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TitleCharacterization of fluorapatite within iron oxide alkali-calcic alteration systems of the Great Bear Magmatic Zone: a potential metasomatic process record
AuthorNormandeau, P X; Harlov, D E; Corriveau, LORCID logo; Paquette, J; McMartin, IORCID logo
SourceCanadian Mineralogist vol. 56, no. 2, 2018 p. 167-187,
Alt SeriesEarth Sciences Sector, Contribution Series 20150365
PublisherMineralogical Association of Canada
Mediapaper; on-line; digital
File formatpdf; html
ProvinceNorthwest Territories
NTS85N; 86C; 86D; 86E; 86F; 86K; 86L
AreaGreat Bear Lake
Lat/Long WENS-120.0000 -116.0000 67.0000 63.0000
Subjectseconomic geology; mineralogy; geochemistry; mineral exploration; exploration methods; mineral deposits; iron oxides; apatite; copper; gold; ore mineral genesis; mineral assemblages; alteration; metasomatism; thermal history; magmatism; fluid dynamics; crystallization; remobilization; precipitation; leaching; allanite; monazite; manganese geochemistry; thin section microscopy; scanning electron microscope analyses; electron probe analyses; mass spectrometer analysis; petrographic analyses; thorium geochemistry; uranium geochemistry; textural analyses; bedrock geology; lithology; igneous rocks; intrusive rocks; granitic rocks; mafic intrusive rocks; intrusions; sills; mineral zoning; Canadian Shield; Great Bear Magmatic Zone; MacTavish Supergroup; Hottah Terrane; Mag Hill mine; Terra mine; JLD showing; Fab system; Brooke Zone deposit; Dennis showing; Precambrian
Illustrationslocation maps; geoscientific sketch maps; tables; photomicrographs; graphs; geochemical plots
ProgramTargeted Geoscience Initiative (TGI-4) Uranium Ore Systems
Released2018 03 31
AbstractFluorapatite-amphibole-magnetite assemblages are a signature alteration product in iron oxide alkali-calcic alteration (IOAA) systems that can host iron oxide-apatite (IOA) and iron oxide Cu-Au (IOCG) deposits. The distinct fluorapatite chemistry that evolves within these systems can record metasomatic processes within IOA and IOCG deposits. Rare earth element (REE)-rich fluorapatite forms during high-temperature sodic-calcic-ferric/ferrous and calcic-ferric/ferrous metasomatism. As temperature declines and the fluid chemistry evolves, localized REE leaching takes place within fluorapatite and leads to secondary allanite and monazite crystallization. Such processes have been observed experimentally and inferred in IOA deposits (e.g., Kiruna district in Sweden, Bafq district in Iran). In the present study, fluorapatite grains in thin section and hand-picked from disaggregated bedrock samples from a number of areas including the Fab system, Brooke Zone, Mag Hill, the JLD showing, the Terra mine, and the Dennis showing from the Great Bear magmatic zone (Northwest Territories, Canada) are examined. Dark irregular zones observed under scanning electron microscope backscattered images have lower REE contents when measured using electron probe microanalyzer and/or laser ablation inductively coupled plasma mass spectrometry. REE remobilization and precipitation of secondary allanite and monazite during metasomatic alteration of the fluorapatite is inferred based on petrographic evidence and dissolution pits in the REE-depleted zones of the fluorapatite. Allanite is the most common secondary REE-rich phase and can be explained by the predominance of calcic alteration during the low-temperature alteration. Fluorapatite grains with Mn content above 500 ppm and where LREE leaching occurred have green or green to yellow cathodoluminescence responses. Single grains of fluorapatite also record an atypical decoupling of U and Th interpreted to be caused by potassic-ferric/ferrous alteration. This metasomatism leads to fluorapatite grains with localized increases in Th content near fractures and edges (e.g., from ~50 to ~1000 ppm). The identification of these textural characteristics, geochemical signatures, and cathodoluminescence responses in fluorapatite from the systems covered by this study is a significant first step in the development of an indicator mineral exploration method for prospecting for IOA and IOCG deposits in the Great Bear magmatic zone and beyond.
Summary(Plain Language Summary, not published)
This NRCan contribution of the Targeted Geoscience Initiative program is part of a series of scientific papers submitted for a special issue on polymetallic iron oxide deposits of Missouri and the Northwest Territories organized by USGS and Geological Survey of Canada scientists. The research topic arises from results of regional till survey in the Great Bear Lake region under the Geomapping for Energy and Minerals program. The research was conducted in collaboration with the Northwest Territories Geological Survey, private sector (Fortune Minerals Ltd), the Tlicho community of Gamètì, and academia. The paper address the signature of minerals rich in rare-earth elements (fluorapatite, monazite, etc.) as a signature alteration product of iron oxide alkali-alteration system and their oxide-apatite and iron oxide copper-gold deposits.

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