Partial least squares-discriminant analysis of trace element compositions of magnetite from various VMS deposit subtypes: Application to mineral exploration |
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| Institution: | 1. Département de Géologie et de Génie Géologique, Université Laval, Quebec, QC G1V0A6, Canada;2. Département de Génie Chimique, Université Laval, Quebec, QC G1V0A6, Canada;3. Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON, N2L3G1, Canada;4. Geological Survey of Canada, Ottawa, ON K1A0E8, Canada;5. Agnico Eagle Mines Limited, Val d''Or, Qc, Canada;1. Institute of Mineralogy, Ural Branch, Russian Academy of Sciences, Miass, Chelyabinsk District 456317, Russia;2. Department of Geology, South Urals State University, 8 Oktyabrya str. 16, Miass 456301, Russia;3. ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Australia;4. Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom;5. Institute of Oceanology, Russian Academy of Sciences, Nakhimosvsky av., 36, Moscow 117997, Russia;6. John de Laeter Centre, Department of Imaging and Applied Physics, Curtin University, GPO Box U 1987, Perth 6845, Western Australia, Australia;1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China;2. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;3. University of Chinese Academy of Sciences, Beijing 100049, China;1. Université du Québec À Chicoutimi, Sciences de la Terre, 555 Blvd. de l''Université, Chicoutimi, QC G7H 2B1, Canada;2. North American Palladium, Metals Exploration Division, 556 Tenth Ave., Thunder Bay, ON P7B 2R2, Canada;3. Present address: TBay Explore Inc, 1100 Memorial Ave. Ste 376, Thunder Bay, ON P7B 4A3, Canada;1. Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China;2. State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing 210093, China |
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| Abstract: | The petrography and mineral chemistry of magnetite from fifteen volcanogenic massive sulfide (VMS) deposits in Canada, and the Lasail VMS deposit in Oman, as well as from two VMS-associated banded iron formations (BIF), Austin Brook (New Brunswick, Canada) and Izok Lake (Nunavut, Canada), were investigated using optical microscopy, electron probe micro-analyzer (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The method of robust estimation for compositional data (robCompositions) was applied to investigate geochemical censored data. Among thirty-seven elements analyzed by EPMA and/or LA-ICP-MS in magnetite from the studied deposits/bedrock lithologies, only the results for Si, Ca, Zr, Al, Mg, Ti, Zn, Co and Ni contain < 40% censored values, and thus could be imputed using robCompositions. Imputed censored data were transformed using centered log-ratios to overcome the closure effect on compositional data. Transformed data were classified by partial least squares-discriminant analysis (PLS-DA) to identify different compositional characteristics of magnetite from VMS deposits and BIFs. The integration of petrography and mineral chemistry identifies three types of magnetite in VMS settings: magmatic, hydrothermal, and metamorphic. Magmatic magnetite in VMS deposit host bedrocks is characterized by ilmenite exsolution and may be overprinted by metamorphism. Some VMS deposits contain hydrothermal magnetite, which is intergrown with sulfides, and shows a metamorphic overprint as it is partly replaced by common metamorphic minerals including chlorite, sericite, anthophyllite, and/or actinolite, whereas the majority of the deposits are characterized by metamorphic magnetite formed by replacing pre-existing sulfides and/or silicates, and is intergrown with metamorphic minerals. Among VMS deposits of the Noranda mining district, the West Ansil deposit is characterized by hydrothermal-metamorphic magnetite zoned by inclusion-free cores and Si- and Mg-rich rims. Magnetite from the studied VMS-associated BIFs is also metamorphic in origin. Aluminum, Ti and Zn contents of magnetite can separate BIF from the other mineralized and un-mineralized bedrock lithologies in the studied VMS settings.PLS-DA shows that variable compositions of magnetite slightly discriminate different studied deposits/bedrock lithologies. The geochemical observations suggest that the variation in magnetite chemistry from different VMS settings might be sourced from differences in: 1) the composition and temperature of parental magmas or hydrothermal fluids, 2) the composition of host bedrocks, 3) the composition of co-forming minerals, and 4) oxygen fugacity. PLS-DA distinguishes magnetite compositions from the studied VMS deposits and BIFs from that of the other ore deposit types including Ni–Cu, porphyry Cu-Mo-Au, iron oxide-copper- gold, iron oxide-apatite, and the Bayan Obo REE-Fe-Nb deposit. Magnetite from the VMS settings on average contains lower concentrations of Si, Zr, Al, Mg, Ti, Zn, Co and Ni relative to that from the other mineral deposit types. PLS-DA of magnetite data from VMS deposits and BIFs of the Bathurst mining camp as well as PLS-DA of magnetite compositions from various mineral deposit types yield discrimination models for application to mineral exploration for VMS deposits using indicator minerals in Quaternary lithified sedimentary rocks. |
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