Sulfur isotope composition of Cu-Pb-Zn mineralization from the pyrite deposit of Campiano (southern Tuscany)

Polymetallic Cu-Pb-Zn(Ag) mineralization of hydrothermal origin commonly occurs through the Southern Tuscany district, often in close association with extensional, late Apenninic structures. In the Boccheggiano area, Cu-Pb-Zn(Ag-Sn) sulfides have been long known along the Boccheggiano fault, a normal structure trending NW-SE and extending downdip (45°NE) for several hundreds of meters. Polymetallic mineralization has been discovered both in the upper portion of the fault (the so-called "Filone quarzoso-cuprifero") and, at depth, in association with a massive pyrite body, still being exploited in the Campiano Mine. This note is focussed on new sulfur isotope data on the Campiano deposit, obtained in the course of an extensive research program, still in progress. Apart from a few samples taken from the contact between anhydrites and massive pyrite or phyllites (characterized by delta34S > 12 per mil), sulfides at Campiano have delta34S values between + 5 and + 10 per mil. The order of enrichment in 34S among the various sulfur compounds, with sulfates>pyrite> pyrrhotite = sphalerite> galena indicates that isotopic equilibrium was approached, at least at large scale, even if only seldom attained. a few galena-sphalerite mineral pairs provided isotopic temperatures in the range 290-350°C, very close to those estimated by sphalerite/stannite geothermometry (320-350°C) and microthermometric analyses on fluid inclusions in sphalerite (Th=310-340°C). If we assume a temperature of 340°C for the galena+ sphalerite equilibrium assemblage, at a sulfur fugacity between 10-8.5-10-.9.5 (calculated from FeS contents of sphalerite associated with pyrite), we can fix a delta34S(total S) = 6.4 per mil for the sulfur in the fluid. Such value is in good agreement with previous estimates from other deposits in Southern Tuscany.