Uranium-lead isotope systematics in uraniferous alkali-rich granites from the Granite Mountains, Wyoming; implications for uranium source rocks

Abstract

Evaluation of U-Th-Pb isotopic analyses of whole-rock granite samples from the Granite Mountains, Wyoming, suggests that uranium has been mobilized at least twice since the intrusion of the granite. Whole-rock Th-Pb, whole-rock Rb-Sr, and zircon U-Pb ages are concordant at 2,600 m.y., but the whole-rock Pb-Pb age is distinctly older. This apparent age is attributed to a 10 to 45 percent uranium loss between 1,400 and 1,700 m.y. ago when the granite was subjected to an ill-defined metamorphic event that was probably of a thermal nature.A second and more extensive uranium loss occurred after the granites were exposed to near-surface conditions by the Laramide event. This loss has left whole-rock U-Pb ages variable and generally impossibly old. Granites that exhibit the largest recent uranium loss are characterized by high silica, thorium, and alkali contents and moderate biotite contents. These features are suggested as more diagnostic of potential granitic uranium source rocks than present-day uranium contents because the general 70 percent or greater recent loss of uranium has left many surface samples with a uranium content that has been considered typical of many granitic units.A few samples were found to have concentrations of some of the mobilized uranium (as much as 1,460 ppm U). Although these samples are not petrographically or chemically distinguishable from those that lost uranium, most are highly fractured. It therefore appears that fracturing may be an important control in forming ore deposits in granites.