North-Central Section - 49th Annual Meeting (19-20 May 2015)

Paper No. 4
Presentation Time: 2:35 PM


ASP, Kristofer, Department of Earth and Environmental Sciences, University of Minnesota-Duluth, 1114 Kirby Drive, 229 Heller Hall, Duluth, MN 55812, SCHARDT, Christian, Department of Earth and Environmental Sciences, University of Minnesota-Duluth, 1114 Kirby Drive, 213 Heller Hall, Duluth, MN 55812 and SPIVAK-BIRNDORF, Lev J., Dept. of Geological Sciences, University of Indiana, 1001 East Tenth Street, Bloomington, IN 47405,

Recent work has shown significant Ni isotopic variations of up to 1.1 ‰ in high-temperature magmatic rocks associated with sulfide mineralization [1,2]. The Duluth Complex in northeastern Minnesota, a gabbroic series of intrusive rocks, contains significant magmatic sulfide mineralization. Selected sulfide deposits in the Duluth Complex were sampled to assess the potential of isotopic fractionation between early cumulates and subsequent Cu-Ni-PGE mineralization. Of particular interest is the exploration potential of Ni isotopes for magmatic sulfides recorded by host rocks, surface weathering, and glacial till. Samples were analyzed at the University of Indiana using the double-spiking method outlined in [1]. Ni isotope ratios are reported relative to the NIST SRM 968 standard with conventional delta notation and a general 2σ error of 0.06 ‰.

Initial results show a spread of δ60/58Ni values from -0.97 ‰ to -0.02 ‰, within the range of Ni isotopic values reported previously [1,2]. The least fractionated values come from unmineralized mafic intrusives (-0.02 ‰), while massive sulfides show values around -0.97 ‰. Till samples record intermediate values ranging from -0.50 ‰ to -0.17 ‰ and weathered surface samples, containing up to 15 % sulfides, range from -0.23 ‰ to -0.10 ‰. The isotopic data suggest a clear isotopic fractionation trend in the Duluth Complex from unfractionated values close to zero for early crystallizing phases to highly fractionated massive sulfides, accumulated at some later stage. Ni isotopic values for till and mineralized surface samples, and their correlation with known deposits, may be useful in distinguishing regions overlying Cu-Ni-PGE mineralization from barren areas. Data may also help to identify the entry point of the mineralizing magma based on the location and isotopic signature of individual sulfide deposits.

1. Wasilenski et al. (2015) Ni isotope fractionation during sorption to ferrihydrite: implications for Ni in banded iron formations. Chemical Geology (accepted for publication)

2. Hiebert RS., Rouxel, O., Houlé, MG., Bekker, A. (2014) Ni isotope fractionation between komatiite and sulfide mineralization at the Neoarchean Hart deposit, Abitibi greenstone belt, Canada. Geological Society of America Abstracts 46: 467