Dunn Colin E. Biogeochemistry in Mineral Exploration

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formed in the plant and may be airborne particulates, crystalline phases of metals can

sometimes be observed within or attached to plant cells. Figure 2-4 shows some heavy

metal phases that have nucleated to form well-deﬁned crystal structures. Figure 2-4a

shows a 2 mm Zn+S crystal (presumably sphalerite) within a twig of western hemlock

from the Carolin (Ladner Creek) Au mine in British Columbia; Figure 2-4b shows

two larger elongate Zn+Mn+O crystals (10 mm) in a needle of lodgepole pine from

the vicinity of the Sullivan Pb-Zn mine at Kimberley in southern British Columbia.

Under the SEM, backscatter images are useful for viewing heavy metals because

they appear as bright spots against a darker background. The higher the atomic

number of an element, the greater the backscatter image contrast, such that very

heavy elem ents like Au, Hg and U appear as the brightest objects on those rare

occasions when they have nucleated to a sufﬁcient degree to be resolved by the SEM.

Once located by this technique, their composition can be determined from X-ray

dispersive analytical instrumentation attached to the SEM.

Of particular note are the photomicrographs shown in Fig. 2-4c, 4d. They depict

the same ﬁelds of view, with 4c showing a reﬂected light image, and 4d a backscatter

image. The photomicrographs are close-up views of the inside of bark collected from

mountain hemlock (Tsuga mertensiana) that was growing on an unexploited gold de-

posit at Mt. Washington on Vancouver Island. The deposit has not been developed as

a mine because it sits on a popular ski-hill, and so there has been little ground dis-

turbance by exploration activities. Each photomicrograph shows a well-formed

trigonal crystal about 1 0.5 mm, which, when the X-ray beam was focused on the

object, proved to be gold. This appears to have crystallized out of the sap within

the tree.

Not all metal phases are crystalline. In fact, the majority appear as poorly deﬁned

irregular aggrega tions that are not readily apparent until examined in backscatter

mode. Figure 2-5 shows a selection of these in trees from several locations.

100 µm

Fig. 2-2. Linear white area is manganese phosphate phase within conifer trunk wood.

32 Plant Function, Chemistry and Mineralogy



Au–Ni phase. This is of interest, because mineralogy texts do not indicate any such

phase as having been recognized in rocks. Consequently, it appears that this phase

has nucleated from the organic acids that coursed through the plant’s xylem and

phloem cells.



Bi phase. The X-ray analysis did not indicate any other element associated with the

Bi. ICP-MS analysis of the bark revealed Bi concentrations well above usual

background values of 20 ppb Bi.



Linear smear of Fe oxide (with a trace of Cl), about 100 mm long in a twig of

mountain hemlock. The smear may be the result of cutting with a diamond knife,

but other FeO phases (notably spherules) within the twig indicate that the mineral

phase formed within the twig.

200µm

20µm

50µm

a: Cross Section of twig from lodgepole pine

(Pinus contorta). Crystals concentrated where

bark is forming around new growth

b: Cross section of twig of western hemlock

(Tsuga heterophylla). Bark at top is crowded

with crystals

c: Ca oxalate crystals (5-10µm) formed in twig

of black spruce (Picea mariana)

d: Ca oxalate crystals (5-10µm) within needle

from western hemlock (Tsu

a heterophylla)

Fig. 2-3. Development of calcium oxalate crystals within plant tissues – (a) Cross section of

twig from lodgepole pine (Pinus contorta). Crystals concentrated where bark is forming around

new growth; (b) Cross section of twig of western hemlock (Tsuga heterophylla). Bark at top is

crowded with crystals; (c) Ca oxalate crystals (5–10 mm) formed in twig of black spruce (Picea

mariana); (d) Ca oxalate crystals (5–10 mm) within needle from western hemlock (Tsuga het-

erophylla).

33Biogeochemistry in Mineral Exploration



Irregular 3 mm diameter patch of Cu, Fe oxide in mountain hemlock bark.



As, Fe, S phase (arsenopyrite?) located in the phyllogen, close to the cambium, of a

sagebrush twig from the near the Nickel Plate (Au skarn) mine, Hedley, British

Columbia. The same specimen contained Fe spheroids in the radial parenchyma

(not shown here).



Ta, Sn phase with peculiar protuberances of different composition from near the

Bernic Lake rare metal pegmatite mine (Li, Ta, Nb, Be, Cs, Rb), Manitoba.

A backscatter image is shown on the right half of the photomicrograph. At other

localities nebulous aggregations of Sn oxide and Sn chloride have been identiﬁed –

for example in a sagebrush twig from near the Nickel Plate mine.

10µm

2µm

50µm

Mn, Zn, O

Fe, Zn, S

Gold

4a: Crystalline Fe+Zn sulphide phase in twig of

western hemlock (Tsuga heterophylla)

4b: Crystalline Zn+Mn oxide in needle of

lodgepole pine (Pinus contorta)

4c:Crystalline gold within mountain hemlock

bark (Tsuga mertensiana)

4d: Crystalline gold within mountain

hemlock bark – backscatter image.

Same field of view as 4c.

Fig. 2-4. SEM photomicrographs of crystalline metal mineral phases within plant tissues – (a)

Crystalline Fe+Zn sulphide phase in twig of western hemlock (Tsuga heterophylla); (b) Crys-

talline Zn+Mn oxide in needle of lodgepole pine (Pinus contorta); (c) Crystalline gold within

mountain hemlock bark (Tsuga mertensiana); (d) Crystalline gold within mountain hemlock

bark – backscatter image. Same ﬁeld of view as 4c (modiﬁed from Dunn, 1995b).

34 Plant Function, Chemistry and Mineralogy

5a: Au-Ni phase in bark of

mountain hemlock (Dunn, 1992)

5b: Bi phase in bark of mountain hemlock

5c: Cross section of conifer twig.

Bright streak is smeared iron oxide

phase ~100 µm (Dunn, 1995)

5d: Cu, Fe oxide in bark of mountain hemlock

5e: As, Fe, S phase in sagebrush

(Artemisia tridentata)

5f: Ta, Sn phase in alder twig (Alnus crispa)

1µm

Au, Ni

10 µm

5 µm

Fig. 2-5. Heavy metal phases in plant structures – (a) Au-Ni phase in bark of mountain

hemlock (Dunn, 1992); (b) Bi phase in bark of mountain hemlock; (c) Cross section of conifer

twig. Bright streak is smeared iron oxide phase 100 mm (Dunn, 1995); (d) Cu, Fe oxide in

bark of mountain hemlock; (e) As, Fe, S phase in sagebrush (Artemisia tridentata); (f) Ta, Sn

phase in alder twig (Alnus crispa).

35Biogeochemistry in Mineral Exploration

Other phases that have been observed include the following:



Cu–Al oxide phase (0.75 mm in diameter) within the cortex of a mountain hemlock

needle,



Iron oxide spherules in twigs and needles of mountain hemlock,



Calcium phosphate in mountain hemlock twigs,



Barium sulphate phase 8 mm in diameter in bark of mountain hemlock and also

in needles of Douglas-ﬁr,



Oxides of Ni, Sn and Zn in the bark of mountain hemlock (all o2 mm),



P and Bi chlorides in the trunk wood of mountain hemlock,



Zinc oxide in trunk wood of mountain hemlock,



Cesium silicate in the stomata of a western hemlock needle,



Ni, Fe and As phases in twigs of sagebrush, and



Crystalline Zn (sulphide?) in a sagebrush twig.

There is a wealth of information yet to be obtained from SEM studies of heavy

metal phases concentrated within plant structures, and there appears to be little

published research on these phases. The important point to note is that in addition to

the chemical evidence of metal concentrations in plant tissues, there is an abundance

of visual evidence when tissues are viewed and analysed with instrument ation that

can resolve micron-sized features. There is compelling evidence that minerals nucle-

ate within plant tissues.

Plant Function, Chemistry and Mineralogy

Chapter 3

FIELD GUIDE 1: CLIMATIC AND GEOGRAPHIC ZONES

SELECTION OF PLANT SPECIES

This section summarizes the common plants to be found throughout various cli-

matic regions of the world. Many, but by no means all, of those listed have been tested

for their effectiveness as biogeochemical exploration sampling media. The lists are far

from exhaustive; they are intended as a general guide to what to look for in each

region, and provide a starting point for conducting a survey in any given area. Since

this book is prepared primarily for those involved in mineral exploration and therefore

those with more of a geological background than a botanical or environmental train-

ing, the plants are referred to by common names with the Latin binomials added for

those seeking more comprehensive information. For many geologists this is the easiest

way to learn about the plants, and ﬁeld assistants generally relate better to simple

descriptions and common names. Nowadays, thanks to the internet, it is a speedy

process to use a search engine to ﬁnd a wealth of information about any of the plants

listed below, including plates in full colour. Addresses of key web sites are provided

that should assist in ﬁnding detailed information on the plants that are described.

Work carried out in the former Soviet regime included some extensive compi-

lations of analytical data (mostly by emission spectroscopy) on a wide variety of

plant species from the Siberian tundra southward through the various vegetation

zones to the Caspian Sea. The emphasis, however, was on the boreal forests of

Siberia. Although in many cases details of relationships to speciﬁc types of miner-

alization were not always obvious (lack of scale and location, because of the per-

ceived political sensitivity of the information at the time), extensive lists of species

relevant to biogeochemical exploration were published (e.g., Kovalevsky, 1987,

1995b, 2001; Kovalevsky and Kovalevskaya, 1989). Typically, such lists were or-

ganized by plant species and tissue types (or ‘bio-objects’ as they were sometimes

referred to) according to their considered sensitivity to mineralization. The lists are

sorted mostly from non-barrier species (plants that can pick up high concentrations

of metals, generally in proportion to concentra tions in the ground – see Chapter 1) to

high-barrier (plants that establish barriers at their roots to the uptake of metals).

These lists provide useful guides, but they are restricted to plants found throughout

the Russian states, and on occasion a plant lis ted as ‘high-barrier’ (i.e., non-inform-

ative) has been found to be quite informative in locations outside of the Russian

environments studied by Kovalevsky and others. This is probably because of

different environmental and/or geological conditions that have permitted the passage

of elements into plant structures classiﬁed as, for example, non-informative in Rus-

sia, yet have proved to be informative elsewhere.

An example of this situation is that of uranium. Out of 65 plant organs studied by

Kovalevsky (1987), 97% were classiﬁed as exhibiting strong barriers to U uptake,

and none fell into his ‘non-barrier’ category. However, other studies from around the

world have reported high concentrations of U in plant tissues (summarized in Dunn

et al., 1985), demonstrating that barriers to the translocation of U from soils to roots,

twigs, and, to a lesser degree, into foliage can quite commonly be minor. In northern

Saskatchewan, Canada, the U-rich Athabasca Sandstone has an extensive cover of

pine and spruce. Twigs of black spruce (Picea mariana) have been found to accu-

mulate high levels of U over deeply buried U deposits, even though the soils in which

they grow have only background levels of U (Dunn, 1983b). However, concentra-

tions in needles growing on these twigs are substantially lower – usually by an order

of magnitude – and so the needles establish a high barrier to U translocation.

The topic of ‘relative’ uptake of metals by different tissues and species is discussed

later, but it is of relevance to point out that in the case of U and the barriers discussed

here, the patterns of relative uptake remain quite constant. Figure 3-3 compares

concentrations in the ash of twigs and needles, each from a single tree, collected along

an 8-km traverse near the eastern margin of the uraniferous Athabasca Sandstone

Group in northern Saskatchewan. Although there are substantially different concen-

trations in the twigs versus the needles from those twigs, the proﬁles are very similar.

This example leads to a concept of fundamental importance to biogeochemical

exploration, involving pattern recognition.



Similar distribution patterns of a single element may be derived from sampling

different sample media provided sampling procedures remain consistent for each

medium.

To elaborate, the close correspondence between the analysis of twigs and needles

illustrated in Fig. 3-1 conﬁrms that provided a set of samples is collected consistently,

and data from different species and/or tissues are not combined into a single da taset

(unless normalized), then areas of anomalous concentrations can be outli ned re-

gardless of whether twigs or needles are collected. This attests to the robustness of the

biogeochemical method for mapping element distributions in many environm ents.

The recognition of spatial relationships of different elements to mineralization,

usually from the analysis of a single species, is a key step in the interpretation of

biogeochemical data. This point will be discussed later.

Orientation

The ﬁrst step in preparing to conduct a biogeochemical survey is to look for the

most widespread species within the area of interest, then determine from published

Field Guide 1: Climatic and Geographic Zones

information if that species is likely to be informative (i.e., do the easy-to-reach parts

accumulate the elements of interest, and have they been shown elsewhere to outline

the presence of mineralization?). If published information is not available, and if time

is not too critical (and assuming that the survey area can be easily accessed), it is

advisable to conduct an orientation survey to collect a few samples of common

species and submi t them for multi-element analysis. This will provide preliminary

and valuable insight to the levels of metals present, and help the geologist/geochemist

ﬁne-tune the analytical package that should be requested for a larger survey. This

knowledge is not a prerequisite, but it will serve to optimise the geochemical contrast

that can be provided by selecting the best sample medium. In the case of the U

example shown in Fig. 3-1, if needles had been selected as the prime sample medium

instead of the twigs, it would not have made a significant difference to the over all

patterns of element distribution, even though U concentrations were much lower in

the needles. The danger here, though, is that because of the lower concentrations in

the needles, some samples from a broader survey area might have yielded values

below the limit of detection, whereas the twigs on which the needles were growing

might yield useful geochemical relief in the data because of their ability to concen-

trate U.

Sometimes one species is not sufﬁciently widespread to sample at every survey

station. It is not uncommon for a proposed survey area to extend through several

vegetation zones. In the temperate forests of North America there may be, for ex-

ample, Douglas-ﬁr at low elevations, with lodgepole pine on surrounding slopes, and

subalpine ﬁr at higher elevations. Similarly, in the boreal forest there may be a

transition from black spruce-dominated forest to jack pine. In these situations it is

necessary to establish overlap zones where two species grow, so that samples of

both species can be collected at a few sites. By establishing the relative element

Uranium in ash of black spruce

100

150

200

250

300

350

123456789101112 1413 15 16 17 18 19 20 21

Sequence Number (traverse)

ppm in twig ash

ppm in needle ash

Twig

Needle

Fig. 3-1. Uranium in ash of black spruce (Picea mariana) twigs (10 years growth) and needles

along a traverse near the eastern edge of the Athabasca Sandstone Group, Saskatchewan,

Canada (modiﬁed from Dunn, 1983c). Analysis by INAA.

39Biogeochemistry in Mineral Exploration

concentrations of pairs of species it is possible to normalize the data to a common

benchmark. However, because of the differences in element requirements and tol-

erances among species (and more so among genera), this normalization commonly

proves to be a rather imprecise exercise.

As a basic premise, for a given survey the same type of plant tissue should be

collected from the same species of tree or shrub, unless there is prior knowledge that

no significant chemical differences occur between two or more species. For some

plant species there may be a negligible difference in the element uptake character-

istics. Such is the case for black spruce (Picea mariana) versus red spruce (Picea

rubens), and for some species of Acacia. However, for others (e.g., black spruce

versus white spruce (Picea glauca)) there are some substantial elemental differences.

In a test to compare the chemistry of outer bark from white spruce with that from

black spruce, samples were collected from adjacent trees at 12 sites in northern

Ontario. Table 3-I shows the average concentration ratios. The data clearly indicate

that most heavy elements are more concentrated in the black spruce, notably Bi, Cd,

Hg, Ni and Pb. For some elements there is little or no difference (e.g., ratios of

0.9–1.1 for Cu, Na, P, Re, S, Ta). The elements that are consistently more concen-

trated in the bark of the white spruce include elements essential for plant nutrition

(B, Ca, K, Mg and Zn) with associated trace elements that show typical geochemical

afﬁnities for these elements – Ba and Sr with Ca; Cs and Rb with K; and Sn, Te and

Tl with less obvious associations.

Substantial differences between species seem to be the exception rather than the

rule, although they must be considered and there are some sobering examples of

extreme differences. In the 1950s, Helen Cannon and her co-workers examined the U

and Se uptake of a wide range of plants, including various species of poison vetch/

locoweed (Astragalus) from the Colorado Plateau (Cannon, 1952, 1964). Among

species of the same genus (Astragalus) they found substantially greater differences in

Se uptake than the relatively subtle differences illustrated in Table 3-I.

Differences among many common plant species are usually more subtle than these

two examples. However, in light of differences that might occur between other pairs of

species, unless the exploration geologist has a ﬁrm handle on elemental similarities and

differences among species, it is better to play it safe and collect only samples of a single

species. Where it is not possible to obtain a systematic coverage of a survey area, a few

comparative tests of the sort shown (Table 3-I) can be undertaken and similar ratios

established between species. When these ratios are available, it is then possible to

normalize the datasets to a common species and establish a meaningful plot of element

distribution patterns. Bear in mind, though, that this procedure is not always fully

satisfactory, and, because of different barriers to element uptake, may simply not be

possible for some pairs of species. Data should be carefully evaluated before jumping

to conclusions and merging data from two species into a single dataset.

Among plant genera there are commonly substantial chemical differences. In that

there are some fundamental morphological and chemi cal differences between plant

Field Guide 1: Climatic and Geographic Zones

genera, the normalization pro cess described above may be less robust than for spe-

cies. This is because a particular genus may establish a more rigorous barri er to the

uptake of certain elements than another genus of the same family. Thus, the rela-

tionship may not be linear, and so a normalization process may have to involve

regression or a more complex mathematical normalization. Clearly, the valid ity of

patterns derived from such processes is increasingly tenuous on moving up the hi-

erarchy of plants : species compared with species may well be possible; genus com-

pared to genus is less robust.

TABLE 3-I

Ratios of elements in outer bark of black spruce (Picea mariana) compared to white spruce

(Picea glauca) – pairs of trees from 12 sites in northern Ontario. Analysis by ICP-MS on an

aqua regia digestion

BS:WS Outer Bark Ave for

same site comparisons

BS:WS Outer Bark Ave for

same site comparisons

n ¼ 12 n ¼ 12

1.4

0.9

1.3

1.5

1.3

1.9

1.2

1.1

0.8

1.8

0.8

0.5

1.8

1.0

0.6

1.0

2.1

1.4

1.6

1.4

1.5

1.2

0.4

0.8

0.5

0.9

1.0

1.6

0.7

1.2

1.5

1.9

1.5

1.2

0.7

0.5

1.6

1.5

0.8

1.6

1.3

0.7

1.5

1.4

Biogeochemistry in Mineral Exploration