RE-OS ISOTOPIC EVIDENCE FOR AN ENRICHED-MANTLE SOURCE FOR THE NORILSK-TYPE, ORE-BEARING INTRUSIONS, SIBERIA

Magmatic Cu-Ni sulfide ores and spatially associated ultramafic and ma fic rocks from the Noril'sk I, Talnakh, and Kharaelakh intrusions are examined for Re-Os isotopic systematics. Neodymium and lead isotopic d ata also are reported for the: ultramafic and mafic rocks. The Re-Os d ata for most samples indicate closed-system behavior since the ca. 250 Ma igneous crystallization age of the intrusions. There are small but significant differences in thr: initial osmium isotopic compositions of samples from the three intrusions. Ores from the Noril'sk I intrusi on have gamma(Os) values that vary from +0.4 to +8.8, but average +5.8 . Ores from the Talnakh intrusion have gamma(Os) values that range fro m +6.7 to +8.2, averaging +7.7. Ores from the Kharaelakh intrusion hav e gamma(Os) values that range from +7.8 to +12.9, with an average valu e of +10.4. The osmium isotopic compositions of the ore samples from t he Main Kharaelakh orebody exhibit minimal overlap with those for the Noril'sk I and Talnakh intrusions, indicating that these Kharaelakh or es were derived from a more radiogenic source of osmium than the other ores. Combined osmium and lead data for major orebodies in the three intrusions plot in three distinct fields, indicating derivation of osm ium and lead from at least three isotopically distinct sources. Some o f the variation in lead isotopic compositions may be the result of min or lower-crustal contamination. However, in contrast to most other iso topic and trace element data, Os-Pb variations are generally inconsist ent with significant crustal contamination or interaction with the sub continental lithosphere. Thus, the osmium and lead isotopic compositio ns of these intrusions probably reflect quite closely the compositions of their mantle source, and suggest that these two isotope systems we re insensitive to lithospheric interaction. Ultramafic and mafic rocks have osmium and lead isotopic compositions that range only slightly b eyond the compositions of the ores. These rocks also have relatively u niform epsilon(Nd) values that range only from -0.8 to +1.1. This limi ted variation in neodymium isotopic composition may reflect the charac teristics of the mantle sources of the rocks; or it may indicate that somehow similar proportions of crust contaminated the parental melts. The osmium, lead, and neodymium isotopic, data for these rocks most cl osely resemble the mantle sources of certain ocean island basalts (GIB ), such as some Hawaiian basalts. Hence, these data are consistent wit h derivation of primary melts from a mantle source similar to that of some types of hotspot activity. The long-term Re/Os enrichment of this and similar mantle sources, relative to chondritic upper mantle, may reflect (1) incorporation of recycled oceanic crust into the source mo re than 1 Ga ago, (2) derivation from a mantle plume that originated a t the outer core-lower mantle interface, or(3) persistence of primordi al stratification of rhenium and osmium in the mantle.