© Author(s) 2024. CC Atribution 4.0 License Mineralogy, geochemistry and genesis of low-grade manganese ores of Anujurhi area, Eastern Ghats, India Mineralogija, geokemija in geneza revnih manganovih rud z območja Anujurhi, Vzhodni Gati, Indija Sujata PATTNAIK1* & Satrughan MAJHI2 1Geological Survey of India, Eastern Region, Bhu-Bijnan bhawan, DK-6, Karunamayee, Sector-2, Salt Lake, Kolkata-700091; *corresponding author: suji.pinky@gmail.com 2Geological Survey of India, State Unit: Odisha, Unit-8, Nayapalli, Bhubaneswar-751012 Prejeto / Received 18. 3. 2024; Sprejeto / Accepted 12. 11. 2024; Objavljeno na spletu / Published online 16. 12. 2024 Key words: Eastern Ghats Mobile Belt (EGMB), Anujurhi, manganese ore, supergene enrichment Ključne besede: Vzhodni Gati, Anujurhi, manganova ruda, supergena obogatitev Abstract The Anujurhi manganese ores occur in the high-grade gneisses of the Precambrian Eastern Ghats Supergroup in Odisha, India. They are characterized by conformable lenses containing minerals such as cryptomelane, romanechite, pyrolusite, todorokite, and pyrophanite, along with other opaque minerals like graphite, goethite and ilmenite. The gangue minerals associated with these ores include quartz, feldspar, garnet, kaolinite, apatite, sillimanite, zircon, biotite, alunite, and gorceixite. The primary elements present in the ore, Si, Mn, Fe, and Al, average at 16.20 %, 15.06 %, 11.94 %, and 6.6 % respectively. Additionally, trace amounts of P, K, Ti, Mg, Ca, and Na were detected. The average Fe/Mn ratio of 0.81 and the Si versus Al plot of the Anujurhi manganese ores suggest a hydrogenous-hydrothermal mixed source for the ferromanganese sediments. The characteristics of the manganese ore bands, absence of carbonate facies of ore, and geochemical association of Mn-Ba together with Na/Mg ratios and CaO-Na2O-MgO ternary plot of the manganese ores strongly indicate that the mineralization is a metamorphosed shallow marine-lacustrine deposit. Following deposition and diagenesis, the manganese minerals underwent at least two phases of Ultra High Temperature (UHT) and granulite facies metamorphism along with the host rocks. Tectonic uplift, erosion, extended exposure to atmospheric oxygen and percolation of meteoric water led to the supergene alteration and remobilization of the primary manganese minerals in a colloidal state, followed by epigenetic replacement along the structural weak planes of the granulite facies rocks, resulting in the formation of the current deposits. This is evidenced by the observed secondary replacement and colloidal textures in the Mn oxides. Izvleček Članek obravnava manganove rude, ki se pojavljajo znotraj visokometamorfnih gnajsov predkambrijske supergrupe Vzhodni Gati na območju Anujurhi v zvezni deželi Odisha, Indija. Zanje so značilne konformne leče, ki vsebujejo minerale, kot so kriptomelan, romanehit, piroluzit, todorokit in pirofanit, skupaj z drugimi neprozornimi minerali, kot so grafit, goethit in ilmenit. Jalovinski minerali v teh rudah vključujejo kremen, glinenec, granat, kaolinit, apatit, silimanit, cirkon, biotit, alunit in gorceiksit. Primarni elementi prisotni v rudi so Si (16,20 %), Mn (15,06 %), Fe (11,94 %) in Al (6,6 %). Dodatno so ugotovili sledne vsebnosti P, K, Ti, Mg, Ca in Na. Povprečno razmerje Fe/Mn, ki znaša 0,81, in diagram primerjave vsebnosti Si z vsebnostmi Al v rudi z območja Anujurhi nakazuje, da feromanganovi sedimenti izvirajo iz mešanih hidrotermalnih virov. Značilnosti manganove rude, kot so odsotnost karbonatnega faciesa rude in geokemična povezava Mn-Ba skupaj z razmerji Na/Mg ter tri komponentnim diagramom CaO-Na2O-MgO rud jasno kažejo, da gre za metamorfozirano plitvomorsko do jezersko nahajališče. Po odlaganju in diagenezi so manganovi minerali prestali vsaj dve fazi ultra visoke temperature (UHT) in metamorfizma granulitnega faciesa skupaj s prikamnino. Tektonski dvig, erozija, dolga izpostavljenost atmosferskemu kisiku in pronicanju meteorne vode so privedli do supergene spremembe in remobilizacije primarnih manganovih mineralov v koloidnem stanju, čemur je sledila epigenetska zamenjava vzdolž strukturno šibkih površin kamnin granulitnega faciesa, kar je povzročilo nastanek sedanjih ležišč. To dokazujejo opazovane sekundarne zamenjave in koloidne teksture v Mn oksidih. GEOLOGIJA 67/2, 285-300, Ljubljana 2024 https://doi.org/10.5474/geologija.2024.014 286 Sujata PATTNAIK & Satrughan MAJHI Introduction Manganese makes up about 0.1 to 0.2 % of the Earth’s crust, ranking as the 10 th most abundant element (Post, 1999). It exists in three different oxidation states in natural systems: +2, +3, and +4, leading to a variety of multivalent phases. Manganese oxide minerals have been utilized for many years by ancient civilizations for pigments and to clarify glass. By the mid-nineteenth centu- ry, manganese became a crucial component in the steel-making industry, which remains the main consumer of manganese ore. Manganese is the pri- mary component in LMO (Lithium manganese ox- ide) and NMC (Lithium nickel manganese cobalt) batteries due to its cost-effectiveness compared to other battery metals and its abundant supply (Parrotti et al., 2023). Over 30 manganese oxide/ hydroxide minerals are found in various geological settings (Post, 1999). The total resources/ reserves of manganese ore in India as of 2015 is 495.87 million tonnes. Odi- sha tops the total reserves/ resources with a 44 % share, followed by Karnataka at 22 % and Madhya Pradesh at 12 %. During 2018–19, the total pro- duction of manganese ore was 2.82 million tonnes with Madhya Pradesh as the leading producer of manganese ore at 33 %, followed by Maharashtra (27 %) and Odisha (16 %). Grade-wise, 68 % of the total production was of lower grade (below 35 % Mn), 21 % of medium grade (35–46 % Mn) and 10 % was of high grade (above 46 % Mn) (Indian Minerals Yearbook 2019). To fulfill the large supply-demand gap of man- ganese due to the splurge in demand in the steel industry; the limited availability of high-grade (+44 % Mn) manganese ores coupled with the lim- itations of the domestic manganese ore, made it obligatory for revision of the threshold value of manganese ores to 10 % Mn in exploration and to exploit low-grade manganese ores. For the de- velopment or upgradation of low-grade ores, it is important to characterize or know the nature of the low-grade ores of our country, including the chemistry, mineralogy and correlation that will serve the future industrial development (Manga- nese Ore: Vision 2020 and beyond, IBM, 2014). Fig. 1. Map of Odisha state showing the important domains (Cratons, Mobile belts and Quaternary sediment cover) and the known manga- nese occurrences (Map source- GSI, SU: Odisha). 287Mineralogy, geochemistry and genesis of low-grade manganese ores of Anujurhi area, Eastern Ghats, India In the state of Odisha, the manganese ore is found to occur in three stratigraphic horizons and spatially distributed wide apart i.e. Iron Ore Group in Bonai-Keonjhar belt (Roy, 2000), Gang- pur Group in Sundergarh district and Eastern Ghats Supergroup in Koraput-Rayagada-Kalahan- di-Balangir belt (Roy, 2000; Mishra et al., 2016). The manganese occurrences in the Eastern Ghat Mobile belt have been studied for their economic potential, mineralogical characteristics and evo- lutional history by several workers like Walker (1902), Dey (1942), Murthy et al. (1971), Narayan- swamy (1966, 1975), Acharya et al. (1994, 1997), Ramakrishna et al. (1998), Nanda & Pati (1991), Jena et al. (1995), Rao et al. (2000), Rickers et al. (2001), Dasgupta and Sengupta (2003), Dash et al. (2005), Bhattacharya et al. (2011), Chetty (2014), Karmakar et al. (2009) and others. The manganese deposits in the Anujurhi-Am- badala-Rukunibori-Loharpadar areas, collective- ly known as the Ambadala occurrence, confirm the presence of a continuous manganese ore zone spanning 150 km from the Kutinga-Nishikhal sec- tor in the Koraput district of Odisha to the Uch- habapalli-Kanaital sector in the Bolangir district of Odisha (Fig. 1). The explored manganese occur- rence is situated approximately 200 m east of Anu- jurhi village in the Rayagada district of Odisha and falls in Survey of India Toposheet no. 65M/5. This paper will provide an overview of the geology, mineralogy, and geochemistry of the manganese deposits in the Eastern Ghat Supergroup of rocks at Anujurhi, with a focus on characterizing the manganese ores as a potential future economic op- portunity. The results obtained will be compared with similar deposits to develop a genetic model that determines the source and formation process of manganese in the Eastern Ghat Mobile Belt. Geological Setting The Eastern Ghats Mobile Belt (EGMB) is a Mesoproterozoic collisional orogen (Chetty, 2014), that extends along the east coast of India for over 900 km with a varying width from 50 km in the south to a maximum of 300 km in the north. EGMB or Eastern Ghats Belt (EGB) denotes a contiguous terrain of granulite facies rocks bounded to the north, west and south by the Singhbhum, Bastar and Dharwar Cratons (Dasgupta, 2019), and to the east it disappears underneath alluvial plains and the Bay of Bengal. The EGB consists of an in- tensely deformed and metamorphosed assemblage of meta-sedimentary and meta-igneous granulite facies rocks, which were subsequently intruded by Proterozoic anorthosite, alkaline rocks and grani- toids. The metasedimentary rocks mainly include garnet-sillimanite gneiss (khondalite), quartzites and calc-granulites, while the meta-igneous rocks range from basic to felsic in composition and are essentially hypersthene-bearing charnockites (Chetty, 2014). The protoliths for the Khondalite Group is believed to be dominantly pelitic with subordinate arenaceous and calc- magnesian com- ponents (Ramakrishnan et al., 1998; Nanda, 2008) and is indicative of their formation in a shallow water stable shelf milieu (Roy, 2000 & 2006). The minimum age of sedimentation of the Khondalite Group points to an Archean event during 2.8 and 2.6 Ga from the available geochronological data (Roy, 2000). Ramakrishnan et al. (1998) divided the East- ern Ghats Mobile Belt into four lithotectonic do- mains longitudinally, viz. Western Charnockite Zone (WCZ), Western Khondalite Zone (WKZ), Central Migmatite Zone (CMZ) and Eastern Khon- dalite Zone (EKZ). A Transition Zone (TZ) at the contact with the Bastar craton to the west is also marked by a prominent frontal thrust (Dasgupta et al., 2013). Rickers et al. (2001) subdivided EGMB into four crustal domains viz. Domain 1(1A & 1B), Domain 2, Domain 3 and Domain 4, whose bound- aries do not match with those of Ramakrishnan et al. (1998) based on Nd-mapping carried out over EGMB. Nd-model ages presented contrasting protolith history in all four crustal domains with domain boundaries marked by prominent shear zones. Later, Dobmeier and Raith (2003) concep- tualized EGMB as a collage of four isotopic prov- inces having distinct geological histories viz. Jey- pore, Krishna, Eastern Ghats Province (EGP) and Rengali Province (Fig. 2). EGB played a dominant role in the configura- tions of at least three supercontinents Columbia (≈ 1.9-1.4 Ga) (Rogers and Santosh, 2002; Zhao et al., 2002, 2004; Karmakar et al., 2009), Rodin- ia (≈ 1.0–0.75 Ga) (Li et al., 2008) and Gondwa- na (≈ 0.55–0.30 Ga). The central domain or EGP (Dobmeier & Raith, 2003) witnessed a prolonged accretion–collision history initiated with rifting and consequent ocean opening and sedimenta- tion at ca. 1.50 Ga during the break-up of Colum- bia (Upadhyay, 2008; Karmakar et al., 2009) and culminated at ca. 0.90 Ga with the formation of supercontinent Rodinia. The latter united cratonic India with east Antarctica as a separate continent Enderbia that existed until about ca. 0.50 Ga (Kar- makar et al., 2009; Dasgupta, 2019). The metamorphic history of EGP was initiat- ed by a high-T/low-P progressive metamorphism and deformation that eventually led to UHT 288 Sujata PATTNAIK & Satrughan MAJHI (~1000 °C) peak (M1-D1) under deep-crustal conditions (8-9 kbar) (Karmakar et al., 2009) ranged from 1.03 Ga (Bose et al., 2011) to 1.13 Ga (Korhonen et al., 2013). A second granulite-grade metamorphism and associated deformation (M2- D2) strongly reworked the deep-crustal granulites and exhumed them to mid-crustal level as evident from decompression-dominated retrogressive segment (Dasgupta & Sengupta, 2003; Dobmeier & Raith, 2003) at 0.95–0.9 Ga with peak condi- tions of around 7 kbar, 850 oC (Bose et al., 2011; Padmaja et al., 2022). M3 is a weak amphibolite grade overprint and mostly localized along ductile shear zones (Karmakar et al., 2009) constrained at ≈0.55-0.50 Ma (Karmakar et al., 2009). Ther- mal imprint associated with M3 is manifested by emplacement of pegmatite crosscutting the M2-D2 foliation (Karmakar et al., 2009). Geology of the Anujurhi area The area forms a part of the Central Migma- tite Zone of Ramakrishnan et al. (1998), Domain 3 of Rickers et al. (2001) and EGP of Dobmeier and Raith (2003). The Khondalite Group comprises a sequence of garnet-sillimanite (+ graphite) schists and gneisses (khondalite senso-stricto) with rel- atively minor quartzite and calc-silicate rocks. The manganiferous horizon at Anujurhi is mainly restricted to the contact of quartz-feldspar-gar- net-sillimanite gneiss and garnetiferous quartzite. The manganese mineralization at Anujurhi is both lithologically and structurally controlled and extends for a strike length of about 900 m. The ore zone is continuous with detached outcrops showing varying width ranging from 5 m to 24 m. Five dis- tinct manganese ore lodes were established for the f irst time at 10 % manganese cut-off in Anujurhi area. The ore bands are narrow, discontinuous and lenticular in shape and parallel to the region- al trend of rocks i.e. N20oE–S20oW with moderate dips towards southeast. The area has undergone polyphase deformation with at least three gener- ation viz. F1 folds are tight to isoclinal folds ob- served in quartzite/manganiferous quartzite and calc-granulite. The S1 axial planar cleavage of F1 folds is parallel to the primary bedding S0. F2 folds are mostly open upright mesoscopic folds recorded in calc-granulite and manganiferous quartzite and F3 folds occur mainly as broad open warps in the khondalites and calc silicates. Materials and Methods As a part of the Annual Programme of Geo- logical Survey of India (GSI) during Field Season 2016–2018, the Anujurhi area was investigated for manganese mineralization by way of mapping in detail on 2000 RF in 2 square kilometers to work out the local stratigraphy, structure and disposi- tion of mineralized zones. Subsurface exploration by core drilling was carried out in eleven boreholes to establish the grade, extension and geometry of the ore bodies. Mapping was supported by labora- tory studies like whole rock geochemistry, miner- al chemistry and petrographic studies of samples from bedrock, pits, trenches and borehole cores. Bedrock samples from in-situ manganese ores and manganiferous quartzite were collected with ut- most care. Pits and trenches were cut perpendicu- lar to the strike of the ore zone to expose the bed- rock and samples were collected by preparing small channels of 50 cm to 1 m in length depending on the width of the zone and mineralogical variation. Fig. 2. (a) Geological map of EGB showing lithological divisions by Ramakrishnan et al. (1998), (b) Isotopic domains of EGB of Rickers et al., 2001, (c) Different provinces and domains of EGB by Dobmeier and Raith, 2003 (EGB- Eastern Ghats Belt, EGP- Eastern Ghats Province, KP- Krishna Province). 289Mineralogy, geochemistry and genesis of low-grade manganese ores of Anujurhi area, Eastern Ghats, India Fig. 3. Geological map of the Anujurhi area showing the manganiferous ore zones. 290 Sujata PATTNAIK & Satrughan MAJHI The core samples of the identif ied manganiferous zones were collected at a sample interval of 1 m from all the boreholes. Initially, the cores were split into half, half is preserved and the rest is sampled. All the samples collected from bedrock, pit, trench and borehole cores were then pow- dered manually using mortar and pestle to -120 mesh size. Two sets of samples are prepared by coning and quartering method where one set is submitted for analyses and a duplicate is pre- served for future use. Major element concentrations of 279 whole rock samples were determined through an X-ray f luorescence Spectrometer (Panalytical-ZETIUM) at the Chemical Laboratory, GSI, SU: Odisha by using the pressed pellet technique. The detection limits for this method range from 0.1 % for ma- jor and minor elements and 1.0 mg/L for trace elements. Samples from the manganiferous zones were studied under an optical microscope (Leica DM 2500P), the textures exhibited by the manga- nese ore are of secondary nature mainly replace- ment and colloform textures. To confirm the minerals identif ied in optical microscopy, ten samples from bedrock, trench and cores were analyzed for X-ray diffractometry (XRD), operating at 40 kV and 30 mA with Cu Kα radiation with 1.54 Å wavelength (PANalyti- cal, Model: X’Pert PRO XRD) at Mineral Physics Division, National Centre of Excellence in Geosci- ence Research, Geological Survey of India, Kolk- ata. Results are added in Supplementary material (Table 1). Quantitative chemical analyses of mineral phases have been undertaken at the National Cen- tre of Excellence in Geoscience Research, Geolog- ical Survey of India, Kolkata with an automated electron probe microanalyzer (SX 100 CAMECA) with five vertical spectrometers, operating at an acceleration voltage of 15 kV and beam current of 12 nA with beam size of 1 μm. Natural as well as synthetic standards were used during the analyses. Fig. 4. Disposition of manganese ore zones in Anujurhi area, (a) Folded and silicified manganese ore within quartzite, (b) Fracture filling ore showing parallel bands of manganese ore and silica east of Anujurhi, (c) Soft, friable manganese ore mainly pyrolusite associated with kaolinite east of Anujurhi, (d) Folded manganiferous quartzite. 291Mineralogy, geochemistry and genesis of low-grade manganese ores of Anujurhi area, Eastern Ghats, India Results Field Observations The ore exhibits a hard, lumpy texture char- acterized by alternating bands of silica, primarily in the form of recrystallized quartz infilling frac- tures. In contrast, where associated with clay min- erals (e.g. kaolinite) within khondalite, and inter- folded with quartzite, the ore becomes soft, friable and powdery. (Fig. 4a, b, c & d). Ore Petrography The petrographic analyses revealed a mineral assemblage composed of primary minerals such as quartz (40–50 %), and garnet (10–15 %) respec- tively followed by Mn-oxides (20–25 %), graphite (5–10 %), goethite (5–10 %), clay minerals (<5 %) and Fe/Mn-Ti oxides (<1 %). The XRD analyses confirm the mineralogy. The chief Mn oxides iden- tified in the area are cryptomelane, romanechite and pyrolusite with minor quantities of todorokite. Fig. 5. (a) Colloform bands of cryptomelane (Cy), romanechite (Rm) and goethite (Go), (b) Mosaic grains of quartz & feldspar with fracture filling romanechite (Ps) and flakes of graphite (Gr), (c) Romanechite (Ps) as fracture filling with quartz (Q) and garnet (Gn), (d) Cryptomelane (Cy), Romanechite (Ps) showing colloform banding with quartz (Q) and garnet (Gn) as gangue minerals, (e) Pyrolusite (Py) associated with clay minerals & (f) Pyrolusite (Py) as fracture filling. 292 Sujata PATTNAIK & Satrughan MAJHI The associated gangue minerals are quartz, gar- net, graphite, goethite, ilmenite, apatite and zir- con, etc. Since the Mn-oxides share very similar physi- cal and optical characteristics, their identification only with the aid of an optical microscope is dif- f icult, hence, the Mn-oxides were distinguished through EPMA. It is observed that pyrolusite is associated mainly with clay minerals like kaolinite and alunite due to alteration of the quartzo-feld- spathic garnet-sillimanite schist or migmatised khondalite. Similarly, a positive correlation is ob- served between cryptomelane and which is further corroborated by EPMA data. Garnet is generally found as equigranular, mosaic grains associated with quartz and Mn-oxides. The chief ore mineral occupying the intergranular spaces is cryptomel- ane which forms colloform banding with romane- chite and goethite (Fig. 5a, b, c & d). Pyrolusite is mainly confined to the cavity fillings and is often found associated with clay minerals (Fig. 5e & f ). Graphite occurs as scales and f lakes within the manganese ores. Goethite is found as gangue showing box work structure, grey with moderate to high ref lectivity. Fine crystals of apatite are found embedded in quartz, feldspar and garnet. Geochemistry Five representative samples of manganese ore and manganiferous quartzite underwent compre- hensive analysis using Electron Probe Microanal- ysis (EPMA). The results clearly demonstrate that the dominant ore minerals present in the miner- alized zones of the Anujurhi area are cryptomel- ane, romanechite and pyrolusite. Cryptomelane is essentially a potassium-bearing manganese oxide with minor amounts of Ba, Ca, Fe and Al. The K2O content ranges from 1.93 to 5.24 %. Romanechite occurs together with cryptomelane as a secondary product of alteration of the primary minerals (Fig. 6a & b). BaO content in romanechite is very high which ranges from 16.88 to 18.71 % and Mn con- tent ranges from 46.75 to 49.6 %. Pyrolusite is also present as veins in the host rock, i.e. quartz-feld- spar garnet sillimanite schist. Mn content ranges from 60.5 to 61.4 % with minor amounts of Ba, Al and Si. With the EPMA results, the chemical for- mulae are cryptomlenane–K0.62(Mn7.65,Fe0.05,Al0.09,- Si0.02)O16, romanechite–(Ba0.59,K0.01)(Mn4.55,Al0.04,- Si0.11)O10 and pyrolusite-(Mn0.97Si0.01 Al0.01Fe0.01)O2. The representative EPMA results for crypto- melane & pyrolusite, romanechite, garnet and F-Mn hydroxides are displayed in Table 1 to Ta- ble 3. Garnet is associated with quartz and contains large amounts of Mn (averages of 18.89 %) and Si (averages of 17.47 %). The average Fe and Ca contents were 3.11 wt% and 9.19 wt% respective- ly. There is an inverse relationship between Mn and (Fe + Ca) results from Mn substitution into spessartines, which have the chemical formulae [(Mn1.9, Fe0.5, Ca0.4) Al2 (SiO4)3] and is similar to the spessartine composition of supergene manganese occurrence at Southern Minas Gerais, Brazil (Par- rotti, 2023). Two compositional varieties of garnet are reported from the manganese deposits of the Anujurhi area, one variety is rich in spessartine garnet (max. 68 %) whereas the other variety is rich in grossular garnet (max. 46 %) as calculated from the EPMA data. Alteration of the garnet pro- ceeds along the grain boundaries and fractures, often leading to a distinct alteration rim with a Table 1. Electron microprobe analyses (wt%) of cryptomelane & pyrolusite. 1 2 3 4 5 6 7 8 9 10 11 SiO2 0.06 0.1 0.15 0.17 0.1 0.06 0.59 0.15 0.81 0.6 0.69 Al2O3 0.31 0.22 0.14 0.24 0.23 0.82 2.68 0.35 0.79 0.86 0.79 MnO 77.84 79.36 78.27 78.4 78.1 75.42 67.39 65.3 78.11 79.32 78.47 MgO 0.07 0.01 0.03 0.01 0.05 0.03 0.04 0.04 0.04 0.06 0.07 Na2O 0.06 0.17 0.1 0.11 0.07 0.02 0.03 0.13 0.04 0.03 0.02 P2O5 0 0 0 0 0 0 0 0 0 0 0 K2O 4.11 3.98 4.02 4.49 3.9 5.24 4.43 2.3 0.61 0.64 0.59 CaO 0.44 0.49 0.53 0.46 0.45 0.62 0.4 0.48 0.16 0.14 0.19 TiO2 0 0.02 0 0.02 0 0 0.01 0 0.02 0 0 FeO 0.11 0.11 0.1 0.09 0.2 0.25 3.47 0.02 0.66 0.7 0.53 Cr2O3 0 0 0.01 0 0 0 0.1 0 0.31 0.11 0.13 NiO 0 0 0.1 0 0.12 0.14 0.05 0 0.22 0 0.17 BaO 0.29 0.25 0.26 0.21 0.24 0.96 1.5 9.03 1.1 1.04 1.24 Cryptomelane-1 to 8, Pyrolusite-9 to 11 293Mineralogy, geochemistry and genesis of low-grade manganese ores of Anujurhi area, Eastern Ghats, India central core (Fig. 6c). In rare cases, complete al- teration of garnet is also observed. It alters to hy- drous oxides/hydroxides of iron and manganese and subsequently cryptomelane (Acharya et al., 1994). Goethite is a common secondary mineral de- rived from the alteration of other iron-rich min- erals, especially magnetite, pyrite, siderite and hematite under oxidizing conditions. Goethite contains considerable amounts of Al, Si, P and Mn. Phosphorus content is the highest, varying from 0.54 to 2.88 % P2O5. Lepidocrocite although less common, has the same origins and they often oc- cur together. Clay minerals are found in abundance in some sections as a result of deep weathering of host rocks. Gorceixite is a hydrated phosphate (BaAl3 (PO4) (PO3OH) (OH)6) that belongs to the alunite group of minerals. It is present as a weathering product of the host rocks like quartz-feldspar-gar- net sillimanite schist in the ore zone. In some of the samples, romanechite can be seen altered to gor- ceixite. P2O5 content ranges from 25.43 to 28.76 % and BaO content ranges from 19.1 to 24.47 %. The high amount of phosphorus in samples from Anujurhi can also be attributed to the presence of gorceixite in most samples. Representative EPMA data of gorceixite are given Supplementary mate- rial (Table 2). Pyrophanite in the Eastern Ghats Group of rocks from the Nishikhal area is also reported by Acha- rya et al. (1994) & in Ambadala area by Pradhan et al. (2016). Pyrophanite is a common accessory mineral associated with metamorphosed manga- nese-rich rocks. A zoned prismatic grain of ilmen- ite-pyrophanite is identified in BSE image within cryptomelane with TiO2 concentration reducing from the center outwards suggesting ilmenite be- ing replaced by Mn-oxides (Fig. 7d). Subhedral prismatic grain of zircons and apatite are rarely found. Representative EPMA data of gorceixite are given in Supplementary material (Table 3). 1 2 3 4 5 SiO2 0.29 0.64 4.4 0.31 0.47 Al2O3 0.46 0.37 0.3 0.51 0.23 MnO 64.06 61.26 61.03 63.85 61.47 MgO 0.05 0.04 0.04 0 0.06 Na2O 0 0.04 0.04 0.07 0.06 P2O5 0 0 0 0 0 K2O 0.07 0 0.01 0.12 0.15 CaO 0.16 0.25 0.29 0.25 0.38 TiO2 0 0 0 0 0 FeO 0.13 0.07 0.07 0.19 0.02 Cr2O3 0 0 0.2 0.05 0.03 NiO 0.07 0.02 0.01 0 0 BaO 16.88 18.71 17.93 17.2 17.02 Table 2. Electron microprobe analyses (wt%) of romanechite. Table 3. Electron microprobe analyses (wt%) of garnet & Fe-Mn hydroxides. 1 2 3 4 5 6 7 SiO2 37.53 37.08 37.61 37.25 2.68 1.17 1.51 Al2O3 20.97 20.51 21.31 20.62 2.95 2.1 1.47 MgO 0.22 0.25 0.24 0.9 0.07 0.03 0.03 Na2O 0.01 0 0.02 0.02 0.09 0.02 0.04 P2O5 0 0 0 0 0.54 1.89 2.88 K2O 0 0.01 0 0 0 0.04 0 CaO 15.99 12.26 18.41 4.8 0.27 0.12 0.04 TiO2 0.11 0.09 0.03 0.2 0.21 0.07 0.04 FeO 2.63 2.63 1.74 9.03 63.12 68.61 69.97 Cr2O3 0.02 0.04 0.03 0 0.07 0.12 0.06 NiO 0 0.02 0 0.1 0.09 0.54 0.22 BaO 0.07 0 0 0.09 0.56 0 0.11 MnO 22.05 27.38 19.45 28.68 7.68 0.9 1.35 Garnet-1 to 4, Fe-Mn hydroxides -5 to 7 294 Sujata PATTNAIK & Satrughan MAJHI Discussion Diagnostic elemental assemblages of major, minor, and trace metals can be utilized to differ- entiate manganese deposits formed in various ge- ological environments (Acharya, 1994 & 1997). Such an approach is well established in non-meta- morphosed ores (Nicholson, 1992 & 1997) but the application of such diagnostic geochemical signa- tures to metamorphosed sediments would assume that this signature has been preserved in the me- ta-sediments of EGB. This argument hinges on the assumption that the original sediment’s chemical composition has been retained despite the recrys- tallization of the mineral assemblage. However, due to the absence of data on minor and trace elements, it was not feasible to accurately define the diagnostic assemblages for different sources of manganese. Upon thorough examination of the chemical data pertaining to the major oxides pres- ent in the ores, exhaustive efforts were undertak- en to formulate a comprehensive genetic model for the manganese ores located in the Anujurhi area. The major elements present in the manganese ores of the study area are Si, Mn, Fe and Al, each averaging at 14.82 wt%, 16.60 wt%, 12.42 wt%, and 6.43 wt%, respectively (Table 4). Other ele- ments such as P, K, Ti, Mg, Ca and Na were found to be present in trace amounts (less than 1 wt %). Phosphorous content averaging 0.68 wt % is very high in comparison to other manganese depos- its e.g., Noamundi-Koira basin, Odisha (Alvi & Mohd., 2021), Tokoro belt, Japan (Choi & Hari- ya, 1992) and is a distinguishing property of the manganese ores of EGMB. Phosphorus is either present as definite mineral phases such as apatite, f luor-apatite which is evident from strong posi- tive correlation with Ca (Fig. 7) and gorceixite or in adsorbed state. In adsorbed state phosphorus also occurs within various manganese and asso- ciated iron mineral phases like cryptomelane and goethite (Rao et al., 2000). There is enrichment of Na-K-Ca-Mg in Anujurhi manganese ores which is diagnostic assemblage for marine manganese ox- ides (Nicholson, 1992). Fig. 6. Back-scattered electron images (EPMA) showing different textures (a) Romanechite (Rm) being altered by clay mineral gorceixite, (b) Romanechite (Rm) as fracture filling in brecciated quartz (qtz), (c) Garnet grains (dark grey) with alteration rims of oxides and/or hydroxides of manganese and iron, enclosed in a matrix of Romanechite (Rm) being altered to Gorceixite (Gx) & (d) ilmenite (Ilm)-pyrophanite (Pyp) zoning within Romanechite (Rm). 295Mineralogy, geochemistry and genesis of low-grade manganese ores of Anujurhi area, Eastern Ghats, India Genesis of manganese ores The four sources of material for sedimentary manganese deposits are hydrothermal, hydrog- enous, detrital, and diagenetic. Sea-f loor hydro- thermal crusts typically have fractionated Fe and Mn concentrations, resulting in either Fe-rich (Fe/Mn > 10) or Mn-rich (Fe/Mn < 0.1) deposits. On the other hand, the Fe/Mn ratio of hydroge- nous ferromanganese sediments, such as deep-sea manganese nodules, averages about unity (Crerar et al., 1982). The Anujurhi manganese ores have an average Fe/Mn ratio of 0.81, ranging from 0.06 to 3.48, suggesting that the major source is hy- drogenous ferromanganese sediments. Hydroge- nous deposits are formed by the slow precipitation of Fe and Mn from seawater and are characterized by Mn/Fe ratios between 0.5 to 5 and a relatively high content of trace metals (Bonatti, 1972; Glas- by, 1997), similar to that of the Nishikhal manga- nese ores (Acharya, 1997). The Si/Al ratio can be used to differentiate be- tween hydrothermal, hydrogenous, and detrital materials and sources (Crerar et al., 1982; Choi and Hariya, 1992). Hydrogenous ferromanganese Table 4. Representative chemical composition of manganese ores (wt%). Sample ANJ/1 ANJ/2 ANJ/3 ANJ/4 ANJ/5 ANJ/6 ANJ/7 ANJ/8 ANJ/9 ANJ/10 Si 12.53 17.37 16.56 20.35 14.52 15.11 8.69 5.40 17.12 14.91 Al 13.01 13.64 12.04 5.93 5.46 14.31 3.02 5.73 3.95 3.68 Mn 10.94 10.33 11.4 13.58 25.29 10.65 17.06 19.30 12.78 15.37 Fe (T) 8.64 6.35 8.37 11.54 4.03 6.69 27.25 24.54 16.61 18.26 Mg 0.16 0.19 0.20 0.14 0.20 0.14 0.07 0.22 0.02 0.03 Na 0.36 0.27 0.20 0.18 0.17 0.16 0.04 0.04 0.00 0.00 Ti 1.04 0.86 0.72 0.17 0.25 0.84 0.06 0.23 0.22 0.19 Ca 0.69 0.36 0.24 0.24 3.31 0.21 0.10 0.69 0.54 0.64 P 1.41 0.47 0.32 0.29 1.65 0.31 0.68 0.53 0.87 0.60 Mn/Fe 1.27 1.63 1.36 1.18 6.28 1.59 0.63 0.79 0.77 0.84 Si/Al 0.96 1.27 1.38 3.43 2.66 1.06 2.87 0.94 4.33 4.05 Fig. 7. Scatterplot correla- tion between major oxides of Anujurhi manganese ores of EGMB. 296 Sujata PATTNAIK & Satrughan MAJHI nodules typically have a Si/Al ratio of about 3, which is characteristic of marine sediment. Ferro- manganese crusts have a mean Si/Al ratio of 5.1, while iron-rich hydrothermal crusts exhibit excep- tionally high Si/Al ratios ranging from 600 to 900, suggesting an additional source of Si in these de- posits. Some hydrothermal manganese-rich crusts also show high Si/Al ratios, ranging from 10 to 20 (Toth, 1980). The Si/Al ratio in the studied man- ganese ore ranges from 0.82 to 37.76, with a mean ratio of 3.38 (Fig. 8). Most samples fall within the hydrogenous field, indicating a source from ferro- manganese crusts and marine sediments. Howev- er, a few samples with high Si/Al ratios fall within the hydrothermal field, suggesting a possible hy- drothermal source for some manganese ores. The significant presence of alumina in certain samples can be attributed to the abundant clay minerals re- sulting from the chemical weathering of host rocks. The scatter plots of Na against Mg clearly dis- tinguishes manganese oxides deposited in marine, shallow marine and freshwater environments (Ni- cholson, 1992). The plots of Na versus Mg (Fig. 9) lie in the freshwater field of Nicholson (1992) sim- ilar to the samples of Nishikhal & Kutinga areas (Acharya, 1994 & 1997). The association of Mn-Ba indicates a probable freshwater origin (Nicholson, 1992). Nonetheless, it’s crucial to acknowledge that Ba is also enriched in hydrothermal minerali- zation (Nicholson, 1992). The CaO-Na2O-MgO ternary plot (after Das- gupta et al., 1999) shows that the Mn oxide ores are associated to both marine sedimentary envi- ronments and freshwater sedimentation in lakes (Fig. 10). In the Anujurhi area, the prevalence of oxide facies without any manganese carbonate mineral further suggests that the ores were depos- ited in highly oxidizing environments in shallow water, near shore, or shelf settings (Roy, 1981; Dasgupta et al., 1993; Nicholson et al., 1997). Ad- ditionally, the Fe-Six2-Mn ternary plot after Toth, 1980 also indicates that manganese originated from Fe-Mn crusts and nodules (Fig. 11). Ti is typ- ically not mobile in hydrothermal solutions and is used to gauge the amount of clastic input (Choi & Hariya, 1992). A high concentration of Ti indicates the mixing of detrital material during precipita- tion, which is backed by a strong correlation be- tween aluminum (Al) and titanium (Ti) as shown in Figure 12. The khondalite succession consists mainly of shallow-water sediments including orthoquartz- ite-carbonate suite, arkoses, and semi-pelites, with manganese beds indicating a passive conti- nental margin assemblage (Acharya, 1997; Ram- akrishnan et al., 1998; Roy, 2006; Nanda, 2008). In the Anujurhi area, well-defined bands of man- ganese ore occur with the same strike and dip as the dominant foliation in the quartzite and quartz-feldspar-garnet-sillimanite gneiss. Addi- tionally, the similar imprints of different phases of folds in manganese ore bodies and the country rocks strongly suggest that the manganese ores have developed as a syngenetic part of the meta- sedimentary sequence of the Eastern Ghats com- plex. The sediments, primarily resulting from conti- nental weathering and containing iron and man- ganese, were transported to deposition sites such as lakes or shallow seas through various mecha- nisms. These include being carried as finely divided particles in river waters, as adsorbed compounds on clay particles, and as sols and gels. The pre- cipitation of iron and manganese in sedimentary Fig. 8. Plots of Aujurhi manganese ore samples in the Si vs Al graph (Choi & Hariya 1992). Fig. 9. Na vs Mg discrimination diagram of Nicholson (1992) showing Anujurhi manganese ore samples. 297Mineralogy, geochemistry and genesis of low-grade manganese ores of Anujurhi area, Eastern Ghats, India environments is significantly inf luenced by Eh- pH conditions. Manganese is soluble at low Eh and precipitates with increasing Eh (under strong- ly oxidizing conditions) at a pH ranging from 5–8, which corresponds to the pH of surface water to seawater (Maynard, 1983; Nicholson et al., 1997). Iron and manganese may have precipitated as iron and manganese hydroxides and oxides (Mn4+) in the presence of free oxygen. During early di- agenesis, Mn4+ oxides are reduced to Mn2+ oxides through anaerobic reduction. The model is supported by geochronologi- cal data indicating that the protolith ages of the meta-sedimentary rocks of Domain 3 in Eastern Ghats are approximately ≈ 2.2 Ga to 1.8 Ga (Rick- ers et al., 2001). This coincides with the worldwide f irst major deposition of sedimentary manganese during the early Paleoproterozoic era, lasting un- til 1.8 Ga, which occurred concurrently with the Great Oxygenation Event (GOE) between 2.45 Ga to 2.1 Ga (Spinks, 2018). These manganese ores have undergone multiple phases of deformation along with the host rocks, as evidenced by the folding of manganese ore bands and their parallel disposition to the S2 foliation. The manganese formations and associated host rocks have experienced at least two phases of ultra-high temperature (UHT) metamorphism at around 1100 Ma and granulite facies meta- morphism at approximately 950-900 Ma, which are related to the breakup of the Supercontinent Columbia and the assembly of the Superconti- nent Rodinia (Karmakar et al., 2009; Bose et al., 201; Dasgupta, 2019). With an increase in pres- sure and temperature during metamorphism, the manganese minerals of higher valency states were transformed into oxides of relatively lower valency (Roy, 1981; Acharya, 1997). After the post-metamorphic period around 950-900 Ma, the Eastern Ghats and Rayner Com- plex amalgamated during the formation of the Supercontinent Rodinia (Karmakar et al., 2009). This caused tectonic uplift and prolonged exposure to atmospheric oxygen and percolation of meteoric water, leading to the alteration of primary miner- als through supergene enrichment. The primary manganese oxide minerals were largely obliterat- ed, evident from the absence of primary manga- nese oxides and alteration of Mn- silicate like sp- essartine. Under these supergene conditions, the strong oxidation effects caused a transformation of the lower valency manganese oxides (primary minerals) into higher valency oxides such as cryp- tomelane, romanechite, and pyrolusite (Acharya et al., 1994). An intriguing example of this trans- formation is the development of romanechite and/ or cryptomelane and goethite from the manganese garnet (spessartine) i.e. supported by petrography and EPMA. The mineral-rich f luid may have jour- neyed through surface run-off or meteoric water to structurally weak planes like shear and fracture planes of the meta-sedimentaries, where Mn and Fig. 10. CaO-Na2O-MgO ternary plot after Dasgupta et al., 1999 shows majority samples falling in marine field for the manganese ore samples of Anujurhi, EGMB, Odisha. Fig. 11. Fe-Six2-Mn ternary plot after Toth, 1980 showing the manganese ore samples showing affiliation towards Fe-Mn crusts and nodules. Fig. 12. TiO2-AI2O3 diagram showing positive correlation for the Anujurhi samples. 298 Sujata PATTNAIK & Satrughan MAJHI Fe reprecipitated as cavity filling and replacement deposits, predominantly containing quartz and feldspar. The replacement and colloidal textures of the secondary manganese oxides provide compel- ling evidence for this origin. The remobilized supergene manganese ores occur as lensoidal ore bodies within the granulite facies of rocks of EGB, persisting for a significant strike length. These ore bodies have been con- f irmed to exist up to a depth of 30–35 m from the surface through drilling. The manganese content in these EGB ores, averaging around 15 %, is lower than that of the Iron Ore Group (Mohapatra et al., 2009) and can be classified as ferruginous man- ganese ore. The high phosphorus content in these ores presents a challenge for commercial recovery, as evidenced in the case of Kutinga-Nishikhal ores (Acharya et al., 1994 & 1997; Rao et al., 2000). However, characterizing these low-grade ores for their mineralogy and association could unlock promising prospects for future utilization. Hence, a thorough study of these ores in the future could be undertaken using Raman Spectroscopy in conjunction with Scanning Electron Microscope (SEM) studies, accompanied by trace element ge- ochemistry analysis. This approach is expected to provide detailed insights into the mineralogy and geochemical behavior of the processes involved during the supergene enrichment and subsequent mobilization to form commercially viable deposits. Acknowledgements The authors would like to express their gratitude to the Deputy Director General of the Geological Survey of India, State Unit: Odisha, for providing administrative and financial support to this work between 2016 and 2018. The study was conducted as part of the mineral exploration project (FSPID: ME/ER/ODS/2016/008 & M2AFGBM-MEP/NC/ER/SU-ODS/2017/12919). The authors are also thankful to Shri Bijay Kumar Sahu, Retired Deputy Director General, GSI, and Shri Manoj Kumar Patel, Retired Deputy Director General, GSI, for their suggestions, guidance, and supervision during f ieldwork. The authors express their appreciation to the personnel working in Petrological Lab of Odisha, the Chemical Lab of Eastern Region, Mineral Physics Divi- sion and EPMA lab of NCEGR at Central Headquarters, GSI, Kolkata, for their timely submission of results and cooperation during lab work. 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