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Geochemical and mineralogical evidences for possible bog iron formation in Podravina region, NE Croatia

Bog iron ores are sedimentary types of iron deposits (RAMANAIDOU & WELLS, 2014), typically occurring in low-lying areas such as swamps, bogs and meadows or river valleys and microdepressions with a groundwater table situated close to the surface (KACZOREK & ZAGÓRSKI, 2007). They represent terrestrial accumulations of Fe ore in the form of various Fe oxides and oxyhyroxides, such as goethite (BANNING, 2008) and are often developed in hydromorphic, loamy, sandy and clayey alluvium and soil (DE GEYTER et al., 1985, LANDUYDT, 1990). During several decades of archaeological investigations throughout the Podravina region, numerous sights of iron smelting workshops, furnaces and iron slag materials were found (VALENT et al., 2017). Archaeological evidences indicate presence of near-by bog iron excavation sites (SEKELJ IVANČAN, 2017). The aim of this study is to present evidences for possible bog iron ore formation in the wider area of the Drava River valley, Podravina region, NE Croatia. Based on high amounts of archaeological evidences and findings pointing toward the usage of bog iron ore in Podravina region, detailed geological exploration was conducted in the area. A total of 44 soil profiles were drilled, most of them in the vicinity of previously established archaeological locations. Six soil profiles, consisting of five Gleysols and one Fluvisol were selected for further mineralogical, geochemical and textural analyses due to visible iron accumulations throughout their depth. The X-ray diffraction (XRD) analyses confirmed goethite, quartz, clay minerals, plagioclase, feldspars, and sporadically dolomite. Chemical analyses indicate variable, but high concentrations of iron oxide in all profiles, generally higher than median values for Podravina region (HALAMIĆ & MIKO, 2009). Kalinovac-Hrastova Greda profile shows highest concentration of Fe2O3 (31.52%) at 60-80 cm depth, while remaining profiles show Fe2O3 concentrations between 3.97−10.90%. Concentrations of As (1.8-563.6 ppm) and P (484- 4513 ppm) show high enrichment values of these elements in the selected soils. Textural analyses indicate high amount of silt and sporadically sand in all profiles, with relatively small amounts of clay. Enrichment factor of Fe2O3 shows significant enrichment in Kalinovac-Hrastova Greda profile, indicating possible ore formation. Based on microelement distribution of Ce, Cs, Hf, La, P and Zr, all soils show a common parent material, but significant differences in concentrations of Fe and other major oxides indicate different formation processes. This is attributed to differences in soil texture of selected soil profiles, oscillations of groundwater table (BRKIĆ & BRIŠKI, 2018), and changes of oxidative and reductive conditions. The results of this study indicate that Podravina region is a suitable area for the formation of bog iron ore, although recent conditions inhibit formations of bog iron ore due to changes in agriculture and melioration which are altering groundwater levels.

bog iron ore ; pedology ; geoarchaeology ; Podravina region ; geochemistry

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