Abstract Core samples from a property in Wyoming were tested for amenability to in situ leaching by acid ferric sulfate. In tests, about 95% recovery of the uranium was achieved. The ore is fortunately low in carbonate, and sulfuric acid consumption of 15#/T is expected using feed solutions of pH 1.8. Factors significantly affecting the yield of uranium are temperature and ferric ion concentration in the lixiviant. Acid concentration, within the pH limits 1.5-2, is significant only in the time required to acidify the formation. Flow rate affects the rate of leaching, but such increase in production rate is balanced by the requirement of handling proportionately larger amounts of more dilute production solution. Some information has been obtained on other ions leached by the process, and an attempt has been made to relate the information to possible changes in clay structures. Certain reactions, such as the conversion of K-feldspar to kaolinite, have been shown to occur very slowly under leach conditions. Introduction A site potentially suitable for in situ leaching of uranium from sandstone was selected from various candidates. Core samples were obtained and blended to give a composite considered representative of the ore. Care was taken to avoid exposure of the ore to oxygen, but subsequent experience showed this precaution to be unnecessary. All tests were performed on this composite sample. GEOLOGY AND MINEROLOGY The ore zone consists of a fluvial sequence of arkosic sandstone and silty claystone, and lies well below the local water table. The majority of the uranium occurs as uraninite, with minor amounts of coffinite. It is believed that the uranium was desposited by the reducing action of humic acid released by compacted sediments. Most of the uranium occurs in hematitic coarser sandstone faces that once contained organic carbon. Its location is related to lithological changes particular to scour and fill environments. The dominant clay is a vermicular kaolinite, with occasional concentrations of chlorite, illite, and montmorillonite. In some areas, kaolinite can be seen altering to illite. APPROACH The leach experiments were primarily designed to provide practical information on stream concentration-history to be expected in field operation using a recycling system. Whenever possible, measurements to elucidate mechanisms were carried out. Inevitably, hindsight has shown a number or desirable measurements not made, but the series of experiments has allowed a definition of a schedule to be followed in testing a uraniferous sandstone ore for its potential for acid ferric sulfate leach. Many auxiliary experiments delineating the behavior of clays, can be suggested; they are particularly necessary to define programs required for studies in site restoration. TECHNIQUES FOR LEACHING The most difficult decision is the kind of experiment to perform.