Abstract

The investigation of this paper focused on whether the composition of hydrothermal ore-related magnetite in till could be used to locate porphyry deposits in terrane where glacial overburden overlies rocks that host porphyry Cu-Au mineralization. This hypothesis was tested using 20 till samples collected in a ~900-km2 area surrounding the Mount Polley porphyry Cu-Au deposit in south-central British Columbia, Canada. At least 100 magnetite grains were randomly selected from the magnetic fraction of each till sample. Nineteen trace elements in ~50 magnetite grains in each sample were measured by laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS). The large beam or raster size used for laser ablation (to 100 μm) homogenizes any heterogeneous trace element distributions in magnetite that result from oxy-exsolution and/or and dissolution/reprecipitation, avoiding this issue with the few micron size of an electron beam. Linear discriminant analysis (LDA) performed on a compilation of magnetite compositions measured by LA-ICP-MS from worldwide porphyry deposits and intrusive igneous rocks define the chemical signature (Mg, Al, Ti, V, Mn, Co, Ni) of hydrothermal magnetite exclusive to porphyry systems. Application of our two LDA models to the 985 magnetite compositions we measured in the till samples surrounding Mount Polley showed anomalous amounts of hydrothermal magnetite grains in till up to 2.5 km west-southwest and 4 km northwest of the deposit—a pattern that is consistent with the ice-flow history of the region. Our LDA models for magnetite trace element compositions have strong potential to be an effective tool in exploration for buried porphyry systems.

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