The role of black shales as a source of sulfur and semimetals in magmatic nickel-copper deposits: Example from the Partridge River Intrusion, Duluth Complex, Minnesota, USA

The basal unit of the Duluth Complex (Minnesota, USA) contains Ni-Cu sulfide deposits. The S in these is thought to be derived from a sulfide-rich black shale unit known as the Bedded Pyrrhotite Unit, a stratigraphic unit within the Virginia Formation host rocks. However, the mechanism of S transfer has not been clearly established. In order to understand how this transfer occurs we have undertaken petrography and whole rock geochemistry of the rocks from the contact aureole and the basal unit. In the contact aureole, the Bedded Pyrrhotite Unit consists of a very fine-grained graphitic shales with thin beds of sulfides consisting of pyrrhotite with minor chalcopyrite (<1%). The basal unit contains numerous Bedded Pyrrhotite Unit xenoliths surrounded by norites. The Bedded Pyrrhotite Unit xenoliths are partially melted and the sulfide beds are disrupted. Leucosomes are present and these contain blebs of sulfides consisting of pyrrhotite, pentlandite, cubanite and chalcopyrite. In the mafic rocks surrounding the xenoliths small patches of sulfide bearing leucosome are found. In addition to being rich in S the Bedded Pyrrhotite Unit is rich in As 38 ppm, Sb 4.1 ppm and Bi 0.6 ppm and Te 0.4 ppm and has high delta S-34 values. The delta S-34, As/S, Bi/S and Sb/S decrease with distance from the xenoliths. Similarly, the Ni/S, Cu/S, Se/S and (platinum-group elements)/S ratios are higher in the mafic rocks and increase with distance from the xenoliths. Our model proposes that droplets of sulfide melt derived from the Bedded Pyrrhotite Unit xenoliths were entrained in the anatectic silicate melt of the xenoliths and transferred to the mafic magma. The sulfide droplets equilibrated with the mafic magma. Those close to the xenoliths did not have the opportunity to react with a large quantity of magma, and hence their composition is similar to the sulfides of the Bedded Pyrrhotite Unit, i.e., rich in semimetals and poor in Ni, Cu and PGE. Farther away from the xenoliths, the sulfide droplets could have reacted with more magma, and the composition of these sulfides approach that of sulfides derived mainly from mafic magma. (C) 2016 Elsevier B.V. All rights reserved.