Abstract
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.
Highlights
Numerous volcanogenic massive sulphide (VMS) deposits with a Cyprus-type metallogenic signature (Cu-Fe-Zn) occur in the Tethyan Jurassic ophiolites of the Italian Northern Apennines
The present study describes this phenomenon more precisely, with the help of detailed fluid inclusion microthermometry, Raman spectro scopy and chlorite thermometry studies in all three types of Boccassuolo veins
Thirty-three representative samples of the sulphide mineralisation were selected for the fluid inclusion study (Table 1), in part they are the same samples analysed in previous works (ZACCARINI & GARUTI, 2008, GARUTI et al, 2009, 2011), in part they are samples studied in detail only during this investigation
Summary
Numerous volcanogenic massive sulphide (VMS) deposits with a Cyprus-type metallogenic signature (Cu-Fe-Zn) occur in the Tethyan Jurassic ophiolites (the Ligurides) of the Italian Northern Apennines. One of these deposits, Boccassuolo, is located close to the city of Modena. Historical mining activity at Boccassuolo enables research at several levels of the stockwork mineralisation This convenient situation allows modelling the vertical and horizontal features of the mineralised hydrothermal system. Only an attempt to evaluate the temperature of the mineralised fluids in Boccassuolo was done by using a chlorite geothermometer (ZACCARINI & GARUTI, 2008), but a detailed description of the characteristics of the hydrothermal fluids is not currently available. Comparison of the temperature obtained using fluid inclusion microthermometry with those calculated with a chlorite geothermometer allows us to evaluate the research efficiency of this latter approach
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