Acta Musei Moraviae

Petrography, hydrothermal alteration and rare earth element mineralization of metagranites and mylonites in the area of the Pb-Zn ore deposit Oskava in the Jeseníky Mountains

Borehole OSK-10, drilled in the 1980s as part of a prospecting survey for sulphide Pb-Zn-Ag ores near Oskava in the Jeseníky Mountains, intersected rocks of the Devonian volcanosedimentary complex (part of the Vrbno Group) and Cadomian granitoids of the Oskava Block (part of the Desná Group). The granitoids are affected by mylonitization. Based on its extent, metagranites and mylonites were distinguished in borehole OSK-10. Rocks of both types are affected by hydrothermal alteration, which is mainly manifested as albitization, sericitization and carbonatization. Hydrothermal alteration together with deformation has led to the formation of sericite-rich domains in these rocks, in which finely flaky mica is mainly accompanied by albite, carbonate of the dolomite-ankerite series and quartz, and to a lesser extent, for example, rutile, siderite, zircon, thorite (or huttonite) and REE-phosphates, specifically monazite-(Ce) and xenotime-(Y). Devonian basic rocks (greenschists) are considered as a possible source of the elements whose contents have been shown to increase by alteration (REE, Ti, Fe, Mg, P and Na). Rare earth mineralization in the metagranites and mylonites of the Oskava Block is economically insignificant due to the low REE-minerals content in these rocks.