S, PB, AND OS ISOTOPIC STUDIES OF MASSIVE NI-CU±PGE SULFIDES HOSTED IN METASEDIMENTARY COUNTRY ROCKS OF THE MIDCONTINENT RIFT SYSTEM, USA
Massive sulfides beneath the Duluth Complex have δ34S of 10.2-17.4 ‰. Disseminated igneous-hosted sulfides have δ34S from 6.0 to 13.0 ‰. Massive sulfides in country rocks at Eagle have δ34S between 2.6 and 3.6 ‰. Igneous-hosted sulfides have δ34S from 0.3 to 4.6 ‰. Country rock-hosted massive sulfides at Tamarack have δ34S from 0.5 to 2.5 ‰, and igneous-hosted sulfides have δ34S between -0.2 and 2.8 ‰. Country rock-hosted massive sulfides from all three sites have near-zero Δ33S. Massive sulfides from Eagle and Tamarack have identical δ34S to samples of igneous-hosted mineralization, but massive sulfides from Duluth have heavier δ34S values than igneous-hosted sulfides. Re-Os compositions of the massive sulfides at Eagle and Tamarack (average γOs1100Ma= +28 and +22, respectively) are virtually identical to igneous-hosted sulfides (average γOs1100Ma= +35 and +25, respectively). At Duluth, country rock-hosted massive sulfides are more radiogenic, with γOs1100Ma from +800 to +1360. Limited analyses of igneous-hosted sulfides have γOs1100Ma of +3 and +557.
Previous S and Os analyses of igneous-hosted sulfides from Tamarack and Eagle have suggested that sulfide saturation was achieved by crustal contamination of mantle melts by Proterozoic country rocks. The identical S and Os isotopes between country rock-hosted massive sulfides and igneous-hosted sulfides at Tamarack and Eagle indicate that the country rock-hosted sulfides are leaked igneous sulfide liquids. At Duluth, crustal contamination has been well documented by S, Pb, and Re-Os studies of igneous sulfides. Massive sulfides near Duluth are interpreted as leaked igneous sulfide liquids, but the heavier S and more radiogenic Pb and Re-Os compositions of the massive sulfides beneath the Duluth Complex suggest the sulfide liquid continued to incorporate sedimentary S after emplacement.