| Title | Origin of chromitites in the Ring of Fire Part II: trace element fingerprinting of contaminants |
| Download | Download (whole publication) |
| Author | Brenan, J M; Woods, K; Mungall, J E; Weston, R |
| Source | Targeted Geoscience Initiative 5, Grant Program interim reports 2018-2019; by Benn, D; Brenan, J M; Fuller, K; Grondahl, C; Layton-Matthews, D; Leybourne, M I; Linnen, R; Martins, T; Milidragovic, D;
Moynihan, D P; Mungall, J E; Nixon, G T; Padget, C D W; Pattison, D R M; Rempel, K U; Scanlan, E J; Scoates, J S; Tsay, A; Voinot, A; Van Wagoner, N A; Weston, R; Williams-Jones, A E; Woods, K; Zajacz, Z; Geological Survey of Canada, Open File 8620,
2019 p. 5-18, https://doi.org/10.4095/314998 (Open Access) |
| Year | 2019 |
| Publisher | Natural Resources Canada |
| Document | open file |
| Lang. | English |
| Media | on-line; digital |
| Related | This publication is contained in Benn, D; Brenan, J M;
Fuller, K; Grondahl, C; Layton-Matthews, D; Leybourne, M I; Linnen, R; Martins, T; Milidragovic, D; Moynihan, D P; Mungall, J E; Nixon, G T; Padget, C D W; Pattison, D R M; Rempel, K U; Scanlan, E J; Scoates, J S; Tsay, A; Voinot, A; Van Wagoner, N
A; Weston, R; Williams-Jones, A E; Woods, K; Zajacz, Z; (2019). Targeted Geoscience Initiative 5, Grant Program interim reports 2018-2019, Geological Survey of Canada, Open File 8620 |
| File format | pdf (Adobe® Reader®) |
| Province | Ontario |
| NTS | 43C/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 |
| Subjects | economic geology; geochemistry; Science and Technology; mineral deposits; chromite; mineral exploration; ore mineral genesis; mineralization; 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; magmas; emplacement; fluid dynamics;
cumulus processes; silicates; olivine; gallium geochemistry; zinc geochemistry; vanadium geochemistry; fractional crystallization; host rocks; alteration; thin section microscopy; Archean; Ring of Fire Intrusive Suite; Black Thor Deposit; Big Daddy
Deposit; Blackbird Deposit; Black Label Deposit; Superior Province; ore systems approach; partitioning; banded iron formations (BIF); Precambrian |
| Illustrations | location maps; geoscientific sketch maps; photographs; plots |
| Program | Knowledge Management Coordination, Targeted Geoscience Initiative (TGI-5) |
| Released | 2019 08 29 |
| Abstract | To expand on the existing database documenting the trace element composition of ROFIS chromites, and to constrain models of their origin, a total of 45 chromite-bearing samples from the Black Thor, Big
Daddy, Blackbird, 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. A search of the element
partitioning literature has shown that of the elements that are readily detectable in the chromites, Ga, Zn and V show contrasting behaviour in their olivine- and chromite-melt partitioning. Specifically, these elements are ambivalent (D ~ 1, Zn) to
moderately incompatible in olivine (V, Ga), but moderately compatible in chromite (D~3-6), and hence their behaviour in magmas will depend on the relative proportions of the two phases that are crystallizing. Simple fractional crystallization models
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 komatiite magma crystallizing olivine and chromite in normal cotectic
proportion, and no evidence of the strong depletion in these elements expected for chromite-only crystallization. The spread in the Ga-V data can be explained if the magma responsible for chromite formation has assimilated up to ~10% of wall-rock
banded iron formation, or granodiorite, or up to 50% of metasediment. The Zn-Ga variation amongst the RoFIS chromitites shows large variation, well outside that expected for the extent of crystallization or assimilation that can reproduce the V-Ga
array. This is interpreted to be the consequence of both the availability and mobility of zinc at subsolidus conditions. A filter applied to the data to exclude the most silicate-rich, and hence 'zinc exchangeable' samples, as well as those with
visible alteration, results in a small subset of samples whose zinc contents are consistent with BIF assimilation, excluding granodiorite or metasediment as important contaminants, at least for the samples considered. Despite the evidence for
contamination, results suggest that the RoFIS chromitites crystallized from normal cotectic proportions of olivine to chromite, and therefore no specific causative link between contamination and chromitite formation. The specific fluid dynamic regime
during magma emplacement may therefore be responsible for crystal sorting and chromite accumulation. Ongoing work will be to conduct laboratory experiments to better constrain the partitioning of Ga, V and Zn between olivine, chromite and komatiite
at conditions relevant to genesis of the RoFIS to further refine the crystallization models developed thus far. |
| GEOSCAN ID | 314998 |
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