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Silicification, moganite and diagenesis of SiO2 minerals in the oldest spelean sinters of the Koněpruské jeskyně Cave, the Bohemian Karst (Czech Republic)


Václav Suchý, Vladimír Machovič, Ladislav Lapčák, Lenka Borecká, Margit Žaloudková, Ivo Světlík

Geoscience Research Reports 51, 2018, pages 3–11
Map sheets: Beroun (12-41)

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Published online: 2 May 2018

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Moganite, a metastable polymorph of SiO2 has been identified in carbonate-siliceous speleothems of the Koněpruské jeskyně Cave, the Bohemian Karst. Moganite was detected by means of Micro Raman spectroscopy in specific partially silicified coralloid speleothems (so-called Koněprusy rosettes) the carbonate matrix of which consists of exotic calcite crystals (e.g. gothic-arch and trigonal prismatic crystals, calcite botryoids reminiscent of a former aragonite precursor, feather-like and chevron-like calcite aggregates). Moganite along with fully crystalline α-quartz and amorphous opal were recognized in abundance in silica-rich insoluble rezidues which resulted from the complete dissolution of carbonate matrix of the Koněprusy rosettes. Particularly high moganite concentrations (> 70 % in some grain samples) were also detected in intensely silicified layered carbonate sinters immediately underlying the Koněprusy rosettes near the cave floor. Mineralogical and luminescence analyses reveal that moganite and α-quartz tend to occur inside of individual speleothems, while its outer and superficial growth zones are commonly rich in amorphous opaline silica, thus confirming the worldwide-known pathway of silica diagenesis.
The silicification of the speleothems was a multi-stage process that occurred repeatedly during the speleothem growth. Much of the silicification clearly post-dated the crystallization of carbonate host as evidenced by silica filling fractures and secondary pores within zoned calcite crystals. A variety of microscopic biomorphic structures recognized in silicified speleothems (e.g. ovoidal and circular siliceous particles 5-20 μm in diameter and hollow siliceous tubes interpreted as remains of coccoid and filamentous microbes, respectively) suggest that microbial processes may have been also instrumental to silica deposition.
The source of silica remains unclear. SiO2-rich solutions may have seeped into the cave from the surface, during periods of warm humid climate and intense weathering in the past. An alternative model for silica precipitation may involve an inflow of hypogene, warm, saline and SiO2-enriched basinal fluids that may have ascended from below and mixed with shallow meteoric waters. Resulting salinities at the level of 35-60 % of seawater may have stimulated silica precipitation coeval with CaCO3 dissolution as suggested by Knauth (1979).
The occurrence of moganite and opaline silica in the speleothems of the Koněpruské jeskyně Cave provides independent mineralogical evidence that the age of the silicification would not be higher than ˜ 200 000 years.


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