Petrogenesis of pelitic xenoliths at the Babbitt CuNi deposit, Duluth Complex, Minnesota, U.S.A.

Abstract

The Babbitt deposit consists of disseminated CuFeNi sulfides found within mafic rocks of the Duluth Complex, generally near contacts with underlying metasedimentary rock types. Host rocks for the deposit include troctolites, olivine gabbros, gabbronorites, norites, and occasionally country rock hornfels. Xenoliths of country rocks are abundant in the deposit, and suggest a relationship between sulfide mineralization and country rock contamination. Country rocks in the Babbitt area include those of the middle Precambrian Biwabik Iron Formation, and both calcareous and non-calcareous pelites of the Virginia Formation. Xenoliths contain the assemblage cordierite-plagioclase-biotite-orthopyroxene, and are thought to have been derived from Virginia Formation protoliths. Comparison of protoliths and xenoliths using composition-volume, element ratio and mass-balance techniques suggests that xenoliths have been strongly depleted in volatiles, alkalis and Si. Footwall rocks show only a depletion in volatiles. Neither fluid-phase transport nor diffusion through an intergranular fluid can account for the mass of material transferred. Extensive partial melting of xenoliths, with residual enrichment of FeO, MgO and Al 2O 3, is the most viable transfer process. The lack of SiO 2 concentration gradients around xenoliths and anomalous igneous rock compositions suggest that desilicification occurred at a time when physical mixing of extracted partial melt and host magma was possible, and prior to final emplacement. Sulfide saturation may have been initiated due to Si assimilation with an auxiliary magma chamber. However, the composition of ores in the Babbitt deposit is consistent with saturation being achieved by addition of sediment-derived volatile sulfur, independent of major-element assimilation.