Abstract

The Dunka Road deposit is one of a series of Cu-Ni occurrences located within troctolitic rocks of the southern portion of the Duluth Complex, Minnesota. Sulfide minerals, principally pyrrhotite, chalcopyrite, cubanite, and minor pentlandite occur interstitially with silicate minerals and average about 2 volume percent in ore-grade horizons. Areas of sulfide concentrations in excess of 1 to 2 percent occur erratically throughout the troctolitic sequence but are in general concentrated near the basal contact with rocks of the metasedimentary Virginia Formation.The delta 34 S values of sulfides in the troctolites range from 0.2 to 15.8 per mil, with an average value near 7.5 per mil. Pyrrhotite in the underlying Virginia Formation hornfels and pyrite from unmetamorphosed Virginia Formation are characterized by a similar range in delta 34 S values. The high positive delta 34 S values and similarity to values found in the Virginia Formation strongly suggest that most of the sulfur in the Dunka Road deposit was derived from the breakdown of pyrite in the Virginia Formation. The sulfur isotopic compositions of minerals in the troctolites show no consistent lateral or vertical trends and are in general characterized by a high degree of variability. Such fluctuations in delta 34 S values indicate that sulfur isotopic homogenization was not attained throughout the troctolitic sequence.Several mechanisms may have contributed to the variations in delta 34 S values found within the troctolites, including mixing of sedimentary and igneous sulfur, assimilation of sedimentary sulfur characterized by variable delta 34 S values, fractionation caused by fluctuations in oxidation states during sulfur volatilization and assimilation, and periodic introduction of isotopically distinct magma batches. In light of the erratic sulfide mineral and delta 34 S distributions, the predominant mechanism is thought to be the introduction of both isotopically and spatially variable country rock sulfur into crystallizing troctolitic magma.

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