Veins fields are fractured domains of the lithosphere that have been infiltrated by hydrothermal fluids. We simulate 3D-isothermal flow, advective-dispersive transport and oxygen isotope equilibrium and kinetic exchange in a fractured, rock matrix. We compare modeling results in two vein fields formed under lithostatic pressure in compressive and extensional tectonic settings. The Val-d'Or auriferous quartz vein field (Abitibi belt, Québec, Canada) formed in a transpressive environment and comprises quartz-tourmaline-carbonate veins distributed over an area of about 45 km x 15 km north of the Cadillac Tectonic Zone (CTZ). The oxygen isotope isopleths of auriferous quartz display a zonation at the scale of the vein field. Oxygen isotope compositions are heavier (~14 ‰) to the south, close to the CTZ, and become lighter towards the north of the vein field and away from the CTZ.

The Kokanee Ag-Pb-Zn vein field (southeastern British Columbia, Canada) formed during Eocene crustal extension and unroofing of the Valhalla metamorphic core complex. The veins are in a fault system that comprises an E-W, S-dipping, low-angle normal fault, the "Main Lode", and several NE-SW steeply-dipping faults. Oxygen isotope isopleths are concentric about the "Main Lode" with a NE-SW deflection, and abut on the Slocan Lake Fault.

In both vein fields, we reproduce the oxygen isotope zonation only if boundary conditions describe a permeable lower boundary invaded by a deep-seated metamorphic fluid that mixes with an upper crustal fluid filling the porosity within the vein field. The fluid is allowed to drain through an impermeable upper boundary along a line that represent a major crustal fault in the vein field. The impermeable boundary represent either low premeabilty lithology, cemented porosity, or structural stress guide. That impermeable boundary is required to separate the hydrostatic from the near- to lithostatic fluid regimes. The role of major crustal faults in these veins fields may be more important as a drain for hydrothermal fluids than as a source.