![]() ![]() dbd.) ion re-oxidizes to the sulphate (SO.sub.4. One of the chief chemical trapping mechanisms is believed to be upward wicking of soluble PM-PGE thiosulfate complexes followed by their reprecipitation as insoluble jarosites as the thiosulfate (S.sub.2 O.sub.3. The jarositic minerals are either wholly or partly absorbed in the microscopic zeolitic pores of the clinoptilolite type clay zeolites present in this type of playa bentonite. In these oxidized valence states, the PM-PGE end up as minor lattice substituents in microscopic insoluble jarosite-type minerals. The PM-PGEs are believed to be present as oxidized salts in which the PM-PGE are in valence states above zero. ![]() Finally, re-oxidation of PM-PGE thiosulfate complexes to jarosites occurs near the evaporative surface. At deeper (deoxygenated) levels the sulfate is re-reduced bacterially to thiosulfate. ![]() Jarosites are gypsum-like minerals believed to be derivative of the desert oxidation of original sulphides-commercially viable borax is available in the area. Evaporative stranding has resulted in the accumulation of Be, B and Li (as well as Na, K, Ca and Mg) as carbonates and jarosites. The geochemistry of several of the dry playa lakebeds of the area also is characterized by zeolization of volcanic ashes in the alkali playas, the transport of PM-PGE ions by creosote and other plant oil ligands in upper groundwaters, and also by alkaline thiosulfate anions, in deoxygenated lower-level groundwaters. ![]() In the past few million years, relatively high enrichment of PM-PGE values in this evaporative trap has been effected by the erosion of Basin and Range Age (Cenozoic) felsic ash volcanos, and the washing of green dioritic ash deposits of the same age directly into the playa, accompanied by the evaporation of late magmatic geothermal solutions. The region constitutes an intensely evaporative regime with challenging metallurgic features. The Amargosa River drainage system of Inyo County, Calif. The deposits are of commercial interest primarily because of the presence of economic amounts of PMs and PGEs although a large suite of other unusual elements is also present. Some such sedimentary systems of commercial interest are those of certain of southeastern California playas. The geological concentration and trapping of metal values in sediments by desert evaporation processes results in a particular class of mineral deposits and the processes of recovery and assay of these sediments must be tailored to their geochemistry. This invention relates to the recovery and assay of gold, silver (PMs) and the platinum group elements (PGEs) from evaporite sediments. Mud leach tails are treated for agricultural use and land reclamation. Recovered flower sulfur and sulfite liquors are used to regenerate recycled thiosulfate. Assay of the product (heads and precipitates) may be effected by high temperature methods developed for these types of deposits. The second stage scavenger precipitate is an impure metallic powder and values are recovered by standard reduction and separation methods. In a second stage, the first stage precipitation liquor is scavenged by metal-exchange at reduced pH for additional metal values and cupellation to the metallic state. The final recovery of the metal values is produced by controlled acid parting into individual precious metals and cupellation to the metallic state. The sponge is de-leaded by firing and cupelling, selective cementation or electrolysis and reduction to dore is best effected by a carburizing distillation. The precipitate may be subjected further to desulfurization and metallic footing to produce a lead-precious metal sponge. In a first stage precious metals and some PGE elements are precipitated by lead acetate or sodium borohydride reagents. After filtration the pregnant solution is subjected to a two-stage precipitation of gold, silver and PGE values. Fine sediments are subjected to a primary leach with a highly caustic sodium thiosulfate leach liquor to produce a metal-pregnant solution. A process is disclosed for the recovery and assay of gold, silver (PMs) and the 6 platinum group elements (PGEs) from playa-evaporite sediments. ![]()
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