engleski

Occurrence of radiating Fe-oxides and Fe- oxyhydroxides crystal groups in the rocks of cores from the borehole Bunjani-59, Croatia

The borehole Bunjani-59 is located in the Sava Depression in the Republic of Croatia, drilled about 10 km SE from the city of Ivanić Grad. The deepest cored rock material from the borehole, at a depth of 715 m, is made of igneous and metamorphic rocks like granites, gneisses and schists which belong to the crystalline basement of the Pannonian Basin. In addition to the major minerals such as quartz and feldspar, the bedrock is also made of iron- rich minerals like biotite, amphibole belonging to the hornblende group, chlorite, cordierite, pyrite and magnetite. The igneous and metamorphic rocks continue vertically in shallow intervals of the borehole in the form of drifted and subsequently processed gravel fragments of rocks, separate sand grains, and minerals in younger sediments like conglomerates, gravelly sandstones and sandstones from 681 to 615 m in depth. By macroscopic examination of cores, reddish, brownish-red and yellow blotchy parts of rocks that occur along the continuously cored intervals of the borehole were observed. With a more detailed study of the rocks, by using the polarizing microscopes Olympus BX51 and Zeiss Axiolab, as well as a scanning electron microscope JEOL JSM-6510 LV, a presence of hematite and goethite was confirmed in these blotchy areas. These are formations of Fe- oxides and Fe-oxyhydroxides in the form of massive earthy coatings and globular clusters and in particular, in the form of the subhedral to euhedral crystals radially arranged like blades and needles. Such different aggregates fill up fractures within the minerals, the interstices among the grains within the gravelly and sandy rock fragments of the basement, and they permeate together with clay minerals within the matrix. The microcrystalline radiating occurrences of hematite and goethite are generally observed directly related to the minerals such as biotite, chlorite, chloritized biotite and hornblende, and are located on the surface of minerals, especially on planes {; ; 001}; ; of mica and chlorite (Fig. 1a). The individual crystals as blades and needles have a length of 2 to 15 μm, while they are 0.30 to 1.25 μm in width (Fig. 1b). They often form snowflake forms. The occurrence of Fe-oxides and Fe-oxyhydroxides is a result of the subsequent chemical changes in the environment caused by weathering of primary rocks and alteration of iron-rich minerals (1) of the crystalline basement of the Pannonian Basin. Their exposure to the air in the zone of weathering crust, i.e. the action of meteoric water, leads to oxidation and leaching of metals (Fe ions) from the rock. Further water fluctuations in eroded and drifted rocks, formed soils and sediments deposited away from their source material, that are found in shallower intervals of the borehole, favor the acidity changes of the environment and additional transitions of hematite to goethite and vice versa.

Fe-oxides ; borehole ; alteration ; radiating aggregates

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