Dunn Colin E. Biogeochemistry in Mineral Exploration

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sometimes occur, e.g., Manuka (tea tree) shrubs growing around the Au-bearing

geothermal ﬁe lds at Waiotapu and Rotokawa in New Zealand have yielded more

than 200 ppm Br in ashed twigs with slightly lower levels in leaves.

Cadmium (Cd) (Fig. 9-11)

The precision of Cd by ICP-MS is excellent. Close to detection (0.01 ppm) Cd

data are likely to exhibit the usual +/– 100%, but this precision improves dramat-

ically at concentrations of just 0.05 ppm. Anomalous values are almost always sub-

stantiated by repeat analyses and so it is unusual to get ‘false positives’ that cannot be

reproduced.

Cadmium in plants has been studied extensively and a concise summary by

Punshon et al. (2005), including almost 200 references, makes excellent reading for

those interested in the complexities of Cd transfer from soils to plants and the

complex interactions that take place within plants.

There are plants known to hyperaccumulate Cd. The small weed Biscutella laevigata

is a montane herb of the Brassica Family found primarily in Eastern Europe that is

reported to contain more than 200 ppm Cd in its leaves, and an order of magnitude more

in its roots (Pielichowska and Wierzbicka, 2004). Although of environmental interest,

this plant is unlikely to be of more than local use in mineral exploration.

Other common trees that have relatively high concentrations of Cd, primarily

because of its ubiquitous geochemical afﬁnity for Zn, are birch, willow and, to a

lesser degree, poplar. Furthermore, a measure of Cd concentrations in most common

plants can assist in delineating zones of Zn enrichment, because unlike Zn, Cd is not

an essential element and tends to better reﬂect zones of Zn enrichment than Zn itself.

Also, Cd is sometimes associated with Au and can be used as a pathﬁnder element for

Au. Figure 9-12 shows concentrations of Cd and Au in dry spruce needles from a

traverse over a till-covered zone of undisturbed Au/Cu/Mo mineralization associated

with diorite and maﬁc volcanics. It is usual for a Cd signature to be spatially broader

(i.e., encompass a wider zone) than that of Au (Fig. 9-12).

Cd ppm

0.05

0.1

0.15

0.2

0.25

0.3

V6 - 30 controls among 600 samples

Concentration

Detection

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

0.24

0.23

0.02

0.23

0.25

0.02

Fig. 9-11. Cadmium – Precision and accuracy on dry tissues – V6 and NIST 1575a

(see Fig. 9-1).

242 Biogeochemical Behaviour of the Elements

Calcium (Ca) (Fig. 9-13)

The precision for Ca is excellent and, because Ca is a fundamental structural

element for cell wal ls and membranes, concentrations are invariably well above the

detection limit by ICP-MS of 0.01% Ca. About 1% Ca is present in the average

plant, and the range of Ca concentrations is normally between 0.1 and 5% dry weight

(Marschner, 1995; see Eden Project data on the CD). Acacias and ﬁgs (Ficus spp.)

have foliage that is relatively enriched in Ca, approaching 5%. Bark and to lesser

degree thin twigs have relatively high concentrations of Ca, because of the accumu-

lation of Ca oxalate crystals in these tissues (see Chapter 2). White and Broadley

(2003) provide a comprehensive overview of the complex roles that Ca plays in plant

structure and metabolism.

In mineral exploration Ca data from plant analyses have limited value, but the

data can assist in lithogeochemical mapping, because plants growing on carbonates

or other Ca-rich substrates are relatively enriched in Ca. For example, a traverse

from granite on to carbonate-rich sediments is reﬂected in an increase in the Ca

Cd ppm Au ppb

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

100 300 500 700 900 1100 1300

South NorthMetres

0.1

0.2

0.3

0.4

0.5

0.6

0.7

100 300 500 700 900 1100 1300

South NorthMetres

Fig. 9-12. Cadmium and Au in dry needles of Engelmann spruce from the Cariboo Zone in

central British Columbia (Dunn et al., 2006a,b).

Ca %

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

0.75

0.73

0.03

0.25

0.27

0.01

0.2

0.4

0.6

0.8

V6 - 30 controls among 600 samples

Concentration

Detection

Fig. 9-13. Calcium – Precision and accuracy on dry tissues – V6 and NIST 1575a (see

Fig. 9-1).

243Biogeochemistry in Mineral Exploration

content of the plant tissues. Furthermore, there is commonly visual evidence of such

a transition, with trees on granites having relatively thin twigs, whereas on the car-

bonates twigs are thicker and exhibit more vigorous and robust growth.

Cerium (Ce) (Fig. 9-14)

See Lanthanum and the Rare Earth Element s (REE).

Precision of analytical data for Ce is excellent at concentrations above 1 ppm Ce.

The poorer precision exhibited for NIST 1575a is not characteristic of other controls

with similar concentrations. A control composed of twigs from mountain hemlock

has a similar mean value to the NIST 1575a (0.08 ppm Ce), yet has an RSD of 8.5%.

Since concentrations of 0.1 ppm Ce or higher are typical for many types of vege-

tation, it can be assumed that the data quality is generally very good unless con-

centrations are close to the detection lim it of 0.02 ppm Ce.

Conifer twigs usually have higher concentrations of Ce than their foliage. The

reverse seems true for deciduous species. Alder leaves (Alnus crispa) from the REE-

rich allanite at Hoidas Lake in northern Saskatchewan have yielded 12 ppm Ce and

the twigs 8.7 ppm Ce. Black spruce from close to the Rottenstone PGE/Ni/Cu de-

posit in Saskatchewan have 0.54 ppm Ce in twigs and 0.05 ppm Ce in needles,

whereas alder from the same location show the same pattern of Ce enrichment in

leaves that was observed at Hoidas Lake.

In South Africa, foliage of Acacia mellifera (Black thorn) has yielded up

to 1.5 ppm Ce adjacent to a kimberlite. Stems from this species had concentrations

of Ce that were half to one-third of those in the leaves. Dwarf birch leaves from

the Ekati area of kimberlites in the Northwest Territories of Canada yielded

average concentrations of 0.08 ppm Ce with a maximum of 0.35 ppm Ce. More

discussion on Ce anomalies related to mineralization is presented in the section on

lanthanum.

Ce ppm

0.5

1.5

V6 - 30 controls among 600 samples

Concentration

Detection

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

1.6

0.1

(0.11)

0.07

0.02

Fig. 9-14. Cerium – Precision and accuracy on dry tissues – V6 and NIST 1575a (see

Fig. 9-1).

244 Biogeochemical Behaviour of the Elements

Cesium (Cs) (Fig. 9-15)

Biogeochemical survey samples usually contain well in excess of the low detection

limit of 5 ppb Cs in dry tissue achieved by ICP-MS. Spikes in the data from survey

samples can be interp reted with conﬁdence that they represent true concentrations,

since sources of contamination are sparse, and replicate analyses of anomalous levels

are nearly always repeatable.

In most geochemical processes, Cs has a strong afﬁnity to K and Rb. In plants this

association with K is usually absent or weak, but there is commonly a good correlation

between Cs and Rb. On occasion, especially in situations where Cs is associated with

some types of mineralization, the Cs/Rb relationship is relatively weak.

An example of modest Cs enrichment spatially related to a Au biogeochemical

anomaly is shown in Chapter 5 (Fig. 5-7). This relationship with Au is not too sur-

prising since the Cs bearing mineral ‘galkhaite’ [Cs

0.6

0.4

3.5

1.5

ZnAs

3.6

0.4

]is

associated with Au deposits at Hemlo (Ontario) and Getchell (Nevada). High levels of

Cs are associated, too, with Au, As and Sb in the areas of geothermal ﬂuid discharge in

New Zealand, and some plants from these areas are significantly enriched in several

elements, including Cs. Table 9-II shows levels of several elements that are enriched in

the ‘Tea Tree’ Manuka (Leptospermum scoparium) growing in the warm mists em-

anating from Champagne Pool and Sulphur Flats on New Zealand’s North Island. In

the case of Champagne Pool, the ﬂuids are over-saturated with respect to the As- and

Sb-bearing minerals orpiment and stibnite (Pope et al., 2004), so some of the metal

enrichments must be directly related to precipitation from the mists that rise from the

Pool and swirl around the Manuka.

Ferns and Ficus (ﬁg) trees both have a strong propensity to accumulate Cs (Eden

Project). In the Chinapintza Au district on the border between Peru and Ecuador

(Cordillera del Co

ndor), around the low sulphidation epithermal prec ious metal vein

system there are enrichments of more than 20 ppm Cs in dry fronds from tree ferns

(Cyathea spp.).

Cs ppm

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

0.03

0.003

0.28

0.29

0.02

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

V6 - 30 controls among 600 samples

Concentration

Detection

Fig. 9-15. Cesium – Precision and accuracy on dry tissues – V6 and NIST 1575a (see

Fig. 9-1).

245Biogeochemistry in Mineral Exploration

Although a Cs/Au association is not always present in vegetation, this relation-

ship is worth scrutiny in biogeochemical studies designed to delineate Au mineral-

ization, because of the potential of Cs to be used as a spatially associated ‘pathﬁnder’

element.

At the Bernic Lake Ta-Li-Cs mine in Manitoba, there are concentrations up to

3600 ppm Cs in ash (358 ppm Cs dry weight) of outer bark from jack pine near the

mine site. Maxima in other tissues (with dry-weight values in parentheses) are black

spruce outer bark with 2300 ppm Cs (212 ppm), and twigs of both pine and spruce up

to 3000 ppm Cs (118 ppm). Concentrations in dry twigs of deciduous species are

5 ppm Cs in birch; 20 ppm Cs in poplar; and mostly greater than 20 ppm Cs in alder.

In other areas remote from Cs mineralization alder has shown a propensity to

accumulate Cs. The Cs signature at Bernic Lake is no doubt related in part to wind-

blown dust from the mining operations. Some of the Cs-bearing particles (mostly

contained in the Cs mica pollucite) probably adhere to the vegetation, but most have

settled to the ground, then been dissolved by rainwater and subsequently taken up by

tree roots. The evidence for this is that analyses of trunk wood yield anomalously

high levels of Cs. Whereas background levels in trunk wood would normally be only

a few ppm Cs in ash, up to 290 ppm Cs is present.

Chromium (Cr) (Fig. 9-16)

Precision of Cr in V6 is good, largely because concentrations at about 4 ppm Cr

are well above the detection limit of 0.05 ppm Cr. The lower precision for NIST 1575a

TABLE 9-II

Concentrations of elements significantly enriched in Manuka (tea tree) samples from sites of

hot springs on New Zealand’s North Island. Concentrations are in ash, determined by INAA.

Ash yield data are provided for estimating concentrations in dry tissue

Waiotapu (Champagne pool) Rotokawa (Sulphur ﬂats) Background

Twig Leaf Twig Leaf

Au (ppb) 983 1740 79 45 5–10

As (ppm) 86 86 38 54 o5

(ppm) 210 180 110 65 o50

Cs (ppm) 140 92 38 36 o1

Rb (ppm) 1500 1100 950 770 o200

(ppm) 270 170 6 4 o1

Ash yield (%) 2.3 3.3 1.9 4.2

Typical ‘background’ levels in plant ash.

Up to 90% of Br and 30% of Sb are likely to have volatilized during ashing.

246 Biogeochemical Behaviour of the Elements

is because it contains less than 1 ppm Cr. Large spikes in ﬁeld survey data are unusual.

When they do occur, they are often due to contamination, and are nearly always

accompanied by spikes in Ni (probably reﬂecting minute metal particles from cutting

blades in grinders or mills commonly used for reducing tissues to a ﬁne powder in

preparation for analysis). In general, the data are very reliable. Chromium is one of

the several elements (including As, S, Se and V) for which some laboratories declare

that low levels cannot be accurately measured by ICP-MS, whereas others claim to

have circumvented this problem and provide consistently precise data.

Earlier, it was noted that there is a loss of up to 70% Cr from some tissues during

reduction of dry material to ash. This factor should be borne in mind in the eval-

uation and interpretation of any data obtained on the analysis of ash. However, it

appears that losses from a particular species are consistent, so that Cr distribution

patterns, when plotted on a map, remain much the same whether the data are derived

from ash or dry plant tissue.

Chromium is an essential trace element to a wide variety of plants, and they

usually absorb Cr in its trivalent state (Cary et al., 1977). Although only usually

present at levels in the low-ppm range, elevated levels of Cr can and do occur,

especially over ultramaﬁc rocks. Over Co–Cr mineralization contained within ultra-

maﬁc rocks of the Bou Azzer region of southern Morocco, samples of the goat-root

Ononis natrix yielded up to 88 ppm Cr in dry stems (Dunn et al., 1996c). This is an

unusual enrichment, and probab ly occurred because Cr mineralization is primarily

contained within the relatively soluble Mg–Cr carbonate stichtite (an alteration

product of chromite) and Cr–mica (fuchsite).

On occasion, Cr exhibits enrichments in plants over zones of Au mineralization.

In Labrador tea (Ledum groenlandicum) growing in a bog adjacent to Au miner-

alization, elevated levels of Au in stems were associated with enrichments of Co, Cr,

Th, U and W (Dunn et al., 1990). This site was later developed as the Jasper Au mine

in northern Saskatchewan.

In general, the foliage of palms and bamboos contain relatively high concentra-

tions of Cr (Eden Project).

Cr ppm

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

4.4

4.3

0.4

(0.4)

1.5

0.4

V6 -30 controls among 600 samples

Concentration

Detection

Fig. 9-16. Chromium – Precision and accuracy on dry tissues – V6 and NIST 1575a (see

Fig. 9-1).

247Biogeochemistry in Mineral Exploration

Chlorine (Cl)

See ‘Halogens’.

Cobalt (Co) (Fig. 9-17)

Precision for Co is generally good and concentrations in plants commonly col-

lected for biogeochemical surveys are well above the detection limit of 0.01 ppm Co.

During uptake by plants Co tends to follow Fe bound to complexing organic com-

pounds (Wiersma and Van Goor, 1979). However, statistical analyses of biogeo-

chemical datasets commonly show that Co follows Ni in geological environments

dominated by maﬁc to ultramaﬁc rocks. In other environments, Co may act inde-

pendently of other elements. On occasion, there is a spatial relationship between Co

and Au deposits, with a zone of Co enrichment peripheral to a central zone of

elevated Au values. The epithermal Au/As/Sb deposit at Mount Washington on

Vancouver Island exhibits this relationship.

In central Africa, the genus Haumaniastrum (a member of the mint Family) has

been described as a ‘copper ﬂower’ because of its ability to hyperaccumulate Cu. It

can also accumulate Co, but it appears that this ability is limited to the Shaban

Copper Arc of Democratic Republic of Congo (formerly Zaı

re) and adjacent parts of

the Zambian Copper belt (Paton and Brooks, 1996).

A study of Co in Black gum (also known as Tulepo, Nyssa sylvatica) from the

United States found that

Foliar concentrations of stable cobalt increase uniformly until senescence. In late Au-

gust, foliage accounts for only 9 percent of total tree weight but 57 percent of total tree

cobalt. Losses of cobalt from trees occur almost entirely by leaf abscission, and the loss

rates of weight and cobalt from decomposing litter are similar. Retention of cobalt in the

biologically active soil layers perpetuates zones of cobalt concentration created by this

species in woodlands (Williams, 1975).

Co ppm

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

4.4

0.03

0.06

0.07

0.02

0.1

0.2

0.3

0.4

0.5

V6 - 30 controls among 600 samples

Concentration

Detection

Fig. 9-17. Cobalt – Precision and accuracy on dry tissues – V6 and NIST 1575a (see Fig. 9-1).

248 Biogeochemical Behaviour of the Elements

These results and conclusions have broad application to understanding biogeo-

chemical cycles, since they demonstrate that biogeochemical enrichment of an ele-

ment, not just Co, can persist for many years through natural recycling from soil to a

plant and back to soil when foliage falls to the ground and decomposes.

Another study of six species of Nyssa from the United States and southeast Asia

found that they all accumulated Co (Brooks, 1983). Other plants that accumulate Co

are from the Rubiaceae (‘Bedstraw’ family – Pentas [Star cluster] with 47 ppm Co in

leaves [Eden Project]) and Zingiberaceae (Ginger family – Costus with 7 ppm Co in

leaves).

In the Co-rich Bou Azzer area of southern Morocco, several plant species grow-

ing on serpentinized ultramaﬁc rocks and near Co mineralization (skutterudite,

safﬂorite and birbirite) were found to accumulate Co to modest levels (Dunn et al.,

1996c). Noteworthy concentrations in dry tissues were leaves of Veronica (24 ppm

Co) and Anvillea garcinii (12 ppm Co).

Copper (Cu) (Fig. 9-18)

Copper concentrations in vegetation are typically 2–3 orders of magnitude above

the detection limit by ICP-MS of 0.01 ppm Cu; hence the precision of Cu is excellent

with RSDs better than 10%.

Copper is one of the most thoroughly investigated elements in plants. It is an

essential element that plays a significant role in several physiological processes and in

disease resistance. Lepp (2005) provides a succinct overview of Cu in plants and

indicates that much work remains to be done, in part because of lack of agreement on

detailed mechanisms of Cu movement and accumulation.

From an exploration perspective, many of the complexities of Cu can be over-

looked without compromising the interpretation of analytical results. No doubt a

greater understanding of Cu metabolism will permit ﬁner tuning of data interpre-

tation, but at this time there is sufﬁcient information that can be extracted from

Cu ppm

Target

Mean

Std. Dev.

RSD %

0.2

2.8

3.3

0.3

V17 - 22 samples among 440 samples

Concentration

Detection

V17 NIST 1575a

Fig. 9-18. Copper – Precision and accuracy on dry tissues – V17 and NIST 1575a (see

Fig. 9-1).

249Biogeochemistry in Mineral Exploration

simple data analysis and plotting of Cu from a speciﬁc plant species in order to

provide the ﬁeld geochemist with some exploration vectors.

There are plant species capable of accumulating high levels of Cu, notably the

‘copper ﬂowers’ of Central Africa (e.g., Becium homblei and Haumaniastrum katang-

ense) – in particular in the Shaba Province of the Democratic Republic of Congo

where concentrations in excess of 1% Cu in dry tissue are reported (Brook s, 1998)

and in Anhui Province of China (Commelina communis – Tang et al., 1997). How-

ever, Lepp (2005) considers that there has not been satisfactory demonstration of Cu

hyperaccumulation under controlled conditions, with the inference that some of

these high accumulations may be attribut able to airborne contamination from cop-

per-mining operations.

Markert’s (1994) ‘Reference Plant’ has 10 ppm Cu. Experience with tissues from

many different common plants used in biogeochemical exploration indicates that

median values are commonly 5–8 ppm Cu, increasing to tens of ppm Cu over min-

eralization. The rubber vine (Cryptostegia grandiﬂora) from the Eden Project yielded

37 ppm Cu, indicating that it has a propensity to accumulate Cu. Concentrations

greater than 50 ppm Cu in uncontaminated areas are exceptional. At the Mt. Polley

Cu/Mo/Au porphyry in central British Columbia, concentrations reach over 30 ppm

Cu in dry twigs and foliage of several conife rs collected from over the original

discovery zone (sampled in 1991 – Dunn, 1995a,b), and over undisturbed prospects

from the surrounding area sampled in 2005 (Dunn et al., 2006a,b). As a broad, but

not deﬁnitive, rule of thumb, values in excess of 15 ppm Cu in conifers are worth

closer examination.

Dysprosium (Dy)

See Lanthanum and Rare Earth Elements.

Dysprosium has no known biological role. In V6 it exhibits good precision with a

mean of 0.09 ppm Dy. Databases are very limited, but it appears that except for near

REE deposits and sometimes with other REE near kimberlites Dy is rarely concen-

trated in plants above its detection limit by ICP-MS of 0.02 ppm Dy. In South Africa,

foliage of Acacia mellifera (Black thorn) has yielded up to 0.42 ppm Dy adjacent to a

kimberlite. Stems from this species had lower concentrations of Dy and most were

below the detection limit. Dwarf birch leaves from the Ekati area of the Northwest

Territories of Canada all yielded concentrations below detection.

Erbium (Er)

See Lanthanum and the Rare Earth Element s.

As for the other REE, Er has no known biological role. In V6 it exhibits near

perfect precision with a mean of 0.04 ppm Er. Databases are very limited, but except

for near REE deposits Er is rarely concentrated in plants above its detection limit by

250

Biogeochemical Behaviour of the Elements

ICP-MS of 0.02 ppm Er. In South Africa, foliage of Acacia mellifera (Black thorn)

has yielded up to 0.2 ppm Er adjacent to a kimberlite. Stems from this species had

lower concentrations of Er and most were below the detection limit. Dwarf birch

leaves from the Ekati area of the Northwest Territories of Canada all yielded con-

centrations below detection.

Europium (Eu)

See Lanthanum and the Rare Earth Element s.

Europium has no known role in plant metabolism. In V6 it exhibits very good

precision with a mean of 0.025 ppm Eu. Europium is rarely concentrated in plants

above its detection limit by ICP-MS of 0.02 ppm Eu, except near REE deposits.

Alder leaves (Alnus crispa) from the REE-rich allanite at Hoidas Lake in northern

Saskatchewan have yielded 0.15 ppm Eu. In South Africa, foliage of Acacia mellifera

(Black thorn) has yielded up to 0.12 ppm Eu adjacent to a kimberlite. Stems and

fruits from this species had lower concentrations of Eu and most were below the

detection limit. Dwarf birch leaves from the Ekati area of the Northwest Territories

of Canada all yielded concentrations below detection. More discussion on Eu anom-

alies related to mineralization is presented in the section on lanthanum.

Fluorine (F)

See ‘Halogens’.

Gadolinium (Gd)

See Lanthanum and the Rare Earth Element s.

Gadolinium has no known biological role. It exhibits good precision with a mean

of 0.13 ppm Gd in V6. As for most of the other REE, databases for Gd are very

limited and it is rarely concentrated in plants above its detection limit by ICP-MS of

0.02 ppm Gd except near REE de posits. In South Africa, foliage of Acacia mellifera

(Black thorn) has yielded up to 0.66 ppm Gd adjacent to a kimberlite. Stems and

fruits from this species had lower concentrations of Gd and most were below de-

tection. Dwarf birch leaves from the Ekat i area of the Northwest Ter ritories of

Canada all yielded concentrations below detection.

Gallium (Ga) (Fig. 9-19)

Although Ga is not a particularly rare element (30th in terms of crustal abun-

dance at 19 ppm Ga), concentrations in vegetation tissues are commonly below the

251

Biogeochemistry in Mineral Exploration