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

Abstract

Sulfur and oxygen isotope compositions of two pelitic Virginia Formation xenoliths and surrounding igneous rocks have been measured in drill core intersects at the Babbitt and Serpentine Cu-Ni deposits, Duluth Complex. The at the Babbitt deposit (xenolith 1) shows compositional and textural evidence for partial melting, whereas the xenolith at the Serpentine deposit (xenolith 1) may still be in part connected to the footwall, and primary sedimentary mineral assemblages are present in its center. The delta 34 S values of the xenoliths range from 6 to 27 per mil. A well-developed vertical profile of upward-increasing delta 34 S values in 2 is suggestive of initial pyrite formation due to bacterial reduction of sulfate in a sedimentary environment where the rate of sulfate reduction was rapid relative to sulfate supply. The extreme variation in delta 34 S values over a vertical interval of less than 20 m illustrates the potential variability of delta 34 S values in the Virginia Formation. The delta 34 S values of magmatic sulfides that formed in response to S assimilation may be equally variable, depending upon local sites of contamination and convective homogenization in the magma. In situ conservation of sulfur is suggested for 2 where the Fe/Mg ratio of biotite decreases with increasing bulk-rock sulfur content. Sulfidation reactions involving iron-bearing silicates or oxide minerals limited the amount of sulfide liberated by the xenolith. Assimilation of country-rock sulfide occurred either at depth within crustal staging chambers or during magma ascent.An oxygen isotope gradient is well-developed at the upper contact between gabbronorite-norite and 2. Oxygen isotope exchange fronts are poorly developed at the lower contact of 2 and at both contacts of 1. At the Babbitt deposit the igneous rock at the lower contact is a compositionally distinct troctolite that shows little evidence for country-rock contamination. Intrusion of distinct, thin, magmatic pulses that cool rapidly, after previous devolatilization and possible partial melting of the xenoliths, can cause the obliteration of well-developed isotopic profiles. The lack of concentration gradients that coincide with the oxygen isotope gradient, together with results of modeling of mass transfer by diffusive processes, suggests that the observed delta 18 O profile was produced during subsolidus cooling. The observed profile may be produced by oxygen isotope exchange via fluid-assisted grain boundary diffusion within 500,000 yr, a duration consistent with the 0.5 to 1 m.y. time span estimated for emplacement of the major intrusions of the Duluth Complex.