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TitleMagnetite as an indcator mineral in porphyry Cu±Au±Mo deposits of British Columbia, Canada
AuthorCanil, D; Pisiak, L; Lacourse, T; Plouffe, AORCID logo; Ferbey, T; Grondahl, C
SourceIndicator minerals in till and stream sediments of the Canadian Cordillera; by Ferbey, T (ed.); Plouffe, AORCID logo (ed.); Hickin, A S (ed.); Geological Association of Canada, Special Paper vol. 50, 2017 p. 161-174
Alt SeriesNatural Resources Canada, Contribution Series 20180026
Alt SeriesMineralogical Association of Canada, Topics in Mineral Sciences 47
PublisherGeological Association of Canada
PublisherMineralogical Association of Canada
RelatedThis publication is contained in Indicator minerals in till and stream sediments of the Canadian Cordillera
ProvinceBritish Columbia
NTS82; 83; 84; 92; 93; 94; 102; 103; 104; 114
Lat/Long WENS-141.0000 -113.0500 60.0000 48.0000
Subjectseconomic geology; surficial geology/geomorphology; mathematical and computational geology; mineralogy; mineral deposits; mineral exploration; prospecting techniques; drift prospecting; porphyry deposits; copper; gold; molybdenum; glacial deposits; tills; till samples; magnetite; hydrothermal systems; bedrock geology; lithology; igneous rocks; intrusive rocks; trace element analyses; mass spectrometer analysis; thermal analyses; statistical analyses; discriminant analyses; petrographic analyses; modelling; anomalies; glacial history; glaciation; deglaciation; ice flow; Canadian Cordillera; Mount Polley Mine; Mount Polley Intrusive Complex; Intermontane Belt; Stikinia Terrane; Quesnellia Terrane; Insular Belt; Wrangellia Terrane; ice-flow directions; Phanerozoic; Cenozoic; Quaternary; Mesozoic
Illustrationsgeoscientific sketch maps; photographs; photomicrographs; plots
ProgramTargeted Geoscience Initiative (TGI-5) Porphyry systems
ProgramTargeted Geoscience Initiative (TGI-4)
ProgramNSERC Natural Sciences and Engineering Research Council of Canada
Released2017 01 01
AbstractGlacial overburden can overlie large areas of Mesozoic intrusive igneous rocks in the Canadian Cordillera that potentially host porphyry Cu-Au mineralization. Magnetite is an accessory mineral in many such deposits, and once eroded from these sources by glacial action and transport, may serve as a useful indicator mineral for prospecting in glaciated terrain. Previous work shows that magnetite from ore deposits has a trace element signature that is unique from that of a common accessory mineral in igneous or metamorphic rocks. To this end we review the geochemical characteristics of magnetite of hydrothermal origin from porphyry deposits in British Columbia. We observe much heterogeneity at the grain and deposit scale for many trace elements (Mo, Cu, Sn, Mn, V, Sc, Ni) in magnetite from five porphyry deposits. Hydrothermal magnetite is consistently low in elements that are temperature-sensitive in the spinel structure (Ti, Al, Mg), and is rich in Sn and sometimes Mo, relative to Sc. A linear discriminant analysis (LDA) of magnetite compositions compiled from worldwide porphyry deposits and intrusive igneous rocks (n=481) permits a rigorous definition of the chemical signature of hydrothermal magnetite from such systems in terms of its Mg, Al, Ti, V, Mn, Co, and Ni contents.
We review an application of magnetite composition as a vector to locate porphyry deposits in glaciated terrain, using 20 subglacial till samples collected in a 900 km2 area surrounding the Mount Polley porphyry Cu-Au deposit, in south-central British Columbia. Twenty trace elements were measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in 50 magnetite grains from each till sample. Application of our LDA models to magnetite populations in the till samples shows that anomalous concentrations of hydrothermal magnetite grains are detected in till up to 2.5 km west-southwest and 4 km northwest of the Mount Polley deposit. The dispersal direction of these grains is consistent with the ice-flow history of the region. The use of LDA to identify hydrothermal magnetite has a strong potential to be an effective indicator in exploration for buried porphyry systems in British Columbia and elsewhere. Further tests of this exploration method should be performed around other known sources of porphyry Cu±Au±Mo mineralization with different ice-flow histories. In addition, improvements to the LDA models we developed for hydrothermal magnetite identification will be possible with more analyses from other porphyry deposits.

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