Fluid inclusion constraints on the Mofete geothermal system

Fluid inclusions (FIs) represent an invaluable source of information for investigating hydrothermal systems, as they preserve composition, temperature and pressure of entrapment of the fluids circulating during mineral growth at depth.

In this study, rock cores of two deep wells (Mofete 3 and Mofete 5) from Mofete geothermal field (Campi Flegrei caldera) were selected to investigate the chemical-physical conditions occurring in the hydrothermal system through fluid inclusion study. Three types of fluid inclusions were identified: Type 1 two-phase liquid -rich fluid inclusions; Type 2 multi-phase liquid + vapor bubble + one or more daughter minerals; Type 3 monophase (vapor) fluid inclusion. Type 1 FIs occur in quartz and calcite at 1324m (Mofete 3) and 1750m (Mofete 5) of depth respectively and can be distinguished petrographically in primary and secondary (Type 1a and Type 1b). Type 2 FIs are hosted within quartz, scapolite and anhydrite of Mofete 5 well at 2492, 2607 and 2698 m of depth. They appear primary Type 3 FIs are prevalently secondary. They occur in Mofete 5 well at 2607 and 2698m of depth within quartz and scapolite

Microthermometric experiments were carried out on Type 1 and Type 2 FIs. At 1324m of depth, Type 1a FIs (Mofete 3) show temperature of homogenization (T_h) between 178 °C and 291 °C with a mode around 215 °C, while Type 1b has T_h between 195 °C and 257 °C with a modal value at 240 °C. Salinity of Type 1a FIs ranges between 2.7 and 5.1 wt.% NaCl_equiv. with a mode at 3.5 wt.% NaCl_equiv.. Type 1b FIs show salinity in the range 3.4-6.9 wt.% NaCl_equiv. with mode around 4.5 wt.% NaCl_equiv..

At 1750m of depth (Mofete 5) T_h of Type 1a FIs are between 250 °C and 273 °C with a modal value at ~255 °C. Type 1b FIs have T_h between 271 °C and 299 °C mode around 285 °C. Salinity of these inclusions has been determined in only 3 Type 1b FIs giving consistent results at 2.9 wt.% NaCl_equiv..

Type 2 FIs revealed T_h in the range 317-453 °C with the mode ~380 °C (2698m of depth Mofete 5) and 331-434 °C mode at ~360 °C (2607m of depth Mofete 5). At 2492m of depth we obtained only 3 data, giving a range between 347 and 383 °C. Salinities determined in Type 2 FIs are: 38-58 wt.% NaCl_equiv. (2698m of depth Mofete 5), 40-65 wt.% NaCl_equiv. (2607m of depth Mofete 5). At 2492m of depth we determined only one salinity data which is 44 wt.% NaCl_equiv..

Our data highlights an increase in the temperature of homogenization from Type 1a to Type 1b FIs. We would also remark that although FIs show an increase of temperature in time, Type-2 FIs are hypersaline fluids (~50% NaCl_eq. Wt%) with T_h comparable to present day measured deep well temperatures, and salinities similar to fluids circulating at these depths.

The increase in T_h of FIs in the shallower hydrothermal system points toward an increase in the activity of the system.