The Sonju Lake intrusion, part of the 1.1 Ga Midcontinent rift-related Beaver Bay Complex, is a 1,200-m-thick, strongly differentiated, layered sequence of mafic cumulates located in northeastern Minnesota. Basal melatroctolite and dunite layers are overlain by troctolite, gabbro, Fe-Ti oxide-rich gabbro, apatite diorite, and monzodiorite. Stratigraphic intervals rich in Pt + Pd, Cu, and S occur over ~500 m in the Fe-Ti oxide-rich gabbro and apatite diorite units. Peak concentrations show offsets that are similar to those found in other tholeiitic layered intrusions. Concentrations of Pd in excess of 100 ppb are confined to the lowermost 25 m of the interval. Copper shows a sharp increase to 630 ppm above the Pd-rich interval. Sulfur contents are low (<375 ppm) in the Cu-rich interval, but they increase to values as high as 3,150 ppm above in the apatite diorite.
Disseminated sulfides in the intrusion have δ34S values that range from –2.2 to 3 per mil Vienna-Canyon Diablo Troilite (V-CDT) and suggest that contamination by country rock sulfur was not an important process in the formation of the metal-rich interval. δ18O values of plagioclase from the intrusion range from 5.6 to 12.0 per mil (V-SMOW) and indicate that a relatively low-18O fluid (δ18O ~3–5 ‰) interacted with the rocks of the intrusion at temperatures less than ~275°C. Clinopyroxene and Fe-Ti oxides (ilmenite with minor amounts of titanomagnetite) show much more restricted ranges in δ18O values (4.6–5.7 and 5.5–6.7 per mil, respectively) and attest to the kinetic control of the oxygen isotope exchange process. The externally derived fluid that interacted with rocks now enriched in platinum group elements (PGE) + Cu- and Fe-sulfide minerals locally liberated sulfur and replaced chalcopyrite and pyrite with goethite. In the Cu-rich zone, goethite that replaces chalcopyrite may contain up to 8.5 weight percent Cu. It is evident that hydrothermal alteration resulted in a decoupling of copper and sulfur, with sulfur being transferred out of the Cu-rich interval.
Interaction between rocks in the PGE-Cu-S interval of the Sonju Lake intrusion and an externally derived fluid at low temperatures modified what appears to have been a primary stratigraphic metal-sulfur zonation. The effects of hydrothermal alteration on PGE and base-metal sulfide mobility and redistribution must be understood before models of primary zonation processes can be meaningfully applied.