ANNEALING HISTORY LIMITS FOR INHOMOGENEOUS, NATIVE GOLD GRAINS AS DETERMINED FROM AU-AG DIFFUSION RATES

Quantitative study of intrinsic inhomogeneities in native gold grains from three deposits in the western United States has revealed concentration profiles that represent the integrated sum of natural diffusion plus original chemical heterogeneity. By assuming that measured natural concentration gradients result solely from diffusion, upper limits may be placed on the temperature-time annealing history of the gold nuggets. This assumption focuses on the end member case in which an initial step-discontinuity is assumed between measured extremes of concentration. Concentration changes of up to 30 weight percent Ag indicate probable deposition temperatures of less than 300°C for electrum from Copper Basin, Arizona, and Alder Gulch, Montana. The gold in the Homestake, South Dakota, deposit probably was formed at temperatures well under 400°C. In support of this study, new data for interdiffusion in the Au-Ag system were obtained from a series of annealing experiments followed by electron microprobe analysis. The interdiffusion coefficient, D, in the range 10-10 to 10-17 cm2 sec-1 was determined from measured profiles across synthetic alloy pairs held at eight fixed temperatures from 297° to 799°C, for periods ranging from 32 hours to 730 days. © 1973 Society of Economic Geologists, Inc.