Dunn Colin E. Biogeochemistry in Mineral Exploration
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preferred trees to sample. Furthermore the stand dominants are usually the most
mature trees, therefore probably represent the best integrated signal of the local
geochemical environment, and by always collecting the tallest and most vigorous of
the trees in a forest stand they provide a consistent sampling medium.
A first step is to spend an hour or so conducting an orientation flight over the
entire survey area to assess the architecture of the forest, the density of the desired
species, and to appraise the ground conditions in general – e.g., noting any newly
burnt areas, cleared areas and sites of potential contamination such as drill pads.
A typical survey procedure for the main survey is to snap a half metre or less from
one or two lateral upper branches or the top of a single tree at each sample location.
Continual communicatio n must be maintained by the three-member crew (sampler,
pilot and the assistant [who undertakes the bagging and GPS entry at pre-designated
waypoints]). This is essential for both safety and efficiency. While in the air the
assistant can on occasion undertake some preliminary culling of excess tissue, but
there is no time for effective reduction of the sample material to the size and con-
sistency required for analysis; time demands that the sample is quickly stuffed into a
pre-numbered cloth sample bag (a polypropylene bag of about 10
00
17
00
[25 42 cm]
is suitable) while the helicopter is positioning for the next sample collection. The bag
must be tough to withstand a quick thrust of the plant stem without ripping, and so
polypropylene bags of the type described in the next section are ideal. Sample trim-
ming to a consistent composition is undertaken by ground personnel immediately
after each flight. Figure 4-7 shows a typical procedure; although for illustration
purposes a somewhat larger sample is shown than is usually required.
Fig. 4-6. Collection of spruce twigs from upper branches – an appropriate sampling method
in some environments.
92 Field Guide 2: Sample Selection and Collection
The two-pers on sampling crew undertakes a flight run to collect treetops, typ-
ically from about 50 trees before the crew s need a short break and the helicopter
needs to be unloaded at a pre-arranged helicopter staging/refuelling location. Dis-
tance between sample stations would normally be 250 or 500 m, although some
reconnaissance surveys have been spaced on 2 2 km grids. Decisions on the size of
the sampling grid should be made by considering the nature of the target miner-
alization. For example, a kimberlite target (e.g., 250 250 m) would require closer
sample spacing than a large porphyry system (several square kilometres). The size of
the grid actually makes little difference to the overall collection time, because of the
speed of the helicopter. Once procedures have been streamlined, and clear commu-
nications have been established among the pilot and the samplers, it takes little more
than one minute per sample station – i.e., from leaving one sample station, flying to
Fig. 4-7. Collection of treetop from a helicopter.
93Biogeochemistry in Mineral Exploration
the next (predetermined using waypoints programmed into a GPS), positioning the
helicopter over the spire of a tree, and reaching out to collect and bag the sample.
Since the helicopter crew only has time to do the sample collection, at least one
person is fully occupied on the ground at the staging post culling the samples to an
appropriate volume and consistency of material for subsequent laboratory prepa-
ration. This person assi sts in unloading the helicopter, organizing and passing on the
next set of bags, and receiving a verbal update on the sampling conditions, any
features or problems of immediate relevance to the sampling programme, and a
general comment on the nature of the forest stand.
The following account uses black spruce as an example. Figure 4-8 shows a
typical field base set-up for a survey conducted in winter to early spring in the boreal
forest. It requires a large clean working top, large pruning shears for trimming the
thick stem (1–2 cm diameter) that is buried deeply in the ball of cones, and smaller
pruning shears for use as required to further trim the samples.
As a guide to the amount of material to collect, the following are approximate
dimensions, weights and yields, which vary by the latitude of a survey area and the
moisture conditions – i.e., the long-term degree of saturation of the ground such that
it is dry (xeric), intermediate (mesic) or boggy. These factors affect growth rates and
they can affect the tree chemistry (e.g., black spruce absorb more Mn in boggy areas
than xeric or mesic sites). A typical amount of material to collect for analysis would
be (a) for black spruce, with its dense top cluster of needles and cones, a single length
of about 30 cm (the length to which it should be trimmed back at the base camp) or
(b) for most other conifers two or three lateral branches of 30–50 cm (because they do
not have the same dense ball of tissues at their tops). This amount of tissue has an
average fresh weight of 400–500 g. The moisture content is 30–50%, so that a 500 g
Fig. 4-8. Boreal forest winter/spring field base set up for sample consolidation.
94 Field Guide 2: Sample Selection and Collection
sample weighs less than 250 g when dry. This dry weight comprises 30% needle, 25%
twig and 45% cones.
Given this breakdown of tissue types, the original sample of black spruce weighing
500 g yields approximately 100 g of dry twig tissue (including the thick stem compris-
ing the spire of the tree, which can represent half of this twig weight). This is plenty of
material if the analytical programme calls for analysis of dry tissue, because only 1 g is
the usual requirement for most multi-element analytical procedures. However, some
survey programmes, such as those designed to determine the very low concentrations
of PGEs that are present in plant tissue, require preconcentration of elements by
reduction to ash. The ash yield of dry spruce twigs is 1.5–2%, so that of the original
large ball of fresh treetop material weighing 500 g, the ash yield of all the twigs and
stems is o2 g of ash. Also, since the stem should be discarded, there is barely 1 g of
twig ash available. Fortunately, most multi-element analytical programmes require
only 0.25 g of ash, or 0.5 g at the most, so there is sufficient ash available.
This example demonstrates two important points of relevance to biogeochemical
exploration.
Although from a 500 g ball of fresh black spruce top there would appear to be
ample material for analysis, it is sobering to realise how very little ash (1 g) that
there is available from that part of the treetop where most of the elements of
economic significance are sequestered – namely in the thin twigs.
Secondly, the often-high concentrations (compared to soils) that are reported from
the ash of plant tissues are a resul t of the localization of elements in specific plant
organs, and their further concentration by reducing the dry matter to ash.
A final point for treetop sampling is that the cost of collection is not as high as
might at first be thought. If an exploration programme has a helicopter on site for
other activities, and the flying cost is, for simplicity of calculations, $1000 /h, once a
sampling crew is up to speed, about 50 samples can be collected in 1 h, for an average
cost of $20 of helicopter time per sample. Large areas can be covered in a short
period of time, regardless of terrain and ground conditions. In the northern forests
the ground is frozen and snow-covered for half the year, thereby precluding the easy
collection of soil samples (a power augur would be required), yet in no way inhibiting
the collection of treetops. Even at 20 1C an appropriately dressed crew is still able
to function quite efficiently, although at temperatures above zero productivity does
improve. Furthermore, in the summer months a ground crew might only collect
10–20 soil samples per day at 250 m intervals in heavily forested and/or rugged
terrain, whereas a helicopter crew can comfortably expect to get 300 samples a da y at
the same sampling interval – with the added bonus of not having to fig ht off the flies
and mosquitoes! Sometimes the vegetation can be the more informative medium, and
sometimes the soils. If, from test samples, the soils are clearly superior, then the extra
effort should go into launching a soil-sampling program me. However, if the veg-
etation is the more informative medium, or it is as equally informative as the soils,
then the expediency of the treetop sampling procedure is well worth considering.
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Biogeochemistry in Mineral Exploration
Cones
The previous section demonstrated that there is a high cone yield from the tops of
black spruce. The typical bulbous top to this species is largely a function of an
exceptionally high abundance of cones, and this phenomenon is not characteristic of
all conifer species: local ly, white spruce, pines and fir have high cone concentrations
in their uppermost parts, but not as consistently or as densely as black spruce. As a
result, for many species any plans to conduct a survey using only the cones might be
thwarted by a lack of cone abundance.
Cones yield even less ash (0.2–0.5%) than twigs (2%), therefore when reduced
to ash, intuitively it would be expected that the metal content would be relatively
high, in which case a strong anomaly to background ratio would make the cones a
desirable medium. However, not all elements make their way into cones in the same
proportions that they do into twigs. The twigs act as a reservoir from which cones
select the desirable and essential elements for further propagation via their enclosed
seeds.
There are now a number of tree canopy tourist attracti ons around the worl d at
which a walkway built at treetop level permits a view of the strange and diverse
ecosystems that evolve at the top of the forest. There is an abundance of new ob-
servations steadily being released to the scientific community and the populace at
large. By the same token, the world at the top of a black spruce tree is very different
from the lateral branches lower down the tree. As described in the previous section,
the dense ball of cones contains a stem, many small lateral twigs laden with needles of
several ages, newly forming small cones, old closed cones, older open cones that have
dispersed their seeds and partially degraded cones that may have simply rotted or
been partially eaten by animals or insects. The occasional insect crawls out and
(rarely) bird feathers are part of the local ecosystem (Fig. 4-9).
An alternative to gathering cones from the air is to collect cones that have
fallen to the ground, but this requires careful sorting by species unless there is a
monoculture stand. In addition, it would be desirable to use only cones of similar
condition, because further complications arise.
Old cones yield higher concentrations of meta ls than new green cones and there-
fore the two should not be mixed when preparing sampl es for analysis.
Cones that have opened up to disperse their seeds have a different composition,
because some elements are concentra ted in the now-scattered seeds. There-
fore, open cones should not be mixed with closed cones – even if they are both
old and suberized (suberin is hardened woody tissue that develops as the tissues
age).
As part of an airborne survey to collect tops from lodgepole pine (Pinus contorta)
in central British Columbia, the suberized cones were separated from the twigs
and needles and sorted according to whether they were (1) Closed (with seeds con-
tained), or (2) Open (seeds lost). A few trees also had some green immature cones.
Table 4-XIX summarizes the data received from INAA of open and closed cones
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Field Guide 2: Sample Selection and Collection
collected from nine trees. This demonstrates that for some elements (e.g., Cr, Cs, Mo,
Rb and Zn) it would not compromise analytical data to mix open and closed cones
as the concentrations proved to be quite similar. However, closed cones tended to
have higher concentrations of K and Ni, and open cones had higher concentrations
of Au, As, Br, Ca, Co, Fe, La, Na and Sb. From these data it appears that much
of the K and Ni were lost with the seeds when the cones opened, whereas other
elements were retained in the woody, dead tissues. Data from the few samples of
immature (green) cones (not included in this summary) showed that they were rel-
atively depleted in As, Cr, Fe, La, Na, Ni, Sb and Zn, but enriched in K (significantly),
Br, Mo and Rb.
Given the complications that surround the collection of a consistent set of cone
samples, it would probably be better to avoid their use as a sample medium for
biogeochemical exploration. Analysis of cones carefully selected from a treetop-
sampling programme could be used as complementary information to substantiate
data obtained from twig or needle tissues.
Stunted trees of the Tundra
Toward the northern margins of the boreal forests, plant life endures extreme cold
and long winters with short days. In this zone (e.g., north of about 601–651) trees are
stunted and add very small increments of growth each year. A 20 cm spruce or birch
might be 50 years old or more. If the black spruce are less than 20 cm high a veg-
etation sample should comprise whatever is practical (e.g., all of the above-ground
Fig. 4-9. Composite sample from the top of a black spruce – typical needles, twigs and cones,
with an unusual presence of matted feathers (a pellet regurgitated by an owl).
97Biogeochemistry in Mineral Exploration
parts), but it remains important to be consistent and maintain a constant maximum
diameter of twig/stem. Back in the laboratory, after the sample is dried and the
needles have been removed, it is then easy to be consistent in sub-sampling the twigs
of similar diameter and age.
In this environment, dwarf birch (Betula nana or Betula glandulosa) or various
species of willow are commonly only a few centimetres high, so a collection of
the spindly twigs with leaves is appropriate. From a combined sample of 50 g, about
40 g is leaf tissue, and for most practical purposes the leaves are the preferred
medium, because of their ease of collection, preparation for analysis and informative
value.
For the small evergreen shrubs (e.g., Labrador tea (Ledum groenlandicum and
L. palustre) and crowberry (Empetrum nigrum)) the simplest procedure is to collect all
of the aerial portions. These two Labrador tea species are very similar in compo-
sition, so as far as is known it is unnecessary to distinguish between the two. Crow-
berry occurs mainly in North America, but also in the southern Andes. About 50 g of
these species will suffice for most survey objectives, and either the leaves or stems can
be used in biogeochemical exploration.
TABLE 4-XIX
Comparison of the relative concentrations of elements in open and closed cones from the same
nine trees
Relatively enriched in: Similar in both (out of 9 trees)
Closed cones Open cones Open and closed
(No. of trees) (No. of trees) (No. of trees)
Au 1 5 3
As 0 9 0
Br 3 5 1
Ca 0 8 1
Co 0 6 3
Cr 0 4 5
Cs 1 1 7
Fe 2 7 0
K5 0 4
La 0 6 3
Mo 1 1 7
Na 0 7 2
Ni 4 3 2
Rb 1 0 8
Sb 0 9 0
Zn 0 1 8
98 Field Guide 2: Sample Selection and Collection
LFH/forest litter/humus
These are not biogeochemical samples sensu stricto. However, since they are
composed primarily of organic material they can be considered to fall under the
broad umbrella of biogeochemical exploration. When dead vegetation decomposes it
can act as a chemical sink for those metals that are readily immobilized by the
generally acidic and reducing conditions associated with the organic matter. This
material can, therefore, serve as a viable sampling medium in some environments.
The terms LFH and Forest Litter are synonymous, but the term humus is re-
stricted to just the decayed material. LFH horizon of soil nomenclature comprises
the uppermost layer of the forest floor, in varying stages of decomposition, and so it
is an environment with a high level of microbial activity. The ‘L’ stands for ‘leaf’, ‘F’
for ‘fibric’ and ‘H’ for ‘humic’. The leaf component is all of the fallen plant debris
that is virtually undecomposed, and comprises leaves, twigs, fragments of bark,
cones, seeds, roots as well as any other organic debris from either the Plant or
Animal Kingdoms. The fibric material is this same material, but partially decom-
posed. The humic component is the well-decomposed material, and it commonly has
an admixture of mineral grains. LFH, several centimetres in thickness, is common in
most forested environments, but it is ephemeral in dry conditions where it is not
always present in sufficient quantities to be used for a geochemical survey. It is
impractical for a geochemical survey to try and separate the L from the F from the
H, so when used it is treated as a single combined sample of the three components.
Typically, a handful of this organic material (commonly 5 cm in thickness) is col-
lected from immediately beneath any moss layer that might be present.
A detailed account of the nature and chemistry of humus and its contained
compounds is given in Stevenson (1994). Given that the humic layer on the forest
floor is made up of a variety of plant species and tissue types, it is to be expected that
some inter-site variations in humus chemistry can be attributed to the various com-
ponents of the forest from which the humus sample is derived. In order to minimise
this variation, an approach is to collect all humus samples, wherever possible, from
beneath a species of tree or shrub that is found throughout a survey area. A survey
conducted in northern Saskatchewan focused on dead tissues of shrub alder (Dunn,
1998c). Field crews were instructed to collect from beneath mature alders. However,
the humus is an inconsistent sample medium and the human factor entered the
equation, generating some inevitable variation in sampling procedures amongst field
crews. On later inspection, many samples were true humus, but others ranged from
handfuls of rotted logs at one end of the spectrum to pebbly material with a high
inorganic co ntent at the other. Admittedly, at the end of a long hot bug-infested day
it is easy to become a little lax in procedures; and in heavy rain and darkened dense
forest it can become difficult to be consistent because the wet humus may be con-
fusing to discern.
In this survey, out of the original collection of samples approximately 20%
were discarded because they were considered inorganic-rich and atypical of the main
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Biogeochemistry in Mineral Exploration
set of humus samples. In preparing the remaining samples selected for analysis, a
simple procedure was devised to place each sample in a plastic colander with 2 mm
openings through which the material was pressed by gently swirling it by hand.
Sieving eliminated pieces of rotted wood and intact twigs and cones. The o2mm
portion of each sieved sample was collected in a large aluminium oven tray, and
shaken by hand to allow gravitational separation of any inorganic particles. This
proved to be a simple, but effective, method for eliminating the majority of the
inorganic material, but certainly not all of the fine particulates. As so often happens
in the real world of conducting a geochemi cal survey, a best compromise has to be
taken.
Figure 4-10 is an example of the results for Au from that survey. Sites of known
Au mineralization were evident, along with other sites yielding relatively anomalous
concentrations worthy of further investigation. A lesson learnt from this study was
that some samples were collected too close to the dusty roads, because plots of Na, K
and Fe (and Fe-related elements – Hf, Sc, Th, REE) clearly follow the trend of the
main unpaved highw ay along which large trucks frequently travel north-eastward to
service the uranium mines of northern Saskatchewan. To avoid the main effects of
this dust, samples of surface soils or vegetation should be collected at least 100 m
from the road.
In western Australia the litter has been found to be an effective medium to use for
exploration (Matthias Cornelius, personal communication), especially in areas dom-
inated by ‘mulga’ (Acacia spp.).
Similarly in Brazil the litter, or mull, accumulates metals and reflects underlying
mineralization (Cornelius et al., 2007). As part of that study litter samples were
collected from the surface by hand following clearing of a small area from living
vegetation. The litter layer was mostly 10–50 mm thick and contained both decaying
and freshly fallen plant matter. Whereas care was taken to minimize the amount of
soil particles trapped in the fine mesh of rootlets that occurred at the base of the
litter, there were some sites where the litter layer was very thin and contained a
considerable amount of soil, possibly washed from upslope. In a situation like this, it
is difficult and time-consuming to completely separate out the inorganic material,
and so allowance for some inorganic contamination must be made when interpreting
the analytical data. Samples were taken over a gossanous base metal deposit. The
element suite that best identified mineralization was Cu, Pb, Zn, As, Mo, Sb and
In. In a few bark samples from Imbauba (Cecropia spp.) the best indicators were Pb,
Mo and Sb.
Flowers, seeds, spores and pollen
Although each of these media can locally accumulate certain metals, they are only
mentioned briefly, because they are rarely in sufficient abundance or are impractical
to collect for a geochemical survey.
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Field Guide 2: Sample Selection and Collection
Fig. 4-10. Gold in dry humus – northern Saskatchewan (Dunn, 1998c).
101Biogeochemistry in Mineral Exploration