New Mexico Geological Society Annual Spring Meeting — Abstracts

The Eocene San Jose Formation is generally considered to consist of widespread arkosic sandstones and interbedded mudstones of fluvial origin. As part of the National Uranium Resource Evaluation (NURE), stream-sediment samples were collected from 439 first-order catchments within the San Jose Formation. Fine-grained fractions (-100 mesh) were analyzed for all major elements, except Si, and a broad spectrum of 25 trace elements. Element distribution maps, prepared from the NURE data, provide a new perception of primary sediment distribution which differs significantly from published geologic maps.

Petrographic identification of sandstone mineralogy, element correlation diagrams, and element distribution maps have been used in combination to delineate four compositionally distinct fluvial systems within the San Jose Formation (see map). The arkosic Carracas Mesa system (A) is distinguished by relatively high concentrations of AI, Ti, Zr, Y, P, Mg and K. Erratic highs of Ti, Zr, and Y reflect placer concentrations of ilmenite, zircon and xenotime along the stream axis. High Al and K, with uniformly low Ti define illitic floodplain deposits (fp). High Mg reflects entrapment of biotite by waning currents in a fluvial-fan subfacies. The Llaves system (B) is distinguished by unusually low AI, which reflects an average estimated SiO₂ content of 89 percent. Kaolinitic floodplain deposits are indicated by locally high variability of Al and very high ratios of Al to alkalis. The Cuba Mesa system (C) is characterized by intermediate concentrations of Al and estimated SiO₂ (84%) in association with relatively thorium-rich zircons (Th/Zr greater than 0.2). The Piedra Peak system (D) is locally defined by slightly elevated boron, indicating traces of tourmaline.

Groundwater uranium anomalies, radiometric anomalies, and weathered uranium occurrences (associated with jarosite) collectively indicate significant potential for leachable Gulf Coast-type uranium deposits at the margins of the 150m-thick Llaves sandstone system. In the future, superconductivity technology utilizing yttrium could make xenotime placers in the Carracas system economically attractive. Recognition of a northeast-trending basement structure, defined by an aeromagnetic lineament and the Carracas system fluvial fan, may help focus exploration for oil and gas in underlying .()~ Cretaceous strata.