READSORPTION OF GOLD DURING THE SELECTIVE EXTRACTION OF THE SOLUBLE ORGANIC-PHASE OF HUMUS, SOIL AND SEDIMENT SAMPLES

Phase selective extractions are employed in the chemical analysis of geological materials to identify the forms in which the element is present and hence to elucidate the mechanisms by which the element has been transported and eventually accumulated in such media, Quantitation of Au bound to the ''soluble organic'' phase of a humus or lake sediment sample would indicate hydromorphic or chemical dispersion as distinct from mechanical dispersion suggested by An in an inorganic or metallic form. The majority of analyses carried out for Au in exploration geochemistry are based upon instrumental neutron activation or Pb fire assay for total Au and aqua regia leaching for ''near total'' Au. Selective leaching of metals scavenged in the surficial environment by the humate and fulvate components of organic material has been successfully carried out for elements such as Zn, Cu, Pb, Co and Ni using alkaline sodium pyrophosphate (Na4P2O7) solution. This paper describes the problems encountered in application of this extraction in batch mode to humus, soil and sediment samples for Au. The concentration of ''organically'' bound Au in a lake sediment control (Red Lake) is shown to be largely dependent upon the sample weight : leach volume ratio. This concentration is determined to be 128 ppb Au (46% of total Au) at 0.5 g subsampling and 59 ppb Au at 3 g subsampling, both leached with 100 ml of 0.1 M Na4P2O7 solution. Thus, the larger the sample taken per unit volume, the lower the proportional amount of gold apparently extracted. Recovery of AuCl3 spike added at the 83 ppb level to 3 g of Red Lake is less than 20%; ''lost'' Au is shown to be present in the sample residue. Similar low recoveries of 12-21% were obtained for four humus samples and slightly higher values of 23-25% were found in application of this leach to a gabbro control, WGB-1. Readsorption of Au in six soil samples was also observed in application of the NH2OH . HCl leach, specific for dissolution of amorphous Fe oxyhydroxides. Thus selective leaching for Au is severely hampered by the difficulty of maintaining Au in solution in the presence of the sample substrate. Separation of the competing kinetics of dissolution and readsorption would more appropriately be addressed by use of a how-through system rather than batch extraction.