Description: 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 evolution of the oceans.

Description: Samples of manganese-rich rock containing two compositional varieties of tokyoite in association with noélbensonite were retrieved from a drill core obtained from the Postmasburg area in the Northern Cape Province of South Africa. The samples consist of fine-grained braunite, hematite, and hausmannite. Within this material abundant vugs are observed that are filled with witherite, baryte, and barytocalcite. In addition, As-rich tokyoite, tokyoite, and noélbensonite occur in the center of the vugs, in fine aggregates 0.1 to 1 mm in size. Individual As-rich tokyoite grains are typically 20–200 μm in size. The outer walls of the vugs are lined with microcrystalline K-feldspar, witherite, and/or sérandite. Textural evidence of the Ba-rich mineral phases in association with As-rich tokyoite suggest an epigenetic mode of formation of the observed assemblages, caused by V- and As-bearing alkali-rich fluids interacting with pre-existing Mn-rich minerals. Electron microprobe analysis (EMPA), electron backscatter diffraction analysis (EBSD), and Raman spectroscopy show that the As-rich tokyoite is a mineral belonging to the brackebuschite mineral group. Its measured mineral composition corresponds to the formula (Ba 1.92 Sr 0.05 Pb 0.03 ) 2.00 (Mn 3+ 0.98 Fe 3+ 0.02 ) 1.00 [(As 1.050 V 0.950 ) 2.00 O 8 )](OH). Electron backscatter diffraction analysis results point to a monoclinic, P 2 1 / m space group, with cell parameters a 9.121 Å, b 6.142 Å, c 7.838 Å, α = = 90°, β = 112°, Z = 2. The structure is therefore similar to gamagarite. Raman spectra of As-rich tokyoite were compared to spectra of arsenbrackebuschite and arsentsumebite, and those of noélbensonite to spectra of hennomartinite and lawsonite. The indexing of Raman peaks in As-rich tokyoite, by similarity with arsentsumebite, suggests a possible ordering of the AsO 4 and VO 4 tetrahedra. This observation, correlated with mineral chemistry and specifically an As:V ratio of ~1:1, suggest that the As-rich tokyoite may in fact represent a possible new mineral with ordering of As and V at tetrahedral positions, or at least an unknown As analogue of tokyoite.

Description: 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 negative ones of the other fractions. The post-GOE Fe-oxides of the stratigraphically higher Hotazel Formation have negative δ56Fe values, which supports a basin-wide Rayleigh fractionation of isotopically heavy-Fe [3]. References: [1] Henkel et al., (2016) Chemical Geology, 421, 93-102 [2] Frost et al., (2007) Contributions to Mineralogy and Petrology 153, 211-235 [3] Tsikos et al., (2010) Earth and Planetary Science Letters, 298, 125-134

Description: 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

Description: Shale gas potential of the Whitehill Formation (Permian), Karoo Basin, South Africa is strongly influenced by widespread Jurassic igneous intrusions. A high-resolution organic petrographic and geochemical study is reported here on two stratigraphic sections, the objective being to gain insights into the chemical structure and alteration of oil-prone sedimentary organic matter during contact metamorphism. One section unaffected by intrusion served as background, while the other section was intruded by two laterally continuous dolerite sills with an average thickness of 10.7 and 9.3 m, preferentially emplaced at the upper and lower contacts of the formation, respectively. Heat flow from the sills caused TOC and S2 values from programmed pyrolysis to decline over a distance of 8.7 m from 3.2 to 0.15% and 4.65 to 0.11 mg HC/g TOC, respectively, and cause the reflectance of vitrinite and solid bitumen to increase from 2.03 to 5.82% and 1.58 to 7.87%. Organic matter in the background samples is dominated by solid bitumen, a secondary thermal conversion product of oil-generative Type II kerogen. In the sill-hosting section, the thermal aureoles can be recognized by systematic changes to the optical texture and chemical structure and composition of solid bitumen. Solid bitumen particles develop anisotropy approaching the sills and become transformed into coke and pyrolytic carbon. δ13Corg values become less negative by up to 3.13‰ VPDB. These data suggest significant devolatilization of organic matter to methane and destruction of petroleum generation and retention potential in the contact aureole of the sills. The trend of these datasets, coupled with the presence of several sills and their laterally extensive nature throughout much of the study area suggests that the intrusions have been significant in increasing methane generation on a basinal scale. Together, this study indicates that the complementary application of combined light and electron microscopy (CLEM), programmed pyrolysis, and isotope ratio mass spectrometry (IRMS) is suitable for a comprehensive characterization of organic matter alteration through the maturation continuum. Results also show that contact metamorphic alteration of organic matter by intrusions produced structural and compositional changes that are distinctive from those expected from geological burial maturation where graphite is the end-product.