Nature of the hydrothermal alteration of the Miocene Sigri Petrified Forest and host pyroclastic rocks, western Lesbos, Greece

Abstract

The early Miocene Sigri Pyroclastic Formation in western Lesbos Island hosts the famous petrified forest of silicified tree fossils, protected as a UNESCO Geopark. The formation consists primarily of flow tuffs, mud flow deposits and stream conglomerate. Hydrothermal veins rich in silica, iron, and manganese are preferentially developed in major fault zones and along paleosols. The purpose of this study is to understand the nature of the hydrothermal system and its relationship to the preservation of tree fossils. Samples were collected from fresh and altered pyroclastic rocks, hydrothermal veins and petrified wood. Rock mineralogy and chemistry were investigated using a petrographic microscope, scanning electron microscope, electron microprobe, Laser Raman spectroscopy, and X-ray powder diffraction. Early devitrification of volcanic glass in tuffs formed opaline silica and locally resulted in smectite. At the same time, fossil trees were partly replaced by amorphous silica. Epithermal alteration and hydrothermal veins include (1) adularia + silica + kaolinite; (2) jarosite + hematite + amorphous silica; and (3) “Fe-oxide” and/or “Mn-oxide” + amorphous silica. The MnSi mixture and Mn-oxides comprise structurally amorphous romanechite with some more crystalline grains of todorokite, ramsdellite and/or nsutite. The mixtures of Fe-oxides and silica are also largely amorphous and include α-cristobalite. Such mixtures replace parts of the petrified wood including cell walls. The various colors observed from the petrified wood are principally related to the abundance of iron and organic matter. Faulting and complex hydrothermal alteration was related to the emplacement of laccoliths and dykes around 18 Ma, 1–3 Ma after deposition of the Sigri Pyroclastic Formation. The observed types of low temperature epithermal alteration are compared with that produced by different circulating waters in the modern Taupo volcanic zone of New Zealand. Hydrothermal deposition of Fe-oxides and silica played an important role in the preservation of the petrified forest.