Origin of Silver-copper-lead Deposits in Red-bed Sequences of Trans- pecos Texas: Tertiary Mineralization in PreCambrian, Permian, and Cretaceous Sandstones.
Silver deposits occur in preCambrian, Permian, and Cretaceous red- bed sequences near Van Horn, Texas. These deposits are geochemically similar and contain economically important quantities of silver, copper, and lead as well as anomalously high amounts of arsenic, zinc, cadmium, and molybdenum. Gold is not enriched. Primary minerals include chalcopyrite, tennantite-tetrahedrite, bornite, galena, sphalerite, acanthite, pyrite, marcasite, barite, and calcite. The deposits are dominantly steeply dipping veins. Strata- bound occurrences are near veins or closely spaced fractures. Structural evidence, including orientations of veins, relative ages of fractures, and relationships to major tectonic events in the region, suggests that the most likely time of mineralization was during late basin and range extensional deformation. Ore deposition probably occurred at least 18 million years after the period of voluminous silicic volcanism (38 to 28 million years ago) in the trans-pecos region. The hypothesis developed in this study on the origin of the veins involves a rise of moderate-temperature, moderately saline hydrothermal fluids along basin and range fractures and precipitation of metal sulfides in response to mixing with shallow ground water. This hypothesis has implications for exploration of additional deposits in trans-pecos Texas and elsewhere.