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Abstract

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.