Marathon-Coldwell Complex


In November of 2010, Stillwater Mining Company completed the acquisition of the Marathon PGM-copper project.  The Marathon project’s PGM-copper resource is situated on one of Canada’s largest PGM-copper geologic provinces.  The Marathon PGM-copper deposit is hosted by the Eastern Gabbro Series of the Proterozoic Coldwell Complex, a large intrusive body that bisects the Archean aged Schreiber-Hemlo Greenstone Belt. The Coldwell is a composite magmatic intrusion and the largest alkaline intrusive complex in North America. It is approximately 25 km in diameter with a surface area of 580 square kilometers. The Coldwell Complex was emplaced in three nested intrusive cycles introduced sequentially during cauldron subsidence near where the northern end of a regional-scale fault system intersected Archean rocks, near the north shore of Lake Superior.

Mineralization at the Marathon PGM-copper deposit is related to a large magmatic system consisting of three or more cross-cutting intrusive olivine gabbro units that comprise the Eastern Gabbro Series of the Coldwell Complex. In order of intrusion, the three gabbroic units consist of Layered Gabbro Series, Layered Magnetite Olivine Cumulate (LMOC) and Two Duck Gabbro (TD Gabbro). Late quartz syenite and augite syenite dikes cut all of the gabbros but form a minor component of the intrusive assemblage. The TD Gabbro is the dominant host rock for PGM-copper mineralization and is a principal focus of exploration in the district. The mineralized zones occur as shallow dipping sub parallel lenses that follow the basal gabbro contact and form distinct mineral horizons designated as Footwall, Main, Hanging Wall and W-Horizon. The Main Zone is the thickest and most continuous zone. Additional accumulations of PGM-copper mineralization are associated with LMOC and occur in the hanging wall of the deposit.