Roll-type uranium deposits are formed by a supergene process that depends on limited oxidation of early formed pyrite in the host rock. Among the initial products of oxidation is a series of unstable soluble sulfur species, principally sulfite, which slowly decompose and ultimately form sulfate and sulfides. Because sulfate is kinetically inert to low-temperature nonbiologic oxidation-reduction reactions, and because the unstable sulfur species and hydrogen sulfide are strong and active reducing agents, the net result of limited oxidation is to produce a strongly reducing environment. Either some of the intermediate sulfur products or hydrogen sulfide may recombine with ferrous iron under these conditions to form the iron sulfide minerals associated with the ores. Laboratory experiments and chemical theory suggest that the entire sequence of reactions can be carried out in a biologically sterile environment and that the sulfur isotopic composition of the reconstituted pyrite might be indistinguishable from pyrite formed by biogenic processes. These unstable intermediate sulfur species may also be involved in the formation of other supergene-enriched sulfide ores as well as in the transportation and deposition of elements such as barium and selenium. © 1969 Society of Economic Geologists, Inc.
