Abstract

Geometry and provenances of the Belt basin at various stages of its 600-m.y. time span (between about 1,450 to 850 m.y. ago) are decipherable for the middle and upper parts of the Ravalli Group, the Helena-Wallace Formations, and the lower part of the Missoula Group. The Revett and overlying St. Regis Formations (middle and upper parts of the Ravalli Group) had a cratonic source in the south and southwest and were deposited in troughs that reflect the west-northwest trend of the Osburn fault zone (or the Lewis and Clark line whose west-central part includes the Osburn fault zone). The Spokane Formation, correlative of St. Regis, had a source in the Canadian Shield and was deposited in a north-to northwest-trending trough in northwestern Montana, west of, but approximately parallel to, the north-northwest trend of the much younger Montana disturbed belt. The Helena and Wallace Formations were deposited in a broad simple northwest-trending basin between the previous troughs, which received clastic sediments from both the east and southwest, and in which extensive carbonate deposits formed on the eastern shelf. Tectonic adjustment in early Missoula time resulted in a long low dome in the Idaho Panhandle area and in the rejuvenation of the north-northwest-trending trough in the east. Clastic sediments containing abundant hematite were deposited on Helena-Wallace carbonate rocks, and Purcell Lava was poured out in the northeast.

Anomalously high amounts of copper (100 or more ppm) are scattered throughout thousands of square miles of Belt terrane. The copper occurs in almost all formations, but it is most common in green strata. This mode of occurrence suggests a syngenetic or diagenetic origin. Stratabound copper ores, however, are known only as disseminations, discrete blebs, and veinlets in white quartzites and siltites of the Revett Formation in a “copper sulfide belt” along the northwestern Montana border. This belt is perpendicular to the Revett trough but parallel to the dome formed in early Missoula time, suggesting post-Revett epigenetic reconcentration of copper.

Geochronologic data as interpreted by several authors are in apparent conflict with geologic history of the basin as read from the sedimentation record. Many of the conflicts can be resolved by reinterpretation of existing geochronologic and geologic data. It is obvious, however, that the dating of many events, including those that may have caused migration of copper, is in need of refinement.

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