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The Galena Mine and the Coeur d'Alene district are hosted almost entirely within rocks of the Belt Supergroup, a sequence of sedimentary rocks of Middle Proterozoic Age, deposited 1.47 to 1.40 billion years ago. It occurs principally in western Montana, Idaho, and southeastern British Columbia. Lesser exposures occur in Washington, Wyoming, and Alberta. The sequence totals at least 21,000 feet in thickness in the Coeur d'Alene district.
Rocks of the Belt Supergroup are clastic sediments, with a minor component of chemical and algal (stromatolitic) dolomites. The clastic facies are dominantly clean to argillic quartzites and quartzose siltites, and argillites. These units are variously colored white, grey, purple, and black, colors which seem to be laterally persistent and thus helpful in defining stratigraphic units.
The Belt Supergroup is regionally subdivided into four units, from youngest to oldest, as shown in Table 1-1 below. In Table 1-1, and in the other tables presented in this discussion, the term "CSV property" refers to the Properties.
In Montana, the Belt rocks are not strongly metamorphosed, and are generally gently dipping, whereas in the Coeur d'Alene district they have been strongly deformed and regionally metamorphosed to lower greenschist facies. Metamorphic biotite and garnet do not occur in these rocks on the Properties, but kaolin has been converted to fine-grained sericite. Thermal metamorphism occurs locally around several Cretaceous granitic stocks in the Coeur d'Alene district, but not near the Properties.
The Coeur d'Alene district lies along the Lewis and Clark Line, a regional fracture zone extending N 020 W from Missoula, Montana to Coeur d'Alene. This fracture zone with its repeated movements has undoubtedly been of great importance in the genesis of ores in the district, and on the Properties.
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