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TitleOrigin of chromitites in the Esker Intrusive Complex, Ring of Fire Intrusive Suite, as revealed by chromite trace element chemistry and simple crystallization models
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LicencePlease note the adoption of the Open Government Licence - Canada supersedes any previous licences.
AuthorBrenan, J M; Woods, K; Mungall, J E; Weston, R
SourceTargeted Geoscience Initiative 5: grant program final reports (2018-2020); by Targeted Geoscience Initiative Coordination Office; Geological Survey of Canada, Open File 8755, 2021 p. 25-43, Open Access logo Open Access
PublisherNatural Resources Canada
Documentopen file
Mediaon-line; digital
RelatedThis publication is contained in Targeted Geoscience Initiative 5: grant program final reports (2018-2020)
RelatedThis publication is related to Origin of chromitites in the Ring of Fire Part II: trace element fingerprinting of contaminants
File formatpdf; xlsx (Microsoft® Excel®)
NTS43C/03; 43C/04; 43C/05; 43C/06; 43C/11; 43C/12; 43C/13; 43C/14; 43D/01; 43D/02; 43D/03; 43D/06; 43D/07; 43D/08; 43D/09; 43D/10; 43D/11; 43D/14; 43D/15; 43D/16; 43E/01; 43E/02; 43E/03; 43E/06; 43E/07; 43E/08; 43F/03; 43F/04; 43F/05; 43F/06
Lat/Long WENS -87.3167 -85.3833 53.3500 52.1500
Subjectseconomic geology; geochemistry; tectonics; Science and Technology; Nature and Environment; mineral deposits; chromite; mineral exploration; ore mineral genesis; mineralization; crystallization; fractional crystallization; fluid dynamics; modelling; bedrock geology; lithology; igneous rocks; intrusive rocks; chromitites; granodiorites; volcanic rocks; komatiites; sedimentary rocks; iron formations; metamorphic rocks; metasedimentary rocks; trace element geochemistry; major element geochemistry; cumulus processes; silicates; olivine; vanadium geochemistry; gallium geochemistry; chromium geochemistry; aluminum geochemistry; iron geochemistry; magnesium geochemistry; host rocks; tectonic setting; magmatism; emplacement; Archean; Ring of Fire Intrusive Suite; Esker Intrusive Complex; Black Thor Deposit; Big Daddy Deposit; Blackbird Deposit; Black Label Deposit; Superior Province; Precambrian
Illustrationslocation maps; geoscientific sketch maps; photographs; photomicrographs; plots; tables
ProgramTargeted Geoscience Initiative (TGI-5) Knowledge Management Coordination
Released2021 10 27
AbstractTo better constrain the origin of the chromitites associated with the Esker Intrusive Complex (EIC) of the Ring of Fire Intrusive Suite (RoFIS), a total of 50 chromite-bearing samples from the Black Thor, Big Daddy, Blackbird, and Black Label chromite deposits have been analysed for major and trace elements. The samples represent three textural groups, as defined by the relative abundance of cumulate silicate phases and chromite. To provide deposit-specific partition coefficients for modeling, we also report on the results of laboratory experiments to measure olivine- and chromite-melt partitioning of V and Ga, which are two elements readily detectable in the chromites analysed.
Comparison of the Cr/Cr+Al and Fe/Fe+Mg of the EIC chromites and compositions from previous experimental studies indicates overlap in Cr/Cr+Al between the natural samples and experiments done at >1400oC, but significant offset of the natural samples to higher Fe/Fe+Mg. This is interpreted to be the result of subsolidus Fe-Mg exchange between chromite and the silicate matrix. However, little change in Cr/Cr+Al from magmatic values, owing to the lack of an exchangeable reservoir for these elements. A comparison of the composition of the EIC chromites and a subset of samples from other tectonic settings reveals a strong similarity to chromites from the similarly-aged Munro Township komatiites.
Partition coefficients for V and Ga are consistent with past results in that both elements are compatible in chromite (DV = 2-4; DGa ~ 3), and incompatible in olivine (DV = 0.01-0.14; DGa ~ 0.02), with values for V increasing with decreasing fO2. Simple fractional crystallization models that use these partition coefficients are developed that monitor the change in element behaviour based on the relative proportions of olivine to chromite in the crystallizing assemblage; from 'normal' cotectic proportions involving predominantly olivine, to chromite-only crystallization. Comparison of models to the natural chromite V-Ga array suggests that the overall positive correlation between these two elements is consistent with chromite formed from a Munro Township-like komatiitic magma crystallizing olivine and chromite in 'normal' cotectic proportions, with no evidence of the strong depletion in these elements expected for chromite-only crystallization. The V-Ga array can be explained if the initial magma responsible for chromite formation is slightly reduced with respect to the FMQ oxygen buffer (~FMQ- 0.5), and has assimilated up to ~20% of wall-rock banded iron formation or granodiorite.
Despite the evidence for contamination, results indicate that the EIC chromitites crystallized from 'normal' cotectic proportions of olivine to chromite, and therefore no specific causative link is made between contamination and chromitite formation. Instead, the development of near- monomineralic chromite layers likely involves the preferential removal of olivine relative to chromite by physical segregation during magma flow. As suggested for some other chromitite-forming systems, the specific fluid dynamic regime during magma emplacement may therefore be responsible for crystal sorting and chromite accumulation.
Summary(Plain Language Summary, not published)
The Targeted Geoscience Initiative (TGI) is a collaborative federal geoscience program that provides industry with the next generation of geoscience knowledge and innovative techniques, which will result in more effective targeting of buried mineral deposits. This compendium is the result of the TGI Grant Recipients 2018-2020.

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