Isotope geochemistry tracks the maturation of submarine massive sulfide mounds (Iberian Pyrite Belt)

The massive sulfide deposits of the southern Iberian Pyrite Belt (IPB) occur as shale-hosted exhalative mounds or shallow sub-seafloor replacive bodies that formed at the Devonian-Carboniferous boundary in anoxic bottoms. The geochemistry of the mineralization and the host shale from some of the major deposits, including a detailed 50-m-thick section through the Migollas orebody, shows a marked textural and vertical zonation that is also reflected in the chemical, textural, and isotopic variations of pyrite and carbonates. Pyrite evolves from primary framboids and colloform aggregates to late euhedral crystals, which accompanies a redistribution of the contained base metals and grain coarsening. SIMS analyses depict an extreme variability of delta S-34 values from -42.3 to +4.4 parts per thousand with a gradual increase that is related to the textural maturation. There is also a systematic rise in the Sr-87/Sr-86 ratios (0.70846 to 0.71354), delta C-13 (-12.2 to -5.2 parts per thousand) and delta O-18 values (+14.1 to +27.8 parts per thousand) of the associated carbonates, which evolve from early dolomite to late siderite. This evolution evidences the maturation of the exhalative mineralization during mixing between two fluids: modified seawater enriched in biogenically derived reduced sulfur and upflowing hydrothermal fluids. Early sulfides precipitated in the seawater-mound interface. Towards the core of the mounds, there was an increasing influence of deep fluids that modified the early mineral textures and the isotopic signatures. This evolution is similar to that observed in modern submarine hydrothermal systems and confirms the importance of mound refining in the evolution of fossil exhalative mounds and the formation of economic ores