Re-Os, Sm-Nd, and Pb isotopic constraints on mantle and crustal contributions to magmatic sulfide mineralization in the Duluth Complex

Previous petrologic and stable isotopic studies of sulfide mineralization i n the Duluth-Complex have led to the premise that sulfide genesis is strong ly linked to the interaction between mantle-derived magmas and sedimentary country rocks in a rift zone environment. In order to more fully evaluate t he nature of this interaction, and to gain an insight into the possible imp ortance of externally derived metals in the ore-forming process, Pb, Sm-Nd, and Re-Os isotopic studies of the Babbitt Cu-Ni deposit were initiated. Ro ck-types examined include low-sulfide troctolite that show very little petr ologic evidence for contamination, disseminated sulfide-bearing troctolitic to gabbroic rocks that occur close to the basal contact with country rocks and contain metasedimentary xenoliths, and massive sulfide. Lead isotopic values of whole rocks, plagioclase mineral separates, and massive sulfides show only subtle differences, and are compatible with 3 to 5% contamination of a mantle-derived melt with a Proterozoic crustal contaminant. epsilon(N d) (1.1 Ga) values of the troctolite and gabbro samples are chondritic, and only the massive sulfides show strong evidence for contamination based on Sm-Nd isotopic values. Massive sulfides tend to be more sensitive indicator s of contamination in the Sm-Nd system because of late-stage incorporation of a light rare earth element-rich fluid into a Ca-PO4 (apatite) component of the immiscible sulfide Liquid. gamma(Os) (1.1 Ga) values are also strong ly anomalous, and range from similar to 500 to 1200 in disseminated sulfide -bearing troctolites and massive sulfides. These values are also consistent with from 1 to 3% contamination by C-rich and strongly radiogenic Proteroz oic sedimentary rocks. Elemental and isotopic mass balance calculations sug gest that up to 50% of the Pb and at least 35% of the Os in the sulfide min eralization have been derived from external sources. A corollary is that ot her metals may also be in part derived from external sources, which would a id in explaining the compositional diversity of the sulfide mineralization. Selective assimilation of country rocks by mantle-derived magmas may occur in crustal staging chambers, during ascent to shallower levels, or in situ as a result of devolatilization reactions and partial melting. Copyright ( C) 1998 Elsevier Science Ltd.