Sassolite Precipitation at the Restless Campi Flegrei Volcano in Italy Points to Hydrothermal Flashing by Deep Boron‐Rich Brines

Studying the behavior of dormant volcanoes is a major challenge in volcanology. Here, we conducted a 10‐year survey of hydrothermal mineralization at Campi Flegrei, an active volcano near Naples in Italy, which last erupted in 1538. Our survey revealed a significant output of boric acid at the hydrothermal sulfate discharge area of Pisciarelli, coinciding with a heightened volcanic unrest (so‐called bradyseisms). This unexpected output shyly began in 2019 before the vanishing of permanent hydrothermal springs and mud‐pools, in the form of transparent, colorless and glassy luster sassolite crystals found within the commonest mineralogical assemblage. High‐boron emissions peaked in July–August 2021, by spraying‐out whitish fine powders rich in sassolite, which has remained a prevalent feature through 2024 while we also measured a decline in NH 4 ‐sulfates and a tenfold increase in mud boron content. Sassolite appearance parallels the increased H 2 S flux at fumaroles, both marking the onset of recent acceleration in ground displacement and the accompanying increasing seismicity. Using new and literature mineral, chemical and isotope (δ 11 B) data, we ascribe sassolite yields to the direct drainage of boron‐rich brines that ascend more efficiently from the deeper to the surface pathways of the geothermal system via fracturing activated by the escalating bradyseism. The shift from closed to open degassing may explain the appearance of sassolite, making it valuable for unraveling the unrest dynamics at Campi Flegrei. Additionally, our findings enlarge present views on traditional pegmatites associated with tourmaline systems.