Manganese Formations Research Articles

Late Mesoproterozoic iron and manganese deposition on a deep-water carbonate platform (Penganga Group), Adilabad, India: Constraints from C, O and Sr isotopes

Abstract The Penganga Group in the Pranhita-Godavari Rift Valley of central India includes a rather unique example of Late Mesoproterozoic iron and manganese formations deposited in a deep-water below storm wave base distal carbonate platform environment. The carbonate rocks of the Penganga Group are mostly lime-mudstones that have been classified into a number of sedimentary cycles with intervals mostly distinguished by color, ranging from brown/pink and siliceous gray to steel-gray and black. At least two about one-meter-thick Mn-Fe formations occur in siliceous gray limestone of the distally steepened part of the Penganga carbonate platform. A number of northwest-southeast striking thrust faults repeat the succession in down dip sections bringing up distal profiles of the Penganga Group against proximal counterparts at the present exposure level. This contribution provides the first comprehensive record of stable C and O and radiogenic Sr isotopes for proximal and distal profiles, allowing for direct comparison and thus, providing context for the environmental conditions of Fe and Mn deposition in a Mesoproterozoic platform environment. Carbon and oxygen isotope compositions determined from the proximal and distal profiles of the platform yield trends that are attributed to variations in circulation pattern and cycles of warm and cold climatic stages across the platform. Secular trends of Sr-isotope compositions suggest a variation in continental input between cold (0.7065) and warm climatic stages (0.7149), similar to that observed for modern marine sediments. The sedimentation of siliceous deposits and ferromanganese deposits in the distal platform environment was likely favored by the onset of upwelling circulation during cold climatic stages. The moderately negative δ13CPDB values (-5.34‰ to -6.34‰) of the Mn-carbonates and variation in δ13Corg values 31.7‰ to -21.7‰ obtained for early diagenetic Mn-carbonate ovoids indicate oxidation of organic matter in the ferromanganese deposits by Mn-oxides during early diagenesis. We conclude that upwelling and platform-wide deposition of ferromanganese deposits on the Penganga carbonate platform may mark the transition from the stagnant Mid-Proterozoic ‘sulphidic’ ocean to a ‘respiring’ open ocean during the Late Mesoproterozoic Era.

Paleo–Mesoarchean sedimentary record in the Dharwar Craton, India: Implications for Archean ocean oxygenation

The early Archean oceans were marked by significant redox changes which have subsequently shaped the Earth’s biosphere. Archean chemical sediments of banded Iron and Manganese formations provide important geochemical proxies for these historical shifts in the redox conditions and to trace the ancient sedimentation patterns and protoliths. In this study, we investigate the proto-ore of the Archean Mn-formations of the Sandur, Chitradurga and Shimoga greenstone belts of Dharwar Craton of southern Peninsular India, which is geochemically characterised as quartz arenites, Mn-arenites, Fe-arenites, Mn-argillites and Fe-argillites. The geochemical systematics suggest their deposition in shallow to deeper shelf in the Archean proto-ocean. The detrital zircon U-Pb systematics of Mn arenites and argillites indicate their maximum depositional age of 3230 ± 52 Ma representing the oldest onset of sedimentation during the Paleo–Mesoarchean timeframe in the Chitradurga Group of Dharwar Supergroup. The detrital influx proxies suggest variations in sedimentation rates associated with the Archean transgressive–regressive cycles and fluctuating hydrodynamic conditions, together reflecting an increasing trend in the contributions of recycled sediment from Sandur to Chitradurga and Shimoga greenstone belts. The available detrital zircon ages of the Mn arenites and argillites from these greenstone belts indicate a ∼ 600 Ma prolonged period of Mn deposition for which high-T hydrothermal fluids from Archean mid-oceanic ridges supplied the manganese. The trace element compositions of the concordant detrital zircons suggest 3.3–3.1 Ga Dharwar basement TTG/granitoid source which is corroborated by the zircon crystallization temperatures of 690–820 °C. The source-normalised α-dose rates of the detrital zircons signify greater degrees of sediment transport and multi-cycle nature which correspond to the earliest episode of crustal growth in the Indian sub-continent associated with the Mesoarchean Ur supercontinent. The clastic-chemogenic sedimentation attained through concomitant detrital sediment–seawater-metalliferous hydrothermal fluid mixing at an epicontinental passive margin resulted in the deposition of Mn-arenites and argillites closer to the higher Eh shore, while the Fe-rich sediments formed at a relatively deeper shelf characterised by comparatively lower Eh and more alkaline conditions. The comprehensive geochemical and geochronological data of the Archean Mn arenite-argillite sequences reveal the significance of regional episodes of ocean oxygenation at the shallow shelves of Archean oceans prior to great oxygenation event (GOE) that was mediated by the prolific growth of ancient microbiota which transformed the Earth to a more habitable planet.

Archean Biogeochemical Cognizance from Dharwar Craton, India – A Review

Abstract The Banded iron formations (BIFs) and manganese formations of Chitradurga, Shimoga and Sandur greenstone belts of Dharwar craton, associated with the stromatolitic carbonates, carbonaceous phyllites and shales along with gold mineralization, are best geological entities to evaluate the Archean biogeochemical processes and transformation of a habitable Earth. The geochemical anomalies along with C, O and S isotopic signatures of stromatolitic carbonates, carbonaceous phyllites and sulphidic BIFs reflect on biogenic signatures, fluctuating Archean ocean temperatures from 25-75°C and anoxic to euxinic redox conditions. The U-Pb detrital zircon ages of these stromatolitic carbonates indicate 3.5 to 2.6 Ga whereas the carbonaceous shales indicate 3.2 – 2.2 Ga reflecting the transportation of organic matter to the ocean basin during the growth of stromatolitic carbonates. The gold content of carbonaceous phyllites and sulphidic BIFs indicates hydrothermal source. The O2 produced due to stromatolitic activity has deposited Mn and Fe of the Archean oceans as BIFs and Mn formations. The biogenic matter of the stromatolites along with other siliciclastic material, gold and sulphides derived from the volcanic activity mixed and formed as carbonaceous shales in the ocean basin under euxinic conditions. The comprehensive geological, geochemical including isotopic studies on these rock types collectively indicate the interaction of lithosphere-hydrosphere-atmosphere-biosphere in the Archean oceans which paved the way for the advanced forms of life.

The origin of manganese formations at Çağalali Hill and Kevri̇kevr Hill related to the late Triassic rifting of the South Neotethys, SE Turkey: The geological, mineralogical and geochemical components

The Koçali Complex includes continental rifting and subduction-related units that make up the southern Neotethys. This complex is a folded, tectonic-segmented succession with lavas, volcaniclastic sediments, pelagic carbonates, radiolarites and manganese deposits (the Tarasa and Konak formations) and ophiolitic units (the Kale formation). Manganese mineralizations in the host formation are found in banded, lenticular and massive forms within red to brown radiolarian cherts. Pyrolusite and polianite are the main manganese minerals, while psilomelane, manganite and minor hausmannite are other manganese minerals. Gangue minerals include quartz and carbonate, with minor barite being observed. The geochemical proofs (MnO: 8.98–73.64%, SiO2: 3.26–86%) originating from the main oxides indicates a hydrothermal rooted source for the manganese mineralization. This is confirmed by anomalies and trace element patterns including very low Fe/Mn (<0.1), low Co/Zn ratios (mean 0.86), very low ∑REE contents (mean 34.67), slightly negative Eu* value (mean 0.74) and low Ce* value (mean −0.23). Manganese-oxide mineralizations observed in the volcanic sedimentary succession of the Koçali Complex have been interpreted as hydrothermal deposits associated with volcanism during the post-rifting seafloor extension of the southern Neotethys during the Late Triassic period.

Iron isotope constraints on the metal source and depositional environment of the Neoproterozoic banded iron- and manganese deposits in Urucum, Brazil

The Urucum area of Brazil hosts a series of Cryogenian ironstones intercalated by oxide-dominated manganese layers. The Urucum iron and manganese formations (IF-MnF) are among the largest sedimentary iron and associated manganese deposits of the Neoproterozoic, however, the depositional model and the source of metals for the IF-MnF in this area are highly controversial. In this study, we performed systematic Fe isotope analysis on fresh and geochemically characterized drill core samples of the Urucum iron and manganese formation deposited in the center of the ancient Urucum graben system. The samples have a large variation in Fe isotope composition, with a δ56Fe range of −2.04‰ to +0.75‰, and exhibit a general trend of decreasing δ56Fe values with increasing manganese contents. The low δ56Fe values of the IF and MnF samples reflect Rayleigh fractionation processes of contineous partial oxidation of aqueous Fe(II) prior to deposition at the sampling site. Using a mixing model and previously published Nd isotope data on the same samples, we estimated that benthic (i.e., porewaters released from submarine sediments in the Urucum basin) Fe fluxes provided 7–50% of total Fe in the Urucum IF-MnF, and the rest of Fe source was from low-temperature hydrothermal vents. Based on combined Fe and Nd isotope data of the Urucum IF-MnF, we propose that low-temperature hydrothermal fluids and benthic fluxes of pore waters were mixed and transported by an upwelling current. The fluid subsequently experienced partial oxidation during the transportation process and became enriched in light Fe isotopes. In the Urucum graben basin, the iron- and manganese-rich oxides deposition occurred progressively under increasingly oxidizing conditions, and such process could have operated repeatedly to produce the alternation of iron and manganese formations. The chemical sediments of the Urucum IF-MnF deposits thus reflect the existence of a sharp redox gradient in the marine environment during the late Cryogenian period.

Iron Bacteria Gallionella in Archean of India

This is the first report on the fossilized fibers found in the Archean banded manganese formations of the Chitradurga Greenstone Belt (2.8 Ga, South India), which are similar to the stalk fibers of modern Gallionella ferruginea Ehrenberg, 1838. We have therefore concluded that the intertwined and assembled stalk fibers provide indirect evidence for the presence of bacteria and biogenic mineral formation, but should not be considered as microorganisms.

МАРГАНЦЕВОЕ РУДООБРАЗОВАНИЕ НА ПОДВОДНОЙ ВОЗВЫШЕННОСТИ БЕЛЯЕВСКОГО В ЯПОНСКОМ МОРЕ (ОБЗОР ЛИТЕРАТУРЫ И НОВЫЕ ДАННЫЕ)

The paper gives systematized and generalized information on manganese formations from the northern and southern summits of Belyaevsky Seamount in the Sea of Japan. This information shows that low-temperature hydrothermal fast-growing crusts are spread on the seamount widely. The crusts have bedded structure. Their textures are collomorphic and fibrous. The main ore minerals are birnessite and todorokite while the accessory minerals include vernadite, manganite, and pyrolusite. There are samples where ore minerals form veinlets in basalts and cement aggregates of clay minerals like smectite-hydromica. Silica minerals and barite crystallized at final mineralization stages. Micro-sized mineral phases of Cu, Zn, Sn, Ni, W, Cr, Ag, Ce, La, and Nd containing in the crusts and classifying by chemical composition as native, sulfide, sulfate, oxide, and intermetallic are obviously superimposed and meet high- and medium-temperature conditions of mineral formation. Crusts’ bulk chemical composition demonstrates abundances of Mn, Ba, Sr, and Li, as well as depletion of Fe, Al, Ti, and many trace elements. Positive Ce anomalies are likely to record the layers formed in the crusts due to the hydrogenic substance supply with hydrothermal ore formation processes being under attenuation.

Grains of Nonferrous and Noble Metals in Iron–Manganese Formations and Igneous Rocks of Submarine Elevations of the Sea of Japan

Iron–manganese formations and igneous rocks of submarine elevations in the Sea of Japan contain overlapping mineral phases (grains) with quite identical morphology, localization, and chemical composition. Most of the grains conform to oxides, intermetallic compounds, native elements, sulfides, and sulfates in terms of the set of nonferrous, noble, and certain other metals (Cu, Zn, Sn, Pb, Ni, Mo, Ag, Pd, and Pt). The main conclusion that postvolcanic hydrothermal fluids are the key sources of metals is based upon a comparison of the data of electron microprobe analysis of iron–manganese formations and igneous rocks dredged at the same submarine elevations in the Sea of Japan.

Geochemical characterisation of Neoproterozoic marine habitats: Evidence from trace elements and Nd isotopes in the Urucum iron and manganese formations, Brazil

The Urucum iron and manganese formation (IF and MnF) in the Neoproterozoic Santa Cruz Formation, Brazil, provides unique insights into Neoproterozoic seawater and the habitat for the evolution of the Ediacaran fauna. Pure Urucum IF drill core samples, i.e. chemical sediments that are devoid of any syn- or post-depositional alteration, are reliable and robust archives of Neoproterozoic seawater proxies. They display low concentrations of rather immobile elements and shale-normalized rare earths and yttrium (REYSN) patterns similar to those of modern seawater. Positive LaSN and GdSN anomalies, enrichment of heavy relative to light REYSN, negative CeSN anomalies and super-chondritic Y/Ho ratios indicate a depositional environment dominated by open ocean water masses and oxic atmosphere–hydrosphere conditions. The REYSN patterns of manganese-rich chemical sediments from the Mn1 and Mn2 horizons, however, show significantly different REY distributions between and within these units. While REYSN patterns from the Mn2 unit are similar to those of the IF, the REY systematics of the Mn1 horizon can be subdivided into two different groups. Fe-poor group A shows rather flat REYSN patterns with large negative CeSN anomalies, while Fe-rich group B shows insignificant to positive CeSN anomalies, YbSN/PrSN ratios below unity and sub-chondritic Y/Ho ratios, suggesting that trace element distributions are controlled by the mineralogical composition rather than by sedimentation rate.Pure IF samples, reflecting Urucum seawater, yield εNd0.635Ga values between −4.56 and −4.08, and are more positive than those of the MnF (−5.52 to −4.66) and associated siliciclastic rocks (−8.35 to −7.69), and are considerably more radiogenic than the crystalline Rio Apa Basement (−13.7). While clastic sediments originated from the Amazonia Craton, the dissolved REY budget of Urucum seawater was derived from terrigenous material of the nearby Neoproterozoic Brasília Belt. There is no evidence for any REY input via high-temperature, hydrothermal fluids or for REY input from a mantle source.

Geochemistry of Mn-bearing rocks hosted in rhyolites from Hadjer Bigli (SE Lake Chad, Cameroon Line): preliminary results

Manganese-bearing rocks consisting of anhedral Mnand Fe-oxyhydroxides (cryptomelane, goethite), kaolinite, tiny cerianite and zircon grains (< 10 μm), and euhedral to subeuhedral quartz crystals, are hosted within rhyolitic rocks of Hadjer Bigli volcano (SE Lake Chad). Their Ce (1328 ppm) and Zn (1563 ppm) contents are increased, with high ∑REE values (1573 ppm).The Mn-rich rocks have been shown to be one of the most important environments for adsorption of some trace elements. The high Zn contents are considered to be the result of water–rock chemical reactions at intermediate to high temperatures. The sedimentary processes led likely to the concentration of manganese in the Lake Chad water and beneath, well before the volcanic activity. According to mineral chemistry and the whole-rock data, rhyolitic magma interacted with the near-surface manganese formations already concentrated by sedimentary processes.

Mineralogy and geochemistry of Neoproterozoic siliceous manganese formations from Ntui–Betamba (Cameroon Pan-African Fold Belt): implications for mineral exploration

The siliceous manganese formations from Ntui–Betamba area have been studied with the aim to constrain their mineralogy, geochemistry, and genesis. Neoproterozoic manganese-bearing rocks at Ntui–Betamba include garnet–galaxite-bearing quartzite and galaxite-bearing quartzite that—besides a characteristic quartz, spessartine-garnet, and galaxite (MnAl2O4) assemblage—comprises also graphite and rutile. Galaxite crystals show chemical zoning from core to rim due to the change in composition from galaxite to cryptomelane. Bulk rock geochemistry reveals that the studied rocks are characterized by high SiO2 content (52.83 wt%). They are enriched in Fe2O3 and Al2O3 and display Mn/Fe ratio of 1.2. MnO concentrations are high in galaxite-bearing quartzite (16 wt%), and moderate (8.03 wt%) in garnet–galaxite-bearing quartzite. The transition trace metal (Co, Cu, and Ni) contents are lower, with Co and Cu markedly depleted relative to Ni, and variable contents of Zn. The REE pattern of garnet–galaxite-bearing quartzite shows Ce-positive anomaly similar to the hydrogenous sediment, but differs from the later with strong Eu-negative anomaly and the lack of weak positive anomaly in Yb, whereas the galaxite-bearing quartzite is characterized by the lack of Ce anomaly and the weak Eu-negative anomaly. Both rocks show HREE content close to that of hydrothermal sediment and exhibit flat HREE trend. The high contents of silica, iron, and aluminum, together with the Co/Zn ratios (~0.3) and the contrasting behavior of REE, suggest that the studied manganese-bearing rocks are of sedimentary origin probably derived from a mixed hydrothermal–hydrogenous source. The high content of Mn-bearing minerals in these rocks represents indicators of concealed or buried mineralization and could be used as exploration guide.

Manganese formations in the accretionary belts of Japan: Implications for subduction–accretion process in an active convergent margin

In the accretionary complexes of Japan, many bedded manganese and iron–manganese ore deposits occur, especially in the Jurassic complexes such as the Chichibu, Tamba, Mino, Ashio and Northern Kitakami belts. The manganese ores in these Jurassic accretionary complexes probably formed from manganese nodule/crust-bearing siliceous sediments on deep-sea floor and were subsequently converted to the manganese ores by metamorphism during the subduction–accretion process. Some of the deposits also show the signatures of younger granitic intrusions. The manganese formations now incorporated within these belts are marker beds of accretionary tectonics associated with plate tectonic processes in convergent margins.

GEOCHEMICAL AND MINERALOGICAL CONSTRAINTS ON THE GENESIS OF THE OTJOSONDU FERROMANGANESE DEPOSIT, NAMIBIA: HYDROTHERMAL EXHALATIVE VERSUS HYDROGENETIC (INCLUDING SNOWBALL-EARTH) ORIGINS

The stratigraphic position of the Otjosondu ferromanganese deposit in the Neoproterozoic Damara Supergroup in the Central Zone of the Pan-African Damara orogenic belt is uncertain. Comparison of the enclosing host rocks with regional profiles in the southern Central Zone would indicate that it is hosted by the Etusis Formation (Fe-rich feldspar-quartz rocks) of the Nosib Group, in a condensed stratigraphic section of the supergroup, and not the Chuos Formation (diamictite, turbidite and thin iron-formation layers) of the Swakop Group, as previously proposed. The Otjosondu ore comprises ferromanganese silicate-oxide assemblages that occur in a feldspathic-quartzite sequence lacking the mixtite facies typical of the Chuos Formation. Hematitic iron formation containing minor hyalophane, spessartine and barite, and hyalophane-bearing spessartine-quartz rock are present in close spatial association with the Mn silicate-oxide rocks. Barite is a widespread and locally abundant mineral in the Mn silicate-oxide ore. Bulk-rock chemical analyses indicate trace-element associations of Ag-As-Cu-Co-Bi-B-P with Mn and Sr-Mo-W with Ba in the Mn silicate-oxide and spessartine-quartz rocks. These element associations are explained by adsorption/substitution processes in an oxygenated alkaline marine setting. Sulfur-isotope compositions of barite from the Mn silicate-oxide ore vary from +10.1 to +16.7 ‰, a range of δ34S values that shows negative correlation with bulk-rock Fe content and is interpreted as mixing of isotopically light hydrothermal S with heavier sea-water S. The protoliths of the ferromanganese rocks were precipitates deposited in a basin-margin setting as a result of mixing of an exhalative hydrothermal fluid with oxygenated, sulfate-bearing sea water, and varying degrees of detrital addition. These conclusions are in contradiction to the deep-water, anoxic, sulfate-depleted oceanic settings required for formation of glacially associated Rapitan-type iron and manganese formations. The formation of the Otjosondu ferromanganese deposit is therefore unrelated to any Chuos/Sturtian global glacial event. Origins as hydrogenetic deposits are mostly excluded due to the base-metal- and P-poor, but Ba-rich nature of the Otjosondu ores. The baritic ferromanganese, hematite-quartz and spessartine-quartz rocks at Otjosondu may be compared to oxidised baritic portions of the Gamsberg (Broken Hill-type) base-metal deposit in South Africa and may represent the distal or shallow-water oxidised equivalents of sediment-hosted exhalative base-metal mineralisation such as the penecontemporaneous Tsongoari and Rosh Pinah deposits.

Chapter 2 A history of Neoproterozoic glacial geology, 1871–1997

Abstract Neoproterozoic glacial records have been discovered on 23 palaeocontinents, their rate of discovery changing little since 1871. Yet, half of all the resulting publications appeared since 2000. The history of research before 1998 is described in five stages defined by publication spikes; subsequent work is not covered because historical perspective is lacking. In stage 1 (1871–1907), ‘Cambrian’ (now Neoproterozoic) glaciation was recognized successively in Scotland, Australia, India, Norway, Svalbard and China. Criteria for recognition included faceted and striated pebbles in matrix-supported conglomerates resting on ice-worn bedrock pavements. In stage 2 (1908–1940), Neoproterozoic glaciation was shown to have been widespread in Africa, Asia and the Americas. Major textbooks summarized these findings, but the rejection of continental drift (to account for late Palaeozoic glacial dynamics) put a chill on research. In stage 3 (1942–1964), the occurrence of glacial deposits within carbonate successions, as well as nascent palaeomagnetic observations, suggested that Neoproterozoic glaciers reached sea-level at low palaeolatitudes, but the belated recognition of sediment gravity flowage caused glacial interpretations to be prematurely abandoned in key areas. In stage 4 (1965–1981), the extent of Neoproterozoic glaciation was rethought in light of plate tectonics. Distinctive chemical sediments (iron±manganese formations and cap carbonates) were identified. In basic climate models, modest lowering of solar luminosity resulted in global glaciation due to ice-albedo feedback, and deglaciation due to greenhouse forcing resulted from silicate-weathering feedback in the carbon cycle. Neoproterozoic glacial geologists were blind to these ideas. In stage 5 (1982–1997), reliable palaeomagnetic data combined with glacial marine sedimentation models confirmed that Neoproterozoic ice sheets reached sea level close to the palaeoequator.

GEOCHEMISTRY AND MINERALOGY OF NEOPROTEROZOIC BANDED IRON-FORMATIONS AND SOME SELECTED, SILICEOUS MANGANESE FORMATIONS FROM THE URUCUM DISTRICT, MATO GROSSO DO SUL, BRAZIL

This study characterizes the precursor mineralogy and geochemistry of the Neoproterozoic iron ore deposits as well as some associated Na-containing manganese assemblages of the Urucum district, Mato Grosso do Sul, Brazil. It is based on ten mineralogically well characterized samples (six of banded iron-formation (BIF), three of manganese formations, and a sample of a veinlet that crosscuts the manganese assemblage), which were carefully selected so as avoid as much as possible supergene enrichment and secondary alteration (weathering), both of which are pervasive in the Urucum district. The six BIF samples are representative of the extensive Urucum BIF sequence from which the rich iron ores were developed by supergene enrichment and are considered to be precursors to the iron ore. The manganese-rich samples are part of unusual siliceous manganese horizons that contain complex silicate assemblages with braunite, cryptomelane, some pyrolusite, and authigenic aegirine. The Urucum BIF sequence is distinctive because it consists almost entirely of hematite and chert (jasper) with almost all of the iron present as only Fe2O3. This is in sharp contrast to the iron chemistry of much older (Archean and Early Proterozoic) BIF, in which a very large proportion of the iron occurs as ferrous iron in magnetite, carbonates, and silicates. As such, the Urucum BIF are essentially identical to those of the Neoproterozoic Rapitan sequence (755–730 Ma) of the Yukon and Northwest Territories of Canada. The Urucum sequence contains abundant dropstones, whereas the Rapitan sequence is set among diamictites but also contains dropstones. The δ 13C values of carbonates at Urucum are low, ranging from –5.2 to –7.0 per mil, which reflects their deposition in a glaciomarine setting. The REE concentrations of the BIF, as well as three Mn formation samples, are very similar and are almost completely lacking positive Eu anomalies (relative to NASC). This is in sharp contrast to the pronounced positive Eu anomalies of Archean and Early Proterozoic iron-formations. The general trend of the REE profiles (in NASC plots), with some enrichment of the heavy REE, is qualitatively very similar to that of modern seawater. The source of the Fe, Mn, and Si is concluded to be from typical ocean water with some deep-sea hydrothermal component. The reappearance of the Neoproterozoic Urucum sequence with BIF and interlayered manganese formations, together with the Rapitan sequence of similar Neoproterozoic age, after an absence of such sedimentary sequences in the geologic record for about 1.1 billion years, is considered to reflect ocean stagnation (with anoxic conditions), which may have been caused by a near-global ice cover, referred to as snowball Earth.

CHEMICAL DEPOSITS ASSOCIATED TO METAVOLCANOSEDIMENTARY SEQUENCES OF THE CENTRAL PORTION OF THE SÃO FRANCISCO CRATON IN THE STATE OF BAHIA, BRAZIL: A REVIEW

Supracrustal suites of metavolcanosedimentary rocks (SC) occur in the central portion of the São Francisco Craton in the State of Bahia. They were primarily formed by calc- alkaline to perialkaline volcanic flows and tuffs, clastic and chemical sediments. The latter are referred to the banded iron (BIF), iron-manganese (BIMF) and manganese (BMF) formations, iron-poor magnesian-rich carbonate formation (IPMRCF), and minor metacherts. BIF and IPMRCF are present in all the reported SC. BIF is typified by the presence of the oxide, carbonate and silicate facies. Occurrences of the sulfide facies are reported for the Sobradinho-Salitre and the Boquira SC. The burial metamorphism and thermal effects pronounced by the Transamazonian migmatization and granitization are accountable for the generation of amphibolites, epidosites, steatite and talc schists, phyllites, sandstones, quartzites, itabirites, metamorphic limestones to include banded carbonates, marbles and calc-silicate rocks. It is suggested that the deposition was similarly processed throughout an extensive crustal segment and that it was part of the cratonic evolution occurred during the end of the Archean to the beginning of the Proterozoic.

Chemical deposits associated to metavolcanosedimentary sequences of the central portion of the São Francisco Craton the state of Bahia, Brazil: a review

Supracrustal suites of metavolcanosedimentary rocks (SC) occur in the central portion of the Sao Francisco Craton in the State of Bahia. They were primarily formed by calc- alkaline to perialkaline volcanic flows and tuffs, clastic and chemical sediments. The latter are referred to the banded iron (BIF), iron-manganese (BIMF) and manganese (BMP) formations, iron-poor magnesian-rich carbonate formation (IPMRCF), and minor metacherts. BIF and IPMRCF are present in all the reported SC. BIF is typified by the presence of the oxide, carbonate and silicate facies. Occurrences of the sulfide facies are reported for the Sobradinho-Salitre and the Boquira SC. The burial metamorphism and thermal effects pronounced by the Transamazonian migmatization and granitization are accountable for the generation of amphibolites, epidosites, steatite and talc schists, phyllites, sandstones, quartzites, itabirites, metamorphic limestones to include banded carbonates, marbles and calc-silicate rocks. It is suggested that the deposition was similarly processed throughout an extensive crustal segment and that it was part of the cratonic evolution occurred during the end of the Archean to the beginning of the Proterozoic.

Metamorphic Evolution of Neoproterozoic Manganese Formations and their Country Rocks at Otjosondu, Namibia

Metamorphic Evolution of Neoproterozoic Manganese Formations and their Country Rocks at Otjosondu, Namibia

Geochemistry of FeMn formations of the Archaean Sandur schist belt, India - mixing of clastic and chemical processes at a shallow shelf

The late Archaean greenstone belts of Karnataka are unique in having manganese formations with mineable ores. An excellent example of these Mn-bearing greenstone belts is the Sandur schist belt where Fe + Mn formations are confined to the basin shelf, clearly separated from the deeper-water manganese-free BIF that accumulated at the basin margin and flanking the marine basin. Sedimentary textures and structures including stromatolites show that Fe + Mn formations have been deposited on a shallow shelf within the photic zone and above wave base. Along with quartz Fe + Mn minerals found in arenites, argillites and carbonates include ripidolite, Mg-chlorite, sericite, Mn-siderite, kutnahorite, dolomite, ferroan dolomite, psilomelane, pyrolusite, cryptomelane, haematite and magnetite. The manganese and iron contents of these rocks range up to 25% and 46%, respectively. Al 2O 3, TiO 2, MgO, CaO, MnO 2, Fe 2O 3 (T) abundances and ratios show a wide variation. Ni, Cr, Co, Zr, V, Sc, Rb, Sr, U, Th, ΣREE, LREE/HREE, La, Ce and Eu anomalies and their binary relationships indicate that wherever the terrigenous component has increased, the concentration of elements of felsic parentage such as Zr and Hf has gone up. Elevated concentrations of Ni, Cr, Co and Sc are contributed by chlorite and other components characteristic of basic volcanic debris. The carbonates, where clastic input is minimal, are extremely depleted in REE and other trace elements. It is proposed that FeO, MnO and SiO 2 probably were added to Archaean ocean water at oceanic ridge vents. The compositional data suggest that the Fe + Mn formations of the Sandur schist belt were generated by chemical and clastic sedimentary processes on a shallow shelf that led into a deeper basin open on one side. In this environment, chemical and clastic sediments and organic matter were mixed and buried. During transgression, chemical precipitation took place at the sediment-water interface, whereas at the time of regression, these chemical sediments were buried by clastic sediments. Redox-potential differences between the watermasses on the shallower and deeper shelf, respectively, fractionated Fe and Mn. The Fe + Mn formations were deposited within the photic zone and above wave base and in a setting where the water column was oxygenated but organic material accumulated. By contrast, BIF was deposited below both wave base and photic zone in a relatively oxygen-deficient setting where formation of stable Mn-oxides was not possible. Fe + Mn formation and BIF provide an important constraint on greenstone tectonics. A shorter distance between Archaean oceanic ridge and shelf environments, namely the smaller plates model, appears to explain several features of Fe + Mn formations, BIFs and greenstone belts.

Late Proterozoic outer shelf manganese and iron deposits at Otjosondu (Namibia) related to the Damaran oceanic opening

The Otjosondu manganese mining area is located at the eastern exposed extent of the inland branch of the late Proterozoic Damara orogen. In spite of intense deformation and a high-grade metamorphic overprint, the sedimentological interpretation of the manganeserich rocks and their associated lithologies has led to a paleoenvironmental model which gives insight into ore genesis. At least two manganese silicate ore horizons (manganese formations) are associated with banded iron-formations. This sequence is sandwiched between supermature metaquartzarenites interpreted as a shoreface deposit derived from a cratonic basement and deposited under high-energy conditions.Manganese ore varieties and their country rocks have been classified using their texture, mineralogy, and geochemistry and have thus been interpreted in sedimentological and facies terms. The evaluation of the vertical facies sequences, their lateral distribution and their contact relationships renders a spatial reconstruction of the environmental setting for the manganese ores possible. The development of facies from a near-shore clastic regime to manganese-bearing hemipelagic sediments and ultimately to pelagic manganostones and ironstones reflects a zonation on a marine shelf. The facies evolution indicates transgressive conditions on a large scale onto a tectonically stable but subsiding shelf, starting with the transgressive quartzarenites overlying an unconformity to the pre-Damara basement, followed by chemical sediments of the ore-bearing horizon and overlying carbonates, and ending up with pelitic schists. On a smaller scale both transgressive and regressive conditions prevailed during manganese precipitation. Small-scale sedimentary cycles are interpreted as due to glaciogene-influenced fluctuations of the sea level.The source of manganese and iron is proposed to have been situated in the deeper oceanic parts of the associated basin. Initial spreading in the Khomas sea and the formation of oceanic crust is documented by tholeiitic volcanics and associated base metal orebodies representing more proximal manifestations of the exhalative activity. These base metal sulfide deposits are associated with pyrite-bearing graphitic pelagics and this was the first site to partition iron and other base metals from manganese. Upwelling along the shelf edge was involved and additional separation of Mn and Fe was due to a change in Eh conditions associated with a redox interface intersecting with the outer shelf environment. The evolution of the Khomas sea provided the ore-forming solutions and also initiated the subsidence of the passive shelf margin. Fluctuations in sea level were due to the basin development on one hand and glacial-interglacial cycles on the other. The latter fluctuations controlled the amount of terrigenous input into an otherwise pelagic regime.