Timescale of fluid migration in orogenic foreland basins

Aqueous fluids are transient yet integral components of the continental crust, essential for understanding tectonic deformation, geochemical processes, and transport and precipitation of metals and minerals. Their presence and migration are governed by porosity and permeability of reservoir rocks, which under favorable conditions can lead to economically valuable ore deposits. Identifying the mechanisms, timing, and pathways of fluid flow—from microscopic to kilometre scales—is therefore critical for exploring critical raw materials. With increasing demand and limited supply, robust transport models grounded on empirical data are needed to explain the origin and distribution of these resources. Fluid movement from the lower to upper crust is driven by pressure gradients, temperature differences, buoyancy, permeability, and tectonic activity, which create and reactivate flow pathways. In foreland basins, this migration has major economic and societal importance, as it controlled ore deposition. Beyond mineralization, fluid dynamics illuminate geohazards such as fault reactivation and inform assessments of subsurface CO2 or nuclear waste storage. Constraining timescales of migration is also essential to reconstruct basin histories, understand interactions between tectonic, sedimentary, and climatic processes, and evaluate mineral resource potential. Mineral veins form when fluids become supersaturated through changes in temperature, pressure, or chemistry, and microscopic-sized pockets of fluid could be trapped as inclusions within growing crystals. These inclusions preserve the composition and conditions of palaeofluids, offering direct evidence of fluid provenance and evolution. Analysing their physical and chemical properties, together with vein mineralogy, provides key insights into fluid chronology and migration. This thesis examines the origin and conditions of fluid migration using stable isotope measurements of aqueous inclusions, combined with petrographic observations, inclusion composition, and in-situ isotopic analysis of vein minerals. Relative timing is constrained through structural and sedimentological analysis, while absolute ages are obtained with the 40Ar/39Ar stepwise crushing technique, which targets radiogenic isotopes in inclusions and host quartz. The late Palaeozoic Variscan Orogeny, a major ore-forming event in Europe, facilitated large-scale upward fluid flow in foreland basins, enriching them in metals and rare earth elements. Quartz veins associated with this event record palaeofluid pathways, pressure-temperature conditions, and tectonic transformations. In this thesis, veins are studied from two key Variscan sites: Rursee in the Rhenohercynian Zone (Germany) and the Almograve coastline in the South Portuguese Zone (Portugal), both rich in quartz veins hosted in deformed sedimentary lithologies. An innovative bulk-crushing approach was developed to measure δ2H and δ18O in fluid inclusions via Cavity Ring-Down Spectroscopy (CRDS). To compare sources and compositions of fluids, analyses combined bulk isotopes, in-situ Secondary Ion Mass Spectroscopy (SIMS) measurements, petrography, microthermometry, and Raman spectroscopy. Results suggest that meteoric and seawater-derived fluids infiltrated and cooled metamorphic fluids during orogeny, with quartz veins forming mainly in reactivated fractures over extended timescales. Chronology reveals further complexity. In Rursee, structural evidence links veins to early Variscan inversion, but 40Ar/39Ar analyses yield Jurassic–Cretaceous ages, possibly reflecting later low-salinity fluids or radiogenic argon from mineral inclusions. In Almograve, relative dating-structural reconstruction ties fluid flow to early foreland basin formation rather than late-stage orogenic phases. Despite a shared structural framework, inclusion geochemistry differs between the two regions, indicating contrasting local and regional fluid histories. In conclusion, this thesis reconstructs the complex evolution of fluid migration in Variscan foreland basins, integrating field-based structural interpretations with precise isotopic and petrographic analyses. The findings refine understanding of crustal fluid dynamics, ore formation, and the geological development of foreland basins across multiple scales.