Preliminary geology of the Blacktail Mountain drilling site, Flathead County, Montana

Open-File Report 79-938




Five argillitic green beds that alternate with six argillitic purple beds in the upper part of the Spokane Formation of the Belt Supergroup of Proterozoic Y age were sampled by 22 shallow core holes. The holes were drilled on a hexagonal pattern over an area about 50 m wide and 150 m long to obtain detailed information on environment of deposition and distribution of anomalous amounts of copper sulfides visible in outcrop, as well as to obtain samples for study of the geochemistry, isotope geology, mineralogy, and physical properties of the rocks and sulfide occurrences. Preliminary megascopic examination of the cores suggests that the purple and green colors represent original oxidizing and reducing environments of deposition on a relatively stable tidal flat and shallow shelf. Two of the five green beds contain zones of copper sulfides, largely bornite and chalcocite with trace amounts of chalcopyrite and covellite. The zones vary somewhat in thickness and stratigraphic position within the mineralized green beds. This report covers only the preliminary results of the ongoing multiple intensive studies of the drilling site and cores. These studies are part of an attempt to develop a model for green-bed copper- silver occurrences in Belt rocks of Proterozoic age. The model is needed to aid appraisal of the mineral potential of these mineral occurrences that are common in much of the 120,000 km2 of Belt terrane.

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Preliminary geology of the Blacktail Mountain drilling site, Flathead County, Montana
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Open-File Report
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U.S. Geological Survey,
ii, 36 p., 1 over-size sheet :ill. ;28 cm.