Carbon isotopic studies of metasedimentary and igneous rocks at the Babbitt CuNi deposit, Duluth complex, Minnesota, U.S.A.

Abstract

Graphitic carbon is a common component of sulfide-bearing igneous rocks of the Babbitt CuNi deposit (Duluth Complex), as well as pelitic metasediments (Virginia Formation) found within the contact aureole or as xenoliths. δ 13C-values of graphite in metasediments of low- to moderate-CaO content range from −34 to −27%. These values are very similar to those of organic carbon in mildly metamorphosed Virginia Formation, which range from −33.6 to -27.9%o. Conversion of organic carbon to graphite has occurred without significant isotopic fractionation. Devolatilization reactions produced a fluid rich in H 2O with X CH 4 / X CO 2 ≥ 1. Carbonate carbon in the Virginia Formation is characterized by δ 13C-values ranging between -21.4 and -15.7%o for high-CaO pelites and calc-silicates. These values are similar to those of carbonate in the underlying Biwabik Iron Formation, but are lighter than limestone ( -7.9%o) found in the upper portion of the Virginia Formation. Although isotopic exchange between reduced organic carbon and carbonate is probable, the carbonates provide a second reservoir of isotopically light carbon in the sedimentary sequence. Graphite in the igneous rock has a δC signature varying between -36.5 and -19.6%o, whereas carbonate carbon is characterized by values from -20.2 to -12.1%o. The light isotopic values, in conjunction with the distribution of carbon in the igneous sequence, suggest that Duluth Complex melts were contaminated by carbon derived from country rocks. The wide range in δC-values, as well as erratic spatial distribution, may be due to: (1) incorporation of a country-rock-derived fluid phase whose isotopic composition was determined by the relative contribution of carbonate vs. organic carbon; (2) mixing of varying proportions of magmatic and sedimentary carbon; and (3) oxidation-reduction redated fractionation between carbon in solid, melt and fluid phases.