Abstract

Recent ground-water studies undertaken by the U. S. Geological Survey in cooperation with the North Carolina Division of Mineral Resources have delineated phosphorite deposits, tentatively regarded as being of middle Miocene age, in Beaufort County. These deposits lie unconformably on limestone of Eocene age and are unconformably overlain by late Miocene marl. The phosphorites, buried beneath strata ranging in thickness from 45 to 250 feet, underlie an area approximating 450 square miles. The total thickness of the phosphorite column throughout the area ranges from several feet to nearly 90 feet.The phosphorite column consists of phosphatic sands and intercalated shell limestones. The sands, composed of pellets of brown sand-size cellophane (probably carbonate-fluorapatite) and sand-size, flat-sided angular quartz with some silt, clay, and organic material, have a median diameter between 0.50 and 0.25 mm. Chemical analyses of representative samples of the raw sand show a variation in P 2 O 5 content from 8 to 31 percent. The P 2 O 5 content is apparently proportionate to the cellophane content throughout the area.Reconstruction of the geologic history suggests that the phosphorites were deposited as chemical precipitates and as in situ replacements in a restricted marine basin where the pH of the water acted as the primary depositional control.Reconstruction of the hydrologic history of the phosphorites indicates that the deposits were preserved under artesian conditions; they were never subjected to alteration under water-table conditions. The absence of postdepositional alteration makes this area a potentially classic one for studies of phosphorite genesis.Chemical analyses of artesian waters from the phosphorites and from limestones overlain by phosphorites reveal significantly greater concentrations of iodide and bromide in solution than are present in water from overlying sediments or from underlying limestones not overlain by phosphorites. Such anomalies, if present in other similar terranes, may indicate the presence of buried phosphorites.The depth and size distribution of the material suggest a recovery method utilizing wells constructed to "pump sand."

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