Molybdenum (Mo) and its isotopes have been used to retrieve palaeoenvironmental information on the ocean–atmosphere system through geological time. Their application has so far been restricted to rocks least affected by severe metamorphism and deformation, which may erase or alter palaeoenvironmental signals. Environmental Mo-isotope signatures can be retrieved if the more manganese (Mn)-enriched rocks are isotopically depleted and the maximum range of δ98Mo values is close to the ~2.7‰ Mo-isotope fractionation known from Mo sorption onto Mn oxides at low temperature. Here, we show that the Morro da Mina Mn-ore deposit in Minas Gerais, Brazil, contains Mn-silicate–carbonate ore and associated graphitic schist that likely preserve δ98Mo of Palaeoproterozoic seawater, despite a metamorphic overprint of at least 600 °C. The extent of Mo-isotope fractionation between the Mn-silicate–carbonate ore and the graphitic schist is similar to modern Mn-oxide precipitates and seawater. Differences in δ98Mo signals are broadly reflected in cerium (Ce) anomalies, which suggest an oxic–anoxic-stratified Palaeoproterozoic ocean.
Assessing when and how oceans became oxygenated is crucial not only to trace the evolution of early life, but also to understand the genesis of metalliferous deposits in marine settings. Metals sensitive to variations in reduction–oxidation (redox) conditions have the potential to resolve the palaeoenvironmental setting in which marine sediments have been deposited1,2,3. Manganese and Mo represent such redox-sensitive metals. The former, soluble as divalent Mn, requires free oxygen in seawater to form Mn-oxide particles. The latter is dissolved as hexavalent Mo in seawater and accumulates in organic-matter-rich sediments such as black shales1,4,5,6, reflecting the Mo-isotope composition (δ98Mo) of seawater only at H2S concentrations that are sufficiently high for the quantitative conversion of molybdate to thiomolybdate, which is then trapped by organic matter in euxinic waters7,8. In contrast, substantial fractionation of −2.7‰ in δ98Mo is associated with the adsorption of Mo onto Mn-oxide particles9.
The application of Mo as a palaeoenvironmental proxy to black shales and other marine sediments has essentially been restricted to sequences that experienced incipient metamorphism and deformation6,8,10,11,12, as they may affect depositional signals. Nevertheless, recent study has indicated that even greenschist-facies metamorphism is unable to erase depositional δ98Mo signals13. This indication is built on a positive correlation between Fe/Mn ratios and Mo-isotope values obtained from the ~2.95-Ga-old Sinqeni iron formation of the Pongola Supergroup, South Africa. Depositional δ98Mo signals have also been retained in the ~1.85-Ga-old Stambaugh Formation of the Paint River, Michigan, USA, which was metamorphosed to greenschist facies14. However, it remains unclear whether such a correlation is universal and realistic for rocks metamorphosed at higher grade, such as those found at the Morro da Mina Mn-ore deposit in Minas Gerais, Brazil. At Morro da Mina, amphibolite-facies metamorphism of marine sediments rich in Mn and organic matter resulted in the formation of queluzite, a Mn-silicate–carbonate rock containing graphite–molybdenite (MoS2) intergrowths, and Mo-bearing graphitic schist15. Both rock types, queluzite and graphitic schist (Fig. S1), should record deposition in euxinic waters to account for Mo enrichment. Thus, Morro da Mina offers a remarkable opportunity to apply the Mo proxy to Mn ore that attained at least 600 °C, as well as to its associated graphitic schist. We report U–Pb ages of detrital zircon grains from (i) quartzite, and (ii) granodioritic dyke in the Mn ore, to bracket the depositional age of the Morro da Mina queluzite in the Barbacena greenstone belt. Furthermore, δ13C and δ98Mo results, in addition to major and trace elements, place new constraints on the palaeoenvironmental conditions of Mn deposition, and demonstrate that Mo-isotope signals can be retrieved after amphibolite-facies metamorphism.
Morro da Mina is a Mn-ore deposit in operation since 1902, located to the south of the Quadrilátero Ferrífero of Minas Gerais, about 35 km southwest of Ouro Preto. Its main ore type is queluzite, which consists mainly of Mn carbonate accompanied by a variety of Mn-silicate minerals including spessartine [Mn2+3Al2(SiO4)3], rhodonite [Mn2+SiO3] and its polymorph pyroxmangite, tephroite [Mn2+2(SiO4)], and widespread dissemination of graphite and alabandite [MnS]16,17,18,19,20,21,22. Queluzite is a characteristic rock of the Lafaiete Formation, a Mn-rich unit that extends for over 100 km23,24, and forms part of the Barbacena greenstone belt25. The queluzite ore contains intercalations of quartz–biotite schist, graphitic schist, quartzite, garnet–amphibole schist and amphibolite (Fig. 1). This rock succession is isoclinally folded and sheared along ductile zones, which are marked by graphitic schist. Ductile shearing shaped queluzite into sigmoidal orebodies, of up 100 m in thickness. The folded succession is locally intruded by granodioritic dykes16,21. The assemblage rhodochrosite–tephroite–pyroxmangite indicates a minimum metamorphic temperature of 600 °C26, but even higher temperatures due to the thermal effect of the granodioritic dykes cannot be excluded. Until now, the ages of the sedimentary protolith of the Morro da Mina queluzite and the tectono-metamorphic overprint have been unclear.
Zircon grains were separated from a quartzite and a queluzite-hosted granodioritic dyke for U–Pb geochronology. Detrital zircon grains from the quartzite sample show cores with a typically magmatic growth zoning, generally surrounded by tiny outgrowths without any zoning (Fig. 2a, inset). The detrital zircon grains yielded Palaeoproterozoic ages, mostly at 2265 Ma, 2150 Ma and 2088 Ma (Table S1, Supplementary Information). The youngest detrital grain gave a concordant age of 2075 ± 12 Ma, which represents the maximum depositional age for the queluzite protolith (Fig. 2a). Zircon grains from the post-tectonic granodioritic dyke yielded a concordant age of 1860 ± 8 Ma (Fig. 2b). Collectively, the ages indicate that the deposition of Mn-rich sediments took place during the Palaeoproterozoic between 2.07 and 1.86 Ga (Fig. 2a).
A sample collection of queluzite (n = 10) and graphitic schist (n = 9) was analysed for their whole-rock major- and trace-element contents, and Mo and C isotopic compositions (Supplementary Table S2). The queluzite samples represent the main Mn-ore type at Morro da Mina. Their Mn contents are between ~15 and 33% (mass), with Fe/Mn ratios varying from 0.06 and 0.35. The samples of graphitic schist exemplify ductile shear zones poorer in Mn (Fe/Mn > 2), but with locally elevated Mn concentrations (Fe/Mn < 2 − i.e., Mn-rich graphitic schist). The queluzite and graphite schist significantly differ in their δ98Mo values, between −1.80 and −0.47‰, and between −0.17 and 0.80‰, respectively (Fig. 3a). The Mn-rich graphitic schist has δ98Mo values between −1.44 and −0.44‰, which are essentially within the queluzite δ98Mo range. The maximum difference in δ98Mo is 2.6‰ (Δ98Mo), which is close to that between modern seawater and seafloor Mn-oxide crusts and nodules (~3.0‰ δ98Mo27,28). The difference of 2.6‰ δ98Mo also agrees with the experimentally determined δ98Mo fractionation through Mo adsorption onto Mn-oxide particles9. The δ98Mo values and Fe/Mn ratios for both queluzite and graphitic schist follow the same positive correlation trend, which was previously delineated by Planavsky et al. (ref.13) for the Archaean Sinqeni iron formation in South Africa. Those authors interpreted this trend to reflect inheritance of the δ98Mo signal originally acquired during Mn oxidation (Fig. 3a). By analogy, this interpretation is extended to Morro da Mina despite tectonic and metamorphic overprint.
Nevertheless, the depositional setting at Morro da Mina differs from that envisaged for the extremely sulfide-poor Archaean Sinqeni iron formation in South Africa, which likely captured its Mo-isotope signals from Mn-oxide particles during their reductive dissolution below the seawater–sediment interface13. Conversely, the reductive dissolution of Mn-oxide particles possibly occurred above the sediment layer within a redox-stratified ocean at Morro da Mina. In such an ocean, Mn-oxide particles that had scavenged Mo from oxic shallow waters became dissolved below a redoxcline, in bottom waters rich in H2S and organic matter. This interpretation is compatible with the ubiquitous dissemination of alabandite and the omnipresence of graphite in the Morro da Mina queluzite, as well as with the finding of high tungsten (W) contents in molybdenite15. Values of δ13C for the queluzite and graphitic schist are respectively between −20.8 and −14.5‰, and between −22.5 and −20.8‰ (Supplementary Table S2), suggesting a largely organic origin for C in both graphite and carbonate (e.g., ref.29). There is a positive correlation between δ13C and the mass ratio of carbonate C to total C (Fig. S2a), which allows to identify the δ13C composition of the carbonate component at about −12‰ (Fig. S2b), in agreement with 6 selective measurements of the δ13C composition of carbonate, which gave an average of −11 ± 2‰. These data suggest an origin via remineralisation by pore fluids. While the dissolved load of divalent Mn was fixed in sediments as Mn carbonate and sulfide (alabandite), released molybdate ions with inherited δ98Mo from Mn-oxide particles above the redoxcline were trapped by S-rich organic matter in euxinic bottom waters7,30. The latter process is expressed as graphite–molybdenite intergrowths in the Morro da Mina queluzite15. It should be mentioned that a shuttle of Mn oxides to the seawater–sediment interface cannot be excluded, but the scenario of Mn redox cycling across a redoxcline appears more compelling if Ce is considered.
Cerium is a key element because of its ability to accumulate in seafloor Mn-oxide deposits as Ce(IV), leading to the mirrored Ce depletion in modern seawater31,32. The Morro da Mina queluzite has positive Ce anomalies (up to 1.8), which indicate mediation by particulate Mn-oxide formed above the redoxcline. On the other hand, the graphitic schist has slightly negative Ce anomalies, which is an indication of precipitation from Ce-depleted seawater above the redoxcline. Therefore, the different Ce anomalies provide evidence that Mn redox cycling took place across an oxic–euxinic water interface. This interpretation is supported by the negative trend between Ce anomaly and δ98Mo (Fig. 3b–d), which is complementary to the positive correlation of δ98Mo vs. Fe/Mn (Fig. 3a), reflecting the amount of particulate Mn oxide during sediment deposition. Such a scenario of Mn-oxide shuttle to euxinic bottom waters would also have led to strong enrichment in authigenic Mo in sediments, but little or no authigenic enrichment in U33. In this context, we note that the Morro da Mina queluzite has enrichment factors between 30 and 500 for Mo, but only between 0.4 and 1.5 for U, compared to the average composition of the upper continental crust.
Placing detailed constraints on the palaeoenvironment at Morro da Mina is problematic due to: (i) the tectonic overprint, which obliterated the metasedimentary stratigraphy; (ii) the substantial clastic component, reflected in the contents of Al2O3 >5% (Supplementary Table S2), which obscures the seawater signature. Because of the tectonic overprint, it is not possible to ascertain how the queluzite and graphitic schist are stratigraphically related to each other, but both rocks were originally sediments deposited under euxinic conditions. It is then possible to advance further interpretations on their Mo-isotope signals and rare-earth-element (REE) patterns. Most queluzite samples have positive Ce anomalies, which can be explained by the reductive dissolution of Mn-oxide particles, from which Ce was released and delivered to Mn-rich sediments (see above). The positive anomalies of Eu likely reflect a seawater signature from its interaction with hot basalt – i.e., high-temperature alteration of basalt, from which Eu is leached by hydrothermal fluids venting into seawater34. Upwelling of Mn-rich bottom waters would carry this Eu signature. Sedimentation of organic-matter-rich mud took place in a deeper part of the basin, having captured the signal of seawater depleted in Ce, but enriched in the heavy Mo isotope (δ98Mo > 0), which are characteristics of Mn-oxide-particle formation in oxic waters. Our particle-shuttle scenario is schematically depicted in Fig. 4, a modern analogue of which is the Baltic Sea, where Mn carbonate and Mn sulfide account for total Mn contents of up to 32% (mass)35, equivalent to the Mn-ore grade of Morro da Mina. The black-shale precursor of the graphitic schist represents, with respect to Mo, Palaeoproterozoic seawater with positive δ98Mo (cf. ref.8), and its essentially flat REE patterns denote clastic input. In summary, the highly metamorphosed Mn-rich rocks at Morro da Mina appear to retain depositional Mo-isotope signatures. However, seawater Mo-isotope values can vary and the lightest and heaviest values are not constrained, but assumed, to be the same age. Although such uncertainties may have influenced the fractionation of Mo isotopes, it is remarkable that the difference between the two end-member δ98Mo values is not only similar to that reported from sites where bedding and other sedimentary features are commonly well preserved12,14, but also close to the experimentally predicted isotopic fractionation of Mo onto Mn oxides at low temperature9. At this point, it is worth mentioning that metamorphic, high-temperature fractionations of Mo isotopes would have been so small that their effect on the predicted low-temperature Δ98Mo is not perceptible.
One important implication refers to the absence of sulfide and ferrous Fe in the water column beneath “local marine oxygen oases” of photosynthetic microorganisms, as a requirement for Mn-oxide-bound Mo to be shuttled to sediments13. Our study indicates that the Mo-isotope signal of Mn oxidation can be retained in euxinic waters below the Mn redoxcline. The aforementioned oxygen oases were not large enough to have formed a Mn redoxcline in Archaean banded-iron-formation (BIF) settings to generate the combined signals of Ce anomalies and Mo-isotope fractionations, as recorded here (Fig. 3b), given the general lack of positive Ce anomalies in Archaean BIF36. Archaean oxygen oases may nevertheless have expanded to a Mn redoxcline forming combined Ce–Mo signals, perhaps in the ca. 3.1-Ga-old Iron Ore Group, which hosts Mn deposits in the Singhbhum craton, eastern India37. Therefore, such Mn deposits in India and elsewhere have the potential of providing palaeoenvironmental information irrespective of metamorphic grade and tectonic overprint. The recognition that the Mo proxy may be retained in rocks containing graphite opens new perspectives of investigating Precambrian terranes that have otherwise been considered unsuitable for palaeoenvironmental studies because of metamorphism and deformation. This point is stressed here with respect to the depositional age for the queluzite sedimentary protolith (~2.07–1.86 Ga), which connects the Morro da Mina Mn deposit to the Lomagundi event38, when extensive removal of organic carbon from seawater was counterbalanced by a high rate of oxygen production39. This connection is further reinforced by the association of Mn deposits with black shales40, known in Palaeoproterozoic metamorphic terranes of West Africa, Gabon, India and northern Brazil37. The preservation of environmental signals – i.e., Mo-isotope fractionations and Ce anomalies – and their connections to such an event can thus be retrieved from medium- to high-grade metamorphic terranes.
Methods are available as electronic Supplementary Information.
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VALE S.A. provided access to, and logistical support at, the Morro da Mina mine. J.M.P. Alves (VALE) assisted A.R.C. during fieldwork. We thank Eva Haluzová and Tomáš Magna for helping us with the Mo-isotope work and MC–ICP–MS measurements, respectively. We also acknowledge B. Čejková and I. Jačková (Czech Geological Survey, Prague) for the preparation of samples and C-isotope analyses. This work was supported by Czech Science Foundation project 17–15700 S to L.A. and J.P., and by the Scientific Program RVO67985831 of the Institute of Geology of the CAS and the Strategic Research Plan of the Czech Geological Survey (2016–2020). Comments by Noah Planavsky and an anonymous reviewer have substantially improved the manuscript and are gratefully acknowledged.