Geochemistry of Famennian to Visean Metapelites from the Iberian Pyrite Belt: Implications for Provenance, Paleo-Redox Conditions and Vectoring to Massive Sulfide Deposits

Abstract

Massive sulfide deposits of the Iberian Pyrite Belt (IPB) document the progression of ore-forming processes in the SW Iberian Variscides, throughout uppermost Devonian to Early Carboniferous. Sulfide ores are usually hosted in volcanic-dominated or shale-dominated successions belonging to the Volcano-Sedimentary Complex (VSC; Late Famennian to Late Visean). Yet, several important orebodies at Neves-Corvo are sitting within upper sections (Famennian) of the early deposited Phyllite–Quartzite Group (PQG). Four key sectors of the Portuguese segment of IPB were revisited and 262 samples collected to characterize the geochemical features of metapelites from PQG and VSC sequences in barren and fertile settings. The selected sectors include active (Neves-Corvo and Aljustrel) and old (Lousal) mines, besides promising brownfield (Gaviao) and greenfield (Sesmarias) prospects, and other lithostratigraphic sections without known mineralization. The composition of PQG and VSC metapelites is similar and mainly controlled by a mixture of clayey-derived and quartz sandy-derived components largely resulting from different sources of granitic to granodiorite/quartz diorite composition. A third component of local volcaniclastic origin could be significant in many samples of upper VSC sections. Mineral transformations during diagenesis and/or hydrothermal alteration/mineralization partially overprinted the primary composition. During sedimentation/diagenesis, prevalent redox conditions were mostly confined to oxic environments, at places transiting to sub-oxic. Subsequent multistage interaction with reducing hydrothermal fluids (to which the sulfide mineralization is related) generated heterogeneous anoxic signs. Consistent increases in Fe2O3/TiO2, (Cu + Zn + Pb)/Sc and (As + Sb)/Sc ratios to values above 10 represent a valuable footprint toward ore horizons.