A major Jurassic (Toarcian Anoxic Event related) Mn ore deposit of Europe is in ┌rkút, consisting of both carbonate and oxide ore sequences. We studied a complete carbonate section, consisting of a black shale sequence with two Mn-bearing horizons. This fine-grained (μm- and nm-sized) sedimentary sequence was traced by systematic m-scale (1-100 m) studies using SEM+EDX and XRD (see poster Cora et al.).
Mn occurs both in carbonate and oxide phases in the carbonate ore. Ore-forming carbonate minerals are fine-grained, dispersed or sometimes nodular, displaying a complex texture. The main carbonate is Ca-rhodochrosite (d104=2.86 Å). The (subordinate) Mn-oxide phase of the carbonate sequence is manganite (Mn3+!). No Mn4+-oxide phase was found.
Fe occurs in reduced and oxidized form, like Mn. Fe-phases are nm-sized goethite and μm-sized euhedral pyrite in the ore, framboidal pyrite in the black shales and Fe-rich sheet silicates (celadonite, K-Fe-smectite). Lamellar ore structure is the result of parallel or exclusive existence of phases recording redox conditions (and their change).
Based on textural, mineralogical features we set up a raw genetic model: Suboxic conditions prevailed near the sediment-water interface in the bowl-shaped basin of limited circulation. Mn was present in reduced, dissolved form (Mn2+aq). Due to occasional inflow of O2-rich currents, Mn2+aq was oxidized to Mn3+, and chemically precipitated as fine-grained manganite (Mn3+O(OH)). Manganite forms in a narrow Eh field (ΔEh=0.2 eV), recurringly set by the O2-rich currents. Manganite can be regarded as the primary ore accumulation form in the basin. After restoration of suboxic conditions, microorganisms could continue oxidation of organic material by the reduction of Mn and Fe. Due to its lower reduction potential, first manganite, then goethite was consumed. As concentration of Mn2+aq and HCO3?aq (org) increased, Ca-rhodochrosite (d104=2.86 Å) precipitated, and allotigenic calcite was metasomatised by Mn2+, too. This two-way Ca-rhodochrosite formation model explains the typical δ13C values (∼ 12-15) and the poor correlation between Mn and δ13C values published by Polgári (1991).
Wherever manganite consumption was complete, microorganisms started to reduce goethite into Fe2+aq, precipitating euhedral pyrite. This simple model explains the paragenesis of the different ore lamellae. Recent examples for Mn-accumulation in O2-depleted pelitic sediments are known from Baltic Sea (Huckriede and Meischner, 1996).
In our bowl-shaped Toarcian basin, the ore is in carbonate (+manganite) form at the bottom whereas it is oxidized (Mn4+-containing oxides) along the rim. The arrangement of the two ore types raise the question, whether the oxidized (Mn4+) Mn ore may have formed only by later (Cretaceous) processes (as generally accepted now) or also by the syngenetic oxidation of the primary manganite (Mn3+!) at shallower, marginal levels of the basin.