T151 Geology and Fluid Dynamics of Cr, Ni-Cu-PGE, and PGE Mineralized Magmatic Plumbing Systems

Tuesday, 6 November 2018
8:00 AM - 12:00 PM
Room 233

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

Paper number: T151-8
Program: Technical Sessions
Time: 9:50 AM – 10:05 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-318573

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