Mechanisms of granite alteration into grus, karkonosze granite, SW Poland

English. Granitic gruses are usually considered as a product of deep weathering, but the influence of hydrothermal fluids was also noticed. In this work, a wide range of mineralogical and chemical methods performed on 43 samples from three representative outcrops and a reference site is used to determine the influence of hydrothermal and weathering fluids on the development of granitic gruses from the coarse grained Karkonosze granite. Four types of altered granites have been distinguished, including compact and friable granite, typical grus, and localized heavy altered zones. Mineralogical observations such as complete albitization of plagioclase and crystallization of secondary quartz revealed an important role of hydrothermal alteration at the early stage of grusification. The origin of smectite and alteration of biotite into muscovite are likely to be connected with circulation of hydrothermal fluids too. Grusification sensu stricto was primarily caused by vermiculitization of biotite which resulted in volumetric expansion and the development of transmineral microcracks with non-accordant pore surfaces. Thus, it requires hydrothermal alteration to be relatively minor, confined to postmagmatic changes, since more advanced alteration of biotite into muscovite rather than vermiculite arrests further development of microcracks and leads to the development of friable granite.

English. Grus; Granite; Clay minerals; Weathering; Hydrothermal alteration

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