Hydromorphic bog iron ore formation in foothill Cambisols: mineralogical and geochemical evidence from Knezovljani, central Croatia

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DOI:

https://doi.org/10.17794/rgn.2026.5.15

Keywords:

hydromorphic soils, geochemistry, correlation matrix, pXRF, geoarchaeology

Abstract

This study examines the formation and geochemical signature of Fe-rich horizons and bog iron-type accumulations in hydromorphic soils at Knezovljani, in the foothills of the Trgovska and Petrova Gora massifs, central Croatia. A pedological trench with three soil profiles was described and sampled, with horizons and a hardened Fe-rich layer analysed using grain size analysis, X-ray diffraction (XRD), portable X-ray fluorescence (pXRF), with XRF and inductive couple plasma mass spectrometry (ICP-MS) for selected hardened Fe samples. The soils contain silt (50–75%) with 10–32% clay, are moderately acidic (pH 4.8–5.9), and They consist of quartz, feldspars, and illite–smectite clays with trace kaolinite, indicating intermediate weathering. One horizon shows pronounced Fe enrichment (33.8 wt% Fe2O3), with high contents of goethite and a laminated Fe-cemented facies, while other horizons contain only 5–9 wt% Fe2O3. Trace elements are elevated in Fe-rich layers, with positive correlation towards Fe2O3. Comparison with regional bog iron ores and Mn-rich occurrences represents Knezovljani as a small, hydrologically focused Fe accumulation pocket formed where Fe-bearing groundwater meets more oxidising conditions at a footslope of a hill, rather than in a microdepression. The Knezovljani Fe horizon is thin and discontinuous, indicating short-lived or spatially restricted Fe mobilisation–precipitation cycles. The findings highlight the importance of microtopography and subsurface flow in formation of secondary Fe ores, providing a framework for further studies on ore types, spatial distribution, and geochemical characterisation towards studies of bog iron ore in central Croatia.

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Published

2026-08-04

Issue

Vol. 41 No. 5 (2026): Forthcoming Issue: No. 81

Section

Geology

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Copyright (c) 2026 Tomislav Brenko, Sibila Borojević Šoštarić, Tena Karavidović, Tajana Sekelj Ivančan

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How to Cite

Brenko, T., Borojević Šoštarić, S., Karavidović, T., & Sekelj Ivančan, T. (2026). Hydromorphic bog iron ore formation in foothill Cambisols: mineralogical and geochemical evidence from Knezovljani, central Croatia. Rudarsko-geološko-Naftni Zbornik, 41(5), Article in press. https://doi.org/10.17794/rgn.2026.5.15