Geochemistry of Magnetite from Hydrothermal Ore Deposits and Host Rocks of the Mesoproterozoic Belt Supergroup, United States

摘要

Magnetite (Fe_3O_4) is a common and widespread accessory mineral in many host rocks and mineral deposits. We used electron microprobe analysis (EMPA), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis, and oxygen isotope analysis to test whether magnetite from the five following geologic settings in western Montana and northern Idaho has distinct geochemical signatures: (1) greenschist facies burial metamorphic rocks of the Middle Proterozoic Belt Supergroup, (2) sediment-hosted stratiform Cu-Ag deposits (Spar Lake and Rock Creek) in Belt Supergroup metasedimentary rocks, (3) hydrothermal Ag-Pb-Zn veins of the Coeur d'Alene district, (4) extensively deformed and partially altered Belt Supergroup host rocks from the Coeur d'Alene district, and (5) two Cretaceous postmetamorphic igneous intrusions. EMPA results show that magnetite from each of these five settings is essentially pure Fe_3O_4, but LA-ICP-MS analyses results show that magnetite from these five settings has trace element concentrations that generally vary over less than one order of magnitude. These magnetite occurrences show subtle compositional differences that generally correlate with temperatures, as determined by oxygen isotope geothermometry. Burial metamorphic magnetite