Precambrian sedimentary rocks

Numerous examples of the finding of graphite-bearing sedimentary rocks in the make-up of the Precambrian sequences are usually given in the latest geochemical review works. 

Rankama (1957) wrote that finely divided carbon in crystalhne schists of mudstone origin is much more widespread in Precambrian rocks than previously believed. The origin of this carbon probably is organic. 

Mason (19 /1) points out that even in very old Archean formations slates are found which contain free carbon of possibly organic origin. In particular, such slates occur in the Gunflint BIF (Lake Superior), with an age of about 1.9 b.y. 

An analysis of sedimentation in the Precambrian made by Tugarinov and Voytkevich (1966) on the basis of a summary of recent geochemical data gives a clear picture of the directionality and irreversibility of the geologic evolution of the Earth s crust. Whereas in the Early Precambrian the composition of the sedimentary-metamorphic complexes is characterized by the predominant development of basic and ultrabasic effusives, altered 

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The question of the distribution of free carbon in Precambrian sedimentary rocks also needs special consideration. Ronov and Migdisov (1970) show that the content in Lower and Upper Proterozoic complexes remained at the same level, but at the boundary of the Proterozoic and Paleozoic there is observed a two-fold increase in the carbon content. However, quantitative data on the carbon content in different types of rocks, including the BIF, are very few. 

At the present time there are sufficiently reliable data on the distribution of the stable isotopes of oxygen, carbon, and sulfur in Precambrian sedimentary rocks. On the basis of analysis of these data, it is possible to obtain additional information on the geochemical history of the ocean and conditions of sedimentation. 

Isotopic fractionation of sulfur (5 S = +8.2 and +3.1%o) has also been established for some samples of sulfides from the graphite-rich ancient ( 3 b.y.) shales of the Soudan formation (Lake Superior). Other determinations of the sulfur isotopic composition of Precambrian sedimentary rocks are known. A wide range of variations of S S has been established in the... 

In recent years some data have been obtained on the content of organic compounds (hydrocarbons, carbohydrates, paraffins and aromatic compounds, alcohols, etc.) in Precambrian sedimentary rocks, including the rocks of the cherty iron-formations. 

Iron formation, the sedimentary rock characterized by the association of chert and megaquartz with silicate, carbonate, and iron oxide minerals, is one of the most distinctive Precambrian sedimentary rocks. These rocks, which reached their peak in abundance during the Paleoproterozoic (Figure 2) (James, 1983 Isley, 1995), consist of both banded iron formations (BIFs) and granular (arenitic) iron formations (GIFs) and... 

Fig. 1. Carbon isotopic values of coexisting carbonate and organic carbon measured in bulk samples of Phanerozoic and Precambrian sedimentary rocks, for the Precambrian represented by data from 100 fossiliferous cherts and shales shown as average values for groups of samples from 50-Ma-long intervals (Strauss and Moore, 1992).

Haskin and Gehl (1962) were the first to notice unusual lanthanide patterns for Precambrian sedimentary rocks. Haskin et al. (1968) confirmed a relative enrichment of Eu in Precambrian sedimentary rocks compared to the North American Shale Composite (NASC) sample. More detailed studies by Wildeman and Haskin (1973) and by Wildeman and Condie (1973) confirmed Eu enrichment and lower total lanthanides for Precambrian sedimentary rocks. Most of the samples for these latter studies came from Archean terrains. These differences in lanthanide patterns between Archean and post-Archean terrigenous sediments have become a crucial observation for models of the evolution of the continental crust. 

India Bihar (State of Orissa) and Mahya (India Pradesh) Precambrian sedimentary rocks 86.4... 

Paleo-placers Conglomerates of quartzite and mica in precambrian sedimentary rocks. May contain gold, silver, platinum and uranium The biggest gold producer in the world, Witivatersrand in South Africa has several mines with this ore type... 

The basement complex for the Patagonian desert and arid northeastern Brazil is formed by metamorphosed Precambrian rocks. Landscapes are characterized by level erosion surfaces of different ages. The landscape is dissected by a large number of valleys. Large depressions are filled with marine and continental beds of sedimentary rocks. Rocks in the Andean system, that stretches the entire length of the west side of the continent, vary greatly. Many depressions are filled with sediments. In addition, many active volcanoes are responsible for periodic lava flows and the deposition of volcanic ash. East of the Andes, the land surface is level and slopes towards the Atlantic Ocean. Broad depressions contain saline or sodic soils. 

The Cretaceous sedimentary rocks of the Western Canada Sedimentary Basin (WCSB), renowned for their hydrocarbon resources, are seldom considered to have potential to host base metal sulphide mineralization. These sedimentary rocks have also discouraged those who presumed that the Precambrian rocks of the Canadian Shield to the east and north are more favourable hosts of base metal mineralization (ctMacqueen 1997). 

Four other significant homologous series present are gem-6 y substituted w-alkanes. The gem prefix designates geminal-substituted compounds, i.e. two substituents on the same atom of a disubstituted compound. Several research groups have reported the presence of 5,5-diethylalkanes, as well as lesser amounts of other g zw-dialkylalkanes, in sedimentary rocks back to the Precambrian and in hydrothermal fluids. 

Iron formation A chemically precipitated Precambrian sedimentary or metamorphosed sedimentary rock consisting of thin layers rich in iron minerals (such as hematite, Fe203, and magnetite, FesOzO and usually quartz (compare with ironstone). 

Veizer J. and Garrett D.E. (1978) Secular variations in the composition of sedimentary rocks, 1. Alkali metals. Precambrian Research 6,367-380. 

In a new summary work by Dimroth and Kimberley (1976) devoted to a comparison of the distribution of carbon and some other elements in Precambrian and Phanerozoic sedimentary rocks, it is mentioned that in most Precambrian sequences the normal negative correlation between grain size and concentrations is established regardless of the depositional environment. That regularity indicates the great importance of the role of plankton in the accumulation of Garrels et al. (1973) came to the same conclusion. 

Investigations of the geochemistry of carbon in recent and ancient sedimentary rocks (Strakhov, 1960 Stashchuk et al., 1964) have demonstrated the special role of this element in the formation of iron minerals. The data obtained by S. Sidorenko (1971) for Precambrian sequences and especially by Plaksenko (1966) for the BIF of the KMA stimulated study of the distribution of carbon and some other elements in the iron-rich rocks, schists, and ore minerals of the Krivoy Rog deposits to a substantial extent. 

Sidorenko, S.A., 1971. Organic Matter in Precambrian Sedimentary-metamorphic Rocks. Diss., Moscow (in Russian). 

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