The surface of placer gold particles are gradually enriched with natural weathering. The relatively less stable metal elements like silicon, potassium, calcium, iron, copper, silver and etc. and the non metal elements like phosphorus, sodium, chlorine and sulfur, and the compounds of both groups are leached from the surface of the gold particle leaving nearly pure gold on the surface.
As Kevin pointed out the smaller the gold particle the greater the surface area to volume ratio. This leads to smaller particles of gold with higher purity compared to larger particles in the same deposit.
Check out the recent work by Dr. Erik Melchiorre (Phd Faculty CSU San Bernardino) on this surface enrichment process and how he has developed methods to quantify the enrichment. Knowledge of surface enrichment has enabled him to calculate the relative deposition ages across placer gold deposits as well as distinguishing two intermingled gold placers as to which geological formation produced which gold and when. There is a lot that can be learned from this basic principle of placer particle enrichment.
Heavy Pans
