GENESIS OF Cu-Ni SULFIDE MINERALIZATION IN THE SOUTH KAWISHIWI INTRUSION, SPRUCE ROAD AREA, DULUTH COMPLEX, MINNESOTA
INSUNG LEE* and EDWARD M. RIPLEY
Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405, U.S.A.
* Current address: Korea Basic Science Center, Isotope Research Group, Yeoeun Dong 224-1, Yusung Ku, Yusung P.O. Box 41, Taejean 305-333, Korea.
Abstract
The South Kawishiwi intrusion is one of several within the Duluth Complex, Minnesota, the principal exposed plutonic component of the 1.1 Ga Midcontinent Rift system. In the Spruce Road area, the South Kawishiwi intrusion is divisible into seven distinct troctolitic to gabbroic units, four of which contain 1 to 5 vol.% of disseminated pyrrhotite, cubanite, chalcopyrite, and pentlandite. Values of 34S in the mineralized units range from 3.8 to 10.2‰, and are distinctly different from those of nonmineralized rocks, which range from -3.4 to 2.8‰. The proposed model for the genesis of the sulfides involves the mixing of sulfur derived from two sources, accumulation of immiscible sulfide liquid, and emplacement of sulfide-saturated magmas. The relatively evolved, high-Al olivine tholeiite parental magmas of the intrusive sequence most likely resulted from fractional crystallization of a mantle- derived primary melt in a chamber in the lower crust or upper mantle. During this process, sulfide saturation would have been achieved, with chalcophile elements removed from the magma into a coexisting sulfide liquid. Emplacement of the sulfide-saturated melt into a higher-level staging chamber permitted the assimilation of sulfur from metasedimentary country-rocks. Reaction between externally derived sulfur and metals initially present in the melt as silicate, oxide, or neutral species would have produced a second generation of immiscible sulfide liquid that mixed with that of mantle origin. Sulfide-rich rocks of the South Kawishiwi intrusion represent melts derived from the high-level chamber that had experienced contamination with sulfur of crustal origin. Because of the wide range in possible contaminant 34S values (e.g., 0 to 29‰ in sulfides from metasedimentary country-rocks), it is difficult to accurately assess the proportion of country-rock sulfur present in the mineralized units, but a range of from 30 to 70% is consistent with available data. Sulfide assemblages are mineralogically zoned, such that pyrrhotite-rich layers are overlain by (cubanite + chalcopyrite)-rich layers. In situ crystallization, controlled by boundary-layer fractionation and upward expulsion of Cu- and Ni-enriched residual liquid, is proposed to explain the zonation. Upward increases in contents of Cu and Ni, as well as of incompatible elements such as Zr, Y, and P, may be controlled by a filter-pressing mechanism, or may result from the decrease in density of an interstitial liquid related to enrichment of volatiles.
Keywords: disseminated Cu-Ni mineralization, anomalous 34S values, sulfide mineral zoning, South Kawishiwi intrusion, Duluth Complex, Minnesota.