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TitleGrain size fraction and number of grains representative of iron oxide composition in till samples: the Sue-Dianne deposit area, Northwest Territories, Canada
AuthorSappin, A -A; Dupuis, C; Beaudoin, G; McMartin, I
SourceGeological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Abstracts Volume vol. 35, 2012 p. 123
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20120035
MeetingGAC-MAC Meeting; St-John's; CA; May 27-29, 2012
ProvinceNorthwest Territories
AreaMazenod Lake; Sarah Lake
Lat/Long WENS-117.0000 -116.5000 64.0000 63.7500
Subjectseconomic geology; surficial geology/geomorphology; grain size analyses; grain size distribution; mineral deposits; mineral occurrences; iron oxides; copper; gold; till analyses; tills; till samples; till geochemistry; Sue-Dianne Deposit
ProgramGEM: Geo-mapping for Energy and Minerals, Iron-oxide Copper-gold (IOCG) / Multiple Metals - Great Bear Lake (NWT)
AbstractThe magnetite to hematite-group Cu-Ag-(Au) Sue-Dianne IOCG deposit, located in the southern part of the Great Bear magmatic zone within the Bear Structural Province of the Canadian Shield, was chosen as a test site to optimize preparation methods for electron microprobe analysis of the ferromagnetic fraction of till samples. The objectives are to determine the optimum grain size fraction and number of grains to measure the composition of magnetite and hematite in the ferromagnetic fraction of till samples. Eight till samples were selected to provide a cross-section from up-ice, proximal to, and down-ice from the Sue-Dianne deposit. For each till sample, the ferromagnetic fraction was sieved in three size fractions: 1) <0.25 mm; 2) 0.25-1 mm; 3) 1-2 mm. The intermediate fraction (0.25-1 mm) was subdivided into sub-samples containing about 200, 100, 50, and 10 grains. The ferromagnetic fractions include magnetite, titano-magnetite, ilmenite, hematite, and other non-ferromagnetic oxides, silicates, and sulfides. At the deposit, hematite is the dominant oxide, whereas up-ice and down-ice of the deposit magnetite and titano-magnetite are the dominant oxides. The grains of the <0.25 mm size fractions are difficult to analyze with the microprobe because their size is too small to obtain accurate results. The 1-2 mm size fraction typically contains less than 110 grains such that analytical results are difficult to compare between samples. The 0.25-1 mm size fraction contains a large number of grains at all sample locations. The time and cost of analyzing 200 grains of the 0.25- 1 mm size fraction limits the number of samples that can be routinely analyzed. Our results show that sub-samples from the 0.25-1 mm size fraction, containing circa 100 grains, yield a representative composition of magnetite contained in till samples. The mineral proportions in the subsamples from the coarse and the intermediate fractions with 50 and 10 grains are variable and, generally, not consistent with the proportions determined for the 100 and 200 grains intermediate fractions. Sub-sample summary statistics of the composition of magnetite show that ca. 100 grains subsamples give the most reliable results. In the Ni/(Cr+Mn) vs. Ti+V and Ca+Al+Mn vs. Ti+V discriminant diagrams, the average magnetite composition of sub-samples from the coarse fraction, and from the 50 and 10 grains intermediate fraction show significant variations, supporting the use of the 100 grains intermediate fraction as the most appropriate sub-fraction to represent the average iron oxide composition of a sample.