International Geologiical Congress - Oslo 2008

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

 

A micro-scale study on the Toarcian black shale-related manganese ore deposit, Urkut, Hungary

 

Ildiko Cora, Eotvos Lorand University (Hungary)
Richard Orban, Eotvos Lorand University (Hungary)
Peter Pekker, Eotvos Lorand University (Hungary)
Gyorgyi Tuba, Eotvos Lorand University (Hungary)
Tamas Vigh, Mangan Kft. (Hungary)
Tamas G. Weiszburg, Eotvos Lorand University (Hungary)
 

 

One of the largest and still mined Jurassic Mn ore deposits of Europe is Urkut. The deposit consists of both carbonate and oxide ore sequences. We studied a complete carbonate section, consisting of lamellar pyritic black shale with two lamellar Mn-bearing horizons. That was the first time when this extremely fine-grained sedimentary sequence was traced by systematic, real micrometer-scale (1-100 m) studies using SEM+EDX and XPD. Some of the mineral phases could be characterized and we also contributed to some of the open genetic questions, but part of the problems remained to be solved via a nm-scale study. The ore forming minerals are fine-grained dispersed or, sometimes, nodular carbonates, displaying complex texture. From among the four genetically different Mn-Ca-carbonate phases we characterized, the dominant is Ca-rhodochrosite (d104=2.86 Å broad, asymmetric reflection), but the local (>1 m) analytical data show a micro-scale continuous substitution between calcite and rhodochrosite with a gap around Mn:Ca=1:1 ratio.
Mn occurs both in carbonate and oxide phases in the carbonate ore. The oxide phase is identified as Mn3+-containing MnO(OH), manganite. Manganite is fine-grained (∼m) and is usually encrusted by Ca-rhodochrosite. No Mn4+-oxide phase was found. Fe is also a dominant element in the ore, both in reduced and oxidized form (pyrite, goethite, Fe-rich phyllosilicates). Goethite has sub-micrometer (nm) grain size and occurs in the 100 m?1 mm laminae in an intimate mixture with Fe-rich phyllosilicates. Framboidal pyrite is present in black shales upper- and underlying the two ore horizons, while euhedral ∼10 m pyrite grains are common in the lower part of the ore. The alternating presence of euhedral pyrite and Fe-oxyhydroxides or Mn-oxyhydroxides is typical.
Two types of dioctahedral Fe-dominant sheet silicates could be found. Celadonite of typical grain size of 0.1-1 m was characterized using sequentially separated bulk samples (Weiszburg et al., 2004). The other dominant sheet silicate is K-Fe-smectite.
Based on parallel or exclusive existence of the described phases, the following raw genetic model was set up: Suboxic conditions prevailed near to sediment-water interface in the bowl-shaped basin. Mn was present in reduced form (Mn2+aq). From time to time, due to the occasional inflow of O2-rich currents, Mn2+aq was oxidised to Mn3+ and precipitated as fine-grained manganite. After restoration of suboxic conditions, microorganisms obtained oxygen necessary to the decomposition of organic material from the hydroxide phases. Due to its lower reduction potential, first manganite, then goethite was consumed. As the concentration of Mn2+aq and HCO3-aq (org) increased, Ca-rhodochrosite (d104=2.86 Å) precipitated, and allotigenic calcite was metasomatised by Mn2+, too. Wherever the consumption of manganite was complete, microorganisms started to reduce goethite into Fe2+aq, precipitating of euhedral pyrite.

 

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