Stable isotope data from hydrothermally altered rocks often show
significant scatter. Such scatter cannot be quantitatively interpreted
by models in which each lithologic unit is assumed to have a uniform
permeability. If a stochastic modeling approach is used instead,
heterogeneous permeability maps can be constructed to approximate the
statistical distributions of natural systems, and both overall isotopic
trends and data scatter can be matched. Three models are presented for
the Alta, Utah, contact aureole to show that the observed scatter in
delta(18)O values is consistent with subhorizontal down-temperature
fluid infiltration through carbonates with heterogeneous permeabilities.
Infiltration through rocks with heterogeneous permeabilities creates
irregular, lobate isotope patterns, so that the idealized isotope
profiles predicted by one-dimensional homogeneous permeability models do
not develop. Localized sampling is unlikely to yield an accurate
estimate of the overall importance of fluid-rock interaction or of
dominant flow directions. In heterogeneous systems, large-scale
hydrothermal alteration and how patterns can best be estimated from
statistically unbiased and spatially distributed data sets.