Limonite iron ores occur in the area of Julian Alps and their foreland. They are found as insoluble residue in karst eluvium of carbonate rocks, accumulated in karst depressions, cave sediments, sediments of unroofed caves, in karst fissures and shafts. Secondary places of occurrence are glacial sediments and fluvial sediments.
We focused on researching mineral composition of magnetic samples of limonite iron ores on every researched location, and on interpretation of process that has enabled their creation.
Creation of nonmagnetic – goethite crustifrom and pisolitic limonite ores is typical for bog, lake, or cave environments on reduction-oxidation geochemical barrier.
We found out that magnetic limonite iron ores are formed by magnetite and/or hematite and in much smaller quantities ilmenite, goethite and amorphous limonite.
Magnetic versions of limonite ore were created by isomorphic replacements - with pseudomorphosis of preceding iron sulphide (pyrite and/or marcasite) with iron oxides (in first phase with magnetite, which has been replaced with hematite in secondary phase). In tertiary phase hematite can be replaced with iron hydroxides (goethite and/or lepidocrocite) or amorphic limonite. When the process of replacement of hematite and magnetite with goethite and lepidocrocite has not yet been concluded, magnetism was preserved.
Origin of iron oxides and iron oxy hydroxides in processes of karst weathering and gradual oxidation of primary iron sulphides was confirmed by extraordinary rare conservation of primary sulphide (pyrite and/or marcasite) in the cores of crystals, presence of sulphur in trace elements analysis, gained by x-ray fluorescence geochemical analysis, and euhedral cross sections of crystal faces of pyrite in polished magnetic ore samples. From the shapes on the surface of some ore samples we can recognize some crystal faces and edges of preceding cubic crystals of pyrite with hexahedral and octahedral crystal faces, which have been replaced by limonite minerals.
Presence of magnetic iron oxides is therefore a consequence of karst weathering and accumulation of insoluble residue of imperfectly oxidized and hydroxylated primary preceding iron disulphides in karst depressions, and with incomplete process of pseudomorphosis of magnetic minerals with nonmagnetic goethite and amorphic limonite.