Sulfur and oxygen isotope studies of melt-country rock interaction, Babbitt Cu-Ni deposit, Duluth Complex, Minnesota

Abstract

The Babbitt Cu-Ni deposit, formerly known as Minnamax, occurs within troctolitic and gabbroic rocks of the Precambrian Duluth Complex. Both massive and disseminated mineralizations are found in the deposit. Pyrrhotite, cubanite, chalcopyrite, and minor pentlandite mineralization occurs near the basal contact with pelitic metasedimentary rocks of the Virginia Formation or higher in the igneous sequence associated with abundant country-rock xenoliths (cloud zones). Sulfur and oxygen isotope studies have been carried out to evaluate the importance of melt-country rock interaction in localizing sulfide mineralization.The delta 34 S values of pyrrhotite plus pyrite from the Virginia Formation located outside the contact aureole range from 5.6 to 13.7 per mil. Pyrrhotite from footwall Virginia Formation hornfels or inclusions in the basal zone exhibits delta 34 S values ranging from 1.5 to 6.2 per mil, whereas delta 34 S values of pyrrhotite from xenoliths in cloud zones range from 2.6 to 21.4 per mil. Isotopic values of basal zone Cu-Ni sulfides range from 6.0 to 13.0 per mil, with a mode at 8 to 9 per mil. Two features highlight basal zone delta 34 S values. First, delta 34 S values of sulfide mineralization are higher than those of pyrrhotite in immediately adjacent Virginia Formation hornfels. Second, higher grade ore areas (> 1% Cu) show 34 S enrichment, with delta 34 S values in the range of 9 to 13 per mil. Sulfides in cloud zones show a wide range in delta 34 S values, from 0.7 to 19.7 per mil. Sulfur isotope data suggest that most of the sulfur found in the deposit has been derived from sedimentary country rocks. However, an in situ process of sulfur derivation is not indicated. Sulfur in basal mineralized zones must have been derived at depth within an auxiliary magma chamber or during magma ascent and emplacement. Isotopic homogenization was not achieved during the sulfide-forming process as evidenced by the spatial variability of delta 34 S values.Oxygen isotope values of both mildly metamorphosed Virginia Formation and footwall hornfels range from 8.2 to 14.0 per mil. Inclusions of Virginia Formation show pronounced isotopic effects due to interaction with melt. Xenolith margins may have values as low as 7 to 8 per mil and are depleted in 18 O with respect to xenolith interiors. The delta 18 O values of igneous rocks from the Babbitt deposit range from 3.9 to 12.2 per mil. Zones of 18 O depletion are rare and suggest that exchange with a low 18 O fluid phase was spatially restricted. Anomalous delta 18 O values in excess of 7 to 7.5 per mil are common throughout the deposit but are limited to within 3 m of Virginia Formation xenoliths. A model of diffusion-controlled 18 exchange between igneous material and xenoliths best describes the delta 18 O distribution. Devolatilization and desilicification of the xenoliths occurred early in their history, when physical mixing in the host magma was possible. Addition of high 18 O material from these processes into a large volume of melt affected delta 18 O distributions only slightly. Continued exchange of oxygen after major cation transfer produced the observed delta 18 O profiles.In the case of basal zone Cu-Ni mineralization at Babbitt, isotopic evidence indicates that sulfide saturation was not caused by in situ addition of sulfur, silica, or oxygen. Sulfide saturation is thought to have been achieved at depth, triggered by assimilation of large quantities of sulfide liberated through devolatilization reactions occurring in country rocks.