27This study examines trace elements in hydrothermal magnetite from five porphyry Cu-Mo-28Au deposits and two skarns in British Columbia, Canada. Trace element concentrations 29 vary several orders of magnitude both within and between magnetite from skarn and 30 porphyry deposit settings. The heterogeneous composition of hydrothermal magnetite may 31 in part be due to the short duration, low temperature and multiple-fluid events that attend 32 the formation of porphyry ore deposits. Principal component analysis shows two dominant 33 patterns of trace element abundances in hydrothermal magnetite. Firstly, positive 34 correlations of Ti, Al and V, which account for nearly 40% of the total variation in 35 magnetite, are inferred to depend on temperature and oxygen fugacity. Secondly, antithetic 36 abundances of lower valence cations (Co, Mn) with higher valence cations (Sn and Mo) 37 may reflect variations in the redox potential, acidity and metal speciation of hydrothermal 38 fluids. The Cu/Fe and Mn/Fe ratios calculated for fluids in equilibrium with the 39 hydrothermal magnetite using experimental partitioning data are similar to those measured 40 directly in brines trapped in quartz-hosted fluid inclusions from porphyry Cu-Mo-Au 41 deposits. 42 43 44 45 46 3
We investigated if the composition of hydrothermal ore-related magnetite in till could be used to locate porphyry deposits in terrain where glacial overburden overlies rocks that host porphyry Cu-Au mineralization. We test this hypothesis using 20 till samples collected in an ~900 km 2 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 Plasma-Mass 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/re-precipitation, 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 LDA models to 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
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