John Callahan

Mercury pollution as a result of gold extraction in North Carolina, U.S.A.

Abstract Water, sediment and panned concentrate from active streams, together with some mosses and well waters, all from the vicinity of old Au operations in North Carolina, were analyzed to determine the extent of pollution from metallic Hg introduced into these areas in the 1800s and early 1900s and by modern “weekend panners”. Heavy mineral concentrates, Au grains, sediment and moss were all found to be indicators of Hg pollution, with concentrations of up to 784,000 μg/kg in heavy mineral concentrates, 7400 μg/kg in sediments, and 4900 μg/kg in moss. Surficial spots on Au grains contained as much as 44.8% Hg. Analyses of fish tissue from several of the drainage channels did not indicate Hg pollution with all values below the North Carolina average of 210 μg/kg. Mercury concentrations in stream and well waters were all below the LLD of 0.2 μg/l. In North Carolina, heavy mineral concentrates appear to be the best indicators of introduced metallic Hg.

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.

Utilizing soil geochemistry, ground magnetic, and radiometric surveys to outline a blue ridge, ultramafic body in North Carolina

Abstract A soil geochemical survey for Ni plus magnetic and radiometric surveys were used to outline a small dunite body on Rich Mountain, North Carolina. The dunite body is poorly exposed and is enclosed by garnet-bearing amphibolite of the Ashe Formation. Of the geophysical techniques used, the magnetic survey gives a better definition of the known extent of the body than the radiometric survey. However, the 400-ppm Ni contour from the soil program appears to best define the dunite and suggests that it extends farther to the north and east than was indicated from geologic mapping. Though HF digestion yields higher Ni values than HNO 3 + HCl digestion, the patterns are the same. The Mn content of the soil at the Rich Mountain locality probably could also be used to outline the dunite body.

A geochemical survey of stream sediments in Watauga County, North Carolina

Abstract Results of a reconnaissance geochemical exploration program in Watauga County, North Carolina, indicate: 1. (1) There is potential for copper in the amphibolite member of the Ashe Formation. 2. (2) There is potential for uranium in the Cranberry Gneiss and Blowing Rock Gneiss, although the uranium in the Cranberry Gneiss may be associated with small pegmatite intrusions and that in the Blowing Rock Gneiss may be associated with high zircon concentrations. 3. (3) A large concentration of zircon in the stream sediments could be the source of higher concentrations of uranium and lead. 4. (4) High specific conductance values are associated with contaminated streams and streams draining the Fries fault zone, possibly reflecting carbonate concentrations in the fault zone. 5. (5) Manganese appears to be concentrated in the finer fractions (minus 80-mesh) of the stream sediments. 6. (6) The zinc and lead, and possibly copper, concentrations of the heavy mineral fraction appear to give a greater contrast between background and anomalous levels in areas of chalcopyrite and galena-sphalerite mineralization than those of the minus 80-mesh fraction.

A rapid field method for extracting the magnetic fraction from stream sediments

Abstract Magnetic fraction concentrates for geochemical analysis can be obtained with an automagnet directly from wet stream sediments, without prior concentration by panning.