SULFUR AND OXYGEN-ISOTOPE STUDIES OF THE INTERACTION BETWEEN PELITIC XENOLITHS AND BASALTIC MAGMA AT THE BABBITT AND SERPENTINE CU-NI DEPOSITS, DULUTH COMPLEX, MINNESOTA

Sulfur and oxygen isotope compositions of two pelitic Virginia Formati on xenoliths and surrounding igneous rocks have been measured in drill core intersects at the Babbitt and Serpentine Cu-Ni deposits, Duluth Complex. The xenolith at the Babbitt deposit (xenolith 1) shows compos itional and textural evidence for partial melting, whereas the ''xenol ith'' at the Serpentine deposit (xenolith 1) may still be in part conn ected to the footwall, and primary sedimentary mineral assemblages are present in its center. The delta(34)S values of the xenoliths range f rom 6 to 27 per mil. A well-developed vertical profile of upward-incre asing delta(34)S values in xenolith 2 is suggestive of initial pyrite formation due to bacterial reduction of sulfate in a sedimentary envir onment where the rate of sulfate reduction was rapid relative to sulfa te supply. The extreme variation in delta(34)S values over a vertical interval of less than 20 m illustrates the potential variability of de lta(34)S values in the Virginia Formation. The delta(34)S values of ma gmatic sulfides that formed in response to S assimilation may be equal ly variable, depending upon local sites of contamination and convectiv e homogenization in the magma. In situ conservation of sulfur is sugge sted for xenolith 2 where the Fe/Mg ratio of biotite decreases with in creasing bulk-rock sulfur content. Sulfidation reactions involving iro n-bearing silicates or oxide minerals limited the amount of sulfide li berated by the xenolith. Assimilation of country-rock sulfide occurred either at depth within crustal staging chambers or during magma ascen t. An oxygen isotope gradient is well-developed at the upper contact b etween gabbronorite-norite and xenolith 2. Oxygen isotope exchange fro nts are poorly developed at the lower contact of xenolith 2 and at bot h contacts of xenolith 1. At the Babbitt deposit the igneous rock at t he lower contact is a compositionally distinct troctolite that shows l ittle evidence for country-rock contamination. Intrusion of distinct, thin, magmatic pulses that cool rapidly, after previous devolatilizati on and possible partial melting of the xenoliths, can cause the oblite ration of well-developed isotopic profiles. The lack of concentration gradients that coincide with the oxygen isotope gradient, together wit h results of modeling of mass transfer by diffusive processes, suggest s that the observed 6 profile was produced during subsolidus cooling. The observed profile may be produced by oxygen isotope exchange via fl uid-assisted grain boundary diffusion within 500,000 yr, a duration co nsistent with the 0.5 to 1 m.y, time span estimated for emplacement of the major intrusions of the Duluth Complex.