Geochemistry of magnetite from hydrothermal ore deposits and their host rocks in the Proterozoic Belt Supergroup, USA

Magnetite (Fe3O4) is a common and widespread accessory mineral in many host rocks and mineral deposits. We used electron microprobe (EM) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analyses to test whether magnetite from four different geological settings in western Montana and northern Idaho has distinct geochemical signatures: (1) greenschist fades burial metamorphic rocks of the Middle Proterozoic Belt Supergroup, (2) sediment-hosted stratiform copper deposits in Belt Supergroup sedimentary rocks, (3) hydrothermal Ag-Pb-Zn veins of the Coeur d'Alene (CDA) district that occur in Belt Supergroup host rocks, and (4) Cretaceous post-metamorphic igneous intrusions. EM results show no significant deviation from stoichiometric magnetite for samples from any of the four occurrences. The lower detection limits afforded by LA-ICP-MS analyses show that magnetite from these four occurrences have very low trace element contents. Factor analysis reveals at least two distinct magnetite varieties: (1) magnetite with an Mg-Mn factor from the Coeur d'Alene district and igneous intrusions, and (2) magnetite with a V-Co-Zn factor from a number of burial metamorphic rocks, as well as from the igneous intrusions and the sediment-hosted Cu-Ag deposits of the Belt Supergroup.