GEOLOGIC AND GEOCHEMICAL STUDIES IN BIG BEND NATIONAL PARK, TEXAS

Recent USGS studies of fresh and altered Tertiary volcanic rocks, carbonate rocks in contact with intrusive rocks, and spring and thermal waters in Big Bend National Park (BBNP) provide a new framework for interpreting past igneous and hydrothermal events and present water-rock interaction processes. The igneous landscape, which dominates much of BBNP, includes, but is not limited to, tectonically-controlled dike systems, shallow intrusive bodies, lava flows, and large ignimbrite-producing calderas. Igneous forms have been variably influenced or controlled by regional tectonism. Nearly all are associated to some degree with extensive fossil hydrothermal systems, which altered and mineralized the surrounding host rocks.

Extensive hydrothermal alteration of volcanic rocks in the Chisos Mountains is characterized by acid leaching to produce kaolinite and other clays and sulfide mineralization. Elsewhere, amygdaloidal flow-tops and pillow margins of basaltic flows are altered to epidote, calcite, zeolites, and silica. Faulted contact zones between intrusive rocks and Cretaceous carbonates are important foci of historic Hg deposits. Altered limestones from contact zones at Mariscal Mine, Study Butte, and the Mariposa district are enriched in Cu, Fe, Pb, and Zn and have carbon isotope values from -2.4 to 2.3 per mil. In the NE portion of the Park, thick diabase sills with minor felsic zones intrude upper Cretaceous limestones, producing skarn deposits.

Six samples of hot spring waters and 26 spring water samples from localities throughout BBNP have been analyzed for trace elements and stable isotopes. Hot spring waters show concentrations (ug/L) of As (13.8), Rb (24), Se (6.6), Sr (4300), Tl (0.7), and W (12) and pH of 7.1-8.2. Non-thermal spring waters have pH from 6.4 to 9.2 (7.7 ave.), and occasionally contain As (9.5ug/L) and Se (11ug/L). Mercury concentrations are <5 ng/L, the limit of determination in waters analyzed. Stable isotopes show that all BBNP waters are meteoric in origin and evaporation produces a wide range of oxygen and hydrogen isotope values.