Fluid inclusion study of the Boccassuolo VMS-related stockwork deposit (Northern Apennine ophiolites, Italy)

Several Cyprus-type volcanogenic massive sulphide (VMS) deposits occur in the Jurassic ophiolitic series of the Northern Apennines. Stratabound, stratiform and stockwork deposits were formed in the western limb of the Neotethys (Ligurian Ocean) and are observed today in basalt, gabbro and serpentinised peridotite host rocks. The studied stockwork deposit at Boccassuolo occurs in basalt and basalt breccia. Detailed petrography, fluid inclusion study, Raman spectroscopy analyses and chlorite thermometry calculations were used to determine the P, T, X conditions of the fluid circulation system. The veins contain three quartz generations, calcite, chlorite, epidote and sericite as gangue minerals and pyrite, chalcopyrite, sphaleritc, pyrrhotite and galena as ore minerals. Based on the fluid inclusion study, the earlier defined three vein types (1, 2 and 3) precipitated from the same type of evolving fluid, though at slightly different stratigraphic positions. The determined ranges of temperature (370-60 degrees C), salinity (6.2-11.4 NaCl equiv. wt%), pressure (30-44 MPa) and methane content (average 0.28 mol/kg) suggest an evolved seawater origin for the hydrothermal fluid, modified by fluid-rock interactions and possibly by mixing of magmatic volatiles. The fluid characteristics and the mineralogical observations have proven a slightly distal position in relation to the centre of the fluid flow for all the studied locations, but more central and more distant blocks were also recognised. The temporal evolution of the system developed into a low temperature event, occurring after the main mineral stage of formation, but still within the same overall process.