Precambrian deposits

Precambrian deposits are rare. Only 3% of the reserves, or 1% of the potential reserves, belong to them. 

The second prime trait, the carbon isotopic composition of microfossil-associated organic matter, is supported by thousands of measurements in hundreds of Precambrian deposits... 

This paper provides a general review of exploration methods for major types of uranium deposits. The author has been active in exploration for deposits hosted by rocks of Precambrian, Palaeozoic, Mesozoic and Caenozoic age. Much of this work involved deposits in sandstones, but the geochemical principles also apply to many of the Precambrian deposits. 

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. 

Sandstone The tertiary, Jurassic and Triassic sandstones of the western Cordillera of the United States account for most of the uranium production in that country. Cretaceous and Permian sandstones are important host rocks in Argentina. Other important deposits are found in carboniferous deltaic sandstones in Niger in Permian Lacustrine siltstones in France and in Permian sandstones of the Alpine region. The deposits in Precambrian marginal marine sandstones in Gabon have also been classified as sandstone deposits. 

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. 

Molecular fossils have been successfully identified in younger Precambrian rocks and linked to certain classes of biological source material. In organic analyses of ancient sediments the cleaned, pulverized rocks are treated with organic solvents to extract a soluble fraction containing the less complex and more easily identifiable compounds. However, this fraction is more subject to contamination since it is not locked within the rock matrix. Normal alkanes have been identified in extracts of the 3 billion year old Fig Tree Shale. These alkanes have a probable biological origin in cellular lipids. The odd and even-numbered alkanes are evenly distributed, a characteristic of alkanes from ancient rocks. It is uncertain, however, whether these compounds were present at the time of deposition or derived from a later source [24]. 

Baker JC, Kassan J, Hamilton PJ (1995) Early diagenetic siderite as an indicator of depositional environment in the Triassic Rewan Group, southern Bowen Basin, eastern Australia. Sedimentology 43 77-88 Barghoom ES (1981) Aspects of Precambrian paleobiology the early Precambrian. Paleobot Paleoecol Evolution 1 1-16... 

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. 

Surface exposures of the San Miguel skarn deposit occur isolated over a 4000 m area, surrounded by intruding Proterozoic granitoid outcrops. The San Miguel skarn is mainly hosted by the Precambrian marbles of the Buenos Aires Complex. 

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. 

FIecht, L. Cuney, M. 2000. Flydrothermal alteration of monazite in the Precambrian crystalline basement of the Athabasca Basin (Saskatchewan, Canada) Implications for the formation of unconformity-related uranium deposits. Mineralium Deposita, 35, 791-795. 

The increased interest in Mo and W due to commodity price increases since 2006 has created a need for a synthesis of information on known deposits and potential exploration environments within the province of Newfoundland and Labrador. This review is drawn from the historical database of exploration and scientific studies, but it also provides some essential geological information concerning recent exploration developments in Newfoundland. Further information is provided in a recent article by Kerr et al. (2009). Molybdenum mineralization also occurs in Precambrian granites in Labrador, but these deposits presently lie within Inuit lands that are... 

Tungsten mineralization in Newfoundland is more restricted in distribution than molybdenum. The best known deposit is in the Grey River area (Fig. 1) and is very close to the Moly Brook deposit (see above). However, it lies within a different package of host rocks, dominated by late Precambrian metasedimentary and metavolcanic rocks. 

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... 

The reservoir rocks that yield crude oil range in age from Precambrian to Recent geologic time but rocks deposited during the Tertiary, Cretaceous, Permian, Pennsylvanian, Mississippian, Devonian, and Ordovician periods are particularly productive. In contrast, rocks of Jurassic, Triassic, Silurian, and Cambrian age are less productive and rocks of Precambrian age yield petroleum only under exceptional circumstances. 

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