Abstract:
Magnetite (Fe₃O₄) is a common accessory mineral in igneous, metamorphic, and sedimentary rocks and an indispensable target for geochemical and petrogenetic studies. Many studies have used magnetites magnetic properties and investigated its geochemical behaviour in igneous environments. However, hydrothermal magnetite, has not yet received rigorous investigation. Many of the physicochemical factors that control variations in minor and trace element contents in igneous magnetite also apply to hydrothermal systems and lead to characteristic trace element signatures depending on the individual formation conditions. Advances in analytical techniques such as laser ablation ICP-MS allow rapid in-situ minor and trace element analysis at a high resolution. My research presents new methods and standards for laser ablation ICP-MS of magnetite. In combination with factor analysis this presents a novel and powerful tool in using the geochemistry of magnetite to fingerprint geological settings such as hydrothermal ore deposits. Electron microprobe, laser ablation ICP-MS and oxygen isotope data of selected case studies in the western United States demonstrate that hydrothermal magnetite is susceptible to its geological environment and formation conditions. The most important factors that govern compositional variations in hydrothermal magnetite are fluid composition, oxygen and sulphur fugacity, host rock buffering, re-equilibration processes and intrinsic crystallographic controls are. I present evidence that matrix mineral inclusions in magnetite can provide valuable genetic information for provenance studies. The geochemistry of magnetite provides useful information about the metamorphic and hydrothermal evolution of specific geological settings such as metamorphic terranes or hydrothermal ore deposits and can also assist exploration geologists in targeting prospective areas.