Os, Nd, O and S isotope constraints on country rock contamination in the conduit-related Eagle Cu–Ni–(PGE) deposit, Midcontinent Rift System, Upper Michigan

• Published in: Geochimica et cosmochimica acta Vol. 89; pp. 10 - 30
• Main Authors: Ding, Xin, Ripley, Edward M., Shirey, Steven B., Li, Chusi
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.07.2012
• Subjects: Conduits; Contamination; Copper; COPPER RESOURCES; crystals; Deposits; developmental stages; igneous rocks; Isotopes; mineralization; mixing; Neodymium; POLLUTION; Rock; SULFIDES; sulfur; swarms; trace elements; Michigan
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2012.04.029

The Eagle Cu–Ni–(PGE) deposit is hosted by mafic to ultramafic intrusive rocks associated with the Marquette–Baraga dike swarm in northern Michigan. Sulfide mineralization formed in a conduit system during early stages in the development of the ∼1.1Ga Midcontinent Rift System. The conduit environment represents a prime location for melt–rock interaction. In order to better assess the extent of country rock contamination in the Eagle system, a combined trace element, Nd, Os, O and S isotope study of country rocks, sulfide-bearing igneous rocks and massive sulfide was undertaken. Both the Eagle and the weakly mineralized East Eagle intrusion show trace element patterns that are similar to those of picritic basalts that formed during early stages of rift development. The trace element, Os, Nd, and O isotopic values of the igneous rocks are consistent with <5% of bulk contamination by Paleoproterozoic and Archean country rocks. Both the Re–Os and Sm–Nd system provide isochrons that are in agreement with the 1107Ma U–Pb baddeleyite age of the intrusive rocks. Calculated γOs(1100) and εNd(1100) values for the magmas are +34 and −2. δ18O values of pyroxene in feldspathic pyroxenite range from 6.5‰ to 6.6‰ and provide the only indication that bulk contamination may locally have exceeded 20%. Sulfur isotopic values of disseminated and massive sulfide in the Eagle intrusion range from 0.3‰ to 4.6‰. The δ34S values are much lower than those that characterize most of the country rocks, but could still be indicative of a contribution of S from country rocks of up to ∼50%. Δ33S values of the disseminated and massive sulfides range from −0.10‰ to 0.09‰ indicating a source in Paleoproterozoic country rocks. Semi-massive sulfide in the Eagle deposit has δ34S values between 2.2‰ and 5.3‰, and Δ33S values show a broad range between −0.86‰ and 0.86‰ indicating a major contribution from an Archean source. Isotopic data from the Eagle deposit strongly indicate that multiple sources of sulfur were involved in the generation of the Ni–Cu–(PGE) mineralization, and magmas which traversed variable paths through the mantle and crust were focused and utilized the same conduit at the level of the Eagle deposit. Our results emphasize the fact that the sulfur isotopic values of immediate country rocks may not be appropriate as end-member values for mixing calculations; sulfur derivation from deeper in the conduit system may have involved rocks with very different sulfur isotope ratios. In addition, sulfur isotopic exchange reactions between passing magma and accumulated crystals or sulfide liquid in the conduit may have led to decreased sulfur isotope variability and δ34S values near those of mantle sulfur.