Dunn Colin E. Biogeochemistry in Mineral Exploration

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by excreting enzymes that assist in breaking down soil particles and microfauna, they

are more efﬁcient at extracting nutrients than the roots themselves.

Fungi play an important role in nutrient transfer into plants. About 300,00 0

species of plants have relationships with fungi that are endotrophic, indicating that

the fungal hyphae penetrate and grow inside a plant’s roots. However, the relation-

ship with a tree is ectotrophic, meaning that a sheath of mycorrhizal fungi encases the

roots. This mass of mycorrhizal fungi is estimated to connect trees with as much as

one thousand times more soil area than the roots themselves ( Luoma, 1999).

The root microenvironment can be highly corrosi ve, with acidity locally below pH

1(Meyer et al., 1973) at the interface with the growing medium. The roots are as

conservative as possible in the use of energy to acquire essential nutrients, thereby

taking the path of least resistance by ﬁrst accepting elements in gaseous form , then

those in solution, and then seeking out additional requirements by selectively ex-

tracting labile elements loosely bonded to soil surfaces, such as the amorphous

manganese and iron oxide coatings to which metals are known to be adsorbed. As a

last resort, roots attack the ﬁne-grained inorganic particles coating bedrock joints,

and the crystalline phases of soils and bedrock.

TABLE 1-II

Average concentrations of selected elements in common plants compared to highest concen-

trations recorded in macro-fungi growing in soils containing background levels of these

elements – modiﬁed after Lepp (1992) with supplementary data

Average plant

(ppm)

Highest fungal

concentration (ppm)

Genus Common name

Ag 0.02 1253

Amanita Death-cap

mushroom

As 0.1 427 Amanita Death-cap

mushroom

Au 0.2 ppb 2250 ppb

Lepiota ‘Shaggy-stalked

parasol’

Cd 0.05 300 Amanita Death-cap

mushroom

Cu 5 469 Amanita Death-cap

mushroom

Hg 0.02 80 Agaricus Common edible

mushroom

Sb 0.1 1423

Chalciporus Peppery bolete

Se 0.02 55

Boletus Edible mushroom

V 0.5 700 Amanita Death-cap

mushroom

From Borovic

ka et al. (2006a,b, in prep.).

12 Introduction

The extent of root systems can be extraordinarily large. Dittmer (1937) estimated

that a single rye plant (Secale cereale), 50 cm tall and with a clump of 80 shoots had a

cumulative total root length of 380 miles (611 km) that included fourteen billion

root hairs. Upon each of these roots, rootlets and root hairs there are myriads of

mycorrhizal fungi continually accessing plant nutrients from the ground while pas-

sively tolerating other elements and passing this soup of material into the plant

structures.

Magnesium is the element that gives chlorophyll its green colour. The remaining

constituents of chlorophyll are the four basic elements of life – hydrogen, carbon,

nitrogen and oxygen. In fact , magnesium is to plant ‘juices’ what iron is blood. There

is close similarity between chlorophyll and haemoglobin; at the hub of every hae-

moglobin molecule is one atom of iron, while in chlorophyll it is one atom of mag-

nesium (Peattie, 1991). Consequently, analysis of a green plant part can be expected

to return a Mg concentration from 500 ppm up to several percent. Woody tissue,

however, such as outer bark and twigs, typically contains only a few hundred ppm

Mg. From an exploration point of view, this example illustrates that some knowledge

of the essentiality of an element and ‘what goes where’ in a plant is useful in de-

veloping an understanding of the levels of elements that might be expected. Whereas

the composition of a typical plant is substantially different from that of a rock, it is of

use to bear in mind the analogy given by Kovalevsky (1987) that the ash of a plant

(i.e., minus all its organic constituents) is similar in composition to a dolomite

(CaCO

MgCO

As to why and how trace elements become ‘locked’ into plant cells, it is sobering

to consider the sequence of events described by Suzuki and Grady (2004) that takes

place during photosynthesis:

When a photon of sunlight hits a chloroplast, one electron is ejected from each molecule

of chlorophyll; this energy excites the molecule which then uses that excitation to carry

out a chemical reaction y the energy released by the ejected electron separates water

into y hydrogen and oxygen y and carbon dioxide into its separate elements. Then the

released carbon, hydrogen, and oxygen recombine to form carbonic acid, which is in-

stantly changed into formic acid y this becomes formaldehyde and hydrogen peroxide,

which immediately breaks down into water, oxygen and glucose.

The fact that carbonic acid, formic acid and hydrogen peroxide are involved

serves to illustrate the complexity of the processes that permit the mobility and

complexing of any trace elements that have been drawn up into the plant structure

via the roots. It is also perhaps relevant that hydrogen peroxide is a strong oxidizing

agent that is the basis of several methods of selectively leaching elements from soils.

In effect, the plant conducts a ‘selective leach’.

Attempts have been made by several researchers to deﬁne the average compo-

sition of plants, e.g., Salisbury and Ross (1969), Lisk (1972), Bollar d (1983), Mars-

chner (1988, 1995), Mengell and Kirkby (1987), Kabata-Pendias and Pendias (1992),

Kabata-Pendias (2001), Mark ert (1992). Markert (1994) reviewed a large amount of

Biogeochemistry in Mineral Exploration

data and published a table listing element concentrations in a world ‘Reference Plant’

(Table 1-III). Over the past decade, there has been a wealth of new data obtained

from low-cost multi-element ICP-MS analyses that have provided much lower de-

tection limits than were readily available for some elements. In light of these new

data, the stated concentrations by Markert have been modiﬁed for those elements

marked with an asterisk.

Markert’s very useful guide was an ambitious and difﬁcult task because there are

such wide variations in composition of the many plant species from around the

globe. It has largely stood the test of time of more than a decade of new data, but the

values should be considered for what the table represents – a broad guide to the

world average composition of all parts of all plants. With the advent of ICP-MS

improved estimates can be made for some elements (notably Au, Ag, Hg, PGEs,

Re, Te, Tl) from many tens of thousands of analyses. These modiﬁcations are

shown in the table in bold font and marked with an asterisk. It is of the utmost

importance to realize, too, that element concentrations among individual tissues

from a single plant are dramatically different. Just as the various components of a

plant – wood, bark, twigs, foliage, ﬂowers/cones, etc. – bear no physical resemblance

to each other, nor do the chemical compositions of each type of tissue. There are

chemical barriers to element translocations between tissues that are discussed in the

next section.

Table 1-IV shows significant differences in the major element content of ash

obtained by igniting conifer needles and twigs to 1000

C. This temperature was

selected to remove both organic components and CO

, and is considerably higher

than the temperature usually set to reduce tissues to ash prior to analysis. Temper-

atures between 470

C and 500

C are used to remove only the organic components

and conserve some potentially volatile elements. This table shows, too, that the

residual ash from high temperature ignition is basically a carbonate-rich material

(dominated by Ca) with substantial ‘impurities’. There are substantial differences

between conifer needles and twigs, with needles having much lower contents of SiO

and commonly Al

, but much higher levels of K

O, P

, and MnO. Every

species of plant and every plant tissue has a different composition (e.g., grasses and

horsetails have high-silica content at the expense of the carbonate) and, understand-

ably, some uptake may be controlled by classic geochemical afﬁnities of elements.

Ranges in composition among species can be as great as the differences, for example,

between rhyolite, dunite, sandstone and limestone.

These data serve to emphasize how important it is in any biogeochemical explo-

ration survey to be consistent in the collection and analysis of plant tissues. This is

because the geochemical exercise being performed is to compare the relative com-

positions within a survey area. Lack of consistency in sampling will give rise to a

classic case of mixing ‘apples with oranges’; this can result in a lot of apparently

‘interesting’ data that can be very misleading. It is akin to mixing A, B and C soil

horizons and expecting to produce a meaningful element distribution map.

Introduction

TABLE 1-III

Element abundances in plants (dry weight) – Summary of estimates of worldwide averages of

all tissues from all plants (modiﬁed after Markert, 1994)

Element Units Concentration

Major elements (>0.1%)

C % 44.5

O % 42.5

H % 6.5

N % 2.5

K % 1.9

Ca % 1

S % 0.3

P % 0.2

Mg % 0.2

Cl % 0.2

Si % 0.1

Trace elements (o1000 ppm)

Ag* ppb 20

Al ppm 80

As ppm 0.1

Au* ppb 0.2

B ppm 40

Ba ppm 40

Be ppb 1

Bi ppb 10

Br ppm 4

Cd ppb 50

Ce ppm 0.5

Co ppm 0.2

Cr ppm 1.5

Cs ppm 0.2

Cu ppm 10

Dy ppb 30

Er ppb 20

Eu ppb 8

F ppm 2

Fe ppm 150

Ga ppm 0.1

Gd ppb 40

Ge ppb 10

Hf ppb 50

Hg* ppb 20

Ho ppb 8

I ppm 3

Continued

15Biogeochemistry in Mineral Exploration

TABLE 1-III Continued

Element Units Concentration

In ppb 1

Ir* ppb 0.01

La ppm 0.2

Li ppm 0.2

Lu ppb 3

Mn ppm 200

Mo ppm 0.5

Na ppm 150

Nb ppb 50

Nd ppm 0.2

Ni ppm 1.5

Os ppb 0.0015

Pa ppb ?

Pb ppm 1

Pd* ppb 0.1

Po ppb ?

Pr ppb 50

Pt ppb 0.005

Ra ppb ?

Rb ppm 50

Re* ppb 0.1

Rh* ppb 0.01

Ru ppb 0.1

Sb ppm 0.1

Sc ppb 20

Se ppb 20

Sm ppb 40

Sn ppm 0.2

Sr ppm 50

Ta ppb 1

Tb ppb 8

Te* ppb 20

Th ppb 5

Ti ppm 5

Tl* ppb 20

Tm ppb 4

U ppb 10

V ppm 0.5

W ppm 0.2

Y ppm 0.2

Yb ppb 20

Zn ppm 50

Zr ppm 0.1

16 Introduction

BARRIER MECHANISMS

Over 30 years ago, Alexander Kovalevsky emphasized the ‘barrier concept’ to

element uptake by plants, and introduced the principle of ‘barrier-free prospecting’

(Kovalevsky, 1974). The concept states that plant species and different plant organs

(his ‘bio-objects’) have varying degrees of resistance to the uptake by roots of el-

ements present in the substrate. ‘Non-barrier’ plants are those that can accumulate

an element in a constant plant-to-soil ratio regardless of the amount of that element

in the ground. These are ideal species for biogeochemical exploration. At the other

end of the scale are the ‘barrier’ plants that accumulate little or none of an element in

underlying soil by establishing mechanisms at the root/soil interface to exclude cer-

tain elements from entering the plant. Such plants are of little use in exploration.

Most plants (about 95%), however, fall somewhere betw een these two extremes and

they are able to accumulate a certain amount of an element before there is an adverse

affect on growth. When this concentration is reached the plant establishes a ‘barrier’

to further uptake by the roots. In addition to the barrier mechanism, many plants

cope with concentrations of elements that are surplus to their requirements by storing

them in a tissue (e.g., outer bark) where a plant’s health will not be adversely af-

fected. Examples of both amorphous and crystalline phases found within plant

structures are given in Chapter 2.

TABLE 1-IV

Major element composition (%) of the ash of conifer needles and twigs ignited at 1000

C. LOI

is the loss on ignition ash obtained at between 475

C and 1000

Mtn. hemlock needles Balsam ﬁr

needles

Jack Pine twigs

n ¼ 3 n ¼ 3 n ¼ 418 n ¼ 3 n ¼ 27 n ¼ 4

SiO

2.39 2.56 3.63 38.96 38.50 37.58

4.28 8.22 0.79 9.61 9.85 6.98

0.55 0.77 0.33 2.90 3.03 2.85

MgO

3.74 3.51 4.37 3.82 4.40 4.27

CaO

25.21 27.17 30.16 21.85 21.85 23.01

0.08 0.08 –0.02 1.34 1.42 1.45

20.65 16.69 19.50 3.27 3.58 3.82

TiO

0.05 0.05 0.01 0.31 0.30 0.34

9.46 6.12 8.27 1.85 1.75 2.04

MnO

5.86 5.93 1.40 0.08 0.12 0.14

LOI

(10001C)

27.60 28.80 26.32 15.80 15.50 15.60

17Biogeochemistry in Mineral Exploration

The barrier mechanism is an important concept, and some text s have attempted

to classify plants, plant tissues, and elements on this basis. However, so many factors

may come into play (e.g., soil acidity, soil moisture, the presence of other elements to

modulate the effects of toxicity, underlying lithology) that one can be misled into

ignoring certain plant species because they have been classiﬁed in some texts as ‘non-

informative’. For example, the Russian literature states that plants establish barriers

to uranium uptake and therefore they are of limited use in biogeochemical explo-

ration for uranium (Kovalevsky, 1987). For many plants and environments this is

true, but now that commercially available ICP-MS provides data for U at ppb levels

with excellent precision and accuracy it has become apparent that slightly elevated

levels of U commonly occur in associati on with many types of mineralization in

many plant species. Also, there are major exceptions to the mantra that most plants

establish barriers to U upta ke. In northern Saskatchewan black spruce (Picea mar-

iana) growing on the Athabasca Sandstone locally have more than 1000 times the

normal background concentrations of U (Dunn, 1983a) indicating that it is a non- or

practically non-barrier specie s. Additional species in this environment yield very high

levels of U (e.g., Labrador tea [Ledum groenlandicum], leather-leaf [Chamaedaphne

calyculata] and other boreal species).

It is necessary to be aware that, whereas one plant may not be responsive to a

certain type of mineralization, another plant may give a strong response. Fortunately,

many plant species may give similar patterns of element distribution with respect

to mineralization, but the absolute concentrations may be dramatically different.

This emphasizes that wherever possible a single plant species should be used for a

survey. On occasion a normalization factor can be applied to allow for the different

absorption characteristics of plant species and tissue types. This possibility is dis-

cussed later.

To reiterate what was stated earlier, a basic premise to be held in mind is that

‘plants do not always provide the same geochemical information as soils’. In an

exploration programme for mineral deposits a soil sample collected for analysis is

usually only a handful of a speciﬁc soil horizon. This soil is usually sieved to obtain a

few grams of a speciﬁc size fraction (e.g., –80 mesh aperture) for analysis. By con-

trast, a plant sample, because of its typically extensive root system, may represent

an integrated signature of several cubic metres of all soil horizons and some-

times bedrock. Furthermore, roots may extract metals directly from migrating

groundwater and accumulate them in their tissues, whereas little or none of that

metal may be adsorbed by the soil. Table 1-V is an extreme example of this situation,

showing a wide range in uranium concentrations in spruce twigs yet no significant

variation in the underlying soils. From the soil survey results, significant uranium

mineralization (world-class Athabasca U deposits) would be missed, whereas

from the biogeochemical data it is clear that the area contains high enrichments of

uranium.

Robert Brooks succinctly summed up the context within which biogeochemical

methods of exploration should be viewed:

Introduction

In the treatment of biogeochemistry, it is not claimed that the method is a universal

panacea for the search for minerals, and indeed under some conditions, it may not be

wise to expect too much from the method. However, if used in the right place by suitably

trained personnel well versed in its potential and application, it will prove to be a

valuable auxiliary or even primary method in the continuing search for minerals

throughout the world. (Brooks 1983, p. 111)

Almost a quarter of a century has passed since that judgement was made. In the

interim extensive research has been conducted and there has been an exponential

increase in the amount of data that is now available to place newly acquired data in

context with newly acquired survey results. To quote another statement of Robert

Brooks (Brooks et al., 1995, p. 239):

potential pitfalls are now better understood, and methods have been streamlined.

Biogeochemical prospecting for minerals is not difﬁcult provided a number of simple

steps and precautions are followed. It is a young science requiring a great deal more

knowledge before its enormous potential can be fulﬁlled.

Progress continues to be made on an ever-expand ing world stage.

TABLE 1-V

Concentrations of U in black spruce (Picea mariana) twigs, and in the Bf-horizon soil un-

derlying each tree, Athabasca Sandstone, Saskatchewan, Canada

Site

U (ppm) in ash of

twigs

U (ppm) – equivalent in

dried twigs

U (ppm) in Bf horizon

soil

0.1

1.9

0.16

2.4

0.4

2.5

0.5

2.0

113

2.3

1.9

226

4.6

1.8

303

1.8

408

8.2

1.8

486

9.7

2.0

886

17.7

1.9

Biogeochemistry in Mineral Exploration

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Chapter 2

PLANT FUNCTION, CH EMISTRY AND MINERALOGY

In this chapter a brief synopsis is provided of a vast amount of literature on the

essential chemical make-up of plants and the intricate processes that take place in

moving elements into, within and out of plants. These processes can be considered as

‘Biogeochemistry’ in the classical sense, and it is a topic dealt with in considerable

detail by Schlesinger (2005).

For the exploration geologist interest ed in using plants to locate concealed min-

eralization, the elem ents of particular relevance are mostly the non-essential ele-

ments. Ideally, the exploration geochemist should have a basic grounding in the

classic aspects of biogeochemistry, but to effectively run a biogeochemical survey for

minerals this is not a requirement. The intention of this short chapter is to ﬁll the gap

between the classical knowledge of the geologist/geochemist and that of the biologist/

botanist/plant chemist with some sali ent information. References are provided for

those wishing to delve deeper into the complexities of the plant world and its classical

biogeochemical studies.

PLANT REQUIREMENTS

There is a distinction that needs to be made as to what different disciplines mean

by the term ‘mineral’. To the geologist a mineral is a natural compound formed by

geological process. A more precise definition is ‘a naturally occurring homogeneous

solid, inorgan ically formed, with a definite chemical composition and an ordered

atomic arrangement’ (Berry and Mason, 1959). To the plant chemist the term ‘min-

eral’ is often used more loosely when referring to a single chemical element. A plant

chemist will indicate that ‘Copper is an essential mineral for plant metabolism’,

meaning simply that it is an essential element. Throughout this book, the term

‘mineral’ refers to its strict geological sense.

In botanical terminology, there are two main groups of elements: (1) Essential, or

nutritional elements and (2) Trace, or non-essential elements. Essential elements are

divided into macronutrients and micronutrients. The macronut rients are further di-

vided into pr imary and secondary. There is also the recognition of a group of el-

ements that are ‘beneﬁcial’. They have not been proven to be essential, but

experimentation has found that plant growth may be enhanced by their presence at

certain concentrations. Sodi um is an element that falls into this category.