FLUID-FLOW AND MASS-TRANSPORT AT THE VALENTINE WOLLASTONITE DEPOSIT, ADIRONDACK MOUNTAINS, NEW-YORK-STATE

The Valentine wollastonite skarn in the north-west Adirondack Mountain s, New York, is a seven million ton deposit which resulted from channe llized infiltration of H2O-rich, silica-bearing fluids. The wollastoni te formed by reaction of these fluids with non-siliceous calcite marbl e. The skarn formed at the contact of the syenitic Diana Complex and w as subsequently overprinted by Grenville-age granulite facies metamorp hism and retrograde hydrothermal alteration during uplift. Calcite mar bles adjacent to the deposit have generally high deltaO-18 values (c. 21 parts per thousand), typical of Grenville marbles which have not ex changed extensively with externally derived fluids. Carbon isotopic fr actionations between coexisting calcite and graphite in the marbles in dicate equilibration at 675-degrees-C, consistent with the conditions of regional metamorphism. Oxygen isotopic ratios from wollastonite ska rn are lower than in the marbles and show a 14 parts per thousand vari ation (-1 parts per thousand to 13 parts per thousand). Some isotopic heterogeneity is preserved from skarn formation, and some represents l ocalized exchange with low-deltaO-18 retrograde fluids. Detailed milli metre- to centimetre-scale isotopic profiles taken across skarn/marble contacts reveal steep deltaO-18 gradients in the skarn, with values i ncreasing towards the marble. The gradients reflect isotopic evolution of the fluid as it reacted with high deltaO-18 calcite to form wollas tonite. Calcite in the marble preserves high deltaO-18 values to withi n <5 mm of the skarn contact. The preservation of high deltaO-18 value s in marbles at skarn contacts and the disequilibrium fractionation be tween wollastonite skarn and calcite marble across these contacts indi cate that the marbles were not infiltrated with significant quantities of the fluid. Thus, the marbles were relatively impermeable during bo th the skarn formation and retrograde alteration. Skarn formation may have been episodic and fluid flow was either chaotic or dominantly par allel 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 thro ugh a permeable rock, such models are not applicable to this hydrother mal 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 buffer ing reactions, and silica mass balance. All three approaches show that large volumes of fluid were necessary to produce the skarn, but silic a mass balance calculations yield the largest minimum flux and are hen ce the most realistic.