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TitleMagnetite in hydrothermal alteration associated to IOCG systems in the Great Bear Magmatic Zone, Canada
AuthorDe Toni, A F; Corriveau, L; Montreuil, J -F
SourceGeological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Programs with Abstracts vol. 35, 2012 p. 1
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20120053
MeetingThe Geological Association of Canada (GAC) and the Mineralogical Association of Canada (MAC)Joint Annual Meeting; St. John's, NL; CA; May 27-29, 2012
ProvinceNorthwest Territories
NTS85N; 86C; 86D; 86E; 86F; 86K; 86L
AreaGreat Bear Lake; Lac la Martre; Hottah Lake
Lat/Long WENS-119.0000 -116.0000 67.0000 63.0000
Subjectseconomic geology; mineral occurrences; mineralization; iron ores; iron oxides; gold; copper; magnetite; hydrothermal alteration; alteration; hydrothermal deposits; Great Bear Magmatic Zone
ProgramGEM: Geo-mapping for Energy and Minerals, Iron-oxide Copper-gold (IOCG) / Multiple Metals - Great Bear Lake (NWT)
AbstractMagnetite is a minor to major constituent of veins, breccias and incipient to intense alteration in iron oxide-copper gold systems of the Great Bear magmatic zone, Northwest Territories. Prevalence of magnetite in most of the known IOCG systems makes it a key mineral to understand and monitor their evolution and development. Multiple magnetite paragenesis associated with numerous textures are observed in these magmatic/hydrothermal systems. The macroscopic and microscopic observation of magnetite-bearing paragenesis is used to understand the processes that have generated the spectrum of textures observed. Mineral assemblages and associated textures can be used as exploration vectors to mineralized zones so distinctions between different paragenesis are essential in the comprehension of IOCG systems. To show the numerous aspects that can exhibit magnetite-associated alteration, a protocol for regional to megascopic description of replacements, veins and breccias has been developed to facilitate the description of these rocks in the field and subsequently on stained and unstained rock slabs and thin sections.
Prevailing paragenesis consists of 1) magnetite (Fe alteration), 2) amphibole-magnetite ± apatite ± albite (high temperature Ca-Fe±Na alteration), 3) K-feldspar-magnetite ± biotite or biotite-magnetite(high temperature K-Fe alteration), with common overprint by hematite. Magnetite replacement is preferentially formed around phenocrysts, within vesicules, in groundmass, or replaces fragments in breccias. These habits suggest that magnetite replacement forms where fluids circulate through porous and permeable rocks and/or minerals. In bedded or layered rocks, magnetite alteration can form selective stratabound replacements of specific horizons. Magnetite also crystallize as breccia cement or fill veins. High temperature K-Fe alteration crystallizes magnetite with K-feldspar in felsic and intermediate igneous rocks, and biotite in siliciclastic sedimentary rocks and mafic rocks. In potassic-altered porphyritic volcanic rocks, K-feldspar first replaces the groundmass and then the phenocrysts with increasing alteration intensity whereas biotite replaces selectively pre-existing amphiboles. Replacements and veins of transitional high temperature Ca-K-Fe and subsequent high temperature K-Fe alteration are commonly followed by the mineralization stage, in which a wide variety of metal can be concentrated (e.g. Cu, Au, Ag, Bi, Co, etc.). Each type of paragenesis and textures exert a control on the aspect that will take subsequent alteration. As an example, amphibole-magnetite veins preferentially cross-cut albitized zone at the expense of the unaltered volcanic rocks. Ultimately, the main objective of the project is to produce an atlas of alteration associated to IOCG that will provide exploration tools and a framework for geologists during the exploration of under-explored terranes.