The Schwartzwalder uranium deposit; II, Age of uranium mineralization and lead isotope constraints on genesis

Abstract

U-Pb isotope analyses of ores from the Schwartzwalder uranium mine, Colorado, show that these ores have high amounts of initial (common) Pb and that the initial Pb was both variable and relatively radiogenic in its Pb isotope ratios ( 206 Pb/ 204 Pb = 26-30). As a result, the only useful approach to dating these ores is with U-Pb isochrons, and even so, some means of dealing with the variable initial Pb isotope ratios is required. Because the common Pb in these ores was apparently derived from sources of similar age and Th/U, the observed 208 Pb/ 204 Pb of these Th-free ores can be used either to identify sample suites with similar initial Pb isotope ratios or to normalize for the variable initial Pb isotope ratios. The resulting U-Pb isochrons indicate an age of mineralization for both Illinois vein and Titan vein ores of 69.3 + or - 1.1 m.y., suggesting that the deposit was formed during the earliest stages of Laramide uplift and under at least 3 km of Phanerozoic cover. The initial Pb isotope systematics of the ores show that the metals in the Schwartzwalder ores were derived from source(s) of 1,730 + or - 130-m.y.-age, with a Th/U of 2.2 + or - 0.2 and 238 U/ 204 Pb of 30 to 60. These restrictions on the source(s) of the metals rule out the possibility that the deposit could have been formed by remobilization of a Proterozoic uranium deposit, and both the 69.3-m.y.-age of the ores and their initial Pb isotope ratios preclude any contribution of metals to the deposit by younger volcanics such as those in the Denver Formation.