Dunn Colin E. Biogeochemistry in Mineral Exploration

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Seeds generally do not represent a suitable sample medium, because of either

difﬁculty of collection or irregular distribution of a species from which seeds can be

collected consistently over a survey area. Acacia seeds from western Australia have

been analyzed and found to contain Ni concentrations higher than those in asso-

ciated litter, twigs and foliage (M. Cornelius, personal communication).

The spores from ferns collected in the western Amazon have been analyzed and

the data compared to the fern leaf tissue (Table 4-XX). With few exceptions elements

were at similar concentrations in both tissues, or higher in the leaf tissue. Since most

fern samples do not have an overabundance of spores, if some prove to be partic-

ularly laden (abundant spore sacs on the underneath of the fronds, and obviou s from

the yellow powder that they generate when dried) it can safely be assumed that the

integrity of the survey will not be compromised, although slightly better comparisons

between samples can be made if surplus spores are not included in the sample that is

to be analyzed.

Flowers are sufﬁciently ephemeral to be disregarded as a suit able sampling me-

dium for a biogeochemical exploration programme. Relevant information on ﬂower

chemistry is scant. However, the pol len from the ﬂowers has been used with some

success using bees as the collecting agent. Bee pollen analyses can help in identifying

metalliferous areas, since metals may accumulate in the pollen and bees rarely forage

more than a few kilom etres from their hives. By setting up a network of hives some

regional biogeochemical prospecting can be undertaken by simply collecting and

analyzing pollen from each hive. A number of researchers experimented with this

procedure in the 1970s and 1980s, but there appears to be little or no published

follow up since that time. Wa rren (1980),andWarren and Horsky (1984) provided

some baseline data from studies in southern British Columbia and collected samples

from the platiniferous Stillwater Complex of Montana; Free et al., (1983) published

data from England; and in Saskatchewan, Wall ace and Jahren (1986) published

results of some of their work. The main ﬁndings of these studies are summarized in

Table 4-XXI.

Summary

Unless there is sufﬁcient prior knowledge of the biogeochemical characteristics of

common species in a proposed survey area, an orientation survey is advisable, wher-

ever feasible and practical. This involves collecting representative species and tissues

from selected sites that preferably include a background area and an area of known

mineralization. Analysis of these samples will determine the following.



ﬁrstly, if the biogeochemical method appears to provide information of value to an

exploration programme and



secondly, the sample medium that provides the best geochemical signature.

102

Field Guide 2: Sample Selection and Collection

TABLE 4-XX

Ferns from the western Amazon. Comparison of fern fronds with the spores retrieved from

each sample. Concentrations in dry tissue by ICP-MS on nitric/aqua regia digestion

Site 1 Site 2 Site 3

Frond Spores Frond Spores Frond Spores

Ag (ppb) 146 204 7 13 122 80

Al (%) 0.14 0.12 0.41 0.08 0.01 0.01

As (ppm) 0.3 5 0.3 0.9 1 1.3

Au (ppb) 16.8 9.7 0.6 0.2 14.4 9.7

B (ppm) 14 9 11 5 19 6

Ba (ppm) 15.7 11.9 25.7 20.9 1.4 0.7

Bi (ppm) 0.02 0.07 0.02 0.02 0.02 0.02

Ca (%) 0.24 0.2 0.06 0.09 0.21 0.09

Cd (ppm) 0.06 0.08 0.07 0.04 0.93 0.34

Co (ppm) 0.06 0.18 0.51 0.49 0.08 0.05

Cr (ppm) 2.13 2.61 2.02 2.02 1.84 1.9

Cs (ppm) 0.329 0.242 1.992 1.126 15.826 8.783

Cu (ppm) 15.65 13.19 18.32 10.59 16.89 10.25

Fe (%) 0.013 0.076 0.004 0.012 0.007 0.009

Ga (ppm) 0.1 0.2 0.1 0.1 0.1 0.1

Hg (ppb) 3943 1066 41 11 786 212

K (%) 1.4 0.93 1.01 0.67 1.56 1.26

La (ppm) 1.31 0.71 6.03 1 1.12 0.21

Mg (%) 0.187 0.211 0.167 0.193 0.165 0.139

Mn (ppm) 659 643 287 453 660 316

Mo (ppm) 0.22 0.39 0.05 0.04 0.12 0.07

Na (%) 0.002 0.002 0.004 0.002 0.002 0.001

Ni (ppm) 0.6 1.4 16.3 12.4 0.2 0.2

P (%) 0.146 0.157 0.114 0.155 0.185 0.189

Pb (ppm) 3.96 10.9 0.34 0.87 2.75 1.72

Rb (ppm) 67.8 44.6 69.5 44.2 181.4 129.6

S (%) 0.15 0.05 0.14 0.05 0.2 0.11

Sb (ppm) 0.02 0.16 0.02 0.04 0.02 0.02

Sc (ppm) 0.2 0.3 0.1 0.3 0.2 0.4

Se (ppm) 0.1 0.1 0.1 0.1 0.1 0.1

Sn (ppm) 0.05 0.11 0.02 0.05 0.02 0.02

Sr (ppm) 9.9 6.1 17.1 11.1 0.6 0.5

Te (ppm) 0.02 0.02 0.02 0.02 0.02 0.02

Th (ppm) 0.01 0.03 0.01 0.01 0.01 0.01

Ti (ppm) 7 18 5 6 7 7

Tl (ppm) 0.02 0.02 0.04 0.02 0.02 0.02

U (ppm) 0.01 0.02 0.01 0.01 0.02 0.01

Continued

103Biogeochemistry in Mineral Exploration

For typical boreal forest regions there are sufﬁcient published studies that can be

referred to for selecting an appropriate sample medium such that this orientation

phase is no longer necessary. However, it may be prudent for an exploration manager

to conduct a brief survey to verify that the biogeochemical method might be an

effective approach for a particular survey area and style of mineralization. In other

environments where there is limited publ ished information, such as central Africa or

the Amazon, there may be sufﬁciently diverse ﬂora that an orientation survey could

save considerable expense.

The various examples presented in the tables and ﬁgures of this and the previous

chapter show how diverse the chemistry can be of different plants and their com-

ponent parts. The example of Mo in the Ama zon (Fig. 4-3) shows that a small survey

can provide valuable focus for conducting a large survey. Furthermore, once a da-

tabase of that sort is ava ilable, quick judgement calls can be made in the ﬁeld as to

what might be suitable to collect should an area of interest not contain any of the

preferred sample media. The western Amazon example shows that, regardless of the

absolute values, the areas of Mo enrichment were clearly discernible from analysis of

each of the four samples that were investigated. Given this similarity of patterns, the

following considerations then came into play for follow-up surveys involving a larger

area and/or a larger number of samples.



Which is the most common plant species in the survey area?



Which is the easiest and least time-consuming to collect? The ‘time’ aspect is not

usually a major consideration, because a great advantage of the biogeochemical

method is that it can be undertaken in considerably less time than for most other

geochemical sample media – it only takes a few seconds to collect a handful of

leaves, and marginally longer to select and snip twigs, or scrape bark scales.



Does each of the sample media provide values substantially above the limit of

detection of the analytical method to be employed, or does one of the sample

media have many values below detection? This can be important, bec ause the

geochemical relief of data at low concentrations can sometimes be of value to an

exploration programme. Wherever possible, select a sample medium that is likely

to provide the highest concentrations of the elements of interest.



Which sample medium is the easiest to process? Additional costs can occur for the

preparation of difﬁcult samples; large samples of wood or chunks of bark involve

additional laboratory costs because of the time it takes to reduce them to a particle

TABLE 4-XX Continued

Site 1 Site 2 Site 3

Frond Spores Frond Spores Frond Spores

V (ppm) 2 2 2 2 2 2

W (ppm) 0.1 0.2 0.1 0.1 0.1 0.1

Zn (ppm) 29.5 34 22.6 10.8 61.3 21.1

104 Field Guide 2: Sample Selection and Collection

TABLE 4-XXI

Analyses of pollen from beehives. Average concentrations in dry pollen

Location Reference Cd

(ppm)

(ppb)

Background areas

Okanagan British Columbia Warren (1980) 0.3 12 77 20 2 32

Omineca British Columbia Warren (1980) 0.6 4 57 39 o354

Fraser Lake British Columbia Warren and Horsky

(1984)

0.5

Wallace and Jahren

(1986)

Gordon Lake Saskatchewan Dunn (unpublished) o0.1 o0.2 0.02

Daffy Green

(Harpenden)

England Free et al. (1983) 11 197 82 1.1 68

Mineralized areas

Trail (near smelter) British Columbia Warren (1980) 4 13 151 353

Kamloops British Columbia Warren (1980) 154 333

Endako (Mo mine) British Columbia Warren and Horsky

(1984)

Ashloo mine British Columbia Warren and Horsky

(1984)

0.1 0.4

Wallace and Jahren

(1986)

Anglo-Rouyn (Au),

Sask.

Saskatchewan Dunn (unpublished) 185 0.4 42 o0.2 1

Cheddar Gorge (Cu/

Pb)

England Free et al. (1983) 17 215 138 3.8 118

Kit Hill (Cu/Pb,

Cornwall)

England Free et al. (1983) 18 116 175 2.5 81

Stillwater PGEs

(samples provided by

H. Warren)

Montana, USA Dunn (unpublished) 1.7 2.9 o2

105Biogeochemistry in Mineral Exploration

size suitable for analysis. In general, the easiest samples to prepare are conifer

needles, deciduous leaves (although some may have tough central veins), followed

by soft bark (conifer scrapings), soft twigs (conifers), hard bark (chunks from

deciduous trees), hard thorny twigs (e.g., acacias) and cones and chunks of wood

being the most time-consuming.



Consider if the survey is to be a one-time event. If it is possible that a second phase

of vegetation sampling might be required, then it might be advisable to collect also

a medium such as outer bark that is alrea dy dead tissue and so seasonal variations

in chemistry are not a consideration.

Finally, all things being equal collect whatever is easiest to obtain. If there is about

as much information from leaves as there is from stems, then while at the sample

station, the leaves can be readily removed from the stems thereby cutting down on

sample bulk, freight costs and sample processing costs. However, if in doubt, collect

stems and leaves, and let the analytical laboratory undertake the processing. One

cautionary word, though, is that leaves from deciduous plants should be in a similar

state of maturity because of seasonal changes in chemistry. This occurs, too, in stems

and leaves of evergreens but is usually less pronounced. A ﬁnal note of caution is that

because of these seasonal changes, resampling of live tissue at a different time of the

year is likely to generate different concentrations of many elements, so do not expect

to get exactly the same analytical results. If returning to a survey area for further

collection, obtain a few samples from trees already sampled during the ﬁrst pass in

order to assist in normalizing the data to a common base.

As a general rule, because of the speed of biogeochemical sampling, collect as

many samples as possible on a ﬁrst pass. It is not necessary to send all for analysis

immediately; however, they should all be dried as soon as possible. Once dry, samples

can be archived for future analysis in the event that interest ing anomalies arise from

the ﬁrst-selected samples. Tests have shown that the trace element chemistry of dried

samples is stable for many years – e.g., samples of Douglas-ﬁr needles collected in

1988 were re-analysed in 2005 and yielded very similar values (Dunn et al., 2006a,b).

SAMPLE COLLECTION

Precautions

Procedures are mostly simple, but before conducting a survey a number of pre-

cautions need to be taken. The basic rule is to ‘be consistent’ – collect the same type

of plant tissue and the same amo unt of growth, all from the same species, and as

much as possible collect from trees of similar appearance and state of health.

A cardinal rule is to avoid contact with metals during sample collection. Jewellery

represents an extremely concentrated point source of the metal of which it is com-

posed, consequently rings (especially precious metals) and other metal jewellery

should not be worn while collecting or handling biogeochemical samples. Even a

106

Field Guide 2: Sample Selection and Collection

bracelet or necklace can be a source of contamination, because by adjusting their ﬁt

or inadvertently touching these items a rich source of metal can be transferred to the

ﬁeld sample. In order to demonstrate the magnitude of the problem to some student

assistants a test was conducted. While wearing no gold jewellery, a sample was ﬁrst

divided into two portions. One half was prepared for analysis (separ ation of leaves

from twigs) and an assistant wearing a gold ring prepared the other half. Remaining

preparation procedures were identi cal, and the two samples were submitted for

analysis. The ‘no ring’ sample yielded an analysis of 20 ppb Au in twig ash, whereas

the sample prepared while wearing the ring returned an analysis of 150 ppb Au in

ash. Such a concentration is sufﬁciently anomalous that, if it were a natural level, it

would warrant some ﬁeld follow-up investigations.

Similarly, lotions, creams and plasters should not come into contact with ﬁeld

samples, because they may contaminate the samples (especially with Zn and B) and

generate false anomalies. The use of anti-dandruff shampoos should be avoided,

because they contai n Se that can be a useful pathﬁnder element for some precious

metal deposits. In short, all reasonable precautions should be maintained and ﬁeld

crews should remain alert to all possible sources of contamination.

Leather gloves provide effective barriers to most sources of vegetation contam-

ination during sampling. The chemical variations among plant tissues are generally

small, and so there is unlikely to be any measurable contamination from saps or leaf

stains that may adhere to gloves. Saps contain much lower element concentrations

than plant tissues, and a smear of sap transferred from one sample to the next is

quantitatively only a very small amount of contamination. To put this into perspec-

tive, the sap smear may weigh only a few milligrams, whereas the ﬁeld sample would

typically weigh about 10,000 times more than this (e.g., 50 g). If, for example, the sap

contained 1 ppb Au (an unusually high concentration for fresh sap), in a worst case

scenario transfer to the next ﬁeld sample would be only about 10,000th of 1 ppb, i.e.,

0.0001 ppb Au which is a concentration well below the detection of ICP-MS analytical

instrumentation, and represents a concentration that is about 1/1000th of a typical

background value of Au in fresh plant tissue. As a result this is not a problem.

Dust from roads and trails can contaminate samples, and even rigorous washing

does not always fully eliminate this ‘anthropogenic’ imprint. If samples are collected

within 100 m of a busy and dusty trail or a paved road, the resulting element dis-

tribution patterns commonly give rise to a string of anomalies that seem to fortu-

itously follow the road. Figure 4-11 shows a plot of ash yields of black spruce twigs

collected at increasing distance into the forest from a very dusty road in northern

Saskatchewan heavily used by large haulage trucks. Typically, in this part of Canada

in forest far removed from any source of contamination, black spruce twigs (most

recent 10 years of growth) yield about 2% ash. The ash yield can be used as an

estimate of the degree of particulate contamination. Figure 4-11 shows that at 25 m

from the dusty road the ash yield was 3.7%, and with increasing depth into the forest

this trailed off to 2% at a distance of 300 m. The area from which these samples were

collected is extremely dusty in the height of summer, with extensive plumes of dust that

107

Biogeochemistry in Mineral Exploration

billow out from behind the large trucks, and so it is an extreme environment of dust

contamination rendering trees close to the road almost white with dust. From other

locations in the northern forests, where the dust levels are lower, it can be expected

that there is little dust contamination at a distance of 100 m from the road. In the

present example (Fig. 4-11), at 125 m from the road the ash yield was down to 2.7%.

Even along little-used forest trails it is advisable to collect samples from 50 m into

the woods. Consequently, as a general rule it is best to sample at least 100 m from a

road and 50 m from a forest trail. Elements that might indicate dust con tamination

are Fe and the high ﬁeld strength elements – notably Ti, Hf, Nb, Th and Zr. If the

road surface is paved with aggregate from a nearby quarry, borrow pit or (even

worse) tailings pile, it can be expected that samples from close to roads will have

elevated levels of the metals contained within the local excavations. It is important to

be constantly aware of this potential problem. Due consideration should be given to

the local geology, and effort should be made to determine what is the most likely

source of road aggregate. Conversations with local people and anecdotal information

can often help to ﬁnd out where the aggregate may have come from.

In one situation a consistent Co anomaly was found to follow the trend of a dusty

road in central British Columbia (Fig. 4-12 – eastern trail). From questioning local

people who knew the long-term history of the area it was established that the road

aggregate probably came from two borrow pits dug into maﬁc/ultramaﬁc bodies.

Analysis of these rocks conﬁrmed that they contained elevated Co levels, and so this

anomalous trend could subsequently be attributed to contamination from road dust.

Another example to demonstrate why samples should not be collected from close

to major highways comes from a plot of Pb in pine bark (Fig. 4-13). In this case there

is a clear relationshi p between Pb in lodgepole pine bark and the location of a major

paved highway. Although Pb in g asoline has been banned in Canada for many years,

it appears that a residual signature remains in the pine bark from samples along the

roadsides. However, it seems, also, that there is a mixed signature in the centre of the

0.5

1.5

2.5

3.5

0 50 100 150 200 250 300 350

Distance from road (metres)

Ash yield %

Fig. 4-11. Northern Saskatchewan – changes in ash yield of black spruce twigs with in-

creasing distance from dusty road.

108 Field Guide 2: Sample Selection and Collection

map, in that high concentrations of Pb occur to the north of the Endako Mo mine.

This area has several sulphide-rich gossans, and the Pb anomalies probably relate to

these occurrences.

Field accessories

Frequently asked questions include what special collecting tools are required?

This includes the type and size of sample bags and the appropri ate tools for scraping

and snipping. Figure 4-14 illustrates accessories for vegetation sampling that are

required in addition to the usual ﬁeld items that a geologist would carry. Shown are

Co ppm

[in ash]

max.27

0 10000 20000

metres

COBALT in Pine Bark

Unpaved road

Fig. 4-12 . Cobalt in the ash of lodgepole pine bark, Nechako area, central British Columbia.

Crosses indicate sample locations.

340000 345000 350000 355000 360000 365000 370000 375000 380000 385000 390000 395000 400000

5990000

5995000

6000000

6005000

6010000

100

150

200

250

Pb ppm

[in ash]

Max. 674

Endako Mo Mine

Fraser Lake

Township

20 km

Lake

Highway

LEAD - Pine Bark

Fraser Lake

Lake

Francois Lake

Fig. 4-13. Lead in ash of lodgepole pine bark, Endako, central British Columbia. Crosses

indicate sample locations.

109Biogeochemistry in Mineral Exploration

pruning snips resting on a ‘kraft’ paper soil ba g and a small polypropylene bag with a

drawstring; a paint scraper (for outer bark scales) and modiﬁed dust-pan resting on a

larger polypropylene bag with drawstring (for large r tw ig+foliage samples). The roll

of masking tape is used for closing the kraft bags. All items are resting on a ﬂour/rice

sack, folded in half, that can be used for easy shipment of the sample collections.

In more detail, these items and others that might be of use are:



A pair of hardened steel pruning snips, preferably Teﬂon-coated anvil type, from

any hardware or gardening store. Some ﬁeld personnel prefer snips with a scissor-

action, and it is really just a matter of preference. Usual ly the anvil type is more

efﬁcient, especially for cutting tough twigs or removing thick branches to gain

access to the required tissue (e.g., bark of a tree).



A hardened steel paint scraper for scraping bark, and either a plastic dustpan or

large paper bag for collecting the ﬂakes of bark. The dustpan is particularly suit-

able because the bark particles tend to ﬂy off in all directions when scraped, and

the dustpan readily catches most of them. Standing in an awkward position on a

windy day it is frustrating if half of the scraped particles fall to the ground. The

dustpan increases the percentage of particles caught, and it is easy to pour the

scrapings into the sample bag. In order to streamline operations, a semi-circle

can be cut from the lip of the dustpan so that it conforms better to the contour

of the tree.

Fig. 4-14. Typical ﬁeld accessories for vegetation sampling.

110 Field Guide 2: Sample Selection and Collection



On occasion a hatchet may be useful in the [unusual ] event that collection of thick

bark is required.



A hunting knife can be substituted for the paint scraper. The blade should be

angled upward when scraping down so that it does not dig into the inner bark.

Inner bark should not be mixed with outer bark because of its substan tially dif-

ferent composition. Although effective, the knife is less efﬁcient than the paint

scraper, and if gloves are not worn it is inevitable that knuckles will be skinned

before the operator learns the optimal procedure.



A roll of masking tape for closure of paper sample bags. Staples are convenient in

the ﬁeld, but tedious for the laboratory to deal with and on occasio n a staple has

been known drop into a sample that is submitted for analysis, generating unusual

metal levels if it goes un-noticed.



Leather gloves prevent any contamination from jewellery or lotions. When work-

ing in arid environments they become virtually essential where thorny species are

present. Also in jungle environments they are useful for brushing off the omni-

present stinging ants.



A large backpack; because if twigs or foliage is the chosen sample medium the

volume of material collected soon becomes quite large, but not heavy. Usually, it is

impractical to separate foliage from twigs in the ﬁeld since separation is most easily

performed back in the laboratory after drying. The less sample handling in the

ﬁeld the better, especially since weather conditions are not always conducive to

spending extra minutes at each sample station and, in extreme weather, it becomes

very tempting for the samplers to take short cuts that could compromise the

sample integrity.



For large surveys, heavy-duty plastic garbage bags or large ﬂour/rice sacks are

useful, and they can be left full of samples at the ends of traverses or cut lines to be

picked up at the end of the day.



A 10 magniﬁcation hand lens to help in species identiﬁcation, and in counting

growth rings on twigs to determine the age of the samples that are collected.

Sample bags

Intuitively, many people embarking on a biogeochemical survey tend to select

plastic bags for containing their samples. However, this is not an appropriate choice

because once samples have been collected moisture is readily released from plant

tissues in the warmth of a backpack, such that by the end of the day the contained

samples become a soggy mess. Moulds and slimes soon develop and sample integrity

is lost as the closed chemical system of organic material contained within the plastic

bag becomes a melting pot of degraded tissue.



For bark scales that are scraped from a tree trunk, ‘Kraft’ (heavy brown paper)

12 28 cm gusset soil sample bags are suitable. The paper has a pH of 5.5 and no

heavy metals are used in the glue. A short strip of masking tape is ideal for closure,

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Biogeochemistry in Mineral Exploration