The hydrothermal system of the Miocene volcanic rocks of Western Lesbos, Greece

Abstract

The Sigri Pyroclastic Formation in the western side of the island of Lesbos consists primarily of pumice flows, mud flows and stream conglomerate. Most of the pyroclastic rocks appear to be derived from a caldera near Vatoussa and shows extensive alteration and mineralization. The purpose of this study is to understand the hydrothermal system and determine the origin of the silica-iron-manganese mineralization. Samples were collected from the Jithra Ignimbrite, layered fine-grained sediments underlying the ignimbrite, a zoned nodule from a fault zone, and silicified wood samples from the Sigri Petrified Forest. Rock mineralogy and chemistry were investigated using a petrographic microscope, scanning electron microscope, electron microprobe, Laser Raman spectroscopy, and X-ray powder diffraction. Hydrothermal alteration minerals and assemblages identified from the altered ignimbrite were: (1) K-feldspar +silica +illite +minor apatite, zircon, TiO₂ minerals; (2) jarosite +hematite +amorphous silica and; (3) Mn-oxide. Three different horizons from the underlying sediments show identical mineral assemblage of smectite +silica +TiO₂ minerals ±monazite, hematite. The presence of hydrothermal quartz and K-feldspar, kaolinite and smectite are closely similar to the alteration assemblages found in the epithermal system of the Taupo volcanic zone which are created by different types of circulating groundwater. The hydrothermal veins and the zoned nodule are predominantly made up of silica +iron +manganese mineralization. The availability of manganese may be related to the decay of organic matter as the study area used to be forested with multiple tree horizons. While the amorphous silica-iron mineralization is mineralogically and chemically comparable to jaspers found in exhalative marine systems. This observation is interesting because there is no evidence for a nearby marine condition in Western Lesbos.