Search

The 2.4Ga Hotazel Formation is a cyclically interlayered sequence of banded iron formation (BIF) and manganese-rich sedimentary rock at the uppermost part of the Neoarchaean-Palaeoproterozoic Transvaal Supergroup in South Africa. It represents an unusual stratigraphic association in the context of the origin of BIF and the coevolution of oxygen and life on early Earth and hence bears special relevance to the environmental conditions and processes that characterized the period leading up to the Great Oxidation Event (GOE) at ca. 2.3Ga. The mineral assemblages that characterize the Hotazel rocks are dominated by carbonate, silicate and oxide minerals, which are traditionally interpreted as predominantly diagenetic in origin, particularly the carbonates. By contrast, primary mineral assemblages are inferred to have been dominated by ferric oxyhydroxides and tetravalent manganese oxides, which show no preservation in the rock record and consequently hinder reconstruction of environmental conditions during sedimentation. Here, we revisit the Hotazel succession with a focus on its bulk-rock and carbonate-specific mineralogical, geochemical and stable isotope (C, Fe) composition by applying for the first time a high-resolution stratigraphic approach to sampling and analysis. Our main aim is to constrain the precursor mineralogy to the Fe- and Mn-rich facies in the Hotazel strata in order to unravel the redox conditions behind the massive cyclic deposition of Fe and Mn at the onset of the GOE. Our carbonate-specific results question traditional diagenetic models for the development of the carbonate fraction of the rocks and instead place the origin of much of the present mineralogy on water-column processes in a stratified basin characterized by successive redox pathways with changing water depth. These pathways exploited a series of thermodynamically predictable electron acceptors for organic carbon recycling, which included – probably for the first time in Earth history –

Rare earth element (REE) abundances in iron formations (IFs) provide key evidence to track the redox evolution of the Precambrian oceans. Deriving information about processes and sources from IFs is complicated by the variable roles of diagenetic dissolution and precipitation that modify primary depositional signals in the rocks. Iron formations are typically mineralogically heterogeneous with varying amounts of carbonate (ankerite + siderite), oxide (predominantly magnetite) and silicate (chert and Fe-silicates). To date, little is known about the respective contributions of these fractions to the bulk-rock REE signal, and which fraction most accurately records primary seawater signals. Here we evaluate an optimized sequential extraction scheme for REE analysis in the ca. 2.46 Ga Asbestos Hills IFs from the Griqualand-West Basin in South Africa. These rocks were deposited immediately before the Great Oxidation Event and are potentially an important archive for large-scale redox changes in the (near) surface environment. To fully evaluate our method, we measured lanthanide elements plus ytrrium (REY) in individual minerals, in bulk rock samples and in three separate fractions using acetate, oxalate and total digestion sequential extractions. The extraction techniques were then verified using a mass balance approach, the first time that this has been comprehensively applied to IFs. Similarities between the carbonate fraction data and modern seawater, coupled with strong measurement reproducibility and systematic stratigraphic variability suggests that carbonate most closely tracks the primary seawater composition. The Asbestos Hills IFs show distinctive REY signatures with strongly variable (Yb/Pr)SN ratios, positive La anomalies, positive and constant Y/Ho, weakly positive Eu anomalies and a lack of Ce anomalies in their carbonate fraction. Stratigraphic variations in these parameters suggest deposition in a stratified fully anoxic ocean, which became

The 1.6-2.0Ga Gamsberg deposit is a >200 Mt zinc reserve located in the Aggeneys-Gamsberg mining district in the Northern Cape Province of South Africa, which is host to a cluster of world-class base metal deposits. These four proximal SEDEX-type deposits were formed in an extensional basin, in which Gamsberg occupies the eastern side of the district. The Gamsberg deposit is characterized by an anomalous enrichment in manganese, which greatly complicates ore re?finement. Previous work has shown a high abundance and variability of Mn in sphalerite minerals throughout the stratigraphy at the West orebody of Gamsberg. This study goes beyond previous work by determining for the fi?rst time sulfur isotope ratio variability in individual sphalerite and iron sul?fide minerals with stratigraphic position through the deposit. LA-ICP-MS analysis of the Fe-sulfi?de- and sphalerite minerals in the samples from the West orebody revealed initially low d34S values around 0‰ that increased abruptly to high d34S values of up to +40‰ relative to Vienna Canyon Diablo Troilite (VCDT). This was interpreted as the source of reduced sul- fur for initial mineralization being decoupled from the source of reduced sulfur after initial mineralization, thermochemcial reduction of seawater derived sulfate. The strongly positive d34S values were interpreted as the result of Gamsberg developing later than the other Aggeneys deposits in the closed depositional basin. Manganese variability is interpreted as a fi?nite sediment enrichment as a distal expression of the other Aggeneys SEDEX deposits which was remobilized and progressively depleted during the formation of the Gamsberg deposit. Metamorphism and associated metasomatism during the Namaquan orogeny (1.06-1.03 Ga) were interpreted as a mechanism for introducing secondary sulfi?de minerals to ore units with an oxidized primary mineral assemblage and mobilizing manganese for incorporation in metamorphic gangue minerals.

The 1.6-2.0Ga Gamsberg deposit is a >200 Mt zinc reserve located in the Aggeneys-Gamsberg mining district in the Northern Cape Province of South Africa, which is host to a cluster of world-class base metal deposits. These four proximal SEDEX-type deposits were formed in an extensional basin, in which Gamsberg occupies the eastern side of the district. The Gamsberg deposit is characterized by an anomalous enrichment in manganese, which greatly complicates ore re?finement. Previous work has shown a high abundance and variability of Mn in sphalerite minerals throughout the stratigraphy at the West orebody of Gamsberg. This study goes beyond previous work by determining for the fi?rst time sulfur isotope ratio variability in individual sphalerite and iron sul?fide minerals with stratigraphic position through the deposit. LA-ICP-MS analysis of the Fe-sulfi?de- and sphalerite minerals in the samples from the West orebody revealed initially low d34S values around 0‰ that increased abruptly to high d34S values of up to +40‰ relative to Vienna Canyon Diablo Troilite (VCDT). This was interpreted as the source of reduced sul- fur for initial mineralization being decoupled from the source of reduced sulfur after initial mineralization, thermochemcial reduction of seawater derived sulfate. The strongly positive d34S values were interpreted as the result of Gamsberg developing later than the other Aggeneys deposits in the closed depositional basin. Manganese variability is interpreted as a fi?nite sediment enrichment as a distal expression of the other Aggeneys SEDEX deposits which was remobilized and progressively depleted during the formation of the Gamsberg deposit. Metamorphism and associated metasomatism during the Namaquan orogeny (1.06-1.03 Ga) were interpreted as a mechanism for introducing secondary sulfi?de minerals to ore units with an oxidized primary mineral assemblage and mobilizing manganese for incorporation in metamorphic gangue minerals.

Iron formations (IF) represent an iron-rich rock type that typifies many Archaean and Proterozoic supracrustal successions and are chemical archives of Precambrian seawater chemistry and post-depositional iron cycling. Given that IF accumulated on the seafloor for over two billion years of Earth's early history, changes in their chemical, mineralogical, and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving Earth. Perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere. Linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes, the supply of nutrients to the oceans, and, ultimately, diversification of the biosphere and complex life forms. Many of the key recent innovations in understanding IF genesis are linked to geobiology, since biologically assisted Fe(II) oxidation, either directly through photoferrotrophy, or indirectly through oxygenic photosynthesis, provides a process for IF deposition from mineral precursors. The abundance and isotope composition of Fe(II)-bearing minerals in IF additionally suggests microbial Fe(III) reduction, a metabolism that is deeply rooted in the Archaea and Bacteria. Linkages among geobiology, hydrothermal systems, and deposition of IF have been traditionally overlooked, but now form a coherent model for this unique rock type. This paper reviews the defining features of IF and their distribution through the Neoarchaean and Palaeoproterozoic. This paper is an update of previous reviews by Bekker et al. (2010, 2014) that will improve the quantitative framework we use to interpret IF deposition. In this work, we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evo

Iron formations (IF) represent an iron-rich rock type that typifies many Archaean and Proterozoic supracrustal successions and are chemical archives of Precambrian seawater chemistry and post-depositional iron cycling. Given that IF accumulated on the seafloor for over two billion years of Earth's early history, changes in their chemical, mineralogical, and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving Earth. Perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere. Linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes, the supply of nutrients to the oceans, and, ultimately, diversification of the biosphere and complex life forms. Many of the key recent innovations in understanding IF genesis are linked to geobiology, since biologically assisted Fe(II) oxidation, either directly through photoferrotrophy, or indirectly through oxygenic photosynthesis, provides a process for IF deposition from mineral precursors. The abundance and isotope composition of Fe(II)-bearing minerals in IF additionally suggests microbial Fe(III) reduction, a metabolism that is deeply rooted in the Archaea and Bacteria. Linkages among geobiology, hydrothermal systems, and deposition of IF have been traditionally overlooked, but now form a coherent model for this unique rock type. This paper reviews the defining features of IF and their distribution through the Neoarchaean and Palaeoproterozoic. This paper is an update of previous reviews by Bekker et al. (2010, 2014) that will improve the quantitative framework we use to interpret IF deposition. In this work, we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evo

Banded iron formations (BIF), deposited prior to and concurrent with the Great Oxidation Event (GOE) at ~2.4 Ga, record changes in the oceanic and atmospheric chemistry during this critical time interval. Three previously unstudied drill-cores from the western Transvaal Basin, South Africa, capturing the rhythmically mesobanded Kuruman BIF and the overlying granular Griquatown BIF, were sampled every ~20 m. along core depth. These samples were analysed for mineralogy, geochemistry and bulk Fe and C-isotopes. Bulk Fe-isotopic values of 50 samples show an apparent relationship with mineralogy. The lower δ56Fe values (< -1.3) correlate with carbonate-rich samples, whereas higher δ56Fe values (>0.0) correspond to samples rich in bulk modal magnetite. To further investigate this relationship, a 3-step sequential extraction protocol was developed to separate the three main Fe-hosting fractions (Fe-carbonates, Fe-oxides and Fe-silicates). Rare Earth Element (REE) patterns were resolved for the individual fractions and using the leachate destruction protocol of Henkel et al. [1] we were able to measure for the first time species specific Fe-isotopes of bulk-BIF samples. Species specific Fe-isotopes are probably a better proxy for the Palaeoproterozoic ocean than bulk-rock values, since the latter are strongly influenced by the modal mineralogy of each sample. We used bulk-rock C-isotope data combined with the species specific REE and δ56Fe to argue that the Fe-carbonates (and possibly Fe-silicates) in the Transvaal BIFs record primary chemical signatures. It follows that chemical signatures can be preserved, through changes of the textural appearance of minerals in BIF during diagenesis and low-grade metamorphism [2]. Preliminary data indicate that the Fe-oxides (dominated by magnetite) are probably formed by recycling and mixing of precursor Fe-(oxy)hydroxides and ferrous sea- or pore-waters, since their positive δ56Fe values deviate strongly and consistently from the nega

Banded iron formations (BIF) deposited during the Great Oxidation Event (GOE) at ~2.4Ga, potentially capture chemical changes in the Paleoproterozoic atmosphere and ocean. The Kuruman and Griquatown BIFs in the Griqualand West Basin, South Africa were deposited concurrent with the GOE. Continuous drill-cores thereof provide important insights into the Paleoproterozoic environment. Bulk-rock samples of these cores show an apparent relationship between modal mineralogy and Fe-isotopes: low δ56Fe-values (<-1.3‰) correlate with samples rich in Fe-carbonate, whereas positive δ56Fe-values correspond to magnetite-rich samples. To interrogate this further, an existing sequential extraction scheme was modified to accommodate the BIF mineralogy [1]. Since ferric oxides are insignificant in the rocks we studied, the hydroxylamine and dithionite leaches could be omitted. The remaining extractions were optimized for the dissolution of three main Fe-hosting mineral fractions (Fe-carbonates, magnetite and Fe-silicates). The final silicate residue was dissolved in HF-HClO4-HNO3. The tests established a 3-step sequential extraction procedure to quantitatively separate the Fe-mineral fractions. To measure the species specific Fe-isotopes a processing-protocol was applied to break down the organic leachates [2]. Combining the species specific data (δ56Fe and REE) with bulk-rock data (C-isotopes and mineralogy) it becomes apparent that the BIFs have recorded primary signals from Paleoproterozoic seawater. References: [1] Poulton & Canfield (2005) Chem. Geo., 214, 209-221. [2] Henkel et al., (2016) Chemical Geology, 421, 93-102

Organic material from the Noordhoek area on the western margin of the Cape Peninsula, South Africa, wasobtained from a ~50 m-long drill-core dominated by fluvio-lacustrine siliciclastic sediments. The aim of thisstudy is to constrain fluctuations in climate and the decline of tropical vegetation elements along the southwesterncoast and the Cape Peninsula of South Africa, during the Late Cenozoic phase, when the Benguela upwellingsystem was established. The approach was to combine palynological, biogeochemical (tetraether lipids) andstable isotope (C, N) studies of the organic-bearing record from the Noordhoek area on the western margin ofthe Cape Peninsula. Bulk C and N isotope data of sediment organic matter, point to a predominantly C3 higherplant source vegetation. Mean annual air temperature (MAT) from the analyses of tetraether lipids (MBT'–CBTindex) was compared with palynomorphs from partly unpublished data of a previously drilled core adjacent tothe study site. The palynomorphs are of subtropical affinities, and suggest that an open riparian forest wouldhave existed in the early to middle Miocene of the southwestern coast of South Africa. Together these datasources allow vegetation and climate reconstructions of subtropical conditions during the early to middleMiocene, which comprised fluctuating open riparian forest and swamp vegetation. Temperatures rose in themiddle Miocene and were higher than those of the present day.

Organic material from the Noordhoek area on the western margin of the Cape Peninsula, South Africa, wasobtained from a ~50 m-long drill-core dominated by fluvio-lacustrine siliciclastic sediments. The aim of thisstudy is to constrain fluctuations in climate and the decline of tropical vegetation elements along the southwesterncoast and the Cape Peninsula of South Africa, during the Late Cenozoic phase, when the Benguela upwellingsystem was established. The approach was to combine palynological, biogeochemical (tetraether lipids) andstable isotope (C, N) studies of the organic-bearing record from the Noordhoek area on the western margin ofthe Cape Peninsula. Bulk C and N isotope data of sediment organic matter, point to a predominantly C3 higherplant source vegetation. Mean annual air temperature (MAT) from the analyses of tetraether lipids (MBT'–CBTindex) was compared with palynomorphs from partly unpublished data of a previously drilled core adjacent tothe study site. The palynomorphs are of subtropical affinities, and suggest that an open riparian forest wouldhave existed in the early to middle Miocene of the southwestern coast of South Africa. Together these datasources allow vegetation and climate reconstructions of subtropical conditions during the early to middleMiocene, which comprised fluctuating open riparian forest and swamp vegetation. Temperatures rose in themiddle Miocene and were higher than those of the present day.

Age determinations were conducted on detrital zircon grains from two stratigraphic levels in the Postmasburg Group, Transvaal Supergroup - the top of the Makganyene Formation and the base of the Hotazel Formation - to constrain the age of the group and for comparison with purported correlatives in the Segwagwa Group of the Kanye basin and Pretoria Group of the Transvaal basin. Detrital zircon grains are interpreted as being derived mainly from the underlying Ghaap Group and its basement of Ventersdorp Supergroup and Kraaipan granite/greenstone rocks, indicating a proximal source to the east and north on the Vryburg arch. The maximum age of the Postmasburg Group was constrained by the youngest detrital-zircon age to 2436. ±. 7. Ma and the broad age range of the group, as a consequence, to somewhere between 2.43 and 2.38. Ga. Comparison with various stratigraphic units in the Kanye and Transvaal basins indicated close similarities in zircon populations with the Duitschland Formation in the Transvaal basin. The Pretoria and Segwagwa Groups contain younger zircon populations at 2350-2320 and 2240-2200. Ma that are related to syn-depositional volcanic activity and are not present in the Postmasburg Group samples, indicating the younger age of these groups. Based on compatibility of zircon populations, new stratigraphic correlations between the upper groups of the Transvaal Supergroup in the Griqualand West, Kanye and Transvaal basins are proposed. These include the probable restriction of the lower Timeball Hill - Hekpoort portion of the Pretoria Group to the Transvaal basin. An older age for the Postmasburg Group - ≈2.4. Ga as opposed to the generally accepted 2.22. Ga - will affect various models proposed for the evolution of the Earth's atmosphere/hydrosphere that have been based on studies of rocks of the group, supporting an older ≈2.4. Ga age for global oxidation transformations.

The formation hosts around 8 000 000 000 tons of Mn resources grading 20-48 wt%, or half of world land-based reserves, in three units interbedded with banded iron formation. Aegirine occurs sporadically at specific stratigraphic intervals, where it appears to replace haematite+quartz. New petrological and geochemical evidence suggests that it was developed by the localised action of late brines and thus post-dates an extensive alteration event that included oxidation, carbonate leaching and residual enrichment of metals in the northernmost part of the orefield. The concept highlights the problems involved in trying to establish a universal model for hydrothermal alteration here and indicates that the deposit's history needs reappraising., The formation hosts around 8 000 000 000 tons of Mn resources grading 20-48 wt%, or half of world land-based reserves, in three units interbedded with banded iron formation. Aegirine occurs sporadically at specific stratigraphic intervals, where it appears to replace haematite+quartz. New petrological and geochemical evidence suggests that it was developed by the localised action of late brines and thus post-dates an extensive alteration event that included oxidation, carbonate leaching and residual enrichment of metals in the northernmost part of the orefield. The concept highlights the problems involved in trying to establish a universal model for hydrothermal alteration here and indicates that the deposit's history needs reappraising.

The Kalahari manganese field in the northern Cape Province, South Africa, is a world-class manganese resource with ca. 8 000 000 000 tons at 20-48% Mn. It consists of three laminated ore units interbedded with iron formation within the Hotazel formation in the uppermost Palaeoproterozoic Transvaal supergroup of 2.65-2.05 Ga. Manganese mining is concentrated in two areas of the Kalahari field, with low-grade carbonate-rich ore in the south having quartz, magnetite and carbonate as the chief mineral constituents and having apparently undergone diagenetic processes involving oxidation of organic carbon. The north of the ore field has high-grade (generally greater than 44 wt % Mn), carbonate-free, oxide-rich ore. Large parts if the northern Kalahari manganese field have Olifantshoek supergroup shales overlying the Hotazel formation with three diverse iron formations formed in different ways. Measurements suggest involvement of an isotopically light fluid, either meteoric water or a low-temperature hydrothermal fluid, in the oxidation, leaching and enrichment of the iron formation. Extensive fluid flow in the Kalahari manganese field may have been related to the Hotazel/Olifantshoek unconformity., The Kalahari manganese field in the northern Cape Province, South Africa, is a world-class manganese resource with ca. 8 000 000 000 tons at 20-48% Mn. It consists of three laminated ore units interbedded with iron formation within the Hotazel formation in the uppermost Palaeoproterozoic Transvaal supergroup of 2.65-2.05 Ga. Manganese mining is concentrated in two areas of the Kalahari field, with low-grade carbonate-rich ore in the south having quartz, magnetite and carbonate as the chief mineral constituents and having apparently undergone diagenetic processes involving oxidation of organic carbon. The north of the ore field has high-grade (generally greater than 44 wt % Mn), carbonate-free, oxide-rich ore. Large parts if the northern Kalahari manganese field have Olifantshoek supergroup shales overlying the Hotazel formation with three diverse iron formations formed in different ways. Measurements suggest involvement of an isotopically light fluid, either meteoric water or a low-temperature hydrothermal fluid, in the oxidation, leaching and enrichment of the iron formation. Extensive fluid flow in the Kalahari manganese field may have been related to the Hotazel/Olifantshoek unconformity.

Proposed models of the giant Mn deposits in the Transvaal cover a diverse spectrum of genetic processes, from large-scale epigenetic replacement mechanisms and submarine volcanogenic-exhalative activity to purely chemical sedimentation whereby the influence of volcanism is of reduced significance. Discussion is presented of the potential metallogenic importance of glacial processes which occurred prior to the development of the Kalahari deposits, as manifested by the occurrence of an older diamictite unit. The snow-ball earth scenario proposed for Neoproterozoic Fe-Mn formations may provide an attractive explanation for the required levels of dissolved Mn in the anoxic parts of the Kalahari deposits. Factors which have yet to be addressed, however, are the primary source of Mn, the processes leading to the characteristic cyclic development of iron formation and Mn sediment, and the likely influence of a postulated sudden oxygenation during the deposition of the ores., Proposed models of the giant Mn deposits in the Transvaal cover a diverse spectrum of genetic processes, from large-scale epigenetic replacement mechanisms and submarine volcanogenic-exhalative activity to purely chemical sedimentation whereby the influence of volcanism is of reduced significance. Discussion is presented of the potential metallogenic importance of glacial processes which occurred prior to the development of the Kalahari deposits, as manifested by the occurrence of an older diamictite unit. The snow-ball earth scenario proposed for Neoproterozoic Fe-Mn formations may provide an attractive explanation for the required levels of dissolved Mn in the anoxic parts of the Kalahari deposits. Factors which have yet to be addressed, however, are the primary source of Mn, the processes leading to the characteristic cyclic development of iron formation and Mn sediment, and the likely influence of a postulated sudden oxygenation during the deposition of the ores.