Why does heavy rainfall help form residual mineral deposits?

Heavy rainfall facilitates the formation of residual mineral deposits by acting as a powerful agent of chemical weathering and dissolution. In warm, tropical regions, high volumes of percolating rainwater dissolve and remove valueless, soluble materials from the upper layers of soil or rock.12

This process, known as leaching, leaves behind a concentrated residuum of less-soluble ore minerals. As soluble substances are carried downward, the remaining material becomes enriched with stable minerals, such as quartz and the hydroxides of iron, manganese, and aluminum. Over time, these accumulated residual minerals can form significant deposits, such as bauxite, or coalesce into tough, impermeable layers known as duricrusts.12