Ironstones, deposition

Finally, this tripartite cyclicity is also seen in the frequency of occurrence of Phanerozoic ironstones and oolites (Figure 10.18). As sea level withdrew from the continents and continental freeboard increased, shallow-water areas with the requisite environmental conditions necessary to form oolite and ironstone deposits decreased in extent. Thus, as calcium carbonate deposidon increased on slopes and in the deep sea, carbonate oolite and ironstone deposition on shelves and banks nearly ceased. 

A clay-sized trioctahedral kaolin mineral, which occurs frequently in sedimentary ironstone deposits and marine mudstones and hence may occur in soils derived therefrom, has been... 

Continental Shelf. Most consohdated mineral deposits found on the continental shelf are identical to those found on land and are only fortuitously submerged. Exceptions include those laid down in shallow marine seas or basins in earlier geochemical environments such as bedded ironstones, limestones, potash, and phosphorites. 

Statement 1. Bituminous sand is an aggregate of sand, clayey matter, oil and water. The sand consists mainly of quartz particles of 50 to 200-mesh size and smaller, but also of particles of other minerals including mica, rutile, ilmenite, tourmaline, zircon, spinel, garnet, pyrite, and lignite. Clay occurs interbedded with the bituminous sand itself. Ironstone nodules of all sizes up to eight inches in diameter occur in the bituminous sand beds, especially in the southern part of the deposit. The oil is viscous, naphthenic, and of a specific... 

Chamosite appears to be the finest grained and most abundant mineral in this group. It occurs in lateritic clay deposits (Brindley, 1951), both as oolites and matrix in sedimentary ironstones (Hallimond, 1925), in hydrothermal deposits (Ruotsala et al., 1964), in shales (Drcnnan, 1963), in Recent shallow-marine deposits (Porrenga, 1966) and in estuarine sediments (Rohrlich et al., 1969). It is probable that chamosite is more abundant than commonly realized however, Drennan (1963) has pointed out that it is extremely unstable in a leached and oxidized environment and is not likely to persist as an allogenic mineral. 

In the Spring Valley and Shavi Members, sedi-mentological facies (Bickle et al. 1975 Martin et al. 1980 Nisbet et al. 1993 Hunter et al. 1998) vary from beach to an assortment of shallow-water settings. REE patterns from oxide-facies ironstones have strong Eu anomalies, but no Ce anomalies. The Spring Valley and Shavi REE are most simply modelled as sediments deposited from water which included (1) ambient sea water of modem aspect but without a Ce anomaly, as well as (2) an admixed hydrothermal (modern black smoker or white smoker) component. The source of the REE may have been distant. 

The model of Fig. 7 is derived in part from the sedimentological facies (Nisbet et al. 1993 Hunter et at. 1998) and in part from the depositional chemistry of banded ironstones (e.g. Drever 1974). In a world where total biological productivity may have been roughly comparable with... 

The Lea Park Formation is the lowermost unit of hydrogeological interest in the area but it is covered by at least 300 m of Belly River sediments. The strata are notable for their low permeability and consist of marine deposited grey sUty shale, thin sandstones, and common sideritic ironstone concretions (Green, 1972). 

One clue to oxygen levels during this period is to be found in the very same iron formations that overlie the shales of the Hamersley Range. Massive sedimentary iron formations were deposited here and around the world in alternating bands of red or black ironstone (haematite and magnetite, respectively), and sediment, typically flint or quartz. The individual bands range in depth from millimetres to metres, while the formations themselves can be up to 600 metres [approximately 2000 feet] thick. Most of these formations were deposited between 2.6 and 1.8 billion years ago, but sporadic outcrops range in age from 3.8 billion to 800 million years. 

If this was the case, the banding of ironstones with flint or quartz could have been produced by seasonal influences, such as higher rates of photosynthesis (and therefore oxygen production) in the summer than in the winter, or seasonal upwellings according to climatic variations. The seasonal fluctuations in iron deposition would have been set against a... 

The principal mineral for the supply of chromium is chromite. It has a variety of names -. chrome ore, chrome-ironstone, or chrome iron ore, Fe0.Cr203, in which the iron and chromium are more or less replaced by magnesium and aluminium. Iron ore with up to about 3 per cent, of chromium is called chromiferous iron ore. The origin of the chromite deposits has been discussed by M. E. Glasser, L. W. Fisher, E. 8ampson, F. Ryba, C. 8. Hitchin, J. 8. DiUer, P. A. Wagner,... 

The pegmatites outcrop into the area s granite and ironstone (laterite) rocks, and the first commercial operations were for tin, tantalum and microlite from then-weathered siufaces, but these alluvial deposits were soon depleted. The waU... 

---