Abstract: When processing a fragmented permeable mass of particles containing oil shale for recovering liquid and gaseous values, a portion of the oil shale is thermally metamorphosed. However, a zone of non-thermally metamorphosed particles can be left in the fragmented mass after completion of processing. To inhibit leaching by water of water-soluble constituents of such non-thermally metamorphosed particles, a heating fluid, such as a mixture of fuel and an oxygen-containing gas, is introduced to the fragmented mass containing non-thermally metamorphosed particles for heating at least a portion of the non-metamorphosed particles to a sufficiently high temperature for forming water-insoluble metamorphic minerals at at least the surfaces of such particles.