The
Middle Proterozoic metasedimentary rocks of the Belt Supergroup contain
important metallic mineral deposits like the diagenetic sediment-hosted
stratiform copper mineralization, including the Troy (Spar
Lake) deposit and the epigenetic
hydrothermal veins in the lower Belt and Ravalli Group in the Coeur d'Alene district.
Magnetite (Fe3O4) is a common and widespread accessory
mineral in these rocks and can have multiple origins including ore stage in
deposits, mobilization stage in source rocks, metamorphism of red beds and
alteration mineral in thin halos enveloping Ag-rich veins in the Coeur d'Alene district. As
shown in several classic studies, the composition of magnetite from igneous
rocks varies significantly in response to controlling factors such as melt
composition, temperature, silica activity, and oxygen and sulphur fugacity.
Therefore, the distinctively different geologic settings within the Belt
Supergroup strongly suggest that there may be differences in the magnetite
compositions. However, initial microprobe analyses detected no variation in
magnetite composition.

Three possibilities may
give an adequate explanation for these initial results:
1.
the magnetite formed primarily as pure
Fe3O4,
2.
one or more tectonothermal events led to regional scale
re-equilibration that affected all rocks in the field area, and
3.
the variations in the composition are below the detection limit of
the microprobe.