Principal component analysis of magnetite composition from volcanogenic massive sulfide deposits: Case studies from the Izok Lake (Nunavut, Canada) and Halfmile Lake (New Brunswick, Canada) deposits

Abstract

Magnetite grains from the Izok Lake (Nunavut, Canada) and the Halfmile Lake (New Brunswick, Canada) volcanogenic massive sulfide deposits, and from till covering the nearby areas were investigated using the scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and optical microscopy. The method of robust estimation for compositional data (rob-composition) was applied to censored geochemical data, and the results were analyzed by principal component analysis (PCA). Textural relationships and mineral association of magnetite reveal the history of formation, and contribute to the explanation of characteristic compositional differences of magnetite from different geological settings. The integration of petrography and mineral chemistry allows discriminating magmatic, metamorphic and hydrothermal magnetite grains in the VMS deposits bedrock samples. Magmatic magnetite is found in Izok Lake gabbro, and Halfmile Lake syenite, felsic ash tuff and gossan samples, whereas magnetite in Izok Lake massive sulfides, gahnite-rich dacite and iron formations formed during the amphibolite facies metamorphism. In Halfmile Lake andesite, magnetite recrystallized during greenschist facies metamorphism. In the magnetite alteration zone associated to the Halfmile Lake deposit, hydrothermal magnetite has been overprinted by metamorphic magnetite. Halfmile Lake massive sulfides in chloritic argillite contain hydrothermal magnetite.PCA identifies discriminator elements and their contributions to magnetite composition from different Izok Lake Lake and Halfmile Lake bedrock samples. The results suggest that Si, Ca, Zr, Al, Ga, Mn, Mg, Ti, Zn, Co, Ni and Cr are discriminator elements for VMS deposits and their host bedrocks. The distinct chemical signatures for magnetite from various bedrock lithologies demonstrate that magnetite grains of the same origin share more similarities in chemistry, as high Ti indicates magmatic sources for magnetite, whereas high Si, Ca and Mg are indicative of hydrothermal settings. Variable compositions of metamorphic magnetite suggest that the chemistry of this type of magnetite is controlled by the composition of host rocks, the grade of metamorphism and oxygen fugacity. PCA of EPMA and LA-ICP-MS data of magnetite from the Izok Lake and Halfmile Lake bedrock samples yield discrimination models for classification of magnetite grains from till. Decreases in the proportion of magnetite grains with the chemical signature of the Izok Lake massive sulfides and gahnite-rich dacite down-ice from the Izok Lake deposit show the use of magnetite chemistry in geochemical exploration. In the Halfmile Lake area, till magnetite grains with the signature of VMS mineralization make a glacial dispersal train more than 2km down-ice from the deposit.