Sandstone occurrence

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. 

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. 

Uranium mineralization in the Windsor area is a typical sandstone roll-front type. The Green Street occurrence that is the focus of this study occurs in thick arkosic sandstones, siltstones, and pebbly conglomerates of the Cheverie Formation (Fig. 2). The occurrence is a remnant tail of a uranium roll front that has subsequently migrated down dip from the outcropping (Ryan O Beirne-Ryan 2007). 

A series of leaching experiments were carried out on mineralized sandstone and siltstone from the Green Street Occurrence (Parsons 2007). The samples were poorly consolidated and easily broken apart by hand and placed into the waters. The samples were placed in distilled water, stream water, and rain water at temperatures ranging from 5 to... 

Analcime occurs only in the upper lagoonal complex in beds 150-300 m thick. It is most widespread in the cement of sandstones and fills the pores of many chemogenic rocks associated with them. The widespread occurrence of analcime in the upper complex and its absence in the. lower complex (despite the similar composition of detrital matter and the identical conditions of formation) would have been unaccountable were it not for one peculiar feature of the heavy mineral fraction. The heavy mineral content of rocks of the upper complex varies from fractions of a percent to 2 or 2.5%. Up ot 50% of the heavy mineral fraction consists of fresh, monoclinic pyroxene and amphibole. 

Coal is Interspersed as individual beds within other types of sedimentary rock beds, including sandstones, limestones, clays, shales, and mixtures of these materials. The plant material that ultimately became coal deposits was accumulated in upland bogs, coastal or near-coastal swamps, or della plains. It is envisioned that the conditions were somewhat similar to the conditions existing today in the Okefenokce Swamp in Georgia or the Everglades of Florida. These areas may have varied from a few acres in several hundreds of square miles (hectares/square kilometers). Hence, the variation in ihe occurrence of coal as we find it today. 

Thornburg, K. and Sahai, N. (2004) Arsenic occurrence, mobility, and retardation in sandstone and dolomite formations of the Fox River valley, eastern Wisconsin. Environmental Science and Technology, 38(19), 5087-94. 

Triplehorn, D.M., 1967. Occurrence of pure, well-crystallized 1M illite in Cambro-Ordovician sandstone from Rhourde El Baguel Field, Algeria. J. Sediment. Petrol, 37 879-884. 

That the atomic weight of uranium lead is extremely variable has already been shown. In order to interpret this variability its sources must be studied both geologically and mineralogically. On the geologic side of the question the uranium ore can be divided in to three principal classes, which are sharply distinct. The definitely crystallized varieties of uraninite occur in coarse pegmatites, associated with feldspar, quartz, mica, beryl, and other minor accessories. The massive pitchblende is found in metalliferous veins, together with sulphide ores of copper, lead, iron, zinc, and so forth. As for camotite, that is a secondary mineral, found commonly as an incrustation on sandstone, and often, also upon fossil wood. There may be other modes of occurrence, but these are the most distinctive. 

The occurrence of highly reactive minerals, such as evaporitic minerals, pyrite and even calcite, in low proportions—a percent or less— in a given rock, e.g., calcareous sandstone, pyritic shale, marl with traces of anhydrite, granite with traces of calcite, may determine the chemical character of stream water (Miller, 1961 Drever, 1988). In a study of 200 streams from monolithologic catchments underlain by various rock types under similar climatic conditions in France, the relative weathering rate based on the cation sum (Meybeck, 1986) ranges from 1 for quartz sandstone to 160 for gypsiferous marl. 

Chemical conditions. The occurrence of glauconites in sandstones (acid) and carbonates (basic) indicates that pH is not of great importance to their formation. They are found in sedimentary rocks ranging in age from Precambrian to the present day. Hence, their formation seems to be constrained by chemical and physical conditions rather than by specific events. There is no apparent time-dependent reaction which transforms them into new phases. 

Figure 9. Conceptual diagram of the important paleohydrologic controls on the occurrence and distribution of MVT mineralization, and arsenic enrichment. This figure is simplified from one originally constructed by T. Hayes (written communication, 1989). Sandstone is. shown as a dot pattern, limestone as a rectangular pattern, dolomite as a trapezoidal pattern, and shale as the dash-dot pattern. The arrows show pathways of fluid migration, and the letters show the locations of ore formation discussed in the text.

Moderate and low arsenic concentrations are found in numerous wells in the FRV, irrespective of the presence of the SCH in the open interval of the well. These levels of arsenic in ground water do not correlate to the intersection of the SCH with the static water level (Fig. 8), indicating that the sulfide oxidation initiated by oxygen introduced at the borehole does not cause all arsenic occurrences. Tfiere is no single stratigraphic unit that controls the low to moderate concentrations wells with open intervals in the Sinnipee Group, the St. Peter Sandstone, the Prairie du Chien Group or the Cambrian sandstones are all affected. 

Uranium U(VI) minerals are most often products of the oxidation and weathering of nearby primary U(IV) ore minerals such as uraninite [U02(c)I and coffinite [USi04(c)l (cf. Pearcy et al. 1994). They also form by evaporative concentration of dissolved U(VI), particulary under arid conditions. Schoepite (/J-UOj 2H2O) is fairly soluble and, therefore, is a rare mineral, whereas carnotite K2(U02)2(V04)2j and tyuyamunite (Ca(U02)2(V04)2j, which have lower solubilities (particularly above pH 5) are the chief oxidized ore minerals of uranium. The plots in Figs. 13.5 and 13.6 indicate that uranyl minerals are least soluble in I0W-CO2 waters, and, therefore, are most likely to precipitate from such waters. This is con.sistent with the occurrence of carnotite and tyuyamunite in oxidized arid environments with poor. soil development (Chap. 7), such as in the calcrete deposits in Western Australia (cf. Mann 1974 Dall Aglio et al. 1974), and in the sandstone-hosted uranium deposits of the arid southwestern United States (cf. Hostetler and Carrels 1962 Nash et al. 1981). The... 

The occurrence of zones along joints and minor faults in the Taveyanne sandstones of the western Alps, in which the laumontite of this unit has been dehydrated and formed other minerals, is taken by Coombs (1971) as a plausible indication of higher hydrostatic pressure within the sandstone than in the joints. 

In devising a scheme for monitoring the rate of deterioration of the sandstone, the assumption was made that any effect that atmospheric pollutants may have on the stones integrity will appear at or near the surface. It is not a common occurrance that masonry is saturated throughout by water. Consequently, an adequate measure of erosion is the rate of recession of the surface. Here, the bonding material is dissolved or disrupted by repeated exposures to moisture which may contain chemicals which accelerate the process. The rate of recession is characterized by the rate at which the loosened quartz grains are subsequently removed. 

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