The Significance of Partial Melting Processes in Hydrothermal Low Sulfide Cu-Ni-PGE Mineralization Within the Footwall of the Sudbury Igneous Complex, Ontario, Canada

Abstract

We present new results and review existing data highlighting different aspects of the genetic relationship between partial melting and hydrothermal mineralizing processes in the contact aureole of the Sudbury Igneous Complex. At the basal contact of the igneous complex, in the footwall breccia, crystallized partial melt pods and veins, referred to as footwall granophyres are abundant and intrude all rock types including the breccia matrix, as well as massive Ni-Cu sulfide ore. The final crystallization of these melts was accompanied by the segregation of high salinity fluids dominantly in the temperature range of 450° to 550°C, as revealed by Ti-inquartz thermometry and studies on primary fluid inclusions. In mineralized parts of the footwall breccia there is clear evidence that interaction of partial melts, exsolved fluids, and preexisting magmatic sulfides occurred. In the deeper footwall of the Sudbury Igneous Complex, similar footwall granophyres occur as networks of veins intruding impact brecciated (Sudbury breccia) country rocks and have an intimate spatial association with hydrothermal, low sulfide assemblages highly enriched in Pt, Pd, and Au (with a unique assemblage of PGM, including malyshevite and lisiguangite). It is suggested that partial melting processes were widespread in the contact and proximal footwall environment of the igneous complex and they were important in providing high salinity magmatic fluids to a hydrothermal system responsible for redistribution of base and precious metals. We describe observations which may evidence the migration of partial melts from the contact into the deeper footwall. Furthermore, we suggest a cogenetic model for the precipitation of sulfide-hydrous silicate assemblages in close spatial association to footwall granophyres. We emphasize that the use of footwall granophyres in mineral exploration for Cu-Ni-PGE ores in the footwall of the Sudbury Igneous Complex is clearly justified. Mapping of footwall granophyre vein networks highlights areas that were proximal to the contact environment and point out structural pathways that may have been used by syn- or postgenetic mineralizing fluids in a given area. Widespread partial melting appears to have occurred in the footwall of other large mafic-ultramafic complexes (such as Bushveld and Duluth) and similar genetic relationships of partial melting and ore-forming hydrothermal processes may have possibly existed also in these systems.