SULFUR ISOTOPE HETEROGENEITY IN DISSEMINATED SULFIDE MINERALIZATION: INSIGHTS FROM THE EAST EAGLE DEPOSIT, NORTHERN MICHIGAN

Sulfur isotope ratios are a key indicator in the assessment of the source of S involved in magmatic Ni-Cu-PGE deposits. Isotope ratios outside of those considered normal for mantle-derived S (δ³⁴S near 0±2‰) denote a contribution of S from country rocks. However, subtleties such as the scale of isotopic equilibration in magmas, and the duration of isotopic exchange, remain poorly understood. The Eagle and East Eagle Ni-Cu-PGE deposits, located near Marquette, Michigan in the Midcontinent Rift System, are closely associated conduit-style deposits whose S isotope systematics allow for evaluation of S isotope variability in mafic magmas. The deposits are hosted by sulfur-rich Paleoproterozoic sedimentary rocks characterized by a wide range in δ³⁴S values from ~0 to >30‰. Archean rocks, primarily orthogneiss, are also present and may contain sulfide minerals characterized by anomalous Δ³³S values. Previous multiple S isotope studies by Ding et al. (2012) have shown that net-textured mineralization at Eagle contains a strong component of Archean sulfur.

At East Eagle Δ³³S values of all three major types of sulfide mineralization range from -0.07‰ to 0.05‰ and are not indicative of the involvement of substantial amounts of S from Archean rocks. Massive and net-textured sulfide mineralization are characterized by a restricted range of δ³⁴S values (1.5‰ to 3.0‰) which may be indicative of isotopic exchange reactions in the conduit involving accumulated sulfide and pristine magma. In sharp contrast, disseminated sulfide mineralization is characterized by a wide range in δ³⁴S values from -4.2‰ to 23.1‰; the range in values provide strong evidence that S from the Paleoproterozoic rocks was involved in sulfide genesis, consistent with the Δ³³S values near 0. Notably, the disseminated sulfide mineralization exhibits isotopic variability to at least the meter scale. Many of the rocks hosting the disseminated mineralization with such extreme δ³⁴S values also show quench textures with elongate and branching plagioclase. The evidence for rapid cooling suggests that the S isotope values have not been affected by exchange in the conduit, and the extreme variability must either reflect multiple inputs of magma with variable δ³⁴S values, or S isotopic heterogeneity at small scales that is generally not preserved in magmatic systems.