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 m.y. after the period of voluminous silicic volcanism (38 to 28 m.y.a.) in the Trans-Pecos region. Other features indicate that, in contradiction to a hypothesis suggested by previous workers, the deposits did not form as a result of middle Tertiary magmatism. (1) Centers of igneous activity were distant from the sites of mineralization. (2) Potentially reactive limestones above and below the ore zones in red beds are generally unmineralized. (3) Zones of argillic, phyllic, and propylitic alteration typical of igneous-hydrothermal veins are absent. (4) Characteristic igneous hydrothermal gangue minerals such as quartz and fluorite are rare or absent. (5) Homogenization temperatures of fluid inclusions in barite and calcite suggest formation temperatures in the range of 120°C to 170°C, that is, lower than temperatures typical in copper-lead-zinc-bearing igneous-hydrothermal veins. These temperatures are higher than those usually attributed to strata-bound, red-bed copper deposits. The elevated formation temperatures are the result of high heat flow in the Basin and Range province of Texas at the time of mineralization; they are not the result of igneous activity. 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.