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North-Central Section - 50th Annual Meeting - 2016

Paper No. 15-2
Presentation Time: 1:50 PM

SULFUR ISOTOPE STUDIES OF NI-CU-PGE SULFIDE MINERALIZATION HOSTED IN METASEDIMENTARY COUNTRY ROCKS OF THE DULUTH COMPLEX, EAGLE INTRUSION AND TAMARACK IGNEOUS COMPLEX, MIDCONTINENT RIFT


PROKOPF, Derek1, SMITH, Joshua M.2, RIPLEY, Edward M.3 and LI, Chusi3, (1)Indiana University, 1001 E. Tenth St., Bloomington, IN 47408, (2)Department of Geological Sciences, Indiana University, Bloomington, IN 47408, (3)Department of Geological Sciences, Indiana University, Bloomington, IN 47405, dprokopf@indiana.edu

Country-rock hosted Ni-Cu-PGE massive sulfides are associated with mineralized mafic intrusions of the Midcontinent Rift System. Massive sulfides occur in the pelitic country rocks beneath the Duluth Complex; massive sulfides are present within the Eagle and Tamarack intrusions, as well as within the country rocks. Locally, the intrusions and the massive sulfides are separated by several meters of country rock, and an igneous origin for these massive sulfides is not intuitive. Prevailing opinion is that country-rock hosted massive sulfides leaked from the main intrusions via faults or fractures, but other genetic models propose hydrothermal or sedimentary processes.

Here we report sulfur isotope ratios of country rock-hosted mineralization, in comparison to values obtained from igneous rock-hosted mineralization. Massive sulfides beneath the Duluth Complex have d34S values from 10.2-17.4 ‰, while igneous rock-hosted mineralization has values from 6.0 to 13.0 ‰. Massive sulfides in country rocks from Eagle have tightly constrained d34S values between 2.6 to 3.6 ‰, and disseminated and net-textured mineralization have values from 0.3 to 4.6 ‰. Country rock-hosted massive sulfides at Tamarack have d34S values from 0.5 to 2.5 ‰, and igneous-hosted mineralization has d34S values from -0.2 to 2.8 ‰. Massive sulfides from Eagle and Tamarack have d34S values that are broadly similar to those of the igneous rock hosted mineralization, but massive sulfides from the Duluth Complex have a larger range of d34S than the igneous-hosted mineralization.

Our preliminary interpretations are that country-rock hosted massive sulfides from Tamarack and Eagle represent sulfides that leaked from the conduits, through unidentified plumbing systems. The massive sulfides beneath the Duluth Complex undoubtedly required input of sedimentary S, although future work will help determine whether this was by direct incorporation into the parental magma and leaking of the sulfides, or by partial melting of sulfidic country rocks. Other studies are also in progress to determine the origin of the country rock-hosted mineralization.

Session No. 15

T32. Structure, Geophysics, Petrology, and Tectonics in the Midcontinent, and Elsewhere
Monday, 18 April 2016: 1:30 PM-5:30 PM
Lincoln Room (I Hotel and Conference Center)
Geological Society of America Abstracts with Programs. Vol. 48, No. 5
doi: 10.1130/abs/2016NC-275191
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