Chert fine-grained

Main opaque minerals are chalcopyrite, pyrite, pyrrhotite, sphalerite and bornite (Table 2.22). These minerals commonly occur in massive, banded and disseminated ores and are usually metamorphosed. Hematite occurs in red chert which is composed of fine grained hematite and aluminosilicates (chlorite, stilpnomelane, amphibole, quartz) and carbonates. The massive sulfide ore bodies are overlain by a thin layer of red ferruginous rock in the Okuki (Watanabe et al., 1970). Minor opaque minerals are cobalt minerals (cobaltite, cobalt pentlandite, cobalt mackinawite, carrollite), tetrahedrite-tennantite, native gold, native silver, chalcocite, acanthite, hessite, silver-rich electrum, cubanite, valleriite , and mawsonite or stannoidite (Table 2.22). 

In order to overcome these problems, interest was focussed on that portion of the organic matter trapped in mineral precipitates which formed synchronously with sedimentation. In these cases, the material is hermetically sealed in the crystalline matter and may survive with relatively little subsequent alteration. Such preservation is common in cherts which are chemical precipitates of silica and now comist of fine grained quartz. These rocks offer the best chance for successful preservation of truely Precambrian molecular fossils. Modem microprobes and spectrophotometer microscopes allow the non-destructive analysis of organic matter enclosed in mineral crystals. Laser bombardment of microscopic... 

Chert is another organic marine sediment, less common than carbonate rocks, but found in huge deposits in some parts of the world. It initially consists of the skeletons of billions of tiny, single-celled animals called radiolaria. These skeletons are composed of microcrystalline quartz or chalcedony (Si02). Dense layers of this material accumulate on the ocean floor, where they are buried and compressed over time. The term chert is sometimes also applied to any compact, very fine-grained siliceous sediment that has resulted from precipitation or consolidation of silica gel. There may be chert lenses or very thin layers within other types of sediments, such as limestone. 

The aim was to investigate deep chert-haematite-bearing veins and fractures. The fractures below 100 m are nearly completely cemented (shallower ones are filled with deep ground water and were not investigated in this study). It is possible to distinguish two types of fractures wide shear fractures several centimetres in diameter filled with breccia of the surrounding host rock cemented by fine-grained haematite and disseminated chert, and fractures 1 mm-1 cm wide with a clear zonation of... 

Table U. The Reclassification of Fine-Grained Chert into Chert and Rhyolitelike Rock on the Basis of INAA Results...

The smaller spheroidal and ellipsoidal bodies of type E, 4-8 jttm in size and preserved due to impregnation with carbonaceous material, fine-grained hematite, or greenalite, differ from the structures that have been described. These structures are found in black cherts, red jasper bands, and granule-containing greenalite cherts (see Fig. 34). The algal structures at the base of the Biwabik formation also contain unique filiform structures of type F their diameter is 1-2, sometimes up to 6 jam. 

Reliance Fm, Belingwe belt, Zimbabwe. About 6.5 mm across photo. Olivine crystals set in fine grained to once-glassy groundmass. For details see Nisbet et al. (1987). Photo W. E. Cameron, (b) Alternating iron-rich and carbon-rich shales. White bands are chert this lithology is transitional to banded ironstone. Approximately, 20 cm across picture. Belingwe belt, Zimbabwe. 

The common silica rocks, chert and chalcedony, are made up of fine-grained quartz and microcrystalline silica. Accordingly, their solubility exceeds that of well-crystallized quartz. Fournier (1985) suggests the temperature function... 

Another mysterious phenomenon is ambient pyrite. Grains a few microns in diameter have moved through solid chert (an extremely fine-grained quartz), leaving a trail of coarse-grained quartz. This was described by Tyler, Knoll, and Barghoorn (172) (Figure 1.9). 

Solid amorphous silica (opal) and quartz crystals, as in geodes, are formed on solid sufaces. In a comprehensive investigation of the behavior of silica in hot-spring water. White Brannock, and Murata (45) pointed out that there was no evidence quartz could be formed within any reasonable time under ordinary conditions. In nature, amorphous silica appears to be deposited from the highest concentration of silica, then chalcedony (extremely fine-grained fibrous form of quartz) from a lower concentration, and Hnally macroscopic quartz crystals from lowest concentrations just exceeding the saturation level for quartz. Over millions of years most amorphous silicas in the presence of water are eventually transformed to chalcedony or chert and quartz. 

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