Depositional Systems, San Angelo Formation (Permian), North Texas: Facies Control of Red-Bed Copper Mineralization

The San Angelo Formation is a mid-Permian sandstone and mudstone sequence about 100 feet thick that crops out in North Texas and dips westward into the Midland Basin; it is composed of two superposed members: the basal Duncan Sandstone Member and the overlying Flowerpot Mudstone Member. Depositional systems within the Duncan Member include the Copper Breaks deltaic system in the north, the Old Glory fluvial-deltaic system in the south, and the intermediate, strike-fed Buzzard Peak sand-rich tidal-flat system. Gradationally above these systems is the Cedar Mountain mud-rich tidal-flat system which is coincident with the Flowerpot Member. Cedar Mountain facies include tidal channel-fill sandstone characterized by flaser bedding, red mudstone of tidal mudflat origin, and algally bound shale and dolomite. Cedar Mountain facies are overlain conformably by the Blaine Formation which was deposited within alternating sabkha and tidal-flat framework of the Blaine sabkha and tidal-flat system. Copper mineralization occurs primarily within narrow, lenticular, organic-rich tidal channel-fill sandstone facies and thin, widespread algal mat shale facies. A sabkha-diagenetic model is used to explain the mineralization. Evaporative discharge from a sabkha creates an upward decrease in hydrodynamic potential with the result that primarily terrestrial ground water moves upward through the sabkha. Hydrogen sulfide, formed by bacteria, precipitates copper as ground water passes through chemically favorable facies. Calculations indicate that 100,000 to 200,000 years may be necessary to form an economic deposit; alternate epigenetic models, involving diffusion over long distances, or movement of hydrothermal solutions along faults and fractures, were evaluated. Copper minerals include chalcocite, covellite, and malachite.