Silicification

Verkieselung,/. silicification. verkiesen, v.t. gravel, ballast pyritize. verkitten, .i. cement, lute seal. 

Considerable changes in the chemical composition of rocks occur during the advanced argillic alteration. For example, Si02 content of highly silicified rocks of the Ugusu silica mine reaches 99% (Iwao, 1962). This silicification is caused by the considerable leaching of elements from the rocks by acid hydrothermal solution except Si and addition of Si from hydrothermal solution. 

Such evolution of a hydrothermal system from acidic sulfate hydrothermal solution to neutral is common in the epithermal system associated with precious metal mineralization. For example, advanced argillic alteration and intense silicification occurred at earlier stage of hydrothermal system in the Seigoshi Au-Ag mine area. 

Coradin, T., Durupthy, O. and Livage, J. (2002) Interactions of amino-containing peptides with sodium silicate and colloidal silica A biomimetic approach to silicification. Langmuir, 18, 2331-2336. 

In fact, such biomimetic molecules demonstrate the ability to tailor the growth of silica nanoparticles in a way that is very similar to diatom-extracted species. However, they demonstrate the same limitations in terms of morphological control of nanoparticle assembly. This is because the diatom shell architecture results not only from interactions of silica precursors with templating molecules but also benefits from a cell-driven molding of the vesicular compartment where silicification occurs [29]. Thus, it is very likely that diatom-like synthetic silica will only be achieved when such confinement/molding effects are taken into account in the design of biomimetic experiments [30]. 

The alternative classification of Kostov (1975) is based on the degree of silicification (Ws ), expressed by the ratio... 

This deposit is located in the north-east of Russia and belongs to a gold-arsenic type of low-sulphide formation (Abramson et al, 1980). It lies within a carbonaceous terrigenous rock of Triassic age and is associated with a dome-shaped uplift in a node of intersecting faults of various directions. The ore bodies consist of zones of silicification and kaolinization with veinlet-disseminated sulphide mineralisation. Gold is present in the form of finely-dispersed dissemination in arsenopyrite and pyrite. As an example. Figure 1 illustrates the distribution of Au and Mn in connection with commercial ore... 

Lentz, D.R. Goodfellow, W.D. 1996. Intense silicification of footwall sedimentary rocks in the stockwork alteration zone beneath the Brunswick No. 12 massive sulphide deposit, Bathurst, New Brunswick. Canadian Journal of Earth Sciences, 33, 284-302. 

Carlin-type gold deposits are currently being targeted and mined in Nevada, USA and in the southwest of China. The deposits are characterized by decarbonation, argillization, sulfidation, and silicification of typically calcareous sedimentary rocks (Hofstra Cline 2000). 

Different types of alterations were reported in this area. Silicification is the most usual alteration whole over the area. Argillitic alteration was obsereved in two types, indogenic, which has extended around mineralization zone and exogenic. 

Pagel, M. 1975. Determination des conditions physico-chimiques de la silicification diagenetique des gres Athabasca (Canada) au moyen des inclusions fluides. Comptes Rendus Academie Sciences Paris, 280, 2301-2304. 

The deposits are localized at fault intersections, are associated with breccia zones, and are within clay mineral and silicification/desilicification host-rock... 

Abstract world class unconformity-related U deposits occur in the Proterozoic McArthur Basin (Northern Territory, Australia) and Athabasca Basin (Saskatchewan, Canada). Widespread pre-to post-ore silicifications in the vicinity of the deposits, allow proper observation of paragenetically well-characterized fluid inclusions. We used a combination of microthermometry, Raman microspectroscopy and Laser Induced Breakdown Spectroscopy (LIBS), to establish the physical-chemical characteristics of the main fluids having circulated at the time of U mineralization. The deduced salinities, cation ratios (Na/Ca, Na/Mg) and P-T conditions, led to the detailed characterization of a NaCI-rich brine, a CaCl2-rich brine and a low-salinity fluid, and to the identification of mixing processes that appear to be key factors for U mineralization. 

Keywords uranium deposits, unconformity, silicifications, fluid inclusions, fluid mixing... 

Chemical questions allied to those of weathering will enter into the problems of the consolidation and alteration of sedimentary rocks, including cementation and recrystallization, the formation of low-temperature veins, silicification, and the growth of concretions. The elastic constants of porous aggregates offer an example of the physical data that are likely to be needed in this same connection. 

Tiger s eye is a yellow and brown-banded gemstone, and Hawk s eye is similar but with blue bands. Both come from South Africa and result from silicification of cross-fiber veins of crocidolite. Cut en cabachon, and parallel to the fiber length. Tiger s eye and Hawk s eye exhibit variations in sheen and color that arise from the not-quite-parallel arrangement of the intimately associated asbestos and quartz. 

High rates of water influx remove SiO at low solution concentrations. Normal ground water and streams carry about 17 ppm SiO and less in high rainfall areas (Davis, 1964). In some weathering profiles silicification or deposition of silica has been observed. Most often the form of the phase deposited is crypto-crystalline, either opal or chalcedony. In these cases quartz grains do not show overgrowths (Elouard and Millot,... 

ELOUARD (P.E.) and MILLOT (G.), 1959. Observations sur les silicifications du Lutetien en Mauritame et dans la vallAe du Senegal. Bull. Ser. Carte Geol. Als. Lorr. 1 2, 15-19. 

SILICIFICATION. An important geochemical process by which certain sedimentary rocks such as limestones and dolomites, or calcareous fossils are partially or entirely replaced by silica, Si02. See also Chert and Flint. 

During silica deposition in diatoms a marked increase in protein concentration in cell walls is observed543 however, incorporation of carbohydrates increases only after Si-deposition. In addition, we have seen that there are consistent trends among specific amino acids in all analyzed species, while the sugars exhibit great variability. These two observations suggest that silicification is protein mediated548. ... 

Rousseau V, Leynaert A, Daoud N, Lancelot C (2002) Diatom succession, silicification and silicic acid availability in Belgian coastal waters (Southern North Sea). Mar Ecol Prog Ser 236 61-73... 

Sullivan, C.W., Silicification by diatoms, in Silicon Biochemistry, Ciba Foundation Symposium 121, John Wiley Sons, Chichester, 1986, p. 59. 

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