Re-Os isotopic variations in carbonaceous pelites hosting the Duluth Complex: Implications for metamorphic and metasomatic processes associated with mafic magma chambers

Previous mineralogic and isotopic studies of sulfide mineralization in the 1.1 Ga Duluth Complex have suggested that sulfur, and possibly metals, have been derived from Proterozoic sedimentary country rocks. The Re-Os isotopi c system offers great potential in the evaluation of crustally derived meta ls in magmatic ore-forming processes, but an understanding of the mechanism s that could cause isotopic variations in country rock contaminants is a pr erequisite. A suite of sulfidic and carbonaceous pelitic rocks of the Virgi nia Formation (country rock to the Duluth Complex), ranging from unmetamorp hosed through strongly recrystallized and partially melted xenoliths, have been analyzed for their Os isotopic composition. Sulfide mineral separates (mainly pyrrhotite), kerogen-graphite isolates, and whole rocks, were analy zed. Whole rocks, kerogen, and graphite from metamorphosed samples plot bet ween 1.85 Ga and 1.1 Ga chondritic reference isochrons. Sulfide separates f rom all metamorphosed samples plot nearer the chondritic 1.1 Ga reference i sochron and require exchange with a fluid or melt characterized by a chondr itic Os isotopic composition (potentially derived from the Duluth Complex). Devolatilization and partial melting may perturb isotopic systematics when accompanied by the loss of Re- and Os-bearing fluids. These processes coul d in part explain the scatter in isotopic values found in kerogen and metam orphosed whole rocks. However, metasomatic addition of Os via interaction w ith a fluid or melt is required to explain the sulfide isotopic ratios and is consistent with sulfide minerals being more susceptible to Os exchange w ith a fluid than kerogen. The relative ease with which pyrrhotite may be re set during fluid interaction, as well as the variable degrees of exchange s hown by kerogen/graphite, suggest that a knowledge of the petrologic and is otopic evaluation of country rocks is necessary before a meaningful assessm ent of the isotopic variations found in igneous rocks or associated ore dep osits can be undertaken. Copyright (C) 2001 Elsevier Science Ltd.