Abstract

Migration of uranium daughters has profoundly affected the U/Pb isotope ages of Tertiary uranium ores and constituent minerals sampled from three mines and three ore types in Shirley Basin, Wyoming. U/Pb ages of pitchblende grains are typically low, show moderate to extreme normal discordance, and are very sensitive to the presence of pyrite impurities. Pyrite separates give typically high U/Pb ages, ranging in discordance from moderate-normal to moderate-reverse. Total ore samples give U/Pb ages intermediate between pitchblende and pyrite and are slightly to moderately discordant. Observed apparent age ranges were (m.y.): 206 Pb/ 238 U 207 Pb/ 235 U 207 Pb/ 206 PbPitchblende 6-30 14-35 270-2,400Pyrite 40-280 39-270 -90-+610Total ore 18-23 21-28 82-810Other key findings are that all pyrite separates from ores contained 130 to 620 ppm unsupported radiogenic lead, and a separate of coalified wood from an ore contained 800 ppm of unsupported radiogenic lead having the astonishing 207 Pb/ 206 Pb value of 0.012 (-5,850 m.y. 207 Pb/ 206 Pb age).These U-Pb isotope systematics are the result of the loss of both lead and radioactive daughters of 238 U from pitchblende and the subsequent migration of these uranium daughters. Migrating lead was both lost from the total ore and incorporated by pyrite, whereas migrating radioactive daughters were both lost from the total ore and trapped by material such as coalified wood.If young uranium ores such as the Shirley Basin ores have experienced neither uranium migration nor a net gain of extraneous uranium daughters, then geochronometry is possible even though the ores have lost lead. Even some ores open to both lead and radioactive uranium-daughter loss can be dated if the time-averaged leakage of radioactive daughters can be estimated. The main requirement of such open system dating is that the isotopic composition of the lead lost by the total ore be similar to that of the lead gained by the pyrite.The time of pitchblende formation in the Shirley Basin for the youngest ore sample analyzed is inferred to be 24 + or - 3 m.y. ago, whereas the oldest sample was apparently formed before 35 m.y. ago. These limits require source rocks for the uranium in the Shirley Basin orebodies to have existed from at least early Oligocene time.

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