Anatomy of a fumarole field; drone remote sensing and petrological approaches reveal the degassing and alteration structure at La Fossa cone, Vulcano Island, Italy

Abstract. Hydrothermal alteration processes can affect the physical and chemical properties of volcanic rocks and develop via complex degassing and fluid flow systems and regimes. Although alteration can have far-reaching consequences for rock stability and permeability, little is known about the detailed structures, extent, and dynamic changes that take place in hydrothermal venting systems. By combining drone-based remote sensing with mineralogical and chemical analyses of rock and gas samples, we analyzed the structure and internal anatomy of a dynamic evolving volcanic degassing and alteration system at the La Fossa cone, Vulcano Island (Italy). From drone image analysis, we revealed a ~70,000 m² sized area subject to hydrothermal activity, for which we could determine distinct alteration gradients. By mineralogical and geochemical sampling of the zones of those alteration gradients, we study the relation between surface coloration and mineralogical and chemical composition. With increasing pixel brightness towards higher alteration gradients, we find a loss of initial mineral fraction and bulk chemical composition and a simultaneous gain in sulfur content. Using this approach, we defined and spatially constrained alteration units and compared them to the present-day thermally active surface and degassing pattern. The combined results permit us to present a detailed anatomy of the La Fossa fumarole field, highlighting 7 major units of alteration and present-day diffuse activity that, next to the high-temperature fumaroles, significantly contribute to the total activity.