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Chapter 14 Oxygen and carbon dioxide in soil air
Geochemical Remote Sensing of the Subsurface Edited by M.Hale
Handbook of Exploration Geochemistry, Vol. 7 (G.J.S. Govett, Editor) 9 Elsevier Science B.V. All rights reserved
451
Chapter 14
OXYGEN AND CARBON DIOXIDE IN SOIL AIR J.S. LOVELL
INTRODUCTION It is self-evident that the oxidation of sulphide minerals entails the consumption of oxygen. The initial source is molecular oxygen from the atmosphere but this must pass into solution in groundwater or soil solutions before any reaction with sulphides is possible. Interstitial air in soils, overburden or porous rocks forms an intermediate reservoir of oxygen between buried sulphides and the free atmosphere. The oxidation may be entirely chemical or may be enhanced by the microbial action of bacteria such as Thiobacillus thiooxidans. The oxidation of sulphides leads to the production of sulphuric acid, which will be neutralised by any available carbonates with the release of gaseous carbon dioxide into the subsurface surroundings and ultimately into the atmosphere. Thus the oxidation of a sulphide ore body will lead to consumption of molecular oxygen and probable production of gaseous carbon dioxide. A porous overburden will form a buffer in which restricted diffusion, dispersion and replenishment will accentuate and retain anomalous gaseous activity. It is rewarding and surprising to calculate the quantities of oxygen consumed by sulphides and the amount of carbon dioxide that may be generated. If the oxidation of pyrite proceeds to goethite thus, 4FeS2 + 1502 + 10H2O
=
2Fe203(H20)
+ 8H2804
then one tonne of pyrite will consume one tonne of oxygen and produce 1.6 t of sulphuric acid. If sufficient carbonate is available then the reaction, CaCO3
+ H2804
=
CaSO4 +
H20 + CO2
will produce 0.72 t of carbon dioxide. The oxygen consumed has a volume of 700 m 3 and the carbon dioxide generated will occupy 370 m 3. With an atmospheric concentration of 20.9% oxygen, this would totally deplete 3350 m 3 of air. If the gas is derived within a rock with a porosity of 20%, then the oxygen will be entirely removed from 16,750 m 3 of cover. As diffusion and mass flow will tend to replace the lost oxygen, an overall fall of 1% oxygen could be produced in 350,000 m 3 of overburden.
452
J.S. Lovell
The carbon dioxide produced would give a concentration of 0.53% in this same volume of rock or soil. The oxidation of sulphides may proceed with extreme rapidity and spontaneous sulphide fires in mines have not been an uncommon occurrence. Bateman (1950) notes that a sulphide vein in a blind and warm stope at the Leonard Mine (Butte, Montana) was oxidised to a depth of 1 m within two years. At the Ely Mine, Nevada, chalcocite ore in a bench in an open pit was oxidised so quickly that, at a depth of 10-15 m, about 15% copper within the ore was removed in solution. Thus, within the overburden above a weathering sulphide deposit, there is a potential for the development of gaseous CO2 and 02 concentrations that are anomalous with respect to the local soil-air regime. This was first considered in the former Soviet Union (Glebovskaya and Glebovskii, 1960), and there are numerous reports in the Russian literature describing the use of these gases in soil air as an exploration method (Kulikova, 1960; Khayretdinov et al., 1965; Kravtsov and Fridman, 1965; Glebovskaya, 1969; Elinson et al., 1970; Dadashev et al., 1971; Fridman and Petrov, 1976). Given that much of this literature describes apparent success in locating buried ore using CO2 and 02 in soil air, it is perhaps difficult to understand why this particular exploration technique was largely ignored in the West prior to the research of Lovell (1979) and Lovell et al. (1980).
OXYGEN AND CARBON DIOXIDE IN THE SUBSURFACE The extent to which oxidising sulphides affect the composition of the subsurface atmosphere will depend upon the rate of oxidation and the intensity of other activities that remove oxygen and generate carbon dioxide. The oxidation of pyrite was reviewed in the introduction to this chapter, and other sulphides more commonly of economic interest would be expected to behave in a similar manner. However, the stabilities of different metallic sulphides vary greatly in the secondary environment and consequently their oxidation rates differ. The rate of oxidation of pyrite has been studied in the greatest detail and may be summarised thus: 9
oxidation proceeds most rapidly below a pH of 3.5, at which point the activity of sulphide-oxidising bacteria becomes the dominant effect, whereas above this pH the oxidation is purely chemical; 9 factors reducing the solubility of iron (high pH, high concentrations of phosphate ions) retard oxidation; 9 a high static water table restricts the circulation of atmospheric oxygen and limits the sulphide oxidation to zones of freshly-oxygenated groundwater; and 9 low temperature reduces the rate of oxidation.
Oxygen and carbon dioxide in soil air
453
It is probable that the conditions that most favour a high rate of oxidation of pyrite are those which are most likely to produce a gaseous expression of sulphide oxidation in the subsurface, i.e. high ambient temperatures, fluctuating water table and emergence of sulphides above the water table from time-to-time. The presence in a sulphide mineral assemblage of minerals relatively unstable under oxidising conditions (pyrite, marcasite, chalcopyrite) is likely to lead to a better gaseous expression in oxygen and carbon dioxide than when only more stable sulphides (such as galena) are present. There are, of course, several processes that can frustrate the detection of such expressions. It is important to consider the rate at which gas enters and leaves the soil air. Baver (1972) quotes several authors and estimates that there would have to be a complete renewal of soil air every hour to a depth of 20 cm in a normal cropped soil in order to maintain its usual average composition and microbiological activity. If mineral deposits are to have adequate expression in the soil air, sulphide oxidation must clearly influence its composition at a rate commensurate with such rapid aeration. Furthermore, the oxidation of sulphides is among the least significant of activities that deplete the oxygen in the soil air and which contribute carbon dioxide. By far the most important is the mineralisation of organic carbon by microbial activity, followed by other biological processes in the soil. Any gaseous expressions of mmeralisation have to be seen against this background, which therefore deserves a brief review here. The rates of oxygen consumption and carbon dioxide production in normal agricultural soils depend upon the activity of plant roots and on microbiological activity, which are in tum dependent upon the soil moisture content and temperature, and the ease of decomposition of the organic matter in the soil. The respiratory quotient of a wellaerated soil (i.e., the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed) is close to unity (Dixon and Bridge, 1968). It should only rise above unity where water or clay minerals restrict free circulation of gases in soils and produce anaerobic pockets. There is a very wide range of published figures for oxygen consumption and carbon dioxide production in soils, because these depend upon a great many factors. For example, Monteith et al. (1964) found that the carbon dioxide flux (i.e., CO2 lost to the free atmosphere) in a non-vegetated Rothamsted clay-loam soil was 1.5 g m -2 day -! in winter and 6.7 g m -2 day -~ in summer and, for the dependence of flux upon temperature, derived the equation, R = IL,Q x/,o where Ro is the flux at 0~ and R is the flux at T~ They found that Q had a value of about 3 and the results of their investigations are given in Fig. 14-1. These figures may be rather low because Currie (1970), using a more direct method, obtained much higher summer figures than Monteith et al. (1964). The figures in Table 14-1 show a much greater rate of gaseous exchange by soil under vegetation.
454
J.S. Lovell June Aua,~
a
6
uly
"O ~N
E a
M a r9c h
X
Sept
F:b
_= O o
A p9r i l .J M a y J 9
4
2t ~Nov Dec I 5
I 10
I 15
M e a n s o i l t e m p . ( o C ) at 10 cm
Fig. 14-1, Relation between the daily soil respiration and mean soil temperature of a bare Rothamsted soil" the two curves are the plot of RoQwl~ for Q = 3 and R0 = 1.2 (upper curve) and R0 = 0.9 (lower curve) (from Russell, 1973).
Ross and Roberts (1970), among others, demonstrated that 0 2 uptake and C O 2 production were significantly positively correlated with the numbers of viable bacteria in soils and negatively correlated, although not usually significantly, with mean annual temperature. The gaseous activity of the soils did not appear to be influenced by differences in the nature of the vegetative cover, the clay or organic carbon content of the soil, or the mean annual rainfall. In a classic early study, Wollny (1881) reported that the CO2 content of the soil at 30"C increases about ten times as the soil moisture content changes from 6.8% to 26.8%. The CO2 content of soil air also increases with depth and the 02 content falls (Table 14II). The gradient is influenced by the nature of the soil: granular soils contain less than
TABLE 14-1 Oxygen consumption and carbon dioxide production from a bare soil and a soil under kale at Rothamsted, UK (g m -2 day -I) Gas exchange Oxygen consumption Carbon dioxide production
Summer (17~ Cropped Bare 24 12 35 16
Winter (3 ~ Cropped Bare 2.0
0.7
3.0
1.2
Oxygen and carbon dioxide in soil air
45 5
2O 16 10
8 0 A t 3 0 cm 20
"'.. . . . . . . . ... . . . . . . "....
............................ 02
e
....... S a n d y loam
15
%
Vol.
811ty c l a y
10
Gas
At 90 cm
20
~o
,.
Jan
.............................
Mar
May
July At
8apt
2
Nov
1 6 0 cm
Fig. 14-2. Oxygen and carbon dioxide content of the soil air at three depths in a sandy-loam and a silty-clay apple orchard.
one half as much CO2 as powdery soils; loams contain more CO2 than sands; and clays contain more CO2 than loams. There are also considerable seasonal fluctuations in the 02 and CO, contents of soil air (Fig. 14-2). The diversity of the phenomena that can affect the soil-air composition and the mutual inter-dependence of these phenomena suggest that the production of a suitable equation to predict the O, and CO2 concentrations in a given soil may be difficult if not
TABLE 14-11 Oxygen and carbon dioxide content of soil under cacao, Rivers Estate, Trinidad (Russell, 1973) Depth (cm) 10 25 45 90 120 Rate*
02 content Wet (I) Dry (2) 13.7 20.6 12.7 19.8 12.2 18.8 7.6 17.3 7.8 16.4
content . . . . . . . . . . Early dry (3) Late dry (4) 1.0 0.5 2.1 !.2 4.3 2.1 6.7 3.7 8.5 5.1 14.8 35.0
CO 2
Wet (1) 6.5 9.7 10.0 9.6 13.4
(I) Oct-Jan; (2) Feb-May; (3) Feb; (4) May; (5) Apr-May. * Observed CO2 diffusion rate from the soil (g m 2 day ~)
CO 2gradient Wet (I) Dry (5) 0.65 0.05 0.13 0.06 0.04 0.07 0.01 0.06 -0.01 0.06
456
.LS. Lovell
impossible. Furthermore, the collection of sufficient data to enable this to be even attempted would be impossible within the context of mineral exploration. Moisture content affects the activity of the soil population in two ways: through the thickness of the water films in the soil and consequently its aeration; and through the reduction in free energy of the water as the films become thinner and the soil drier. Bacteria can move only in water films and, although many are smaller than 1 ~tm in size, they only appear to be readily mobile in films appreciably thicker than this. The rate of movement becomes slower as the soil becomes drier, probably because the water-film pathway between two points in the soil becomes more tortuous (Hamdi, 1971). A point that does not appear to have received attention (Russell, 1973) is that microbial activity always involves the excretion of by-products that are toxic to the organism and, as the soil becomes drier and the water films thinner, these will diffuse into the surroundings increasingly slowly. Microbial activity tends to increase with increasing moisture content, with a corresponding increase in CO2 production. The composition of the soil air upon the microbiological population is not important until the 02 content falls below 1%. Until this level, the soil behaves as a fully aerobic environment (Greenwood, 1967). Vegetated soils have higher CO2 contents, due to root respiration, than bare soils, and the presence of decomposable organic matter increases 02 uptake and hence CO2 production (Table 14-III). The very process of preparing soils for cultivation increases their aeration and rates of both gaseous exchange and organic matter mineralisation.
TABLE 14-I11 Composition of the air in soils, percent by volume (Russell, 1973) Soil type Arable, no dung for 12 months Pasture land Arable, uncropped, no manure: - sandy soil - loam soil - moor soil Sandy soil dunged and cropped: potatoes, 15 cm - serradella, 15 cm Arable land: - fallow unmanured - dunged Grassland -
-
Usual composition 02 CO2 19-20 0.9 18-20 0.5-1.5
Extreme limits 02 CO2 10-20
0.5-11.5
20.6 20.6 20.0
0.16 0.23 0.65
20.4-20.8 20.0-20.9 19.2-20.5
0.05-0.30 0.07-0.55 0.28-1.40
20.3 20.7
0.61 0.18
19.8-21.0 20.4-20.9
0.09-0.94 0.12-0.38
20.7 20.4 20.3
0.1 0.2 0.4
20.4-21.1 18.0-22.3 15.7-21.2 16.7-20.5
0.02-0.38 0.01-1.4 0.03-3.2 0.3-3.3
Oxygen and carbon dioxide in soil air
457
SAMPLING AND ANALYTICAL METHODS A great deal of the research that has been carried out into the concentrations of 0 2 and CO2 in soil air has been for the purposes of agriculture. There is a large body of literature describing different sampling methods (e.g., Yamaguchi et al., 1962; Tackett, 1968; Dowdell et al., 1972; Burford and Stefanson, 1973; Bunting and Campbell, 1975). The Russian literature is unfortunately deficient in adequate descriptions of the analytical systems first used in mineral exploration. However, amongst the methods that have been mentioned are interferometry (Khayretdinov et al., 1965), thermal conductivity (Glebovskaya, 1969; Kulikova, 1960), gas chromatography (Dadashev et al., 1971) and mass spectrometry (Glebovskaya, 1969). The chief characteristic that distinguishes O2 and CO2 from other gases that have been used in mineral exploration is that the levels and changes in concentrations encountered in soil air are in the percentage range. The analysis is thus much simpler and can, where appropriate, rely upon the use of portable instruments.
Oxygen and carbon dioxide analysers In a number of surveys, Lovell (1979), Lovell and Hale (1983) and Lovell et al. (1979, 1983) used a Taylor Servomex DA272 oxygen analyser and a Lab-Line 2245 carbon dioxide analyser. These instruments were connected in series and samples of soil air from a probe driven about 1 m into the ground were introduced by means of a hand pump. The Taylor Servomex DA272 oxygen analyser exploits a property that distinguishes 02 from most common gases" oxygen is paramagnetic and hence tends to move to the strongest part of a non-uniform magnetic field. This movement can be measured in a Pauling cell, in which two nitrogen-filled diamagnetic spheres of glass are mounted at the end of a bar to form a dumb-bell. This dumb-bell is mounted horizontally on a vertical torsion suspension. The whole measuring cell operates within a strong, non-uniform magnetic field. The spheres are repelled from the strongest part of the field and so rotate until the force produced by the twist of the suspension is equal to the force acting on the spheres (Fig. 14-3). The strength of the magnetic field varies with the magnetic properties of the gas with which the cell is filled and this will thus govern equilibrium positions attained by the dumb-bell. The analyser uses a platinum torsion suspension, which imparts physical strength and is able to conduct a current to an electromagnetic coil used to maintain the dumb-cell at a zero position. The current required to maintain this zero position is a function of the 02 content of the gas present in the cell. The instrument can be calibrated easily in the field by the use of pure nitrogen for the zero and dry air for the span (up to 21% O2). Table 14-IV gives the magnetic susceptibilities of some common gases compared with 02. It can be seen that none of those with a susceptibility likely to interfere with the measurement would be expected to
458
J.S. Lovell
\ Force
on S p h e r e
~.. Pole Pie
\
(a)
/:.
,
..~-_"_--7 T F,o, d
..-7---- ~'"...
Restoring F o r c e of Suspension
(b)
Fig. 14-3. Determination of oxygen concentration using its paramagnetic properties: (a) principle of the Pauling cell; (b) analytical cell of Taylor Servomex OA272 oxygen analyser.
occur in significant quantities in soil air. The instrument is fitted with a scale expansion system that permits a full-scale response in the range 16-21% 02. With this system it is possible to detect changes in the 02 content of the soil air of 0.05%. The carbon dioxide analyser exploits the fact that CO2 is about 40% less efficient at conducting heat than air. This difference in thermal conductivity is measured by means of a pair of thermistors in cells which are mounted in a massive aluminium block and which form the two arms of a sensitive bridge circuit. These thermistors respond to temperature changes with comparatively large changes in resistance. The thermistors are held at an elevated temperature relative to the aluminium by a fixed current. Heat is lost to the block via the gas surrounding the thermistors. The equilibrium temperature of each thermistor, and hence its resistance, is therefore dependent upon the thermal conductivity of the surrounding gas. Air fills one cell, which forms the reference arm of the bridge, and the sample fills the other. The sample and reference air are dried prior to analysis by means of a silica gel absorption tube. For use in the field, it was found necessary to fit the instrument in an insulated polished metal box, to minimise drift due to the effects of wind and direct sunlight. The instrument is calibrated in two ranges, 0-10% and 0-20% CO,. With multiple measurements it is possible to read the meter to within 0.05% CO2 with a detection limit of 0.05% CO2. However, extreme care is needed to obtain reliable measurements at this level. A disadvantage of both instruments is that, in the field, repeated and tedious zeroing is necessary before and after each measurement in order to ensure the requisite precision.
Oxygen and carbon dioxide in soil air
459
TABLE 14-IV Magnetic susceptibilities of common gases at 100% concentration in sample, expressed in terms of oxygen equivalent Gas Acetylene Ammonia Argon n-butane iso-butane l-butane Carbon dioxide Carbon monoxide Ethane Ethylene Helium Hydrogen
Percent 02 equivalent -0.24 -0.26 -0.22 - 1.3 - 1.3 -0.85 -0.27 +0.01 +0.46 -0.26 +0.30 +0.24
Gas Hydrogen sulphide Methane Neon Nitric oxide Nitrogen Nitrogen dioxide Oxygen n-pentane iso-pentane Propane Propylene Water
Percent 0 2 equivalent -0.39 -0.2 +0.13 +43 0 +28 + 100 - 1.45 - 1.49 -0.86 -0.54 -0.02
Orstat gas analyser Ball et al. (1983a, 1990) pumped soil air from a hollow probe into a modified Orstat gas analysis apparatus. A known volume of soil air is first pumped into the gas burette of the apparatus, and subsequently transferred to integral absorption vessels (Fig. 14-4). The absorbent for CO2 is 40% aqueous KOH. The volume of gas absorbed is recorded from the gas burette. The remaining gas is exposed to a mixture of saturated aqueous ammonium chloride and ammonia in contact with copper coils for absorption of 02. Again the gas volume reduction is recorded from the gas burette. Practical limits of detection under field conditions are 0.01-0.1% for each gas.
Draeger tubes For the determination of CO2 only, Romer and Finlay (1984) pumped 100 cm 3 of soil air from a probe through a calibrated glass tube containing reactants which respond to CO2 with colour change (Draeger type CH31401). Each tube is used once and then discarded. Its pre-sealed ends are snapped off before it is attached to the pump, and the CO2 that passes through the tube changes the colour of the reactant from white to blue over a part of the length of the tube. The length of the colour change is proportional to CO2 concentrations over the range 0.5-10% by volume.
460
J.S. Lovell
I1,, ,, II ,,',
,
/\
It
/\ i
OB8 burette
t
,,/
II II
II II
Absn.
II
vo.,e!
Absn.
~
I ve.,.l
t
m
i , I
Levelling i bottle
J
Flexible tubing
,
,
[
!
I
Fig. 14-4. Schematic of the Orstat gas analyser (from Ball et al., 1990).
Gas chromatography and mass spectrometry The determination of 0 2 and C O 2 c a n be performed with considerable sensitivity by gas chromatography (Tackett, 1968; Bailey and Beauchamp, 1973; Bunting and Campbell, 1975; Blackmer and Bremner, 1977). Lovell and Reid (1989), using a hollow sampling probe driven into the ground, drew 30 c m 3 soil-air samples into a syringe and injected these into air-evacuated sealed cylinders for later gas chromatographic determination of 02 and CO 2 in the laboratory. At concentrations above about 0.1% v/v, 02 and CO 2 can be measured by mass spectrometry (Anderson et al., 1972; Nerken, 1972; Ball et al., 1973; McFadden, 1973; Newton et al., 1975; Robertson and Bracewell, 1979). McCarthy and Bigelow (1990) developed a truck-mounted mass spectrometer which McCarthy and McGuire (1998) used for on-site determination of CO 2 and other soil gases in the Carlin trend of Nevada. Soil-air samples were extracted from a hollow sampling probe by means of a syringe, from which they were injected directly into the mobile mass spectrometer.
461
Oxygen and carbon dioxide in soil air
b-surface 0 (3
air
1 14
13
12
1 1 10
9
8
7
6
5
4
3
2
1
Sample points 8-8E
N-NW
~!~:..:.:l
3 :;F__~,~~,~.~i'~,~ ~~,~.V'~~i ! . - ~ . 6 ~ r 9t~)~:--~
,,
Ey~i-
"
-~i,!
t
./.
Lower Cretaceous (Balel suite)
.
. .........
sandstone deposits
Lower Cretaceous sandstone-conglomerate deposits (Tergen suite)
Varlacsn
9.-- " "
(Unda) granltolda
Ore zones
Fig. 14-5. Relation of carbon dioxide in soil air and geology at Balei gold veins, Baykal region, Russia (reproduced with permission from Kulikova, 1960).
CASE HISTORIES
Russia
Kulikova (1960) describes the results of a gas survey over the Balei deposits in the Baykal region of Russia. The mineralisation is gold in quartz veins and the source of the gas is believed to be emissions of CO2 from depth along mineralised fractures within Lower Cretaceous arenaceous sediments. The area is covered by alluvial Quatemary deposits that are up to 10 m thick on the fluvial plains and may reach 30 m on the terraces. The samples were collected in bottles with water seals of saturated NaCI. Despite the fact that the area of investigation is underlain by continuous permafrost at depths of 3-40 m, there is a good expression of the mineralised fractures in the soil-gas data from a depth of 1.5-2 m, with values over the ore in the range 1.0-2.0% CO2 against a background of 0.2-0.8% CO2 (Fig. 14-5). Soil samples were collected and analysed for sorbed subsurface gas, which also showed evidence of an enhanced CO2 flux.
J.S. Lovell
462
5.0
~ ~
4.0 r 0
3.0 2.0 1.0
, 1966 ~""~-
0
,,
,,
,,,,,
, *,,,
,
, ,
,
,
,
,
~ ...... ,
, ,
, ,
,
June, ,
Teat
1966 points
eluvl81 and colluvl81 sediments
aleurolltlc
clays
z o n e of f o l d i n g a n d I n t e n s i v e dlcklte mineralization
fracture
of r o c k s
with
z o n e of f o l d i n g a n d i n t e n s i v e calcite mineralization
fracture
of r o c k s
with
orebodles faults
Fig. 14-6. Relation of carbon dioxide in soil air and geology at Sakhalinsk mercury deposit, northwest Caucasus, Russia (reproduced with permission from Ovchinnikov et al., 1972).
Ovchinnikov et al. (1972) give only a few details of a survey over the Sakhalinsk mercury deposit, northwest Caucasus, but its inclusion here is of interest as it illustrates the influence of a nearby earthquake. The initial survey showed a weU-arked anomaly over the mineralisation with a maximum value of 4.8% CO2 against a background of 0.5-1.5% CO2 (Fig. 14-6). The second survey, some two months later, showed values elevated by approximately a factor of three. The influence of seasonal factors cannot be ruled out, but there was a very marked change in the ~3C ratio and gaseous anomalies associated with suboutcropping faults became more marked. The source of additional CO2 is thought to be abyssal gas preferentially moving from depth along the mineralised fractures.
Azerbaijan Dadashev et al. (1971) describe a soil-gas survey over the Filizchai pyrite deposit in Azerbaijan. They report a definite relation between the position of the ore body and the composition of the soil air. As the survey line enters the ore field in the southwest, there
Oxygen and carbon dioxide in soil air
463
Fig. 14-7. Oxygen and carbon dioxide in soil air over the Filizchai pyrite deposit, Azerbaijan (reproduced with permission from Dadashev et al., 1971).
is a steady increase in the C O 2 content of soil air and a decrease in the 0 2 content. There is a marked anomaly in the composition of the soil gas northeastwards, with the O2 content falling and the CO2 content rising until the ore body plunges to greater depth (Fig. 14-7). The changes in gas concentrations are ascribed to the effects of the oxidation of sulphide minerals; the patterns provoke interest in whether the anomaly maxima coincide with the position of maximum sulphide oxidation.
Kyrgyzstan
A survey carried out over a polymetallic ore deposit in Kyrgyzstan is described by Glebovskaya and Glebovskii (1960). The traverse shows an increase over the background CO2 concentration of 1-1.5% to a diffuse anomaly of 2-3.5% over the suboutcrop of the ore zone. There is an intense 02 anomaly, with the 02 concentration falling sharply from a consistent background of 20-20.5% 02 immediately over the mineralisation (Fig. 14-8).
Namibia
At Witvlei, 160 km west of Windhoek in central Namibia, copper mineralisation occurs within a sedimentary sequence ranging from conglomerates through grits, sandstones and arkoses to siltstones and claystones. The area is underlain by a sandstone-siltstone sequence dipping at about 40 ~ to the southeast. Trenching revealed that surficial geochemical copper anomalies are due to chalcopyrite mineralisation within certain horizons. Limited drilling showed that the sulphides have been oxidised to malachite and chrysocolla to a depth of about 30 m, and that the mineralised units are
J.S. Lovell
464 % CO 2
% 02
8.0 7.0
21.0
6.0
20.0
5.0
19.0
4.0
18.0
3.0
17.0
2.0 i
16.0
1.0
15.0
0
02
CO 2 v 15
!
u
u
v
10
0
-5
-10
, -15
| -20
i -26
9
.-,'of,.'.; ""
9
. . . . : . ~: .... : .p... 9 ;. :.;: .-.~:
9 , . . , , .,., : . . . . : ' ~ . . :
9 4
...'-~ ., .. .- , ", .
gneisaoid granite leucocratlc granite ~
tectonic dislocation
quartz porphyry
~
ore body
daclte
~
un9
deposits
Fig. 14-8. Oxygen and carbon dioxide in soil air (from !.5 m) over polymetallic ore deposits in Kyrgyzstan (reproduced with permission from Glebovskaya and Glebovskii, 1960).
interbedded with calcareous arkose. There is a cover of up to 2 m of bright-red Kalahari sand with a basal rubble horizon and occasional calcrete; outcrop is rare. Within this area two soil-gas traverses were conducted; one of these is shown in Fig. 14-9. Using a convention in which 02 concentration is reported as AO2, the reduction in the 02 concentration from the background atmospheric concentration of 21%, the results show a single, low-contrast anomaly at the site of the oxide/sulphide interface with a AO2 value of 0.3% and CO2 value of 0.4%, set against a quiet background of 0.1-0.2% AO2 and 0-0.1% CO2. The other traverse carried out during this survey revealed a similar pattern, and gave a good expression of the mineralisation.
Johnson Camp, Arizona The mineralisation at Johnson Camp (see also Chapter 8) consists of chalcopyrite, sphalerite, bomite and pyrite. It was pyrometasomatically-introduced into limestones of the Naco Group of Upper Pennsylvanian to Lower Permian age by the nearby Tertiary Texas Canyon quartz monzonite intrusive (Cooper and Silver, 1964). The mineralisation has invaded the host rock along minor fractures and as disseminations. The mineralised zones have been partly oxidised near their suboutcrops and, adjacent to the innumerable mineralised minor fractures, along much of their down-dip extensions. The overall
465
Oxygen and carbon dioxide in soil air
0.4
." ~ 16 CO 2 ~ , - - - - , ~ 11, ~O 2
~
.f~
l~
,
, ,oo . ,,o.
OO t ~O2. 0., 2400
. 2:!O0
2200
/,.a. 2100
2000
,~
"
t 800
1800
Cu
-~.....,,,~... 1700
! 000
1800
$400
1800
.~ 1200
zone
................................................................. D~
L o w e r limit o f o o p p e r
oxide mlnorele
SECTION FROM TRENCH. LINE 2, DAHEIM
Fig. 14-9. Oxygen and carbon dioxide in soil air over sedimentary copper mineralisation at Witvlei, Namibia (from Lovell et al., 1983).
sulphide concentration is approximately 3%. Three zones of mineralisation have been outlined by drilling. These are concealed by pediment gravels and alluvium that increase in thickness to the northeast from 10 m over the most southerly ore zone to some 225 m over the most northerly. A soil-gas survey was carried out across this area during a dry summer (Lovell et al., 1983). Three traverses were completed, with soil air being collected from a depth of 50-100 cm; one of these traverses is shown in Fig. 14-10. The suboutcrop and down-dip extension of the shallowest body of mineralisation (Zone I) is marked by a discontinuous series of anomalous values, with maximum values of 0.75% AO2 and 0.9% CO2. These maximum values do not occur over the actual suboutcrop, but at a position where the sulphide zone is some 60 m below the surface. This is then followed by a series of background values in the range 0.15-0.3% AO2 and 0-0.15% CO2, although the traverse is still partially underlain by the down-dip extension. It is interesting to note here that this particular mineralised zone is believed to be an extension, along strike, of the old Peabody Mine (J. Kantor, pers. commun.). Scott (1916) reports that in the mine the ore was oxidised to 60 m. If the same conditions extend into the survey area, then the highest AO2 and CO2 soil-air values are to be found over the projected interface of oxide and sulphide ore. The suboutcrop of the central zone of mineralisation (Zone II), lying to the northeast and covered by 90-150 m of overburden, has a good expression in the soil-gas data, but the down-dip extension is not indicated. This is perhaps a result of oxidation being confined to the upper sections of the mineralisation by the greater depth of cover. The two other traverses conducted during this survey showed that Zones I and II, the shallowest zones, have a consistent expression in the soil air, but that Zone III, the deepest zone, covered by 225 m overburden, had no expression. A subsequent soil-air survey, following a very wet summer, revealed similar patterns in the data, but the values were greatly reduced. The background values for both AO2
J.S. Lovell
466 % CO 2 end ,~02 1.0
N
o.
0.8
0.7
9
t
2 /
o., ~ , . 0.4
Ore -,ones
/\ I !
o.s
~
o.,
~t"~,
~ ~.~,,-~ , , / ~ / ~
~
~
'~T'~
v
I/",,/ ~,,~ ,~',..,,.~
'~'--
....~" .
.
~,,,/.
~
.
,
,~..
.,
,,02
;i
o
P e d i m e n t g r a v e l s end c s l i c h e
1O0
P a l a e o z o i c ,,ediments
,oo
\\~o.,, i
0
|
100
i
200
|
300
1
400
i
600
,, i
800
700
1
800
coo
,0;0
,1;0
1200
,300
14;0
1800
1coo
, 700 '
1800
1.00
Metere
Fig. 14-10. Oxygen and carbon dioxide in soil air over pyrometasomatic mineralisation at Johnson Camp, Arizona (from Lovell et al., 1983).
and CO2 were below the limits of detection, with maximum values of 0.2-0.4% over the mineralisation (B.W. Oakes, pers. commun.).
Colorado Plateau, Arizona Mineralised breccia pipes occur in Palaeozoic sedimentary rocks of the Colorado Plateau in northern Arizona (Weinrich, 1985). These pipes are usually circular or oval at suboutcrop, have horizontal dimensions that are typically a few tens of metres and vertical dimensions that may extend to 1000 m. They appear to have developed over solution-collapse structures resulting from karstic weathering of Lower Mississippian carbonate sediments between the Upper Mississippian and Triassic. The mineralisation, comprising pitchblende and sulphides of Fe, Cu, Mo, Pb and Zn, is found beneath a massive pyrite cap, several hundreds of metres below surface. Some of the ore bodies are, or have been, mined for uranium. Gangue minerals include calcite and dolomite. The surface features of the collapse structures include: concentric, inward-dipping beds; circular areas of brecciated rock; circular bleached or ferruginous tonal anomalies; circular vegetation anomalies and circular concave topography. However, not all of these structures are mineralised, and the concentration of CO: in the overlying soils has proved to be a valuable guide to the extent of the concealed mineralisation, which may lie as much as 500 m below the surface. The development of a gaseous expression of mineralisation at such considerable depths is facilitated by a number of features. The sulphide content of the ore is very high (tens of percent) and its oxidation far below the
Oxygen and carbon dioxide in soil air
467
Fig. 14-1 I. Carbon dioxide in soil air (sample sites and contours in %) over mineralised breccia pipe, northern Arizona (from Lovell and Reid, 1989).
ground surface is promoted by a low water table and continuous supply of oxygenated groundwater, as a result of the deep canyons that developed during the uplift of the Colorado Plateau. In addition, the isolated and transgressive nature of the breccia structures appears to favour vertical gaseous movement and to limit the gaseous anomalies to the confines of the pipe. The low organic content and the poorly-developed nature of the surface soils yields a low CO2 background and permits subtle expressions of the mineralisation to be recognised. Figure 14-11 shows contoured soil-air CO2 data collected from a depth of 1 m, overlying a breccia pipe which is heavily mineralised at depths of 150-250 m. The anomaly peak is only 0.3% CO2 (only ten times the atmospheric background) but the circular gas halo is centred over the mineralisation (Lovell and Reid, 1989; Reid and Rasmussen, 1990). The fact that these low concentrations accurately reflect mineralisation at these depths is a testament to the sensitivity and precision of the gaschromatographic method of analysis that was used, and to the low background. This technique was subsequently routinely adopted as an exploration tool in northern Arizona. However, prior to its adoption, a series of trials were carried out to determine its applicability and confidence levels in this environment. Of 14 prospects that were tested, 12 were subsequently drilled. From the soil-air CO2 data it was possible to predict correctly both the presence and tenor of the underlying mineralisation in 11 out of the 12
468
J.S. Lovell
Fig. 14-12 Variation of CO2 patterns in soil air over mineralised breccia pipe, northern Arizona: a) early summer; b) 18 days later; c) a further 10 days later (from Lovell and Reid, 1989).
cases (A.R. Reid, pers. commun.), an excellent success rate for any exploration technique. Further surveys at different times of the year proved disappointing: a series of exploration targets failed to return any detectable concentrations of CO2 in the soil air; and re-sampling of areas that had previously been shown to contain CO2 related to mineralisation also failed to yield a response. As these further surveys had been carried out during the dry summer months, it was surmised that the dry soils had no capacity to retain CO2 in concentrations significantly out of equilibrium with the atmosphere. This phenomenon was studied by monitoring, for several weeks, a grid of soil sites over mineralisation (Fig. 14-12). As the soil dried, the anomaly over mineralisation faded and shifted. A later survey after heavy rainfall showed that the anomaly was re-established, and demonstrates the continuous evolution of CO2 from its source. Other surveys after winter rain and snow also revealed anomalies, including an anomaly over the one mineralised breccia pipe that was not anomalous in the initial CO2 survey of prospects.
DISCUSSION Since the techniques described here measure transient fluxes of 02 and CO2 in soil air, it is hardly surprising that measurements are not always reproducible over long
Oxygen and carbon dioxide in soil air
469
periods of time. At Johnson Camp, Arizona, surveys in different seasons yielded comparable patterns but at different concentration levels. Over the breccia pipes of the Colorado Plateau the distinctive but subtle anomalies found in one season were completely lost in another. Ball et al. (1990) found a similar seasonal pattern over auriferous sulphides in West Africa, with CO2 anomalies in soil air detected during the wet season disappearing by the end of the dry season. Even in the temperate climate of the UK, Ball et al. (1983b) found the best 02 and CO2 responses over mineralisation in the wetter winter months. The inference that dryness of the soil plays a significant role in soil-air CO2 anomaly persistence gains support from Hinkle (1990), who found that, amongst a considerable number of soil-air environment factors investigated, soil moisture content influences the CO2 level in soil air the most, although its effects are rather unpredictable. It is likely that isotopic analysis to determine the ~3C and ~80 content of various soilair samples offers a means of distinguishing those containing some CO2 generated during sulphide oxidation at depth from those whose CO2 derives solely from the atmosphere and plant respiration. This has been attempted by Alpers et al. (1990) at Crandon, Wisconsin, where McCarthy et al. (1986) found a CO2 anomaly in soil air over massive sulphides hosted by Proterozoic limestones and concealed beneath glacial till containing clasts of Palaeozoic limestone. In fact the CO2 in soil air over the mineralisation was isotopically indistinguishable from that in background samples 1-2 km away. This finding is, however, far less conclusive than it first seems to be: at the time when the samples where taken for isotopic analysis, the CO2 anomaly over the mineralisation was not particularly well developed due to recent precipitation.
CONCLUSIONS There is a substantial body of evidence to support the belief that the concentrations of 02 and CO2 in soil air can reveal the position of concealed mineralisation, even through considerable thicknesses of overburden. Surveys can be conducted using any one of several proven field and laboratory methods. The equipment required is, for the most part, inexpensive and readily available. However, there is a very wide range of concentrations of 02 and CO2 in the soil air of different environments. Also, measurements can vary markedly with the seasons. Thus it is extremely important when applying the technique in a given area to carry out local orientation tests in order to establish the true background and the contrast that may be expected.
Handbook of Exploration Geochemistry, Vol. 7 (G.J.S. Govett, Editor) 9 Elsevier Science B.V. All rights reserved
451
Chapter 14
OXYGEN AND CARBON DIOXIDE IN SOIL AIR J.S. LOVELL
INTRODUCTION It is self-evident that the oxidation of sulphide minerals entails the consumption of oxygen. The initial source is molecular oxygen from the atmosphere but this must pass into solution in groundwater or soil solutions before any reaction with sulphides is possible. Interstitial air in soils, overburden or porous rocks forms an intermediate reservoir of oxygen between buried sulphides and the free atmosphere. The oxidation may be entirely chemical or may be enhanced by the microbial action of bacteria such as Thiobacillus thiooxidans. The oxidation of sulphides leads to the production of sulphuric acid, which will be neutralised by any available carbonates with the release of gaseous carbon dioxide into the subsurface surroundings and ultimately into the atmosphere. Thus the oxidation of a sulphide ore body will lead to consumption of molecular oxygen and probable production of gaseous carbon dioxide. A porous overburden will form a buffer in which restricted diffusion, dispersion and replenishment will accentuate and retain anomalous gaseous activity. It is rewarding and surprising to calculate the quantities of oxygen consumed by sulphides and the amount of carbon dioxide that may be generated. If the oxidation of pyrite proceeds to goethite thus, 4FeS2 + 1502 + 10H2O
=
2Fe203(H20)
+ 8H2804
then one tonne of pyrite will consume one tonne of oxygen and produce 1.6 t of sulphuric acid. If sufficient carbonate is available then the reaction, CaCO3
+ H2804
=
CaSO4 +
H20 + CO2
will produce 0.72 t of carbon dioxide. The oxygen consumed has a volume of 700 m 3 and the carbon dioxide generated will occupy 370 m 3. With an atmospheric concentration of 20.9% oxygen, this would totally deplete 3350 m 3 of air. If the gas is derived within a rock with a porosity of 20%, then the oxygen will be entirely removed from 16,750 m 3 of cover. As diffusion and mass flow will tend to replace the lost oxygen, an overall fall of 1% oxygen could be produced in 350,000 m 3 of overburden.
452
J.S. Lovell
The carbon dioxide produced would give a concentration of 0.53% in this same volume of rock or soil. The oxidation of sulphides may proceed with extreme rapidity and spontaneous sulphide fires in mines have not been an uncommon occurrence. Bateman (1950) notes that a sulphide vein in a blind and warm stope at the Leonard Mine (Butte, Montana) was oxidised to a depth of 1 m within two years. At the Ely Mine, Nevada, chalcocite ore in a bench in an open pit was oxidised so quickly that, at a depth of 10-15 m, about 15% copper within the ore was removed in solution. Thus, within the overburden above a weathering sulphide deposit, there is a potential for the development of gaseous CO2 and 02 concentrations that are anomalous with respect to the local soil-air regime. This was first considered in the former Soviet Union (Glebovskaya and Glebovskii, 1960), and there are numerous reports in the Russian literature describing the use of these gases in soil air as an exploration method (Kulikova, 1960; Khayretdinov et al., 1965; Kravtsov and Fridman, 1965; Glebovskaya, 1969; Elinson et al., 1970; Dadashev et al., 1971; Fridman and Petrov, 1976). Given that much of this literature describes apparent success in locating buried ore using CO2 and 02 in soil air, it is perhaps difficult to understand why this particular exploration technique was largely ignored in the West prior to the research of Lovell (1979) and Lovell et al. (1980).
OXYGEN AND CARBON DIOXIDE IN THE SUBSURFACE The extent to which oxidising sulphides affect the composition of the subsurface atmosphere will depend upon the rate of oxidation and the intensity of other activities that remove oxygen and generate carbon dioxide. The oxidation of pyrite was reviewed in the introduction to this chapter, and other sulphides more commonly of economic interest would be expected to behave in a similar manner. However, the stabilities of different metallic sulphides vary greatly in the secondary environment and consequently their oxidation rates differ. The rate of oxidation of pyrite has been studied in the greatest detail and may be summarised thus: 9
oxidation proceeds most rapidly below a pH of 3.5, at which point the activity of sulphide-oxidising bacteria becomes the dominant effect, whereas above this pH the oxidation is purely chemical; 9 factors reducing the solubility of iron (high pH, high concentrations of phosphate ions) retard oxidation; 9 a high static water table restricts the circulation of atmospheric oxygen and limits the sulphide oxidation to zones of freshly-oxygenated groundwater; and 9 low temperature reduces the rate of oxidation.
Oxygen and carbon dioxide in soil air
453
It is probable that the conditions that most favour a high rate of oxidation of pyrite are those which are most likely to produce a gaseous expression of sulphide oxidation in the subsurface, i.e. high ambient temperatures, fluctuating water table and emergence of sulphides above the water table from time-to-time. The presence in a sulphide mineral assemblage of minerals relatively unstable under oxidising conditions (pyrite, marcasite, chalcopyrite) is likely to lead to a better gaseous expression in oxygen and carbon dioxide than when only more stable sulphides (such as galena) are present. There are, of course, several processes that can frustrate the detection of such expressions. It is important to consider the rate at which gas enters and leaves the soil air. Baver (1972) quotes several authors and estimates that there would have to be a complete renewal of soil air every hour to a depth of 20 cm in a normal cropped soil in order to maintain its usual average composition and microbiological activity. If mineral deposits are to have adequate expression in the soil air, sulphide oxidation must clearly influence its composition at a rate commensurate with such rapid aeration. Furthermore, the oxidation of sulphides is among the least significant of activities that deplete the oxygen in the soil air and which contribute carbon dioxide. By far the most important is the mineralisation of organic carbon by microbial activity, followed by other biological processes in the soil. Any gaseous expressions of mmeralisation have to be seen against this background, which therefore deserves a brief review here. The rates of oxygen consumption and carbon dioxide production in normal agricultural soils depend upon the activity of plant roots and on microbiological activity, which are in tum dependent upon the soil moisture content and temperature, and the ease of decomposition of the organic matter in the soil. The respiratory quotient of a wellaerated soil (i.e., the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed) is close to unity (Dixon and Bridge, 1968). It should only rise above unity where water or clay minerals restrict free circulation of gases in soils and produce anaerobic pockets. There is a very wide range of published figures for oxygen consumption and carbon dioxide production in soils, because these depend upon a great many factors. For example, Monteith et al. (1964) found that the carbon dioxide flux (i.e., CO2 lost to the free atmosphere) in a non-vegetated Rothamsted clay-loam soil was 1.5 g m -2 day -! in winter and 6.7 g m -2 day -~ in summer and, for the dependence of flux upon temperature, derived the equation, R = IL,Q x/,o where Ro is the flux at 0~ and R is the flux at T~ They found that Q had a value of about 3 and the results of their investigations are given in Fig. 14-1. These figures may be rather low because Currie (1970), using a more direct method, obtained much higher summer figures than Monteith et al. (1964). The figures in Table 14-1 show a much greater rate of gaseous exchange by soil under vegetation.
454
J.S. Lovell June Aua,~
a
6
uly
"O ~N
E a
M a r9c h
X
Sept
F:b
_= O o
A p9r i l .J M a y J 9
4
2t ~Nov Dec I 5
I 10
I 15
M e a n s o i l t e m p . ( o C ) at 10 cm
Fig. 14-1, Relation between the daily soil respiration and mean soil temperature of a bare Rothamsted soil" the two curves are the plot of RoQwl~ for Q = 3 and R0 = 1.2 (upper curve) and R0 = 0.9 (lower curve) (from Russell, 1973).
Ross and Roberts (1970), among others, demonstrated that 0 2 uptake and C O 2 production were significantly positively correlated with the numbers of viable bacteria in soils and negatively correlated, although not usually significantly, with mean annual temperature. The gaseous activity of the soils did not appear to be influenced by differences in the nature of the vegetative cover, the clay or organic carbon content of the soil, or the mean annual rainfall. In a classic early study, Wollny (1881) reported that the CO2 content of the soil at 30"C increases about ten times as the soil moisture content changes from 6.8% to 26.8%. The CO2 content of soil air also increases with depth and the 02 content falls (Table 14II). The gradient is influenced by the nature of the soil: granular soils contain less than
TABLE 14-1 Oxygen consumption and carbon dioxide production from a bare soil and a soil under kale at Rothamsted, UK (g m -2 day -I) Gas exchange Oxygen consumption Carbon dioxide production
Summer (17~ Cropped Bare 24 12 35 16
Winter (3 ~ Cropped Bare 2.0
0.7
3.0
1.2
Oxygen and carbon dioxide in soil air
45 5
2O 16 10
8 0 A t 3 0 cm 20
"'.. . . . . . . . ... . . . . . . "....
............................ 02
e
....... S a n d y loam
15
%
Vol.
811ty c l a y
10
Gas
At 90 cm
20
~o
,.
Jan
.............................
Mar
May
July At
8apt
2
Nov
1 6 0 cm
Fig. 14-2. Oxygen and carbon dioxide content of the soil air at three depths in a sandy-loam and a silty-clay apple orchard.
one half as much CO2 as powdery soils; loams contain more CO2 than sands; and clays contain more CO2 than loams. There are also considerable seasonal fluctuations in the 02 and CO, contents of soil air (Fig. 14-2). The diversity of the phenomena that can affect the soil-air composition and the mutual inter-dependence of these phenomena suggest that the production of a suitable equation to predict the O, and CO2 concentrations in a given soil may be difficult if not
TABLE 14-11 Oxygen and carbon dioxide content of soil under cacao, Rivers Estate, Trinidad (Russell, 1973) Depth (cm) 10 25 45 90 120 Rate*
02 content Wet (I) Dry (2) 13.7 20.6 12.7 19.8 12.2 18.8 7.6 17.3 7.8 16.4
content . . . . . . . . . . Early dry (3) Late dry (4) 1.0 0.5 2.1 !.2 4.3 2.1 6.7 3.7 8.5 5.1 14.8 35.0
CO 2
Wet (1) 6.5 9.7 10.0 9.6 13.4
(I) Oct-Jan; (2) Feb-May; (3) Feb; (4) May; (5) Apr-May. * Observed CO2 diffusion rate from the soil (g m 2 day ~)
CO 2gradient Wet (I) Dry (5) 0.65 0.05 0.13 0.06 0.04 0.07 0.01 0.06 -0.01 0.06
456
.LS. Lovell
impossible. Furthermore, the collection of sufficient data to enable this to be even attempted would be impossible within the context of mineral exploration. Moisture content affects the activity of the soil population in two ways: through the thickness of the water films in the soil and consequently its aeration; and through the reduction in free energy of the water as the films become thinner and the soil drier. Bacteria can move only in water films and, although many are smaller than 1 ~tm in size, they only appear to be readily mobile in films appreciably thicker than this. The rate of movement becomes slower as the soil becomes drier, probably because the water-film pathway between two points in the soil becomes more tortuous (Hamdi, 1971). A point that does not appear to have received attention (Russell, 1973) is that microbial activity always involves the excretion of by-products that are toxic to the organism and, as the soil becomes drier and the water films thinner, these will diffuse into the surroundings increasingly slowly. Microbial activity tends to increase with increasing moisture content, with a corresponding increase in CO2 production. The composition of the soil air upon the microbiological population is not important until the 02 content falls below 1%. Until this level, the soil behaves as a fully aerobic environment (Greenwood, 1967). Vegetated soils have higher CO2 contents, due to root respiration, than bare soils, and the presence of decomposable organic matter increases 02 uptake and hence CO2 production (Table 14-III). The very process of preparing soils for cultivation increases their aeration and rates of both gaseous exchange and organic matter mineralisation.
TABLE 14-I11 Composition of the air in soils, percent by volume (Russell, 1973) Soil type Arable, no dung for 12 months Pasture land Arable, uncropped, no manure: - sandy soil - loam soil - moor soil Sandy soil dunged and cropped: potatoes, 15 cm - serradella, 15 cm Arable land: - fallow unmanured - dunged Grassland -
-
Usual composition 02 CO2 19-20 0.9 18-20 0.5-1.5
Extreme limits 02 CO2 10-20
0.5-11.5
20.6 20.6 20.0
0.16 0.23 0.65
20.4-20.8 20.0-20.9 19.2-20.5
0.05-0.30 0.07-0.55 0.28-1.40
20.3 20.7
0.61 0.18
19.8-21.0 20.4-20.9
0.09-0.94 0.12-0.38
20.7 20.4 20.3
0.1 0.2 0.4
20.4-21.1 18.0-22.3 15.7-21.2 16.7-20.5
0.02-0.38 0.01-1.4 0.03-3.2 0.3-3.3
Oxygen and carbon dioxide in soil air
457
SAMPLING AND ANALYTICAL METHODS A great deal of the research that has been carried out into the concentrations of 0 2 and CO2 in soil air has been for the purposes of agriculture. There is a large body of literature describing different sampling methods (e.g., Yamaguchi et al., 1962; Tackett, 1968; Dowdell et al., 1972; Burford and Stefanson, 1973; Bunting and Campbell, 1975). The Russian literature is unfortunately deficient in adequate descriptions of the analytical systems first used in mineral exploration. However, amongst the methods that have been mentioned are interferometry (Khayretdinov et al., 1965), thermal conductivity (Glebovskaya, 1969; Kulikova, 1960), gas chromatography (Dadashev et al., 1971) and mass spectrometry (Glebovskaya, 1969). The chief characteristic that distinguishes O2 and CO2 from other gases that have been used in mineral exploration is that the levels and changes in concentrations encountered in soil air are in the percentage range. The analysis is thus much simpler and can, where appropriate, rely upon the use of portable instruments.
Oxygen and carbon dioxide analysers In a number of surveys, Lovell (1979), Lovell and Hale (1983) and Lovell et al. (1979, 1983) used a Taylor Servomex DA272 oxygen analyser and a Lab-Line 2245 carbon dioxide analyser. These instruments were connected in series and samples of soil air from a probe driven about 1 m into the ground were introduced by means of a hand pump. The Taylor Servomex DA272 oxygen analyser exploits a property that distinguishes 02 from most common gases" oxygen is paramagnetic and hence tends to move to the strongest part of a non-uniform magnetic field. This movement can be measured in a Pauling cell, in which two nitrogen-filled diamagnetic spheres of glass are mounted at the end of a bar to form a dumb-bell. This dumb-bell is mounted horizontally on a vertical torsion suspension. The whole measuring cell operates within a strong, non-uniform magnetic field. The spheres are repelled from the strongest part of the field and so rotate until the force produced by the twist of the suspension is equal to the force acting on the spheres (Fig. 14-3). The strength of the magnetic field varies with the magnetic properties of the gas with which the cell is filled and this will thus govern equilibrium positions attained by the dumb-bell. The analyser uses a platinum torsion suspension, which imparts physical strength and is able to conduct a current to an electromagnetic coil used to maintain the dumb-cell at a zero position. The current required to maintain this zero position is a function of the 02 content of the gas present in the cell. The instrument can be calibrated easily in the field by the use of pure nitrogen for the zero and dry air for the span (up to 21% O2). Table 14-IV gives the magnetic susceptibilities of some common gases compared with 02. It can be seen that none of those with a susceptibility likely to interfere with the measurement would be expected to
458
J.S. Lovell
\ Force
on S p h e r e
~.. Pole Pie
\
(a)
/:.
,
..~-_"_--7 T F,o, d
..-7---- ~'"...
Restoring F o r c e of Suspension
(b)
Fig. 14-3. Determination of oxygen concentration using its paramagnetic properties: (a) principle of the Pauling cell; (b) analytical cell of Taylor Servomex OA272 oxygen analyser.
occur in significant quantities in soil air. The instrument is fitted with a scale expansion system that permits a full-scale response in the range 16-21% 02. With this system it is possible to detect changes in the 02 content of the soil air of 0.05%. The carbon dioxide analyser exploits the fact that CO2 is about 40% less efficient at conducting heat than air. This difference in thermal conductivity is measured by means of a pair of thermistors in cells which are mounted in a massive aluminium block and which form the two arms of a sensitive bridge circuit. These thermistors respond to temperature changes with comparatively large changes in resistance. The thermistors are held at an elevated temperature relative to the aluminium by a fixed current. Heat is lost to the block via the gas surrounding the thermistors. The equilibrium temperature of each thermistor, and hence its resistance, is therefore dependent upon the thermal conductivity of the surrounding gas. Air fills one cell, which forms the reference arm of the bridge, and the sample fills the other. The sample and reference air are dried prior to analysis by means of a silica gel absorption tube. For use in the field, it was found necessary to fit the instrument in an insulated polished metal box, to minimise drift due to the effects of wind and direct sunlight. The instrument is calibrated in two ranges, 0-10% and 0-20% CO,. With multiple measurements it is possible to read the meter to within 0.05% CO2 with a detection limit of 0.05% CO2. However, extreme care is needed to obtain reliable measurements at this level. A disadvantage of both instruments is that, in the field, repeated and tedious zeroing is necessary before and after each measurement in order to ensure the requisite precision.
Oxygen and carbon dioxide in soil air
459
TABLE 14-IV Magnetic susceptibilities of common gases at 100% concentration in sample, expressed in terms of oxygen equivalent Gas Acetylene Ammonia Argon n-butane iso-butane l-butane Carbon dioxide Carbon monoxide Ethane Ethylene Helium Hydrogen
Percent 02 equivalent -0.24 -0.26 -0.22 - 1.3 - 1.3 -0.85 -0.27 +0.01 +0.46 -0.26 +0.30 +0.24
Gas Hydrogen sulphide Methane Neon Nitric oxide Nitrogen Nitrogen dioxide Oxygen n-pentane iso-pentane Propane Propylene Water
Percent 0 2 equivalent -0.39 -0.2 +0.13 +43 0 +28 + 100 - 1.45 - 1.49 -0.86 -0.54 -0.02
Orstat gas analyser Ball et al. (1983a, 1990) pumped soil air from a hollow probe into a modified Orstat gas analysis apparatus. A known volume of soil air is first pumped into the gas burette of the apparatus, and subsequently transferred to integral absorption vessels (Fig. 14-4). The absorbent for CO2 is 40% aqueous KOH. The volume of gas absorbed is recorded from the gas burette. The remaining gas is exposed to a mixture of saturated aqueous ammonium chloride and ammonia in contact with copper coils for absorption of 02. Again the gas volume reduction is recorded from the gas burette. Practical limits of detection under field conditions are 0.01-0.1% for each gas.
Draeger tubes For the determination of CO2 only, Romer and Finlay (1984) pumped 100 cm 3 of soil air from a probe through a calibrated glass tube containing reactants which respond to CO2 with colour change (Draeger type CH31401). Each tube is used once and then discarded. Its pre-sealed ends are snapped off before it is attached to the pump, and the CO2 that passes through the tube changes the colour of the reactant from white to blue over a part of the length of the tube. The length of the colour change is proportional to CO2 concentrations over the range 0.5-10% by volume.
460
J.S. Lovell
I1,, ,, II ,,',
,
/\
It
/\ i
OB8 burette
t
,,/
II II
II II
Absn.
II
vo.,e!
Absn.
~
I ve.,.l
t
m
i , I
Levelling i bottle
J
Flexible tubing
,
,
[
!
I
Fig. 14-4. Schematic of the Orstat gas analyser (from Ball et al., 1990).
Gas chromatography and mass spectrometry The determination of 0 2 and C O 2 c a n be performed with considerable sensitivity by gas chromatography (Tackett, 1968; Bailey and Beauchamp, 1973; Bunting and Campbell, 1975; Blackmer and Bremner, 1977). Lovell and Reid (1989), using a hollow sampling probe driven into the ground, drew 30 c m 3 soil-air samples into a syringe and injected these into air-evacuated sealed cylinders for later gas chromatographic determination of 02 and CO 2 in the laboratory. At concentrations above about 0.1% v/v, 02 and CO 2 can be measured by mass spectrometry (Anderson et al., 1972; Nerken, 1972; Ball et al., 1973; McFadden, 1973; Newton et al., 1975; Robertson and Bracewell, 1979). McCarthy and Bigelow (1990) developed a truck-mounted mass spectrometer which McCarthy and McGuire (1998) used for on-site determination of CO 2 and other soil gases in the Carlin trend of Nevada. Soil-air samples were extracted from a hollow sampling probe by means of a syringe, from which they were injected directly into the mobile mass spectrometer.
461
Oxygen and carbon dioxide in soil air
b-surface 0 (3
air
1 14
13
12
1 1 10
9
8
7
6
5
4
3
2
1
Sample points 8-8E
N-NW
~!~:..:.:l
3 :;F__~,~~,~.~i'~,~ ~~,~.V'~~i ! . - ~ . 6 ~ r 9t~)~:--~
,,
Ey~i-
"
-~i,!
t
./.
Lower Cretaceous (Balel suite)
.
. .........
sandstone deposits
Lower Cretaceous sandstone-conglomerate deposits (Tergen suite)
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(Unda) granltolda
Ore zones
Fig. 14-5. Relation of carbon dioxide in soil air and geology at Balei gold veins, Baykal region, Russia (reproduced with permission from Kulikova, 1960).
CASE HISTORIES
Russia
Kulikova (1960) describes the results of a gas survey over the Balei deposits in the Baykal region of Russia. The mineralisation is gold in quartz veins and the source of the gas is believed to be emissions of CO2 from depth along mineralised fractures within Lower Cretaceous arenaceous sediments. The area is covered by alluvial Quatemary deposits that are up to 10 m thick on the fluvial plains and may reach 30 m on the terraces. The samples were collected in bottles with water seals of saturated NaCI. Despite the fact that the area of investigation is underlain by continuous permafrost at depths of 3-40 m, there is a good expression of the mineralised fractures in the soil-gas data from a depth of 1.5-2 m, with values over the ore in the range 1.0-2.0% CO2 against a background of 0.2-0.8% CO2 (Fig. 14-5). Soil samples were collected and analysed for sorbed subsurface gas, which also showed evidence of an enhanced CO2 flux.
J.S. Lovell
462
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Fig. 14-6. Relation of carbon dioxide in soil air and geology at Sakhalinsk mercury deposit, northwest Caucasus, Russia (reproduced with permission from Ovchinnikov et al., 1972).
Ovchinnikov et al. (1972) give only a few details of a survey over the Sakhalinsk mercury deposit, northwest Caucasus, but its inclusion here is of interest as it illustrates the influence of a nearby earthquake. The initial survey showed a weU-arked anomaly over the mineralisation with a maximum value of 4.8% CO2 against a background of 0.5-1.5% CO2 (Fig. 14-6). The second survey, some two months later, showed values elevated by approximately a factor of three. The influence of seasonal factors cannot be ruled out, but there was a very marked change in the ~3C ratio and gaseous anomalies associated with suboutcropping faults became more marked. The source of additional CO2 is thought to be abyssal gas preferentially moving from depth along the mineralised fractures.
Azerbaijan Dadashev et al. (1971) describe a soil-gas survey over the Filizchai pyrite deposit in Azerbaijan. They report a definite relation between the position of the ore body and the composition of the soil air. As the survey line enters the ore field in the southwest, there
Oxygen and carbon dioxide in soil air
463
Fig. 14-7. Oxygen and carbon dioxide in soil air over the Filizchai pyrite deposit, Azerbaijan (reproduced with permission from Dadashev et al., 1971).
is a steady increase in the C O 2 content of soil air and a decrease in the 0 2 content. There is a marked anomaly in the composition of the soil gas northeastwards, with the O2 content falling and the CO2 content rising until the ore body plunges to greater depth (Fig. 14-7). The changes in gas concentrations are ascribed to the effects of the oxidation of sulphide minerals; the patterns provoke interest in whether the anomaly maxima coincide with the position of maximum sulphide oxidation.
Kyrgyzstan
A survey carried out over a polymetallic ore deposit in Kyrgyzstan is described by Glebovskaya and Glebovskii (1960). The traverse shows an increase over the background CO2 concentration of 1-1.5% to a diffuse anomaly of 2-3.5% over the suboutcrop of the ore zone. There is an intense 02 anomaly, with the 02 concentration falling sharply from a consistent background of 20-20.5% 02 immediately over the mineralisation (Fig. 14-8).
Namibia
At Witvlei, 160 km west of Windhoek in central Namibia, copper mineralisation occurs within a sedimentary sequence ranging from conglomerates through grits, sandstones and arkoses to siltstones and claystones. The area is underlain by a sandstone-siltstone sequence dipping at about 40 ~ to the southeast. Trenching revealed that surficial geochemical copper anomalies are due to chalcopyrite mineralisation within certain horizons. Limited drilling showed that the sulphides have been oxidised to malachite and chrysocolla to a depth of about 30 m, and that the mineralised units are
J.S. Lovell
464 % CO 2
% 02
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Fig. 14-8. Oxygen and carbon dioxide in soil air (from !.5 m) over polymetallic ore deposits in Kyrgyzstan (reproduced with permission from Glebovskaya and Glebovskii, 1960).
interbedded with calcareous arkose. There is a cover of up to 2 m of bright-red Kalahari sand with a basal rubble horizon and occasional calcrete; outcrop is rare. Within this area two soil-gas traverses were conducted; one of these is shown in Fig. 14-9. Using a convention in which 02 concentration is reported as AO2, the reduction in the 02 concentration from the background atmospheric concentration of 21%, the results show a single, low-contrast anomaly at the site of the oxide/sulphide interface with a AO2 value of 0.3% and CO2 value of 0.4%, set against a quiet background of 0.1-0.2% AO2 and 0-0.1% CO2. The other traverse carried out during this survey revealed a similar pattern, and gave a good expression of the mineralisation.
Johnson Camp, Arizona The mineralisation at Johnson Camp (see also Chapter 8) consists of chalcopyrite, sphalerite, bomite and pyrite. It was pyrometasomatically-introduced into limestones of the Naco Group of Upper Pennsylvanian to Lower Permian age by the nearby Tertiary Texas Canyon quartz monzonite intrusive (Cooper and Silver, 1964). The mineralisation has invaded the host rock along minor fractures and as disseminations. The mineralised zones have been partly oxidised near their suboutcrops and, adjacent to the innumerable mineralised minor fractures, along much of their down-dip extensions. The overall
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Fig. 14-9. Oxygen and carbon dioxide in soil air over sedimentary copper mineralisation at Witvlei, Namibia (from Lovell et al., 1983).
sulphide concentration is approximately 3%. Three zones of mineralisation have been outlined by drilling. These are concealed by pediment gravels and alluvium that increase in thickness to the northeast from 10 m over the most southerly ore zone to some 225 m over the most northerly. A soil-gas survey was carried out across this area during a dry summer (Lovell et al., 1983). Three traverses were completed, with soil air being collected from a depth of 50-100 cm; one of these traverses is shown in Fig. 14-10. The suboutcrop and down-dip extension of the shallowest body of mineralisation (Zone I) is marked by a discontinuous series of anomalous values, with maximum values of 0.75% AO2 and 0.9% CO2. These maximum values do not occur over the actual suboutcrop, but at a position where the sulphide zone is some 60 m below the surface. This is then followed by a series of background values in the range 0.15-0.3% AO2 and 0-0.15% CO2, although the traverse is still partially underlain by the down-dip extension. It is interesting to note here that this particular mineralised zone is believed to be an extension, along strike, of the old Peabody Mine (J. Kantor, pers. commun.). Scott (1916) reports that in the mine the ore was oxidised to 60 m. If the same conditions extend into the survey area, then the highest AO2 and CO2 soil-air values are to be found over the projected interface of oxide and sulphide ore. The suboutcrop of the central zone of mineralisation (Zone II), lying to the northeast and covered by 90-150 m of overburden, has a good expression in the soil-gas data, but the down-dip extension is not indicated. This is perhaps a result of oxidation being confined to the upper sections of the mineralisation by the greater depth of cover. The two other traverses conducted during this survey showed that Zones I and II, the shallowest zones, have a consistent expression in the soil air, but that Zone III, the deepest zone, covered by 225 m overburden, had no expression. A subsequent soil-air survey, following a very wet summer, revealed similar patterns in the data, but the values were greatly reduced. The background values for both AO2
J.S. Lovell
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Fig. 14-10. Oxygen and carbon dioxide in soil air over pyrometasomatic mineralisation at Johnson Camp, Arizona (from Lovell et al., 1983).
and CO2 were below the limits of detection, with maximum values of 0.2-0.4% over the mineralisation (B.W. Oakes, pers. commun.).
Colorado Plateau, Arizona Mineralised breccia pipes occur in Palaeozoic sedimentary rocks of the Colorado Plateau in northern Arizona (Weinrich, 1985). These pipes are usually circular or oval at suboutcrop, have horizontal dimensions that are typically a few tens of metres and vertical dimensions that may extend to 1000 m. They appear to have developed over solution-collapse structures resulting from karstic weathering of Lower Mississippian carbonate sediments between the Upper Mississippian and Triassic. The mineralisation, comprising pitchblende and sulphides of Fe, Cu, Mo, Pb and Zn, is found beneath a massive pyrite cap, several hundreds of metres below surface. Some of the ore bodies are, or have been, mined for uranium. Gangue minerals include calcite and dolomite. The surface features of the collapse structures include: concentric, inward-dipping beds; circular areas of brecciated rock; circular bleached or ferruginous tonal anomalies; circular vegetation anomalies and circular concave topography. However, not all of these structures are mineralised, and the concentration of CO: in the overlying soils has proved to be a valuable guide to the extent of the concealed mineralisation, which may lie as much as 500 m below the surface. The development of a gaseous expression of mineralisation at such considerable depths is facilitated by a number of features. The sulphide content of the ore is very high (tens of percent) and its oxidation far below the
Oxygen and carbon dioxide in soil air
467
Fig. 14-1 I. Carbon dioxide in soil air (sample sites and contours in %) over mineralised breccia pipe, northern Arizona (from Lovell and Reid, 1989).
ground surface is promoted by a low water table and continuous supply of oxygenated groundwater, as a result of the deep canyons that developed during the uplift of the Colorado Plateau. In addition, the isolated and transgressive nature of the breccia structures appears to favour vertical gaseous movement and to limit the gaseous anomalies to the confines of the pipe. The low organic content and the poorly-developed nature of the surface soils yields a low CO2 background and permits subtle expressions of the mineralisation to be recognised. Figure 14-11 shows contoured soil-air CO2 data collected from a depth of 1 m, overlying a breccia pipe which is heavily mineralised at depths of 150-250 m. The anomaly peak is only 0.3% CO2 (only ten times the atmospheric background) but the circular gas halo is centred over the mineralisation (Lovell and Reid, 1989; Reid and Rasmussen, 1990). The fact that these low concentrations accurately reflect mineralisation at these depths is a testament to the sensitivity and precision of the gaschromatographic method of analysis that was used, and to the low background. This technique was subsequently routinely adopted as an exploration tool in northern Arizona. However, prior to its adoption, a series of trials were carried out to determine its applicability and confidence levels in this environment. Of 14 prospects that were tested, 12 were subsequently drilled. From the soil-air CO2 data it was possible to predict correctly both the presence and tenor of the underlying mineralisation in 11 out of the 12
468
J.S. Lovell
Fig. 14-12 Variation of CO2 patterns in soil air over mineralised breccia pipe, northern Arizona: a) early summer; b) 18 days later; c) a further 10 days later (from Lovell and Reid, 1989).
cases (A.R. Reid, pers. commun.), an excellent success rate for any exploration technique. Further surveys at different times of the year proved disappointing: a series of exploration targets failed to return any detectable concentrations of CO2 in the soil air; and re-sampling of areas that had previously been shown to contain CO2 related to mineralisation also failed to yield a response. As these further surveys had been carried out during the dry summer months, it was surmised that the dry soils had no capacity to retain CO2 in concentrations significantly out of equilibrium with the atmosphere. This phenomenon was studied by monitoring, for several weeks, a grid of soil sites over mineralisation (Fig. 14-12). As the soil dried, the anomaly over mineralisation faded and shifted. A later survey after heavy rainfall showed that the anomaly was re-established, and demonstrates the continuous evolution of CO2 from its source. Other surveys after winter rain and snow also revealed anomalies, including an anomaly over the one mineralised breccia pipe that was not anomalous in the initial CO2 survey of prospects.
DISCUSSION Since the techniques described here measure transient fluxes of 02 and CO2 in soil air, it is hardly surprising that measurements are not always reproducible over long
Oxygen and carbon dioxide in soil air
469
periods of time. At Johnson Camp, Arizona, surveys in different seasons yielded comparable patterns but at different concentration levels. Over the breccia pipes of the Colorado Plateau the distinctive but subtle anomalies found in one season were completely lost in another. Ball et al. (1990) found a similar seasonal pattern over auriferous sulphides in West Africa, with CO2 anomalies in soil air detected during the wet season disappearing by the end of the dry season. Even in the temperate climate of the UK, Ball et al. (1983b) found the best 02 and CO2 responses over mineralisation in the wetter winter months. The inference that dryness of the soil plays a significant role in soil-air CO2 anomaly persistence gains support from Hinkle (1990), who found that, amongst a considerable number of soil-air environment factors investigated, soil moisture content influences the CO2 level in soil air the most, although its effects are rather unpredictable. It is likely that isotopic analysis to determine the ~3C and ~80 content of various soilair samples offers a means of distinguishing those containing some CO2 generated during sulphide oxidation at depth from those whose CO2 derives solely from the atmosphere and plant respiration. This has been attempted by Alpers et al. (1990) at Crandon, Wisconsin, where McCarthy et al. (1986) found a CO2 anomaly in soil air over massive sulphides hosted by Proterozoic limestones and concealed beneath glacial till containing clasts of Palaeozoic limestone. In fact the CO2 in soil air over the mineralisation was isotopically indistinguishable from that in background samples 1-2 km away. This finding is, however, far less conclusive than it first seems to be: at the time when the samples where taken for isotopic analysis, the CO2 anomaly over the mineralisation was not particularly well developed due to recent precipitation.
CONCLUSIONS There is a substantial body of evidence to support the belief that the concentrations of 02 and CO2 in soil air can reveal the position of concealed mineralisation, even through considerable thicknesses of overburden. Surveys can be conducted using any one of several proven field and laboratory methods. The equipment required is, for the most part, inexpensive and readily available. However, there is a very wide range of concentrations of 02 and CO2 in the soil air of different environments. Also, measurements can vary markedly with the seasons. Thus it is extremely important when applying the technique in a given area to carry out local orientation tests in order to establish the true background and the contrast that may be expected.