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

Several Cyprus-type volcanogenic massive sulphide (VMS) deposits are found 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 found today in basalt, gabbro and serpentinised peridotite host rocks. The studied stockwork deposit at Boccassuolo is found 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, sphalerite, 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 stratigraphical positions. The determined ranges of temperature (370 60°C), salinity (6.2 11.4 NaCl equiv. wt%), pressure (30-44 MPa) ranges and methane content (average 0.28 mol/kg) suggest an evolved seawater origin for the hydrothermal fluid, modified by fluid-rock interaction and possibly by mixing of magmatic volatile. The fluid characteristics and the mineralogical observations have proven a slightly distal position in relation to the centre of the fluid flow for all studied locations, but less and more distal blocks were also recognised. The temporal evolution of the system developed into a low temperature event, occurring after the main mineral stage, but still participating to the same process as a whole.</p

Hydrothermal processes related to Triassic and Jurassic submarine basaltic complexes in northeastern Hungary and in the Dinarides and Hellenides

Comparative studies on hydrothermal alteration of submarine peperitic basalt occurrences related to the Triassic early rifting of the Neotethys were carried out in various parts of the Dinarides and Hellenides. The study areas included the displaced fragments of the Dinarides in the Darnó Unit, NE Hungary, the Kalnik Mts in Croatia and the Vares-Šmreka area in Bosnia and Herzegovina. In the Hellenides, similar environments were studied in the Stragopetra Mts., Greece. Jurassic pillow basalts formed in a back-arc-basin of the Neotethys were also studied (in the Szarvaskő Unit, NE Hungary, which also represents a displaced unit of Dinaridic origin). Within the submarine basaltic lava flows, six volcanic facies were distinguished. The hydrothermal alteration was characterized according to those facies. The first process was the albitization of the rock-forming plagioclase at ~300°C temperature in all localities. During the higher temperature stage of the subsequent cooling, chloritization in the ground mass is typical for all types of basalts, however chlorite and rarely quartz formed in the fractures and amygdales of the Triassic basalts, while chlorite, quartz and prehnite precipitated in the fractures of the Jurassic rocks. At lower temperatures of this cooling-related process, calcite is a common mineral filling up the larger amygdales, jig-saw type fractures and other open spaces, but some epidote, pumpellyite, prehnite and laumontite also occur in the Triassic basalts. The late stage alteration (happened at the lowest temperature) is characterized by argillitization at every locality. The observed hydrothermal alteration patterns also show slight differences according to the volcanic facies as a function of the distal/proximal setting in relation to the eruptive centers and the presence/absence of water-saturated and unconsolidated carbonate or siliciclastic sediments at the time of the emplacement of lava flows. The study revealed that the most important factors influencing mineralogy and zoning of hydrothermal alteration in these short living local hydrothermal systems are the rapid cooling of the hydrothermal fluid, the dominance of the not much evolved seawater as the source of hydrothermal fluid and the local, i.e. effective water/rock ratio, determined by the degree of fracturing in the rock. The mineralogical-textural peculiarities of the highly localized hydrothermal fluid/rock interaction in the studied submarine sea-mount type volcanoes are clearly different from the products of the large-scaled hydrothermal processes occurring at mid-oceanic ridges. Recognition of these differences is important in the evaluation of ore potential of Neotethyan realm or other areas with occurrences of submarine basaltic unit