Dunn Colin E. Biogeochemistry in Mineral Exploration
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Boreal (northern) forest
This section elaborates on some features of plants that are common in northern
forests, and introduces some details on considerations that should be made during
the collection of a few of the most common species from this en vironment. More
details are given in the next chapter.
Large tracts of the northern latitudes are covered with boreal forest. Fortunately,
there are only a few common tree and shrub species that are likely to occur in
sufficient abundance to be of use for a biogeochemical survey. This makes the boreal
forest one of the most favourable regimes for biogeochemical prospecting. The most
common conife rs are spruce, pine, fir and tamarack. Birch and aspen are the dom-
inant deciduous trees and the most common shrubs are alder, willow and Labrador
tea. Discussions in this section focus on species present in North America, but a
similar range of trees and shrubs occurs across vast tracts of Siberia, Russia and
Scandinavia. For detailed lists of species the reader is referred to the extensive pub-
lications by Kovalevsky, of which much of the key information is summarized in
English in Kovalevsky 1987 and 1995.
For those contemplating a biogeochemical survey as an exploration option in the
boreal forest, and who have not much knowledge of tree identification, a useful first
step is to learn to differentiate between the three principal conifers found in the
northern forests – spruce, pine and fir. They do have characteristic profiles (e.g., a
bulbous top to black spruce, and a ‘layered branch’ appearance to balsam fir), but
these are not fully diagnostic and serve as a first pass to zero in on the desired species.
Closer examination of twigs and needles reveals some marked differences. Figure 3-2
shows typical examples of the three genera, from left to right fir (Abies), pine (Pinus)
and spruce (Picea).
Fig. 3-2. The three principal conifers of the northern forests: fir (left), pine (centre) and spruce
(right).
42 Field Guide 1: Climatic and Geographic Zones
Fir (left) has flat, soft needles. The photo shows 5 years of growth, with a new pair
of branches each year.
Pine (centre) has long sharp needles. The number of years of growth is more
difficult to discern. This sample is about 3 years old.
Spruce (right) has short, abundant and usually spiky needles (especially white
spruce; black spruce needles are softer). The photo shows 4 years of growth.
A short list of the most favourable species for biogeochem ical prospecting in the
boreal forests of North America includes the following.
Black spruce (Picea mariana)
White spruce (Picea glauca)
Red spruce (Picea rubens) [only present in eastern North America, notably in the
Maritime Provinces where it is the Pr ovincial tree of Nova Scotia; chemically, it is
very similar to black spruce with which it is thought to hybridise]
Jack pine (Pinus banksiana)
Tamarack (Larix laricina)
Balsam fir (Abies balsamea)
Paper birch (Betula papyrifera)
Mountain alder (Alnus crispa)
Speckled (river) alder (Alnus rugosa)
Labrador tea (Ledum groenlandicum)
Aspen (Populus tremuloides)
Out of the above, the black spruce, balsam fir or mountain alder are the prefer red
species, and aspen is the least useful. Jack pine can also be very informative (as can
all of the above to lesser or greater degrees), but in many areas it is less widespread
than the spruce or alder.
In the boreal forest of Sweden, the principal biogeochemical sample medium
recognized and utilized by the Geological Survey of Sweden (SGU) is stream peat –
organic material consisting of aquatic mosses and roots of aquatic higher plants.
They are barrier-free with respect to trace metal uptake and integrate the long-term
geochemical signature of stream water (Brundin and Nairis, 1972; Brundin et al.,
1987; Selinus 1988; Lax and Selinus, 2005). The biogeochemical database at SGU
contains analyses of more than 30 elements in 35,262 samples (Lax and Selinus,
2005). Most samples are composed of (in descending order) roots of sedge
(Carex L.), willow moss (the bryophyte Fontinalis antipyretica) and the roots of
meadowsweet (Filipendula ulmaria). The last mentioned is a member of the rose
family and, like other members of this family, it contains cyanogenic glycosides. It
is important to be aware that plants containing these compounds should not be
reduced to ash if accurate concentrations of Au are required. This is because during
the dry ashing process, Au may combine with the cyanogenic compounds (of which
43
Biogeochemistry in Mineral Exploration
there are many) to form Au cyanide that volatilizes long before the usual ashing
temperature of about 475
o
C is reached (Girling et al., 1979).
Black spruce
This tree is extremely widespread in the boreal forests and has proved to be one of
the most responsive species to metal enrichments in the substrate, commonly accu-
mulating higher levels of many elements than most other species. It tends to accu-
mulate a wide range of elements in its tissues, and can therefore be considered the
‘biotite’ of the botanical world, because by analogy biotite can incorporate in its
crystal lattice the ‘left-over’ elements from a rock-forming liquid.
Black spruce is dealt with here in some detail and serves as an example of how to
go about collecting a sample and why certain recommendations for collection are
made. This sets the scene on the thought process that needs to be employed in
biogeochemical sample selection and collection (see Chapter 4).
Black spruce occurs in the boreal forests of North America and in stunted form in
the tundra. A field method to help in identifying this tree is to use a 10 mag-
nification hand lens to examine the twig surface. If it appears hairy, then it is a black
spruce (red spruce is similar, but only occurs in eastern North America); if it is
smooth, with no hairs, it is a white spruce.
Figure 3-3 shows scanning electron micrographs, at two magnifications, of the
surface of a black spruce twig. In the left hand photomicrograph the tentacle-like
protuberances are the hairs; the ultra-fine anastomosing web is a network of fungal
hyphae. Fungal hyphae, only 2 mm in diameter, extend continuously at a rate
that can be extremely rapid – up to 40 mm per minute under optimal laboratory
conditions. Each hypha is supported by the continuous movement of materials into
the tip from older regions of the hyphae. So a fungal hypha constitutes a contin-
uously moving mass of protoplasm in a continuously extending tube, which, on a
micron scale, is continuously redistributing the elemental composition of the twig
surface. In Chapter 1 it was shown that macro-fungi are capable of concentrating
Fig. 3-3. Scanning electron micrographs of the surface of a black spruce twig. Left: stalk to
which a needle attaches, and fine hairs that are characteristic of black spruce twigs (0.2 mm),
with ultra-fine web-like fungal hyphae. Right: close up of the 0.2 mm hairs.
44 Field Guide 1: Climatic and Geographic Zones
extraordinarily high levels of many metals. It is to be expected, therefore, that in the
micro-environment of a twig surface there is a constantly evolving chemical com-
position and metals are being drawn from the twig surface. If, therefore, a sample is
washed prior to analys is, a considerable portion of the fungal filaments may be
removed, thereby removing some of the natural geochemical signature of the twig.
This factor needs to be considered when deciding whether or not to wash a sample,
and is discussed in detail later (Chapter 6).
In the right hand close-up photomicrograph of Fig. 3-3 the fine hairs, mostly
0.1–0.2 mm in length, give the effect of a surreal landscape and show that these little
hairs have even finer protuberances.
In most environments of the boreal forest, a 25–30 cm length of black spruce twig
comprises 10 years of growth. This is an appropriate amount to collect, because
variations in the geochemical signature over 10 years are integrated. Figure 3-4 shows
U concentrations in dry black spruce twigs, plotted against the age of the twig.
These variations may be caused, for example, partly by a relatively wet and/or hot
summer, but more importantly the chan ging ratio of twig wood to twig bark as the
trees grow. For those elements, such as U, that concentrate in the bark, the older the
twig the higher the proportion of wood (which has low U concentrations) compared
to bark (high U concentrations).
A definitive study to demonstrate the location of U in the plant structure was
undertaken by Apps et al. (1987), using fission track mapping of black spruce twigs.
One of the samples tested was a U-rich twig that was provided to the authors from a
1981 collection made by the Saskatchewan Geological Survey. The tree from which
the sample was collected was growing in virgin forest near the eastern margin of the
Athabasca Sandstone, some 10 km from the Rabbit Lake U mine. At the time of
collection Rabbit Lake was the only mining operation within a radius of 250 km.
Chemical analysis of that sample had yielded 1260 ppm U in ash (approximately
Uranium in dry twigs
0
0.5
1
1.5
2
2.5
012345678910
Year of growth
U ppm.
Fig. 3-4. Uranium in dry twigs, dissected into years of growth. Black spruce from the Atha-
basca Sandstone, northern Saskatchewan.
45Biogeochemistry in Mineral Exploration
25 ppm U in dry tissue) and so it was sufficiently enriched to be a good candidate for
the fission track analysis.
The basic procedure involved placing Lexan polycarbonate plastic in contact with
a clean flat surface of twig and irradiating in a SlowPoke nuclear reactor for 4 h. Full
details are given in Apps et al. (1987). The images (Fig. 3-5) show on the left a
photograph of the twig cross section. The rings define approximately 10 years of
growth, marginal to which are the cork cambium (pale grey with no obvious struc-
ture) and the dark outer bark (the rhytidome). The bark is defined as all tissue
outside the vascular cambium – a very thin layer (comprising only 2–4 cells) sep-
arating it from the inner woody xylem (i.e., growth rings). The dark area at the
bottom of the cross section is an emerging small branch. The figure on the right
shows the fission track image of the same field of view. The dark areas show the
greatest irradiation etching, and from this it is evident that the great major ity of the
U (>90% from the authors’ calculations) occurs in the outer bark. Even the bark
forming around the small twig that is branching at the bottom of the photograph is
defined by the intensity of the irradiation damage.
This example provides compelling evidence that the U is not concentrated in the
woody tissue and, as the authors point out, the pattern shown in the previous figure
Fig. 3-5. Cross section of black spruce twig rich in uranium. Left: photograph to show plant
structure; right: fission track image showing areas (black) of radioactive damage from the U
content of the tissues. Annotated from photographs courtesy of Dr. Mike Apps.
46 Field Guide 1: Climatic and Geographic Zones
(Fig. 3-4) of decreasing U concentration with increasing age can be explained by the
steadily increa sing biomass with age – i.e., as the twig grows the ratio of bark to
woody tissue increases. Give n this phenomenon, it is evident why it is of importance
in a bio geochemical survey to maintain a constant thickness and length of twig at
each sample station. In a given survey using twigs, the length and thickness of twig
that is collected is not critical provided a consistent sample is obtained at all sample
stations. Some surveys have mistakenly collected only the thick part of a twig. Al-
though this does not optimize the element concentrations, it gives a level playing field
in that the distribution patterns can be plotted with confidence that a consistent
sample medium has been collected. The collection should be viewed in the same way
as the collection of soils: there is no point in compari ng the analysis of a 80 mesh
fraction from one sample station with a –250 mesh fraction from another.
Depending upon how vigorous the black spruce twig growth is, five to seven live
twigs from around the circumference of a tree usually provides more than enough
representative material. Dead twigs should be avoided, because they may have par-
tially shed their bark and, as noted, there are substantial differences between the
chemistry of the bark and that of the twig wood. In areas of particularly vigorous
growth (southern boreal forests) or of stunted growth (dwarfed trees of the tundra) it
may be practical to collect a lesser or greater number of years of growth, respectively.
Needles do not need to be removed from the twigs in the field, because once they are
dried in the laboratory they readily fall off. As an alternative the outer bark can be
collected as a suitable sample medium. More details on the selection of plant tissues
are given in the next chapter.
A question often asked is, should more than one tree be sampled and the twigs
pooled into one sample to give a more representative evaluation of the ground
chemistry? Ideally, the answer is yes; however, in the real world some complications
may arise. Firstly, by pooling several twigs from several trees quite a large sample
needs to be processed, and so costs are increased. Secondly, whereas at one sample
station there may be several appropriate trees for sampling, at another there may be
only a single tree. This would introduce a bias to the sampling. Studies have indicated
that there is little or nothing to be gained from sampling more than one tree. The key
factor is to be consistent.
Balsam fir
This coniferous tree (Abies balsamea) is common eastward from north-eastern
British Columbia, across the northern forests to the Maritime Provinces, and into the
north-central United States. A related species, the subalpine fir (Abies lasiocarpa)
from central and western British Columbia, is also sometimes referred to as balsam
fir. Reconnaissance surveys have been conducted covering some 20,000 square
kilometres of Nova Scotia using twigs of balsam fir as the sample medium (e.g.,
Dunn et al., 1989, 1994a, 1996a,b).
47
Biogeochemistry in Mineral Exploration
Unlike the spruce in which many elements are significantly more concentrated
in twigs than needles, the difference between these two types of tissue in balsam fir
(and firs in general) is less pronounced. Cohen et al. (1987) found dry balsam fir
needles to comprise an effective medium for delineating Au mineralization at Hemlo
in Ontario. Similarly, surveys in the Bathurst Camp of New Brunswick have
made effective use of balsam fir needles. Fir samples need to be thoroughly
dried before attempting to separate the needles, because there is a tenacious
tissue (the abscission tissue) that holds needles to the twig. Once dried, they
separate quite easily. The thin, smooth bark, with its many blisters of sticky sap (the
yellow, oily resinous ‘Canada balsam’ that is used for mounting petrographic thin
sections) is not a suitable sample medium, because it is both messy and poorly
informative.
Alder
In the boreal forests of North America there are two species of this large shrub
(typically 2–3 m, but it can be up to 5 m tall). To the non-botanically minded field
geologist it can be recognized as the common, tall shrub that is frustrating to try and
walk through, because of the springiness and moderate density of the stems which
impede progress.
Mountain, sitka or green alder (Alnus crispa) is common to relatively dry areas,
whereas river or speckled alder (Alnus rugosa) occurs in wet areas. The mountain
alder is known by various other botanical names, but the consensus among taxon-
omists is generally the species name crispa.
Mountain alder is usually the more widespread of the two species and, although
they have only minor chemical differences (more Co in twigs of the river alder) it is
preferable to collect just one species. A twig length of 30 cm is commonly 3 years of
growth. For most exploration purposes it is best to collect only this newer growth
(shiny reddish surface), because that is where many trace metals of interest (e.g., Au
and notably Mo) are concentrated. Older growth has grey bark and contains rel-
atively high concentrations of Zn, Ca, Ba and REEs. Either twigs or leaves can be
used, but the chemistry of leaves changes quite rapidly early in the growing season so
a survey at that time should be conducted in as short a time as possible – preferably
no more than 2 weeks, once the leaves have grown to their full size.
Ways for the non-botanist to quickly diff erentiate between the two species in the
field are as follows:
1.
New growth of mountain alder has reddish twigs; twigs of river alder are green.
2.
Leaves of mountain alder have a finely serrated margin; leaves of river alder are
much more jagged.
3.
Examination of a snipped cross section of an alder twig will usually reveal a small
brownish triangle in the middle of the river alder; this is white in the mountain
alder.
48
Field Guide 1: Climatic and Geographic Zones
Temperate coniferous forest
Temperate rain forests only occur in seven regions around the world: the Pacific
Northwest, the Valdivian forests of southern Chile, southern New Zealand and
Tasmania, northwest Europe, southern Japan and between the Black Sea and the
Caspian Sea.
In most temperate coniferous forests, evergreen conifers predominate, while some
are a mix of conifers and broadleaf evergreen trees and/or broadleaf deciduous trees.
Temperate evergreen fores ts are common in the coastal areas of regions that have
mild winters and heavy rainfall, or inland in drier climates or mountain areas. Many
species of trees inhabit these fores ts including cedar, fir, spruce, pine, hemlock and
Douglas-fir, with lesser numbers of juniper, cypress, redwood and yew. In the
southern hemisphere dominant trees are pine, kauri and podocarpus. The understory
also contains a wide variety of herbaceous and shrub species, such as salal, buffa-
loberry (‘soopalallie’) and alder in western North America.
The largest area of temperate forest is western North America, extending from
northern California to southern Alaska. The diversity of flora is greater than that of
the boreal forest, making this a more complex area within which to conduct biogeo-
chemical surveys. The dominant north–south trend of the mountain ranges and
valleys in western North America gives rise to different climatic zones within which
there are considerable differences in rainfall and temperature. The result is a series of
north–south-trending zones of vegetation that have been defined as biogeoclimatic
zones – i.e., an ecological classification based on vegetation, soils and climate (Pojar
et al., 1987). Each zone has its characteristic flora, and locally, within a distance of
only a few hundred metres, because of change in elevation or aspect there may be a
different assemblage of trees. However, on a property-scale there are a number of
common tree species that, according to the biogeoclimatic zone, can be of consid-
erable use to a geochemical survey.
Lodgepole pine Pinus contorta
Ponderosa pine Pinus ponderosa
Pacific silver fir Abies amabilis
Sub-alpine fir Abies lasiocarpa
Western hemlock Tsuga heterophylla
Mountain hemlock Tsuga mertensiana
Engelmann spruce Picea engelmannii
White spruce Picea glauca
Douglas-fir Pseudotsuga menziesii
Western redcedar Thuja plicata
Western larch Larix occidentalis
In different climatic settings there occur other species of pine, spruce, fir, larch,
yew and cedar. Of the many deciduous species that may occur, red alder, birch, maple,
49
Biogeochemistry in Mineral Exploration
willow and poplar are the most common. The choice of sample medium is guided by
temperature and moisture regime, and the elevation. In coastal regions at moderate to
low elevations Douglas-fir, western redcedar and western hemlock are three of the
most common and useful tree species. Farther inland and at higher elevations there is
lodgepole pine (one of the most useful trees), Pacific silver fir, mountain hemlock, sub-
alpine fir, Engelmann spruce and white spruce. In the drier interior there is Ponderosa
pine, characterized by its soft, thick orange to grey bark that falls off in large scales.
For most of these species either the twigs with foliage (maintaining a consistent
number of years of growth) or the outer bark can be sampled.
The Scandinavian coastal conifer forest is made up mostly of Norway spruce
(Picea abies) and, to a somewhat lesser extent, Scots pine (Pinus sylvestris). Juniper
(Juniperus spp.) is also common. There is a rich understory of mosses and ferns , and
there are also broad leaf trees scattered in this forest, including birch, willow, poplar
and alder.
The Sierra Jua
´
rez and San Pedro Ma
´
rtir pine-oak forests is a small ecoregion that
covers the higher elevations of these ranges of the northern Baja California Peninsula
in Mexico, near the border with California. The forests are predominantly pine (10
species), juniper, fir and oak (5 species).
In South America, the Valdivian and Magellanic temperate rain forests are the
second largest in the world, after the Pacific temperate rain forests of North America.
They share many plant famili es with the temperate rain forests of New Zealand,
Tasmania and Australia, and vary from dominantly deciduous to evergreen forest
(notably southern beech, Nothofagus betuloides), to coniferous with the monkey-
puzzle (Araucaria) and Podocarpus nubigena.
In the temperate forests of southern Australasia, pine and southern beech are two
genera of plant that are among those most studied for their biogeochemical explo-
ration potential. Timperley et al. (1970, 1972) provide discussion and details of
various species from New Zealand. Their findings are summarized in Brooks (1983).
Temperate deciduous forest
Most temperate deciduous forests are located in the eastern United States, Can-
ada, Europe, China, Japan and parts of Russia. Numerous studies have been con-
ducted in these areas, commonly with an environmental bent, and results include a
wealth of information from many European countries that can be of use to the
explorationist.
Biogeochemical surveys in deciduous forests are commonly more difficult to
conduct than those in the coniferous forests. Typically, the deciduous forest has a
diverse flora and inconsistent distributions of species such that the desired species
may be absent from many of the proposed sample stations. Commonly, tree bark is
smooth, branches are out of reach and growth increments are irregular and more
difficult to discern than those of the conifers. Furthermore, many tree specie s
50
Field Guide 1: Climatic and Geographic Zones
(poplar, maple, oak, elm, beech and red alder) take up relatively low concentrations
of the elements that are of interest for locating mineral deposits. Exceptions are
willow, birch, beech and sometimes oak. As a broad generalization, the trees of the
temperate deciduous forest generate biogeochemical anomalies that are of relatively
low magnitude, and therefore more difficult to de tect. Leaves may be more inform-
ative than the twigs (especially for Hg), but surveys should be conducted within a
short time frame because of the rapid changes in leaf chemistry that take place.
Growing amongst the trees, there is commonly an understory of shrubs and
bushes that may be both more informative than the trees and more practical to
collect. Shrubs that have been used with some success include beaked hazelnut
(Corylus cornuta) and red-osier dogwood (Cornus stolonifera).
Locally, there may be many other possible choices of shrub that could be used.
Since comprehensive databases on many plant species are not available, when con-
fronted with a field situation where data are lacking for a widespread species, an
orientation survey to sample that species (or several species) should be conducted.
Provided appropriate precautions are taken to collect similar growth from the same
species at each test site, the sensitivity of a plant to mineralization or to different
substrates and environmental conditions can be determined. Altho ugh there are some
plant species that are better than others for biogeochemical surveys, there are few (or,
perhaps, none) that are completely unin formative when chemically analysed.
In temperate forests of Australia and New Zealand some of the more important
species of use are
Monterey pine
Blackwood
‘Cauliflower bush’
Southern beech
Pinus radiata (especially the outer bark)
Acacia melanoxylon
Cassinia aculeate
Nothofagus (especially leaves)
Tundra
The shrubby groundcover of the tundra is dominated by very slow-growing dwarf
spruce, birch and willow. Species that are commonly the most useful are the fol-
lowing:
Black spruce
Dwarf birch
Labrador tea
Black crowberry
Leather leaf
Bearberry
Bog blueberry
Willow
Picea mariana
Betula glandulosa
Ledum groenlandicum, L. palustre, L .decumbens
Empetrum nigrum
Chamaedaphne calyculata
Arctostaphylos alpina
Vaccinium uliginosum
Salix lanata and other willow species
51
Biogeochemistry in Mineral Exploration