Geochemical and petrologic data from contact aureoles consistently
document fluid focusing through small-scale permeable structures. We use
stochastic representations of permeability in a series of transient
numerical simulations to assess how such small-scale rock
heterogeneities influence kilometer-scale fluid convection around a
shallow crustal pluton. The sensitivity study considers different
permeability scenarios by varying statistical characteristics of the
permeability distribution (mean, variance, and spatial correlation).
Large-scale convective flow patterns in heterogeneous contact aureoles
are shown to deviate significantly from equivalent homogeneous aureoles
in ways that cannot be predicted without detailed mapping of the
permeability field. Fluid focused through high-permeability zones
results in cumulative fluid fluxes as high as 1 x 10(8) kg m(-2) after 2
x 10(5) years, while nearby low-permeability regions experience
time-integrated fluxes <<10(5) kg m(-2). Spatially variable flow
patterns preclude extrapolation of flow directions or fluxes determined
from one portion of the aureole to another. Several simulations show
high-permeability zones developing isolated long-lived convection cells
far from the intrusion. Reduced permeability contrasts (low variance) or
anisotropic permeabilities inhibit development of such hydrologically
isolated regions. When average aureole permeability is 10(-16) m(2),
maximum metamorphic temperatures deviate by less than or equal to 30
degrees from conductive cooling models despite locally high fluxes.
Advective temperature displacements as large as 170 degrees occur only
when average permeability is 10(-15) m2 and can be detected using
isotopic or petrologic thermometry. These temperature deviations reflect
local permeability heterogeneities and could be useful in identifying
high-permeability paleochannels. These results also imply, however, that
temperature profiles taken from only one portion of a heterogeneous
contact aureole may not be regionally representative or used to infer
the overall advective-conductive thermal structure of the large-scale
hydrothermal system.