Acta Musei Moraviae

A pronounced diagenetic alteration of heavy minerals in sandstones from the locality Lukoveček (Rača Unit, Flysch Belt of the Outer Western Carpathians, Czech Republic)

An electron microprobe study was conducted on polished sections prepared from Upper Cretaceous-to-Paleogene sandstone sampled at the locality Lukoveček (Soláň Formation, Rača Unit, Flysch Belt of the Outer Western Carpathians, Czech Republic). The studied rock can be classified as a quartzose to arkosic sandstone with basal calcite cement and very small amount of grains of heavy minerals. Most detrital phases (quartz, feldspars, garnet, apatite, monazite, zircon, Ti-phase) show less or more intense signs of diagenetic dissolution, with loss reaching ~0 to ~80 vol. % of original grain volume. Abundant cracks in detrital phases are fulfilled by calcite. Muscovite, biotite and chrome spinel are the only phases without signs of dissolution. The microprobe analyses show that the detrital garnet belongs to almandine (Alm_53-83Grs_2-25Prp_8-42Sps_1-8), apatite to fluorapatite, and chrome spinel to spinel (0.70 apfu Mg, 0.48 apfu Cr); the compositions are comparable with published data from Czech, Polish and Slovak parts of the Flysch Belt of the Western Carpathians. There is observed a very variable level of dissolution of different grains of individual heavy minerals in mm scale. This implies that corrosion was widespread but focused (local) process, which was likely associated with late-diagenetic fluid episode(s) postdating the cementation of primary porosity of the sandstone. In this scenario, the alteration was restricted only to mineral grains which were accessed to corrosive fluids due to actually formed open cracks, whereas uncracked clasts completely enclosed in early calcite cement remained essentially untouched. The corroding fluids were likely alkaline aqueous solutions, perhaps enriched in sulphate and/or carbonate anions, enabling dissolution of hardly soluble Zr-, Ti- and REE-minerals. The pronounced diagenetic dissolution of both less stable (garnet, apatite) and more stable (zircon, monazite, Ti-mineral) heavy minerals, if common in the area of Flysch Belt, could help to explain the earlier observations on wide fluctuations in content and composition of heavy mineral fractions.