Origin Of Precambrian Iron Formations Research Articles

Carbon isotope stratigraphy of Precambrian iron formations and possible significance for the early biological pump

The origin of Precambrian iron-formations (IF) remains contentious, particularly with respect to the mineralogy of primary precipitates and the exact processes and conditions leading to their formation. Despite the uncertainties, prevailing hypotheses range from biological precipitation of ferrihydrite to abiotic water-column formation of greenalite. By contrast, iron carbonate minerals (siderite, ankerite) in IF have traditionally been attributed to diagenetic origins based on textural and isotopic relationships. Recent studies on IF from the Neoarchaean-Paleoproterozoic Transvaal Supergroup of South Africa have revealed evidence for apparently primary, low-δ13C, Fe/Mn-bearing Mg calcite as precursor to iron carbonate formation and as a potentially underestimated pathway of isotopically light carbon burial during IF deposition. Here, we present whole-rock δ13C data and carbonate-specific geochemical analyses for samples from five drill cores that capture the entire stratigraphic extent of the Kuruman and Griquatown IF of the Transvaal Supergroup. Our results demonstrate remarkable consistency in stratigraphic profiles among the locations for the trends and magnitudes of bulk δ13C values that are independent of paragenetic association, modal mineralogy, and chemical composition of the bulk carbonate fraction of each sample. We interpret these records as resulting from water-column abiotic carbonate formation that was accompanied by kinetic isotopic effects associated with fluctuating conditions (pH, alkalinity) controlling carbonate supersaturation in ambient seawater. Although our interpretation provides strong support for abiotic, anoxic models for IF genesis prior to the Great Oxidation Event (GOE), it does not entirely preclude additional biological mechanisms of primary ferric oxyhydroxide formation and its possible role in an early biological pump.

Shallow-marine ironstones formed by microaerophilic iron-oxidizing bacteria in terminal Paleoproterozoic

The microbial origin of Precambrian iron formations is debated due to the lack of direct fossil evidence. In order to reveal the genesis of ironstones under low-oxygen levels, integrative studies of sedimentology, petrography, mineralogy, and geochemistry were conducted on the intertidal to shallow subtidal ooidal and stromatolitic ironstones from the terminal Paleoproterozoic Chuanlinggou Formation (ca. 1.65–1.64 Ga) of North China, using microscopy, SEM, EDS, ICP-OES, ICP-MS and MC-ICP-MS techniques. Mineralogical study shows that the Fe-rich mineral is predominantly hematite that resulted from dehydration of amorphous Fe-oxyhydroxide during diagenesis. Petrographic observation indicates that the iron was oxidized and precipitated from seawater rather than sourced from terrestrial detritus. Basinward increases of the ironstone abundance, Eu anomalies (from 1.39 to 1.56) and δ56Fe values of the ironstones (from +0.5‰ to +1.0‰) suggest that the iron was mainly sourced from seafloor hydrothermal fluids, and partially oxidized and precipitated in shallow subtidal to intertidal environments. The common existence of Fe-oxide coated sheaths, spiral stalks, residual extracellular polymer substances (EPS) and other biogenic fabrics indicates that microaerophilic iron-oxidizing bacteria (FeOB) may have played an important role in precipitating the Chuanlinggou ironstones. The extremely low oxygen concentrations implied by the proliferation of microaerophilic FeOB in the shallow waters, the weak positive Ce anomalies (0.94–1.12) and low Mn concentrations in the ironstones are broadly consistent with the previous result of a Cr isotope study. Thus the establishment of a microaerophilic FeOB genetic model for the widespread Chuanlinggou ironstones in North China provides new insight into the origin of Precambrian iron formations and the redox evolution of ocean-atmosphere systems during the “Boring Billion”.

Phototrophs in high iron microbial mats: microstructure of mats in iron-depositing hot springs.

Chocolate Pots Hot Springs in Yellowstone National Park are high in ferrous iron, silica and bicarbonate. The springs are contributing to the active development of an iron formation. The microstructure of photosynthetic microbial mats in these springs was studied with conventional optical microscopy, confocal laser scanning microscopy and transmission electron microscopy. The dominant mats at the highest temperatures (48-54 degrees C) were composed of Synechococcus and Chloroflexus or Pseudanabaena and Mastigocladus. At lower temperatures (36-45 degrees C), a narrow Oscillatoria dominated olive green cyanobacterial mats covering most of the iron deposit. Vertically oriented cyanobacterial filaments were abundant in the top 0.5 mm of the mats. Mineral deposits accumulated beneath this surface layer. The filamentous microstructure and gliding motility may contribute to binding the iron minerals. These activities and heavy mineral encrustation of cyanobacteria may contribute to the growth of the iron deposit. Chocolate Pots Hot Springs provide a model for studying the potential role of photosynthetic prokaryotes in the origin of Precambrian iron formations.

Phototrophs in high iron microbial mats: microstructure of mats in iron-depositing hot springs

Chocolate Pots Hot Springs in Yellowstone National Park are high in ferrous iron, silica and bicarbonate. The springs are contributing to the active development of an iron formation. The microstructure of photosynthetic microbial mats in these springs was studied with conventional optical microscopy, confocal laser scanning microscopy and transmission electron microscopy. The dominant mats at the highest temperatures (48–54°C) were composed of Synechococcus and Chloroflexus or Pseudanabaena and Mastigocladus. At lower temperatures (36–45°C), a narrow Oscillatoria dominated olive green cyanobacterial mats covering most of the iron deposit. Vertically oriented cyanobacterial filaments were abundant in the top 0.5 mm of the mats. Mineral deposits accumulated beneath this surface layer. The filamentous microstructure and gliding motility may contribute to binding the iron minerals. These activities and heavy mineral encrustation of cyanobacteria may contribute to the growth of the iron deposit. Chocolate Pots Hot Springs provide a model for studying the potential role of photosynthetic prokaryotes in the origin of Precambrian iron formations.

Origin of Precambrian iron-formations in the Lake Superior region

The Precambrian banded, cherty iron-formations in the Lake Superior region were deposited on broad continental shelves, and deposits of the supratidal, intertidal, and subtidal zones can be recognized in the well-known facies of the iron-formations. The supratidal deposits were primarily calcitic, aragonitic, and sideritic muds. Other minerals, in order of decreasing abundance, may have been gypsum, anhydrite, ankerite, dolomite, quartz, apatite, and halite. Intraclasts were produced by desiccation-granulation, and some of this material was redeposited in the intertidal and subtidal zones. The intertidal zone was characterized by the development of extensive algal reefs and digital-sized and larger stromatolitic mounds. Desiccation-granulation and tidal rip of mineralized stromatolites further added to the accumulation of intraclasts. The intertidal zone was the domain of granules that were derived by abrasion and rounding of intraclasts. The original mineralogy of the intertidal deposits was similar to that of the supratidal zone; however, because of photosynthesis, the sea water was oxygenated. Lozenge-shaped grains are considered to be secondary, silicified, and iron-oxide replicates of original gypsum crystals, and, by comparison with recent evaporites, some anhydrite and halite may also have been present. The deposits of the intertidal and subtidal zones are considered to be comparable to the present-day Abu Dhabi complex in the Persian Gulf. Spherical microstructures are abundant and well preserved in the chert of the subtidal deposits. Pyrite, which characterizes these deposits, probably was formed largely by the bacterial reduction of sulfate derived from the intertidal zone. The laminations in the iron-formations resulted from several causes that were controlled by chemical and mechanical processes, such as the rates and localization of the precipitation of calcitic, aragonitic, and sideritic muds and the mechanical breakup and redistribution of these sediments as sand, silt, and mud. These processes resulted in compositional and textural laminations. The diversity of the banded, cherty iron-formations, recognized in the different facies, then, can be explained largely in the diagenetic changes. The large-scale pervasive diagenetic changes were the replacement of calcite and aragonite by chert and the formation of iron silicates by the reactions between ionic silica and siderite and iron-bearing dolomite. Vein-like structures and the development of magnetite along stylolites indicate mobility of iron in late diagenetic stages, and iron-organic chelates are suggested to account for the mobility. Depending on the composition of the laminae and the chemical environment, silicification during diagenesis produced the hematite, magnetite, siderite, pyrite, and mixed-mineral facies.

Origin of Precambrian iron formations

The statistical analysis of chemical data by Lepp and Goldich (Econ. Geology, v. 59, p. 1025-1060, 1964) does not adequately support their conclusions because (1) it excludes post-Precambrian cherty iron formation and Precambrian minette types; (2) in attributing apparent uniformity to control in some areas, it ignores geologic evidence such as that suggesting contemporaneous vulcanism; (3) it does not satisfactorily demonstrate that significant replacement of calcium carbonate by silica has actually occurred; and (4) the assumption that the iron-calcium ratio of source areas is the same as that of the lithosphere is not necessarily valid.

Origin of Precambrian iron formations

Discussion of a paper by Henry Lepp and Samuel S. Goldich (Econ. Geol. v. 59, p. 1025-1060, 1964), in which incorrect use of chemical data, narrow restriction to iron formations, and failure to take certain important characteristics into account are criticized.

Origin of Precambrian iron formations

Bulk composition of Precambrian iron-formations is remarkably similar, despite differences in mineral facies. They differ from post-Precambrian iron formations in their low contents of alumina, titania, manganese, phosphorus, and lime. Uniformity of iron content, despite likelihood of variation due to sedimentary processes and depositional environment, suggests fundamental control at sources of materials. A model is proposed by which lateritic weathering in a reducing atmosphere yields solutions with Fe and Ca in approximately their 1:1 ratio of crustal abundance. These elements are deposited as banded calcite-siderite rocks. Different mineral facies are largely due to differing amounts of silica replacement, oxidation, etc., during diagenesis and metamorphism.