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在南达科他州布特郡黑山山脉北东翼,有几个产于白恶纪否汝河(Fall River)砂岩中的铀矿床。矿床系产在连续、细粒和分选良好的底部砂岩内。该砂岩厚度为2-18呎,倾向的NE、倾角约4°。围绕矿床约2平方哩的详细填图表明,全部铀矿化和多数高放射值地区都产于底部砂岩中,其颜色从上覆红色变为下覆黄—灰或淡黄色的地方。放射性测量表明,铀几乎沿着曲折红—淡黄色接触带连续分布达5哩以上。试验室工作证明,红色是由高价铁的含水氧化物和赤铁矿(由低价铁矿物转变而成)所引起的。红—淡黄色接触带与铀矿床的紧密共生表明,二者系由同一溶液形成的。近地表的地下水氧化溶液沿倾斜向下运动,到深部时逐渐变成弱还原溶液,遂可能发生铀的原始沉积。铀的聚集系由于局部的还原条件。这些还原条件可能是区域倾斜层上的构造,如砂岩的局部变薄或渗透性的局部减弱,也可能是含黄铁矿和含碳物质的局部富集。红色变化可能是前渐新世风化的结果。在渗透性较好的岩层中,其风化深度达古老侵蚀面以下约200呎。在现代风化旋迥中,原生铀氧化形成了钾钒铀矿和其他可能的次生铀矿物。
In the northern east wing of the Black Hills of Butte County, South Dakota, there are several uranium deposits that originated in the white sandstone of the Nomai River. The deposit is produced in a continuous, fine-grained and well-sorted bottom sandstone. The thickness of the sandstone is 2-18 ft, with a dip of NE of about 4 °. Detailed mapping about 2 square miles around the deposit shows that all uranium mineralization and most of the high-emission areas occur in the bottom sandstone and change their color from overlying red to underlying yellow-gray or yellowish. Radiometric measurements showed that uranium was distributed almost continuously over 5 miles along the zigzag red-yellow contact zone. Laboratory work has shown that red is caused by the oxidation of high-valent hydrous oxides and hematites, which are made from low-cost iron minerals. The close coexistence of the red-light yellow belt with the uranium deposits suggests that both are formed from the same solution. Near-surface groundwater oxidation solution moves downwardly in inclination and gradually becomes a weak reduction solution in the deep part, so that the original deposition of uranium may occur. Uranium accumulation system due to the local reduction conditions. These reduction conditions may be tectonic structures on regional bevels, such as local thinning or permeability weakening of sandstone, or local enrichment of pyrite-bearing and carbonaceous materials. The red change may be the result of pre-Oligocene weathering. In more permeable formations, the weathering depth is about 200 feet below the ancient erosion surface. In modern weathering, primary uranium oxidation forms potassium-vanadium uranium deposits and other possible secondary uranium minerals.