Partial Melting Processes and Cu-Ni-PGE Mineralization in the Footwall of the South Kawishiwi Intrusion at the Spruce Road Deposit, Duluth Complex, Minnesota

Abstract

A strong relationship was documented in the charnockitic footwall at the Spruce Road Cu-Ni sulfide deposit (South Kawishiwi intrusion, Duluth Complex) between the contact metamorphic processes, mineralogy, texture, and ore mineral assemblages. Textural evidences of partial melting, including partial melt films, granophyric intergrowth of feldspar and quartz, as well as in situ partial melt pockets and veins, indicate that partial melting occurred in the granitic footwall even up to 125 m from the footwall-intrusion contact. Temperature in the proximal 10-m segment of the footwall was around 850° to 920°C based on the observed mineral assemblage and two-pyroxene thermometry and decreases to around 700°C 100 m from the intrusion footwall contact. Partial melting of the footwall granite in this segment gave rise to sulfide liquid to sink and infiltrate into the partially molten rock. Fe- and Cu-rich sulfide mineral assemblages have been distinguished in the partially molten footwall. In the proximal 10-m part of the footwall, the sulfide assemblage is pyrrhotite and pentlandite rich, whereas in the distal part, 10 to 100 m from the contact the dominant minerals are chalcopyrite, bornite, millerite, platinum group minerals (PGMs), native gold, and other base and semimetal minerals. Systematic variation of Pd/Rh and Au/Pd ratios indicate that the formation of the sulfide mineral zonation can be interpreted with fractional crystallization of a parent sulfide liquid. Biotite, associated with the Cu-rich assemblage, indicates that fluids may have expelled from the fractionated sulfide liquid. Variation of the Cl/F ratio in the biotite and apatite that crystallized during the peak metamorphism indicates that fluids may have also played a role in the metal and sulfur transport.