Abstract. 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 smallscale 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 displacements as large as 170 ø occur only when average permeability is 10 -25 m 2 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.

▪ Abstract  Stable isotopic, mineralogical, and chemical alteration in metamorphic terranes is evidence for reactive fluid flow during metamorphism. In many cases, the amount and spatial distribution of the alteration can be quantitatively interpreted using transport theory in terms of fundamental properties of metamorphic flow systems such as time-integrated flux, flow direction, and Peclet number. Many estimates of time-integrated flux in the upper and middle crust are surprisingly large, 105–106 cm3 fluid/cm2 rock; estimates for the lower crust are much smaller. Rather than pervasive and uniform, reactive fluid flow in all metamorphic environments is channelized on scales of <1–104 m. Channelization results from heterogeneous permeability structures controlled by features such as lithologic layering, contacts, folds, fractures, and faults. Consequently flow may be in the direction of either decreasing or increasing temperature or isothermal. Site-specific thermal-hydrologic models of metamorphic terranes that explicitly consider chemical reactions and dynamic permeability structures will help resolve outstanding questions with regard to the driving forces and duration of flow, metamorphic permeability distributions, and how deformation controls fluid flow.

The Valentine wollastonite skarn in the north-west Adirondack Mountains, New York, is a seven million ton deposit which resulted from channellized infiltration of HzO-rich, silica-bearing fluids. The wollastonite formed by reaction of these fluids with non-siliceous calcite marble. The skarn formed at the contact of the syenitic Diana Complex and was subsequently overprinted by Grenville-age granulite facies metamorphism and retrograde hydrothermal alteration during uplift.Calcite marbles adjacent to the deposit have generally high 6"O values ( c . 21%0), typical of Grenville marbles which have not exchanged extensively with externally derived fluids. Carbon isotopic fractionations between coexisting calcite and graphite in the marbles indicate equilibration at 675" C, consistent with the conditions of regional metamorphism. Oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14% variation (-1% to 13%). Some isotopic heterogeneity is preserved from skarn formation, and some represents localized exchange with low4 ''0 retrograde fluids.Detailed millimetre-to centimetre-scale isotopic profiles taken across skarn/marble contacts reveal steep 8I8O gradients in the skarn, with values increasing towards the marble. The gradients reflect isotopic evolution of the fluid as it reacted with high 6"O calcite to form wollastonite. Calcite in the marble preserves high 6"O values to within <5 mm of the skarn contact. The preservation of high 6"O values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid. Thus, the marbles were relatively impermeable during both the skarn formation and retrograde alteration. Skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts. Although these steep isotope gradients resemble fluid infiltration fronts, they actually represent the sides of the major flow system. Because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock, such models are not applicable to this hydrothermal system and, by extension, to many other metamorphic systems where low-permeability rocks restrict fluid migration pathways.Minimum time-integrated fluid fluxes have been calculated at the Valentine deposit using oxygen isotopic mass balance, reaction progress of fluid buffering reactions, and silica mass balance. All three approaches show that large volumes of fluid were necessary to produce the skarn, but silica mass balance calculations yield the largest minimum flux and are hence the most realistic.

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