Magmatic Cu–Ni–PGE sulfide deposits often show evidence for the importance of crustal contamination processes in their genesis. The Re–Os isotopic system has proven to be a very useful tracer of contamination in these types of deposits. Previous studies of Cu–Ni mineralization in the 1.1 Ga Duluth Complex showed that massive sulfides are characterized by Os isotope ratios that indicate a high degree of crustal contamination and gamma(Os) values (percent deviation from the bulk silicate Earth value at a specific time, here 1.1 Ga) up to 1175. Sulfide and organic carbon-bearing sedimentary rocks of the Virginia Formation occur in the footwall of the Duluth Complex and as xenoliths and are likely contaminants of the mantle-derived parental magmas that produced the Complex. Re–Os isotope studies of the Virginia Formation (showed that xenoliths and rocks in the contact aureole were characterized by Os isotope ratios that were indicative of open-system processes. Curiously, this led to the interpretation that both the Duluth Complex magmas, from which the sulfide mineralization was generated, and the likely country rock contaminant were characterized by perturbed Os isotope ratios.
Williams C. D., Ripley E. M., Li C. (2009) Variations in Os isotope ratios of pyrrhotite as a result of water–rock and magma–rock interaction: Constraints from Virginia Formation–Duluth Complex contact zones. Geochimica et Cosmochimica Acta 74, 4772-4792. [PDF]
