Deciphering the Douro Carboniferous Basin (Pejão Coalfield, Portugal): Geochemical, petrological, and mineralogical evidence of depositional, diagenetic, and hydrothermal processes

The Douro Carboniferous Basin (DCB) in Portugal hosts significant coal deposits, with the Pejão Coalfield being among the most important. This study integrates petrographic, geochemical, and mineralogical analyses to investigate the sedimentation, hydrothermal influences, and paleoclimatic conditions of the DCB. Petrographic analysis classifies these coals as high-rank anthracite A, with vitrinite as the dominant maceral (>90 %). Geochemical analyses reveal trace element enrichments (Ag, As, Pb, Sb, and V), suggesting a combination of primary sedimentary input and later hydrothermal overprinting, as indicated by the presence of REE-bearing phosphates (florencite, monazite, xenotime) and sulfosalts (tetrahedrite, bournonite).

Paleoweathering indices (CIA, PIA, WIP, MIA) suggest intense weathering and subtle sediment recycling, while paleosalinity proxies indicate periodic evaporation events linked to ephemeral lacustrine zones, possibly playa-type systems. The REE distribution patterns and provenance indicators suggest a limited zircon enrichment, supporting minimal sedimentary recycling, especially intracarboniferous breccia and sandstone lithologies. These findings provide new insights into the basin's depositional environment, diagenetic history, and hydrothermal alteration, contributing to a broader understanding of Carboniferous intermontane basins.