Veins fields are fractured domains of the lithosphere that have been infiltrated by hydrothermal fluids. The Val-d'Or auriferous vein field consists of a large number of quartz-tourmaline-carbonate (QTC) veins distributed over an area of about 45 km x 15 km north of the Cadillac Tectonic Zone (CTZ). QTC veins occur in subvertical reverse shear zones and related subhorizontal tension fractures that formed an interconnected network that channeled auriferous fluid flow. 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. This oxygen isotope zonation is interpreted to result from infiltration of a deep-seated metamorphic fluid with a  d¹⁸O value of 9 ‰ into the interconnected fracture network where  the metamorphic fluid reacts with volcanic hostrocks and mixes with an upper crustal fluid with a  d¹⁸O value of about 4 ‰.

         We use the finite-element model FRAC3DVS to simulate 3D-isothermal fluid flow and advective-dispersive transport in discretely-fractured porous media. The model computes oxygen isotope compositions during equilibrium and kinetic exchange between the fluids and the rock mass. The porous matrix is discretized with 3D elements and is intersected by vertical and inclined (45 degrees) interconnected planar high permeability zones that schematically reproduce the fracture pattern in the Val-d'Or vein field. We assume isothermal conditions at 350 C. The volcanic rock matrix is assigned a homogeneous porosity and oxygen isotope composition. 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. Several boundary conditions and material properties are adjusted to reproduce the oxygen isotope zonation found in the Val-d'Or vein field. The most critical parameters are the boundary conditions describing the geometrical configuration of the source and drain of the hydrothermal fluids.