Magnetite as an Indicator Mineral in the Exploration of Porphyry Deposits: A Case Study in Till near the Mount Polley Cu-Au Deposit, British Columbia, Canada

Abstract

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

Trace Elements in Magnetite from the Pagoni Rachi Porphyry Prospect, NE Greece: Implications for Ore Genesis and Exploration

“…Magnetite is a common mineral in porphyry deposits. Magnetite of magmatic origin is commonly disseminated in the unaltered intrusive rocks or partly as a relict in propylitic alteration that surrounds the mineralized center [14,18]. The composition of magnetite seems to be a function of several factors including temperature, f S 2 and/or f O 2 , cooling rate, melt composition, and element partitioning between co-precipitating phases [1,2,12,13,16,68].…”

“…Based on detailed textural characterization and high-resolution in situ compositional analyses, Wen et al [17] proposed that a V/Ti versus Fe diagram should be used to discriminate between pristine igneous magnetite, re-equilibrated magnetite, and hydrothermal magnetite. Finally, Pisiak et al [18], via the use of linear discriminant analysis, calculated discriminant factors based on the composition of magnetite in barren and ore-related igneous rocks as well as hydrothermal magnetite, suggesting that the method was a useful tool when exploring for ore-related, magnetite-bearing intrusions.…”

“…Based on detailed textural characterization and high-resolution in situ compositional analyses, Wen et al [17] proposed that a V/Ti versus Fe diagram should be used to discriminate between pristine igneous magnetite, re-equilibrated magnetite, and hydrothermal magnetite. Finally, Pisiak et al [18], via the use of linear discriminant analysis, calculated discriminant factors based on the composition of magnetite in barren and ore-related igneous rocks as well as hydrothermal magnetite, suggesting that the method was a useful tool when exploring for ore-related, magnetite-bearing intrusions.Porphyry-type deposits, are not generally exposed at the surface and so, one aspect of exploring for such deposits has used potential indicator minerals such as apatite, epidote, garnet, and tourmaline as a guide (e.g., [19][20][21][22]). However, the chemical composition of igneous and hydrothermal magnetite and its association with the porphyry deposit subtypes and the magmatic affinity of the host intrusive rocks remains poorly constrained.…”

Trace elements in magnetite from porphyry Cu–Mo–Au deposits in British Columbia, Canada

“…Polley. Based on their Ti, Ni and Cr contents and the classification of Dare et al (2014), 181 more than 90% of these till grains are from igneous bedrock sources (Pisiak et al, 2014). 182…”

“…6b). For example, magnetite from the skarn deposits is impoverished in 276 these latter three elements, relative to those from porphyries and from the igneous grains 277 that make up the majority of the till (Pisiak et al, 2014). 278 279 5.…”