	0.34	<0.01	0.04	<0.01	<0.01	<0.01	1.20	0.09	59.85	0.85	98.63
O10	Cryptomelane	35.72	0.25	0.26	<0.01	0.02	<0.01	<0.01	<0.01	1.84	0.08	59.30	0.82	98.30
O10	Cryptomelane	36.74	0.62	0.39	<0.01	0.03	<0.01	<0.01	<0.01	1.04	0.11	59.26	0.86	99.05
O3	Romanechite	34.78	0.11	0.08	0.02	0.02	0.05	11.09	0.02	0.18	0.38	51.89	0.11	98.72
O3	Romanechite	35.19	0.08	0.08	0.02	0.02	0.06	11.26	0.02	0.17	0.40	51.57	0.07	98.93
O10	Romanechite	35.45	0.09	0.08	0.02	0.01	0.06	11.09	0.02	0.18	0.41	52.24	0.10	99.75
O10	Romanechite	35.42	0.14	0.07	0.02	0.02	0.05	10.65	0.03	0.20	0.36	51.83	0.13	98.91
O3	Hollandite	37.32	0.72	0.05	0.18	0.06	<0.01	3.29	<0.01	3.26	0.11	54.71	0.11	99.81
O3	Hollandite	36.56	0.77	0.06	0.20	0.05	<0.01	3.23	<0.01	3.25	0.11	54.95	0.10	99.29
O10	Hollandite	36.82	0.83	0.06	0.20	0.06	<0.01	3.40	<0.01	3.18	0.11	54.54	0.14	99.35
O10	Hollandite	36.19	0.58	0.06	0.25	0.05	<0.01	3.12	<0.01	3.34	0.10	55.45	0.12	99.27
O3	Pyrolusite	35.83	0.25	0.80	0.01	0.02	0.09	0.17	<0.01	<0.01	<0.01	62.24	0.42	99.83
O3	Pyrolusite	36.06	0.23	0.68	0.01	0.03	0.10	0.21	<0.01	<0.01	<0.01	61.96	0.46	99.74
O10	Pyrolusite	36.12	0.26	0.71	0.01	0.03	0.11	0.15	<0.01	<0.01	<0.01	61.42	0.40	99.21
O10	Pyrolusite	36.23	0.27	0.73	0.01	0.03	0.08	0.20	<0.01	<0.01	<0.01	61.37	0.51	99.43
O3	Lithiophorite	43.00	13.94	0.26	<0.01	0.04	0.04	0.44	0.14	0.05	0.18	35.64	4.51	98.24
O3	Lithiophorite	42.22	14.35	0.28	<0.01	0.05	0.03	0.55	0.16	0.06	0.20	35.97	4.18	98.05
O10	Lithiophorite	41.95	13.40	0.22	0.02	0.04	0.04	0.40	0.01	0.09	0.05	37.04	4.95	98.21
O10	Lithiophorite	42.55	13.64	0.12	0.01	0.03	0.01	0.51	0.00	0.10	0.03	36.84	4.83	98.67
Careaçu
C1	Spessartine	39.02	11.13	16.45	0.01	0.87	9.67	<0.01	0.04	<0.01	<0.01	19.46	3.12	99.76
C1	Spessartine	38.95	11.06	16.43	0.01	0.85	9.10	0.02	0.04	<0.01	<0.01	20.28	2.92	99.64
C2	Spessartine	39.16	11.04	16.05	0.01	<0.01	9.32	0.01	0.02	<0.01	0.01	19.31	4.21	99.14
C2	Spessartine	39.23	10.76	16.10	0.03	0.03	9.15	0.01	<0.01	0.01	0.01	19.46	4.24	99.03
C1	Cryptomelane	35.81	0.03	0.05	0.12	0.01	<0.01	<0.01	<0.01	2.66	0.19	58.97	0.59	98.43
C1	Cryptomelane	36.70	0.15	0.06	0.13	0.02	<0.01	<0.01	<0.01	2.82	0.19	58.07	0.65	98.79
C2	Cryptomelane	36.91	0.14	0.08	0.17	0.02	<0.01	<0.01	<0.01	2.95	0.24	57.56	0.63	98.70
C2	Cryptomelane	36.60	0.22	0.18	0.07	0.04	<0.01	<0.01	<0.01	2.35	0.17	58.51	0.94	99.08
C1	Hollandite	36.64	0.16	0.11	0.04	0.03	0.09	4.47	<0.01	1.59	0.20	55.23	0.59	99.15
C1	Hollandite	36.01	0.14	0.08	0.17	0.05	0.03	4.99	<0.01	2.66	0.19	54.05	0.65	99.02
C2	Hollandite	36.60	0.22	0.18	0.07	0.02	0.05	4.04	<0.01	2.32	0.24	54.93	0.36	99.03
C2	Hollandite	36.03	0.17	0.11	0.07	0.05	0.03	4.82	<0.01	2.26	0.17	55.29	0.63	99.63
C1	Pyrolusite	35.10	0.21	0.74	0.01	0.03	0.10	0.18	<0.01	<0.01	<0.01	63.06	0.41	99.84
C1	Pyrolusite	35.22	0.24	0.69	0.01	0.02	0.11	0.20	<0.01	<0.01	<0.01	62.88	0.50	99.87
C2	Pyrolusite	34.89	0.22	0.70	0.01	0.03	0.10	0.17	<0.01	<0.01	<0.01	62.63	0.44	99.19
C2	Pyrolusite	35.16	0.23	0.72	0.01	0.02	0.09	0.19	<0.01	<0.01	<0.01	62.91	0.47	99.80
C1	Lithiophorite	45.09	13.21	0.02	0.02	0.01	0.02	0.06	<0.01	0.02	<0.01	38.21	2.07	98.73
C1	Lithiophorite	44.66	12.93	0.08	0.01	0.02	0.02	0.04	<0.01	0.03	<0.01	38.15	2.08	98.02
C2	Lithiophorite	45.82	12.28	0.12	0.01	0.02	0.02	0.01	<0.01	0.06	<0.01	38.01	2.07	98.42
C2	Lithiophorite	45.88	12.42	0.02	0.02	0.02	0.03	0.05	<0.01	0.03	<0.01	37.77	2.10	98.34

shows the EMPA results for spessartine and Mn-oxides (n = 48 spots).

The supergene Mn occurrences have elevated SiO₂ and Al₂O₃ contents in relation to the other major oxides, ranging from 35.70 to 39.10 wt% and from 12.20 to 14.90 wt% at Ouro Fino and from 32.50 to 39.60 wt% and from 16.30 to 19.90 wt% at Careaçu, respectively. In the study areas, the MnO content decreases from the Ouro Fino to the Careaçu samples, varying from 25.50 to 28.40 wt% and from 16.80 to 21.20 wt%, respectively. In contrast, the CaO and Fe₂O₃ contents are higher in the samples collected at Careaçu than at Ouro Fino. In addition, LOI contents in the samples collected at Ouro Fino (average of 11.96 wt%) are higher than at Careaçu (average of 7.30 wt%).

Spessartines contain large amounts of Mn (averages of 27.30 wt% at Ouro Fino and 19.63 wt% at Careaçu) and Si (averages of 15.23 wt% at Ouro Fino and 16.26 wt% at Careaçu). The average Fe and Ca contents were 3.35 wt% at Ouro Fino and 3.62 wt% at Careaçu, and 2.14 wt% at Ouro Fino and 9.31 wt% at Careaçu, respectively. The inverse relationship between Mn and (Fe + Ca) (Figure 6a) results from Mn substitution into spessartines, which have the chemical formulae [(Mn_1.8,Ca_0.9,Fe_0.3)Al₂(SiO₄)₃] at Ouro Fino and [(Mn_2.5,Fe_0.3,Ca_0.2)Al₂(SiO₄)₃] at Careaçu.

Figure 6b illustrates the relationship between Mn and K in Mn-oxides. The EMPA analyses reveal that hollandites contain K averages of 3.25 wt% at Ouro Fino and 2.21 wt% at Careaçu, while the cryptomelanes have K averages of 1.42 wt% at Ouro Fino and 2.69 wt% at Careaçu. Romanechites, pyrolusites and lithiophorites have low average K contents, <0.03, <0.01 and <0.10 wt%, respectively. Romanechite has high Ba (11.02 wt%), while a high Al content (13.83 wt%) identifies lithiophorite bands. The Mn contents vary from ~36 to ~63 wt%, reflecting the various manganese minerals present. With the EMPA results, the chemical formulae are: (a) cryptomelane—K_0.27(Mn_7.66,Fe_0.11,Al_0.08,Si_0.10)O₁₆, romanechte—(Ba_0.40,K_0.02)(Mn_4.77,Al_0.02,Si_0.01)O₁₀, hollandite—(K_0.63,Ba_0.18,Na_0.17)(Mn_7.54,Al_0.20,Fe_0.02,Si_0.02)O₁₆, pyrolusite—(Mn_0.97Si_0.03)O₂ and lithiophorite—(Al_0.59,Fe_0.10)(Mn_0.93,Si_0,01)O₂(OH)₂ at Ouro Fino; (b) cryptomelane—K_0.55(Mn_7.68,Fe_0.09,Al_0.04,Si_0.01)O₁₆, hollandite—(K_0.43,Ba_0.25,Na_0.04)(Mn_7.57,Al_0.50,Fe_0.08,Si_0.03)O₁₆, pyrolusite—(Mn_0.97Si_0.03)O₂ and lithiophorite—(Al_0.58,Fe_0.05)(Mn_1.05)O₂(OH)₂ at Careaçu.

5. Discussion

5.1. Comparison of the Supergene Mn Occurrences in the SBO and Other Brazilian Deposits

Several supergene Mn occurrences outcrop can be found in the SBO, as illustrated in Figure 1b. These occurrences are derived from chemical weathering of gondites. Based on the Mn content, the Mn ore can be classified into three categories: high-grade Mn ore, with Mn contents higher than 35%; ferruginous Mn ore, with Mn contents between 10 and 35%; and manganiferous Fe ore, with Mn contents between 5 and 10% [48]. Among the supergene Mn occurrences in the SBO, the main area studied is the Cocho Stream Mine, located in the municipally of Itapira (São Paulo State), which has an average of 23% MnO₂, with the main mineralogy composed of cryptomelane (20%), spessartine (20%), lithiophorite (20%), quartz (15%) and pyrolusite (10%) [39]. Our results showed that the supergene Mn occurrences in the Ouro Fino and Careaçu have an average of ~27 and ~20 wt% MnO₂, respectively. Therefore, the supergene Mn occurrences in the SBO are classified as ferruginous Mn ores, indicating their relevance for the possible opening of mines for the commercialization of Mn.

As mentioned previously, Brazil has significant reserves of Mn distributed throughout its territory. However, the main Mn reserves are concentrated in the states of MG, MS and PA. The Morro da Mina Mn deposit is located in the MG and inserted in the Rio das Velhas Supergroup, specifically within the Lafaiete Formation, identified as a volcanic-sedimentary sequence of the greenstone belt type in the Quadrilátero Ferrífero [49]. The Urucum Mn deposit is located in the MS and associated with a sedimentary deposit characterized by Neoproterozoic banded iron formations (BIFs) with primary Mn-oxides hosted in the Santa Cruz Formation [50,51]. The Azul Mn deposit, located in the central-western Carajás Mineral Province (PA), has an Mn ore originated from sedimentary (mainly rhodochrosite) and supergene origins [52]. The Mn contents in these deposits are ~ 32, 43 and 38% of MnO₂ at the Morro da mina [53], Urucum [54] and Azul [52] Mn deposits, respectively, these values being higher than the Mn contents in the supergene Mn occurrences in the SBO.

The ⁴⁰Ar/³⁹Ar geochronological data for the Azul Mn deposit indicated different chemical weathering episodes ranging from ~70 to ~10 Ma [55,56]. In the Quadrilátero Ferrífero, the ⁴⁰Ar/³⁹Ar geochronological data proposed a prolonged history of weathering between 62 and 14 Ma, with the majority of MnOx precipitated between 51 and 41 Ma [57]. In both areas, the MnOx precipitation peak was at ~47 Ma [56,57]. Urucum was deposited between 800 and 595 Ma, with metamorphism and hydrothermal alteration at ~575–512 Ma, and their summit rocks have been exposed to weathering for at least ~70 Ma [58,59]. Unfortunately, there are no ⁴⁰Ar/³⁹Ar geochronological data for the supergene Mn occurrences in the SBO. Our chemical mineral results indicated a relatively high K content in cryptomelanes and hollandites, associated with their good crystallinity and lack of contaminants contributing to reliable ⁴⁰Ar/³⁹Ar geochronological data, allowing the comparison of MnOx precipitation ages in the SBO with other Mn deposits in Brazil.

5.2. Origin of the Supergene Mn Occurrences

Due to chemical weathering processes, the more soluble ions are moved by the surface waters to the oceans, while the less soluble ions form secondary minerals [60,61,62,63]. Parental rocks, climate, biosphere, relief and time play important roles during chemical weathering processes [60,61]. The temperature and runoff are the main parameters controlling the chemical weathering of igneous and metamorphic rocks, but other factors must be considered to explain the water/rock interactions, such as the variations in landforms and landscapes [64,65,66,67,68,69]. Both study areas have the same geological, climatic and geomorphological settings, as well as savannah vegetation.

Gondites are lenses interbedded in gneisses belonging to the SBO basement. When the basement reached the Earth’s surface, the action of meteoric agents started promoting the chemical weathering of these rocks (Figure 7a). Atmospheric reactions (CO₂ + H₂O → H₂CO₃ → H₊ + HCO₃⁻) and oxidation of organic matter in soils (HCOOH + 0.5 O₂ → CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻) produce carbonic acid, which is the primary reactant driving chemical weathering processes. During the chemical weathering of gneisses, over a sufficient period of time, deep stratified weathering profiles are formed, and the meteoric waters become enriched in alkali and alkali-earth elements (Figure 7b). The elements dissolved in the upper horizons are transported to lower horizons and depending on the local pH-Eh condition, some elements may reprecipitate in secondary minerals within the system. Gondites are essentially composed of quartz and spessartine. During the interaction between gondites and the enriched meteoric water within saprolites, quartz is not weathered due to its low dissolution rates [70] and spessartines have a low chemical weathering rate [71], leading to supergene Mn occurrences (Figure 7c).

In general, the spessartines are well preserved in the study areas (Figure 4a,c), but their edges exhibit different stages of weathering (Figure 4b,d). The chemical weathering process related to spessartine is the incongruent dissolution, where the Al and SiO₂ released produce kaolinite, Fe forms goethite and Ca is leached into the groundwater. Mn is one of the first elements to be released during the chemical weathering of spessartines, and under oxidizing conditions, the Mn²⁺ changes to Mn³⁺ or Mn⁴⁺ [1,72]. Therefore, most of the supergene Mn minerals formed are formed as Mn⁴⁺, with some Mn³⁺ [3,6]. Mn⁴⁺ forms pyrolusite, while the cryptomelane, hollandite, romanechite and lithiophorite formation results from the recombination of K⁺, Ba²⁺, Al³⁺ and Li⁺ released from the chemical weathering of gneisses and Mn²⁺, Mn³⁺ and Mn⁴⁺ from spessartines.

Table 3. Theoretical chemical weathering reactions showing the most important processes during the interaction between enriched meteoric water and spessartines, originating the supergene Mn occurrences in the SBO.

(Mn,Ca,Fe)3Al2(SiO4)3 (spessartine) + 12 H+ (aq) → Mn2+ (aq) + Ca2+ (aq) + Fe2+ (aq) + 2 Al3+ (aq) + 3 H4SiO4 (aq)

2 Al3+ (aq) + 3 (OH)2- (aq) → 2 Al(OH)3 (aq)

2 Al(OH)3 (aq) + 2 H4SiO4 (aq) → Al2Si2O5(OH)4 (kaolinite) + 5 H2O (liq)

4 Fe2+ (aqs) + O2 (aq) + 6 H2O (liq) → 4 FeOOH (goethite) + 8 H+ (aq)

Mn2+ (aq) + 0.5 O2 (aq) + H2O (liq) → MnO2 (pyrolusite) + 2 H+ (aq)

K+ (aq) + 8 Mn2+ (aq) + 8 H2O (liq) + 4 O2 (aq) + (OH)− (aq) → KMn8O16(OH) (cryptomelane) +16 H+ (aq)

K+ (aq) + Ba2+ (aq) + 8 Mn2+ (aq) + 8 H2O (liq) + 3.5 O2 (aq) + 3 (OH)− (aq) → (Ba,K)Mn8O16(OH) (hollandite) + 16 H+ (aq) + H2O (liq)

Ba2+ (aq) + 5 Mn2+ (aq) + 5 H2O (liq) + 2.5 O2 (aq) + 2 (OH)− (aq) → BaMn5O10(H2O) (romanechite) + 10 H+ (aq) + 0.5 O2 (aq)

Al3+ (aq) + Li+1 (aq) + Mn2+ (aq) + 2 H2O (liq) + 2 (OH)- (aq) → (Al,Li)MnO2(OH)2 (lithiophorite) + 4 H+ (aq)

summarizes the main chemical weathering reactions associated with spessartines. The petrography and SEM-EDS analyses confirm the chemical weathering processes proposed here.

The presence of kaolinite indicates that the supergene Mn occurrences in SBO are typical deposits formed under the warm and humid tropical paleoclimatic conditions (moderate chemical weathering) found in southeastern Brazil’s passive margin. Warm and moderately humid conditions in a savannah-type environment promote active chemical weathering with the advance of the weathering front and low physical erosion, which removes the upper horizons of soils, and the supergene Mn occurrences outcrop at the Earth’s surface (Figure 7d). Figure 7e,f illustrate the concomitant outcrops of soil and supergene Mn occurrences. Extensive and systematic future studies involving the ⁴⁰Ar/³⁹Ar geochronology, as already carried out in other areas in Brazil [55,56,57,58,59,73,74,75] and elsewhere [76,77,78,79,80,81,82], will provide valuable insights on the weathering history and paleoclimatic conditions during the precipitation of these supergene Mn minerals, as well as on the landscape evolution in the SBO. Likewise, the current climatic conditions also suggest moderate chemical weathering in the study areas, as already proposed for different rock types in southeastern Brazil [83,84,85,86,87,88,89,90,91,92,93,94,95]. Recently, Meneguel et al. [96] and Conceição et al. [97] studied the moderate chemical weathering processes to form the raw material used in the porcelain stoneware industry in the São Paulo State and the supergene P, Ti, Nb and REE deposits in the Tapira and Catalão I alkaline-carbonatites complexes, respectively.

The supergene Mn occurrences at Ouro Fino and Careaçu are fine-grained and their mineral phases are difficult to distinguish visually in the field or in hand specimens, as well as in petrography analyses carried out on polished thin sections. Due to their poorly crystalline presentation, the XRD patterns fail to identify the different mineral phases, as also proposed by Ling et al. [98]. Consequently, the SEM-EDS was used here to characterize the Mn-oxides collected at Ouro Fino and Careaçu, even though this technique has experimental limitations [47]. Given the poor crystalline and chemical variability of Mn-Oxides, associated with fine-scale intergrowths of two or more phases, the minerals identified by SEM-EDS should be interpreted carefully. Raman spectroscopy has several advantages for identifying tunnel- or layer-structure Mn-oxides, as well as for investigating changes in response to certain redox, cation-exchange, and other reactions [11,13]. Consequently, future studies using Raman spectroscopy, associated with SEM-EDS and EMPA, will provide unparalleled insights into the micro-mineralogy and chemistry of these complex samples.

6. Conclusions

The supergene Mn occurrences at Ouro Fino and Careaçu are inserted in the SBO basement, and they are found as tabular and massive bodies. The collected samples presented a black color and the petrographic analyses provided important information about the mineralogical composition and physical characteristics of these occurrences. In general, the samples are predominantly equigranular to inequigranular, with a mineral assemblage composed of quartz, spessartine, Mn-oxides, goethite and kaolinite. The MnO contents showed averages of ~27 wt% at Ouro Fino and ~20 wt% at Careaçu, with the supergene Mn occurrences being classified as ferruginous Mn ores, indicating the possibility of opening mines for the commercialization of Mn. The Mn contents in other Mn deposits in Brazil are higher than the Mn contents in the supergene Mn occurrences in the SBO. The dominant meteoric alteration (chemical weathering processes and/or pedogenesis) originates the supergene Mn occurrences, while the physical erosion of the upper horizons of soil is responsible for outcrops of these Mn deposits at the Earth’s surface. The Mn contents vary from ~36 to ~63 wt%, reflecting the various manganese minerals. In order to confirm the Mn-oxides, future studies must be carried out using Raman spectroscopy. Finally, the K contents in hollandites and cryptomelanes suggest that these minerals can be used for a reliable ⁴⁰Ar/³⁹Ar geochronology, allowing the comparison of MnOx precipitation ages, weathering history, paleoclimatic conditions and landscape evolution in the SBO.