Abstract
The Vijenac limestone quarry, near Tuzla in Bosnia and Herzegovina, is composed of carbonate rocks locally embedding tectonically disturbed siltite and sandstone with Fe-Mn concretions. The quarry itself represents a part of Dinaric overstep sequences (the Pogari Formation) unconformably overlying ophiolite melange and ophiolite trust-scheets. Petrographic, chemical and mineralogical analyses had shown that the concretions may be divided into two types: (i) Mn-rich concretions with ≈ 17 wt.% of Mn and compact texture and (ii) Mn–poor with ≈ 8 wt.% of Mn and porous texture. The amount of Ca, Fe and Mg in both concretion types are similar. Nickel and Cr positively correlate with Fe and Mn, respectively. According to petrographic and mineralogical analyses, concretions are composed of calcite, dolomite, hematite, todorokite and takanelite. Petrographic study confi rmed the development of concretions within three stages including two generations of calcite. Studied concretions are formed within consolidated sandstones inheriting their sedimentary textures – therefore a late diagenetic process is assumed.
Highlights
A concretion is a compact mass of mineral matter, usually spherical or disk–shaped, embedded in a host rock of a different mineralogical composition
This study presented the ¿rst evidence of the Fe-Mn concretions within sedimentary rocks of the Dinaric Ophiolite Zone
Results of the atomic absorption spectroscopy (AAS) analyses show that the bulk composition of the concretions consists of Mn (7–17 wt.%), Ca (39-42 wt.%) and Fe (4–6 wt.%) while the rest corresponds to silica residue
Summary
A concretion is a compact mass of mineral matter, usually spherical or disk–shaped, embedded in a host rock of a different mineralogical composition. Concretions show distinct internal structures with concentric zonation (i.e. Gasparatos, 2005). Concretions vary in mineralogical composition, size, shape, hardness, and colour. They are commonly composed of calcite, dolomite, siderite, pyrite, barite, ferromanganese oxides or gypsum. Concretions occur when a considerable amount of cementing material precipitates locally around a nucleus, which is usually some other mineral or fossil remains (Mozley, 2003). The Fe-Mn concretions and nodules occur in a variety of geological environments like the ocean Àoor, river sediments, lake sediments, soils and sedimentary rocks. One of their biggest importance is keeping records of diagenetic and post-depositional changes of the sedimentary units, since they preserve evidence of groundwater Àow and the interactions on water-rock boundary
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