Dunn Colin E. Biogeochemistry in Mineral Exploration

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Ag on stump surfaces from over a silver deposit in Siberia and suggested that stump

surfaces might represent an appropriate biogeochemical medium for Ag exploration

(pers. comm., ca. 1990).

With regard to normal environmental conditions, foliage tends to have higher

concentrations of Hg than twigs. Figure 9-35 shows this relationship from a plot of

Hg concentrations in tissues from the tops of Douglas-ﬁr, southern British Colum-

bia. There is usually a very high correlation between Hg in twigs and Hg in foliage

that becomes evident when concentrations are in the tens of ppb Hg or higher. In the

example shown the relationship is moderately good, but the proﬁle for Hg in twigs is

subdued because levels are close to detection.

In acidic aquatic environments bryophytes have been shown to accumulate HgS

(metacinnabar) in cell walls (Satake et al., 1990). Two liverworts (Jungermannia

vulcanicola and Scapania undulata) occurri ng in Hg-contaminated acid waters were

investigated by several high-powered microscopic techniques and coagulated crys-

talline phases of HgS were identiﬁed attesting to the ability of the plants to absorb

Hg in stream waters and nucleate crystals within their cell structures.

A study of mercury in 1208 dry plant samples (11 species) showed that all samples

from the mercury-bearing Pinchi Fault (central British Columbia) contained 200 to

1600 ppb Hg (Warren et al., 1983). This contrasted with background concentrations

from non-mineralized ground that rarely yielded in excess of 150 ppb Hg. A later

study found 560 to 43,000 ppb Hg in the moss Pleurozium schreberi from the Pinchi

mine site, and along the same structure 125 km to the north at the former Bralorne

Takla mine the same species yielded 420–8560 ppb Hg (Plouffe et al., 2004). At the

same time as the study by Plouffe et al., samples of vascular plants and mosses were

collected from these areas, and scanning electron microscope investigation of the

same species reveal ed the presence of small irregular grains of cinnabar (approxi-

mately 2 mm) that undoubtedly accounted for much, if not all, of the elevated levels in

the moss. Outer bark of lodgepole pine and Douglas-ﬁr yielded up to 5000 ppb Hg,

Needles v. Twigs

Sequence of samples

Hg ppb.

Hg in needles

Hg in twigs

Averages

Fig. 9-35. Mercury in dry twigs (5 years growth) and needles from the tops of Douglas-ﬁr

(Pseudotsuga menziesii) from near the Shuswap River in southern British Columbia.

272 Biogeochemical Behaviour of the Elements

and various tissues from deciduous species at the mine site had from 40 to 1600 ppb

Hg. These data are a sobering reminder that biogeochemical anomalies from im-

mediately around a mine site may be related to airborne particulates of ore minerals

derived from mining operations.

Russian studies have suggested that vegetation ash from samples over ore zones

contain a non-volatile chemical species of mercu ry (perhaps mercury carbide) that

bears a distinct relationship to the extent of the underlying ore. Kovalevsky (1986)

reports a phenomenal 300,000 ppb Hg in ash from plants overlying mercury ore and

up to 4300 ppb Hg in plant ash from sampl es over pyrite-polymetallic ore. The most

sensitive plant tissues were found to be bark of birch (Betula platyphylla) and larch

(Larix dahurica). The question here arises, are these truly Hg concentrations derived

from Hg absorbed by the plants? Or could they be related to Hg associated with

airborne particulates adhering to plant surface? From a personal database that in-

cludes many tens of thousan ds of analyses of ashed plant tissues, the only anomal ous

Hg in ash samples have been the following:

In a reconnaissance survey of western Nova Scotia a single sample of red spruce

bark ash yielded a high and analytically repeatable level of 4000 ppb Hg. Upon

investigation it was found that this sample also contained 640 ppm As, 2600 ppm

Sb and 6% Pb – vastly more than the typical levels for the area of o5 ppm As and

Sb and o200 ppm Pb in ash. The conclusion arrived at was that this tree must

have got in the way of a hunter with a riﬂe, and that fortuitously the bark sample

included a splattering of Pb shot!

In a reconnaissance level biogeochemical survey of a 200 km

area of northern

Newfoundland, twigs and outer bark of black spruce were the principal sampl e

media. All samples were reduced to ash by ignition at 470

C, and analysed by

INAA and ICP-ES for a wide range of elements including Hg. At the time of the

survey, mine tailings were exposed at an abandoned cop per/gold deposit (Con-

solidated Rambler). Around this site, and for a distance of 3–4 km to the south-

east, both bark and twigs of black spruce tissue yielded anomalous concentrations

of Hg in ash with maxima of 255 ppb Hg in outer bark and 150 ppb Hg twigs.

High values were coincident with enrichments of Au, As and Sb.

In the latter example, the Hg values, although repeatable, seemed suspicious

because Hg in vegetation usually volatilizes completely at between 150 and 200

C. It

is tempting to assign these coincident anomalies to new and undiscovered zones of

gold minerali zation, especially in light of the observations by Kovalevsky (1986), and

also because the area is scattered with small Au deposits. However, in the case of the

Newfoundland study a more probable explanation is that the metals are associated

with ﬁne particles of wind-blown sulphide-rich dust, derived from the large open area

of mine tailings, which have become lodged in the tree tissues. This explanation is

supported from anecdotal evidence that foresters in the area complained that their

chain saws became dulled rapidly when cutting trees from the area down-wind from

the mine site, suggesting that there was a relatively high abundance of wind-blown

273

Biogeochemistry in Mineral Exploration

dust particulates lodged in the bark. Mercury is associated with the ore, and is mostly

associated with pyrite an d pyrrhotite. Analysis of the mine tailings showed that most

of the Hg, Au and As is present in very ﬁne particles (o–230 mesh) that become

readily airborne by winds (Table 9-IV). During the ashing process, not all the mer-

cury volatilized because it was tightly locked within the crystal lattices of the sulphide

grains. Despite careful washing of vegetation samples, it is not always possible to

remove all dust because some becomes ﬁrmly embedded and with time tissue may

grow around and over some grains, such that they become incorporated within the

plant structure. This factor should always be considered as a possibility when in-

terpreting biogeochemical data from samples collected close to mine sites.

In South America, studies in the Amazon have found high levels of Hg around the

garimpo artisanal workings where the garimpeiros use Hg to recover the Au. The

shrub species Vassoura de Bota

o, which is a pioneer species capable of surviving and

invading disrupted environments, has been found to contain up to 4600 ppb Hg in dry

tissue (Dunn and Angelica

, 2000). Of the 23 plants common to many areas of the

Amazon it was found that the highest Hg concentrations were in foliage. Imbauba

(Cecropia spp.), the most common species of the survey area, does not accumulate high

levels of Hg (nor the other elements determined) but it reveals relative Hg enrichments

around the garimpos for a distance of 200–300 m. Other species indicate that there is

airborne contamination around the garimpos for a distance of at least 500 m.

Three thousand kilometres to the west, in the headwaters of the Amazon, lies the

Chinapintza district in the Cordillera del Co

ndor, located on the border between

Peru and Ecuador. The epithermal Au deposits of this area have associated with

them a strong Hg anomaly in the tree ferns (Cyathea spp.). The median Hg con-

centration for fern fronds throughout an area of approximately 120 km

is 76 ppb

Hg. At Chinapintza concentrations reach almost 4000 ppb Hg with concentrations

greater than 300 ppb Hg extendi ng over an area of at least 1 km

and trending for

TABLE 9-IV

Metal concentrations in various size fractions of tailings from the former consolidated Ram-

bler Mine, Baie Verte, Newfoundland, Canada

ASTM ‘Tyler’ sieve size Hg

(ppb)

(ppm)

Coarse (>–30 mesh

[600 mm])

110 200 33 2 8 18 170

–30 to –80 175 300 70 8 15 31 160

–80 to –150 195 700 390 19 87 28 95

–150 to –230 485 750 600 26 120 19 45

Fine (o–230 mesh

[63 mm])

1170 1700 670 41 120 37 190

274 Biogeochemical Behaviour of the Elements

several kilometres to provide a target for more de tailed exploration. Elsewhere in the

survey area other anomalous Hg trends are evident. Whereas consideration should be

given to the possibility that the highest Hg co ncentrations may be derived from

airborne particulates, regional trends serve as targets on which to focus exploration

activities.

In light of the results from many recent studies on the use of Hg data to assist in

mineral exploration, much of the advice given by Kovalevsky (1986) remains sound.

In particular, that Hg data can be used to elucidate concealed structures (folds,

faults, fractures, zones of brecciation), and it is advisable to use Hg biogeochemical

data in prospecting mainly for non-mercury deposits. As shown above, the presence

of cinnabar in outcrop or open pit is likely to generate false Hg anomalies from

airborne particulates. Mercury haloes may emanate from deep-seated ore-bodies that

have Hg associated with them. Kovalevsky (1986) suggests that because of the vol-

atility of Hg, ore-bodies at depths of 200–2000 m can generate Hg haloes that can be

reﬂected in plants. Foliage and conifer bark are the preferred sample media.

Molybdenum (Mo) (Figs. 9-36 and 9-37)

Reproducibility of Mo in analytical controls and duplicates is excellent at the

levels of Mo typically found in vegetation. Howev er, the data for NIST 1575a in-

dicate poorer precision at the much lower levels present in that control material.

Molybdenum is an essential micronutrient in plant enzymes. Its most important

function is the reduction of soil nitrate

but only small traces are required. Roots are

commonly relatively enriched in Mo (especially nodulated roots such as those found

in alder), and N-ﬁxing micro-organisms have especially large requirements for Mo.

In plants, Mo is one of the most mobile elements, but the plant tissues in which

there is relative enrichment of Mo vary by species. In general, the twigs of conifers

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

0.28

0.27

0.02

(0.01)

0.02

0.01

Mo ppm

0.05

0.1

0.15

0.2

0.25

0.3

0.35

V6 - 30 controls among 600 samples

Concentration

Detection

Fig. 9-36. Molybdenum – Precision and accuracy on dry tissues – V6 and NIST 1575a (see

Fig. 9-1).

275Biogeochemistry in Mineral Exploration

have higher Mo concentrations than the foliage. In deciduous species values are

sometimes higher in the leaves and sometimes in the twigs, but the pattern is con-

sistent for a particular species. Differences are not great, with one tissue type rarely

having more than double the Mo concentration of another, although an exception

appears to be plants that tend to accumulate Mo, such as alder. Molybdenum in

alder twigs can be more than four times enriched than alder leaves, and especially

enriched in the latest year of twig growth. Conversely, the genus Combretum (bush-

willow), widespread throughout much of sub-Sahara Africa, has more Mo in leaves

than twigs. Acacia from the North Mara district of Tanzania and also from Western

Australia contains slightly more Mo in the twigs than the foliage.

Molybdenum is a significant pathﬁnder for Cu–Mo, Cu–Au–Mo, W–Mo and

other Mo-bearing deposits. Copper–Mo porphyries are usually better deﬁned from

the Mo content of the vegetation than by that of Cu. Kovalevsky (1987) investigated

the exploration characteristics with respect to Mo of 506 species and parts of plants

and found most to be highly informative. From their Mo content suberized (woody)

pine cones from the ﬂoor of Siberian forests were found to be particularly effective for

focussing on exploration targets. However, because there are so many plant tissues

that do not establish significant barriers to Mo uptake, most parts of most species are

suitable for biogeochemical surveys with the exception of inner bark (bast). Inner

bark is rarely an informative or practical medium to collect for any element.

Data from the Eden Project demonstrate the greatly varying capabilities of dif-

ferent plant species to accumulate Mo, with plants showing a range from 0.03 ppm

Mo in Protea to 10.8 ppm Mo in Acacia grown in similar soils under a controlled

environment.

Molybdenum is one of the few elements for which studies claim to be success ful in

quantifying the depth through overburden to or e. Kovalevsky (1987) reports that

Mo in dry twigs - Precision

y = 1.1416x - 0.0068

= 0.9877

0.05

0.1

0.15

0.2

0.25

0.3

0 0.1 0.2 0.3

Mo ppm

Fig. 9-37. Precision (analytical duplicates) of Mo in dry Douglas-ﬁr twigs. Nineteen repeat

analyses within batch of 555 samples, of which 10 analytical pairs returned identical values.

276 Biogeochemical Behaviour of the Elements

if the minimum y measure of the anomalies is their threefold excess over local back-

ground, then the maximum [overburden] depth of detecting lithogeochemical haloes

having a relative intensity of 500 compared with the local background (i.e.,

ore ¼ 1000 ppm and background ¼ 2 ppm), is 22 m for the lower parts of the stems of

Dahurian rhododendron [Rhododendron dauricum], 17 m for the upper parts, and 16 m

for shoots and leaves.

This bold prediction of depth to mineralization, estimated from sampling differ-

ent tissues in a ‘non-barrier’ plant, he deﬁned as ‘biogeochemical logging’. This

quantiﬁcation of depth to mineralization is an exciting concept, but it does not

appear to have been tested outside of Siberia.

A survey over a Mo porphyry in the western Amazon showed that the Mo

anomalies derived from four different plant parts (foliage from bamboo, a vine, tree-

fern, and bark from the fern) were similar and spatially related to mineralization.

However, concentrations were substantially higher in the fern (foliage and bark) than

in the other two species (Fig. 4-3).

The ready availability of Mo causes high levels of Mo uptake by plants in con-

taminated sites. As a consequence, there are commonly large biogeochemical Mo

haloes around Mo mining operations. Windblown Mo-bearing dust can settle on the

ground and be dissolved by rain and leach into the soil. It is then absorbed by plant

roots to give a substantial enrichment in plant tissues. This process probably ac-

counts in large part for the extensive Mo halo that surrounds the Endako mining

operations in central British Columbia, where reconnaissance level surveys using the

outer bark of lodgepole pine (Pinus contorta) have outlined the regional extent of

molybdenum enrichment. This survey involved reducing bark to ash which, in

lodgepole pine throughout much of central British Columbia, has background levels

of 5–10 ppm Mo (0.1–0.2 ppm Mo dry weight [DW] equivalent). The median value

for a dataset of 217 samples was 78 ppm (1.5 ppm Mo DW), and values in excess of

the median extended over an area of approximately 5000 km

. Values greater than

the 95th percentile value of 717 ppm Mo (34 ppm Mo DW) extended over an area of

500 km

attaining a maximum of 1.5% Mo in ash (300 ppm Mo DW) from trees close

to the mine (Fig. 9-38).

Although dust from the mining operations contributes to the high levels encoun-

tered, historic records indicate similar levels in plants collected from the mine site

around 1950 prior to any mining operations (Warren et al., 1953). At that time values

of 65 ppm Mo in dry alpine ﬁr needles were reported (equivalent to 2000 ppm Mo in

ash) from an undisclosed site which from discussions with Professor Warren in the

1990s proved to be Endako. Clearing of the mine site for production did not begin

until 1964. Analyses published in 1965 ( Warren and Delavault, 1965) of a variety of

species reported maxima in ash of 9600 ppm Mo in willow leaves and 17,000 ppm Mo

in the annual plant ‘ﬁreweed’ (Epilobium angustifolium). Sa mples of ﬁreweed col-

lected more than 30 years later from the mine site yielded 2950 ppm Mo in ash or

376 ppm Mo in dry tissue (Dunn, 1998b).

277

Biogeochemistry in Mineral Exploration

At the Lucky Ship prospect located 100 km farther west, outside of the large

Endako anomaly, concentrations of more than 1000 ppm Mo in the ash of second-

year growth of alpine ﬁr (Abies lasiocarpa) needles were reported by Hornbrook

(1969). In oven-dry needles of lodgepole pine and alpine ﬁr concentrations exceeded

20 ppm Mo over the deposit, and anomalous values extended for 500 by 750 m.

Subsequently, various companies held the property and undertook a limited amount

of prospecting. In 2005, Cantech Ventures Inc. commenc ed a drilling programme and

grades of >0.089% Mo were encountered within its ﬁrst two holes. Drilling in 2006

‘encountered Mo mineralization over its entire length with the best grade between 6.0

and 266.0 metres which averaged 0.084% Mo (including the ﬁrst 36 metres which

averaged 0.163% Mo)’ (www.newcantech.com, August 9, 2006, press release).

Molybdenum serves as a good example of an element that can be very useful as a

biogeochemical indicator of mineralization – both Mo deposits and Mo-bearing

deposits, such as Cu–Mo–Au porphyries. Analytical data are precise and accurate,

and because it is highly mobile Mo can be readily taken up by plants. However,

because of its mobility, caution must be exercised to carefully evaluate the environ-

ment within which samples are collected (espec ially noting any nearby mine work-

ings); and there should be vigilance maintained when interpreting results, because an

anthropogenic ‘footprint’ can obscure the na tural signature derived directly from

concealed mineralizati on.

Neodymium (Nd)

See Lanthanum and the Rare Earth Element s.

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

mean of 0.7 ppm Nd. Databases are limited but sufﬁcient to establish that Nd is

usually concentrated in plants at levels just above its detection limit by ICP-MS of

0.02 ppm Nd in dry tissue. In South Africa, foliage of Acacia mellifera (Black thorn)

140

282

717

Mo ppm

[in ash]

Max. 15,000

Endako Mo Mine

Fraser Lake

Township

20 km

Lake

Highway

MOLYBDENUM - Pine Bark

Fraser Lake

Lake

Francois Lake

Fig. 9-38. Molybdenum in the ash of lodgepole pine bark – reconnaissance survey in the

vicinity of the Endako Mo mine. Analysis by INA.

278 Biogeochemical Behaviour of the Elements

has yielded up to 2.7 ppm Nd adjacent to a kimberlite. Stems from this species had

concentrations 50–70% lower. Dwarf birch leaves from the Ekati area of the North-

west Territories of Canada yielded an average concentration of 0.03 ppm Nd, with a

maximum of 0.18 ppm Nd. The sagebrush Artemisia collected from near the Bayan

Obo REE mine in northern China yielded a phenomenal 600 ppm Nd in dry tissue

(3300 ppm Nd in ash). However, from the unusually high ash yield of this sample

(18%) it is strongly suspected that a co nsiderable proportion of the Nd was derived

from airborne particulates.

On occasion, Nd enrichment has been observed in samples spatially related to

zones of Au mineralization. Further details are given in the section on ‘Lanthanum’.

Nickel (Ni) (Fig. 9-39)

The analytical precision for Ni by ICP-MS is very good and concentrations in plant

tissues are commonly well in excess of the detection limit of 0.1 ppm Ni.

For some plants, Ni is an essential element present in the enzyme urease, and it

probably plays a role in the translocation of nitrogen. It can also occur as a citrate,

and it is in this form that some extraordinarily high concentrations of Ni in plants

have been reported. The blue sap reported on the surface of bark from the serpen-

tine-endemic tree ‘Se

ve bleue’ (Sebertia acuminata) from New Caledonia obtains its

colour from Ni. Analyses have yielded 25.7% Ni in dried sap and 11.2% Ni in fresh

sap (Jaffre

et al., 1976). The Ni contents of other dried plant tissues from this tree

were 1.17% Ni in leaves, 2.45% Ni in bark from the trunk, 1.12% Ni in bark from

the twigs, 0.20% Ni in fruits and 0.17% Ni in trunk wood.

Lee et al. (1977, 1978) established that the Ni content of the sap in Se

ve bleue, and

in the leaves of other New Caledonian and Zimbabwean species, was a citrate–nickel

complex. In other species Ni is present as a malate complex. There are more plants

that hyperaccumulate Ni than any other metal and Brooks (1998) lists 256 species

noting that another 48 are known from Cuba. Subsequently, more have been

Ni ppm

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

3.3

3.2

0.25

1.5

1.6

0.16

V6 -30 controls among 600 samples

Concentration

Detection

Fig. 9-39. Nickel – Precision and accuracy on dry tissues – V6 and NIST 1575a (see Fig. 9-1).

279Biogeochemistry in Mineral Exploration

identiﬁed bringing the number now to 318. The dominant plant families that have

species capable of hy peraccumulating Ni are the Buxaceae, Euphorbiaceae,

Flacourtiaceae and (notably) the Brassicaceae. Examples are as follows:



Buxaceae (e.g., boxwood) from Cuba. Up to 2.5% Ni in Buxus vacciniodes.



Euphorbiaceae (ﬂowering shrubs) – especially Leucocroton from Cuba (up to 6%

Ni) and Phyllanthus from New Caledonia (up to 3.8% Ni).



Flacourtiaceae (ﬂowering trees and shrubs) from New Caledonia. Especially

Homalium and Xylosoma with up to 1.5% Ni.

The Brassicaceae (herbs and forbs) are mainly from southern Europe. Most no-

table are Alyssum (48 species – up to 2.9% Ni – especially Alyssum bertonii) and

Thlaspi (Pennycress – 23 species with up to 3.1% Ni). The species Berkheya coddii has

been identiﬁed as a prime candidate for phytomining because of its high biomass (2 m

tall) and its ability to concentrate >2% Ni.

With respect to mineral exploration surveys the above examples of Ni-hyperac-

cumulation are somewhat academic. Most of the Ni-hyperaccumulator plants are

curiosities and not sufﬁciently abundant to conduct an exploration biogeochemistry

survey. In the vast boreal and temperate forests of the world, no common plant is

known to be a Ni-hyperaccumulating species. Foliage of plants from the Eden

Project show more typical levels, with a range from 0.2 ppm Ni in pine needles to

5.2 ppm Ni in bamboo leaves.

In a survey using scrapings of outer bark from 455 Douglas-ﬁr trees (Pseudotsuga

menziesii) in the vicinity of the Old Nick showings in southern British Columbia it was

found that the median concentration was 1 ppm Ni with a maximum of 4.6 ppm Ni.

Mineralization is associated with pyrrhotite and pentlandite disseminated within

serpentinite units and metasediments. Mineralization is fairly uniform with an average

range of 0.15–0.2% Ni. The bark signature is, therefore, quite subtle although linear

trends related to mineralization and bedrock are apparent from contoured bark data of

values greater than 1.5 ppm Ni (British Columbia Dept. of Mines 2004 assessment

report 27,579). By comparison, a large dataset of lodgepole pine outer bark samples

from central British Columbia where no Ni deposits are reported yielded concentrations

lower than those in Douglas-ﬁr at Old Nick, with a median value of 0.3 ppm Ni and a

maximum of 0.85 ppm Ni.

Nickel has a tendency to accumulate at the top of conifer trees, so values are

higher than in bark. A set of 35 Douglas-ﬁr top samples from a background area of

south-central British Columbia (Shusw ap River) yielded up to 4.4 ppm Ni in dry

needles and 5.6 ppm Ni in ﬁve years growth of twigs. A broader survey of the area

involving 562 tree tops had a maximum of 9.6 ppm Ni in twigs.

In northern Saskatchewan, the small ultramaﬁc Rottenstone deposit that was

mined out in the 1960s has some strong Ni anomalies within an area of more than

100 km

that are not related to mining activities and have yet to be explored. At the old

mine site several common species of the northern forests are enriched in Ni and the

280

Biogeochemical Behaviour of the Elements

data serve to evaluate their relative uptake of Ni (Table 9-V). The high concentration

in alder leaves attests to the sensitivity of this species to the presence of Ni. Leaves were

collected from 5-year-old shrubs almost 40 years after mining operations had ﬁnished.

In Manitoba, Canada, the Thompson Nickel Belt yields an abundance of geo-

physical conductors, but geophysics alone cannot conﬁdently differentiate between

those that represent barren sulphides and those that are nickeliferous. Black spruce

(Picea mariana) top twigs were collected from over several of these conductors and

analysed for 53 elements. Nickel concentrations were elevated, reaching a maximum

of 24 ppm Ni in three years growth of twigs over several conductors, and subsequent

drilling has found them to be Ni-bearing. There was very little difference in the Ni

content of each year of growth of either the twigs or needles. However, the twigs

contained more than double the Ni content of the needles or cones. Figure 9-40

TABLE 9-V

Rottenstone ultramaﬁc deposit (Ni–Cu–PGEs–Au), Saskatchewan. Nickel (ppm in dry tissue)

in common species from close to the abandoned mine site

Species Latin binomial Tissue Ni (ppm)

Black spruce Picea mariana Top twigs 35

Black spruce Picea mariana Twigs 32

Black spruce Picea mariana Needles 14

Labrador tea Ledum groenlandicum Twigs 18

Labrador tea Ledum groenlandicum Needles 21

Mountain alder Alnus crispa Twigs 25

Mountain alder Alnus crispa Leaves 60

- 500 1,000 1,500 2,000

metres

Conductor

Ni ppm

S 2,500

Fig. 9-40. Nickel in top twigs of black spruce (analysis by ICP-MS of 3 years growth of dry

tissue). Thompson Nickel Belt, Manitoba, Canada. Data courtesy of Anglo American Ex-

ploration Ltd.

281Biogeochemistry in Mineral Exploration