Carbonate Precambrian

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

At the end of 2003, new research results led to sensational headlines Minerals Cooked Up in the Laboratory Call Ancient Microfossils Into Question was the title chosen by Richard A. Kerr for his article in Science dealing with synthetically prepared silicate carbonates. Their microstructures show morphologies which look exactly like those of filaments which had been assigned as cyanobacterial microfossils of the Precambrian Warrawoona chert formation in western Australia. The synthetic structures consist of silicate-encapsulated carbonate crystals, and in part have a helically twisted morphology reminiscent of biological objects. Simple... 

Extensive, horizontal sandstone plateaus occur in tropical shield areas. Well-known examples are the Precambrian Roraima sandstone formations on the Guiana Shield and the Voltaian sandstone formations in Western Africa. Major occurrences of consolidated sands are found in Northern Africa, in Guyana and Surinam, eastern Peru, northeastern Brazil and in Liberia (western Africa). These sandstone formations have a history of tropical weathering in common they all have a deep weathering mantle of bleached, white sands that are very rich in quartz, poor in clay and excessively drained. Electrolyte contents differ by region In arid and semi-arid areas where evaporation exceeds precipitation, salts and carbonates may accumulate at or near the surface of the soil. 

Auler, A.S. Smart, P.L. 2003. The influence of bed-rock derived acidity in the development of surface and underground karst evidence from the Precambrian carbonates of semi-arid northeastern Brazil , Earth Surface Processes and Landforms, 28, 157-168... 

Klein C (1974) Greenalite, stilpnomelane, minnesotaite, crocidolite, and carbonates in a very low-grade metamorphic Precambrian iron-formation. Can Min 12 475-498 Klein C (1978) Regional metamorphism of Proterozoic iron-formation, Labrador Trough, Canada. Am Min 63 898-912... 

Considerable geographic variability exists in the distribution of the source rocks contributing salts to river and groundwaters. As shown in Table 21.3, most of the evaporites, which are the dominant natural source of Na and Cl in river water, lie in marginal and endorheic (internal) seas. Some of these subsurfece evaporite deposits dissolve into groundwaters, which eventually carry Na and Cl into the ocean. Carbonates are the prevalent rock type between 15°N and 65°N. Precambrian-age crustal rocks and meta-morphic minerals predominate between 25°S and 15°N and north of 55°N. Shales and sandstones represent on average 16% of the terrestrial surfece lithology. 

Precambrian carbonate rocks in the San Miguel area allowed the formation of an isolated skarn deposit related to the Proterozoic magmatic hydrothermal activity and associated with the granite intrusions. Similar processes occurred at the Punta Tota skarn. These deposits at the present time constitute the only two skarns recognized in the Tandilia Belt of Buenos Aires Province. 

Baker AJ, Fallick AE (1989) Heavy carbon in two-biUion-year-old marbles from Lofoten-Vesteralen, Norway implications for the Precambrian carbon cycle. Geochim Cosmochim Acta 53 1111-1115... 

We further conclude that calcified tissues were not developed for carbonate deposition. They were secreted as components of the cell wall or as excretory byproducts of metabolic processes. As a consequence of a changing habitat, these materials started to calcify or introduced inorganic carbonate deposition. The sudden occurence of highly developed metazoan populations at the boundary Cambrian-Precambrian is frequently explained in terms of an explosive evolution347). For those who are familiar with the mechanism of calcification this deus ex machina explanation is not necessarily the most satisfying one. It may simply be a reflection of the universal adaption of many forms of life to a new environmental setting that developed at that time. Only a slight modification in pH is required (see Fig. 14) to... 

In conclusion, cellular extraction of carbonate, phosphate and silica appeared almost simultaneously at about the Precambrian-Cambrian boundary. Which of these three modes of biomineralization is the most ancestoral one is not clear largely due to the metastability of some biominerals, the coexistence of different minerals within some primitive organism557,558), and uncertainties of stratigraphic correlation. 

Hinton, M. J., S. L. Schiff, and M. C. English. 1997. The significance of storms for the concentration and export of dissolved organic carbon from two Precambrian Shield catchments. Biogeochemistry 36 67—88. 

Schiff, S., R. Aravena, E. Mewhinney, R. Elgood, B. Warner, P. Dillon, and S. Trumbore. 1998. Precambrian Shield wetlands Hydrologic control of the sources and export of dissolved organic carbon. Climatic Change 40 167—188. 

Rapid embedding in silica is particularly favourable for the good preservation of the organic matter, and this was surely the case with the well-preserved microstructures. But there is much indication from the thin section petrography that the Precambrian banded iron-formations were primarily deposited as carbonates (Lougheed, 1983)The origin of the embedding chert has been a major problem,... 

Kerogens isolated from the Fig Tree cherts produced very complex mixtures of pyrolysis products, dominated by a series of methyl branched alkenes with each member of the series having 3 carbon atoms more than the previous member. At each carbon number a highly complex mixture of branched alkanes and alkenes plus various substituted aromatic compounds was found. The highly branched structures may have actually incorporated isoprenoids originally present in the Precambrian microorganisms (Philp Van DeMent, 1983)6>. 

Models for the formation of Precambrian sediments suggest that the chemical sediments (such as cherts) of the Isua supracrustal belt have formed as shallow water deposits. This is in agreement with structures locally preserved in the metacherts of the sequence. After deposition, the supracrustals were folded and metamorphosed. Finally, the metamorphism reached lower amphibolite facies and in consequence, most of the primary minerals became recrystallized. As a result all chert now appears as quartzite. But apparently metacherts, magnetite iron formation and quartz carbonate rocks have retained their major element chemistry largely unaltered during metamorphism (Nutman et al., 1984) 119). 

Porter SM, Knoll AH, Affaton P (2004) Chemostratigraphy of Neoprotero-zoic cap carbonates from the Volta Basin, West Africa. Precambrian Res 130 99-112... 

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