**8. Conclusion**

Peperites are commonly developed in submarine environments with contemporaneous volcanic activity and sedimentation. And although the occurrence and complex processes of formation have been studied and explained in many modern and ancient geological settings worldwide [4, 10–14], and particularly in the

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

mineral is chlorite or interlayered chlorite-smectite.

The alteration of peperites from the Smrekovec Volcanic Complex indicates close relationship to the rock composition and texture, and therefore, the style of peperite formation. Porosity and permeability of peperites is, in general, lower than that of the host sediment. The types involving fine-grained host sediments have low permeability and low porosity, and in the stratovolcano-hosted hydrothermal system with convective-advective flow regime (**Figure 3**) they must have functioned as aquicludes unable to drain effectively the advective flow of hydrothermal fluids that largely controlled hydrothermal alteration of volcanic deposits [37]. In such hydrothermal system, peperites involving coarse-grained host sediments should be more extensively altered and contained authigenic minerals with higher temperature stability ranges than the types involving fine-grained host-sediments, but that is not the case. On contrary, in this type of peperites significant authigenic calcium-aluminosilicate minerals are often lacking and the most common alteration

*Authigenic calcium-aluminosilicate minerals that commonly serve as geothermometers in volcanichydrothermal systems, their chemical formulae and temperature stability ranges, compiled from [52, 55–65].*

In dispersed blocky peperite, matrix composed of siliciclastic host sediment is usually unaltered. The alteration of juvenile clasts often indicates only the reactions of devitrification of volcanic glass, and only juvenile clasts with perlitic cracks can be altered to laumontite and Fe-oxides (**Figure 9A**). The activity of hot fluids originating from heated pore waters can be assumed, but chemical gradients favourable for the formation of laumontite were attained only inside the juvenile clasts with perlitic cracks. Perlitic cracks apparently served as conduits for hot fluids that leached volcanic glass during the flow, and in this manner underwent the changes in chemical composition (e.g. [67]) that finally resulted in crystallisation of laumontite. Far more extensive alteration of blocky peperites involving fine-grained pyroclastic host sediment supports the forementioned explanation. The interaction of heated pore fluids and highly reactive host sediment apparently controlled geochemical evolution of so-formed hydrothermal solutions and the related extensive alteration of all constituents of peperite. Laumontite or mineral assemblages of laumontite, albite, quartz, pumpellyite, incipient epidote and chlorite, or laumontite, prehnite, quartz, chlorite and incipient epidote, or laumontite, analcime and interlayered chlorite-smectite indicate that temperature gradients were prerequisite but not sufficient for alteration to occur and that the main controlling factor were

Microglobular peperite has been interpreted as a frozen example of a fuelcoolant interaction (FCI) between magma and fluidised host sediment [5], and the

**318**

**Figure 14.**

geochemical gradients.

Carpathian-Pannonian region [9, 15–17] where the Smrekovec Volcanic Complex belongs to [29], the studies related to their alteration have been relatively scarce [15–20]. The present study gives evidence of the existence of localised and ephemeral hydrothermal conditions related to and persisting during the formation of peperites and resulting in their distinct alteration.

The Smrekovec Volcanic Complex is a remnant of an Oligocene submarine stratovolcano characterised by a complex development of an over 2500 m thick succession of volcanic rocks. Submarine environment, the style of eruptions, morphology and the abundance of pyroclastic and syn-eruptively resedimented volcaniclastic deposits were favourable for the formation of peperites that are particularly abundant in medial-zone lithofacies associations. In a succession of volcanic deposits studied in detail in two sections Krnes and Smrekovec G34 attaining 400 m and 470 m, respectively, the alteration of peperites indicates that authigenic minerals have higher temperature stability ranges than those in the adjacent underlying and overlying deposits irrespectively of their lithofacies.

The alteration of peperites indicates close relationship to the rock composition and texture, and therefore, the style of peperite formation. Dispersed blocky peperite involving siliciclastic host sediment is commonly poorly altered. Volcanic glass in juvenile clasts is usually devitrified or hydrated, and only some clasts with perlitic texture can be altered to laumontite. In the textural types with fine-grained pyroclastic host sediment laumontite, or the assemblages of laumontite, albite, quartz, pumpellyite, incipient epidote and chlorite, or laumontite, prehnite, quartz, chlorite and incipient epidote, or laumontite, analcime and interlayered chloritesmectite may occur. The alteration minerals indicate that thermal gradients were prerequisite but not sufficient for alteration to occur and that the main controlling factor were geochemical gradients in reacting fluids.

Microglobular peperite developed by interaction between magma and fluidised sediment [5], and the temperatures of alteration must have been higher than in blocky peperite. Smaller microglobules are often completely altered to authigenic mineral assemblages with higher temperature stability ranges (e.g. pumpellyite, epidote) while the larger microglobules remained incompletely altered or altered to authigenic minerals with lower temperature stability ranges (e.g. laumontite). The relationship indicates that high-temperature conditions could not persist for a long period of time and had ephemeral character.

Authigenic mineral assemblages developed in peperites indicate that their formation is specific and related to the formation of parent rock itself. Thermal stability ranges of actinolite and pumpellyite indicate the highest temperatures possibly exceeded 280°C and decreased when the parent lava flow and the associated peperite underwent cooling. Hydrothermal fluids mainly originated from heated pore fluids although deuteric fluids could have been locally admixed. The evolution of fluids circulating in peperite was essential for extensive alteration to occur and that was possibly attained by interaction with unstable and highly reactive host sediment. Many peperites have been developed as low-porosity and low-permeability layers, and therefore contemporaneous and later hydrothermal activity related to the stratovolcano-hosted hydrothermal system with convective-advective flow could not have exerted any critical thermal or geochemical impact.

The alteration of peperites in the Smrekovec Volcanic Complex can be regarded as syn-formational, hydrothermal, ephemeral, localised and depending on many factors such as the extent and time span of thermal regime, the process of formation of parent rock and thermal and geochemical evolution of circulating hydrothermal fluids. And although peperite deposits are not rare in similar volcanic-sedimentary settings worldwide, the alteration as recognised in the present study has not been

**321**

**Author details**

Polona Kralj

*Submarine Stratovolcano Peperite Syn-Formational Alteration - A Case Study of the Oligocene…*

reported yet and indicates the formation and alteration of peperites are complex

Slovenian Research Agency (ARRS) is acknowledged for granting the research (Programme Mineral Resources P-0025). The editorial work of Academic Editor Dr. Károly Németh and helpful comments of anonymous reviewers are greatly appreciated. Many thanks to Mrs. Mia Vulovic, Author Service Manager for her kind assistance. I thank Ms. Staška Čertalič and Mr. Mladen Štumergar from

Geological Survey of Slovenia, Dimičeva ulica 14, 1000, Ljubljana, Slovenia

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: polona.kralj@geo-zs.si

provided the original work is properly cited.

*DOI: http://dx.doi.org/10.5772/intechopen.95480*

and distinctive and interrelated processes.

Geological Survey of Slovenia for technical support.

**Acknowledgements**

*Submarine Stratovolcano Peperite Syn-Formational Alteration - A Case Study of the Oligocene… DOI: http://dx.doi.org/10.5772/intechopen.95480*

reported yet and indicates the formation and alteration of peperites are complex and distinctive and interrelated processes.
