Summary:
The Marathon deposit is one of several mafic to ultramafic intrusive bodies in the Mid-continent Rift System that host significant copper, nickel or Platinum Group Metals sulphide mineralization. These intrusions include the Yellow Dog peridotite (Eagle Deposit), the Tamarack Deposit, the Current Lake Intrusive Complex (Thunder Bay North Deposit), and the numerous intrusions located along the base of the Duluth Complex.
The intrusion and deposition of sulphides within magma conduits has recently been accepted as the dominant mineralization process chosen to explain rift related deposits and has been proposed for the Marathon, Thunder Bay North and the Eagle Deposits. The magma conduit model has grown in favour since it was proposed to explain deposits in the Noril’sk region and the deposits at Voisey’s Bay, Newfoundland and Labrador, Canada.
Comparisons between the Mid-continent Rift System and the Voisey’s Bay and Noril'sk settings point to several similarities that suggest that the Mid-continent Rift System is a likely setting for Ni-Cu mineralization. The continental rifting and associated voluminous igneous activity in all three regions formed in response to the rise of a hot plume of mantle material from deep in the Earth, fracturing the overlying continental crust. In the Mid-continent Rift System, melting of the plume produced more than 2 million cubic km of mostly basalt lava flows and related intrusions.
The Marathon Property is situated along the eastern margin of the Proterozoic Coldwell Complex, which is part of the Keweenawan Supergroup of igneous, volcanic and sedimentary rocks.
The Marathon Deposit is hosted by the Two Duck Lake Gabbro (“TDL Gabbro”), a late intrusive phase of the Eastern Gabbro. The Eastern Gabbro is a composite intrusion and occurs along the northern and eastern margin of the CC, which intrudes the much older Archean Schreiber-Hemlo Greenstone Belt. The entire CC is believed to have intruded over a relatively short period of time between 1108 and 1094 Ma.
The Marathon deposit consists of several large, thick and continuous zones of disseminated sulphide mineralization hosted within the Two Duck Lake Gabbro. The mineralized zones occur as shallow dipping sub-parallel lenses that follow the basal gabbro contact and are labeled as footwall, main, hanging wall zones and the W-Horizon. The Main Zone is the thickest and most continuous zone. For 418 drill hole intersections with mineralized intervals greater than 4 m thick, the average thickness is 42 m, and the maximum is 205 m.
Sulphides in the Two Duck Lake Gabbro consist predominantly of chalcopyrite, pyrrhotite and minor amounts of bornite, pentlandite, cobaltite and pyrite. The proportions of sulphide minerals as determined in a QEMSCAN survey of a bulk sample are 2.75% pyrrhotite, 0.79% copper-iron 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; however, in general, the sulphide assemblage changes gradually up section from the base to the top of mineralized zones. Sulphides at the base of the Two Duck Lake Gabbro consist predominantly of pyrrhotite and minor chalcopyrite but 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. In general, the variations in the chalcopyrite to pyrrhotite ratio across the deposit, and from bottom to top of the deposit, correlates with variations in the copper/palladium ratio, with the highest concentrations of palladium occurring in samples with copper-rich sulphide assemblages.
The model that best explains the Marathon deposit is based on the accumulation of sulphides in basins and troughs of a magma conduit which underwent significant upgrading of copper and Platinum Group Metals content by the process of multistage dissolution grading that was described for similar disseminated mineralization in the Noril'sk region, Russia by Kerr and Leitch (2005).
In addition to the Marathon deposit, the Property hosts other Platinum Group Metals deposits / mineralization in four additional areas – Geordie, Sally, Boyer and Four Dams.