Stuckless and Nkomo reported in 1978 that 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 P.,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.
The authors concluded that three sources have been proposed for the uranium deposits of central Wyoming: granites. arkosic sands derived from the granite, and volcanic ash. They also concluded that the granites of the Granite Mountains were a major source of uranium for several reasons: the
major uranium districts are disposed about the flanks of the mountains; large amounts of uranium have been lost from the currently exposed, and presumably from the previously eroded, granites; if most of the labile uranium is removed from the granite prior to its mechanical disintegration, then the arkosic sands derived from the granite would make a poor source for uranium; Harshman (1972) has reported that altered sands in the Shirley Basin, through which the ore-bearing solutions presumably moved, contain more uranium than the unaltered arkoses; radiometric dating of the ores (Ludwig, 1978) suggests that they are, at least in part, older than the proposed volcanic source rocks (Zielinski and Naeser, 1977; Love, 1970); and the timing for at least part of the uranium loss from the granite agrees with radiometric dates of the ores.
Uranium contents of all favorable granitic source rock, although still anomalously high when compared to average granites (Rogers and Adams, 1969),
have been drastically depleted for all, surface samples analyzed in this study and for those reported by Rosholt and others (1973). Hence, present-day uranium contents may not be a good indicator of favorable granitic source rocks. Other features common to most of the samples that lost large amounts of uranium in the relatively recent past include high alkali, silica, and thorium contents, and moderate biotite contents.
A few thin zones of uranium reconcentration within the granite were analyzed. These contained 20 to 1,460 ppm uranium. The only feature common to
these samples and therefore possibly controlling uranium deposition appears to be intense fracturing. The fact that all samples from the granite of Lankin Dome appear to have lost uranium in response to an apparently mild mid-Precambrian thermal event may have important consequences for uranium
exploration in crystalline terrains. If the suggestion of Killeen and Heier (1975) is followed, then the mobilized uranium may have migrated away from
the thermal front and may have been reconcentrated in the crystalline rocks farther north. As a corollary, whole-rock Pb-Pb ages, which are apparently too old, may indicate terrains in which uranium has been mobilized in the past. Conversely Pb-Pb ages in complex Precambrian terrains may be misleading,
if the rocks failed to act as a closed system during thermal or structural events or during periods of exposure and weathering. Finally, the Th-Pb ages do
not seem to be as sensitive to these effects as the PbPb ages.
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