03 – SEDIMENTARY MANGANESE DEPOSITS IN CARAJÁS, BRAZIL

Ano 09 (2022) – Número 02 Artigos

 10.31419/ISSN.2594-942X.v92022i2a3MLC

 

 

Marcondes Lima da Costa^1*

Oscar Jesus Choque Fernandez²

Carlos Eduardo Reinaldo Delgado³

Luiz Cláudio Gonçalves da Costa⁴

Marlis Elena Ramirez Requelme²

 

¹Federal University of Pará, Belém, PA, Brazil, marcondeslc@gmail.com

²Federal Institute of Technolgy Pará, Belém, PA, Brazil, ochoque.fernandez@gmail.com, marliselena@yahoo.com.br

³Vale S.A, Carajás, Brazil,  carlos.delgado@vale.com

⁴Vale S.A. at the time the research was carried out

*Corresponding Author

 

Highlights:

• Lenses of carbonaceous and Mn-oxides-rich shales hosted by reddish siltstones;
• Fine amorphous Mn-oxides deposited in a shallow platform approximately 2.0 Ga;
• Stromatolite-like structures rich in Mn-oxyhydroxides;
• Tectonic-hosted Mn-oxyhydroxides + kaolin + pyrite; and
• Cryptomelane and cryptomelane-hollandite are the main Mn ore minerals.

 

GRAPHIC ABSTRACT

 

ABSTRACT

The Carajás Mineral Province in Brazil is one of the most important in the world because it contains world-class mineral deposits (Fe, Cu, Au, Ni and Mn), with Fe, Ni and Mn being mainly of lateritic enrichment. In Carajás, at least four distinct Mn deposits are known (Azul, Buritirama, Buriti and Sereno), and the best known and still important are represented by the Igarapé Azul. The main mines are located in Azul and Buritirama. The Azul deposits are the earliest known and were initially considered as typically lateritic. Further studies have shown that much of the ore manganese reserves are classically associated with carbonaceous grey to black shales represented by oxyhydroxides, where cryptomelane and cryptomelane-hollandite are the major ore mineral. All together as large lenses, they are hosted by thick packets of reddish siltstones and sandstones of Paleoproterozoic (approximately 2.1 Ga). The entire sedimentary package was not affected by metamorphism, perhaps at most anchimetamorphism, as suggested by the discreet presence of sericite and partly chlorite, which can also be diagenetic. The mineralogy dominated by Mn-oxyhydroxides and whole chemical composition of rocks and ore and specific geochemical diagrams after them, as well as the abundance of carbonaceous organic matter and the presence of stromatolite-like structures, show deposition in a shallow marine platform. The source of manganese is both hydrogenous and hydrothermal. The incursion of hydrothermal Mn contribution, in part abyssal, is indicated by high concentration of Co-Ni-Cu and Ba, as well as REE and slight positive Ce and Eu anomalies. Rhodochrosite is restricted and early diagenetic, likely pyrite, as confirmed by the distinct isotopic data. The deposition of Mn oxides, mainly amorphous, appears to be microbial mediated, probably silicium bacteria (Si-O-Mn-C), and close to ocean redoxcline, being mostly oxic conditions.  Most of the crystalline Mn-oxyhydroxides are of diagenetic origin and in part epigenetic. The restricted presence of carbonates and the marked presence of carbonaceous organic matter shows that, during the diagenesis, the original conditions of sedimentation prevailed, only locally reducing, when rhodochrosites and pyrites were formed. This is reinforced by the isotopic composition of the organic matter. Tectonic deformations reached the entire package during two regional events (Trans-Amazonic and Brazilian) and provided the remobilization and reprecipitation of Mn-oxyhydroxides of high content and crystallinity (pyrolusite and manganite) along faults, fractures and apical folds.

All these data show that the Mn deposits of Azul find parallels in several world deposits formed in the first Great Oxidation Event of the Earth’s atmosphere, between 2.4 and 2.1 Ga.  They also find some parallels in the deposits of Neoproterozoic until Cenozoic. The great differential is the mastery of Mn oxyhydroxides and the absence of metamorphism, when confronted with the older ones.

Keywords: cryptomelane; hollandite; stromatolite-like structures; carbonaceous organic matter; stable isotopes; tectonic-hosted Mn ores.

INTRODUCTION

The strong economic and industrial development of Asian countries, especially China and India, has increased the demand for manganese ores sharply, without considering that some famous mines, such as Serra do Navio and Conselheiro Lafaiete in Brazil have run out. Brazil, until then one of the main players of Mn, was restricted in general terms only to the mines of the Azul and Buritirama (Salgado et al., 2019) in Carajás Mineral Province (CMP) (Figure 1), and Urucum in Mato Grosso do Sul (Biondi & Lopez, 2017), besides small deposits in Bahia. In view of this global and national framework, it was imperative to develop geological studies to understand the genesis of manganese deposits, aiming to increase the reserves of the known deposits and discover new ones.

The manganese ore of the Azul (Figure 1) was considered to be of lateritic origin from alteration of rhodochrosite-rich proto-ore related to two rhodochrosite units (Lower and Upper Units) (Figure 2) (Anderson et al., 1974; Valarelli et al., 1978; Bernardelli & Beisiegel, 1978, Bernardelli, 1982, Beauvais, 1984, Coelho & Rodrigues, 1986, Beauvais et al., 1987, Silva, 1988, Vasconcelos et al., 1994, Ruffet et al., 1996, Weber, 1997; Dardenne & Schobbenhaus 2001).

Costa et al. (2005), however, showed that the main ore then mined fifteen years ago corresponded to several packets of Mn-oxyhydroxides enveloped by dark grey to black shale lenses. These lenses then converge externally to grey shales hosted by thick packets of red siltstones to sandstones that dominated the entire region of the mine and even its surroundings. They also showed that these rocks were affected by tectonic deformations that favour the formation of Mn-oxyhydroxide mineralizations hosted by faults, folds and veins and strongly associated with kaolin venules. In the end, they proposed that these sedimentary rocks in fact belonged to the formation Águas Claras, with an age approximately 2.1 Ga (Costa et al., 2005; Fabre et al., 2011). They also verified that rhodochrosite was a very restricted mineral.  Araújo & Sousa (2018) in a large report, presented a detailed study of the manganese ore deposits in Carajás and its turnaround. For Azul they followed the origin proposed by Costa et al (2005) by using a detailed geological description for the mine front and drill holes, and presented optical mineralogical and texture analyses, XRD and MEV/EDS mineralogical data, and several instances of U-Pb dating using detrital zircons, structural analyses and even geophysical surveys.  They then reinforced the sedimentary and supergene importance for manganese ore and admitted that the sedimentary ore is associated with the Archean sequence formed by pelitic rocks. Although manganese deposits involve up to four different formation environments (sedimentary, volcanic-sedimentary, metamorphosed, hydrothermally modified, hydrothermal and supergene), sedimentary deposits are economically the most important and are mainly distributed from the Proterozoic through the Quaternary (Laznicka, 1992; Fan & Yang, 1999). For manganese deposits in the CMP, considered as supergene (lateritic), new research shows that at least the Azul deposits mined in the last 16 years are mainly sedimentary. The estimated ore reserves are of the order of 150 Mt and therefore a deposit of medium size.

In general, the oxide manganese deposits are considered to be formed by supergenic alteration over manganese carbonate or silicates (Weber, 1997; Maynard, 2004), but some deposits, even of large amounts to oxyhydroxides are typically sedimentary (Haralyi & Walde, 1986), such as those of Ukraine, Australia, South Africa (Kuleshov, 2017), and even in part of Gabon (Maynard, 2004).

 

 

The manganese of Azul is not, however, a unique manganese deposit in the region of CMP. The next most important is Buritirama located to the north of the Carajás block (Figure 1), bordered by the Bacajá block (Salgado et al., 2019), whose production level is already comparable to that of Azul. Although very rarely investigated from the point of view of its origin the manganese ore is considered to be lateritic (supergene ore) and locally hydrothermal (Araújo & Sousa, 2018; Salgado et al., 2019), formed from carbonate rocks (kutnohorite, rhodochrosite and Mn-calcite) and manganese silicates (rhodonite, spessartine, Mn-diopside) rocks with orthoquartzites and mica schists (Valarelli et al., 1978; Salgado et al., 2019).

 

 

Buriti is another manganese deposit in CMP, though smaller than the other ones (Figure 1), just south of Buritirama. Here, the ore occupies higher relief and is layered and hosted by compact but friable sandstones. The ore is massive, stratified, dense, and composed mainly by cryptomelane (Figure 3 A and B). The top is covered by colluvium (Figure 3 A) of this ore material. This deposit was not the subject of academic studies.

Finally, there are Mn-oxyhydroxide deposits of Sereno (Figure 1) which occupy the cliffs of a large valley (Figure 3 C) in the northern region of Serra Leste, next to the Serra Pelada domain area, which is distinguished by the richness of gold associated with manganese and carbonaceous shales. Manganese ore locally is very thick and presents as metric pockets massive to cavernous (Figure 3 D), embedded in fine quartzites that are strongly tectonically deformed. The only geological information available on this deposit is reported by Araújo & Sousa (2018), who classified the manganese ore as lateritic.

 

 

The main objective of this work is then to present the dominant presence of the eminently sedimentary and restrictly superimposed tectonic ore mineralizations, on which the lateritic and its degradation (detrital and pisolithic) have been developed, which largely support the current landscape of the Azul Mn deposits in the CMP. It will present new mineralogical data and mineral chemistry, especially for cryptomelane and cryptomelane-hollandite, rock geochemistry and geochronology, isotopic geochemistry and the occurrence of stromatolite-like structures. The new data were only possible after the studies carried out over the last 15 years, through fieldwork on several occasions, including the description of several drilling holes and collection of representative samples.

With this perspective of a sedimentary origin for the manganese ore from Azul, a new environment is opened to prospect new deposits and increase reserves of this ore in the region, where laterite Mn-bearing crusts may be strong geological indicators.

 

Azul’s manganese deposits are located in the CMP (Figure 1), southeast of the State of Pará. This province comprises relatively elevated terrain for the Amazonian pattern, represented by kilometer-long plateaus, with altitudes varying between 600 and 900 m and valleys and undulating surfaces between the plateaus, watershed, from 300 to 400 m in altitude. The tops of the plateaus are slightly undulated to almost flat. The Azul manganese deposits are located on the slopes of an east-west kilometre plateau with up to 600 m in altitude and exposed in the depth during mining in the last 16 years. The manganese ore also cropped out on the slopes and at the bottom of the valleys of the Azul creek.

An extensive Paleoproterozoic metasedimentary sequence, mainly clastic, marine to fluvial, denominated as the Águas Claras Formation (Nogueira et al., 1995), covers a large Archean to Paleoproterozoic area of ​​CMP (Figure 1). This sedimentary cover carries the manganese ore of the Azul and is, at the same time, the source material of the lateritic manganese ore.

The Águas Claras Formation occupies an area of ​​900 km² in the central part of the Carajás Transcurrent System, the primary ore bearer, consisting essentially of shales (siltstones and mudstones) with different degrees of lithification, being locally deformed by faults and folds related to this system in the mineralized zone. The lower mudstone and siltstone members were deposited on a marine platform system, and the sandstones and conglomerates of the upper member are of fluvial and shallow coastal origin (Nogueira et al., 1995).

Most of the surficial terrain of the Azul manganese mines are laid on lateritic materials, mainly arising as iron-aluminum crusts, manganese crusts, or clay-like argisols with Fe and Mn-oxyhydroxide spherulites and fragments (Figures 4 through 7). These laterite formations correspond to old, thick lateritic profiles, equivalent to those that formed part of the iron, aluminium, gold, and nickel ores, among others, in Carajás and Amazonia in general (Costa, 1997; Costa et al., 2005).

 

 

MATERIALS AND METHODS

Fieldwork and sampling – The fieldwork consisted of several mapping trips in and out of the pits, describing shallow to 200- m in depth boreholes (Figure 4). Sampling was carried out on the mining fronts and along the drill holes (Figures 4 and 5, Table 1), representing the materials of the lateritic profiles and sedimentary rocks and their products of tectonic deformation, mainly along faults, folds, and fractures.

Optical Microscopy – It was used for mineralogical identification and textural analysis on both thin and polished sections for the different minerals, in which opaque (manganese ore minerals) are dominant. For the identification of the mineralogical phases, a ZEISS microscope, model AXIOLAB POL, with reflected light, ocular of 2.5x, 10x, 20x and 50x of the laboratory of Gemology of the Group of Mineralogy and Applied Geochemistry of the Institute of Geosciences of the University Federal University of Pará (IG-UFPA) and a UNION microscope with 20x and 40x objective of the metallography laboratory of the Technical Course in Metallurgy of the Federal Institute of Education, Science and Technology of Pará (IFPA), both coupled with a camera and image system, Canon and Fujitsu, respectively, were utilized.

X-ray-Diffraction – It was of vital importance for the mineralogical characterization of most samples, formed from cryptocrystalline to microcrystalline materials. For this purpose, an X-ray diffractometer model X’PERT PRO MPD (PW 3040/60) from PANalytical, with PW3050/60 (Theta/Theta) goniometer was used. The analytical conditions comprise a copper anode X-ray tube (λCuKα1 = 1,54060Å), model PW3373 / 00, long fine focus, under conditions of 2200W, 60kv, with a Kβ Ni filter. The detector used is of type RTMS, X’Celerator. The logs were obtained in the exposure range from 4 to 75º 2θ for a total sample. The data acquisition of the records was performed with the software X’Pert Data Collector, version 2.1a, and the treatment of the data was performed with the X’Pert HighScore software version 2.1b, also of PANalytical, consulting the PDF database (Powder Diffraction File) of the ICDD (International Center for Diffraction Data). These analyses were carried out at the X-ray Diffraction Laboratory of the Geosciences Institute of the Federal University of Pará (IG-UFPA).

Infrared Spectroscopy – The technique was also used to aid in the mineralogical identification, mainly of the cryptocrystalline and / or amorphous phases, frequent in the investigated materials. The equipment used for these analyses was a Perkin-Elmer, model 1760 X FT-IR spectrophotometer, coupled to a microcomputer, with records in the range of 4000 to 400 cm -1, with measurements every 4 cm -1. Data were interpreted using Perkin-Elmer’s Graph Server software version 1.60. These analyses were also carried out in the Infrared Laboratory of the IG-UFPA

Thermal Analysis – It was used strictly, also as an auxiliary technique in the identification of Mn oxyhydroxides and in the clay minerals. Equipment was used from the laboratories of the Geowissenschaften Institut, Halle-Wittenberg Universität in Germany and IG/UFPA. A thermal analyzer, model PL Thermal Sciences with simultaneous thermal analyser STA 1000/1500 from Stanton Redcroft Ltd., with vertical cylindrical furnace and digital converter coupled to a microcomputer, was utilized. Approximately 10 mg of each sample was placed in a platinum crucible and run under an N₂ flow rate of 50 cm³/min. The heating rate was 20 ° C/min, with an initial temperature of 20 ° C and final temperature of 1100 ° C.

Scanning Electron Microscopy with Dispersive Energy System (SEM/EDS) – It was also used continuously to aid in the various mineralogical and micromorphological identifications, being crucial for the characterization of the Mn and Fe oxyhydroxides and the clay minerals. Equipment from IG-UFPA, Goeldi Museum and ZWL-Lauf/Nürnberg laboratories was used. The analyses were performed using a software-controlled LEO 1450VP Scanning Electron Microscope through an X-ray dispersion spectrometry system (SED 500 DP).

Electron Probe Micro-Analyser (EPMA) – It was used to determine chemical composition mainly of oxyhydroxides of Mn (cryptotomelane), carbonates (rhodochrosite) and tentatively some fined-grains silicates (chlorite and illite). The analyses were carried out at the Institute of Physics of the Federal University of Minas, Belo Horizonte, Brazil, equipped with a JEOL electronic microprobe, model 8900RL. The electron beams (diameter of 5 μm) used during the measurements were generated from 20 kv, with a current intensity of 25 nA and integration time of 6 s.

Whole Chemical Analyses – The chemical composition of total samples and isolated fractions was determined mainly in commercial laboratories. The techniques employed were mainly fusion with metaborate and / or lithium tetraborate, followed by digestion with nitric and / or hydrochloric acid and determination by ICP-OES and ICP-MS. These analyses were performed in the ACME and ACTLab laboratories.

Geochronology – Selected samples of Mn oxyhydroxides from the sedimentary rock were subjected to the ⁴⁰K/³⁹Ar method intermediated by ACTLab and ⁴⁰Ar/³⁹Ar through the laboratories of Prof. Dr. Paulo Vasconcelos from the University of Queensland, Australia.

Mass Spectrometry – It was used for the determination of d³⁴S in sulphides, mainly pyrite, as well as d¹³C in carbonaceous material, contained in sedimentary rocks mineralized in Mn oxyhydroxides. It was also used for the determination of d¹⁸O and D in kaolinite from kaolin venules that section the Mn-oxyhydroxides ore hosted by tectonic structures. These analyses were performed in ACTLab. The same technique was used to determine d¹³C and d¹⁸O in carbonates. Analyses were carried out at GNS in New Zealand.

Leco – It was used for the determination of organic carbon in the rock samples in the ACTLabs.

 

Table 1 – List of samples collected and analyzed with the respective analytical procedures carried out. DH: Drill hole; XRD: X ray diffraction; OM: optical microscope; EPMA: Electron microprobe analysis; SEM/EDS: Scanning electron microscope/Energy dispersive X spectrometry; FTIR: Fourier-transformed infrared; COM: carbonaceous organic matter; X: Indicates the respective analytical technique that was performed on each sample.

Sample Nr.

Mine

DRILL HOLE-ID

XRD

Chemistry

OM

EPMA

SEM/EDX

K/Ar dating

δ 13C e  δ 18O

δ13C PDB

δ 34S isotope

FTIR

COM

Azul 06

1

DH167P27.30

X

X

Azul 09

1

DH167P62.80

X

X

Azul 10

1

DH167P68.50

X

X

X

Azul 13

1

DH167P105.45

X

X

Azul 15

1

DH167P107.85

X

X

Azul 18

1

DH277P4.70

X

X

Azul 24

1

DH277P68.25

X

X

Azul 25

1

DH277P77.00

X

X

Azul 50

B540E

X

X

Azul 81

1

DH5P183.83

X

X

Azul 82

1

DH5P202.00

X

X

X

Azul 83

1

DH5P202.00

X

X

X

Azul 83

1

DH5P216.30

X

X

X

X

X

Azul 84

1

DH5P220.00

X

X

Azul 88

1

DH6p128.16

X

X

X

X

Azul 89

1

DH6P136.40

X

X

X

X

X

Azul 93

1

DH63P121.00

X

X

Azul 98

1

DH86P63.60

X

X

Azul 102

1

DH90P82.40

X

X

Azul 110

1

DH174P60.40

X

X

Azul 114

3

DH304P92.00

X

X

Azul 115

3

DH304P94.00

X

X

Azul 120

1

DH318P73.00

X

X

X

X

Azul 130

1

DH382 P22.60

X

X

X

Azul 140

1

DH382 P73.40

X

X

Azul 142

1

DH382 P 93.70

X

X

Azul 153

1

DH382 P150.15

X

X

X

Azul 155

1

DH382 P162.60

X

X

X

Azul 156

1

DH382 P168.50

X

X

Azul158

1

DH382 P176.10

X

X

X

Azul160

1

DH382 P185.35

X

X

X

Azul 161

1

DH382 P188.45

X

X

X

X

Azul 162

1

DH382P195.05

X

X

X

X

X

X

Azul 163

1

DH382P198.25

X

X

X

Azul 164

1

DH382P199.20

X

X

X

X

X

X

Azul 165

1

DH382P202

X

X

X

X

X

X

Azul 166

1

DH382P206.55

X

X

X

X

X

X

Azul 173*

1

DH387 P18.00

X

X

X

Azul 175*

1

DH387 P25.00

X

X

Azul 176*

1

DH387 P27.60

X

X

X

Azul 178

1

DH387 P33.00

X

X

Azul 199

1

DH387 P114.20

X

X

X

X

X

Azul 201

1

DH396P79.70

X

X

X

X

X

Azul 202

1

DH396P83.50

X

X

X

Azul 206

Hand sample 4

X

X

X

X

Azul 225

B476 pelite

X

X

X

X

1

DH303/P69.50m-c

X

X

1

DH303/P69.50m-d

X

1

DH303/P87.55m-a

X

X

X

1

DH303/P87.55m-f

X

X

X

1

DH326/P57.25m-a

X

X

X

1

DH326/P57.25m-c

X

1

DH326/P57.25m-d

X

X

1

DH348/P33.80

X

X

1

DH560

X

X

DH728/P2.66m-a

X

X

DH728/P12.35m-d

X

X

MF-14

X

X

MF-22

X

X

MF-24

X

X

MN1-8

1

X

X

MN1-22

1

X

X

MN1-25A

1

X

X

MN1-29A

1

X

X

X

X

MN1-30

1

X

X

X

MN1-33

1

X

X

MUGEO2460

Carrapato

X

X

X

X

ESTMn01

Carrapato

X

X

X

X

MF-42

X

MN1-31

1

X

MF-24

X

MF-19

X

MF-10

X

MF-17

X

MN1-24

1

X

X

X

MF-23

X

MF-21

X

MN1-1

1

X

X

MN1-27

1

X

X

MN1-32

1

X

X

MN1-35

1

X

X

MN3-1

3

X

X

MN3-2

3

X

X

X

MN3-3

3

X

X

X

MN3-7

3

X

X

X

MN1-18

1

X

X

MN1-10

1

X

X

MN1-31

1

X

X

MN1-24

1

X

X

 

RESULTS

Ore Geology – The outcrop and subsurface (approximately up to 50 m deep) manganese ore expressed by Mn-oxyhydroxides in the mines of Azul were mined in the first 16 years (1984 through 2000). From the 2000s onwards, the dominant ore was at a higher depth. The former was considered to be supergenic (lateritic), and the other was primary (Costa et al., 2005). The outcropped and subsurface mineralized materials, manganese ores, involve ferrous-manganesiferous, and manganesiferous crusts and ferrous-aluminum-manganesiferous pisolites and fragments (Figures 4 through 7). Underlying these materials or even outcropping in the lower part or border of the mines is the manganese-bearing or manganese-free black shales and siltstones/mudstones and sandstones (Figures 4 through 7). Supergenic ores were part of the full to erosional truncated lateritic profile. On the other side is the manganese oxyhydroxide ore mined in the last 16 years in the Azul deposit hosted by dark grey to black shales, finely laminated, which forms lenses up to 4 m thick and a hundred metres in length. They are hosted by thick reddish mudstones/siltstones and sandstones, sometimes striped (Figures7 and 8 C), or micro faulted (Figure 8 C). The contact between these rocks and the shales is gradational (Figures 7 and 8 A and B), in which the Mn oxyhydroxides give rise to dark grey carbonaceous material (Figure 8 A, B and D) forming the grey siltstones/mudstones. In addition to the Mn oxyhydroxides, mainly cryptomelane and birnessite, these rocks are formed by illite/sericite, chlorite, kaolinite, and quartz (Table 2) and, as appropriate, fine laminated carbonaceous material (Figure 8 A and B). The red siltstones, formed by quartz, kaolinite, illite and microcrystalline haematite, may contain centimetre to decimetre large nodules of Mn oxyhydroxides, or millimetre to centimetre stratified to laminated lenses of those same materials (Figure 8 C). These lenses become more and more frequent as they approach the zone of contact with the greasy grey shales (Figure 8 D).

 

 

 

 

 

Table 2 – The minerals identified and their general evolution in the manganese deposits of Azul, Carajás Mineral Province. (?): The information needs further investigation to be fully confirmed.

Minerals	Environmental processes
	Syngenetic (sedimentary)	Early diagenetic (anchimetamorphic)	Epigenetic (tectonic)	Supergene (lateritic) after Costa et al. (2005)
Illite
Sericite
Kaolinite
Chlorite
Smectite
K-Feldspar
Quartz
Pyrite
Chalcopyrite
Amorphous Mn- oxyhydroxides
Rhodochrosite
Carbonaceous organic matter
Cryptomelane
Cryptomelane-hollandite
Coronadite
Birnessite	?
Hollandite
Pyrolusite
Nsutite
Todorokite
Lithiophorite
Gibbsite
Goethite
Hematite

 

This sedimentary succession, which reaches approximately 200 m in thickness in mine 1 (Figure 5), is correlated with the Águas Claras of Araújo et al., 1988; and Nogueira et al., 1995. Araújo and Sousa (2018) proposed that the maximum age for mineralization would be approximately 2,609 Ma from U-Pb dating of detrital zircons. These minerals were extracted from rocks of the Águas Claras formation within the Azul mine and the data confirm the Neoarchean age (Costa et al., 1995). Previous studies show, however, that the sediments of the Águas Claras Formation pointed out by indirect geochronological data have a maximum age of 1.8 Ga (Beauvais et al., 1987).  Our t five geochronological analyses performed (Table 3) showed ages compatible with those presented in the previous one, with three of them approximately 2.1 Ga.

 

Table 3 – ⁴⁰K/³⁹Ar geochronologic ages get in red siltstones hosting the sedimentary manganese ore (dark gray to black shales) in the Azul deposits.

Sample ID:	40K/39Ar Ages
MN1-8	2359±62
MN1-22	2006±51
MN1-25A	2044±53
MN1-29A	1792±47
MN1-30	2058±53

 

The grey colour intensifies with the increase of the content of carbonaceous organic matter (COM), while black is correlated with the Mn-oxyhydroxides (Figures 7 and 8). The transition from red siltstones/mudstones to black shales occurs both laterally and at the top and bottom contacts (Figures 7 and 8 A and B). Dark siltstones and black shales behave as real lenses, mainly from the intermediate zone to the bottom of mine 1, having a hundred metres of apparent length and a maximum thickness of 15 m on the mining fronts (Figure 7). The geological section presented by Bernardelli and Beisiegel (1978) and modified by Beauvais et al. (1987) already foresee the mineralization associated with the mega-lenses body form (Figure 2).

Most of the mega-lenses exposed during manganese ore mining in 2009, 180 m below the top of the plateau surface, that is, at the topographic level of 400 m, consisted of Mn-oxy-hydroxides (cryptomelane), with a massive to laminar aspect, with kaolin blades, venules and sulphide pockets, occurring within a pack of black shales grading to dominant striped or striped, red siltstones (Figures 7 through 9 C). This mode of occurrence suggests that the Mn-oxyhydroxides of the mega-lens of black shale are contemporaneous with sedimentation or early diagenesis (Table 2). In the Azul mines, this material is special due to its high content, and it is classified as dioxide, dominated by cryptomelane. Similar lenses extend through almost all mines at different vertical levels and horizontal positions hosted by reddish siltstones to sandstones that extend along all the mine´s area (Figures 6 and 7). Grey siltstones/mudstones and grey to black shales, with carbonaceous organic matter (COM), contain kaolin (kaolinite, quartz and illite) in fine layers parallel to the bedding or cross cutting the same (Figure 8), generally a characteristic feature of them. They can also present venules and nodules/concretions or framboids of pyrites, sometimes in cubic crystals and/or chalcopyrite masses, and even gold. The distribution of the COM is concordant with the bedding, and gives the grey colour, fissiliness and greasiness to the sediments. They can be well correlated to black shales present in global manganese sedimentary mineralizations, already proposed by Beauvais (1984).

In contrast to the pioneering work in the Azul deposits, which began with drill hole 5, which allowed for identification of primary manganese units constituted by Mn carbonate (Bernadelli & Beisiegel, 1978 illustrated in Figure 2; Valarelli et al., 1978), the data obtained in this work during several field works, and deep drill holes did not find these rocks. The geological observations in the mine in recent years when it reached greater depths and therefore fresh rocks, in addition to more recent deep drill holes (up to 200m in depth), the depth of carbonates in drill hole 5, show that the previous considered proto-ore (the marls: carbonate-bearing shales, mainly rhodochrosite) were no longer found. This sequence described here in the lower zone of the actual pits and in the deepest drilling holes does not present any evidence of origin of these oxyhydroxides from the chemical weathering of rhodochrosite, which is always restricted to millimeter laminae and mostly veinlets.

Deformation-related Mineralization – The sedimentary packet (red siltstones/mudstones, sandstones, and the manganese, mineralized or not, black shales in the mines of Azul) was intensely tectonically deformed. The set as a whole is folded, with main manganese ore from central mine 1 occupying the upper portion of the east-west anticline (Bernardelli & Beisiegel, 1978) (Figures 5 through 7). The folding was more intense and tighter at the Carrapato and Mine 3 (Figure 9 A and B). North-south and northwest-southeast faults and shear zones overprinted the entire sequence (Araujo & Sousa, 2018). The tectonic deformation also developed intense cracking, micro faults, folds and micro folds (Figures 9 C and D; 10 A through E). At the intersections between failures and fractures developed massive to colloform, locally druse form, very dense, sometimes cavernous, ore bodies, made of pyrolusite, manganite, and perhaps nsutite (Figure 10 C). These minerals were not found in the dark grey to black shales and non-deformed siltstones. In the shear zones (mine 3) (Figure 9), these manifestations extend for tens of metres in length. Here the manganese oxyhydroxides are massive to crystalline and form rich ore. The hinges of the tight folds served as trapping for Mn-oxyhydroxides accumulation in high contents (Figure 10 A through C), besides the pockets. Millimetric to centimetric veinlets fulfilled with kaolin and Mn-oxyhydroxides (Figure 10 E), as well as (Fe,Cu)-sulphides have been formed.

 

 

 

Micromorphology and Mineralogy – Because the present work emphasizes the sedimentary sequence and its diagenetic and tectonic alteration, greater emphasis will be given only to the minerals and their micromorphological aspects that make up these materials.  Those related to lateritic cover will only be mentioned. The sedimentary rocks mineralized in Mn-oxyhydroxides (black shales and dark grey and black siltstones) and their hosting rocks (red siltstones/mudstones and striped, red sandstones) are formed by quartz, illite/sericite, chlorite, and kaolinite and are distinguished by concentrations of Mn-oxyhydroxides, carbonaceous organic material (COM) and haematite (Table 2). COM is typical of the lenses of grey shales and in the transition zone between manganese-rich shales (Mn ore) and red siltstones/sandstones, and it is represented by very fine grey material composed mostly of amorphous carbon. The Mn-oxyhydroxides are the main minerals of black siltstones and shales and in part of the transitional zone. They are represented by cryptomelane and cryptomelane-hollandite (Figure 11), the most typical manganese mineral of the sedimentary sequence, displaying distinct micromorphological aspects (Figure 12) that are birnessite restricted (Table 2). Haematite is characteristic of siltstones and red sandstones, striped or not, and is microcrystalline and finely disseminated in these rocks. Pyrite, chalcopyrite and rhodochrosite occur discreetly, usually as venules and microbands concordant with stratification. Locally, they were identified by SEM/EDS Ti oxide needles, which by the mode of occurrence may correspond to rutile.

In the pockets, faulting, and shear zones, and even in the hinges of the mega-folds, cryptomelane persists, however pyrolusite is more abundant, besides the nsutite. Pyrolusite occurs in mm to cm crystals in typical druse intergrowths. Therefore, this mineral is a guide to the deformed zones in the Azul mine and at the same time constitutes locally rich concentrations of Mn. The kaolin veins and venules are formed obviously by kaolinite (Figure 10 E), which may contain quartz, pyrite, chalcopyrite and sometimes rhodochrosite. The kaolinite in these venules and bands shows well-developed micrometric pseudo-hexagonal crystals. Quartz tends to be milky to microcrystalline, rarely macrocrystalline. The rhodochrosite forms micrometric mass, milky white, slightly transparent and with grey shade occurring together with quartz and pyrite in a micritic texture, as well as, in veinlets in the black shales. In the quartz venules that intercept the micritic masses with rhodochrosite, this mineral generally borders the quartz venular body.

Microprobe analyses (Table 4) of the rhodochrosite masses show important contents of Fe (FeO), 4.84 to 5.88%, while Mg (MgO) contents ranged from 0.02 to 0.36 and Ca (CaO) from 0.02 to 0.93, suggesting the possible presence of kutnohorite, and / or ankerite and siderite, or even calcite, found sporadically in venules.

The AZUL-164 sample was one of the few samples of black shales with micritic rhodochrosite ranging from main mineral to accessory, alongside quartz, illite, chlorite and pyrite. Calcite is an accessory. This mineralogical composition can be deduced from the results of chemical analyses by microprobe at different points in this sample (Table 5).

 

Table 4 – EPMA chemical composition of rhodochrosite from Mn-bearing shales.

Rhodochrosites	1	2	3	4	5	6	7	8	9	10	Theoretic
Na2O	nd	nd	nd	nd	nd	0.02	nd	nd	nd	nd
SiO2	0.02	nd	0.03	0.02	0.03	0.05	nd	0.02	0.04	0.06
Al2O3	0.04	0.03	nd	0.02	nd	0.06	0.01	0.01	0.04	0.04
MgO	0.11	0.20	0.22	0.03	0.02	0.10	0.11	0.26	0.36	0.05
K2O	nd	nd	nd	nd	0.02	0.01	0.03	nd	0.01	0.01
MnO	52.77	52.46	53.12	52.50	52.73	52.66	52.91	51.70	51.29	53.08	61.71
PbO2	nd	nd	0.08	0.04	0.04	0.19	nd	nd	nd	nd
CaO	0.38	0.87	0.66	0.14	0.02	0.11	0.32	0.81	0.93	0.08
TiO2	0.03	0.02	nd	nd	nd	nd	nd	nd	nd	0.03
SO3	0.04	nd	0.03	0.03	nd	0.01	0.03	0.07	0.02	0.05
Cl	nd	0.02	0.02	0.02	0.01	0.01	0.01	0.01	0.02	Nd
FeO	5.17	5.15	5.36	5.18	5.30	4.84	4.91	5.47	5.88	5.42
BaO	nd	nd	0.17	nd	nd	nd	nd	nd	nd	0.09
Total	59.80	60.00	60.95	59.20	59.42	59.29	59.57	59.60	59.84	60.15

 

In addition, these analyses allow for the identification of microcrystalline intergrowth between illite and chlorite and make it possible to delimit chlorite domains (Table 6).

 

Table 5 – EPMA chemical analyses of the manganese-bearing shales with microcrystalline to micritic rhodochrosite and calcite, and chlorite, illite, pyrite and quartz.

Samples/Mineralogy/ Main oxide elements	Na2O	SiO2	Al2O3	MgO	K2O	MnO	PbO2	CaO	TiO2	SO3	Cl	FeO	BaO	H2O	Total
AZUL164-C2A, chlorite, illite, rhodochrosite, pyrite, calcite	0.05	29.07	16.45	14.95	0.98	6.93	nd	0.86	0.05	4.96	0.02	6.02	nd	3.32	83.66
AZUL164-I, chlorite, illite, rhodochrosite, pyrite, calcite	0.05	38.49	15.89	12.94	1.34	6.82	nd	1.27	0.07	3.16	0.04	6.41	0.11	3.61	90.20
AZUL164-D, chlorite,  illite, rhodochrosite, quartz, pyrite	0.05	33.82	16.46	13.56	3.53	6.36	nd	0.90	0.10	2.88	0.03	5.56	nd	3.41	86.66
AZUL164-B, rhodochrosite, illite, calcite, quartz, pyrite	0.04	20.89	8.18	0.63	3.80	18.20	nd	2.78	0.28	2.03	0.06	1.10	nd	2.08	60.06
AZUL164-E, chlorite, rhodochrosite, (illite), quartz, pyrite, calcite	0.07	26.73	11.83	8.59	0.52	12.50	nd	4.74	0.03	4.03	0.03	6.09	0.04	2.93	78.12
AZUL164-C2B, quartz, chlorite, rhodochrosite	0.04	53.68	10.73	8.84	0.10	4.32	nd	0.04	0.05	0.26	0.02	4.60	nd	3.69	86.36
AZUL164-A, chlorite, rhodochrosite, illite, pyrite	0.03	32.15	12.77	12.87	3.19	3.77	nd	0.75	0.13	0.83	0.02	2.14	0.13	2.89	71.65
AZUL164-C, quartz, chlorite(?), rhodochrosite	0.04	82.72	2.18	2.63	0.02	2.49	0.04	1.69	nd	0.17	0.02	0.97	0.09	4.45	97.52

 

Table 6 – EPMA chemical analyses of illite + chlorite, and chlorite and in grey shales.

Analyszed points	1	2	3	4	5	5	Webmineral*
	Illite + chlorite (?)	Illte + chlorite (?)	Chlorite (?)+	Chlorite (?) +	Chlorite (?) +	Chlorite	Chlorite
Na2O	0.01	0.01	0.05	0.03	0.00	0.01
SiO2	39.87	38.23	29.07	32.15	31.54	30.17	30.28
Al2O3	18.76	18.24	16.45	12.77	17.13	17.81	17.13
MgO	19.49	18.86	14.95	12.87	19.62	25.22	25.39
K2O	7.90	7.93	0.98	3.19	0.00	0.32
MnOa	2.28	2.54	6.93	3.77	0.00	7.29
PbO2	0.00	0.00	0.00	0.00	0.00	0.00
CaO	0.08	0.01	0.86	0.75	0.00	0.02
TiO2	0.44	0.44	0.05	0.13	0.00	0.04
SO3	0.01	0.07	4.96	0.83	0.00	0.02
Cl	0.00	0.04	0.02	0.02	0.00	0.01
FeOb	4.80	4.69	6.02	2.14	8.17	5.51	15.09
BaO	0.00	0.00	0.00	0.13	0.12	0.04
H2O	3.83	3.71	3.32	2.89	3.24	3.51	12.11
Total	97.47	94.75	83.66	71.65	79.83	89.96

^aTotal Mn as MnO; ^b Total Fe as FeO. *Webmineral, http://www.webmineral.com/data/Clinochlore.shtml#.XnjQBIhKjIU, accessed March 23, 2020.

 

Cryptomelane and cryptomelane-hollandite, the main mineral of Mn in the Azul deposits, presents in masses that are finely stratified or still of massive aspect (Figure 12). In the sedimentary package of Azul the cryptomelane occurs associated with illite/sericite, sometimes quartz, kaolinite, pyrite and COM (Table 2). Its chemical composition changes in terms of the most common members of the group, K, Ba, Sr and Pb, where the major highlight is for Ba, locally represented by the cryptomelane-hollandite series (Tables 7 and 8), generally associated with both discordant and concordant venules (Figure 12). Cryptomelane also appears as an ore mineral in the venules and filling pockets and fissures. However, no differences in its chemical composition were observed between them due to these different modes of occurrences. In turn, they are cryptomelane-hollandites poor in SrO (Table 8), and coronadite (Pb) was identified even more restrictively by both XRD and EPMA. The EPMA chemical composition of cryptomelane-hollandites from Azul, when compared to those other deposits, diverge in contents of Al, Fe, Ni, Co, Cu and Zn. (Table 9), generally higher.

 

 

 

 

Table 7 – EPMA chemical composition (oxide Wt.%) and calculated chemical formulas for cryptomelane, considering 16 oxygens pro formula.

Point analysed	1	2	3	4	5	6	7	8	9	10
SiO2	0.89	0.78	0.93	0.35	0.37	0.72	0.91	0.41	0.85	0.90
Al2O3	7.40	6.90	7.71	5.40	5.56	7.23	8.76	4.69	7.46	8.14
FeO	5.80	0.60	1.67	2.53	3.89	3.57	1.99	2.30	1.12	1.49
MnO2	84.42	87.03	86.03	84.27	82.01	83.74	77.98	86.64	84.72	78.99
MgO	0.02	0.00	0.16	0.29	0.35	0.46	0.14	0.23	0.06	0.15
CaO	0.08	0.04	0.08	0.20	0.17	0.24	0.10	0.18	0.07	0.11
Na2O	0.08	0.10	0.10	0.15	0.08	0.10	0.18	0.12	0.11	0.12
K2O	2.73	4.09	3.49	2.07	1.96	2.47	3.08	2.54	4.35	3.05
TiO2	0.13	0.02	0.03	0.04	0.06	0.11	0.04	0.11	0.06	0.02
P2O5	0.15	0.11	0.11	0.11	0.12	0.21	0.20	0.17	0.10	0.12
SO3	0.02	0.01	0.00	0.05	0.08	0.05	0.10	0.03	0.01	0.07
V2O3	0.21	0.00	0.08	0.00	0.00	0.00	0.15	0.02	0.02	0.00
CoO	0.08	0.11	0.13	0.16	0.16	0.20	0.13	0.17	0.15	0.13
NiO	0.03	0.00	0.07	0.08	0.07	0.19	0.03	0.08	0.00	0.03
CuO	0.24	0.30	0.21	0.24	0.22	0.25	0.24	0.23	0.31	0.26
ZnO	0.06	0.14	0.18	0.06	0.02	0.11	0.15	0.09	0.16	0.11
As2O5	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
SrO	0.07	0.06	0.04	0.06	0.04	0.06	0.07	0.07	0.05	0.02
ZrO2	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.05	0.00	0.00
BaO	0.20	0.00	0.02	0.03	0.00	0.06	0.02	0.04	0.00	0.05
PbO	0.07	0.03	0.03	0.05	0.00	0.00	0.01	0.22	0.00	0.02
Total	98.68	100.31	101.06	94.14	95.17	99.75	94.28	98.39	99.57	93.75
				B cations
Al3+	1.041	0.943	1.043	0.772	0.806	0.996	1.265	0.659	1.028	1.181
Fe2+	0.579	0.059	0.161	0.257	0.400	0.349	0.204	0.229	0.109	0.154
Mn4+	6.633	6.972	6.829	7.068	6.969	6.763	6.604	7.134	6.849	6.725
Mg2+	0.003	0.000	0.028	0.052	0.064	0.079	0.025	0.040	0.010	0.027
Co2+	0.008	0.010	0.012	0.016	0.016	0.019	0.012	0.016	0.014	0.013
Ni2+	0.002	0.000	0.006	0.008	0.007	0.018	0.003	0.007	0.000	0.002
Cu2+	0.022	0.026	0.018	0.022	0.021	0.022	0.022	0.021	0.028	0.024
Zn2+	0.005	0.012	0.015	0.005	0.002	0.009	0.014	0.007	0.014	0.010
Total	8.294	8.021	8.113	8.200	8.284	8.254	8.150	8.114	8.055	8.137
				A cations
Ca2+	0.010	0.005	0.010	0.025	0.023	0.030	0.013	0.023	0.009	0.015
Na+	0.019	0.022	0.022	0.035	0.020	0.022	0.043	0.029	0.024	0.029
K+	0.416	0.605	0.511	0.321	0.308	0.368	0.482	0.385	0.650	0.479
Sr2+	0.005	0.004	0.002	0.004	0.002	0.004	0.005	0.005	0.003	0.001
Ba2+	0.009	0.000	0.001	0.002	0.000	0.003	0.001	0.002	0.000	0.002
Pb2+	0.002	0.001	0.001	0.002	0.000	0.000	0.000	0.007	0.000	0.000
Total	0.461	0.636	0.547	0.389	0.353	0.427	0.544	0.451	0.686	0.527

 

Table 8 – EPMA chemical composition (Wt.%) and calculated chemical formulas for the series cryptomelane-hollandite, considering 16 oxygens pro formula.

Point analyzed	1	2	3	4	5	6	7	8	9
SiO2	0.18	0.21	0.28	0.22	0.12	0.38	0.14	0.35	1.28
Al2O3	4.36	4.57	6.53	5.27	2.26	5.61	3.15	5.97	7.43
FeO	3.51	3.43	0.05	0.07	0.03	0.15	0.21	1.16	4.45
MnO2	85.53	82.71	83.56	83.26	90.71	81.92	87.70	84.47	70.07
MgO	0.01	0.00	0.02	0.01	0.05	0.06	0.10	0.14	0.05
CaO	0.03	0.05	0.01	0.05	0.15	0.05	0.13	0.12	0.17
Na2O	0.00	0.07	0.05	0.06	0.10	0.00	0.00	0.04	0.04
K2O	3.06	2.96	3.94	3.76	5.14	3.90	3.26	3.78	3.09
TiO2	0.16	0.14	0.05	0.06	0.02	0.07	0.06	0.09	0.26
P2O5	0.52	0.45	0.81	0.66	0.65	0.89	0.27	0.91	1.27
SO3	0.02	0.04	0.00	0.01	0.00	0.03	0.06	0.11	0.28
V2O3	0.25	0.33	0.63	0.33	0.40	0.48	0.10	0.44	0.82
CoO	0.09	0.13	0.53	0.83	0.21	0.70	0.28	0.11	0.53
NiO	0.00	0.03	0.03	0.08	0.05	0.10	0.09	0.12	0.00
CuO	0.55	0.56	1.00	1.37	1.41	1.44	0.84	0.68	0.81
ZnO	0.00	0.05	0.02	0.05	0.10	0.08	0.01	0.14	0.03
As2O5	0.00	0.10	0.00	0.00	0.00	0.00	0.00	0.00	0.00
SrO	0.00	0.00	0.04	0.00	0.02	0.00	0.01	0.01	0.00
ZrO2	0.00	0.00	0.00	0.09	0.05	0.04	0.00	0.00	0.01
BaO	2.49	2.65	1.21	1.35	0.34	1.71	1.56	2.00	2.11
PbO	0.32	0.28	0.08	0.00	0.02	0.00	0.06	0.00	0.04
Total	101.08	98.76	98.82	97.52	101.84	97.62	98.01	100.64	92.74
				B cations
Al3+	0.612	0.657	0.915	0.754	0.312	0.803	0.450	0.827	1.120
Fe2+	0.349	0.350	0.005	0.007	0.003	0.015	0.02 1	0.114	0.476
Mn4+	7.034	6.971	6.869	6.989	7.337	6.872	7.342	6.859	6.192
Mg2+	0.001	0.000	0.004	0.002	0.009	0.011	0.017	0.024	0.010
Co2+	0.008	0.013	0.05 1	0.081	0.019	0.068	0.027	0.010	0.055
Ni2+	0.000	0.003	0.003	0.008	0.005	0.010	0.009	0.012	0.000
Cu2+	0.050	0.052	0.089	0.125	0.124	0.132	0.077	0.060	0.079
Zn2+	0.000	0.004	0.001	0.005	0.009	0.007	0.001	0.012	0.003
Total	8.054	8.050	7.938	7.969	7.819	7.919	7.944	7.917	7.933
				C cations
Ca2+	0.003	0.007	0.001	0.007	0.018	0.006	0.017	0.015	0.023
Na+	0.000	0.016	0.010	0.015	0.024	0.000	0.000	0.008	0.010
K+	0.464	0.461	0.598	0.583	0.767	0.605	0.504	0.567	0.504
Sr2+	0.000	0.000	0.002	0.000	0.002	0.000	0.000	0.001	0.000
Ba2+	0.116	0.126	0.057	0.064	0.016	0.081	0.074	0.092	0.106
Pb2+	0.010	0.009	0.003	0.000	0.001	0.000	0.002	0.000	0.001
Total	0.594	0.619	0.670	0.669	0.827	0.692	0.597	0.684	0.644

 

 

Table 9 – EPMA average chemical compositions (Wt.%) of cryptomelane-hollandite from manganese deposit Azul compared with cryptomelane-hollandite and/or hollandite from other manganese ore deposits. nd: not detected. CRY-HOL – cryptomelane-hollandite; HOL – hollandite. (1) – cryptomelane-hollandite from St Marcel-Praborna, Italia (Perseil, 1998). (2) – cryptomelane-hollandite from Manganese deposit of Vani, Milos – Greece (Liakopoulos et al., 2001). (3) – hollandite from St. Marcel-Praborna, Italy (Perseil, 1998).

	CRY-HOL			CRY-HOL		HOL
Oxides	Ore -13A	Ore -26	Ore 3-7B	(1)	(2)	(3)
SiO2	0.35	0.32	0.22	0.22	0.46	0.41
Al2O3	5.02	7.31	1.99	0.00	0.38	0.00
FeO	1.45	0.05	1.63	nd	nd	Nd
MnO2	83.32	84.52	85.12	83.94	83.52	75.68
MgO	0.05	0.01	0.30	0.00	0.26	0.00
CaO	0.08	0.04	0.56	0.06	0.45	0.30
Na2O	0.04	0.16	0.16	0.26	0.27	0.09
K2O	3.66	3.85	3.38	4.25	3.39	0.31
TiO2	0.10	0.05	0.03	0.20	0.00	1.04
P2O5	0.72	1.01	0.04	nd	nd	Nd
SO3	0.06	0.02	0.04	nd	nd	Nd
V2O3	0.42	0.37	0.09	nd	nd	Nd
CoO	0.38	0.29	0.09	nd	nd	Nd
NiO	0.06	0.03	0.09	0.00	0.10	0.00
CuO	0.96	0.64	0.10	0.00	0.17	0.00
ZnO	0.05	0.03	0.12	0.00	0.63	0.00
As2O5	0.01	0.03	0.00	nd	nd	Nd
SrO	0.01	0.01	0.11	6.41	0.00	1.66
ZrO2	0.02	0.04	0.00	nd	nd	Nd
BaO	1.71	0.98	0.82	1.52	6.46	15.91
PbO	0.09	0.06	0.02	0.00	0.30	0.00
Total	98.56	99.83	94.90	98.34	96.90	98.34

nd: not detected. CRY-HOL – cryptomelane-hollandite; HOL – hollandite. (1) – cryptomelane-hollandite from St Marcel-Praborna, Italia (Perseil 1998). (2) – cryptomelane-hollandite from Manganese deposit of Vani, Milos – Greece (Liakopoulos et al 2001). (3) – hollandite from St. Marcel-Praborna, Italy (Perseil 1998).

 

Sulphides, mainly pyrite and chalcopyrite, although small in quantity, occur in manganese-rich or not black shales (carbonaceous) and venules. They respond to the relative high concentrations of Cu, Pb, Zn, Ni, Co, Mo, As, Sb and Ag (Tables 10 and 11) together with COM. Pyrite framboids occur, associated with COM masses, sometimes with arsenopyrite and even argentite. Framboidal pyrite can be related to a series of mineralized microbial produced micro-textures (Yu et al., 2019).

Carbonaceous Organic Matter (COM) – The carbonaceous organic matter that frequently occurs in the transition from the grey to dark grey or black shales (Figures 8 B) reaches 4.7% in quantity. It is not crystalline. No graphite has been identified yet, which shows that these sedimentary rocks were not affected even by very low metamorphism, strengthened by the presence of illite (sometimes sericite) and still kaolinite.

The content of organic matter content (COM) is extremely variable (Figure 13), being more abundant, of course, in the carbonaceous shales with or without Mn-oxyhydroxides, which, together with the disseminated sulphides and the illites/sericites, give the grey colour to these rocks.

Extracts obtained by decantation in distilled water and submitted to analysis by scanning electron microscopy (SEM/EDS) and XRD confirmed the presence of amorphous carbon (Figure 14 a, b, c).

The SEM/EDS chemical analytical spectra of extracted organic matter (Figure 14 a) indicates as high amount of carbon and some Fe, Mn, K, Mg, Al, S and O, which can be related to pyrite (Fe and S), rare rhodochrosite (Mn, Mg, C, O), kaolinite (Al, Si, O) and cryptomelane (K, Mn, O) (Figure 14 b).

 

 

 

Whole-Rock Geochemistry – Manganese-rich dark grey shales (Table 10) are distinguished by high but variable manganese oxides (up to 80.7% MnO), followed by SiO₂ (3 to 53%), Al₂O₃ (1.8 to 23%), Fe₂O₃ (1.7 to 12.7%), MgO (0.1 to 4.3), K₂O (0.5 to 2.9%) and TiO₂ (0.1 to 0.8%). Alternately, they are relatively poor in CaO (<1.0%), Na₂O (0.2%) and P₂O₅ (0.2%), except for some isolated samples. The values ​​of MnO correspond mainly to cryptomelane, while K₂O corresponds to this mineral and illite/sericite, which is reinforced by the MgO contents; the SiO₂ content of this mineral, quartz and still kaolinite. All Al₂O₃ represents aluminosilicates such as kaolinite and illite/sericite (Table 8). The highest values ​​of CaO are referenced to the random presence of calcite and/or rhodochrosite and kutnohorite, which were randomly identified in venules and microbands in these rocks. Those more pronounced P₂O₅ are interpreted as possible apatites, but the mineral occurs sporadically. The TiO₂ values ​​are compatible with the rutile needles, as outlined by SEM/EDS.

 

Table 10 – Chemical composition (Wt.%) of manganese ore related to shale (dark grey shales). Cont´n.

SAMPLE ID:	SiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Na2O	K20	TiO2	P205	LOI
Azul 06	3.7	9.04	4.11	65.32	0.11	0.04	0.07	2,00	0.37	0.08	13.8
Azul 09	5.04	6.39	4.41	65.94	0.23	0.19	0.08	2.35	0.42	0.44	13.25
Azul 10	0.83	1.83	1.7	80.74	0.12	0.14	0.1	1.26	0.17	0.42	13.05
Azul 18	15.29	13.14	9.97	41.2	0.72	0.39	0.09	1.41	0.56	0.11	15.68
Azul 50	14.05	17.67	9.07	38.5	0.45	0.19	0.04	0.62	0.736	0.07	18.96
Azul 81	35.72	8.06	5.46	23.36	1.88	0.52	0.14	1.9	0.424	0.06	22.3
Azul 82	31.22	5.13	6.05	29.11	1.49	0.58	0.15	0.92	0.273	0.05	24.05
Azul 83	17.42	2.54	3.46	45.16	1.48	1.52	0.18	0.5	0.112	0.06	27.92
Azul 88	46.63	9.67	5.13	15.85	2.62	0.38	0.1	2.79	0.551	0.1	15.52
Azul 89	23.31	4.44	2.44	39.46	1.39	1.18	0.19	1.06	0.319	0.06	24.85
Azul 120	2.56	3.38	1.94	75.18	0.12	0.21	0.25	2.27	0.186	0.47	13.79
Azul 130	18.15	19.92	8.18	35.17	0.36	0.14	0.03	0.65	0.808	0.09	16.46
Azul 140*	14.78	15.88	8.25	40.96	0.58	0.23	0.11	0.72	0.829	0.1	17.24
Azul 142*	6.44	13.15	5.93	53.28	0.55	0.22	0.04	0.93	0.587	0.1	17.7
Azul 162	42.61	9.85	16.52	4.72	4.29	0.37	0.07	2.96	0.566	0.01	16.99
Azul 163	46.72	9.61	12.7	6.82	3.85	0.75	0.33	2.46	0.464	0.37	15.02
Azul 164	53.39	11.04	7.29	6.26	3.71	0.46	0.05	3.39	0.586	0.16	12.74
Azul 165	39.53	7.77	9.79	16.85	3.08	1.33	0.04	2.47	0.392	0.16	18.71
Azul 166	33.38	6.67	3.74	28.68	3.12	1.25	nd	2.38	0.361	0.17	20.26
Azul 173*	7.99	14.58	8.1	49.96	0.26	0.13	0.05	0.92	0.443	0.1	17.03
Azul 175*	17.17	16.03	8.87	41,00	0.2	0.17	0.19	1.13	0.741	0.18	13.53
Azul 176*	9.45	15.72	9.03	47.49	0.2	0.12	0.03	0.87	0.718	0.18	16.32
Azul 178	24.43	23.32	12.72	21.84	0.23	0.13	0.13	0.76	0.987	0.19	14.31
Azul 199	43.22	9.42	7.66	16.65	1.79	0.35	0.27	2.31	0.518	0.1	16.42
Azul 201	34.33	7.43	2.33	31.25	0.88	0.72	0.14	1.28	0.283	0.08	20.35
Azul 202	43.16	9,00	2.78	24.13	1.03	0.36	0.02	1.93	0.345	0.05	16.33

 

SAMPLE ID.		V		Cr		Co		Ni		Cu		Zn		Ga		Ge		As		Rb		Sr		Y		Zr		Nb		Mo		Ag		In		Sn		Sb		Cs		Ba
Azul 06		160		24		526		122		503		226		27		1.1		48		33		431		43		82		4.2		27		1.9		nd		1		0.6		1.2		2440
Azul 09		208		31		513		589		716		1790		20		0.9		53		56		226		30		42		3.3		131		1.2		nd		nd		2.7		2.8		2890
Azul 10		127		32		358		545		362		1860		13		nd		72		31		140		14		24		1.9		133		0.6		nd		nd		1.2		0.5		409
Azul 18		204		35		292		304		302		315		26		1		83		66		623		29		167		6.5		39		0.5		nd		2		3.1		4.2		2950
Azul 50		142		nd		334		390		295		173		26		nd		25		51		150		41		194		7		12		nd		nd		2		5.1		5.2		1890
Azul 81		168		90		112		212		254		761		13		nd		30		71		19		18		nd		6		19		nd		nd		nd		2.1		2.7		241
Azul 82		130		88		162		325		225		551		11		nd		64		40		16		10		nd		4		65		0.6		nd		8		5.4		1.4		174
Azul 83		135		35		64		340		84		673		10		nd		22		17		15		5		nd		3		20		nd		nd		nd		1.1		0.5		151
Azul 88		174		105		70		128		293		492		16		nd		25		106		30		20		131		7		16		0.6		nd		nd		2.5		3.8		368
Azul 89		173		46		57		337		50		439		12		nd		22		42		19		5		nd		3		19		nd		nd		nd		1.6		1.1		284
Azul 120		127		25		309		470		554		1.490		21		nd		47		51		136		11		nd		1		34		0.8		nd		nd		1.3		1.3		2160
Azul 130		184		32		234		316		243		199		32		nd		27		43		167		35		245		11		14		nd		nd		3		3.2		4.5		1900
Azul 140*		206		81		432		515		745		220		35		nd		27		64		180		45		nd		10		26		nd		nd		3		2.5		5.4		1250
Azul 142*		282		-20		431		520		828		320		40		nd		49		68		171		44		nd		11		23		0.9		nd		2		2.9		5.6		1290
Azul 162		165		96		62		131		nd		619		13		nd		16		97		23		26		109		5		-2		nd		nd		nd		nd		3		278
Azul 163		233		162		105		198		273		961		14		2		89		103		35		32		106		4		23		1.6		nd		nd		3.2		3.6		316
Azul 164		226		119		76		116		105		793		17		nd		29		116		35		24		124		5		8		0.9		nd		nd		1		3.2		395
Azul 165		152		82		110		119		13		1070		16		nd		19		81		21		28		91		4		3		0.6		nd		nd		nd		2.3		265
Azul 166		207		74		103		166		195		943		19		nd		93		74		25		17		41		3		27		nd		nd		nd		1.7		1		292
Azul 173*		169		119		478		483		643		413		47		1		34		34		176		31		nd		8		28		2.3		nd		2		2.6		3.1		3560
Azul 175*		208		56		308		252		252		317		31		nd		28		33		130		33		nd		9		25		2.7		nd		2		4.6		2.8		1920
Azul 176*		233		73		306		346		454		684		39		nd		76		31		113		30		nd		10		53		2.1		nd		3		3.3		2.7		1600
Azul 178		241		144		188		166		276		330		30		1		30		22		84		32		334		11		32		2.8		nd		3		3.2		1.3		3100
Azul 199		182		117		137		216		216		555		14		1		31		111		39		17		116		60		22		3.2		nd		nd		1.3		3.7		352
Azul 201		120		75		167		220		107		695		15		1		10		59		14		12		70		5		17		1.4		nd		1		1.3		2.7		167
Azul 202		74		67		79		172		38		233		16		1		6		73		12		10		103		5		12		nd		nd		nd		nd		2.9		179
SAMPLE ID.	La		Ce		Pr		Nd		Sm		Eu		Gd		Tb		Dy		Ho		Er		Tm		Yb		Lu		Hf		Ta		W		Tl		Pb		Bi		Th		U
Azul 06	118		474		33		131		27		6.6		17		2.9		16		2.7		6.8		0.9		5.9		0.7		3.5		0.4		2.2		5		68		Nd		8.7		4.1
Azul 09	184		166		33		98		13		2.8		9		1.3		6.5		1.1		2.7		0.4		2.4		0.3		2.4		0.3		10		4.7		133		Nd		4.8		2.5
Azul 10	32		18		6.9		23		3.9		1		3.3		0.5		2.6		0.5		1.2		0.2		1		0.1		0.9		0.1		3		1.1		21		Nd		1.5		1.2
Azul 18	78		178		14		50		9.3		2.5		6.5		1		5.6		1.1		2.8		0.4		2.6		0.4		5		0.6		3.8		5.5		144		0.4		10		4.1
Azul 50	47		150		14		56		14		3.6		12		1.9		10		1.7		5		0.8		4.6		0.6		5.3		0.7		3		2.3		122		0.7		10		2
Azul 81	17		40		3		13		2.7		0.9		2.8		0.5		2.7		0.5		1.6		0.2		1.5		0.2		4.2		0.5		6		0.5		69		Nd		5		2.2
Azul 82	12		31		2.2		9.2		1.9		0.5		1.8		0.3		1.8		0.3		1.1		0.2		1		0.2		2.9		0.3		6		0.5		48		Nd		3.7		1.5
Azul 83	7.5		21		1.3		5.5		1.2		0.4		1.1		0.2		1.1		0.2		0.7		0.1		0.6		0.1		1.5		0.1		3		0.2		37		Nd		1.7		0.8
Azul 88	21		49		3.8		16		3.4		1		3.4		0.6		3.4		0.6		2		0.3		1.8		0.3		4.3		0.6		7		1		51		Nd		6.3		2.2
Azul 89	7.8		22		1.5		6.6		1.5		0.4		1.5		0.3		1.6		0.3		1		0.1		0.9		0.1		1.9		0.2		4		0.3		12		Nd		2.5		1.7
Azul 120	35		42		7.4		34		7.5		1.9		7.3		1.2		6.9		1.2		2.7		0.3		2		0.3		1.4		0.2		nd		3.3		175		Nd		2.6		2
Azul 130	62		135		13		49		10		2.3		7.8		1.3		7		1.3		4		0.6		3.4		0.5		8.3		0.9		6		2.1		159		1.2		14		2.9
Azul 140*	58		114		13		50		10		2.6		9		1.5		8		1.6		4.7		0.7		4		0.6		8.6		0.9		24		2.1		176		1.4		15		3.1
Azul 142*	62		104		14		55		12		2.9		10		1.7		8.9		1.7		4.9		0.7		4.1		0.5		5.9		0.6		16		3.9		283		1.3		9.8		2.4
Azul 162	24		48		4.8		19		3.8		1		3.6		0.6		3.5		0.7		2.2		0.3		1.8		0.2		3.3		0.4		6		0.1		56		Nd		5.6		2.3
Azul 163	21		44		4.5		19		4.1		1.1		4.3		0.7		4.1		0.8		2.5		0.3		1.9		0.3		2.8		0.4		6		0.8		966		11		5.5		2.7
Azul 164	22		48		4.7		19		3.8		1		3.4		0.6		3.3		0.7		2		0.3		1.8		0.3		3.7		0.5		8		0.6		93		Nd		6.8		2.8
Azul 165	20		41		4.2		18		3.8		0.9		3.7		0.6		3.6		0.7		2.1		0.3		1.7		0.2		3.1		0.3		5		0.2		44		Nd		5.1		1.9
Azul 166	15		36		3		13		2.7		0.9		2.5		0.4		2.4		0.5		1.4		0.2		1.1		0.2		2.1		0.2		6		0.7		38		Nd		3.7		2.4
Azul 173*	65		247		18		68		16		3.3		9.9		1.6		8.5		1.5		4.2		0.6		3.7		0.5		5.1		0.5		18		7		165		2		18		3.4
Azul 175*	44		176		11		47		12		2.8		8.1		1.4		7.8		1.4		4.2		0.6		3.9		0.5		7.8		0.9		8		1.5		162		1.1		15		4.7
Azul 176*	39		116		8.8		34		7.1		1.8		5.6		0.9		5.4		1.1		3.2		0.5		2.8		0.4		6.1		0.8		26		3		216		2.5		14		3.4
Azul 178	37		117		9		38		8.9		2.3		7.1		1.3		6.7		1.3		3.8		0.6		3.4		0.5		9.7		1.1		7		2.5		266		1.3		19		4.5
Azul 199	21		51		4.3		17		3.2		0.9		3		0.5		2.8		0.6		1.7		0.3		1.5		0.2		3.9		1.3		4		0.4		52		Nd		6.8		17
Azul 201	14		32		2.8		11		2		0.5		1.9		0.4		2		0.4		1.3		0.2		1.1		0.2		3.3		0.3		3		0.5		94		Nd		6.7		2.1
Azul 202	13		26		2.2		8.6		1.7		0.5		1.5		0.3		1.6		0.3		1.1		0.2		1.1		0.2		3.8		0.4		3		0.6		12		Nd		5.7		1.6

 

 

Table 11 – Chemical composition of manganese-poor gray shales and siltstones, in general pyrite poor. Cont´n.

SAMPLE ID.				SiO2				Al2O3					Fe2O3					MnO				MgO						CaO				Na2O					K20					TiO2				P2O5				LOI
Azul 13				65.12				16.26					5.93					0.04				0.80						0.03				nd					2.25					1.52				0.09				7.18
Azul 15				68.73				13.13					7.95					0.02				0.32						0.02				0.01					1.06					1.26				0.13				6.16
Azul 24				72.71				10.53					5.26					0.10				0.64						0.02				0.02					1.72					0.72				0.09				7.09
Azul 25				57.35				10.43					21.28					0.14				0.09						0.04				0.02					0.20					0.53				0.22				8.55
Azul 93				64.87				11.45					15.17					0.04				0.16						0.02				0.09					0.47					0.52				0.13				7.30
Azul 98				62.91				11.47					9.86					0.08				0.82						0.02				0.08					2.20					0.56				0.17				10.87
Azul 102				72.37				15.15					2.57					0.13				0.97						nd				0.08					2.35					0.72				0.04				6.02
Azul 110				71.96				13.68					4.42					0.07				0.36						0.06				0.22					1.26					0.57				0.10				6.26
Azul 114				63.98				14.22					5.15					0.31				2.83						0.14				0.09					6.23					0.94				0.02				6.42
Azul 115				60.31				13.09					8.44					0.27				2.25						0.32				0.09					3.64					0.79				0.01				10.44
Azul 153				68.00				14.11					3.51					0.08				1.14						0.02				0.12					4.30					0.58				0.04				6.95
Azul 156				67.20				14.31					3.09					0.09				1.48						nd				0.15					4.23					0.70				0.04				7.69
Azul 158				61.83				13.35					5.71					0.44				4.37						0.02				0.04					3.90					0.65				0.03				9.05
Azul 160				63.20				13.07					4.78					0.57				3.40						0.02				0.04					4.21					0.63				0.03				8.96
Azul 161				54.80				11.86					10.27					1.30				3.29						0.05				0.06					3.67					0.55				nd				12.97
Azul 84				65.59				14.28					4.09					0.32				3.89						0.02				0.04					1.19					0.89				0.04				8.82
SAMPLE ID.			V				Cr				Co				Ni						Cu				Zn						Ga				Ge				As				Rb				Sr				Y
Azul 13			228.98				101.97				1.19				20.68						21.73				-30.00						20.60				1.99				21.13				91.38				25.25				68.91
Azul 15			159.29				92.44				nd				Nd						33.28				37.43						17.73				1.79				25.38				45.33				32.36				53.37
Azul 24			228.68				125.81				2.51				24.42						141.31				-30.00						16.43				2.69				126.55				67.45				33.05				26.58
Azul 25			290.38				104.90				2.28				22.28						269.48				-30.00						11.90				1.97				9.42				9.09				39.00				23.61
Azul 93			102.25				102.01				4.73				41.02						34.70				92.51						17.18				2.21				13.92				27.17				10.52				17.55
Azul 98			264.13				108.46				12.58				40.52						317.31				50.24						13.48				1.55				70.83				93.47				30.58				31.03
Azul 102			93.92				139.20				6.45				43.34						46.68				55.57						19.30				1.29				nd				111.26				6.88				13.07
Azul 110			67.60				95.14				nd				Nd						85.45				nd						20.60				1.71				nd				49.61				17.27				65.94
Azul 114			115.26				169.48				17.40				35.19						104.32				144.09						20.79				1.67				10.66				157.30				46.12				9.63
Azul 115			107.87				173.44				27.82				112.57						90.17				222.41						20.58				nd				nd				111.99				26.76				9.75
Azul 153			100.00				113.00				9.00				44.00						118.00				117.00						16.00				2.00				10.00				134.00				43.00				24.00
Azul 156			143.00				125.00				7.00				46.00						86.00				147.00						17.00				2.00				9.00				153.00				37.00				21.00
Azul 158			149.00				114.00				31.00				81.00						189.00				526.00						16.00				nd				15.00				144.00				29.00				26.00
Azul 160			242.00				121.00				32.00				108.00						465.00				386.00						16.00				nd				36.00				154.00				33.00				24.00
Azul 161			148.00				114.00				35.00				115.00						12.00				382.00						14.00				nd				156.00				123.00				35.00				31.00
Azul 84			110.26				185.62				33.32				109.71						91.25				106.36						18.02				2.03				nd				55.58				12.62				15.83
SAMPLE ID.		Zr				Nb				Mo				Ag			In			Sn			Sb			Cs			Ba					Hf				Ta			W			Tl				Pb				Bi
Azul 13		215.16				9.27				10.35				3.53			0.13			1.93			0.98			3.27			447.28					5.85				0.74			1.94			0.19				9.75				nd
Azul 15		262.56				9.51				4.94				2.27			Nd			1.18			1.39			1.72			666.95					7.04				0.75			3.02			0.22				26.54				nd
Azul 24		154.04				7.75				24.09				7.94			Nd			1.35			5.51			2.69			233.30					4.60				0.60			7.80			0.20				22.69				nd
Azul 25		86.14				5.59				23.66				5.25			0.21			Nd			1.45			0.53			241.80					2.67				0.35			12.69			0.07				72.71				nd
Azul 93		191.38				8.56				2.66				Nd			Nd			1.13			1.01			1.02			349.30					5.16				0.83			2.05			0.12				24.90				nd
Azul 98		143.28				7.31				38.35				0.90			Nd			Nd			3.40			4.27			244.97					4.55				0.63			2.73			0.73				110.66				nd
Azul 102		234.81				9.28				7.31				Nd			Nd			1.74			Nd			5.72			89.50					6.71				1.02			5.74			0.51				24.92				nd
Azul 110		235.27				9.60				nd				Nd			Nd			Nd			1.26			1.73			189.45					6.48				0.75			2.07			0.25				14.76				nd
Azul 114		219.41				8.45				2.60				Nd			Nd			1.65			1.10			6.20			603.91					6.35				0.99			2.50			1.67				34.75				nd
Azul 115		180.97				6.83				nd				Nd			Nd			1.31			1.27			5.56			512.91					5.20				0.67			2.44			1.27				25.48				0.43
Azul 153		176.00				6.00				20.00				Nd			Nd			1.00			0.70			4.00			578.00					4.70				0.50			3.00			1.10				59.00				nd
Azul 156		188.00				6.00				15.00				Nd			Nd			1.00			1.00			5.00			620.00					5.10				0.60			3.00			0.70				139.00				2.10
Azul 158		156.00				7.00				4.00				0.70			Nd			1.00			0.90			5.40			467.00					4.60				0.60			5.00			1.60				50.00				nd
Azul 160		169.00				7.00				18.00				0.70			Nd			1.00			4.20			4.90			475.00					4.90				0.60			6.00			1.60				63.00				nd
Azul 161		137.00				7.00				2.00				0.70			Nd			Nd			0.60			3.70			428.00					4.00				0.50			5.00			0.20				40.00				nd
Azul 84		211.14				9.02				nd				Nd			Nd			1.28			1.65			2.71			253.70					5.96				0.85			5.44			0.42				22.65				nd
SAMPLE ID.	Th				U				La			Ce				Pr			Nd					Sm			Eu			Gd			Tb			Dy				Ho			Er		Tm				Yb				Lu
Azul 13	9.77				3.61				17.57			28.87				4.08			21.37					5.63			1.66			6.64			1.22			7.29				1.67			4.96		0.77				4.52				0.64
Azul 15	11.03				3.49				22.47			36.05				5.15			24.40					4.99			1.47			5.52			1.01			6.03				1.36			4.02		0.64				3.62				0.52
Azul 24	8.67				3.25				33.89			60.90				6.90			25.90					4.93			1.40			4.53			0.73			3.80				0.80			2.13		0.32				1.89				0.28
Azul 25	4.62				4.34				23.74			51.31				4.23			15.69					3.01			1.16			2.75			0.51			3.02				0.70			1.94		0.29				1.72				0.25
Azul 93	11.33				3.00				13.22			19.98				2.17			9.66					2.57			0.74			2.97			0.52			3.00				0.63			1.98		0.30				1.92				0.30
Azul 98	7.99				3.00				31.57			64.28				6.38			29.57					6.44			2.01			6.70			1.05			5.23				0.95			2.63		0.33				2.02				0.28
Azul 102	9.69				2.28				31.97			68.76				5.65			22.77					3.96			0.92			2.89			0.44			2.40				0.47			1.61		0.24				1.58				0.23
Azul 110	13.25				6.76				26.62			47.30				4.72			20.71					4.82			1.41			6.45			1.43			9.79				2.15			6.39		1.00				6.21				0.92
Azul 114	8.47				3.09				11.39			22.91				2.22			10.02					2.15			0.67			2.12			0.36			2.02				0.39			1.28		0.20				1.37				0.21
Azul 115	6.94				2.75				14.93			31.05				2.90			12.74					2.61			0.67			2.21			0.36			1.93				0.37			1.18		0.18				1.13				0.18
Azul 153	9.30				3.30				32.60			57.90				7.12			27.30					5.10			1.28			4.80			0.70			3.90				0.80			2.30		0.32				1.80				0.29
Azul 156	8.50				3.00				25.00			41.70				4.63			18.20					3.50			0.95			3.30			0.50			2.90				0.60			1.90		0.29				1.70				0.26
Azul 158	7.70				2.70				24.80			51.90				5.51			22.50					4.30			1.11			3.70			0.60			3.60				0.70			2.20		0.32				2.00				0.26
Azul 160	8.20				3.30				26.20			51.00				5.32			22.00					4.40			1.18			4.10			0.70			3.50				0.70			2.10		0.31				1.90				0.27
Azul 161	6.90				2.60				22.70			45.20				4.72			19.20					4.10			1.04			4.40			0.80			4.40				0.90			2.60		0.36				1.90				0.27
Azul 84	8.78				3.07				20.37			41.69				3.91			16.88					3.71			0.95			3.45			0.54			3.06				0.57			1.83		0.27				1.61				0.24

 

The content of the trace elements (Table 10), when compared to the shales generally and Mn-poor black shales from Azul specifically (Table 11) and to the crustal average, the manganese-rich dark grey shales of Azul are enriched in Ni, Cu, Zn, As, Mo, Ag and Sb and they reflect the dominance of carbonaceous organic matter with sulphides. The contents of Ni, Co and Cu also show a significant positive correlation to Mn contents, in this case to Mn-oxyhydroxides. These manganese-rich rocks/ores are also relatively enriched in Ba, Pb, and Sr and even REE, showing an additional slightly good positive correlation to MnO. Of these elements, Ba is by far the most abundant, and forms the cryptomelane-hollandite series. Among the REE, the samples that present rhodochrosite with much more frequency are poorer in these elements, being below the Earth crust and PAAS levels, while in the samples dominated by cryptomelane and cryptomelane-hollandite, REE, especially cerium, are slightly enriched.

Manganese-poor grey shales, composed of illite-sericite, quartz, chlorite and carbonaceous organic matter, usually contain pyrite and are then dominated by SiO₂, Al₂O₃, Fe₂O₃, MgO (0.3 to 4.4%) and K₂O (0.2 to 6.2%), in addition to TiO₂ (0.5 to 1.5%) (Table 10). In turn they are comparatively poor in MnO, CaO, Na₂O and P₂O₅ (0.01 to 0.22).

In terms of trace elements, the dark grey shales are distinguished from manganese-rich dark or black shales mainly by their lower contents of Co, Ni, Cu, Zn, As, Mo, Ag and Sb, and Ba, Sr, Pb and REE. This is explained by the absence of Mn-oxyhydroxides, mainly cryptomelane-hollandite, which are carrier of Cu, Co and Ni, as well as Ba, Sr, Pb and REE. Alternately, these Mn-poor shales display much more pyrite and organic matter, that carries (As, Mo, Sb, Ag).

 

DISCUSSIONS

Geological Records – The Azul manganese oxyhydroxides lenses hosted by carbonaceous black shales, siltstones/mudstones and red fine sandstones, can be well correlated to well-known Paleoproterozoic to Neoproterozoic manganese deposits worldwide, especially those of 2.4 to 2.0 Ga, such as Kalahari in Africa (Roy, 2006; Kuleshov, 2017 Dubois et al., 2017) and the Upper Proterozoic marine sequence of the Penganga Group, Andhra Pradesh, India (Roy et al., 1990). They are associated with black shales, deposited in the platform shelf by transgression episodes, at the time of the increase of the oxygen in the terrestrial atmosphere and burial of the organic matter in increasing production.  These conditions can be compared to those described by Frakes and Bolton (1992), combining specific ocean chemistry, sea level and climate on the formation of primary sedimentary manganese deposits. The age obtained here, approximately 2.1 Ga, is in accordance with the data reported by Mougeot et al (1996) and Fabre et al. (2011) and discussed by Cabral et al (2013, 2017), considering mainly the black shales associated with Carajás BIF Formation. The siltstones and red sandstones represent the partial terrestrial contribution and reflect the availability of oxygen for Fe and Mn oxidation, and for organic matter hosted in the dark to black shales (Roy, 2006; Dubois et al., 2017).

Even in the Mesoarchean deposits, in which the sedimentary sequence has already been modified by metamorphism, tectonic deformations and hydrothermal activities, this picture is still possible to observe (Ghosh et al. 2015). The mode of geological occurrence of these layers in Azul is fully in agreement with the dominant general picture in the Early Proterozoic in all general aspects, including the accumulation of organic matter and the marked presence of black shales and Mn oxides. Some deposits in turn are dominated by Mn carbonates (rhodochrosite, manganoan dolomites, kutnahorite and Mn-calcite), such as the Paleoproterozoic deposits from Franceville in Gabon (Dubois et al., 2017) and Kalahari (Gutzmer & Beukes, 1996; Roy, 2006; Gutzmer et al., 2012); the Neoproterozoic  deposits from Datangpo formation, South China (Yu et al., 2016, 2019), and Molango in Mexico (Roy, 2006); Mesozoic deposits from Úrkút in Hungary (Polgári et al., 2016);  and Cenozoic deposits from Nikopol (Ukraine) and Chiatura (Georgia) (Roy, 2006). In all of these deposits over a wide geological time, rhodochrosite and the other carbonate minerals are of diagenetic formation, always related to oxidation of organic matter and reduction of previous amorphous manganese oxides (Tang & Liu, 1999; Roy, 2006; Gutzmer et al., 2012, Polgári et al, 2016; Yu et al., 2019).

In the Azul manganese deposits, rhodochrosite, however, is rare. This statement contrasts with what was presented by Beauvais (1984) and Beauvais et al. (1987) after Bernardelli and Beisiegel (1978), rhodochrosite being the primary source for lateritic supergene manganese ore from the Azul deposit. Two manganese-bearing units, rhodochrosite being the main mineral, were represented by carbonaceous manganese-bearing limestones or marls (Bernardelli & Beisiegel, 1978; cited by Beauvais et al., 1987). These two units were only found in a single drill hole (DH 5, Figure 2) carried out in the early 1970s. In the countless deep drill holes made later, these two rhodochrosite-bearing units were not identified. However, rhodochrosite has been identified as a sporadic mineral in isolated samples of black shales and in venules, but never as a main rock mineral in Azul deposits in the last four decades. The extreme variations in the chemical composition of rhodochrosites or even manganoan dolomites are common in the most diverse manganese deposits and are always linked to the diagenetic formation in the sedimentary package from Paleoproterozoic through Cenozoic (Fan & Yang, 1999; Varentsov, 2002; Roy, 2006; Polgári et al., 2016; Yu et al., 2016; Dubois et al., 2017; Kuleshov, 2017; Rajabzadeh et al., 2017).

Economic sedimentary deposits of manganese oxyhydroxides mainly as cryptomelane are also known in the Neoproterozoic, with strong evidence of mediation by microorganisms, such as Urucum, in Mato Grosso, Brazil (Biondi et al., 2020),

Several of these Early Proterozoic deposits are metamorphosed (India and Brazil; in Brazil: Conselheiro Lafaiete, Buritirama, Sereno, Serra do Navio and Jequié) in general Archaean (≥ 2.6 Ga) (Roy, 2006; Kuleshov, 2017; Araújo & Sousa, 2018; Salgado et al., 2019). The presence of illite, kaolinite and amorphous organic matter clearly shows that the Azul manganese deposits, and its hosting rocks were not metamorphosed, although the sedimentary packets show deformation structures. The studies of Fabre et al (2011) came to the same conclusions, with maximal to anchimetamorphic conditions indicated by sericite formation. Some ⁴⁰K/³⁹Ar ages performed on red hosting siltstones (Table 3) show ages compatible with deposits in the range of 2.3 to 2.0 Ga., in the range of single age of 1.8 Ga presented by Gomes et al (1975), Bonhomme et al (1982), Mougeot et al (1996) and Fabre et al. (2011).

The oldest folding consists of tight folds with E-W axis (Pinheiro & Holdsworth, 2000), observed in the Mine Carrapato and Mine 1 (Figures 5 and 6) and is compatible with the Trans-Amazonian (Transamazônica) tectonic deformation vector (2.1 to 1.8 Ga) (Cabral et al., 2017). The strong degree of deformation, paradoxically incompatible with the absence of metamorphism or anchimetamorphism only, can be interpreted as an active deformation that occurred when the sediments were still unconsolidated, providing a predominantly plastic deformation, which is consistent with the age of the Águas Claras sediments. This is indicated by the common presence of convolute structures observed in the mudstones/siltstones.

The next folding event is formed by smooth folds of the N-S axis, with a long wavelength, which forms sequences of synforms and antiforms, exemplified by the large antiform of the western portion of Mine 1 (Figures 2 and 5). This deformation is compatible with the Brazilian (Brasiliana) tectonics recognized by Pinheiro & Holdsworth (2000).

The main fault zone in the region is the Carajás Fault installed at 2.7 Ga, which was reactivated at 1.8 Ga, affecting the manganese mineralization by remobilization Mn-oxyhydroxides (pyrolusite, nsutite, manganite) followed by an intense silicification and kaolinization of the sediments established along faults and apical fold zones.

 

Mineralogical Records – In addition to the predominant cryptomelane and/or cryptomelane-hollandite, other Mn-oxyhydroxides, such as birnessite, todorokite, lithiophorite, nsutite, pyrolusite, manganite, etc., constitute the ore manganese minerals in the shales and their fault-deformed bodies. This mineralogical composition, although varying in the proportions of each species, and in the presence or absence of some of them, is typical of most of the world’s deposits of sedimentary manganese, from Proterozoic to Cenozoic (Roy, 2006; Polgári et al., 2012; Johnson et al., 2016: Biondi and Lopez; Kuleshov, 2017; Molnár et al., 2017).

In the Azul deposits, cryptomelane and cryptomelane-hollandite (Table 2) are the most abundant manganese ore mineral, however in the world deposits according to Kuleshov (2017) they lose to rhodochrosite and braunite. Braunite was not identified in Azul, and rhodochrosite is very restricted, although mentioned as protore for supergene ore formation in first publications (Bernardelli & Beisiegel, 1978 and Beauvais et al., 1987) Although rhodochrosite may constitute an important mineral ore, it is also the Mn-proto mineral for supergene enrichment, which is dominated by manganese oxyhydroxides in Africa, China and Brazil (Maynard, 2004; Chisonga et al., 2012; Putter et al., 2015; Deng et al., 2016; Kuleshov, 2017; Scarpelli & Horikava, 2017). However, cryptomelane would be the most abundant when we consider the deposits formed only by Mn oxyhydroxides. Cryptomelane and braunite are also the main manganese ores in the large Neoproterozoic Urucum deposit, which is diagenetic in origin (Biondi and Lopez, 2017); of the Neoproterozoic Mn-oxyhydroxides, although without reporting on which minerals, of Nanhua basin Mn deposits, South China, which are related to hydrothermal activity and glaciation (Yu et al., 2016) and even in Miocene Mn-oxide hydrothermal mineralizations of Boléo, Mexico (Conly et al, 2011).

As exposed, the minerals of the cryptomelane group, i.e.  ryptomelane (K) hollandite (Ba), strontiumelane (Sr) and coronadite (Pb), especially the first two minerals, have been reported in several manganese deposits from Proterozoic to Cenozoic, being of sedimentary (early diagenesis), metamorphic, hydrothermal and supergene origin. For Johnson et al (2016), the manganese mineral formation begins with the deposition of Mn(IV) oxides, leading to the early diagenesis production of carbonate phases, such as rhodochrosite and so on. At Azul, Mn oxides alone predominate, that is, the conditions for Mn(IV) reduction and OM oxidation did not prevail.

In the Azul deposits, EPMA analyses show the chemical variations characteristic of the main members of the group in which the trace elements (Co, Ni, Cu and Zn) tend to concentrate in the hollandite member (Figure 15 for Zn, Ni and Cu), Cu in cryptomelane. Based on the K₂O and BaO values ​​obtained by EPMA, it was possible to individualize three chemically distinct cryptomelane-hollandites (Figure 16): Ba-free cryptomelane (group 1) representing those typically related to amorphous Mn-oxyhydroxides and sedimentary in nature; K₂O/BaO = 3.0 cryptomelane-hollandites (group 2), related to deformation structures and epigenetic in nature (Table 2); and K2O/BaO variable cryptomelane-hollandites, which may be related to early diagenetic recrystallization of amorphous Mn-oxyhydroxides (Table 2).

 

 

 

Costa et al. (2005) and Araújo & Sousa (2018) found that cryptomelane and cryptomelane-hollandite also constituted the main minerals of supergene deposits of lateritic origin (Table 2), together with todorokite, lithiophorite, pyrolusite, nsutite and Mn-free minerals such as gibbsite, kaolinite, goethite, and haematite.   For rhodochrosite, even as a rare mineral in the Azul mine, its diagenetic origin should be similar to those found in most manganese deposits associated with Mn-carbonate-rich shales, marl and limestones from Proterozoic through Cenozoic. The extreme variation of the chemical composition of Azul rhodochrosite indicates also its diagenetic origin as observed by Kuleshov (2017) in other deposits in a limited anoxic environment (Du et al., 2011). The diagenetic activities in Azul were more restricted, certainly as a consequence of the high oxygen availability at their precipitation time. The large set of red siltstones to sandstones that host the mineralization, confirm the oxygen availability, consuming much of it and protecting the in-built lenses of carbonaceous shales.

The constant presence of pyrite, dispersed in the shales and manganese-rich black shales, support a possible, however restricted, microbial marine sulphate reduction during the early diagenesis reactions, preserving a good portion of the carbonaceous organic matter. Pyrite and kaolinite veinlets, in turn, indicate the tectonic remobilization and epigenetic formation (Table 2).

 

Microbial Activity and Stromatolite-Like Structures – Stromatolites are organic sedimentary structures produced by sediment trapping, binding and/or precipitation as a result of the growth and metabolic activity of microoorganisms (Perry et al., 2007). The ubiquity of carbonaceous organic matter (COM) associated with manganese mineralizations is undeniable and has been linked to it since the Archean, represented by bacteria, algae, and protozoa (Roy, 2006; Maynard, 2004; Kuleshov, 2017; among many others), compatible with its relationship with the black shales. The abiotic oxidation of Mn(II) is kinetically inhibited below a pH of 9, and therefore Mn(II) oxidation in circumneutral natural aquatic environments is predominantly driven by biological processes (Zeiner et al., 2006). Stromatolite-like structures and microbialites have also been reported in several ancient manganese deposits, still recorded in the Archean and extending to the Proterozoic in Chitradurga Group (India), Gandarela Formation (Brazil) and South Africa (Roy, 2006; Kuleshov, 2017); at some extension in the Neoproterozoic deposit at Urucum, in Mato Grosso, Brazil (Biondi And Lopez, 2017 and 2020) and Datangpo Formation manganese deposits, South China (Yu et al., 2019); in the Mesozoic deposits of Úrkút (Polgári et al., 2012, 2016; Molnár et al., 2017); and Abadeh-Tashk area in Iran (Rajabzadeh et al., 2017) and even in recent deposits in deep-sea nodules (Akai et al., 2013). COM are a fundamental feature of sedimentary manganese formations throughout the Earth’s evolution (Maynard, 2004; Roy, 2006; Kuleshov, 2017). Economic sedimentary manganese oxyhydroxide deposits of Urucum show evidence of microbial mediation by microorganisms, especially the layers of manganese ore Mn-2 and Mn-3 with kremydillite (Biondi et al., 2020) and stromatolite-like structure just below the Mn-1 layer (Biondi and Lopez, 2017).  Even in the youngest deposits of the Jurassic-Cretaceous, syngenetic and diagenetic manganese oxyhydroxides have formed, almost mediated by microorganisms, as well as in recent deep-sea nodule deposits (Akai et al., 2013). Microbial mediated reactions between manganese oxide and organic matter promote the formation of the manganese carbonate in several manganese deposits from Paleoproterozoic through Cenozoic (Gutzmer and Beukers, 1996: Fan et al., 1999; Polgári et al., 2012; Dubois et al., 2017; Yu et al., 2019). Stromatolite-like macroscopic features with Fe-rich biomats as Fe-microbialites have been described in Iranian manganese deposits (Rajabzadeh et al, 2017). Additionally, to Mn-oxyhydroxides occur silicates and carbonates, being the oxyhydroxides in part amorphous (Rajabzadeh et al, 2017), similarly to the world-class Jurassic manganese Úrkút deposit, in Hungary, with cherty Fe-Mn oxide ore being related to stromatolite-like structures.  The bulk mineralogy of this ore comprises ferrihydrite, goethite, hollandite, pyrolusite, cryptomelane, todorokite, or birnessite, hausmannite and manganite, (Polgári et al, 2012; Molnár et al., 2017), even in the manganese black shales with predominate rhodochrosite microbialites (Polgári et al., 2016).

Recently, manganese rock material with stromatolite-like structures was found in mine 2 (Figures 5 and 17) of the Azul manganese deposits. Two samples were collected, ESTMn01 and MUGEO 2460. The large and most representative sample MUGEO 2460 was deposited at Museu de Geociências (Geosciences Museum) of Federal University of Pará, in Brazil. This sample (Figure 17) is dark grey in colour, very dense, and dominated by Mn-oxyhydroxides. Externally, it consists of several mat-mounds (Figure 17 A) (see Polgári et al., 2012 p. 96 Figure 7i and p. 98 Figure 8 e-h for comparison) formed by plane-parallel fine layers (Figure 17 A and B). They are sectioned by kaolin and quartz venules, sometimes with pyrite. Internally, the mounds converge first to a mass that is slightly stratified and then to a mass formed by centimetre large nodules of the same material (Figure 17 C and D), which describe layers, in part cracked and cross-cut by massive Mn-oxides (with metallic lustre and grey in colour) and Fe-hydroxides (brownish) (Figure 17 C and D). Microdiapires and microcrystalline quartz or quartz fine sand injections can also be observed (Figure 17 C and D). As a whole, part of this sample can be correlated to that described by Molnár et al. (2017, p. 844). Cryptomelane and todorokite are the main mineral of the sample MUGEO 2460, being cryptomelane-hollandite in the sample ESTMn01.  Cryptomelane is a unique mineral of the massive grey crack’s infillings (Figure 17 C) displayed in MUGEO 2460 (Figure 17 C), while quartz, kaolinite and goethite are the other minerals of both samples, which generally constitute a thin capping of the entire structure (Figure 17 A).

 

 

The chemical composition of two Mn-rich stromatolite-like samples (Table 12) shows content similarities for (Ca, Mg, Na, Mn, P, LOI, C, S, Cr, Cs, Ga, Rb, Sn, Y, As, Bi, In, Sb, Co, Li, Zn, La, Gd, Tb, Tm, Yb, Lu); however, the sample ESTMn01 shows higher values for Al, Fe, K, Ti, Cr, Ba, Sr, Rb, Zr, Nb, Ta, Hf, Sc, Th, U, Hg, Tl, Ce, La, Nd, Pr and Sm. Aluminium reflects the presence of kaolinite and K, Ba, Sr, Rb, Tl and LREE (Ce through Sm) + Sc of cryptomelane-hollandite, the main mineral of the samples; Zr and Hf, Nb, Ta, Th and U exhibit some isolated zircon grains and Fe, Cr and Hg the goethite. What draws much attention in these two samples is the high levels of SiO₂ and MnO, which together and alone make up 64.8 and 73.0%, respectively, and the contents of Fe₂O₃ and Al₂O₃ are below 5.9%. The sample MUGEO 2460 presents the higher values ​​of Si, V, Cs, Dy, Er, Eu, Gd, Tb, Yb, Ho, Lu, Y, As, In, Sb, Se, Te, Cd, Co, Ni, Cu, Mo, and Pb. This sample is dominated by todorokite and cryptomelane, both (mainly todorokite) being carriers of HREE + Y and V, Cs, As, In, Sb, Co, Ni, Cu, Pb, Mo, Se, Te and Cd. The two bulk samples show very low contents of P, C and S (Table 12). The order of high magnitude of the contents of much of these elements (V, Co, Ni, Cu, Pb, Mo, Se, Te and Cd) for sample MUGEO 2460 may support a possible hydrothermal contribution and/or still hydrogenous nodules from the ocean bottom (Glasby et al., 2005; Polgári et al., 2012; Jones et al., 2013; Molnár et al., 2017) as the primary source for manganese. These chemical elements together with many others such as Mn, Fe, Mg, S, P, and so on (As, Co, Ce, Ba, Sr), also belong to bio-essential elements reported by Polgári et al. (2012) and Zarasvandi et al. (2013) in the Jurassic Mn-carbonate-bearing black shales with microbial activity from Úrkút (Hungary). Therefore, the mineralogical and chemical differences between the two stromatolite-like structures are clear, showing that the microbial activity and the continental and hydrothermal contributions changed locally, the latter being perhaps the main source of these metals. It also became clear that the mineralogy of the Mn-oxyhydroxides in turn controls the fixation of these chemical elements. The high contents of Si and Mn suggest a microbial silicium bacteria.

 

Table 12 – Chemical composition of two equal manganese-rich stromatolites-like structures (samples 1: ESTMn01 and 2: MUGEO 2460) from Azul mine 2 composed of distinct manganese oxides (cryptomelane-hollandite, and todorokite and cryptomelane, respectively).

SAMPLE ID:	SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O	K2O	Cr2O3	TiO2	MnO	P2O5	SrO	BaO	LOI	Total
1.) ESTMn01	18.7	5.9	5.07	0.04	0.32	0.06	2.13	0.01	0.24	46.1	0.15	0.01	0.83	11.4	90.96
2.) Mugeo2460	24.7	3.95	1.73	0.04	0.29	0.07	0.89	0.01	0.16	48.3	0.11	<0.01	0.01	10.3	90.56
	C	S	Ba	Ce	Cr	Cs	Dy	Er	Eu	Ga	Gd	Ge	Hf	Ho	La
1.) ESTMn01	0.09	0.01	7120	62.1	50	1.95	1.98	1.07	0.53	30.7	2.19	<5	2.8	0.35	14
2.) Mugeo2460	0.09	0.02	101	31.6	40	2.26	2.74	1.35	0.62	30.7	2.39	<5	1.6	0.55	12.6
	Lu	Nb	Nd	Pr	Rb	Sm	Sn	Sr	Ta	Tb	Th	Tm	U	V	W
1.) ESTMn01	0.13	2.7	13.7	3.92	54.5	3.09	1	70.3	0.2	0.35	4.61	0.2	1.65	55	<1
2.) Mugeo2460	0.15	1.7	9.6	2.71	40.9	2.16	1	18.5	0.1	0.4	3.11	0.21	0.85	201	<1
	Y	Yb	Zr	As	Bi	Hg	In	Re	Sb	Se	Te	Tl	Ag	Cd	Co
1.) ESTMn01	10.6	1.09	102	14.2	0.06	1.045	0.016	0.002	0.74	0.5	0.11	4.77	<0.5	6.6	262
2.) Mugeo2460	13.6	1.13	58	17.4	0.06	0.055	0.019	<0.001	0.85	3.2	0.27	0.17	<0.5	22.6	295
	Cu	Li	Mo	Ni	Pb	Sc	Zn
1.) ESTMn01	302	10	1	62	78	15	667
2.) Mugeo2460	719	10	35	190	223	5	655

 

SEM scattering images allow some microbial structures in the manganese/silicium-rich stromatolite-like structures to be individualized (Figure 18). The SEM/EDS analyses also support the Mn-bearing silicium bacteria and Mn-bearing protozoan. Zeiner et al. (2006) experimentally discussed the importance of microorganisms, such as tetrahymena thermophyla protozoa and oxidant bacteria such as leptothrix discophora SS-1, in the formation of manganese oxides on the basis of predator and prey, respectively, and concluded that the extracellular Mn-oxide coating protects cells from predators. The importance of microbial activity for fixing and oxidizing Mn in solution and fixing in sediments, is widely discussed and has been recognized in manganese deposits from Paleoproterozoic to the Recent (Yu et al. 2016; Shiraishi et al., 2016; Sinisi et al., 2018).

 

 

After the micromorphology and micrometre chemical analyses, we interpret these possible microbes as being equivalent to a silicium bacterium, bacterial colonies (moulds, bulbous and micro-nodules) and locally fungi-like based on Nayak et al. (2013), Rajabzadeh et al. (2017) and Park et al. (2018). All of the analysed points show the presence of carbon, reaching up to 38%, with silicon ranging from 5 to 52% (point 7 manganese from 0.8 to 45.8%, potassium from 0.10 to 1.70% and barium, 0.19%, in addition to Fe, Al, Ti, P and Na, and of course, oxygen (Table 13). As the contents of Mn and K (?) increase, those of Si and C decrease, suggesting a capture of Mn and K by exchange with Si and C, while those of oxygen practically do not vary (Figure 19). These chemical elements are the major chemical components of the manganese ore of Azul. Similar EDS chemical analyses of Mn-bearing microbes are discussed by Nayak et al. (2013), as well as by Biondi & Lopez (2017) for several microbialites related to kremydillites, some of them rich in C, Si and Mn. The marked presence of carbon and features suggestive of microbial activity leave no doubt as to the participation of microorganisms in the fixation and deposition of manganese oxides in the Azul deposit during Early Proterozoic, at least partially, similarly to most manganese deposits around the world (Roy, 2006; Polgári et al., 2012, 2016; Kuleshov, 2017; Rajabzadeh et al, 2017; Molnár et al., 2017; Biondi and Lopez, 2017; Biondi et al., 2020).

 

Table 13 – SEM/EDS chemical analyses (Wt. % on 100% basis) of probable protozoan and bacteria and some carbonaceous records in the Mn-bearing stromatolite-like structure (Sample MUGEO 2460) from Azul manganese deposit.  Point analyses on the SEM image are indicated in the figure (Figure 18). nd: not detected.

Elements	Point 1	Point 4	Point 3	Point 7	Point 2	Point 5	Point 6
	Mn-rich Si bacteria/ protozoan + COM	Mn-bearing	Mn-bearing	Silicium bacteria	Mn-rich Silicium bacteria	Mn-rich Silicium bacteria	Mn-rich Silicium bacteria
		Silicium bacteria + COM	Silicium bacteria + COMmatter
Carbon	23.75	38.37	16.92	1.12	3.12	2.55	2.24
Oxygen	39.02	39.55	44.17	44.63	35.94	34.61	32.39
Silicium	5.10	9.86	20.74	52.18	7.57	14.34	6.85
Aluminum	2.46	3.24	2.79	0.55	5.34	4.36	5.18
Manganese	20.43	5.56	13.04	0.80	43.09	40.34	45.80
Iron	5.33	0.67	0.61	0.12	1.93	1.45	4.38
Magnesium	0.24	0.31	0.27	0.03	0.37	0.42	0.55
Potassium	0.96	0.99	0.79	0.10	1.70	1.20	1.50
Barium	0.07	nd	0.13	0.17	0.08	0.11	0.19
Sodium	1.11	0.39	0.12	nd	0.24	0.07	Nd
Phosphor	1.12	0.55	0.27	0.22	0.44	0.40	0.50
Titanium	0.06	0.07	nd	nd	0.06	nd	0.38

 

 

 

Fault-Hosted Manganese Ore: Tectonic Mineralization – The Trans-Amazonian tectonic deformation, which reactivated the Archean Carajás fault, seems to have hit the manganese-rich sediments of Azul right at the end of its sedimentation. However, north-south and northwest-southeast faults and shear zones supposedly correlated to the Brazilian tectonic deformation Cycle overprinted the entire sequence (Araujo & Sousa, 2018). They developed intense cracking, micro faults, folds and micro folds, whose voids were infilled with pyrolusite, manganite and perhaps nsutite, being massive to crystalline, with high manganese ore content. Structural controls of fault-hosted manganese are noticed at Woodie Woodie, East Pilbara, Australia (Jones et al., 2013), Hotazel formation, Kalahari manganese field, South Africa (Gutzmer and Beukes, 1996; Roy, 2006; Vafeas et al, 2018).

Kaolin Veinlets and Bedding Laminae – The grey shales and the dark grey manganese-bearing shales and manganese ores were strongly affected by pockets and venules of white kaolin in almost all of their extensions and especially in zones of greater tectonic deformation represented by faults, folds and shear zones. In general, these venules may contain isolated crystals from sulphides, commonly pyrite, and rhodochrosite. The main mineral of kaolin venules is obviously kaolinite, which is microcrystalline, in well-formed crystals, such as platelets or stacking of pseudo-hexagonal platelets, at the level of almost booklets, with no preferred direction (Figure 20). The chemical analyses by EDS confirm the chemical composition for kaolinite, and the picks for Mn and K show their relationship with the hosting ore represented by cryptomelane. These characteristics of kaolinite and of the fault-hosted Mn-oxyhydroxides that were shown earlier, reinforce the significant range of the tectonic deformation related to faults and fault zones, mainly.

 

 

Major and Trace Element Contribution –The chemical composition of the manganese-bearing shales, when compared to some world-class deposits, in terms of K, Mg, Ca and Ti, are similar to hydrothermal and/or hydrogenous nodules, modern deposits, and the Chinese Neoproterozoic (Table 14). Alternately, the low levels of Na correlate with the giant deposits of Kalahari (Maynard, 2004), the metasedimentary manganese (Late Archean) of Western Dharwar Craton, southern India (Sethumadhav et al., 2010), and Urucum (Viehmann et al., 2016). Based on the Al₂O₃ and SiO₂ concentrations, the Mn-rich Azul sediments and ore would involve both normal and abyssal sedimentation (Figure 21 A). As shown in the diagram of Si-Fe-Mn (Figure 22 A), the Mn-bearing black shales do not plot in any of the main source Mn field but come closer to hydrogenous.

 

Table 14 – Chemical composition of some world important manganese deposits and compared with Past Archean Shales. Modified after Maynard (2004).

Chemical		Kalahari S.Africa	Tanganshan China	Xiangtan China	Molango Mexico	Hydrothermal Pacific	Hydrogenous Pacific	Hydrogeneous Nodules	Post Archean
Elements	Units	Paleoprot.	Neoprot.	Neoprot.	Jurassic	Modern	Modern	Modern	shales
Mn	%	36.5	44.9	42.3	28.0	37.0	22.1	18.6	MnO	0.11
Fe	%	4.22	2.66	1.54	8.53	1.87	15.1	12.5	FeO	6.5
Na	%	0.01	0.03	0.01	0.00	2.37	1.6	1.7	Na2O	1.2
Mg	%	1.93	1.09	1.23	5.10	1.95	1.26	1.6	MgO	2.2
Al	%	0.09	0.74	0.44	1.43	1.57	1.01	2.7	Al2O3	18.9
Si	%	2.40	1.54	4.75	5.36	7.73	3.69	7.7	SiO2	62.8
P	%	0.01	0.12	0.07	0.06	0.13	1.18	0.25	P2O5	0.16
K	%	0.00	0.01	0.17	0.00	1.01	0.56	0.7	K2O	3.7
Ca	%	12.2	2.72	1.34	1.73	2.48	4.13	2.3	CaO	1.3
Ti	%	0.01	0.14	0.10	0.06	0.15	0.77	0.67	TiO2	1.0
C organic	%		2.90	0.60
C carbonate	%	3.15	8.70	8.70				0.1
d13Corg	per mil				-27.3
d13Corg	per mil	-9.1			-13.1
d18Ccarb	per mil	-10.4			-3.6
S total	%	0.04	2.00	0.80	0.20	0.06	0.305	0.47
d34S	per mil		26.7	52.5	3.2
Sc	ppm		13	12	2.12	3.8	6.4	10	16
V	ppm	6	80	53	67	225	515	500	150
Cr	ppm	16	23	24	15	48	22	35	110
Co	ppm	50	64	44	132	72	6400	2700	23
Ni	ppm	19	45	24	60	287	5400	6600	55
Cu	ppm	<5	16	11	7	228	1080	4500	50
Zn	ppm	74	19	266	48	238	680	1200	85
As	ppm		48	34	31	33	165	140
Se	ppm		2.0	0.5		0.12	0.4	0.6
Rb	ppm	1	1	14	<0.2	<0.2	<0.2	17	160
Sr	ppm	146	69	101	40	555	1210	830	200
Y	ppm	5.3	41	33	8.6	17	166	150
Zr	ppm	2	33	29	15	23	172	560	210
Nb	ppm	<0.1	25.0	17.5	1.5	9.9	<0.1	50	19
Mo	ppm	<2	16.7	6.5	2	327	445	400	1.0
Sn	ppm	<1	6.5	4.0	<1			2	4.0
Sb	ppm		14	2.8	1.1	25	24	40
Ba	ppm	360	50	259	45	1380	1700	2300	650
Pb	ppm	6	68	8	6	45	1780	900	20
Bi	ppm		0.5	<0.1				7.0	0.25
Th	ppm	0.4	3.3	1.2	1.4	0.7	33.0	30.0	14.6
U	ppm	0.2	0.9	0.7	1.6	2.1	9.6	5.0	3.1
La	ppm	3.18	32.0	26	11.8	18.9	202	157	38
Ce	ppm	3.3	98.8	79.4	18.1	16.3	1100	530	80
Pr	ppm	0.5	8.0	6.7	2.4		106	36	8.9
Nd	ppm	2.1	32.8	27.8	9.5	7.2	162	158	32
Sm	ppm	0.33	7.60	5.50	1.70	0.99	42	35	5.6
Eu	ppm	0.16	2.35	1.14	0.43	0.28	9.90	9.00	1.1
Gd	ppm	0.58	7.52	5.56	1.87		26	32	4.4
Tb	ppm	0.09	1.24	0.88	0.29	0.25	7.53	5.40	0.77
Dy	ppm	0.55	7.99	5.85	1.49		57.8	31.0	4.4
Ho	ppm	0.14	1.55	1.23	0.31		6.60	7.00	1.0
Er	ppm	0.42	4.54	3.28	0.85		31.9	18.0	2.9
Tm	ppm	0.06	0.58	0.46	0.11		4.30	2.30	0.40
Yb	ppm	0.45	3.50	2.75	0.75	0.78	17.7	20.0	2.8
Lu	ppm	0.08	0.43	0.38	0.15	0.14	3.34	1.80	0.43
Sources		a,b	a	a	a,c,d	e	e	F	g

 

 

This chemical composition is compatible with mineralogy dominated by quartz and illite/sericite, and partly chlorite, in addition to pyrite. TiO₂ levels are higher than in manganese black shales. The near absence of CaO is remarkable, which denotes the absence, for example, of calcite and/or dolomite, common in Mn-rich carbonate rocks (Gutzmer and Beukes, 1996). They can be correlated to the Post Archaean Shales, but they are impoverished in Ca and Na, mainly, thus showing a certain partial signature with the manganiferous shales. These shales in the Azul deposits, besides Mn, differ markedly in term of the higher contents of Si, Al, and Ti, thus demonstrating their greater contribution in terrigenous material (quartz, illite/sericite and kaolinite).

As regards Co, Ni, Cu and Zn, as well as As, Mo, Sb and Ag, their levels in the manganese-bearing shales allow for comparison with the hydrothermal-type Mn-oxyhydroxide deposits, mainly and in part to the modern hydrogenous nodules. In the diagrams of (Co+Ni+Cu)_*10-Fe-Mn and Zn-Ni-Co (Figures 22 B and C) they truly fall in the field between hydrogenous and hydrothermal sources, suggesting both metal sources for Mn in Azul, as already demonstrated by aluminosilicate minerals. The Neoproterozoic Datangpo manganese deposit in China, after analysis of these elements, shows a hydrogenous source with hydrothermal contribution after other geochemical proxies, for instance, REE (Yu et al., 2016), while the Mesozoic of Iran hydrothermal (Rajabzadeh et al., 2017) and those of Úrkút, Hungary, are typically hydrothermal and diagenetic bacterial (Polgári et al., 2012).

The contents of REE, in turn, allow us to confront the manganese shales partially with the Neoproterozoic deposits of China, far from the classic Kalahari deposit (Maynard, 2004, Table 10). In turn, Ba, Pb and Sr reinforce the relationship with the hydrothermal and modern hydrogenous deposits. Likewise, the values ​​of Zr, Y, Th, Uand V, however far from Kalahari and Neoproterozoic China deposits (Table 14). In the REE – Al₂O₃ diagram (Figure 21 B), the sediments and Mn ores of Azul plot mainly as terrigenous input, microorganism enrichment and Mn-oxyhydroxides absorption. This reinforces what has already been demonstrated, strengthening the importance of the cryptomelane-hollandite, todorokite and birnessite, sedimentary precursor and early diagenesis formed.

The carbonaceous dark shales and dark grey to black shales sedimentary rocks chemically correspond to black shales, normal shales and carbonaceous shales (Taylor & McLennan, 1985). The manganese-rich shales are distinguished by the much higher values ​​of Co, Ni, Cu, Zn, Ba, Pb, Sr and in part Ce (Figure 23). The high values ​​of these elements demonstrate that the source of much of them would have been hydrothermal sources, as shown by comparisons with modern ocean deposits and modern hydrogenous deposits (Table 11) and in the previous diagrams (Figures 21 B and C). In turn, the great volume of siltstones and red sandstones demonstrate, at the same time, a great terrestrial, continental contribution. In general, the Mn-rich black shales and the manganese ore of Azul are relatively enriched, and the Mn-poor dark shales are impoverished. They range from relatively low-REE-content Post-Archean Shales to rich-REE-content hydrogenous deposits (Table 11), much higher than the REE contents of Paleoproterozoic biotite schists and graphite schists associated with Mn-carbonate and silicates marbles at Serra do Navio manganese deposits, considered to be derived from black shales and siltstones that are locally carbonaceous (Chisonga et al., 2012), and much higher than Mesozoic manganese mineralizations in the Abadeh-Tashk area (Rajabzadeh et al., 2017).

The distribution of REE in Mn-rich Azul rocks and ores normalized to PAAS sediments (Figure 24) displays a slightly positive Ce and Eu anomaly with middle rare earth subtle enrichment similarly to Neoproterozoic Dapangto Mn deposits in China; they differ only in concentration, which is higher in Azul Mn ores. A similar pattern showed the Archean black shales REE associated with banded iron formation in Carajás, however chondrite-normalized, displaying a hydrothermal signature in an anoxic and euxinic marine environment (Cabral et al., 2013). This differs from what was observed in the Mn and Fe Neoproterozoic deposits of the Urucum/Santa Cruz, which have a strong negative Ce anomaly (Klein and Ladeira, 2004; Angerer et al., 2016 and Biondi & Lopez, 2017). After Klein and Ladeira (2004) the general trend of the REE NASC-normalized plots showing some enrichment of the heavy REE can be well compared to that of modern seawater, concluding that the Fe, Mn, and Si is from typical ocean water with some deep-sea hydrothermal input. Alternately, these behaviour patterns of REE with a positive Ce anomaly (Figure 24) are consistent with those of the phosphatized Fe-Mn nodules, hydrogenetic Fe-Mn nodules and hydrogenetic Fe-Mn crusts (Xiao et al., 2017), which again differ in concentration, with it being higher in Azul. This suggests that Mn and Fe oxyhydroxides are also deposited as amorphous and poorly crystalline phases on a solid or accretion substrate, very slowly, initially sediment-free, Fe-bearing vernadite (d-MnO₂) and FeOH, respectively (Xiao et al., 2017), usually as colloidal particles with a surface charge, slightly positively surface-charged Fe oxides and negatively surface-charged Mn oxides (Xiao et al., 2017). This must have happened in Azul’s manganese deposit, similarly to Neoproterozoic Datangpo-type sedimentary manganese deposits in Guizhou Province, China (Xiao et al., 2017), which contrary to Azul, went into Mn carbonate type on later diagenesis. It is well known that the Mn-oxyhydroxides have a capacity to scavenge several trace elements from sea-water (Bau and Koschinsky, 2009). The wide presence of cryptomelane demonstrates intense diagenetic activity, with interaction between the surface-charged amorphous Mn-oxides and cations, such as of the elements K, REE, Co, Ni, Cu, Zn, and so on. Ba⁺⁺ seems to have come from percolating fluids along fractures and faults, forming hollandite. Some samples with positive Ce anomalies perhaps represent the strong oxidizing conditions that favoured the Ce oxidation and even the discrete formation of cerianite terrestrial weathering, before coming to Azul basin.

 

 

 

 

 

Isotopic Composition for d ¹³C and d ¹⁸O in Mn-Carbonates – The d ¹³C values ​​of the Azul Mn carbonates (Table 15) are equivalent to those of modern Baltic Sea deposits (d ¹³C per mil _PDB), which are richer in Mn and poorer in Ca and Mg (Maynard, 2004), indicating an environment linked to shallow and near-shore sediments.

The isotopic compositions d ¹³C_PDB and d ¹³O _SMOW of the Azul Mn-carbonates (mainly rhodochrosite), slightly depleted in d ¹⁸O _SMOW, demonstrate a formation during an earlier diagenesis (Table 15) (Figure 25) by comparison to Paratethys deposits (Kuleshov, 2003; 2017).  Microbially mediated reactions between manganese oxide and organic matter promote the formation of the manganese carbonate in the deposits (Kuleshov, 2017; Yu et al., 2019). This mechanism also resulted in the negative δ13C signals preserved in the Paleoproterozoic manganese deposits (Kalahari, Nsuta, Katanga and Moanda and Serra do Navio (Putter et al., 2018); Neoproterozoic manganese deposits, Datangpo in China (Yu et al., 2017, 2019) and Mesozoic Úrkút in Hungary (Polgári et al., 2012; Molnár et al., 2017). In these conditions, the amorphous and poor crystalline (Mn,Fe)-oxyhydroxides remain partially and an expressive part of the organic matter laminae, stromatolite-like structure (Molnár et al., 2017). This stromatolite-like structure has also been locally observed in the Azul manganese deposits.  In the Serra do Navio manganese deposits, besides the positive Ce anomalies, the δ¹³C _PDB values of −4.3 to −9.4 per mill suggest that Mn-carbonates derived during suboxic diagenesis from sedimentary Mn(IV) oxyhydroxide precipitates (Chisonga et al., 2012).

Even if present, rhodochrosite is not an abundant mineral in the Azul manganese deposits, although it has been described as forming two manganese units in a single drill hole carried out in the early geological explorations. The presence of diagenetic carbonates presupposes that of a carbonaceous organic matter. However, the Mn deposits of Azul still have organic carbonaceous material, but the restricted formation of rhodochrosite, suggests that these deposits did not stay for a long time below the redoxcline surface (water/sediment interface), which would favour the reduction of Mn and oxidation of organic matter (Molnár et al., 2017).

 

Table 15 – Values of isotopic composition for d ¹³C e d ¹⁸O in manganese carbonates from Azul deposit. d ¹⁸O_smow = 1.03091. d¹⁶O_PDB + 30.01.

Sample ID:	d13C PDB	d18O PDB	d18O SMOW
AZUL 83	-11.29	-5.97	23.85
AZUL 88	-11.14	-4.6	25.27
AZUL 89	-12.31	-5.64	24.19
AZUL162	-16.24	-7.32	22.46
AZUL165	-12.11	-5.35	24.49
AZUL166	-12.56	-5.94	23.89
AZUL201	-8.99	-5.26	24.59
MF-14	-12.33	-5.41	24.43
MF-22	-11.74	-5.42	24.42
MF-24	-12.27	-5.26	24.59
Average	-12.10	-5.617	24.018

 

 

 

δ¹³C _PDB for Organic Matter – The values ​​of δ¹³C _PDB for organic matter contained in the grey shales associated with the undisturbed red siltstones show strong depletion (Table 16), a geochemistry signature found in the Azul black shales and in other similar manganese sedimentary deposits. Stable δ¹³C_PDB isotope analyses presented almost constant δ¹³C_PDB values (32.8‰ through 34.6‰, N=10) in the range of the numerous values ​​of the Paratethys Early Oligocene manganese deposits, which vary from 25 to 35 ‰ (Lan et al., 2019; Kuleshov, 2003, 2017); the Vendian-Cambrian, North Iran, which shows high concentrations of Mn, P and Ba and  high abundances of fossil phytoplankton associated with black shale (Kimura et al., 1997); the organic carbon-rich sediments in southern high latitude deep sea  in the range of 16.5 through 28.7‰ (Sackett, 1986); and  those of the Molango Mn deposits in Mexico (Maynard, 2004). Negative shifts in the δ¹³C values have been interpreted to reflect recycling of respired CO₂ in a more localized reservoir (Sageman, 2004), with OM probably being deposited contemporaneously with the Mn-oxyhydroxides in Azul´s manganese sedimentary environment.

Meanwhile, the negative and practically homogeneous δ¹³C _PDB values in the sediments of the manganese deposits in Azul suggests homogeneity in the source of organic matter, therefore without isotopic fractionation. Part and locally, this OM was oxidized and allowed the formation of Mn-carbonate, by reducing the Mn of the oxyhydroxides, during the diagenesis, demonstrated by the formation of rhodochrosite strictly.

 

Table 16 – Values of isotopic composition for δ¹³C _PDB for organic matter from carbonaceous shales.

Samples ID	δ 13C PDB
AZUL 153	-33.5
AZUL 155	-33.9
AZUL160	-34.1
AZUL161	-33.1
AZUL162	-32.8
AZUL164	-33.4
AZUL165	-34.1
AZUL166	-33.3
AZUL199	-34.6
AZUL 201	-34.2
Average	-33.7

 

δ ³⁴S Pyrite Isotopic Composition – The values ​​of the isotopic composition δ ³⁴S of pyrite from grey and dark grey shales varied between 3.85 and 8.54 per mill (Table 17), in the range of the Archean (2.7 Ga) black shales associated with BIF in Carajás, Brazil presented by Cabral et al (2013, 2017). The δ³⁴S-isotope values range between +0.3 and +10.7‰, suggesting microbial sulphate reduction. The Fe and S isotope analyses carried out in pyrites contained in Águas Claras sandstones by Fabre et al. (2011) showed a mean value of ^δ34S = 11.97 ‰, which is ​​only slightly above that for the black shale pyrites analysed here. They conclude that a near-complete reduction of sulphate could be expected in the sediment and would result in a δ³⁴S signature for the diagenetic pyrites nearly identical to the signature of seawater sulphate. The value δ³⁴S = 11.97 ‰ measured by Fabre et al. (2011) is close to the range in δ³⁴S values that was suggested as representative for Paleoproterozoic seawater sulphate approximately 2.1 Ga ago (Schröder et al., 2008). These values ​​are low and are compatible with the atmosphere of the Paleoproterozoic or even Archean because atmospheric oxygen was too low, insufficient to oxidize the reduced sulphur to form sulphate in seawater (Goldhaber, 2004), a possible source of sulphur for Mn-rich carbonaceous shales of Azul deposits. This slightly higher isotopic composition allows us to speculate regarding the increase of atmospheric oxygen already in the Proterozoic approximately 2.1 Ga, the age of the Águas Claras Formation, which carries the manganese mineralization. It also strengthens the idea that sulphides, such as pyrite, are of late diagenetic origin, certainly linked to the partial oxidation of organic matter to the partial formation of carbonates, such as rhodochrosite in areas affected by microfractures and fissure venules of pyrite (pyrite + rhodochrosite/calcite + quartz). Scott et al. (2014) by using pyrite sulphur-multiple isotopes from Proterozoic black shales, found a large range in the δ³⁴S and concluded that there is evidence for rapid expansion and contraction of the early Paleoproterozoic seawater sulphate reservoir, with the rising of seawater sulphate (Schröder et al., 2008) expanding during the Great Oxidation Event ca. 2.3 Ga and contracting at 2.06 Ga, just close to Águas Claras sedimentation

 

Table 17 – Values of isotopic composition for δ ³⁴S in pyrites from dark shales in Azul manganese deposit.

Samples ID	δ 34S
AZUL155	3.85
AZUL158	6.43
AZUL161	8.17
AZUL162	7.79
AZUL164	8.54
AZUL165	8.25
AZUL199	6.90
Average	7.13

 

 

Remarks on the Environmental Azul Sedimentary Manganese Deposits – The Azul manganese ore deposits were deposited probably as amorphous or low crystallinity Mn(IV)-oxyhydroxides on a shallow marine platform with great production of carbonaceous organic matter and some microbial activity, which in part allowed the formation of Mn-rich stromatolite-like structures, therefore above the redoxcline line. Sedimentation occurred during the Paleoproterozoic approximately 2.1 Ga when microbial activity promoted certainly the oxidation of dissolved Mn(II) in marine water. The stromatolite-like structures indicate shallow and flat deposition ambience. The various black shale lenses carrying Mn-oxyhydroxides located in different lateral and vertical positions and displaying variable lengths, suggest micro-basin formations favourable to the precipitation of Mn oxyhydroxides over time and space. The sedimentation was accompanied by a large terrestrial contribution of sediments with oxidized iron minerals (oxides) indicated by thick and large packets of reddish mudstones/siltstones and sandstones. The major Mn source comes from continents (hydrogenous) indicated by chemical composition diagrams as oxyhydroxides supporting a deep chemical weathering taking place at the time of the sedimentation or soluble and then oxidized via microorganism, for deposition of amorphous Mn-oxyhydroxides. However, a fraction of Mn also received hydrothermal contributions, in part in abyssal deposition, as suggested by the chemical diagrams and indicated by high concentrations of Ba, Cu, Ni and Co, in addition to high concentration of REE. During the diagenesis, in part probably mediated by carbon-silicium bacteria, cryptomelane and other Mn-oxyhydroxides were formed. Chlorite and pyrite were also formed, certainly also mediated by bacteria, reducing restrictive conditions, similarly for rhodochrosite, which demonstrates that the microbiological reducion activity was local, which is also indicated by the presence of carbonaceous organic matter associated with Mn-oxyhydroxide ore, as in typical early diagenesis. The lower constant negative d¹³C isotopic data of carbonaceous matter account for its deposition contemporaneously with the Mn-oxyhydroxides. The isotopic composition of carbonates shows that rhodochrosite, in the same manner as most deposits, is typically diagenetic, depicted by d ¹³C_PDB and d ¹³O _SMOW isotopic composition. At the time of early diagenesis reactions, there was low oxygen availability in the environment because the oxidation of carbonaceous organic matter had been reduced, as well as the sulphate and iron, apparently, under an oxic marine water indicated by pyrite framboids, certainly microbial mediated. This is in part confirmed by low positive δ³⁴S-isotope values in pyrite.

However, these Mn-mineralized sediments were greatly affected during the Brazilian Tectonic Cycle that occurred in the Neoproterozoic. Most of the hollandite belongs to this phase but may still have an early diagenesis component. As a consequence, probably sericite, perhaps chlorite (anchimetamorphism), and innumerable kaolin, pyrite and other sulphides and rhodochrosites have also been developed. In addition, Mn oxyhydroxides were remobilized and re-precipitated in faults, fractures and pockets, usually as pyrolusite, forming high-grade ore locally. The association of OM + Mn-oxyhydroxides (mainly cryptomelane) + pyrite + illite/sericite ± chlorite) shows that the sedimentary sequence represented by the dark grey to black shales embedded in the primary striped red siltstones to sandstones shows the superposition of the different geological events and processes (deposition, early diagenesis and epigenesis) that overlapped the Azul sedimentary sequence belonging to the Águas Claras formation.

 

CONCLUSIONS

Azul’s manganese ore deposits are of Paleoproterozoic age and typically sedimentary-diagenetic associated with black shales hosted by reddish mudstones/siltstones and sandstones deposited in a shallow marine platform with expressive carbonaceous organic matter. Among CMP’s manganese deposits, it was the largest, with reserves in the process of being exhausted. It is dominated by Mn-oxyhydroxides, mainly cryptomelane and cryptomelane-hollandite, and its precursors are probably amorphous and/or low crystallinity Mn-oxyhydroxides. Mn-carbonates, for example, rhodochrosite, are rare and when present, of early diagenetic origin, and even epigenetic. The Mn-bearing sedimentary sequence is not metamorphized, with at most anchimetamorphic alteration suggested by the presence of sericite, sometimes chlorite, which may also be linked to diagenesis and tectonic deformations during two events. The source of Mn is mainly hydrogenous, also with a hydrothermal contribution demonstrated by the chemical composition, mainly by trace elements and REE and isotopic evaluations. The deposition and fixation of Mn from sea water was partly microbial mediated, for example, silicium bacteria, by oxidation, close to the redoxcline line, which locally gave rise to stromatolite-like structures. Pyrite framboids are present and organic in origin, with marine sulphate being reduced by bacterial action after isotopic data formed during the diagenesis.

With these characteristics, the Azul manganese deposits belong to deposits formed during the first Great Oxidation Event in the Earth’s atmosphere, between 2.4 and 2.1 Ga and are well correlated with several deposits from this period and in part with others from Neoproterozoic to Cenozoic.

 

Acknowledgements

The authors would like to thank ADIMB/FINEP for their financial support of this research; CNPQ for support through scholarships and research grants (303233/2003-4; 307901/2006-8; 304519/2009-0; 305015/2016-8); the ZWL in the name of Dr. Jürgen Göske and the University of Halle-Wittenberg for analytical support in the name of prof. Dr. Herbert Pöllmann; and the chemist Dr. Suyanne Flavia Santos Rodrigues and geologists Alessandro Sabá Leite and Daiveson Serrão for  graphic support of the illustrations; and especially the two anonymous reviewers for the careful review of the manuscript as a whole.

 

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