Summary:
The Marathon deposit is hosted by the Two Duck Lake (TDL) gabbro, a late intrusive phase of the Eastern Gabbro.
The TDL gabbro is the host rock for the Marathon deposit. It occurs as a massive and poorly layered unit approximately 50 to 250 m thick that strikes near north for >6 km and in general dips west at 5° to 45°.
The Marathon deposit is one of several mafic to ultramafic intrusive bodies in the mid-continent rift systems (MRS) that host significant copper, nickel or PGM sulphide mineralization. These intrusions are the Yellow Dog peridotite (Eagle deposit), the Tamarack Intrusive Complex (Tamarack deposit), the Current Lake Intrusive Complex (Thunder Bay North deposit), and the numerous intrusions located along the base of the Duluth Complex.
Mineralized Zones
The Marathon deposit consists of several large, thick, and continuous zones of disseminated sulphide mineralization hosted within the TDL gabbro. The mineralized zones occur as shallow-dipping sub-parallel lenses that follow the basal gabbro contact and are labelled as footwall, main, and hangingwall zones, and the W-Horizon. The Main Zone is the thickest and most continuous zone. For 393 drill hole intersections with mineralized intervals >4 m thick, the average thickness is 42.8 m and the maximum thickness is 205.1 m.
Sulphide Mineralization
Sulphides in the TDL gabbro consist predominantly of chalcopyrite, pyrrhotite and minor amounts of bornite, pentlandite, cobaltite, and pyrite. The sulphides occur in between primary silicates and to a less extent in association with secondary calcite and hydrous silicates, such as chlorite and serpentine (Watkinson and Ohnenstetter, 1992). Chalcopyrite occurs as separate grains or as replacement rims on pyrrhotite grains. Some chalcopyrite is intergrown with highly calcic plagioclase (An70 to An80) in replacement zones at the margins of plagioclase crystals (Good and Crocket, 1994).
The modal mineralogy of a composite sample that is representative of the Marathon deposit mineralization (and TDL gabbro) was determined in a QEMSCANTM survey by XPS (Kormos, 2008). A total of nine aliquots of material were analyzed. In decreasing order of abundance, the sulphide component of the composite sample consists of 2.75% pyrrhotite, 0.79% Cu-Fe sulphides (chalcopyrite and bornite), 0.09% pentlandite, and trace amounts of pyrite, galena, and sphalerite.
The relative proportions of pyrrhotite and chalcopyrite vary significantly across the Marathon deposit. In general, the sulphide assemblage changes gradually up the section from the base to the top of mineralized zones. Sulphides at the base of the TDL gabbro consist predominantly of pyrrhotite and minor chalcopyrite. The relative proportion of chalcopyrite increases up section to nearly 100% chalcopyrite near the top. In the W-Horizon, sulphides consist mainly of chalcopyrite and bornite and minor to trace amounts of pentlandite, cobaltite, pyrite, and pyrrhotite.
There is a relationship between mineralization and the paleo-topography of the footwall contact. For example, mineralization is best developed within basins or troughs (b and c) of the footwall and thins or pinches out above prominent footwall ridges. Although the mineralized zones are almost continuous from the north to south extents of the Marathon deposit, assays with the best grades (combined Pd+Cu recalculated and presented as NSR), plot along trends that mimic the alignment of troughs or ridges.