Veins fields are fractured domains of the lithosphere that have been infiltrated by hydrothermal fluids. We use the finite-element model FRAC3DVS to simulate 3D-isothermal fluid flow and advective-dispersive transport in a fractured porous matrix. Fractures are represented in the model by vertical and inclined planar high permeability zones, which intersect the matrix and schematically reproduce the fracture pattern in the vein field. The model computes oxygen isotope compositions during isothermal equilibrium and kinetic exchange between the fluids and the rock mass.

        We compare modeling results in two vein fields formed in compressive and extensional tectonic settings. The Val-d'Or auriferous quartz vein field (Abitibi belt, Québec, Canada) formed in a transpressive environment and it comprises a large number of 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 composition of auriferous quartz displays a zonation at the scale of the vein field. d¹⁸O values are higher (~14 ‰) to the south, close to the CTZ, and decrease regularly to values of about 9 ‰ towards the north of the vein field and away from the CTZ. The porosity of the rock matrix is filled with the upper crustal fluid with a d¹⁸O value of 4 ‰. The rock matrix and high permeability planes are infiltrated by the deep-seated metamorphic fluid (d¹⁸O = 9 ‰), forced to flow upwards by a pressure gradient.

        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. We simulate infiltration of a deep-seated fluid (d¹⁸O = 8 ‰) along the Slocan Lake Fault at the base of the continental crust. A pressure gradient forces the fluid to flow towards higher crustal levels where the fluid enters the fractures in the Slocan Group slates (d¹⁸O = 18 ‰). Porosity in the Slocan Group slates and interconnected fractures are filled with evolved meteoric water (d¹⁸O = 3 ‰).

        Several boundary conditions and material properties are adjusted in the model to reproduce the oxygen isotope zonation found in theVal-d'Or and Kokanee vein fields. We find that the most critical parameters to reproduce field observations are the boundary conditions describing the geometrical configuration of the source and drain of the hydrothermal fluids.