International Geologiical Congress - Oslo 2008

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MRD-13 Ore deposits associated with black shales: from their origin to their environmental impacts

 

Fossile Fe-Mn-oxide chimneys?

 

Márta Polgári, Institute for Geochemical Research, Hungarian Academy of Sciences (Hungary)
Attila Lajos Tóth, Research Institute For Technical Physics And Materials Science, Hungarian Academy of Sciences (Hungary)
Mária Tóth, Institute for Geochemical Research, Hungarian Academy of Sciences (Hungary)
Tibor Németh, Institute for Geochemical Research, Hungarian Academy of Sciences (Hungary)
Tamás Vigh, Mangán Ltd. (Hungary)
Lóránt Bíró, Eötvös Loránd University (Hungary)
 

 

Jurassic cherty Mn-Fe-oxide deposition as a distal facies of the black shale hosted huge early diagenetic Mn-carbonate ore bed (úrkút, Central Europe) was investigated by XRD, OM, ICP, EPMA-SEM methods. Geochemical characteristics, formation of the ore body explain structurally determined marine bacterially mediated hydrothermal formation of the Mn carbonate deposit. It has been proposed that hydrothermal fluids venting into the depositional basin was involved in the mineralization, but this idea is controversial. A hydrothermal origin may be supported by several lines of evidence such as the fractionation of Mn and Fe, the in situ precipitation of celadonite, and the accumulation of huge amounts of Mn, Fe, Si. The aim of this study is to look for evidence for a hydrothermal plumbing system by study of cherty Fe-Mn oxide formation, which has a basic significance in elaboration of element transport processes. This mineralization occurres in varicoloured metalliferous clays along the block faulting zones of NW-SE direction on a strongly dissolved Lower Liass locally manganized limestone. Recently ore samples from the supposed local vent area úrkút-Csárdahegy open pit were analysed. The samples are black-brown blocky or noduliferous with a diameter of 10-15 cm, cryptocrystalline, often with vugs infilled by coccoid like forms. Fe-rich outer covering on the blocks are characteristic. Polished sections showed stromatolitic textures around channels built up by Fe- and Mn-oxides by macroscopic and optical microscopic observations. Mineralogy of the inner black metalliferous parts of samples is formed by hollandite, pyroluzite, todorokite, cryptomelane, goethite, phyllosilicates, and quartz (tr). The outer brown parts consist of goethite, manganite, phyllosilicates and quartz. The bulk chemical composition (wt. %) of brown parts show a wide range of Fe and Mn contents, with mean values of 45.8% Fe, 3.8% Mn, 0.2% K. The black inner parts are characterized by mean values of 18.1% Fe, 29.3% Mn, 1.5%K content. Both parts show a mean value of 4.4% Si, 1.9% Al, 0.4% Ca, 0.5% Mg and 0.3% P. Ba (max: 0.2%), Sr (0.9%) and Co (0.06%) are enriched in Fe-Mn oxides. Electron microprobe studies showed a snow ball like Fe-oxide texture with high porosity filled with Mn oxide. The diameter of the balls is a few tens of micrometers. P content homogeniously enriched (mean 0.35%) on the Fe oxide spheres. No detrital phases were determined. The stromatolitic mounds were grown at the sediment/water interface, around chimney-like micro channels with similar morphology to recent mineralized hydrothermal ones. Our data suggests that the cherty Fe and Mn oxides were deposited from deep-sourced fluids circulating through basement rocks. Circulation along zones of structural weakness was likely driven by high geothermal gradients. In this sense, the Jurassic submarine Hungarian Mn mineralizations might be indicative of former sites of proto-rift environments. (OTKA-NKTH No. K 68992).

 

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