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

Abstract

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, 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 stratigraphic positions. The determined ranges of temperature (370-60°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.

Reconstruction of Hydrothermal Processes in the Cyprus Type Fe-Cu-Zn Deposits of the Italian Northern Apennines: Results of Combined Fluid Inclusion Microthermometry, SEM-CL Imaging and Trace Element Analyses by LA-ICP-MS

“…The formation temperature of chlorite from the various stockwork, stratiform and stratabound deposits vary between 50-360 • C [37] pointing to various conditions during the hydrothermal process. These data are in good agreement with the preliminary fluid inclusion data for Boccassuolo (minimum formation temperature of quartz between 70-360 • C [38]).…”

“…: methane content was not possible to check due to the small size of inclusions or strong luminescence of calcite. *: data including the previous preliminary results of [38].…”

“…These deposits have been known since the Bronze Age [30] and detailed mineralogical, petrographic and geochemical studies completed on them since the 1950s [31][32][33][34][35][36][37] provide a solid basis for their interpretation as Cyprus type VMS deposits. However, recent studies have mainly addressed the mineral chemical, geochemical and geotectonic aspects of ore deposit formation [33][34][35][36][37] and only a few have focused on the reconstruction of hydrothermal processes by fluid inclusion studies [38]. In addition to this, these locations provide an excellent opportunity to model the fluid evolution of Cyprus type VMS deposits.…”

Petrological factors controlling abiotic methane formation in mafic-ultramafic rocks

“…As previous studies have shown, abiotic methane is associated with various petrogenetic processes (e.g. Sachan et al, 2007, Normand 2010, Miura et al, 2011, Arai et al, 2012, Blamey et al, 2012, Kiss et al, 2015, especially secondary/alteration processes (e.g. Abrajano et al, 1988, Etiope et al, 2011, 2013a, 2013b, Boschetti et al, 2013, Szponar et al, 2013, Etiope et al, 2018.…”

“…Most of the researches, suggest that the gas measured during pulverisation of the host lithologies should derive from Aluid inclusions (e.g. Sachan et al, 2007, Normand 2010, Miura et al, 2011, Arai et al, 2012, Blamey et al, 2012, Kiss et al, 2015, but only few mention that methane can occur within secondary pores or micro-fractures (e.g. Konnikov et al 2000, Okland et al, 2014, Etiope et al, 2018.…”