Dunn Colin E. Biogeochemistry in Mineral Exploration

Подождите немного. Документ загружается.

Recently, the concept of a ‘functional plant nutrient’ has been introduced (Sub-

barao et al., 2003). In their paper they note that the concept of ‘essentiality’ was

deﬁned by Arnon and Stout (1939) as

those elements necessary to complete the life cycle of a plant. A few other elements such

as Na have an ubiquitous presence in soils and waters and are widely taken up and

utilized by plants, but are not considered as plant nutrients because they do not meet the

strict definition of ‘essentiality’. Sodium has a very speciﬁc function in the concentration

of carbon dioxide in a limited number of C4 plants and thus is essential to these plants,

but this in itself is insufﬁcient to generalize that Na is essential for higher plants. The

unique set of roles that Na can play in plant metabolism suggests that the basic concept

of what comprises a plant nutrient should be re-examined. We contend that the class of

plant mineral nutrients should be comprised not only of those elements necessary for

completing the life cycle, but also those elements which promote maximal biomass yield

and/or which reduce the requirement (critical level) of an essential element. We suggest

that nutrients functioning in this latter manner should be termed ‘functional nutrients.’

Thus plant mineral nutrients would be comprised of two major groups, ‘essential nu-

trients’ and ‘functional nutrients.’ y other elements such as Si and Se may also conﬁrm

to the proposed category of ‘functional nutrients.’

All 90 naturally occurring elements are found in most plant tissues, but with

many at extremely low concentrations. In addition to C, H and O, only 16 elements

are truly essential for growth of all plants (Table 2-I, ﬁrst column). An additional

16 elements shown in the remaining columns of Table 2-I are essential to some

plants and/or beneﬁcial to the metabolism of others. The remaining 55 elements are

mostly taken up passively in small quantities (traces or ultra-traces) as plants absorb

the nutrient elements that they need for growth and reproduction. At this time

they are not known to play an essential role in plants, but at ultra-trace levels

more elements may prove to be essential. This large group of trace elements can be

tolerated by most plants, with some species tolerating higher concentrations than

others.

In extreme cases ‘hyperaccumulation’ of a non-essential elem ent, such as Tl, can

take place although elements that can hyperaccumulate are mostly those that are

essential to many plants (e.g., Ni, Cu, Zn, Co, Mn). Hyperaccumulation of metals

(e.g., 1% Ni in Alyssum bertolonii) was ﬁrst recognized by Minguzzi and Vergnano in

1948, but it was not until 1977 that the term was introduced by Robert Brooks and

his co-workers (Brooks et al., 1977). Brooks (1998) summarized the numbers of

plants that are known to be hyperaccumulators of eight elements (Table 2-II ). Since

that publication appeared there have been additional plants recognized (e.g., a sec-

ond genus that can hyperaccumulate Tl).

Some elements that are considered non-essential can, in some situations, substi-

tute for an essential element and perform a surrogate ‘essential’ role. Molybdenum

Plant Function, Chemistry and Mineralogy

and W are examples. A primary function of Mo is in nitrate reductase, an important

enzyme that converts nitrate to ammonium. Tungsten can substitute for Mo in the

synthesis of nitrate reductase with reduced effectiveness, but still maintaining some

function. Similarly, Br can substitute for Cl in some plants.

It is advantageous to have some knowledge of these plant requirements when it

comes to interpreting biogeochemical data with respect to mineral exploration. For

example, concentrations of Zn in plants normally appear quite high when compared

to other elements or to Zn concentrations in soils. Part of this enrichment is because

Zn is an essential element and, therefore, will be present at moderate concentrations

even when there is no Zn mineralization in the substrate. As a result, the geochem-

ically similar element Cd is likely to be a better ‘pathﬁnder’ element for Zn min-

eralization than Zn itself. However, if there is substantial sphalerite mineralization

near surface it is highly likely that it will be reﬂected as enhanced concentrations of

Zn in tissues of trees that are rooted within the geochemical envelope of Zn enrich-

ment emanating from the mineralization.

TABLE 2-I

Essential elements

The biogeochemistry of life – essential elements

Essential to all

animals and plants

Essential to several

classes of animals

and plants

Essential to a wide

variety of species in

one class

Essential to only a

few species

Recent work

indicates

essentiality, but of

unknown function

Hydrogen(H) Silicon (Si) Boron (B) Lithium (Li) Rubidium (Rb)

Carbon (C) Vanadium (V) Fluorine (F) Aluminium(Al) Tin (Sn)

Nitrogen (N) Cobalt (Co) Chromium (Cr) Nickel (Ni)

Oxygen (O) Molybdenum(Mo) Bromine (Br) Strontium (Sr)

Sodium (Na) Iodine (I) Barium (Ba)

Magnesium(Mg)

Phosphorus(P)

Sulfur(S)

Chlorine (Cl)

Potassium (K)

Calcium (Ca)

Manganese (Mn)

Iron (Fe)

Copper (Cu)

Zinc (Zn)

Selenium (Se)

Note: Elements in bold type are generally considered to be trace elements in plants.

Source: The United States Geological Survey website http://geology.er.usgs.gov/eastern/

environment/environ.html#1 (8th October 2006).

23Biogeochemistry in Mineral Exploration

ELEMENT UPTAKE AND FUNCTION

The uptake of elements by plants is dependent upon ionic size and charge, and the

microscopic structure of root surfaces.



some elements can go in and out at will,



some elements are physically excluded,



some elements are actively pulled through the root walls (i.e., osmosis), and



some elements are actively excluded.

An element can ‘plug’ a root wall aperture, and subsequently exclude other el-

ements from passing through the wall or it may change the charge of the wall. This is

the ‘barrier’ mechanism described in the Russian literature (e.g., Kovalevsky, 1987).

Furthermore, the size of the apertures depends upon the temperatur e and availability

of water, and can change during the course of the year, giving rise to seasonal

variability in plant composition.

The role of various elements in plant metabolism is summarized in Table 2-III.As

noted above, many elements play a role in plant metabolism yet others may have no

function. Gold, for example, is not known to be of any use to plants, yet in plants

growing over mineralization it can be absorbed and concentrated a thousand-fold

over its usual background levels.

Additional information on the forms and principal functions of trace elements

essential for plants is provided by, amongst others, Kabata-Pendias and Pendias

(1992), Kabata-Pendias (2001), and by Chapin and Eviner (2005).

Photosynthesis is one of the most metal-sensitive processes of plant metabolism,

hence when a plant absorbs unusually large amounts of some metals (e.g., Cu)

photosynthesis can be affected, such that a plant may show stress, yellowing of leaf

TABLE 2-II

Plant hyperaccumulators of eight elements and the families in which they are most often found

(after Brooks, 1998)

Element No. of species Family

Cadmium 1 Brassicaceae

Cobalt 26 Lamiaceae, Scrophulariaceae

Copper 24 Cyperaceae, Lamiaceae, Poaceae, Scrophulariaceae

Manganese 11 Apocynaceae, Cunoniaceae, Protaceae

Nickel 290 Brassicaceae, Cunoniaceae, Euphorbiaceae,

Flacourtiaceae, Violaceae

Selenium 19 Fabaceae

Thallium 1 Brassicaceae

Zinc 16 Brassicaceae, Violaceae

24 Plant Function, Chemistry and Mineralogy

tips (chlorosis), or subtle changes in spectral reﬂectance that hyperspectral imagery

may be able to differentiate.

Uptake of an excess of trace elements can result in the induction of deﬁciencies of

other essential elements. These interactions trigger secondary respon ses, involving

enzymes, which either protect membranes against further damage or may partially

bypass metal-sensitive reactions. As a result the physiological state of the cell may be

altered and the plant may become more metal-tolerant.

Within a plant, elements may be variously mobile. The mobility determines where

deﬁciency or toxic symptoms will be expressed and how elements will accumulate

with increasing age. Table 2-IV lists the uptake mechanisms and relative mobility in

TABLE 2-III

The roles of essential and beneﬁcial elements in vascular plants (‘S’ ¼ structural;

‘E’ ¼ enzymatic). Modiﬁed after compilation by Scagel (in Dunn et al., 1993a)

Element Class Use

Al E Colloidal properties of cells

B S, E Carbohydrate metabolism, ﬂavinoid synthesis

Ca S Cell walls, N-metabolism

Cl E Enzymes, osmotic functions, stomatal movement

Co E Coenzyme

Cu S, E Enzymes, coenzymes, phosphorylation

F E Respiration

Fe S, E Chloroplasts, respiratory enzymes, phosphorylation

I E Protein function

K E Enzyme activity, osmotic regulation, stomatal movement

Li E Salt metabolism

Mg S, E Central element in chlorophyll molecule, enzymes,

phosphorylation

Mn E Chlorophyll synthesis, enzyme activator

Mo E Nitrogen metabolism, enzymes

N S Amino acids, proteins, cell membranes, enzymes, chlorophyll

synthesis, protein synthesis

Ni E Translocation of N

P S, E Nucleoproteins, phospholipids, high-energy phosphate bonds,

energy transfer, phosphorylation

Rb E Partial analogue for K

S S Amino acids, proteins, coenzymes, phosphorylation

Si S Cell walls

Sr S Partial analogue for Ca

Ti E Nitrogen ﬁxation

V E Nitrogen metabolism

Zn E Enzymes, hormone synthesis

25Biogeochemistry in Mineral Exploration

the phloem (downward movement) of essential and trace elements in plants. These

results (Bukova c and Wittw er, 1957) are complicated by the fact that the mobility of

elements classiﬁed as ‘intermediate’ varies with species, the stage of growth and the

concentration in the plant (Loneragan, 1975). Consequently, this classiﬁcation

should be viewed as only a general indication of the relative ability of element

movements. Elements that are mobile will express deﬁciency symptoms in older

plant parts and toxic symptoms in the youngest parts. Low-mobility elements will

TABLE 2-IV

Classiﬁcation of some chemical elements found in plants according to their uptake, transport

and mobility in the phloem (after Bukovac and Wittwer, 1957)

Element Quantities Uptake transport Phloem mobility

Ag Trace Passive

Al Micro Active

B Micro Active Low mobility

Ba Trace Passive Low mobility

Ca Macro Active Low mobility

Cl Micro Passive Mobile

Co Micro Passive

Cr Trace Passive

Cu Micro Active Intermediate

Fe Micro Active Intermediate

I Trace Passive

K Macro Active Mobile

Li Trace Passive Low mobility

Mg Macro Active Mobile

Mn Micro Active Intermediate

Mo Micro Active Intermediate

N Macro Active Mobile

Na Trace Passive Mobile

Ni Micro Active

P Macro Active Mobile

Rb Trace Passive Mobile

REE Trace Passive

S Macro Active Mobile

Se Trace Passive

Si Micro Active

Sr Trace Passive Low mobility

Ti Trace Passive

U Trace Passive

V Micro Passive

Zn Micro Active Intermediate

26 Plant Function, Chemistry and Mineralogy

express deﬁciency symptoms in the youngest plant parts and toxic symptoms in the

oldest plant parts.

ROOT FORM AND CONTROLS ON ELEMENT UPTAKE

Roots are complex structures that are highly variable in their morphology and in

their lateral extent and depth of penetration. Revi ews of roots and chemical inter-

actions in their rhizosphere are given by Richards (1986), Jones (1998) and Arienzo

(2005). The combined length of roots, rootlets and their mycorrhizal fungi (con-

sidered to be essential for tree survival) can be immense. Roots have been reported at

a depth of 53 m (Phillips, 1963) and there are anecdotal reports of roots penetrating

to greater depths from their presence in the roofs of mine galleries. It has been

estimated that a single rye plant, just over 1 m tall can have a combined length of

these roots, rootlets and mycorrhizae totalling over 600 km, and through millions of

microscopic apertures a root system constantly extracts from the soil and ground-

water those nutrients that the plant requires along with other elements that the plant

is able to tolerate. The additional roles that bacteria play are discussed in Chapter 12.

Thus, the root system of a single plant can integrate the geochemical signature of

many cubic metres of soil, groundwater and sometimes the precipitated oxides that

coat the surfaces of joints and faults in bedrock, and mineral surfaces. Locally, the

extraordinary physical power of roots can be observed in road-cuts or cliff faces

revealing the way the root system of a tree strives to ﬁrmly anchor itself, sometimes

by wedging apart and propagating rock fractures while seeking out sufﬁcient water

and nourishment for its very existence. Furthermore, on a hot sunny day 100–150 l of

water (with its dissolved elements) may be transported through the roots and stem to

the leaves of a large tree (Kramer and Kozlowski, 1979). Given these sobering sta-

tistics it is evident that a tree can be considered as an efﬁcient integrator of the

chemistry of a large volume of soil, groundwater and sometimes bedrock, and

thereby reﬂects a comprehensive geo chemical signature of the substrate.

Practically all of the movement of elements from roots to foliage occurs in the

xylem sap, carried by mass ﬂow in the transpiration stream with subsequent ﬂow

down the phloem to complete the ﬂuid cycle. Not all elements move in the xylem at

the same rate, with some slowed by adsorption on cell walls during xylem ﬂow. Thus,

some elements get to the growing parts of the plant more quickly than others, thereby

accounting for seasonal ﬂuctuations in concentrations of some elem ents. In the au-

tumn, soon before the leaves fall, some elements return to twigs where they are stored

over the winter months before they are required for the next year’s growth cycle.

There are a number of environmental factors that can modify the capabilities of

roots to absorb e lements. The pH immediately around the roots is generally a mi-

croenvironment that can be considerably more acidic than the surrounding rooting

environment of the soil, and values as low as pH 1 have been reported. The non-

woody roots and root tips are surrounded by a mucilage sh eath that is acidic and

Biogeochemistry in Mineral Exploration

may modify element ﬂuxes. Many trace elements are relatively stable at slightly acidic

pH due to the stability of the ligand chelates with which they are associated

(Pb>Cu>Ni>Co>Zn>Cd>Mn). Seasonal variation in pH inﬂuences the avail-

ability of several elements (e.g., Cu, Zn, Pb, Cd, Cr, Ni) thereby further accounting

for some of the seasonal variations in metal uptake and emphasizing the importance

of conducting a survey using live tissues in as short a time frame as possible (few

weeks). The relative mobility of elements is shown in Table 2-V.

The salt content of a groundwater solution determines what elements are taken up

or not in competing for active sites on the roots. In particular Na is readily taken up

by many plants, but blocks the subsequent uptake of K. Notes should be kept of sites

from near alkaline and seaside locations because the uptake of some elements may be

modiﬁed by such conditions. Along major roadways in northern climates, the salt

content of plants may be elevated because of the application of salt to road surfaces

in winter months to dissolve ice.

Redox potential determines the availability of many elements to plants. For in-

stance, arsenic becomes readily available under low redox conditions. Elements that

are particularly affected are those that require active uptake. Plant root respiration

cannot occur at low redox potentials and elements cannot be actively transported.

Redox potential also determines the form (i.e., chemical species) of an element in the

soil solution.

Decaying organic matter tends to bind trace elements and remove them from soil

solution, thereby preventing their uptake by plants. Consequently, consideration

should be given to the environmental conditions of a survey area, because by com-

bining samples from trees growing on organic-rich and inorganic soils, there may be

differences in distribution patterns of some elements that are largely attributable to

local ground conditions.

Several factors can inﬂuence root developm ent and subsequent metal uptake.



Slope and aspect can modify thermal conditions increasing the length of the

growing season and regulating transpiration rates.



Moisture levels that can regulate the length of the growing season and degree of

soil aeration.



History – stand age and tree age. Young trees have a wider lateral to depth ratio of

roots than older trees.



Soil depth and texture.



Pathology – insects and fungi.



Climatic events – drought, winter desiccation and frost.

SUMMARY COMMENTS ON CHEMICAL REQUIREMENTS OF PLANTS

There is a formidable amount of literature on plant physiology, nutritional re-

quirements and plant chemistry in general. Clearly, controlled pot and greenhouse

tests have done much to determine the conditions under which metals can accumulate

Plant Function, Chemistry and Mineralogy

in plants and the levels that can occur prior to the onset of toxicity. However, ex-

periments in the natural forest are far less common, and of necessity they need to be

extremely long term – tens if not hundreds of years.

It appears that in natural forest, over a number of years plants establish equi-

librium with their environment. Many factors can interact to govern element uptake,

TABLE 2-V

Relative mobility of different trace elements in relation to pH. Immobilizing factors: Fe/

Mn ¼ Fe/Mn oxides; OM ¼ organic matter; C ¼ clay; Red. ¼ reducing conditions; Ca ¼ cal-

cium carbonate (modiﬁed after compilation by Scagel, in Dunn et al., 1993a)

Element Acidic (opH

5.5)

Weakly acidic to

neutral (pH 5.5–7.0)

Alkaline

(pH>7.0)

Immobilizing

factors

Ag High Medium–low Very low Fe/Mn, OM, Red.

As Medium Medium Medium Fe/Mn, C

Au Low mobility Low mobility Low mobility

Ba Low Low Low Red., Ca, C

Be Low Low Low Fe/Mn; OM; C

Bi Low Low Low Fe/Mn, Red.

Cd Medium Medium Medium Red., OM

Ce Poorly soluble Poorly soluble Poorly soluble

Co High Medium–low Very low

Cu High Medium–low Very low Fe/Mn, OM

F High High High

Hg Medium Low Low Fe/Mn

Li Low Low Low Fe/Mn, C

Mo High High High Fe/Mn, Ca, Red.

Nb Poorly soluble Poorly soluble Poorly soluble

Ni High Medium–low Very low

Pb Low Low Low Ca, Red.

Pt Poorly soluble Poorly soluble Poorly soluble

Ra High High High Fe/Mn, OM

Rb Very high Very high Very high

Sb Low Low Low Fe/Mn, Red.

Se High High Very high Fe/Mn, Red.

Sn Poorly soluble Poorly soluble Poorly soluble

Ta Poorly soluble Poorly soluble Poorly soluble

Te Very. low Very low Very low

Th Very low Very low Very low C

U Low–medium High Very high Fe/Mn, OM, Red.

V High High High Red.

W Poorly soluble Poorly soluble Poorly soluble Fe/Mn

Zn High High–medium Low–Very low Fe/Mn, OM, Ca

29Biogeochemistry in Mineral Exploration

but there is sufﬁcient stability in plant chemistry for the biogeochemical method to be

a robust approach to mineral exploration provided consistency in collection, prep-

aration and analysis of tissues is maintained. An awareness of the factors that can

modify element uptake needs to be constantly maintained, and there should be a

general appreciation of the complexities of the natural environment. Consequently,

brief notes on ﬁeld conditions should be made and, if possible, broadly quantiﬁed in

order to assist in data interpretation. Quantiﬁcation of ﬁeld parameters need only be

a simple broad categorization. For example, aspect can be recorded as ‘south facing’

and degree of slope can be simply recorded as steep, moderate, gentle or ﬂat. Thus a

ﬁeld notation of 1S would indicate a gentle south-facing slope, whereas 3N would

denote a steep north-facing slope. By recording parameters in this manner they can

be related to chemical factors (by simple observation or statistically) during inter-

pretation of the subsequent elemental data.

MINERALOGY OF PLANTS

Skinner and Jahren (2005) gives a comprehensive overview of biomineralization

in plants and animals, eloquently describing those phases related primarily to sul-

phur, iron, silica, phosphorus and carbonates. It has long been established that

inorganic phases, such as silica phytoliths, develop on plant surfaces, giving grasses

and horsetails (Equisetum) their roughness. Figure 2-1 shows examples of some forms

of phytolith associated with the cultivar ‘Einkorn wheat’, from the Gramineae

Family, and provides clear evidence of why cereal s and grasses can be harsh to the

touch.

The scanning electron microscope (SEM) is required for detailed observation of

plants for crystalline and other metal phases, because they are rarely more than

100 mm in size. Figure 2-2 shows the development of a manganese phosphate phase,

about 100 mm 10 mm, which developed within the inner trunk wood of the conif-

erous tree ‘mountain hemlock’ (Tsuga mertensiana) from Mt. Washington on Van-

couver Island. This phase appears to be non-crystalline, and is wedged between the

longitudinal ﬁbres.

There are more than 100 families of tropical plants that contain silica and 215

families that contain calcium oxalates (Skinner and Jahren, 2005). The most fre-

quently occurring mineral phases observed in plant structures are crystals of calcium

oxalate (CaC

). They are commonly most concentrated in the bark (Fig. 2-3a, 3b),

although they occur, too, within twigs and foliage (Fig. 2-3c, 3d). Whereas Ca oxa-

late is a simple mixture of Ca, O and C, it is not absolutely pure. Pharmaceutical

analyses indicate that it typically has 0.5% Ba and 0.4% ‘Poorly soluble residues’.

These poorly soluble residues are likely to be primarily Si, Al, Fe, K, Mg and Na (all

of which can form independent oxalates), but in addition minor and trace elements

may be located within the crystal lattices of Ca oxalates in plants. Those reported

Plant Function, Chemistry and Mineralogy

include Sr, Rb, REE, S, F, Cl and Br. To date, no multi-element ICP-MS analysis of

Ca oxalate resid ues from plants appears to have been published, so it is quite feasible

that many additional trace and ultra-trace elements may be sequestered within the Ca

oxalate crystal structures.

On most modern SEMs the practical resolution is an object approximately 0.1 mm

in diameter. Examination of plant structures under the SEM quite commonly reveals

metal phases that occur as crystals or amorphous nucleations that are o2 mmin

diameter. Whereas some mineral phases seen on plant surfaces are not deﬁnitively

(a) (b)

Fig. 2-1. Scanning electron micrographs (SEM) of silica phytoliths on einkorn wheat

(Triticum monococcum) – (a) hair cell; (b) lamina – abaxial section; (c) and (d) inﬂorescence,

top and side views.

Source: http://home.byu.net/tbb/tball/index2.html (Terry Ball, Brigham Young University),

Feb. 2004.

31Biogeochemistry in Mineral Exploration