Lead Isotope Systematics and Uranium Depletion in the Granite Mountains, Wyoming

Research Article| March 01, 1973 Lead Isotope Systematics and Uranium Depletion in the Granite Mountains, Wyoming J. N. ROSHOLT; J. N. ROSHOLT 1U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar R. E. ZARTMAN; R. E. ZARTMAN 1U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar I. T. NKOMO I. T. NKOMO 1U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information J. N. ROSHOLT 1U.S. Geological Survey, Denver, Colorado 80225 R. E. ZARTMAN 1U.S. Geological Survey, Denver, Colorado 80225 I. T. NKOMO 1U.S. Geological Survey, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (3): 989–1002. https://doi.org/10.1130/0016-7606(1973)84<989:LISAUD>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation J. N. ROSHOLT, R. E. ZARTMAN, I. T. NKOMO; Lead Isotope Systematics and Uranium Depletion in the Granite Mountains, Wyoming. GSA Bulletin 1973;; 84 (3): 989–1002. doi: https://doi.org/10.1130/0016-7606(1973)84<989:LISAUD>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Isotopic composition and concentration of lead in whole rock and microcline and concentration of uranium and thorium in whole-rock samples of granite from the Granite Mountains, Wyoming, have been determined. The lead isotopic composition in the whole rocks was found to be highly radiogenic with a range in Pb206/Pb204 of 19.58 to 42.27; the corresponding range in microclines is 15.39 to 22.44. A Pb206/Pb204 versus Pb207/Pb204 plot of the whole-rock data yields an apparent isochron age of 2,790 ± 80 m.y. as the time of crystallization of the granite. Chemically determined values of U238/Pb204 in the whole rocks lie between 3.3 and 18.4 and are too low to account for the amount of radiogenic lead observed. A material balance of lead, thorium, and uranium components indicates that an average of approximately 75 percent of the amount of uranium required to produce the radiogenic lead was removed from the rocks, whereas, on the average, there was no apparent loss of thorium. Loss of uranium from the granite is demonstrated to extend at least to a depth of 165 ft in a drill core. The average uranium loss from the samples analyzed represents about 20 g uranium per 1,000 kg of rock that apparently was removed during the Cenozoic and that probably constitutes the major source of uranium now in ore deposits in central Wyoming basins.The lead isotopic composition of the microclines indicates that lead was mobilized within the granite and was isolated in the feldspar during a thermal event about 1,640 + 120 m.y. ago. However, there is no evidence that the whole rocks themselves became open systems at that time. Whole-rock and microcline isochrons intersect at Pb206/Pb204 and Pb207/Pb204 of 13.77 and 14.86, respectively, indicating a characteristic U238/Pb204 of 8.96 in the source region of the granite magma. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.