T151 Geology and Fluid Dynamics of Cr, Ni-Cu-PGE, and PGE Mineralized Magmatic Plumbing Systems

Tuesday, 6 November 2018
8:00 AM - 12:00 PM
Room 233

S, PB, AND OS ISOTOPIC STUDIES OF MASSIVE NI-CU±PGE SULFIDES HOSTED IN METASEDIMENTARY COUNTRY ROCKS OF THE MIDCONTINENT RIFT SYSTEM, USA

Paper number: T151-12
Program: Technical Sessions
Time: 10:55 AM – 11:10 AM

Authors

Abstract

Country rock-hosted Ni-Cu±PGE massive sulfides are found near mafic-ultramafic intrusions of the 1.1 Ga Midcontinent Rift. Massive sulfides occur in Proterozoic country rocks beneath the sheet-style Duluth Complex and adjacent to and below the conduit-style Eagle and Tamarack intrusions. Locally, the intrusions and the massive sulfides are separated by several meters of country rock, and, until now, an igneous origin for the massive sulfides has not been confirmed.

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-320745

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