The discovery of hydrothermal fields on the seafloor was one of the greatest scientific discoveries of the last century. However, despite the incredible progresses made over the last few decades, still little is known about how mineral deposits associated with hydrothermal systems are formed on the seafloor. This lack of knowledge is even more pronounced for ultramafic-hosted hydrothermal systems, likely to hold more significant mineral deposits than basalt-hosted systems. This project aims to contribute to (1) our understanding of the geochemical and mineralogical changes that take place within hydrothermal systems, (2) clarify the role of serpentinization reactions in the formation of mineral deposits and (3) provide new insights into the formation of seafloor massive sulphide deposits, characterizing the main differences between mafic and ultramafic hydrothermal systems. Moreover, by characterizing the chemical processes operating in ultramafic hydrothermal settings, the project aims to contribute towards the development of new survey geochemical methods to be used in the detection of ultramafic-hosted hydrothermal fields.