New Mexico Geological Society Annual Spring Meeting — Abstracts

Geochronologic studies of Colorado Plateau uranium deposits began around 1950 during the early years of the "uranium boom," but detailed studies in the Grants district were not published until more than thirty years later, after most mining had ended. Early work in Utah and Colorado yielded mostly discordant U-Pb ages; these nevertheless suggested a late Cretaceous to early Tertiary (Laramide) age for several deposits, similar to the vein type deposits of the Colorado Front Range. This idea was influential throughout the 1950s and early 1960s. Geologic studies in Grants and other districts, and slowly accumulating new geochronologic data, then suggested that many Plateau deposits are in reality much older, often approaching the age of their host rocks. Subsequent work has supported this latter interpretation.

The following conclusions have been reached for the Grants district from geochronologic studies published since around 1980. At Ambrosia Lake and Smith Lake, primary (trend) ore fonned early in the history of the host sandstones of the-Upper Jurassic Morrison Fonnation, at 130 Ma or earlier, based on U-Pb data and Rb-Sr and K-Ar ages of clay minerals penecontemporaneous with uranium minerals. Redistributed (stack) ore and an oxidized uranium mineral (uranophane) at Ambrosia Lake have late Tertiary U-Pb ages of 3 to 12 Ma, consistent with geologic evidence on remobilization of uranium. Redistributed Morrison ore in the Church Rock area is as young as Pleistocene; redistribution was not extensive at Smith Lake. Primary deposits in the Morrison at Laguna may be younger than at Ambrosia Lake, but limited age data are inconclusive. The age of primary deposits in the Middle Jurassic Todilto Limestone is 150-155 Ma, close to the age ofthe Todilto; these deposits are older than those in the Morrison. Secondary Todilto uranophane yields U-Pb ages of 3 to 7 Ma, confirming Tertiary redistribution of uranium in this formation as well.

Most U-Pb ages from all ore-bearing horizons are discordant, attributed mainly to open-system behavior involving migration of radiogenic Pb and relatively long-lived intermediate daughter isotopes in the ²³⁸U decay chain. Under this interpretation the ²⁰⁷Pb/²³⁵U age is the most reliable, and generally yields a minimum age. However, some ages are concordant. In addition, concordia diagrams provide meaningful conclusions from discordant age data. Migration of ²³⁸U daughters and subsequent decay to ²⁰⁶Pb produces anomalous radiogenic ²⁰⁷Pb/²⁰⁶Pb ratios and resulting apparent ages in many samples. Some ²⁰⁷Pb/²⁰⁶Pb "ages" are greater than 1000 Ma in Morrison and Todilto samples that are deficient in ²⁰⁶Pb. By contrast, excess ²⁰⁶Pb produces a negative or "future" age of -840 Ma for one Todilto sample; four ages ranging from -19 to -580 Ma for Ambrosia Lake, Smith Lake and Church Rock; and an extreme of -13000 Ma for one Ambrosia Lake sample (²⁰⁷Pb/²⁰⁶Pb =0.008). Such anomalies were not observed in early studies, and provide especially strong evidence of complex open-system behavior.