Mercury interactions in a simulated gold placer

A simulated stream sediment bed was constructed in a laboratory to determine whether dissolved Hg could be transported through sediment and deposited as amalgam on Au grains. Metallic Hg was placed in a sump at one end of a tank filled with gravel (quartz sand, granules, and pebbles), and Au grains were buried in the gravel at the other end. Water was circulated in a continuous closed loop over the Hg and through the gravel that included the Au grains for more than 850 days. The Hg content of the water increased from nil at the beginning to approximately 0.5 μg/l after approximately 22 days. The Hg content on the rims of Au grains went from nil to approximately 0.2 wt.% over 22 days reaching a maximum 0.48 wt.% Hg after 14 days. Subsequent measurements indicated a persistent decrease of the maximum Hg on Au grains to ⩽0.19 wt.% Hg at 552 days and to ⩽0.05 wt.% at 851 days. Deposition of Hg on the Au grains indicates that amalgams can form without actual contact of Hg on Au in stream sediments. Why Hg first deposited on Au and then dissolved from the Au is unknown, but a paucity of microbiota early in the experiment and subsequent development of microbiota that could facilitate dissolution of Hg is suspected. The simulated Au placer, with its coarse sediments and free water flow, is analogous to streams that have measurable (>0.2 μg/l) Hg in the water and no amalgams on Au grains within the sediments. An example of a Au mining region with similar water concentrations of Hg would be the Amazon Basin, although information on the presence of amalgam rims on Au grains is lacking, as for most regions. Lower but still measurable (⩽0.55 μg/l Hg) concentrations of Hg in stream water and a lack of amalgams on Au grains occur in Au placers near Talladega, Alabama. The opposite case would be streams with less-than-measurable (<0.2 μg/l) Hg in the water but amalgams on Au grains, where conditions are less aerated and Hg would be more likely to remain in the substrate. This situation is analogous to Au placers in the North Carolina Piedmont (South Mountains, Robbins, and High Point), where Hg is not detected in stream water (<0.2 μg/l) and Au grains possess amalgamated rims. Mercury concentrations in the air over the tank (41–465 ng/m3) varied inversely with barometric pressure (1012–1033 mb @ SL), with a positive response to light, which is consistent with the work of other researchers. The positive photo effect on Hg concentration in the air was obvious at lower barometric pressures (∼ave. 1015 mb @ SL) but subdued or nonexistent at higher barometric pressures (∼ave 1025 mb @ SL). Mercury concentrations were as much as 3 times as high during daylight hours compared to nighttime concentrations over the tank at relatively low barometric pressures. The Hg content of the water remained relatively low (<1 μg/l) through the first 200 days and then abruptly increased where it oscillated between 4 and 17 μg/l to the end of the experiment (851 days). Meanwhile, the rate of loss of Hg from the tank averaged approximately 1 μg/cm2 day with a high of 1.54 μg/cm2 day. Apparently the release of Hg is little affected by the Hg content of the water, as long as a minimum amount of Hg is maintained in the water. The release rate of Hg from the tank experiment is approximately 10 times higher than those reported by other workers but probably represents a maximum due to ideal, oxidizing, high water flow conditions in the tank.