Abstract
Geologic observations coupled with laboratory studies indicate several periods of mineralization in the Grants mineral belt. The earliest mineralization is from trend ore in the Ambrosia Lake and Smith Lake districts; Rb-Sr radiometric ages on chlorite formed penecontemporaneously with primary uranium minerals range from 135 to 138 ± 8 m.y. This period of mineralization is within the limits of error for the age of sedimentation obtained on barren ground montmorillonite of 140 to 145 ± 10 m.y., but cross-cutting ore indicates early epigenetic as opposed to syngenetic mineralization. Early formed ore in the Laguna district was remobilized and reprecipitated during some mid-Cretaceous event at 110 to 115 m.y. determined on the basis of Rb-Sr dating. Ore was not derived from the overlying Dakota Formation (Cretaceous), as the Rb-Sr dates for the Dakota and Mancos formations are 92 ± 6 m.y. and 85 ± 10 m.y., respectively (in excellent agreement with U.S. Geological Survey K-Ar dating). Mineralization is present in the Dakota Sandstone, but whether the ore was syngenetic or epigenetic is unknown. Much of the stack ore was apparently formed during the Laramide orogeny about 60 m.y. ago, usually in close proximity to a redox front. Post-Laramide ore is proposed for several deposits in reduced ground at this redox front, some of which is apparently Tertiary although remobilized Jurassic ore cannot be distinguished from that from much younger sources even though reworked Jurassic ore is supported by high 87Sr/86Sr ratios. Ore possibly formed during the Tertiary from a southerly source for some deposits, and some remobilized ore, possibly of Pleistocene age, is common in minor amounts.
