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Authigenic Fe-minerals as indicators of the Late Permian and Early Triassic depositional conditions (Velebit Mt., Croatia)

The uppermost Permian deposits in the Velebit Mt. area (Croatia) are characterized by the stratigraphic break caused by emergence and subsequent transgression which continued through the Permian–Triassic Boundary (PTB) into the Early Triassic. Two informal lithologic units can be distinguished – Transitional Dolomite (TD), relatively rich in fossils, and Sandy Dolomite (SD), characterized by recrystallized grains, scarce fossils, and enhanced input of terrigenous siliciclastic material [1, 2]. Selected TD and SD samples were disintegrated, or cut and polished and analyzed by optical microscopy, SEM, EDS, and XRD. Preliminary studies of the authigenic Fe-minerals found in the TD and SD units have shown characteristic mineral associations connected to the changes in the depositional environment. Fossiliferous TD deposits contain randomly dispersed red hematite grains. Highly recrystallized SD deposits above the lithologic change only occasionally contain hematite. Further up in the SD unit, due to the increased siliciclastic input and scarce fossils after the PTB, iron oxides form hematite, identical as in the TD unit, and magnetite. Magnetite, in framboidal form, in places accompanied with larger cubic crystals, is confined to recrystallized calcite and do not appear in the dolomite. The interstices between framboid microcrystals are filled with calcite ; some magnetite hexahedra intergrow with calcite rhombohedra or contain calcite crystals. Fine granulation, random dispersion, and association with fossil remains imply early diagenetic origin of hematite, when pore waters were more oxygenated and iron migrated by diffusion and not by pore water flow. Magnetite was formed during late diagenesis when more oxygen depleted waters penetrated into the sediment and mobilized iron from hematite. Locally concentrated magnetite suggests ion migration with pore water flow. Mineral stability fields of hematite and magnetite in an iron–carbonate–water system display magnetite as dominant mineral at high pH and low Eh conditions. In the absence of oxygen and at the high pH, precipitation was driven through OH¯ addition. As precipitation reduces OH¯ concentration and lowers pH, a small amount of carbonates was dissolved and restored equilibrium conditions within a solution.

Fe-minerals; Permian-Triassic boundary; Velebit Mt.; Croatia

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