Perlite Deposits Of The Central Slovakia Volcanic Field (Western Carpathians): Geology And Properties

Perlites in the Central Slovakia Volcanic Field are associated with with rhyolite dykes, cryptodomes, extrusive domes, coulees and volcanoclastic rocks of the Jastraba Fm. (12.3-11.4 Ma). From numerous occurrences only the Lehotka pod Brehmi (LPB) and Jastraba (JST) represent deposits of economic interest. The LPB deposit exploits a pile of extruded hyaloclastite breccia composed of grey porous and dark dense fragments. The JST deposit exploits glassy rhyolite breccia composed of grey porous fragments associated with an extrusive dome/coulee. The perlites at both deposits are peraluminous, calc-alkaline of the high-K type, poor in phenocrysts (around 5 %) of plagioclase, biotite and minor amphibole (LPB) or sanidine/anorthoclase (JST). Glass at both deposits is silica rich (75.4-79.5 wt. % dry) with Al2O3, K2O and Na2O as other major constituents. It is inhomogeneous showing domains enriched in Na2O or K2O. Glass water content (3.0-6.0 wt. 4/0) shows a weak positive correlation with its silica content and a negative correlation with its Nap content. Perlites show porosities of 5-16 % (dark dense), 16-30 % (grey porous) and 30-44 % (pale grey pumiceous). Narrow stretched pores represent remnants after outgassing of ascending magma while open undeformed pores grew at a low pressure before quenching. The transformation of volcanic glass into perlite took place owing to the hydration by heated fluids of meteoric origin. The hydration was supported by a significant porosity with interconnected pores and by sustained elevated temperature. Perlites at both deposits show a low content of tightly-bound water and a low Na/K ratio. These properties are responsible for their relatively low degree of expansion. On the other hand, due to the same reason, the perlites have a good mechanical stability.