GEOLOGICAL SURVEY CIRCULAR 609 

Mercury in Soil Gas and AirA Potential Tool in 

Mineral Exploration 

Mercury in Soil Gas and AirA Potential Tool in 

Mineral Exploration 

By J. H. McCarthy, Jr., W. W. Vaughn 

R. E. Learned, and J. L. Meuschke 

GEOLOGICAL SURVEY CIRCULAR 609 

Washington r 969 
United States Department of the Interior 

WALTER J. HICKEL, Secretary 

Geological Survey 

William T. Pecora, Director 

First printing 1969 

Second printing 1969 

Third printing 1970 

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 
Abstract --------------------------------

Introduction -----------------------------

Acknowledgments ------------------------

Apparatus and technique for measuring 

mercury in soil gas -------------------- Ground experiments------------------

Apparatus and technique for measuring 

mercury in air---·----------------------

CONTENTS 

Page 

1 

1 

1 

1 

1 

2 

Apparatus and technique for measuring 

mercury in air-.. Continued 

Experiments from ground-based 

vehicles---------------------------

Aircraft experiments-----------------

Conclusions -----------------------------

Significance ----------------------------- References------------------------------

ILLUSTRATIONS 

Page 

2 

3 

3 

4 

4 

Page 

Figure 1. Photograph of transparent plexiglass tent------------------------------------------- 7 

2. Photograph of wire basket on top oftent-------------------------------------------- 7 

3. Map showing mercury content in soil gas at Cortez, Nev ----------------------------- 8 

4. Map showing mercury content in soil at Cortez, Nev--------------------------------- 9 

5-7. Scatter diagrams showingS. Mercury content in soil gas versus mercury content in soil---------------------- 10 

6. Mercury content in soil gas versus amount of mercury released from soils 

at 60°C ------------------------------------------------------------------ 11 

7. Relation of barometric pressure to mercury content in soil gas ------------------ 12 

8-11. Graphs showingS. Variation of mercury content in soil gas with time of collection ------------------ 13 

9. Relation of thickness of overburden to mercury content in soil gas---------------- 14 

10. Reproducibility of measurements of mercury in soil gas------------------------- 15 

11. Mercury in air as a function ofaltitude---------------------------------------- 16 

TABLE 

Page 

Table 1. Summary of data for mercury in air collected by means of aircraft---------------------- 3 

III 

Mercury in Soil Gas and Air-A Potential Tool in 

Mineral Exploration 

By J. H. McCarthy, Jr., W. W. Vaughn, R. E. Learned, 

and J. L. Meuschke 

Abstract 

The mercury content in soil gas and in the atmosphere was 

measured in several mining districts to test the possibility that 

the mercury content in the atmosphere is higher over ore deposits 

than over barren ground. At Cortez, Nev., the distribution of 

anomalous amounts of mercury in the air collected at ground level 

(soil gas) correlates well with the distribution of gold-bearing 

rocks that are covered by as much as 100 feet of gravel. The 

mercury content in the atmosphere collected at an altitude of 200 

feet by an aircraft was 20 times background over a mercury deposit and 10 times background over two porphyry copper deposits. 

Measurement of mercury in soil gas and air may prove to be a 

valuable exploration tool. 

INTRODUCTION 

Mercury anomalies have been found in rocks and 

soils of many base- and precious-metal ore deposits 

(Gott and McCarthy, 1966; Gott and others, 1967; 

Hawkes and Williston, 1962; Ozerova, 1962), and their 

successful use as a pathfinder has been demonstrated 

(Erickson and others, 1966). Sergeev (1961) has demonstrated a correlation between the concentration of 

mercury in soil and the soil gases that overlie mercury 

deposits. The high vapor pressure of mercury suggests 

that mercury vapor would be released from a source 

of mercury at depth. The diurnal variation in atmospheric pressure and temperature would cause pulsations in the atmosphere and soil gas and would result 

in the ex h a I at ion of mercury through the earth's 11 breathing process. 11 

In the preliminary experiments described here, the 

mercury in soil gas and in the atmosphere was collected by amalgamation on gold or silver foil and was 

subsequently released and measured by an atomicabsorption instrument (Vaughn, 1967). A small aircraft was used to sample air at altitudes of 100 to 

1,000 feet. 

These experiments were undertaken as part of the 

Heavy Metals program of the U.S. Geological Survey 

in an effort to develop new methods of geochemical 

exploration. The data collected to date, though few, 

are encouraging. Additional experiments are underway 

to evaluate this technique further. 

ACKNOWLEDGMENTS 

It is a pleasure to acknowledge the cooperation of 

Mr. Lee Stoiser and Mr. Hatfield Goudey, of the 

American Exploration & Mining Co., for supplying 

maps and information on depth of overbltrden in the 

Cortez area, and Mr. W. C. Lawson, of the PhelpsDodge Corp., andMr. J. H. Courtright, of the American Smelting and Refining Co., for their help .in arranging low-level flying over the copper deposits at 

Ajo and Silver Bell. 

APPARATUS AND TECHNIQUE FOR MEASURING 

MERCURY IN SOIL GAS 

Pyramidal tents of transparent plastic were used to 

funnel soil gas through gold or silver f~' 1 to t r a p 

mercury. The tents (fig. 1) were constructed of four 

pieces of 1/8-inch-thick plexiglass, which were 2 feet 

wide at the base and tapered to 2 inches wide at the top; 

vertical height of the tent was 22 inches. T'· e tent was 

formed by joining the plexiglass panels with masking 

tape. A basket made of 80-mesh stainless-steel wire 

cloth was placed over the opening in the top and was 

secured with tape (fig. 2). The tent was then placed 

on the ground and soil was banked around its base to 

exclude free movement of air into it from the outside. 

Gold or silver flakes (about 1 gram) we1·e spread 

evenly in the wire basket, and a canvas s~mple sack 

was taped loosely over the top to prevent the flakes 

from blowing away (fig. I). The flakes were 0.007 inch 

thick and about 1/16 inch square. Under th~ influence 

of the sun's rays, the air temperature inside the tent 

immediately began to rise, setting up convection currents that carried the air in the tent upwErd through 

the trap. The tent was left in position for 2 rours, during which time 175 cubic feet of air passed through 

the trap. In practice, about 10 tents were placed along 

a traverse within a period of 2 hours and were then 

collected in order. 

Ground Experiments 

The recently discovered gold deposit at Cortez, 

Nev., was chosen for study because rocks in the area 

contain anomalous concentrations of mercl'ry closely 
associated with the gold (Erickson and others, 1966). 

Much of the area is covered by recent gravels. Measurements of mercury in soil gas were undertaken to 

determine (1) if measurable amounts of mercury vapor 

from the gold deposit were migrating through the gravel 

into the atmosphere and (2) if a significant difference 

existed between the mercury content in soil gas over 

barren bedrock and that over gold-bea:ring bedrock. 

The results of this study show that greater amounts 

of mercury in the soil gas are found in the area of 

known gold mineralization and that the configuration 

of the mercury anomaly is similar to the configuration of the gold deposit in the bedrock (fig. 3). 

Soil samples were also taken at each tent locality 

and analyzed for mercury. The results (fig. 4) show 

that the mercury content in the soil does not reflect 

the known gold deposit as well as does the mercury 

content in the soil gas. The mercury collected from 

soil gas during a 2-hour period ranged from 2 ng 

(nanograms) to 175 ng, with background being about 

5 ng. The maximum anomaly-to-background ratio, 

therefore, was about 35: 1. The background mercury 

content in soil in the area is about 30 ppb (parts per 

billion) and the highest amount found was 200ppb; thus 

the maximum anomaly-to-background ratio is 

about 7:1. 

Although the configurations of the anomalies for 

mercury in soil gas are different from those in soil, 

they are sufficiently similar to suggest that the mercury detected·. in the soil gas may have been derived 

from the soil. However, the lack of correlation between mercury in soil gas and mercury in soil, as 

shown in figure 5, indicates that the source of the 

mercury found in the soil gas is not the surface soil. 

The conforll},ity of the soil-gas anomaly to known mineralized bedrock suggests that the mercury was derived from bedrock. 

In order to further test the possibility that the mercury in soil gas may have been derived from the soil 

itself, 10 g of soil collected at the tent sites was heated 

at 60°C for 2 hours and the expelled mercury was collected by amalgamation on gold or silver and measured; 

the temperature was similar to that actually attained 

within the tents during the field experiments, and the 

heating time was the same. The values obtained were 

plotted against those of mercury in soil gas that was 

collected at corresponding field sites (fig. 6). The apparent randomness of the plot, as in the experimemt 

previously described, suggests that the soils were not 

the source of the mercury detected in the soil gas. This 

conclusion is further substantiated by comparison ,of 

the total amount of mercury in the soil and the total 

amount in the soil gas, as show,, in figure 6. A maximum of 6 ng of mercury was released from the soils 

in a 2-hour period; as much as 175 ng of mercury, 

however, was trapped from the soil gas in the same 

period of time. 

The relation between the mercury content in several 

soil-gas samples and the barometric pressure at the 

2 

time of collection is plotted in figure 7. Although insufficient data have been collected to clearly demonstrate the dependence of mercury in soil gas on change 

in barometric pressure, the available data suggest 

that more mercury is detected at lower t"an at higher 

pressure. 

Daily measurement of barometric pre~sure with a 

microbarograph has revealed a consistent d i u r n a I 

variation in all areas studied. The pressure begins to 

drop at about 8:00-9:00 a.m. and falls steadily until 

about 6:00-7:00 p.m.; then it begins to rise steadily 

through the night. Thus, if no atmospheric disturbances 

exist, the pressure record transcribes an approximate 

sine wave with maximum rate of drop about midday. 

Variation of mercury content in soil gar' collected at 

different times during the day is shown in fig u r e 8. 

These data show that the maximum amou,t of mercury 

is collected about mjdday, a period corresponding to 

the maximum. rate of fall in barometric p r e s s u r e. 

The effect of the thickness of overburden on soilgas measurements was investigated in the Cortez area. 

Thickness of overburden is plotted against mercury 

content in soil gas in figure 9. The data suggest that 

more mercury is found in the soil gas over deeper 

overburden, but additional data are needed to confirm 

or deny such a correlation. However, VS'riable thicknesses of overburden did not seem to han per the successful use of mercury in soil gas in o·.ttlining mineralized bedrock at Cortez. 

The reproducibility of the soil-gas rreasurements 

was determined by repeating, measurements at eight 

sites in the Cortez area on different days; the data 

are plotted in figure 10. With the exception of the 

measurements at site 1, the pattern obtained was the 

same each day. 

Mercury in the soil gases in several other mining 

districts was investigated. In the 1 vanhoe district north 

of Battle Mountain, Nev., mercury insoilgaswas collected by means of the plastic tents. A tent set up on 

soil over the projection of a cinnabar-bearing vein collected 600 ng of mercury in 2 hours, wher~as a tent set 

up 50 feet away from the vein collected 100 ng of mercury during the same period. At the Silv~r Cloud mine 

in the same district, plastic tents were used to collect 

mercury in soil gas overdisseminatedandbandedcinnabar occurring in s i 1 i c i fie d volcanic tuff and ash 

(opalite). As much as 1,300ngofmercurywas collected 

in a 2-hour period, whereas on 1 y 4-5 r-r of mercury 

was collected in tents in areas away from the opalite. 

Mercury in soil gas was collected at several sites in 

the Coeur d'Alene district of Idaho. As nuch as 60 ng 

of mercury was found in soil gas collect,edin a 2-hour 

period. The highest amounts coincided with known mineralized areas. 

APPARATUS AND TECHNIQUE FOR MEASURING· 

MERCURY IN AIR 

The ground experiments demonstraterl that rr..ercury 

vapor was released to the atmosphere. T'1e next logical 
step was to determine how high into the atmosphere the 

mercury vapor ascended and if it too reflected bedrock 

mineralization. If it could be demonstrated that mercury vapor was found in the above-surface air, it might 

make possible the collection of mercury in air from 

ground-based vehicles or aircraft and greatly broaden 

the potential application of this exploration technique. 

First, it was necessary to determine if m e r c u r y 

could be found in the air above the ground. Traps 

were set up above the bedrock surface of cinnabarbearing opalite at the Silver Cloud mine. The traps 

were mounted on a pole at heights of 1 foot, 7 feet, and 

14 feet. The following data were obtained: 

Height above surface 

(in feet) 

14------------------ 7------------------ 1------------------

Replicate measurements 

of mercury collected 

in a 2-hour period 

1 

26 

13 

(in nanograms) 

2 

17 

19 

12 

3 

43 

60 

13 

The volume of air passing thro:..tgh the traps was not 

measured; however, the experiment showed thatmercury could be detected at least as much as 14 feet above 

the surface. The smaller amounts collected as 1 foot 

above may have been the result of less air pas sing 

through the traps because of frictional drag near the 

surface, and indeed, the greater variability in amounts 

of mercury found in the higher traps suggests greater 

variability in the volume of air passing through the traps. 

Experiments from Ground-based Vehicles 

After demonstrating that mercury vapor was fo:..tnd 

in air above the ground at the Silver Cloud mine, the 

next step was to determjne if differences in the mercury content in this air reflected known mineralization. 

Air sampling was done from a moving vehicle along a 

40-mile traverse through the Ivanhoe mining district. 

Air was captured through a 6-inch funnel which was 

mounted on the front bumper of the vehicle; the air 

was then brought into the cab through plastic tubing and 

was passed through gold flakes to trap ther mercury. 

The traps were changed at 3-mile intervals. The lowest 

amount obtained was 2 ng and the highest, 19 ng. The 

highest amounts were collected over the area of known 

mercury production. 

Aircraft Experiments 

Because significant quantities of mercury had been 

found as much as 14 feet above the s u r fa c e of the 

ground, additional experiments were carried out to 

determine ( 1) the altitude to which mercury ascends in 

air and (2) the correlation, if any, of mercury in air 

at higher altitudes with known mJ.neral deposits. 

An intake tube was installed in a single-engine Beaver aircraft to bring air inside the cabin, and a meas3 

ured volume of air was ptssed through no'-)le-metal 

foil traps. In order to determine the optimun altitude 

for mercury collection, air was .collected at different 

altitudes from 100 to 1,000 feet. Thedataare shown in 

figure 11. These data show a tapering off of mercury 

collected above 200 feet, and therefore all subsequen't 

flying in these experiments was done at an rltitude of 

200 feet. 

Mercury in air was measured in Arizona over two 

mercury deposits and two porphyry copper deposits, 

and several long traverses were flown along the highway from Quartzsite, Ariz., to the Colorad<' River at 

Blythe, Calif., in order to determine the br~kground 

concentration of mercury in air. The data ob<:ained are 

summarized in table l, One mercury depo~it was in 

the Superstition Mountains east of Phoenix; the other, 

in the Dome Rock Mountains near Quartzsiw. where 

cinnabar occurs in Cambrian schist. Air collected over 

these two deposits contained 10-20 tim•es m-:-~re mercury than background air. 

Mercury was collected in airovertheporphyrycopper deposits of the New Comella pit at Ajo and the 

Tiro and Oxide pits at Silver Bell. AtAjo, the fact that 

the greatest concentration of mercury collected ( 30 ng 

per cubic meter) was over the edge of the pit suggests 

a possible peripheral enrichment of mercury. High~r 

concentrations of mercury were found in air over the 

open pits at Silver Bell than at Ajo. At ~th copper deposits the mercury in air was 5-l 0 times the mercury 

in background. 

CONCLUSIONS 

Mercury in soil gas collected at the suyface has 

delineated part of the kn·;)'Wll gold deposits rt: Cortez, 

Nev., through as much as 100 feet ofgravel. Mineralized veins and faults are readily detectable by measurement of mercury in soil gas. 

The single experiment tn which mercury in air was 

collected from a moving vehicle showed that the areas 

of the highest concentrations of mercury coincided 

with the known mineralized areas in the Iva"lhoe district, Nevada. 

Table 1.--Summary of data for mercury in air 

collected by means of aircraft 

[Results are in nanograms per cubic meter] 

Area 

Superstition Mountains··-·· 

Dome Rock Mountains--··-·--

Ajo-------------------··--· 

Silver Bell-----------··--· 

Colorado River at Blythe, 

Calif .• to Quartzsite, 

Ariz. 1-------------------

l/Background concentration. 

Min 

58 

12 

12 

18.5 

1.6 

Max 

66 

57.5 

30 

53.2 

7.2 

Avg 

62 

31.4 

18.8 

27.6 

4 •. s 
Measurable amounts of mercury were detected in 

air as high as 1,000 feet above the surface, and the 

greatest concentrations were found below 300 f e e t. 

Higher concentrations of mercury are found in air over 

base- and precious-metal ore deposits than in air over 

unmineralized bedrock. 

SIGNIFICANCE 

Experience in geochemical exploration has shown 

that dispersion halos, which reflect mineralized rock, 

are usually limited to bedrock or residual overburden. 

Consequently, geochemical methods have not been successfully applied in areas of transported or postmineralization overburden. If, however, postmineralization overburden is permeable to the passage of mercury vapor, the measurement of mercury in soil gas 

or air may be useful in detecting concealed mineral 

deposits. The nature of the overburden and perhaps 

even its thickness may be of minor importance. Such 

a technique might be used to investigate the mineral 

potential of pediments in the Basin and Range province 

or of the vast areas of the southwest that are overlain 

by volcanic rocks or alluvium. 

The use of an aircraft to detect mercury may make 

it possible to explore large and in access i b 1 e areas 

rapidly. It may be possible to delineate or extend 

metallogenic provinces by using mercury as an indicator element, or it may be possible to detect new 

large regional mineral belts or trends, particularly if 

the data on mercury are used in conjunction with regional geophysical data. 

4 

REFERENCES 

Erickson, R. L., Van Sickle, G. H., Nakag~wa, H. M., 

McCarthy, J. H., Jr., and ~eong, K. W., 1966, Gold 

geochemical an om a 1 y in the Cortez d i strict, 

Nevada: U.S. Geol. Survey Circ. 534, 9 p. 

Gott, G. B., and McCarthy, J. H., Jr., 1966, Distribution of gold, silver, tellurium, and mercury in the 

Ely mining district, White Pine County, Nevada: 

U.S. Geol. Survey Circ. 535, 5 p., 10 maps. 

Gott, G. B., McCarthy, J. H., Jr., Van Sickle, G. H., 

and McHugh, J. B., 1967, Distribution of gold, tellurium, silver, and mercury in part of the Cripple 

Creek district, Colorado: U.S. Geol. SPrvey Circ. 

543, 9 p. 

Hawkes, H. E., and Williston, S. H., 1962, Mercury 

vapor as a guide tolead-zinc-silverdeposits: Mining Cong. Jour., v. 48, no. 12, p. 30-32. 

Ozerova, N. A., 1962, Problems of geoch~mistry; 4, 

Primary dispersion halos of mercury: Akad. Nauk 

SSSR Inst. Geol. Rudnykh Mestorozhde""'iy, Petrografii, Mineralogii i Geokhimii Tn•dy, no. 72, 

p. 3-135. 

Sergeev, E. A., 1961, Methods of mercurometric investigations: Internat. Geology Rev., v. 3, no. 2, 

p. 93-99. 

Vaughn, W. W., 1967, A simple mercury varor detector 

for geochemical prospecting: U.S. Gecl. Survey 

Circ. 540, 8 p. 
FIGURES 1-ll 
if 
Figure 1. --Photograph of transparent plexiglass tent. Tent is 2 feet wide at base. 

Figure 2.--Photograph of wire basket on top of tent. Basket is 2 inches long on each side. 

7 
• 

• 

• 

EXPLANATION 

• 

• 

• 

Sample locality 

Nanograms of mercury 

collected in 2 hours 

by means of plastic 

tents r ..... ....... ... , 

>100 

f&1 

50-100 

f7//1 

30-50 

Approxiaate outline 

of part of the 

gold deposit 

• 

• 

• 

• 

• 

0 500 FEET 

• 

• 

Figure 3. -Mercury content in soil gas at Cortez, Nev. 

8 

Cortez 

• 

NEVAO.\ 

• 
40°11'30" 

EXPLANATION 

• Sample locality • • • 

Mercury in soil (ppb) 

f· ... ·J • . . . . ·. • 

>100 • 

~ 50-100 0 500 FEET ....__,_,_~ 

Approximate outline 

of part of the • • 

gold deposit • 

Figure 4.--Mercury content in soil at Cortez, Nev. 

9 
250 

• 

z 

0 

1-t 200 ....:I • ....:I 

1-t 

lXI 

~ 

~ • j:l... 

Cl) 

E-t 

~ 

< • • j:l... 

z 150 • 1-t 

.. • • ~ 

E-t 

1-t • • Cl) 

E-t 

z 

IJ1 • -· E-t 

E-t 100 

< • ....:I • .. 1-t • 0 - Cl) • 

z • 1-t 

>-

:::::> 

~ ••• 

•• 

• •• 

- u so ...... ...... • • ~ 

~ 

:::E .. • -• • - • 

... 

... 

.. • • • • • 

0 so 100 150 200 250 

MERCURY IN SOIL GAS, IN NANOGRAMS 

Figure 5.--Scatter diagram showing mercury content in soil gas versus mercury content in soil. 

10 
180 

• 

160 

140 

• 

U) 120 

C.!:) 

~ 0 

z 

<( 

z 

z 100 H 

.. U) 

<( 

C.!:) 

....:l 

H 

0 

U) 80 • z 

H • 

>-

~ 

::::> 

u 

~ • ~ 

~ 60 

• • 

40 • 

• • 

• • • 

• 

• 20 • • • • • • • • • • 

0 1.0 2.0 3.0 4.0 s.o 6.0 7.0 

MERCURY RELEASED FROM SOIL SAMPLES HEATED 2 HOURS AT 60°C, IN NANOGRAMS 

Figure 6.--Scatter diagram showing mercury content in soil gas versus amount of mercury re- leased from soils at 60°C. 

11 
30.5 

• 

• • • Cl) 

~ 

:c 30.4 u 

z 

1-4 

z 

1-4 • .. fQ 

g:: 

::;:, 

.. Cl) 

Cl) 30.3 .... fQ 

•••• g:: 

p.. • • u 

1-4 • 

g:: 

6-4 

fQ 

::E 

0 

g:: 30.2 < 

~ 

• • • • • - 30.10 20 40 60 80 10C 120 

MERCURY IN SOIL GAS, IN NANOGRAMS 

Figure 7. -Relation of barometric pressure to mercury content in soil gas. 

12 
14 

tl) 

~ t.!l 

0 12 • z 

c( 

~ 

z 

..... 

.. )'I 

c:ll:: 10 ::> 

u 

c:ll:: 

~ 

8 

6 

10:00 11:00 12:00 1:00 2:00 3:00 4:00 5:00 

A.M. TIME OF DAY P.M. 

Figure 8.--Variation of mercury content in soil gas with time of collection. 

13 
1SO 

E-o 

~ 

~ 

~ 

z 

~ 100 .. • 

z 

~ • • 0 • 

0:: 

::::::> 

• ~ 

0:: • ~ 

> 0 • ~ 

0 • • • • • U) 

U) • ~ so z 

~ • u • ~ 

E-o 

::I: .. 

• • • • 

• • • 

• 

• • • • 

0 so 100 150 200 

MERCURY IN SOIL GAS, IN NANOGRAMS 

Figure 9.--Relation of thickness of overburden to mercury content in soil gas. 

14 
100 

V'J 

::E: 

~ (.!) 

0 

:z: 

< :z: 

:z: 10 1-1 

~ ;:::, 

u 

~ 

P-1 

:::g 

2 3 4 

oFirst-day measurements 

6Second-day measurements 

5 6 7 

SITB NUMBER 

Figure 10.--Reproducibility of measurements of mercury in soil gas. 

15 

8 
7.r------.-------r------~------r------.-------r------~------r------.------~ 

6· ~ 

u.:l 

E-t 

u.:l 

::E 

u s· ..... 

,:Q • ::> 

u 

~ 

u.:l 

~ 4 Cl) 

t!) 

~ 0 

~ 3 

:z: 

:z: 

..... 

.. 

~ 2. 

::> 

u 

~ 

u.:l 

::E 

1 

0 100 200 300 400 sod 600 700 800 900 iooo 

ALTtTUDE, IN FEET 

Figure 11.--Mercury in air as a function of altitude. 

16 

* U.S. GOVERNMENT PRINTING OFFICE : 1970 0 - 403-258