Stratigraphy of the Proterozoic Revett Formation, Coeur d'Alene District, Idaho

Abstract

The Proterozoic Revett Formation of the Belt Supergroup contains three informal members that can be identified throughout the Coeur d'Alene mining district of northern Idaho. The lower Revett Formation is dominated by quartzite, but also contains intervals of siltite. The middle Revett consists predominantly of siltite, though quartzite and argillite locally form significant intervals. The upper Revett consists of intervals of quartzite that alternate with intervals of siltite and/or thin-bedded argillite. These units show dramatic changes in thickness and sedimentary facies within the Coeur d'Alene mining district; changes that are more abrupt and extreme than seen elsewhere in the Belt basin. The regionally significant Osburn fault bisects the district, with 20 to 30 km of post-mineralization right-lateral strike-slip offset. South of this fault, the upper Revett is 640 m thick at the Bunker Hill mine in the west, 450 to 500 m thick in the centrally located Silver Belt, and over 550 m thick at the Reindeer Queen deposit to the east. North of the Osburn fault, the upper Revett is approximately 120 m thick in the vicinity of the Lucky Friday mine, but abruptly thins to 45 to 90 m to the north and northeast, in the southern end of the western Montana copper sulfide belt. The middle Revett Formation south of the Osburn fault appears to be 400 to 450 m thick. North of the Osburn Fault, the middle Revett thins to approximately 120 m in the Lucky Friday area, and to approximately 60 m at Military Gulch. The lower Revett Formation is approximately 1650 m thick south of the Osburn fault, but thins to 400 to 450 m thick to the north of the Osburn fault. Observed thickness changes support previous hypotheses that the current Osburn fault coincides with a Proterozoic synsedimentary fault that controlled sedimentation in this region.