The mineralogy and chemistry of heavy mineral concentrates recovered from in-stream sediments has only more recently received attention, especially compared to heavy minerals recovered from till (e.g. Ottesen and Theobald 1994 Leybourne and Goodfellow 2003 Leybourne and Johannesson 2008). Stream sediment geochemistry has a long history of use in mineral exploration because of the relative ease of sample collection and because stream sediments represent a time-integrated signal of both mechanical and hydromorphic dispersion (e.g. Supplementary material: Laser ablation data for major, minor and trace elements in magnetite from bedrock and stream sediment samples from Casino are available at Thematic collection: This article is part of the Applications of Innovations in Geochemical Data Analysis collection available at: We show that the chemical compositions of magnetite recovered from stream sediments can be used to explore for porphyry systems. Copper in magmatic-hydrothermal magnetite, present as minute inclusions of sulfide minerals such as chalcopyrite or substituted within the magnetite crystal lattice, is a strong indicator of Cu mineralization. Magmatic-hydrothermal magnetite with a trace element composition similar to that from the potassic alteration zone at Casino is identifiable in stream sediments draining the deposit. Ni/Cr are used to discriminate between magmatic-hydrothermal magnetite from the potassic alteration zone and igneous magnetite from granodiorite and quartz monzonite hosting the deposit.

Linear discriminant analysis of 12 elements (Mg, Al, Ti, V, Mn, Co, Cr, Ni, Cu, Zn, Ga and Ge) and plots of Ti v. The trace element composition of detrital magnetite grains recovered from six local streams around the Casino high-grade porphyry Cu–Au–Mo deposit, west-central Yukon, is compared with igneous and magmatic-hydrothermal magnetite recovered from mineralized and unmineralized host rocks at the deposit.

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