An Investigation of Ni and Cu Isotopic Fractionation in Basal Duluth Complex Cu-Ni-PGE Mineralization, Northeastern Minnesota

Abstract

Cu-Ni-PGE magmatic sulfide-style mineralization occurs along the western margin of the Duluth Complex in northeastern Minnesota. Previous studies have demonstrated a notable fractionation of 60Ni and 58Ni in terrestrial materials, including both primary and secondary phases, with a total range of up to 2.1 ‰. Other work has indicated a fractionation of 65Cu and 63Cu, with pronounced differences between primary copper sulfides and secondary copper phases in a variety of deposit types. Prior to this study, no δ60/58Ni or δ65/63Cu values have been measured in Duluth Complex rocks. The primary goal of this study is to measure Ni and Cu isotope values in a variety of Duluth Complex samples, and develop a possible model for the δ60/58Ni isotopic system in this geologic terrane. Based on the findings of previous studies, samples were collected to determine the isotopic differences between sulfide-bearing and sulfide-barren material. Samples were collected from a variety of locations in the basal Duluth Complex, including glacial till beds and surface outcrops in the vicinity of the I, II, Serpentine, Mesaba, and NorthMet deposits. Additional drill core material was obtained from the III, Wetlegs, and Wyman Creek deposits. A detailed characterization of till, weathered surface, and primary drill core samples revealed three main sources of nickel in Duluth Complex material: silicate, sulfide, and secondary oxide. The 24 δ60/58Ni values have an overall range from -0.97 to 0.22 ‰, but are correspondingly distinct in each type of material: silicate (-0.03 ‰ average), sulfide (-0.36 ‰ average), secondary oxide (-0.50 ‰ average). Further geochemical and microprobe work, along with the isotopic values, indicate two main stages of Ni fractionation in basal Duluth Complex rocks: a high temperature stage during crystallization, and a low temperature stage during surficial weathering. High-T fractionation is defined by a preferential incorporation of 58Ni into sulfide, while silicates, especially olivine, are reflective of the Bulk Silicate Earth value. Low-T fractionation results in a preferential incorporation of 58Ni into secondary oxide, while 60Ni possibly enters solution and leaves the system. The 22 measured Duluth Complex δ65/63Cu values have an overall range from -1.28 ‰ to 0.36 ‰, with an overall average of -0.35 ‰. Further work is necessary to better define and interpret the involved fractionation processes in the Duluth Complex.