首页|Magnetite as an indicator mineral in porphyry Cu+-Au+-Mo deposits of British Columbia, Canada
Magnetite as an indicator mineral in porphyry Cu+-Au+-Mo deposits of British Columbia, Canada
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Glacial overburden can overlie large areas of Mesozoic intrusive igneous rocks in the Canadian Cordillera that potentially host porphyry Cu-Au mineralization. Magnetite is an acceBory mineral in many such deposits, and once eroded from these sources by glacial action and transport, may serve as a useful indicator mineral for prospecting in glaciated terrain. Previous work shows that magnetite from ore deposits has a trace element signature that is unique from that of a common acceBory mineral in igneous or metamorphic rocks. To this end we review the geochemical characteristics of magnetite of hydrothermal origin from porphyry deposits in British Columbia. We observe much heterogeneity at the grain and deposit scale for many trace elements (Mo, Cu, Sn, Mn, V, Sc, Ni) in magnetite from five porphyry deposits. Hydrothermal magnetite is consistently low in elements that are temperature-sensitive in the spinel structure (Ti, Al, Mg), and is rich in Sn and sometimes Mo, relative to Sc. A linear discriminant analysis (LDA) of magnetite compositions compiled from worldwide porphyry deposits and intrusive igneous rocks (n=481) permits a rigorous definition of the chemical signature of hydrothermal magnetite from such systems in terms of its Mg, Al, Ti, V, Mn, Co, and Ni contents.We review an application of magnetite composition as a vector to locate porphyry deposits in glaciated terrain, using 20 subglacial till samples collected in a 900 km2 area surrounding the Mount Polley porphyry Cu-Au deposit, in south-central British Columbia. Twenty trace elements were measured by laser ablation inductively coupled plasma maB spectrometry (LA-ICP-MS) in 50 magnetite grains from each till sample. Application of our LDA models to magnetite populations in the till samples shows that anomalous concentrations of hydrothermal magnetite grains are detected in till up to 2.5 km west-southwest and 4 km northwest of the Mount Polley deposit. The dispersal direction of these grains is consistent with the ice-flow history of the region. The use of LDA to identify hydrothermal magnetite has a strong potential to be an effective indicator in exploration for buried porphyry systems jn British Columbia and elsewhere. Further tests of this exploration method should be performed around other known sources of porphyry Cu+-Au+-Mo mineralization with different ice-flow histories. In addition, improvements to the LDA models we developed for hydrothermal magnetite identification will be poBible with more analyses from other porphyry deposits.
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