**6. Peperite alteration in the succession of volcanic deposits and the stratovolcano-hosted hydrothermal system in the Smrekovec Volcanic Complex**

The stratovolcano-hosted hydrothermal system with convective-advective flow regime of hydrothermal fluids (**Figure 3**), and another important event in the evolution of hydrothermal alteration of volcanic deposits was the emplacement of the Kramarica Sill. Consequently, volcanic deposits underwent alteration related to diverse processes and different superimposed stages of hydrothermal activity [37].

The largest source of geothermal energy and hydrothermal fluids in the time span of volcanic activity some 28-23 mya [43] was a deep igneous body. The alteration resulting from an elevated geothermal gradient is characterised by clinoptilolite, heulandite, analcime, smectite and interstratified smectite-chlorite. The convective flow of hydrothermal fluids occurred primarily through fracture systems and the most typical mineral formed owing to hydrothermal activity is laumontite. Where the fractures were densely distributed, the adjacent rock was altered as well. Laumontite occurs as interstitial cement and replaces volcanic glass and intermediate plagioclases in assemblage with albite, and the principal phyllosilicate mineral is chlorite or interstratified chlorite-smectite with over 80% of chlorite layers. Advective outflow of hydrothermal fluids preferentially occurred through highpermeability layers of the stratovolcano edifice, and laumontite, chlorite, albite and more rarely prehnite are typical minerals encountered in coarse-grained rocks such as volcaniclastic breccias. The adjacent, lower-permeability layers contain authigenic minerals with lower temperature stability ranges, namely clinoptilolite, heulandite, analcime and interlayered chlorite-smectite (**Figures 5** and **6**) [37, 38]. Stilbite locally occurs as vein mineral and was developed during late-stage of hydrothermal activity.

The emplacement of the Kramarica Sill (**Figure 2**) was related to the formation of new vent along the Periadriatic Line. Thermal effects of the emplacement promoted a number of progressive alteration reactions such as from laumontite to prehnite, laumontite to yugawaralite, or interlayered chlorite-smectite to chlorite. During the cooling of the sill, local hydrothermal conditions persisted and controlled retrograde reactions such as from prehnite to yugawaralite, from prehnite to laumontite, from laumontite to heulandite or analcime and from chlorite to interlayered corrensite-chlorite [37, 38, 54].
