On mineralogical and geochemical criteria in the genesis of red beds

The article discusses a concept on the origin of hematite in red-bed deposits as a result of the alteration of ferrihydrite (a hydrous iron oxide with a composition 2.5 Fe₂O₃ · 4.5 H₂O). The formation of ferrihydrite is associated with a predominantly bacterial oxidation of ferrous iron contained in discharged groundwaters. Clastogenic material and ferrihydrite flakes have been supplied to the site of sedimentation from different sources. The climate in the areas of their deposition has been hot and arid. Owing to an absence of organic matter during the diagenesis, the iron remained in the form of an Fe³⁺ oxide.Sedimentary rocks united under the name of “red beds” are of different ages. A great number of papers deals with the conditions of formation of these rocks and yet the genesis of the hematite contained in them has not been satisfactorily explained. The significance of red beds in paleoclimatology is also not clear. This paper is an attempt to discuss these problems on the basis of mineralogical and geochemical data.     

Mineralogical criteria in the origin of marine iron-manganese nodules

In iron-manganese nodules from the floor of Pacific ocean, Baltic, White Sea and Kara Sea, iron bydroxide '-FeOOH was analysed in the laboratory. In buried ooze, reduction processes generate Fe(HCO₃)₂ which migrates into the upper part of the bottom ooze and into near bottom sea water where Fe(OH)₂ is formed. The oxidation process of Fe²⁺ to Fe³⁺, without participation of iron bacteria, leads to the topotactic transformation of Fe(OH)₂ to '-FeOOH. Marine water does not contain Fe²⁺ and cannot be a direct source of iron deposited in the nodules. Discovery of '-FeOOH in marine nodules permits the consideration that both iron and manganese were derived from the buried bottom mud, which during diagenetic processes led to the transfer of these metals in solutions and their upward migration.     

Contributions to the mineralogy of authigenic manganese phases from marine manganese deposits

The formation of authigenic manganese minerals and ores in the pelagic regions of the ocean is related to oxidation of Mn²⁺ extracted from basalts and other rocks with heated seawater. For littoral parts of the ocean and lakes mobilization of Mn²⁺ and Fe²⁺ is admitted finding its way to the bottom sediments (along with the organic substances) from land in the form of Mn⁴⁺. The main manganese mineral of oceanic and continental basins is vernadite. Its deposition is considered a result of the activity of microorganisms.     

The isotopic composition of plant sulfur

The isotopic composition of sulfur has been studied in plants representative of various regions of the U.S.S.R., two oceanic islands, and atmospheric precipitations on land and in marine areas. In soils, the isotopic composition of sulfur in the atmospheric water varies as a result of sulfate reduction (increase of δ³⁴S of the soil sulfate) and sulfate regeneration from hydrogen sulfide. The sulfur in plants from the oceanic islands has characteristically higher values of δ³⁴S than the sulfur in the plants and in the atmospheric water of the continents. Compared to sea water, the sulfur from the island plants that were studied contains a considerably lesser proportion of the ³⁴S isotope. This can be explained by the significant role in such plants of the sulfur of the atmospheric air masses coming from the continents.     

On the isotopic sulfur composition of some Precambrian strata

A study of minerals from Proterozoic and Archean series of various regions has been carried out. Sulfate and sulfide sulfur, having doubtlessly passed through the cycle of isotopic fractionation, has been determined in Archean rocks of the Baikal region. In the lazurite deposit of Malaya Bystraya, the country rocks are Precambrian stinking (fetid) dolomites which contain ancient hydrogen sulfide with ³⁴S from +19.0 to +33.4, and sulfate with ³⁴S=+44.9. Carbonate rocks have served as sources of sulfide and sulfate sulfur for a number of minerals (lazurite, scapolites, apatite, pyrrhotite, pyrite, a. o.). A connection between the enrichment of sulfides of metamorphic strata in the ³²S isotope or the ³⁴S isotope and the presence in rocks of increased amounts of graphite — the product of metamorphism of the organic matter of ancient sediments — is established. A conclusion has been drawn regarding the isotopic fractionation about 3·10⁹ y. ago.

---

Zusammenfassung Schwefelisotopenfraktionierung durch den Sulfid-Sulfat-Zyklus findet nur dort statt, wo freier Sauerstoff vorhanden ist. Diese Tatsache wird benutzt, um das Eintreten von freiem Sauerstoff und den Beginn organischen Lebens im Präkambrium festzusetzen. Fraktionierter Sulfid- und Sulfatschwefel in archäischen Gesteinen vom Baikalsee wurde gemessen und ein Zusammenhang zwischen dem Schwefelisotopenverhältnis und einem zunehmenden Gehalt an Graphit festgestellt, der dem Gehalt an organischer Substanz entstammt. Diese Isotopenfraktionierung datiert ungefä hr 3:10⁹ M.J. her.

     

On the genesis problem of thermal sedimentary iron ore deposits

The author unites deposits, the material of which had been supplied as thermal solutions and deposited predominantly on the sea floor, under the name of thermal-sedimentary. It is assumed that the iron of such deposits had been extracted as Fe²⁺ from rocks by waters heated by the subterranian heat. The ferruginous precipitates producing thermal-sedimentary iron ores are formed, in the author's opinion, at discharge points of thermal solutions. Iron contained in them is precipitated as ferrihydrite (2.5Fe₂O₃·4.5H₂O), which spontaneously transforms into hematite.

---

Zusammenfassung Der Autor faßt unter dem Begriff thermalsedimentär Lagerstätten zusammen, deren Stoffbestand in Thermallösungen zugeführt und hauptsächlich auf dem Meeresboden abgelagert wurde. Es wird angenommen, daß das Eisen derartiger Lagerstätten durch zirkulierende Wässer, die ihre Erwärmung dem unterirdischen Kreislauf verdanken, als Fe²⁺ aus den durchflossenen Gesteinen herausgelöst worden ist. Die eisenhaltigen Ablagerungen, die die thermal-sedimentären Eisenerze bilden, werden nach Ansicht des Autors an den Austrittsstellen der Thermallösungen gebildet. Das Eisen, das in ihnen enthalten ist, fällt als Ferrihydrit aus, welches unmittelbar in Hämatit übergeführt wird.