Oxygen and Multiple S Isotope Investigation of Magma-Country Rock Interaction During Early Stages of Magmatism in the Midcontinent Rift System: Constraints From the Eagle Deposit

Indiana University, Bloomington IN

Investigators

Abstract

Intellectual merit: The Eagle deposit, located in the Baraga Basin of northern Michigan, consists of semi-massive and massive Fe-Ni-Cu sulfides found within a dike-like peridotite body. The rocks are associated with the Proterozoic Midcontinent Rift System (MRS), and are thought to represent feeders to early-stage, rift-related volcanics. Country rocks include Proterozoic metasedimentary rocks that are locally graphitic and sulfidic; these rocks appeared to be likely sources for S contained in the igneous rocks, However, sulfides in the igneous rocks are characterized by d34S values less than 4 per mil, whereas sulfides in the Proterozoic rocks show a d34S range from ~10 to 40 per mil. Preliminary S isotopic analyses indicate a range of d33S values from 0.02 to -0.7 per mil, indicating significant mass independent fractionation of S isotopes that to date has only been documented in Archean rocks. Thus, most of the S contained in the Eagle deposit must be derived from Archean rocks, rather than from the sulfide-rich Proterozoic rocks. To verify and understand this surprising result, a detailed investigation of d33S variations is proposed for the Eagle deposit, as well as both Proterozoic and Archean country rocks. Multiple S isotope studies have proven to be important to the continuing evaluation of Earth's early atmosphere. However, recent studies have shown that mass independently fractionated S isotopes (MIF-S) may not be restricted to rocks greater that ~2 Ga in age. The proposed stratigraphically controlled study at Eagle will provide information on the distribution of d33S values in the igneous rocks and related sulfide mineralization, in the Proterozoic (two sequences, one of ~ 2.2-2.4 Ga and the other of ~1.8-1.9 Ga, which will cover important intervals in Earth history with respect to MIF-S), and in the Archean rocks. We also plan to analyze gabbroic rocks with associated Cu-Ni sulfide mineralization found in the Duluth Complex of Minnesota, located on the north side of the rift. In these samples d34S values of the sulfide accumulations are similar to the values of iron sulfides in carbonaceous, 1.8 to 1.9 Ga country rocks. This association is much different from that at Eagle, and presents an opportunity to contrast multiple S isotope systematics in mineralized systems that formed at different times in the development of the MRS. Oxygen isotope analyses of olivine and plagioclase in the rocks at Eagle will also be undertaken to investigate the relative importance of devolatilization and partial melting of country rocks in transferring MIF-S to the magmatic system. Results of the proposed study will increase our understanding of magmatic sulfide ore formation in the MRS, will shed light on the nature of interaction between mantle-derived magmas and crustal rocks at various times in the developing rift, and provide insight into S mass independent fractionation on Earth. Broader impacts. The proposed research represents a collaboration between academia and the minerals industry. Kennecott will fund a graduate student fellowship as part of the project, as well as limited geochronologic, mineralogical and geochemical studies. In addition, reserves of Ni and platinum-group elements in the United States are low and studies of this type are needed to evaluate known resources and to develop exploration models. Both graduate and undergraduate students will benefit from the collaboration with mining industry, and will receive training in the utilization of state-of-the-art analytical instrumentation and in methods of scientific investigation.