Tectonic control on hydrothermal circulation and fluid evolution in the Pietratonda–Poggio Peloso (southern Tuscany, Italy) carbonate-hosted Sb-mineralization

Abstract

This paper deals with the development of an exhumed Sb-ore deposit associated with Neogene–Quaternary magmatism of southern Tuscany. This epithermal mineralization represents a fossil hydrothermal system that was probably active during the volcanic activity that produced rhyolitic lava flows (~ 2.3 Ma), presently exposed close to the study area (Roccastrada volcanic complex). Volcanism was coeval with strike-/oblique-slip and normal faults that controlled the hydrothermal circulation. Faults were active until the Pleistocene. Our study, based on fluid inclusion and structural data, demonstrates that fluids were channeled in damage zones of the Pliocene–Quaternary strike-/oblique-slip and normal faults and permeated within damage zones of Middle–Late Miocene extensional detachments, consisting of fractured carbonate rock masses sandwiched between successions with very low permeability. Fluids produced a diffuse hydrothermal alteration both in the cataclasite and in carbonate damage zones where jasperoid strata bounds developed. Fluid inclusion investigation allowed tracing the evolution of the hydrothermal fluids that is interpreted in terms of a progressive cooling of the hydrothermal system. Such evolution could be related to large infiltration of shallow waters from the early mineralizing stage (fluids at about 200–225 °C, salinity around 3 wt.% NaCl equiv.) down to the later post-ore stages, characterized by more diluted fluids (salinity of about 2.1 wt.% NaCl equiv) having lower temperatures (T h around 155 °C). The latest hydrothermal activity in the area is indicated by fluid inclusions hosted in late calcite veins that are characterized by T h values of about 125 °C. This fossil hydrothermal system may provide a close analog to the active geothermal systems occurring in southern Tuscany, thus the results are useful for better understanding the relationships between brittle structures and fluids migration.