Why are ores containing calcite or dolomite treated with alkaline leaching?

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Enrichment of uranium — uranium processing How uranium is enriched to make a nuclear weapon.

Ores containing significant amounts of basic minerals, such as calcite or dolomite, are treated with alkaline leaching because these minerals react excessively with acidic leachants. Using an acid-based process on such ores would consume an uneconomical amount of acid, whereas alkaline leaching—typically using sodium carbonate solutions—effectively extracts the uranium by forming a soluble uranyl carbonate complex ion, $[U O_{2} (C O_{3})_{3}]^{4 -}$ .1

In this process, oxygen is generally used as the oxidant to ensure the uranium is in the hexavalent state required for successful leaching. Once the uranium is dissolved, it can be recovered from the resulting carbonate leach liquors using ion-exchange resins.12