Hydrothermal fluid evolution in the "Botro ai Marmi" quartz-monzonitic intrusion (Campiglia Marittima, Tuscany, Italy). Evidence from a fluid inclusion investigation

Abstract: The quartz-monzonitic intrusion of "Botro ai Marmi" (Tuscany, Italy) can be considered to be a typical example of an intrusion-centered magmatic hydrothermal system. The evolution of hydrothermal fluids in the "Botro ai Marmi" intrusion was investigated using fluid inclusion analyses to provide suitable physico-chemical constraints on the fluids involved in the late- to postmagmatic hydrothermal activity that affected the intrusion, providing inferences on their origin and variations of temperature and pressure with time. This work demonstrates that the earliest fluids circulating in the "Botro ai Marmi" intrusion were high-temperature brines exsolved directly from the crystallizing magma. This fluid circulated in the intrusion under lithostatic conditions (P > 90 MPa, T > 540°C). A second evolutionary stage of the magmatic hydrothermal system is marked by the transition from lithostatic (>90 MPa) to hydrostatic dominated conditions (50 to 10 MPa). In this stage the fluids are also interpreted to be mainly orthomagmatic in origin but unmixed in a high-salinity brine and in a low-salinity vapor aqueous phase, at a temperature ranging from about 500°C to 300°C. These fluids were responsible for the potassic alteration facies. At a later stage of hydrothermal evolution, abundant meteoric dominated fluids entered the system and are associated with propylitic alteration. This event marks the transition from a magmatic-hydrothermal system to a typical hydrothermal (“geothermal”) system, which can be assumed to be similar to some extent to the nearby active high-enthalpy geothermal system of Larderello. Low-temperature and low-salinity meteoric water-dominated fluids characterize the latest stage of the "Botro ai Marmi" hydrothermal system.