Optimal ferromagnetic fraction in till samples along ice-flow paths: case studies from the Sue-Dianne and Thompson deposits, Canada

Abstract

Iron oxides are minerals resistant to chemical alteration and mechanical abrasion, and which have ferromagnetic properties and a range of chemical compositions. These characteristics are useful as indicator minerals in exploration, for example using till in glaciated terrains. Iron oxide proportions, grain size, and chemical composition of till samples collected near the Sue-Dianne Cu-Au-Ag IOCG deposit in the Great Bear magmatic zone (Northwest Territories, Canada) and magmatic Ni-Cu deposits in the Thompson Nickel Belt (Manitoba, Canada) show that subsamples containing c. 100 grains from the 0.25–1.0 mm grain size ferromagnetic fraction yield a representative mineralogical and compositional range of oxide grains from a till sample. Subsamples with less than 100 grains yield statistically less representative data. The 1–2 mm grain size fraction typically contains too few iron oxide grains and thus using this fraction is not statistically representative. The composition of iron oxides from eight till and five bedrock samples was determined along transects up- and down-ice of the Cu-Au-Ag Sue-Dianne IOCG deposit. At, and immediately down-ice of, the deposit, hematite is the principal oxide and shows dominant BIF and IOCG chemical signatures in the Ca+Al+Mn v. Ti+V discriminant diagram. Up-ice and farther down-ice of the deposit, magnetite and titanomagnetite are the dominant oxides and magnetite shows dominant Kiruna and IOCG signatures. The composition of iron oxides from six till samples along a north–south transect and 11 till samples from a 180 km-long east–west transect, along the older and younger directions of ice-flow, respectively, was determined in the Thompson Nickel Belt (Manitoba, Canada). The proportion of magnetite in till with the signature of Ni-Cu deposits increases for at least 1 km south of the Pipe Ni-Cu deposit along the direction of the older southward ice flow, whereas the glacial dispersal of magnetite with a chemical signature typical of Ni-Cu deposits was limited during the younger westerly ice flow.