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Cordilleran geochronology deduced from hydrothermal leads Small, William David


A total of 34 lead ore samples from selected hydrothermal deposits in Wyoming, Montana, and Idaho have been isotopically analyzed and geochronological interpretations made from the results. Leads from along the southeastern flank of the Idaho Batholith appear to have a primary component 2500 my old. Leads from Butte and Cassia counties, Idaho, may be interpreted as having this same primary component with an added component that is estimated to be 1400 to 1600 my old. The radiogenic component of leads along the southeastern flank of the Idaho Batholith commenced development in a closed system 2500 my ago. Radiogenic components of the leads from Butte and Cassia counties commenced development 1900 and 2700 my ago respectively. Preliminary results of analyses from the south end of the Wind River Mtns, Wyoming, and the Little Belt Mtns, Montana, show primary lead ages of about 3200 and 2200 my respectively. Common lead geochronology indicates that the basement rocks of Southern Idaho may be assigned to the Superior Province of North America as defined by Kanasewich (1965). A second Precambrian event was recorded by a change in the lead isotope abundances during the Penokian era. Thus, Southern Idaho had been subjected to several uplifts during parts of Early and Middle Precambrian time. The ages of the anomalous leads from Butte and Cassia counties could represent the times of formation of sedimentary layers which remained closed systems until the time of formation of the ore bodies. A model for continental accretion and growth is discussed. The contribution of the present report to tectonic development models is in the suggestion of a geological sequence which may give rise to anomalous lead suites. This geological sequence is concerned with regional tectonic events which take place in the lower crust and are manifested by igneous activity. Examples of leads with apparent enrichment in the 208 isotope were found during this study and other instances are mentioned. The enrichment is tentatively considered to occur as a result of concentration of the thorium decay product in sedimentary basins. This could occur if the thorium is in more easily weathered minerals than are the uranium isotopes. Evidence supporting naturally occurring lead isotope enrichment phenomena is cited.

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