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Formation of cherty iron sediments

Now it remains to put together the partial conclusions obtained and attempt to present a generalized physicochemical model of the formation of banded cherty iron sediments. It should be kept in mind that the transport and deposition of substantial amounts of iron and silica from low- [Pg.179]

When high iron and silica concentrations were reached, chemical deposition of iron and silica became possible, on the arrival of supersaturated volcanic solutions. The oldest (Archean) BIF, closely associated with volcanic rocks (formations of Algoma type) were deposited from supersaturated solutions formed when acid thermal waters mixed with ocean waters saturated with carbonic acid but devoid of free oxygen. [Pg.180]

In the original thermal waters (as in the hot waters of recent fumaroles) both iron and silica occurred in true dissolved (ionic or molecular) form, producing rather high concentrations (of the order of n lO - -10° g/1) the environment was acid (pH 2-3) and reducing ( /i 0.0--1-0.2 V / Oj 2 Fe ion predominated). [Pg.180]

In the case of terrestrial effusions, when acid volcanic waters mixed with surface river waters and especially with sea waters in the near-shore zones, the pH value gradually increased due to dilution and neutralization and approached the equilibrium value, controlled by carbonate-silicate buffer reactions (pH within 6-8 depending on the type of buffer). As a result, colloidal silica already had begun to coagulate in the near-shore zones, and clots were deposited along with clastic components or were carried on by currents. However, maximum chemogenic deposition began some distance [Pg.180]

After passing near-shore geochemical barriers the waters contained hardly any colloidal silica and in the deep parts of the basins the intensity of accumulation of chemogenic cherty sediments would have fallen off sharply. [Pg.181]


At pH = 6, which presumably occurred in the ancient Precambrian sedimentary basins, migration of iron could have been accomplished only in the form of Fe " at an Eh not lower than —0.10 and not higher than 0.0 to + 0.10 V. A lowering of Eh would have led to massive deposition-of pyrite, and an increase in deposition of Fe hydroxides. The actual conditions of chemogenic formation of cherty iron sediments consisting of siderite, iron... [Pg.114]

Physicochemical model of the formation of cherty iron sediments in the Precambrian... [Pg.174]


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