MULTIPLE SULFUR ISOTOPE STUDY OF EAGLE EAST, MICHIGAN: UNDERSTANDING THE GENESIS OF NI-CU-PGE DEPOSITS

The Eagle East deposit is a high-grade Ni-Cu sulfide deposit located near Marquette, Michigan. The deposit is hosted by mafic-ultramafic intrusions associated with the Baraga dike swarm. Previous research on the nearby Eagle deposit indicated that crustal sulfur contamination likely induced sulfide saturation and led to the deposit’s formation. While the two intrusions are similar, they are associated with different conduits and may reflect different degrees and styles of crustal contamination. Thus, this system provides a natural laboratory for the investigation of multiple S sources in magmatic sulfide ore genesis.

Fifty core samples from disseminated, massive, and semi-massive sulfides were measured for sulfur isotope composition. δ³⁴S values range from 1.4‰ to 3.2‰ for massive sulfides, 0.7‰ to 5.5‰ for semi-massive sulfides, and 2.1‰ to 13‰ for disseminated sulfides. Δ³³S is defined as defined as ∆³³S = δ³³S – 1000 ((1 + δ³⁴S/1000)^λ – 1), where λ = 0.515. Values ranged from -0.09‰ to 0.01‰ for massive sulfides, -0.095‰ to 0.12‰ for semi-massive sulfides, and -0.09‰ to 0.035‰ for disseminated sulfides.

Sulfur derived from uncontaminated mantle is characterized by a δ³⁴S of 0±2‰. Proterozoic country rocks from the Baraga Basin are characterized by a wide range of δ³⁴S values, from ~4‰ to 29‰ depending on the formation. δ³⁴S values from Eagle East indicate that crustal sulfur from these Proterozoic country rocks was important for promoting sulfide liquid saturation in the mafic magmas that host the sulfide mineralization. This is particularly evident from the elevated δ³⁴S observed in the disseminated mineralization, which suggest sulfide derivation from high-³⁴S Proterozoic country rocks and emphasize multiple magma injections (characterized by distinct S isotope values) in the formation of the ore system. ∆³³S values that deviate from 0±0.05‰ are indicative of the presence of S derived from Archean rocks. Only ten Eagle East samples show such values. That these samples come from all mineralization types is suggestive of localized contributions of S from Archean rocks. Thus, although some sulfide was derived from these deep Archean rocks, more significant magma/country rock interaction occurred in shallower portions of the system, characterized by the presence of sulfide-rich, Proterozoic country rocks.