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Miocene fluvial sands from an outcrop at Voervadsbro in central Jutland were studied to assess the Fe-Ti oxides and their alteration products forming during a warm, humid climate and under the influence of scattered organic material. The opaque minerals and their alteration products were investigated by optical light microscope, reflection microscope, microprobe, scanning electron microscope and X-ray diffraction. The detrital Fe-Ti oxides consist of, in decreasing order, ilmenite, titanomagnetite, magnetite, rutile, hematite and silicified leucoxene. Degradation of organic matter created mildly reducing and neutral-acid conditions under which the Fe-Ti oxides (ilmenite, titanomagnetite, magnetite and hematite) were unstable. Ilmenite has a three-step alteration process: ilmenite → pseudorutile → fine leucoxene → coarse leucoxene (single crystals of rutile or anatase). Alteration of titanomagnetite commonly resulted in coarse leucoxene in a trellis texture. Alteration of ilmenite lamellae in titanomagnetite is typically complete, probably because of their small size compared to ilmenite grains. Colloidal leucoxene is an alteration product of ilmenite and titanomagnetite. The formation of colloidal leucoxene seems to be related to organic matter or elements associated with it. Magnetite has been partly dissolved, preferentially around the rim and along fissures. Hematite is rarely a detrital grain due to intensive dissolution, and the exsolution lamellae of hematite are invariably more altered than the ilmenite host. Oxidising conditions prevailed locally e.g. in coarse-grained foresets without organic material and at the atmospheric interface of bogs. In this environment dissolved iron (originating from the alteration of Fe-Ti oxides) precipitated mainly as goethite.

Alteration of detrital Fe-Ti oxides in Miocene fluvial deposits, central Jutland, Denmark