Tungsten abundance in Precambrian iron-formations and other sedimentary rocks

1978 ◽  
Vol 73 (6) ◽  
pp. 1167-1170 ◽  
Author(s):  
K. A. Harmon ◽  
D. M. Shaw ◽  
J. H. Crocket
Keyword(s):  
Sedimentary Rocks ◽  
Iron Formations

Iron Formations as Palaeoenvironmental Archives

2021 ◽  
Author(s):  
Kaarel Mänd ◽  
Leslie J. Robbins ◽  
Noah J. Planavsky ◽  
Andrey Bekker ◽  
Kurt O. Konhauser
Keyword(s):  
Sedimentary Rocks ◽  
Scientific Understanding ◽  
Significant Part ◽  
Formation Mechanisms ◽  
Redox States ◽  
Elemental Cycling ◽  
Ancient Iron ◽  
Iron Formations ◽  
Palaeoenvironmental Conditions ◽  
Chemical Sediments

Ancient iron formations - iron and silica-rich chemical sedimentary rocks that formed throughout the Precambrian eons - provide a significant part of the evidence for the modern scientific understanding of palaeoenvironmental conditions in Archaean (4.0–2.5 billion years ago) and Proterozoic (2.5–0.539 billion years ago) times. Despite controversies regarding their formation mechanisms, iron formations are a testament to the influence of the Precambrian biosphere on early ocean chemistry. As many iron formations are pure chemical sediments that reflect the composition of the waters from which they precipitated, they can also serve as nuanced geochemical archives for the study of ancient marine temperatures, redox states, and elemental cycling, if proper care is taken to understand their sedimentological context.

Precambrian atmospheric oxygen: evidence in the sedimentary distributions of carbon, sulfur, uranium, and iron

1976 ◽  
Vol 13 (9) ◽  
pp. 1161-1185 ◽  
Author(s):  
Erich Dimroth ◽  
Michael M. Kimberley
Keyword(s):  
Oxygen Pressure ◽  
Ferrous Iron ◽  
Atmospheric Oxygen ◽  
Sedimentary Rocks ◽  
Major Change ◽  
Free Atmosphere ◽  
Ambient Oxygen ◽  
Iron Formations ◽  
Organic Productivity ◽  
Sedimentary Pyrite

The sedimentary distributions of carbon, sulfur, uranium, and ferric and ferrous iron depend greatly upon ambient oxygen pressure and should reflect any major change in proportion of oxygen in the atmosphere or hydrosphere. The similar distributions of these elements in sedimentary rocks of all ages are here interpreted to indicate the existence of a Precambrian atmosphere containing much oxygen.Organic carbon contents and distributions are similar in Precambrian and Quaternary sedimentary rocks and sediments, although distributions in both would have been sensitive to variations in rates of organic productivity and atmospheric oxygen pressure. Sedimentary pyrite is almost invariably closely associated with organic carbon, suggestive of formation by sulfate reduction, in sedimentary rocks of any age. Archean and Middle Precambrian cherty iron formations and uranium ores resemble Phanerozoic ores and probably formed similarly by diagenetic concentration. In general, we find no evidence in the sedimentary distributions of carbon, sulfur, uranium, or iron, that an oxygen-free atmosphere has existed at any time during the span of geological history recorded in well preserved sedimentary rocks.

Age Determinations in the Circum – Ungava Geosyncline and the Evolution of Precambrian Banded Iron-Formations

1972 ◽  
Vol 9 (6) ◽  
pp. 652-663 ◽  
Author(s):  
B. J. Fryer
Keyword(s):  
Large Scale ◽  
Sedimentary Rocks ◽  
Life Forms ◽  
Hudson Bay ◽  
Fold Belt ◽  
Banded Iron Formations ◽  
Unique Nature ◽  
Valuable Marker ◽  
Iron Formations ◽  
Age Determinations

Rb-Sr whole-rock isochron studies on volcanic and sedimentary rocks from the Belcher Fold Belt in Hudson Bay, and sedimentary rocks from the Labrador Trough and the Lake Mistassini area of Quebec, yield ages of 1800, 1870, and 1790 m.y., respectively. These age determinations substantiate previous correlations between these areas based on stratigraphic and structural similarities. Consequently, the concept of a Circum – Ungava Geosyncline appears to be valid on geochronological grounds, with geosynclinal development terminating with the onset of the Hudsonian orogeny at about 1800 m.y.The Circum – Ungava Geosyncline contains banded iron-formations exhibiting shallow-water sedimentary structures which distinguish them from older banded iron-formations. These deposits probably represent the hydrospheric response to the initial large scale introduction of oxygen to the atmosphere by evolving life forms. Because of their probable unique nature these younger banded iron-formations may serve as a valuable marker horizon for global correlations of Proterozoic strata.

Direct Observation of Crystalline Ordering In Naturally Graphitized Carbon Produced by Low Grade Metamorphism

1977 ◽  
Vol 35 ◽  
pp. 162-163
Author(s):  
Thomas R. McKee ◽  
Peter R. Buseck
Keyword(s):  
Crystallite Size ◽  
Sedimentary Rocks ◽  
Carbonaceous Material ◽  
Low Grade ◽  
X Ray ◽  
Graphitized Carbon ◽  
Transmission Electron ◽  
Heat Treated ◽  
Commercial Carbon ◽  
Metamorphic Zone

Sediments commonly contain organic material which appears as refractory carbonaceous material in metamorphosed sedimentary rocks. Grew and others have shown that relative carbon content, crystallite size, X-ray crystallinity and development of well-ordered graphite crystal structure of the carbonaceous material increases with increasing metamorphic grade. The graphitization process is irreversible and appears to be continous from the amorphous to the completely graphitized stage. The most dramatic chemical and crystallographic changes take place within the chlorite metamorphic zone.The detailed X-ray investigation of crystallite size and crystalline ordering is complex and can best be investigated by other means such as high resolution transmission electron microscopy (HRTEM). The natural graphitization series is similar to that for heat-treated commercial carbon blacks, which have been successfully studied by HRTEM (Ban and others).

Stabilizing pyritic material

1989 ◽  
Vol 4 ◽  
pp. 244-248 ◽  
Author(s):  
Donald L. Wolberg
Keyword(s):  
Significant Contribution ◽  
Organic Materials ◽  
Sedimentary Rocks ◽  
Iron Sulfides ◽  
Decomposition Products ◽  
Iron Minerals ◽  
Pyrite Formation ◽  
Organic Sediments ◽  
Freshwater Environments

The minerals pyrite and marcasite (broadly termed pyritic minerals) are iron sulfides that are common if not ubiquitous in sedimentary rocks, especially in association with organic materials (Berner, 1970). In most marine sedimentary associations, pyrite and marcasite are associated with organic sediments rich in dissolved sulfate and iron minerals. Because of the rapid consumption of sulfate in freshwater environments, however, pyrite formation is more restricted in nonmarine sediments (Berner, 1983). The origin of the sulfur in nonmarine environments must lie within pre-existing rocks or volcanic detritus; a relatively small, but significant contribution may derive from plant and animal decomposition products.

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