Genetic types, geochemistry and the archaeometallurgical importance of bog iron ore deposits from Inner Somogy

Abstract

In the region of Inner Somogy, Late Pleistocene – Early Holocene fluvial freshwater marshes were created by active neotectonic deformations. Bog iron ore deposits were formed in these freshwater marshes due to the precipitation of Fe(III) minerals; this occurred during the microbial and chemical oxidation of fluids containing Fe(II), while they were streaming beneath the surface. The bog iron ore deposits can be grouped into three types based on their genetics: (i) microbial bog iron ore lenses made by Fe(II)-oxidizing microorganisms in the back shallows of the southern part of Nagyberek (this is now an uplifted area); (ii) limonitic indications formed by chemical oxidation on the margins of back marshes filled up with sandy sediments and on the  uplifted areas; and (iii) in the area which uplifted from the Early Holocene microbial bog iron ore lenses were redeposited by creeks and later developed as redeposited bog iron ore layers. Bog iron ores from the microbial bog iron ore lenses and from the redeposited bog iron ore layers were smelted intensively during the period of Avar settlement of Hungary and also later, during the age of the Conquest and after. This was due to the abundance and high quality of the ores. However, the bog iron ores of Somogy had a high phosphorus content, and so the phosphoric iron extracted by the smelting of these ores had poor mechanical properties. However, due to the presence of calcium carbonate in the area, the phosphorus content of the iron blooms could be decreased.