Dunn Colin E. Biogeochemistry in Mineral Exploration

Подождите немного. Документ загружается.

This page intentionally left blank

Chapter 4

FIELD GUIDE 2: SAMPLE SELECTION AND COLLECTION

GENERAL CONSIDERATIONS

The rationale for applying biogeochemical methods to mineral exp loration is that

plants absorb metals present in the ground and transfer these metals via their root

systems to the growing plant. Quite simply, the more of a metal that there is in the

ground, the more it is likely to be taken up in solution by a plant and will be

deposited within that plant. There are, however, processes that take place that pre-

vent this from being a linear association.

Roots have to ﬁnd water and dissolved nutrients in order to sustain a plant’s life.

A single large tree can pump more than a hundred litres of water from the ground

every day and transport it via its xylem up to its extremities. Several processes

interact to transfer a solution from the soil environment up into a tree. Osmosis

explains the pull of water into the roots, and some minor movement can take place

by capillary action. However, the princip le movement through a tree is best explained

by evaporation through stomata of its foliage. This evaporation creates a vacuum

that then permits more water and its dissolved constituents to be drawn into the

plant. Some volatile compounds escape with the water vapour, but most remain

within the plant structure, in the phloem or deposited on cell walls, to be cycled back

to the ground, as illustrated in Chapter 2.

Metals are absorbed from soil, from groundwater and locally from bedrock where

roots penetrate faults, joints, cleavages and the interstices or boundaries between

mineral grains. The significant advantage of applying plant chemistry to exploration

is that the root system of a plant may penetrate through many cubic metres of the

substrate, and therefore integrate the geochemical signature of a large vo lume of all

soil horizons, the contained groundwater, gaseous emanations and bedrock where it

is covered by only a few metres of overburden. The live roots occasionally found in

mine ga lleries deep underground probably only occur where there is a very low water

table in an arid environment. Intuitively, it would seem that roots need to probe

deeply into the substrate in order to extract all the nutrients that they require.

However, depth of root pe netration is not critical for a biogeochemical response,

because elements can migrate upward from considerable depth in solution, by dif-

fusion, in electrochemical cells, and possibly by seismic pumping (i.e., release of

metals due to earth tremors) to be accessed by root systems. Consequently, there is

commonly not a good correlation between plant and soil chemistry, especially in

areas where there is exotic overburden.

The microscopic mycorrhizal fungi on root surfaces effectively transfer nutrients

into plant structures. Discussion of these mechanisms is given in Chapter 1, where it is

noted that the complex microenvironment surrounding roots can be highly corrosive

and soil acidity derived from organic acids can be as low as pH 1. Furthermore, simply

because plants minimize the energy output required to ﬂourish, roots will take the path

of least resistance and ﬁrst accept elements in gaseous form, then those in solution, and

then seek out additional requirements by selectively extracting labile elements that are

loosely bonded to soil surfaces or rock fractures. Russian workers have indicated that

gases are absorbed by plants 3000 times more readily than elements in solution, and

the latter are absorbed 300 time more readily than elements locked in the crystal

lattices of minerals comprising rocks or soils (Kovalevsky, 1974). Loosely bound

elements are mostly adsorbed to soil coatings of amorphous manganese and iron oxide

coatings. These coatings are the targets, also, of various soil selective extraction tech-

niques used in exploration geochemistry. Consequently, the plant can be considered as

a type of selective leach process. Once the sources of elements in gases, in solutions and

adsorbed on surface coatings have been exhausted, further plant requirements are met

by attacking the less labile components of the substrate – the crystalline phases of soils

and bedrock.

Many texts suggest that for biogeochemical exploration to be successful there

should be a high correlation between the metal content of the soil and that of the

plant (Bro oks, 1983). This is a valid concept for some parts of the world where there

are residual soils. However, as noted in the ﬁrst chapter, plants establish barriers to

metal uptake in order to protect themselves from potential toxicity so that over a

broad range of concentrations the metal content of a plant may not be proportional

to the metal content of the soil (Kovalevsky, 1987, 1995a). Consequently, a good

positive correlation between plant and soil chemi stry does not always occur, espe-

cially where exotic overburden such as lacustrine clay, alluvial plain silt, glacially

derived material, or wind-blown loess has been deposited on mineralized bedrock.

This situation may be further complicated by elements that remain dissolved in

groundwater and taken up directly by plant roots without precipitating in the soil

medium. This is particularly true of highly soluble elements (e.g., halogens and some

U complexes) that can remain in solution until intercepted by the rhizosphere (root

zone) of a tree. Furthermore, although some elements may be absorbed directly from

the interaction of their roots with the groundwater and/or the capillary fringe of the

water table, others may be taken up in gaseous form (e.g., Hg and halogens). In

summary, whereas the physicochemical environment of the soil may not be co ndu-

cive to element adsorption from groundwater or gaseous phases, plant roots can

absorb elements directly from these phases and concentrate them in the plant tissues.

For these reasons, whereas plant to soil coefﬁcients can be established in lab-

oratory experiments, the real world is rarely that simple. In attempting to determine

the relationship between the chemistry of the soil and that of a tree, the usual

Field Guide 2: Sample Selection and Collection

procedure is to collect a bag of soil and a bag of tree tissue. However, there arise

some fundamental considerations.



Which soil horizon should be collected?



Which size fraction of the selected soil horizon should be analysed and by which

analytical method?



Which type of plant tissue (and from which species of plant and location on that

plant – top or bottom, north or south) should be collected for comparison with

that fraction of the underlying soil that has been selected for analysis?



What are the effects of topography and ground conditions in determini ng the

uptake by a plant of the various elements?

Typically, each soil horizon has a different metal content, as does each size frac-

tion of that soil. Similarly, each vegetation tissue type has a different ability to collect

and store metals; and concentrations in living tissue change with the seasons . The

problem is compounded by the fact that a soil sample is usually no more than a

handful of a single horizon, and as such represents a miniscule sample compared to

the volume of material sampled by the root system of a large tree. Furthermore, the

relationship between plant and soil chemistry improves with depth down the soil

proﬁle, so the tree is the ‘natural drill’ that collects metals from depth.

Table 4-I shows the relationship between Au and As in outer bark and the soils in

which the trees were growing. The bark of both the Douglas-ﬁr and Engelmann

spruce has strongest correlations with the C horizon soils, indicating that this horizon

is the primary source of these metals that wer e drawn into the trees. In effect, the

trees have efﬁciently drawn up the Au and As and sequestered them in their bark,

thereby providing a sim ple window to the chemistry of the C horizon, without having

to dig a pit – which in this area would have required digging to a depth of up to 1 m.

TABLE 4-I

Correlation coefﬁcients (r) between Au and As in bark ash and underlying soil horizons near

the Nickel Plate past-producing mine (gold skarn) at Hedley, southern British Columbia

Soil horizon

Douglas-ﬁr bark Engelmann spruce bark

n ¼ 12 n ¼ 13

Au As Au As

Forest litter

0.13 (ns) 0.10 (ns) 0.48 (ns) 0.58 (s)

A horizon

0.63 (s) 0.63 (s) 0.65 (s*) 0.65 (s*)

B horizon

0.60 (s) 0.55 (s) 0.79 (s**) 0.80 (s**)

C horizon

0.76 (s*) 0.64 (s) 0.90 (s**) 0.88 (s**)

Note: ns – not significant (P o0.05); s – significant (P >0.05–o0.01); s* – highly significant (P

>0.01–o0.001); s** – very highly significant (P >0.001).

65Biogeochemistry in Mineral Exploration

Plants are complex structures that apply extraordinarily sophisticated mecha-

nisms to select those elements that they require for efﬁcient metabolic function, while

tolerating other elements and sequestering them out of harm’s way, and excluding

other elements that could have significant toxic effects. Each species of plant is

unique in its chemical composition and, therefore, its value to biogeochemical ex-

ploration. For any given species for which there is no information available on its

propensity to accumulate elements, an orientation survey is highly advisable in order

to optimize the value of the biogeochemical method to exploration. It is as well to be

aware that in many situations the information supplied by plant c hemistry will be

different from that derived from analysis of soils or other surﬁcial deposits – each

provides its own ‘layer’ of geochemical information in the same way that different

geophysical measurements provide different types of information on the physics of

the Earth.

Seasonal variations

In Chapter 2, there is a brief introd uction to botanical aspects of seasonal var-

iations in plant chemistry. This section now puts these complex interactions into a

practical perspective and provides examples of variations and how to deal with them

so that a biogeochemical survey is a practical proposition. An often-overlooked

factor in soil sampling is that soils, too, exhibit seasonal variations in composition

(Victoria et al., 1989; Kaiser et al., 2001), so geochemists should always be conscious

of this phenomenon.

To reiterat e what has previously been noted, numerous studies have established

that throughout the year, the elemental composition of living plant tissues can vary

considerably as a plant grows. Consequently, it is advisable to conduct a biogeo-

chemical survey in as short a time frame as possible – preferably 2–3 weeks during the

growing season. During the winter months a plant’s metabolism is quite stable, and

so a sample collection programme can be of longer durati on. Dead tissues, such as

outer bark, provide the exception, because with time there is little or no change in

their composition, and therefore they represent valuable media to use for any surveys

that are to be conducted over a long period (several months or even years).

Each plant species exhibits its own compositional varia tions throughout the year,

such that for some, but not all, elements a tree or shrub sampled in the spring yields

concentrations different from those in the same plant during the summer.

For biogeochemical exploration, the potential complexities created by seasonal

variations in plant chemistry can be significant, but provided appropriate consid-

erations are applied, they are manageable. The literature contains details of various

studies that appear to offer some conﬂicting advice. For example, Brooks (1983)

notes that ‘it seems to be well established that the elemental content of many

deciduous leaves rises to a maximum just before exfoliation’, and he provides as

evidence a series of plots from a study by Guha (1961). Scrutiny of Guha’s plots

Field Guide 2: Sample Selection and Collection

shows that this is true for some elemen ts in some species (e.g., Mn and B in syc-

amore), but the opposite is true for Cu in the three species that were studied, since all

yielded lowest concentrations just before exfoliation. In short, the patterns are not

consistent.

Subsequent studies have provided more consistent results with the spring emerg-

ing as the period when maximum concentrations occur – especially of Au. Studies of

Au in a variety of plant species from the Hemlo area of Ontario demonstrated that,

in trees growing over mineralization, the highest Au values were recorded in the

spring (Cohen et al., 1987). Similarly, in Colorado Stednick et al. (1987) and Stednick

and Riese (1987) conﬁrmed this pattern for Au, but found no time dependence for

Cu and Zn. These ﬁndings are in accord with previous studies in the more northerly

latitudes of the La Ronge Gold belt in Saskatchewan (Dunn, 1985), where 17

mountain alder shrubs (Alnus crispa) located approximately 250 m apart were

revisited on four occasions from June 1984 to April 1985. Each shrub was of

similar maturity and height (about 2 m), and on each occasion all 17 were sampled in

the same day. The tissues selected for analysis comprised the most recent three

years of twig grow th, and leaves from these twigs were obtained dur ing 1984.

Alder is deciduous and so the leaves represented only the current year’s growth,

and in April 1985, when there was still snow on the ground, the leaves had not yet

emerged.

At each of the 17 sample sites the changes in composition were consistent

(Table 4-II) – i.e., in early August all of the shrubs recorded lower Au concentrations

than in early June. Similarly, in September, all had higher concentrations than in the

early August sampling. By the following April, with the ﬁrst rise in sap, consistently

and substantially higher Au levels were recorded (Table 4-II).

In this study the twigs were the ﬁrst to be analysed, and it was only at a later date

that the leaves were recovered from storage for analysis. Intuitively, it was considered

likely that the losses of Au in the twigs from June to August might have been a result

of the Au migrating from the twigs into the leaves resulting in higher concentrations

in the leaves. However, this proved not to be the case and, if this process did take

place, the Au was soon leached out of the leaf tissue. There are several possibilities

that could explain this loss of Au.



Au dissolved in the plant sap cycles through the plant and is eventually returned

through the cells to the ground.



Girling and Peterson (1978), from an experiment involving doping some soils with

a radioactive Au isotope, demonstrated that Au has an acropetal tendency – i.e., it

migrates to leaf tips.

During evapotranspiration, the form ation of salts on leaf surfaces, and the spal-

ling of tissues during hot summer days the Au could be released to the atmosphere

from the microcosm that forms the entity of a plant. An SEM of the surface of a

black spruce twig (Fig. 4-1) shows the ﬂaking of cuticle (particles of abo ut 5 mmin

Biogeochemistry in Mineral Exploration

diameter) that would readily be removed by wind or rain. Since there is evidence that

some elements migrate to plant extremit ies, then trace elements can be lost from a

plant in this manner.

An alternative explanation of the acropetal tendency observed by Girling and

Peterson (1978) is that the plants reacted to the radioactive isotope of Au in a manner

different from the way that they react to naturally occurring gold (Au

197

), by seques-

tering the more toxic radioactive form at the furthest extremities of its structure where

it could do the least harm. Recent studies have dramatically illustrated that different

chemical forms of an element are ﬁxed in different locations in plant structures. A

study of Se in the poison vetch Astragalus, using the laser light source synchrotron

instrumentation, has clearly demonstrated that organoselenium occurs in different

locations in leaves and twigs than Se complexed as a selenate (Pickering et al., 2000).

TABLE 4-II

Gold in the ash of alder (Alnus crispa). Same 17 shrubs collected from northern Saskatchewan

on four occasions during a one-year period

Site Alder twigs gold (ppb) in ash Alder leaves gold (ppb) in ash

1984 1985 1984

June August September April June August September

1 32 7 23 250 nd nd nd

2 53 6 17 47 nd nd nd

3 58 9 20 130 43 6 19

4 34 6 15 166 48 7 15

5 29 8 10 37 27 18 11

6 35 7 11 34 21 6 12

7 23 6 13 57 25 7 13

8 25 8 13 41 21 13 13

9 251120 27211116

10 29 20 23 75 11 8 22

11 35 10 22 58 8 7 8

12 23 8 14 51 6 6 9

13 12 17 18 33 10 8 8

14 24 10 11 53 14 7 18

15 25 11 12 42 5 10 13

16 14 11 9 66 13 3 13

17 21 10 38 48 8 7 14

Average in ash 29 10 17 71 19 8 14

Average dry

weight

0.6 0.2 0.34 1.4 1 0.4 0.7

Note: nd ¼ not determined.

68 Field Guide 2: Sample Selection and Collection

Whereas Table 4-II shows that substantial seasonal variations in Au occurred in

alder, for Mo these changes were not evident. Table 4-III shows data for the June

and August collections, and includes also data for outer bark of black spruce. At

most sites concentrations were similar and variations were within the precision levels

of the analytical method.

From Table 4-II it is evident that the Au values from alder samples collected at

different times of the year cannot be directly compared, although it might be possible

to normalize the data from a regression equati on. Conversely, Table 4-III shows that

the Mo data from the different seasons could be merged without compromising the

integrity of the survey. For alder, as a broad generalization, it seems that concen-

trations of most elements are reasonably consistent throughout the year and exhibit

only minor seasonal differences (e.g., As, Ba, Ca, Cs, Mo, Rb, REE (rare-earth

elements), Sb, Sr, Th, U, Zn). For a few elements there is substantial variation with

highest concentrations occurring in association with the rising sap, then tailing off

quite quickly as the summer progresses (e.g., Au, Co, Cr, Fe, Ni). The outer bark of

the spruce is dead tissue, and so seasonal variations are not a consideration.

At an elevation of 1700 m in the northern Cordillera of the United States (Oregon),

Ashton and Riese (1989) studied seasonal variations of As and Au in Ponderosa pine

(Pinus ponderosa) and white ﬁr (Abies concolor). In this environment they found minor

seasonal differences in As, but substantially higher Au concentrations in the spring and

fall than in the summer and winter.

Fig. 4-1. SEM of the surface of a black spruce twig (Picea mariana) showing natural spalling

of cuticle (Dunn, 1993, 1995).

69Biogeochemistry in Mineral Exploration

In the hotter environment of central India seasonal variations of U were not

clearly deﬁned, although the common plant Lagerstroemia parviﬂora (myrtle family)

in its dormant stage exhibited better background to anomaly contrast than other

neighbouring species that were in a dynamic grow th phase (Pande et al., 1993).

In a study of seasonal variation of 20 elements in ﬁrst and second year needles of

Norway spruce (Picea abies), the data fell into 3 groups (1) Ca, Sr, Ba and Mn; (2) Al,

Br, Co, Fe, Hg, La, Sc, Sb and Zn; 3) K, Rb, Cs, P and Cl. Elements in groups 1 and 2

increased from spring to summer, while those from group 3 decreased (Wyttenbach

and Tobler, 1988).

Another study monitored the variations in needles of balsam ﬁr (Abies balsamea),

collected from a single site every week for 6 weeks from early July to late August

(Leybourne et al., 1999). Analytical determinations were undertaken in the labora-

tories of the Geological Survey of Canada in Ottawa using ICP-ES and ICP-MS on

plant ash for 10 major elements and Ba, Be, Co, Cr, Cu, Ni, Sc, Sr, V, Zn, REE, Ag,

Bi, Cd, Cs, Ga, Hf, In, Mo, Nb, Pb, Rb, Ta, Th, Tl, U and Zr. In general, variations

were quite subtle (+/– a few percent), except for some trends during the sampling

TABLE 4-III

Molybdenum in the ash of alder (Alnus crispa) and black spruce (Picea mariana). Same 17

shrubs collected from northern Saskatchewan in spring and summer

Site Alder twigs Mo (ppm)

in ash

Alder leaves Mo (ppm)

in ash

Spruce bark Mo (ppm)

in ash

June August June August June August

138ndndndnd

23222–11

374ndndndnd

424541–1

512103211

614743ndnd

719224223

89543ndnd

911181311

1089311–1

11592111

12 23 14 1 1 1 2

132241–11

141064433

15124211

16 –1 4 1 –1 3 –1

17 –1 –1 4 –1 1 2

Note: nd ¼ not determined.

70 Field Guide 2: Sample Selection and Collection

period of increases in Ba, Ca, Mg, Mn, Sr and Zn by about 20%, and decreases of

similar magnitude in Fe, K, P and Rb (Fig. 4-2). Except for Fe, these trends are in

general accord with those noted above for Norway spruce needles.

For some elements the tree water ﬂow-rate (related to evapotranspiration) has

been shown to affect the seasonal variations of metal concentrations in tree crowns.

Sailerova and Fedikow (2004) noted that the best correlation between seasonal

Ba in balsam fir needles

y= 30.714x + 925.71

= 0.5807

800

900

1000

1100

1200

1300

1234567

Time sequence (weeks)

1234567

Time sequence (weeks)

1234567

Time sequence (weeks)

1234567

Time sequence (weeks)

1234567

Time sequence (weeks)

1234567

Time sequence (weeks)

1234567

Time sequence (weeks)

1234567

Time sequence (weeks)

ppm in ash

Zn in balsam fir needles

y = 31.429x + 982.86

= 0.4681

800

900

1000

1100

1200

1300

ppm in ash

CaO in balsam fir needles

25.00

27.00

29.00

31.00

33.00

35.00

% in ash

Sr in balsam fir needles

400

430

460

490

520

550

ppm in ash

IRON in balsam fir needles

0.00

0.10

0.20

0.30

0.40

0.50

% in ash

POTASSIUM in balsam fir needles

y = -0.6893x+ 17.914

= 0.6768

0.00

5.00

10.00

15.00

20.00

% in ash

RUBIDIUM in balsam fir needles

100

120

140

oom in ash

PHOSPHORUS in balsam fir needles

y= -0.0729x + 7.7414

=0.0347

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

% in ash

y = 15.357x + 425.71

= 0.4876

y = -0.0279x + 0.4357

=0.5576

y = 0.6x+ 28.971

= 0.5876

y= -4.6071x + 123.86

=0.3724

Fig. 4-2. Variations in element content of balsam ﬁr needles (Abies balsamea) from New

Brunswick, Canada, over a 6-week period. Concentrations in ash.

71Biogeochemistry in Mineral Exploration