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Sulfide mineralogy and sulfur isotope geochemistry of layered sills in the Deer Lake Complex, Minnesota

Abstract

The Deer Lake Complex, located in north-central Minnesota, consists of a series of layered peridotite-pyroxenite-gabbro sills. Sulfide minerals occur as fine disseminations throughout pyroxenite and gabbro units, and occur more sporadically in peridotite and basal chilled margin units. Sulfide volume percentage rarely exceeds 0.5. A distinct zonation in sulfide mineralogy and sulfur isotopic composition characterizes the sills. Cobaltian pentlandite is the dominant sulfide mineral in peridotite (pd) units, with Ni-enrichment most likely linked to the serpentinization process. δ34Spd values are variable, ranging from −3.5 to +2.8‰. Sulfide assemblages in pyroxenite (px) and lower gabbro units consist of chalcopyrite, pyrrhotite, and minor pentlandite. δ34Spx values range from −1 to +1 ‰. Pyrite is the principal sulfide mineral in upper gabbro (μg) units. Its origin may be related to increased f02 conditions of the remaining melt and to reaction between a S-bearing volatile phase and mafic silicates. δ34Sug values range from 1 to 3.5 ‰. Sulfur isotopic values of chilled margin (2–9 ‰) and peridotite units, together with the erratic spatial distribution of sulfide minerals in these zones, suggests that the parent magma was not initially saturated with sulfur, and that local sulfide concentrations are the result of incorporation of sulfur derived from metasedimentary country rocks. Sulfide saturation was more uniformly reached during pyroxenite formation, with contained sulfur being of magmatic origin. Enrichment in 34S of pyrite from upper gabbro may be explained by buildup of isotopically heavy sulfur following a Rayleigh process, coupled with possible involvement of a SO2-rich fluid phase during hydrothermal alteration.

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Ripley, E.M. Sulfide mineralogy and sulfur isotope geochemistry of layered sills in the Deer Lake Complex, Minnesota. Mineral. Deposita 18, 3–15 (1983). https://doi.org/10.1007/BF00206691

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Keywords

  • Pyrite
  • Sulfide Mineral
  • Sulfur Isotope
  • Sulfur Isotopic Composition
  • Sulfide Saturation