MAGNETITE IN THE BELT SUPERGROUP

In the Lemhi subbasin of the Belt basin near Salmon, Idaho, magnetite is concentrated in lower members of the Apple Creek Formation: sands of the Lem Peak and correlative Jahnke Lake members, mud-cracked deposits of the coarse siltite member, and diamictite. Large positive aeromagnetic anomalies conform to their exposure distribution, indicating that magnetite is characteristic of those units. Median magnetic susceptibility (MS) in the coarse siltite member is 6.9 (x10^-3 SI; n=375). Values from 0.3 to 48 across a magnetite-rich 1-meter interval were mostly conformable, consistent with variation during deposition. Magnetite is absent or sparse in overlying units of pinch and swell, sand, silt, and crinkle-cracked mud of the banded siltite member (median MS = 0.21, n=307), and biotite-rich sand of the Lake Mountain and Trapper Gulch members (median MS = 0.20, n=59). We consider those to have been deposited in ever deepening waters. Notably, the Iron Creek Cu-Co deposit is at the contact between the magnetite-rich and magnetite-poor strata. Some magnetite is clearly detrital as it, or hematite, which may have replaced it, is present in laminae with other heavy minerals such as zircon. Elsewhere it is disseminated and less clearly detrital, possibly having formed by metamorphism of hematite or Fe hydroxides. SEM results indicate that the fine-grained layers of the banded siltite member are largely biotite. Geochemical data are consistent with similar iron content in both the coarse and banded siltite members. If all the sediment had a common source, absence of magnetite in the banded siltite unit may reflect conversion of original magnetite to other Fe-bearing minerals such as biotite. Alternatively, magnetite may have been concentrated in the shallow to subaerial deposits when transport energy waned but was still strong enough to carry detrital biotite and/or Fe-bearing clay minerals into deeper environment of the banded siltite and higher members. Although strata of the main Belt basin units in northern Idaho have much lower magnetite content (median MS = 0.14, n=208), the Ravalli Group and its correlative in Canada have the highest magnetite content and are like the lower Apple Creek strata in that deposition was in a shallow water to subaerial environment.