R. C. G. S. Jorge

Detrital zircon geochronology of the Carboniferous Baixo Alentejo Flysch Group (South Portugal); constraints on the provenance and geodynamic evolution of the South Portuguese Zone

U–Pb detrital zircon data from the Baixo Alentejo Flysch Group in the South Portuguese Zone show significant age differences between formations. The Visean Mértola Formation and Serpukhovian to early Bashkirian Mira Formation are dominated by zircons in the 316–388 Ma age range, whereas the late Bashkirian to late Moscovian Brejeira Formation is dominated by zircons with an age range of 498–687 Ma. Detrital zircons spanning an age range of 0.9–1.1 Ga are present in the Brejeira Formation but are absent in the Mértola and Mira formations. Detrital zircon ages of the Mértola and Mira formations indicate provenance from an extra-basinal source (Ossa–Morena Zone) with a minor intra-basinal contribution (South Portuguese Zone). The abundant presence of detrital zircon with age ranges of 500–750 and 0.9–1.1 Ga in the Brejeira Formation suggests a sediment source from the Avalon–Meguma terranes with limited recycling from the SW Portugal Domain. The different inferred source areas for the Baixo Alentejo Flysch Group formations are attributed to the presence of a forebulge that was formed in Mid-Visean times during the foreland phase of the South Portuguese Zone. The forebulge acted as a physical barrier separating sub-basins that accumulated the Mértola–Mira and Brejeira sediments respectively.

Osmium isotope systematics in the Iberian Pyrite Belt

New Re and Os analyses of pyrites from Aljustrel, Lagoa Salgada and Neves-Corvo deposits yield concentrations that are similar to those previously reported for the Rio Tinto and Tharsis deposits, all at the Iberian Pyrite Belt. Osmium contents in IPB pyrites increase from the deeper stockwork zones towards the upper massive sulphides. Pyrite samples from IPB sulphide orebodies plot on a major 187Re/188Os-187Os/188O linear array, yielding an age of 359±26 Ma and an initial 187Os/188O ratio of ca. 0.57. Five samples from Neves-Corvo sulphide-rich stockwork ores yield an age of 358 ±29 Ma and an initial 187Os/188O ratio of 0.49 ±0.07, in accord with palynological and isotopic age constraints for ore formation at the IPB. Pyrites from Neves-Corvo copper-tin ores have Re/Os ratios and osmium concentrations that are similar to those in pyrites from sulphide-rich stockwork ores; however, their highly radiogenic initial 187Os/188O ratios (4.89–7.85, if preserving original isotope system-atics) preclude derivation from the same sources as the remaining IPB sulphides. The overall Re-Os geochemical data conform to previous Sr, Nd and Pb isotopic studies supporting multiple sources for IPB metallogenesis.

Silica gel microtextures in siliceous exhalites at the Soloviejo manganese deposit, Spain

Well-preserved primary textural features were recognized in cherts and jaspers that form one of the manganese ore-hosting units of the Soloviejo Mn deposit, in the Iberian Pyrite Belt. These include (i) spherulitic, (ii) banded and (iii) brecciated. The spherulites (0.04—0.3 mm) consits of a central core of minute grains of hematite or quartz, rimmed by chalcedony or microcrystalline quartz. Banded jaspers show no spherulitic textures. Banded textures are made up by alterations of micro- to meso-bands of silica and hematite. They often show soft-sediment deformation features, such as microslumps, pinch-and-swell, and load cast. Diagenetic brecciation overprints these pristine textures. Breccias consist of jasper fragments (mm to cm), affected by total or partial recrystalization, cemented by different proportions of microcrystalline quartz. Primary textures in siliceous hydrothermal precipitates reflect different stages of formation and diagenetic maturation of its silica gel precursor. Their interpretation helps in constraining the initial conditions assisting the hydrothermal activity at the Soloviejo deposit, and its early evolving stages.

Reply to Discussion on ‘Detrital zircon geochronology of the Carboniferous Baixo Alentejo Flysch Group (South Portugal); constraints on the provenance and geodynamic evolution of the South Portuguese Zone, Journal of the Geological Society, 172, 294–308

We would like to thank Pereira (2015) for his interest in our work and appreciate the opportunity to clarify some aspects of the Rodrigues et al . (2015) study. Here we dispute several fundamental assertions made in the discussion of our original paper.