Abstract

The Water Hen intrusion is a small (2 miles long, 1 mile wide) body of layered rocks which has been intruded into the basal portion of the Duluth Complex, near Hoyt Lakes, Minnesota. Cyclical units of mineral-graded layers are well developed in the intrusion and range from dunites at their bases through troctolites to anorthosites at their tops. Contaminated igneous rocks and associated footwall inclusions are found predominantly in the lower third of the intrusion. The contaminated zones contain cordierite, orthopyroxene, aluminous spinel, graphite, and sulfides.Cu-Ni sulfide mineralization occurs mainly in a basal dunite unit with lesser amounts found in contaminated dunites and peridotites associated with mineral-graded units higher in the intrusion. Pyrrhotite, chalcopyrite, cubanite, and pentlandite are the essential sulfide minerals.Sulfur isotopic values determined on sulfides in metasedimentary footwall rocks give delta 34 S values of around +18 per mil while those sulfides associated with contaminated rocks in the intrusion give delta 34 S values of about +15 per mil. Sulfides associated with the basal dunite, not obviously contaminated, give delta 34 S values of about +11 per mil.The occurrence of footwall inclusions and contaminated rocks in the intrusion taken together with sulfur isotopic evidence suggests that part and perhaps a large portion of the sulfur in the intrusion was derived from the metasedimentary footwall rocks.

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