Size and Shape of Gold and Platinum Grains

Abstract

Size analyses were made of two samples of gold concentrates, one sample of gold- bearing placer gravel, and two samples of platinum-bearing concentrates. The precious metals were separated from the sized fractions by use of an elutriator. Grain-shape analyses were made from grain dimensions measured microscopically. Breadth and length of each grain were measured by ocular micrometer intercepts; grain thickness was determined from the difference between readings of the fine focusing vernier when focused on the top of the grain and on the grain substrate. These four figures were translated into grain dimensions by a computer program that also calculated various shape factors and ratios between dimensions and mechanically plotted the results in different ways. The Corey shape factor that was plotted against the breadth of the grains, the dimension that controls response to sieving, conveyed the most useful information.

Relations between median sizes of black sand and precious metals, taking the effect of shape factors into account, suggest that the gold concentrate from one sluice accumulated under conditions of hydraulic equivalence and that another did not. Sharp breaks at the find ends of the precious metal cumulative curves seem to indicate the smallest size of effective accumulation for each sluice. The size is smaller for the material believed to have been accumulated under conditions of hydraulic equivalence. The size-distribution curves of a gold-bearing gravel indicate that all the gold could be concentrated in 35 percent of the gravel by sieving at a critical size.

The two gold samples reported have average Corey shape factors of less than 0. 4 and the two platinum samples have factors greater than 0. 4, indicating that the platinum is more spherical than gold. On the shape factor versus size plots, the grains become more spherical as the size decreases although there are some highly spherical, coarse grains of gold in one sample. One of the platinum samples shows a decrease in shape factor as the grain size decreases. The shape data suggest two populations of grains in one of the gold and in one of the platinum samples, though there are two possible methods of interpretation that identify two slightly different pairs of populations.

Sedimentologic data on detrital precious metal grains and host sediments can be useful in designing recovery processes and in interpreting nature and genesis of placer deposits.

Publication authorized by the Director, U. S. Geological Survey.