Investigation of non pristine volcanic structures acting as probable hosts to epithermal gold mineralization in the back arc region of the active Aegean arc, using combined satellite imagery and field data: examples from Lesvos volcanic terrain

Abstract

Abstract The island of Lesvos, located in the northeastern Aegean, just offshore Asia Minor, is mostly covered by Miocene calkalkaline, and predominantly shoshonitic volcanic products. Pyroclastic deposits are voluminous, indicating intense explosive activity. In the eastern part of the island outcrops the Polychnitos Ignimbrite. The western part of Lesvos is covered by the Sigri Pyroclastic formation, the host to the renown lithified forest. The prolific 21.5-17 Ma volcanic activity requires well developed subvolcanic magma chambers and their surfacial expression in the form of sizeable caldera structures, two of which they have been previously inferred from intense alteration zones. Remote sensing was combined with field geology and geochemical methods to appreciate the actual geometry and number of these volcanic structures, which are severely affected by erosion and fault tectonics. Landsat TM and SPOT satellite images and the Digital Elevation Model (DEM) of the targeted areas were digitally processed in order to reveal specific geological characteristics associated with calderas such as rims and floors, radial and ring faults, areas of hydrothermal alteration, drainage network and lava domes both internal and external to caldera structures. In order to convey as much spectral information as possible and to enhance the outlines of Lesvos volcanic features, we produced a slope map with a resolution of 1 0 derived from DEM of the study area. As a result we have recognized seven caldera structures of which six new ones. The highlights of these investigations are: (a) The Stipsi and Sigri calderas, related with extensive argillic alteration zones and probably high-sulphidation epithermal precious metal mineralization, (b) The source of the Sigri pyroclastics is a (14×10 km) large caldera structure, the Sigri caldera, that encloses the previously recognized Vatoussa structure (nested caldera).