Supriya Roy

Late Archean initiation of manganese metallogenesis: its significance and environmental controls

Abstract Manganese metallogenesis was initiated in a modest scale during the Late Archean time selectively in India and Brazil. Sedimentary deposits occur in both greenstone belts and high grade (granulite) terranes in shallow-water shelf regimes only and are often intimately associated with stromatolites. The changes in the rates of continental crustal growth, mantle heat flux and outgassing along with the evolution of tectonic regimes, sedimentary environments, the state of the biosphere and finally the compositions of interacting atmosphere–hydrosphere system from the Early to the Late Archean time have been traced as interdependent parameters and interpreted to explain the first appearance of sedimentary manganese deposits promoted by possibly localized oxygenated environments. Compared to the spectacular ‘manganese event’ represented by the giant Paleoproterozoic Kalahari deposit, South Africa which is interpreted to have been formed as a consequence of the rapid increase in atmospheric oxygen level ca. 2.5–2.4 Ga ago, the modest initiation of the Late Archean manganese depositions was triggered by localized photosynthetic oxygen supply forming basin-margin oxygen oases as a prologue for the succeeding stratified ocean system. The dissolved manganese in the basin water that was oxidized to form shelf-type shallow-water deposits is suspected (by analogy with contemporary banded iron formations (BIFs)), but not proved, to be of hydrothermal ancestry. The initiation of concentrated manganese sedimentation during the Late Archean by itself, provides an evidence for an increase in the oxygen content in the contemporary atmosphere–hydrosphere system compared to that in the earlier period, supporting the geochemical indications from paleoweathering profiles.

Geochemistry of braunite and associated phases in metamorphosed non-calcareous manganese ores of India

In the metamorphosed manganese oxide ores of India, braunite is ubiquitous in all assemblages from chlorite to sillimanite grades. Chemical analyses of braunite from different prograde assemblages confirm the presence of a fixed R2+ (=Mn2++Mg+Ca) SiO3 molecule in the mineral. Element partitioning between coexisting braunite and bixbyite indicates a near-ideal mixing of Fe+3/ -Mn+3 in the phases. This also indicates that braunite became relatively ferrian while equilibrating with associated phases such as bixbyite, hollandite and jacobsite during prograde reactions. Petrogenetic studies show that as a general trend, prograde lower oxide phases appeared by deoxidation of higher oxide phases. But braunite, a more reduced phase than bixbyite, appeared early from deoxidation of pyrolusite in presence of quartz. Bixbyite could appear later from the reacting pyrolusite-braunite-quartz assemblage. Inferred mineral reaction paths and the general trend of pro-grade deoxidation reactions suggest that the composition of ambient fluid phase was internally buffered during metamorphism.

Late diagenetic changes in manganese ores of the Upper Proterozoic Penganga Group, India

Sedimentary manganese-oxide ores occur interstratified with chert and minor calcareous shale and enclosed in limestone in the transgressive Upper Proterozoic marine sequence of the Penganga Group, Andhra Pradesh, India. This sequence shows no evidence of volcanism or hydrothermal activity and is not metamorphosed. The manganeseoxide ores show unusual coexistence of the primary sedimentary minerals, todorokite and birnessite, with their diagenetic conversion products, manganite, braunite and bixbyite, The mineralogical characters and chemical compositions of the important phases as well as their texture are discussed. Braunite occurs in two generations, one earlier or contemporaneous with bixbyite and the other later than bixbyite. The different stages of conversion of todorokite and birnessite to braunite and todorokite to manganite are preserved. The general observations suggest that bixbyite was also produced by conversion of todorokite-birnessite. Pyrolusite and cryptomelane, conventionally considered as diagenetic products of Mn-oxide/hydroxide precursors deposited as primary sediments, are here rarely present and then only as supergene derivatives. This documentation of diagenetic transformation of primary todorokite and birnessite directly to braunite and bixbyite should be considered while extrapolating the progenitor mineralogy of the metamorphosed manganese-oxide ores that abound in the Precambrian.

Atypical ferromanganese nodules from pelagic areas of the Central Indian Basin, equatorial Indian Ocean

Abstract Two box core samples, one each from red clay and siliceous ooze sediment provinces from the Central Indian Basin (Indian Ocean) were studied. Both cores contained ferromanganese nodules at their tops. The red clay sediment also hosts buried nodules at a depth of 82 cm. Chemical characterization of the sediments indicates that early diagenetic remobilization of transitional metals is pronounced in the siliceous ooze sediment, while in the red clay no such effect is discernible. This is further corroborated by the abundance and dissolution patterns of microorganisms and ferromanganese micronodules occurring in the two sediment columns. Bulk chemical analyses indicate that all the nodules are of the hydrogenetic B-type. Vernadite and todorokite are the major minerals, both of which constitute laminated, nondirectional colloform and cuspate microstructures. Petrographic studies yield evidence of recrystallization of todorokite and partial transformation of vernadite to todorokite, the former process being more advanced in buried nodules. Microprobe analyses revealed that primary todorokite has a lower Mn Fe ratio and higher Ni content than recrystallized todorokite in the same sample. The mineralogical reconstitution in the nodules is attributed to postdepositional intranodule diagenesis. The present study does not reveal the correlation between mineralogy and microstructure in the nodules which has been suggested for the nodules from the Pacific, neither can any mineral or microstructure be considered specific for any particular genetic model. The studied nodules are, therefore, atypical. The mineralogical and chemical reordering in these ferromanganese nodules raises the question of the viability of radiometric methods in determining growth rates of such nodules.

Manganese metallogenesis: A review

Abstract Metallogenesis of manganese in space and time is reviewed here in the light of the progressive development of the atmosphere, the hydrosphere and the lithosphere attendant with the varied styles of tectonism. Economic deposits of manganese first appeared c.3000 Ma ago, postdating by at least 800 million years the oldest known geological sequence containing iron-formation and base-metal sulfide ores. The development of manganese deposits in the Archean as a whole, vis-a-vis that of iron-formation and stratiform massive sulfides, was minor. This is possibly a reflection of the composition of the then endogenic exhalations and/or the character of the atmosphere and the hydrosphere. The geologic setting of the manganese deposits of this age was always atypical of the Archean period. Deposition of manganese was intensified with the advent of the Proterozoic with the changing tectonic style leading to stabilization of the cratons and oxygenation of the hydrosphere and the atmosphere. Large to superlarge deposits were formed mainly through terrigenous input during this period. The Mesozoic era ushered in the supremacy of manganese deposition and the peak was reached in Cenozoic time. This was largely due to the formation of giant shallow-water deposits in areas of marine transgression as well as deposition of manganese-rich nodules and crusts in deep-sea environments. Three major aspects of manganese metallogenesis stand out as most important but enigmatic. These are: (a) the extreme fractionation of iron and manganese in nature and their reverse trend of metallogenic development; (b) the universal record of shallow-water deposition of manganese in land-based deposits in contrast with the deep-sea milieu observed in modern basins; and (c) the common evidence of biological activity in close association with manganese deposition which could be either causal or casual. All these aspects merit further in-depth study and metallogenic analysis in a broad spectrum.

Contrasting parageneses in the manganese silicate-carbonate rocks from Parseoni, Sausar Group, India and their interpretation

Mn silicate-carbonate rocks at Parseoni occur as conformable lenses within metapelites and calc-silicate rocks of the Precambrian Sausar Group, India. The host rocks are estimated to have been metamorphosed at uppermost P-T conditions of 500–550°C and 3–4 kbar. The Mn-rich rocks contain appreciable Fe, reflected in the occurrence of magnetite(1) (MnO 1%), magnetite(2) (MnO 15%) and magnetite(3) (MnO 10%). Two contrasting associations of pyroxmangite, with and without tephroite, developed in the Mn silicate-carbonate rocks under isothermal-isobaric conditions. The former assemblage formed in relatively Fe-rich bulk compositions and equilibrated with a metamorphic fluid having a low XCO2 (<0.2), and the latter equilibrated with a CO2-rich fluid. Rhodochrosite+magnetite(1)+quartz protoliths produced the observed mineral assemblages on metamorphism. Partitioning of major elements between coexisting phases is somewhat variable. Fe shows preference for tephroite over pyroxmangite at the ambient physical conditions of metamorphism. Oxygen fugacity during metamorphism was monitored at or near the QFM buffer in tephroite bearing domains, and the fluid composition was buffered by mineral reactions in respective domains. As compared to other metamorphosed Mn deposits of the Sausar Group, the Mn silicate-carbonate rocks at Parseoni were, therefore, metamorphosed at much lower fO2 through complex mineral-fluid interactions.

Petrogenesis of metemorphosed manganese deposits and the nature of the precursor sediments

This paper reviews the current concepts on petrogenesis of metamorphosed manganese-rich sedimentary rocks with the aim to identify the processes, their various controls and the possible progenitors. On the basis of mineralogical composition, five distinct rock types have been identified which show 21 mineral assemblages in total. Mineralogical, textural and chemical data on naturally occurring phases of these five rock types are also presented. These data, together with the results of experimental studies are used to interpret the metamorphic evolution of the different rock types. Finally, an attempt has been made to determine the possible precursor sediment types.

Mineralogy and genesis of the metamorphosed manganese silicate rocks (gondite) of Gowari Wadhona, Madhya Pradesh, India

Manganese silicate rocks, interbanded with manganese oxide orebodies, constitute an important stratigraphic horizon in the Mansar formation of the Sausar Group of Precambrian age in India. The manganese silicate rocks of Gowari Wadhona occupy the westernmost flank of the manganese belt of the Sausar Group. These rocks are constituted of spessartite, calcium-rich rhodonite, quartz, manganoan diopside, blanfordite (manganese bearing member of diopside-acmite series), brown manganese pyroxene (manganese bearing aegirine-augite), winchite (manganese bearing richterite-tremolite), juddite (manganese bearing amphibole with richterite, tremolite, magnesioriebeckite and glaucophane molecules), tirodite (manganese bearing amphibole with richterite, cummingtonite and glaucophane molecules), manganophyllite, alurgite, piedmontite, braunite, hollandite (and other lower oxides of manganese) with minor apatite, plagioclase, calcite, dolomite and microcline. A complete mineralogical account of the manganese-bearing phases has been given in the text. It has been shown that the juxtaposition of manganese silicate rocks with dolomitic marble, regional metamorphism to almandine-amphibolite facies and assimilation of pegmatite veins cutting across the manganese formation, were responsible for the development of these manganese silicate rocks and the unusual chemical composition of some of the constituent minerals. It has been concluded that the manganese silicate rocks of Gowari Wadhona were originally laid down as sediments comprising manganese oxides admixed with clay, silica etc. and were later regionally metamorphosed to almandine-amphibolite facies. All evidences indicate that rhodochrosite was not present in the original sediment and the bulk composition of the sediments was rich in manganese. These rocks agree entirely to the detailed nomenclature of the gondites enunciated by Fermor (1909) and amplified by Roy and Mitra (1964) and Roy (1966).

Controls of evolution of mineral assemblages in ultrahigh-temperature metamorphosed Mn-carbonate-silicate rocks from the Eastern Ghats Belt, India

A unique suite of Mn-silicate-carbonate rocks from the Eastern Ghats Belt, India, were metamorphosed under ultrahigh temperatures and exhibit contrasting mineral assemblages in closely spaced domains. Development of contrasting assemblages under isothermal-isobaric conditions is attributed to initial variations in fluid composition particularly a CO2 , and bulk-rock Mn:Mg:A1:Si:Ca ratios. The fO 2 was buffered near the QFM buffer during peak metamorphism. Garnet shows a sharp increase in Fe +3 content towards the rim, which is interpreted to be the result of increased a CO2 during later stages of evolutions of the rocks due to channelled fluid flux. At the terminal stage of evolution, the rocks suffered hydration at lower temperatures.

Genetic reinterpretation of crystallographicintergrowths of jacobsite andhausmannite from natural assemblages

SummaryCrystallographic intergrowths of jacobsite and hausmannite (“vredenburgite”) occur in association with braunite in the Precambrian Sausar Group of rocks, India, that were metamorphosed under 600-700°C and P ∼ 6 kb. Quartz, hematite, rhodochrosite and a later hausmannite may occasionally occur as minor associates. Detailed characterization of the intergrown phases reveals that hausmannite lamellae, oriented in 4 or 5 crystallographic directions in the jacobsite host, show a wide variation in thickness and tapered intersections at low angles. The lamellae may be locally deformed. Analytical data reveal that the composition of natural hausmannite and jacobsite in the intergrowths cannot be approximated within the system Fe3O4 -Mn3O4, as has been conventionally done. These really belong to the Fe2O3-Mn3O4 subsystem. In the two phase intergrowths, hausmannite is depleted and the jacobsite is enriched in Fe in higher grade rocks. Mineral associations and petrographic considerations suggest that the jacobsite-hausmannite intergrowth originated through prograde decarbonation-oxidation reactions of a carbonatic precursor in an unbuffered X CO2 situation, but f O2 was held between hematite-magnetite and bixbyite-hausmannite buffers at the ambient physical conditions of metamorphism. Subsequent oxidation yielded a strong oxygenbuffering assemblage jacobsite, hausmannite, braunite, hematite and quartz. This study negates the commonly held idea that hausmannite jacobsite crystallographic intergrowth (“vredenburgite”) originates through unmixing of a high ([ldvredenburgite”) originates through unmixing of a high temperature spinelss temperature spinelss during cooling.ZusammenfassungKristallographische Verwachsungen von Jakobsit und Hausmannit („Vredenburgit”) treten in Verbindung mit Braunit in Gesteinen der präkambrischen Sausargruppe in Indien auf. Die Gesteine wurden bei Drucken von ca. 6 kbar und Termperaturen von 600–700°C metamorphosiert. Quarz, Hämatit, Rhodochrosit, und Hausmannit als Spätphase treten gelegentlich als untergeordnete Gemengteile auf. Hausmannit-Lamellen, die in vier oder fünf kristallographischen Richtungen in Jakobsit orientiert sind, haben sehr unterschiedliche Durchmesser und bilden versetzte Zwickel in kleinem Winkel mit dem Jakobsit. Die Lamellen können lokal deformiert sein. Analytische Daten zeigen, daß die Zusammensetzung von natürlichem Hausmannit und Jakobsit in Verwachsungen nicht, wie bisher angenommen, in dem System Fe3O4-Mn3O4 dargestellt werden kann. Diese Verwachsungen gehören vielmehr in das Fe2O3-Mn3O4 System. In höher-gradigen metamorphen Gesteinen ist Fe in zwei-phasigen Verwachsungen im Hausmannit ab- und im Jakobsit angereichert. Die Mineralzusammensetzung und petrographische Gesichtspunkte lassen darauf schließen, daß die Jakobsit/Hausmannit Verwachsung durch prograde Dekarbonatisierung/Oxydationsreaktion eines karbonatischen Vorläufers in einem nicht gepufferten X CO2 Milieu entstanden ist. F O2 wurde durch Hämatit-Magnetit und Bixbyit-Hausmannit Puffer unter den gegebenen physikalischen Bedingungen der Metamorphose stabil gehalten. Eine nachfolgende Oxydation führte zu einer starken Sauerstoff-puffernden Assoziation von Jakobsit, Hausmannit, Braunit, Hämatit und Quarz. Diese Untersuchungen widerlegen die allgemein verbreitete Ansicht, daß die kristallographische Verwachsung von Hausmannit und Jakobsit (Vredenburgit) durch Entmischung eines Hochtemperatur-Spinells während der Abkühlung entstanden ist.