STRONTIUM AND OXYGEN-ISOTOPE PROFILES ACROSS MARBLE SILICATE CONTACTS, LIZZIES BASIN, EAST HUMBOLDT RANGE, NEVADA - CONSTRAINTS ON METAMORPHIC PERMEABILITY CONTRASTS AND FLUID-FLOW

Sr isotope profiles across marble-silicate rock contacts are used in conjunction with previously published oxygen isotope profiles to constrain fluid movement, porosity and permeability contrasts in migmatitic rocks from Lizzies Basin in the East Humboldt Range, Nevada. The O-18/O-16 systematics in the high-grade sequence have been interpreted to reflect infiltration of similar to 2 x 10(2) m(3)/m(2) of a relatively low O-18 hydrous fluid through the sequence, but with preservation of delta(18)O anomalies in thin marble bands due to a 30-fold lower porosity in the marble compared with silicate rocks (Wickham and Peters 1992). The Sr isotope profiles confirm that tracer exchange between marble and silicate rock was primarily by diffusion, and in one case, indicate that porosities differed by less than a factor of four in the similar to 10 cm boundary layer which exhibits diffusive modification of Sr-87/Sr-86 ratios. This contrasts with modelling of the oxygen isotope profiles which imply porosity contrasts > 10 for one marble band and > 50 for a second marble band. Either strontium and oxygen isotope diffusion reflect different events (possible if fluid Sr contents varied with time) or porosity varied substantially with the silicate rocks. Oxygen isotope profiles in the deeper part of the metamorphic section in which delta(18)O values of silicate rocks have been homogenised and lowered, indicate similar diffusion distances (and thus porosity-time evolution) to oxygen isotopic profiles higher in the section. Comparison of strontium and oxygen isotope diffusion distances constrains fluid Sr contents to between similar to 50 and similar to 500 ppm deep in the section, but less than similar to 10 ppm higher in the section. The difference is related to release of relatively saline, Sr-rich fluids, by the abundant leucogranites and associated skarns deep in the section (cf. Peters and Wickham 1995).