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Uranium concentrations of 42 to 169 ppm in fibrous calcite from spring-deposited tufa in the middle Miocene Barstow Formation, California, are among the highest reported for calcite. Fission-track maps of multiple bands of uniformly dull-luminescent fibrous calcite show that the concentration of U increases in the outward growth direction of the calcite of each individual band. Homogeneous dull luminescence in the fibrous calcite indicates no change in redox conditions of the fluid from which the calcite was precipitated. It is proposed that the cyclic pattern of increase in U concentration reflects a cyclical change in the U/Ca ratio in the fluid. Episodic mixing between Ca-rich spring water with a low U/Ca ratio and Ca-poor saline alkaline lake water with a high U/Ca ratio could produce the episodes of formation of fibrous calcite with increasing U concentrations. The spring water supplies the Ca for calcite precipitation, and the U concentrations increase with the decreasing fraction of spring water and increasing fraction of lake water. These cycles reflect a variable recharge of groundwater into the lake by springs. The large variation in U concentrations suggests mixing of 0% to 45% spring water with the lake, while the narrow range in delta O-18 of -6.37 parts per thousand to -6.87 parts per thousand limits the variation of the proportions of spring and lake water to 5%.