SCANNING ELECTRON MICROSCOPY INVESTIGATION OF GOLD IN A LATERITE SAMPLE FROM THE GUYANA SHIELD OF VENEZUELAN: A CASE FOR AUTHIGENIC DEPOSITION

Debate exists as to whether or not gold may mineralize via authigenic processes in fluvial or soil environments. By convention gold is essentially regarded as insoluble in near earth surface environments, however if it can be demonstrated that authigenic gold exists in fluvial or soil environments then a fundamental process has been overlooked and potentially new chemical pathways may exist to extract gold from ore.

A gold sample consisting of an apparent nugget embedded in a laterite matrix from the Guyana shield of Venezuela was investigated to determine if gold from this sample was authigenic or detrital. Scanning electron microscopy (SEM) clearly shows unique textures that are indicative of authigenic processes. SEM and energy dispersive spectroscopy (EDS) clearly show a diffusion rind of discrete micrometer to nanometer scale gold particles adjacent to the nugget. The nanoparticles show no mechanical depositional features that indicate transportation. A spatial gap of particles between the nugget and the rind, combined with intricate textures of gold particles in the rind indicate growth in place. Some nanoparticles show evidence of potential Oswald ripening where 1-3 micrometer sized particles are surrounding a 5 micrometer gold particle within the laterite. EDS analyses indicated concentrations of Cl, Fe, Cu, and Ca associated with the gold nanoparticles. The exact mechanism of authigenesis is unclear, however the results have significant pragmatic implications. The occurrence of gold in laterites in Venezuela and Africa is a common setting and samples from a variety of regions should be investigated to determine if this is a common or rare texture. Micrometer and nanometer scale gold may not be recovered in traditional placer mining which is used in laterite ores thus many sites may have waste or ore that has recoverable gold. Future work will focus on determining the prevalence of this texture in Venezuelan samples and attempts to identify geochemical mechanisms.