Source and evolution of ore-forming hydrothermal fluids in the northern Iberian Pyrite Belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes

A fluid inclusion and stable isotopic study has been undertaken on some massive sulphide deposits (Aguas Teñidas Este, Concepción, San Miguel, San Telmo and Cueva de la Mora) located in the northern Iberian Pyrite Belt. The isotopic analyses were mainly performed on quartz, chlorite, carbonate and whole rock samples from the stockworks and altered footwall zones of the deposits, and also on some fluid inclusion waters. Homogenization temperatures of fluid inclusions in quartz mostly range from 120 to 280 °C. Salinity of most fluid inclusions ranges from 2 to 14 wt% NaCl equiv. A few cases with Th=80–110 °C and salinity of 16–24 wt% NaCl equiv., have been also recognized. In addition, fluid inclusions from the Soloviejo Mn–Fe-jaspers (160–190 °C and ≈6 wt% NaCl equiv.) and some Late to Post-Hercynian quartz veins (130–270 °C and ≈4 wt% NaCl equiv.) were also studied. Isotopic results indicate that fluids in equilibrium with measured quartz (δ18Ofluid ≈–2 to 4‰), chlorites (δ18Ofluid ≈8–14‰, δDfluid ≈–45 to –27‰), whole rocks (δ18Ofluid ≈4–7‰, δDfluid ≈–15 to –10‰), and carbonates (δ18Oankerite ≈14.5–16‰, δ13Cfluid =–11 to –5‰) evolved isotopically during the lifetime of the hydrothermal systems, following a waxing/waning cycle at different temperatures and water/rock ratios. The results (fluid inclusions, δ18O, δD and δ13C values) point to a highly evolved seawater, along with a variable (but significant) contribution of other fluid reservoirs such as magmatic and/or deep metamorphic waters, as the most probable sources for the ore-forming fluids. These fluids interacted with the underlying volcanic and sedimentary rocks during convective circulation through the upper crust.