Dunn Colin E. Biogeochemistry in Mineral Exploration
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Fig. 4-4. Collection of outer bark scales from typical conifer. A paint scraper (hardened steel)
of the type shown is an effective tool for this purpose.
Fig. 4-5. Felled Clarisia racemosa (‘oity’) from the Amazon, showing scraped bark and a
sample of leaves from the same tree.
82 Field Guide 2: Sample Selection and Collection
the two bark layers. Most elements of economic significance are concentrated in the
outer bark, as was shown earlier with regard to PGE in black spruce (Table 4-VII).
This is an important fact to recognize when conducting a biogeochemical survey.
Table 4-XII shows that in an area of Au mineralization in eastern Canada, the
TABLE 4-XI
Element concentrations in dry samples of outer bark (grey scales on a bright red inner bark –
Fig. 4-5) and leaves from a single tree of Clarisia racemosa rooted in gold-bearing rocks at a
‘garimpo’ trenched site in Para
´
State, Brazil
Bark Leaves Bark Leaves
Au (ppb) 22.1 0.9 Na (ppm) 56.2 53
As (ppm) 0.12 0.02 Ni (ppm) 2 2
Ba (ppm) 9 5 Rb (ppm) 3 12
Br (ppm) 20 17 Sb (ppm) 0.01 0.012
Ca (%) 0.1 0.26 Sc (ppm) 0.17 0.01
Ce (ppm) 2.5 0.1 Se (ppm) 0.4 0.2
C (ppm)r 1 0.3 Sr (ppm) 19 21
Fe (%) 0.063 0.005 Ta (ppm) 0.05 0.05
Hf (ppn) 0.24 0.05 Th (ppm) 0.9 0.1
Hg (ppb) 70 150 U (ppm) 0.08 0.01
K (%) 0.057 0.5 W (ppm) 0.05 0.05
La (ppm) 1.1 0.02 Zn (ppm) 6 13
Mo (ppm) 0.08 0.05
TABLE 4-XII
Concentrations (in ash) of elements in inner and outer bark from two red spruce trees, central
Nova Scotia (ash yield approximately 2%). Analysis by INAA
Tree A Tree B
Inner bark Outer bark Inner bark Outer bark
Au (ppb) o5 51 9 126
As (ppm) 2 56 93 300
Sb (ppm) 0.1 10 0.7 3.5
Cr (ppm) 1 41 7 18
Fe (ppm) 500 16,000 2200 16,000
La (ppm) 0.5 16 3 18
Ba (ppm) 3600 1500 5100 2500
Zn (ppm) 3300 1600 9200 3900
Ca (%) 30 18 32 28
83Biogeochemistry in Mineral Exploration
concentrations of Au, As, Sb, Cr, Fe and La (and the other REE) were significantly
higher in the outer than in the inner bark of red spruce. However, the reverse was
true for Ba, Zn and Ca, all of which concentrate in woody tissue. This pattern is
similar for most tree species.
Mature Douglas-fir have very thick development of their outer bark (the ‘rhyti-
dome’), up to several centimetres in thickness. A question that might arise is should a
chunk representing the entire outer bark be sampled, or is a simple scraping of the
outermost material going to provide similar information? Table 4-XIII provides some
answers to this question by comparing samples from two trees. The data show that for
Au, As, Co, Mo, Rb and Zn the concentrations are similar; Ca is much higher in the
entire rhytidome probably because of high concentrations of Ca-oxalate crystals; the
remaining elements in the list (Br, Cr, Fe, K, La, Sb) are higher in the outer bark
scrapings. The higher Br in the outer bark may be more a function of salt spray from
proximity to the ocean than natural variations due to the trees’ metabolism.
In the case of paper birch (Betula papyrifera) there are more than two layers of
bark (Table 4-XIV). The tree sampled was rooted in a fault zone that extended from
an open pit from which Au and Cu had once been extracted. In this example the
middle bark layer contained the highest levels of Au, As, Ba, La and Zn, with lowest
levels in the bast of the inner bark. Rubidium was quite evenly distributed across the
bark layers.
TABLE 4-XIII
Douglas-fir bark – Bowen Island, Howe Sound, southern British Columbia. Concentrations
are in ash determined by INAA
Site 1 Site 2
Bark surface Entire rhytidome Bark surface Entire rhytidome
Au (ppb) 56 64 81 78
As (ppm) 53 40 39 34
Ba (ppm) 2100 3300 2200 2100
Br (ppm) 140 28 84 16
Ca (%) 18 27.5 12.2 20.5
Co (ppm) 10 7 9 7
Cr (ppm) 44 26 51 39
Fe (%) 1.76 0.79 1.87 1.25
K (%) 8.06 4.31 3.41 2.26
La (ppm) 39 20 51 35
Mo (ppm) 9 4 6 7
Rb (ppm) 64 73 59 45
Sb (ppm) 63 34 62 26
Zn (ppm) 2200 2500 1200 1400
84 Field Guide 2: Sample Selection and Collection
Layers of bark can be readily peeled from paper birch, and the implication from
these data is that a combined collection of the outer and middle bark would comprise
the optimal sampling medium. This is fortunate, because collection of the inner bark
requires a somewhat tedious process of cutting with a knife to remove a segment of
the spongy material that it comprises, and it leaves a dark scar on the tree that can
last for many years until new bark is formed. Of course, complete stripping of the
inner bark from around the tree should be avoided as it would result in its death,
because the conduit for the rising sap would be lost.
Twigs
In Chapter 3, the variations in the chemistry of individual twigs were discussed. It
was demonstrated why surveys using twigs should comprise a similar number of
years of growth; in particular, because the chemistry of a twig varies along its length
and the highest concentrations of heaviest and toxic metals occur toward the tips.
This is probably because the ratio of twig bark (containing most of the metals) to
twig wood increases as the twig diameter decreases. Consequently, if 3 years of
growth is collected at one site and 10 years grow th from another, a comparison of the
element concentrations would give misleading results because of the differences in
composition of twigs of differing ages.
In the boreal forest, 10 years of growth is commonly 25–30 cm in length, with a
maximum twig diameter of 3–5 mm. A similar length and diameter of twig from
temperate or tropical climates will usually represent far fewer years of growth. A length
of 25–30 cm is a practical size to collect, and 7–10 twigs of this size are usually more
than sufficient for most analytical programmes.
TABLE 4-XIV
Birch bark (Betula papyrifera) – concentrations (in ash determined by INAA) of bark layers.
Anglo-Rouyn mine area, northern Saskatchewan
Outer Middle Inner
Au (ppb) 108 153 10
As (ppm) 22 18 0.9
Ba (ppm) 450 870 1700
Ca (%) 5.2 12.2 28.7
Cr (ppm) 38 14 3
Fe (%) 2.76 0.71 0.05
La (ppm) 20 7 2
Na (ppm) 12,000 3410 506
Rb (ppm) 120 160 190
Zn (ppm) 3000 16,000 8800
85Biogeochemistry in Mineral Exploration
To elaborate on the varia tions of chemistry with years of growth, data
(Table 4-X V) illustrate changes in composition along a branch of western hemlock
(Tsuga heterophylla) from above Au mineralization at the Ladner Creek Gold de-
posit (formerly the Carolin Mine) in British Columbia. The differences in Au, As and
Cr distributions are particularly striking, with each being most concentrated toward
the twig ends. As noted in the case of inner and outer bark, not all elements follow
the same trend. Calcium is more enriched in the thick part of the branch, whereas Sr
and Zn are homogeneously distributed.
Foliage
Whereas metal concentrations in the needles of conifers are generally lower
than concentrations in twigs, metals accumulate in the leaves of many deciduous
and evergreen species and, on a dry weight basis, the con centrations are commonly
greater than the concentrations in twigs. This seems especially true of plants from
the tropics. Table 4-XVI shows results from the analysis of dry tissues of Acacia
from the vicinity of the North Mara Au mine, situated in the Lake Victoria Green-
stone Belt in northern Tanzania. With few exceptions, there are substantially
higher concentrations in the leaves than the stems; in particular Hg, Sb, REEs, Nb
and Ag.
TABLE 4-XV
Variations in the distribution of selected elements along branches of a single western hemlock
at the Ladner Creek Au deposit, near Hope, southern British Columbia. Concentrations in ash
determined by INAA (approximately 2% ash yield) (from Dunn and Ray, 1995)
Thick (oldest) Medium Thin (youngest)
(>10 mm diameter) (5–10 mm diameter) (o5 mm diameter)
As (ppm) 22 31 82
Au (ppb) 530 650 1590
Br (ppm) 19 18 18
Ca (%) 29 24 14
Co (ppm) 11 12 21
Cr (ppm) 32 26 84
Cs (ppm) 2 2 2
Fe (%) 0.8 1.1 2.3
La (ppm) 2 3 6
Na (%) 0.4 0.4 1.1
Sr (ppm) 430 480 450
Zn (ppm) 1500 1400 1900
86 Field Guide 2: Sample Selection and Collection
TABLE 4-XVI
Comparison of element concentrations in leaves and stems of Acacia spp. from northern
Tanzania. Determinations of dry tissue by ICP-MS (nitric acid followed by aqua regia di-
gestion). Data from Paul Taufen, courtesy of Placer Dome Tanzania
Stems Leaves Relative concentration
n ¼ 37 n ¼ 37 Leaf: stem
Ag (ppb) 3 23 8.4
Al (%) 0.006 0.037 6.0
As (ppm) 0.07 0.20 2.7
Au (ppb) 0.4 1.7 4.3
B (ppm) 16 48 3.0
Ba (ppm) 35 52 1.5
Be (ppm) 0.05 0.07 1.5
Bi (ppm) 0.010 0.015 1.5
Ca (%) 1.667 2.178 1.3
Cd (ppm) 0.009 0.013 1.4
Ce (ppm) 0.23 1.95 8.7
Co (ppm) 0.05 0.25 5.1
Cr (ppm) 2.6 3.2 1.3
Cs (ppm) 0.04 0.09 2.3
Cu (ppm) 9.1 8.4 0.9
Fe (%) 0.006 0.041 6.7
Ga (ppm) 0.050 0.149 3.0
Ge (ppm) 0.009 0.011 1.2
Hf (ppm) 0.004 0.028 7.5
Hg (ppb) 1.3 15.4 11.7
In (ppm) o0.01 o0.01
K (%) 1.007 1.214 1.2
La (ppm) 0.22 1.51 6.8
Li (ppm) 0.22 0.37 1.7
Mg (%) 0.185 0.286 1.5
Mn (%) 20 61 3.0
Mo (ppm) 3.2 2.8 0.9
Na (%) 0.011 0.026 2.3
Nb (ppm) 0.04 0.40 10.0
Ni (ppm) 1.3 1.4 1.1
P (%) 0.102 0.180 1.8
Pb (ppm) 0.06 0.425 6.7
Rb (ppm) 15 20 1.3
Re (ppb) 3 20 6.2
S (%) 0.114 0.221 1.9
Sb (ppm) 0.08 0.79 10.2
Sc (ppm) 0.09 0.13 1.3
Continued
87Biogeochemistry in Mineral Exploration
In the case of Acacia, there is not a great diff erence between the ash yield of stems
(commonly 5–7%) and that of the foliage, so the leaves would be the preferred
medium for a number of reasons.
They are readily removed from the stems after drying.
They are easy to handle (no thorns) and are easy to mill to a fine powder.
They contain significantly higher concentrations than stems of most elements.
However, if analysis is to be performed on the ash of some species (notably the
conifers), then the lower ash yield of the twigs/stems results in concentrations of
elements in twig ash that may be higher than in foliage – i.e., 2% ash yield from twigs
represents a 50-fold concentration of elements compared to the dry tissue, whereas
the 5% ash yield from some types of foliage represents concentrations of only 20-
fold. Suc h factors need to be taken into account when designing a sampling pro-
gramme and deciding upon the appropriate analytical protocols.
One more consideration is that seasonal changes in the chemistry of leaves are
typically more rapid than for twigs – especially for deciduous species. The chemical
requirements for leaf cell growth change as the plant enters its growth cycle. Hor-
ticulturalists are keenly aware that there are evolving needs for both major and trace
elements during the growing season, and appropriate fertilization is required to opt-
imize growth. The chemistry of newly unfurling leaves is different from that of mature
leaves, later in the year. There is a complex chemical balance between what a plant
requires at any one time, and what it is able to tolerate. By analogy, a small child is less
tolerant to poisons entering the body than is an adult. For trees and shrubs chemical
changes are less intense for the twigs than the leaves, because twigs integrate changes
TABLE 4-XVI Continued
Stems Leaves Relative concentration
n ¼ 37 n ¼ 37 Leaf: stem
Se (ppm) 0.23 0.43 1.9
Sn (ppm) 0.13 0.09 0.7
Sr (ppm) 243 264 1.1
Ta (ppm) 0.001 0.005 4.0
Te (ppm) 0.010 0.012 1.1
Th (ppm) 0.009 0.064 7.4
Ti (ppm) 3.6 14.1 4.0
Tl (ppm) 0.013 0.020 1.6
U (ppm) 0.005 0.026 5.1
V (ppm) o1 o1
W (ppm) o0.1 o0.1
Y (ppm) 0.05 0.6 11.3
Zn (ppm) 15 21 1.4
Zr (ppm) 0.16 1.32 8.3
88 Field Guide 2: Sample Selection and Collection
in plant chemistry over a number of years, whereas deciduous leaves are short-lived.
This explains why the recommendation for twig collection is to obtain several years of
growth. A contributing factor to the more pronounced seasonal changes in deciduous
leaves compared to twig chemistry is that in hot weather there is appreciable tran-
spiration through the leaf stomata, with the result that there is nucleation of salts that
are readily washed from leaves during rains or blown away with spalling cuticle.
These pro cesses further explain why a biogeochemical survey that uses deciduous
leaves should be conducted within a short time frame (a week or two), and any heavy
rains, hot days or high winds should be noted. Note that throughout this last dis-
cussion, ‘leaves’ have been consistently prefixed by ‘deciduous’. For evergreen leaves,
it seems that, because of their multi-year existence, their chemistry is more stable,
although it is wise not to mix newly unfurled pale green leaves with those of darker
foliage that has been present for several years.
Trunk wood
Concentrations of most elements of exploration significance are substantially
lower in trunk wood than elsewhere within tree structures. An exception is Ag in
conifer trunk wood, since 10 ppm Ag in ash is a common background concentration,
whereas twigs have o2 ppm Ag. This difference is less obvious in the analysis of dry
tissues, because the ash yield of conifer wood is usually about 0.5%, whereas that of
twigs is close to 2%. When these values are considered on a dry weight (DW) basis,
10 ppm Ag in wood is 50 ppb Ag DW and 2 ppm Ag in twigs is 40 ppb Ag. For many
other elements the relative concentrations in wood compared to twigs are much lower.
If there is uncertainty as to whether twigs or bark might be contaminated with
airborne dust, an analysis of wood ash might be a viable approach and help to
resolve this problem. Table 4-XVII shows concentrations of Au, Ag and As in
lodgepole pine from the vicinity of the former Nickel Plate mine. Typical back-
ground levels in the ash of these tissues are o10 ppb Au, o5 ppm As and variable
levels of Ag with o2 ppm in both inner and outer bark, but about 10 ppm in trunk
wood. The data demonstrate unusual enrichment of metals on the outside and
inside of the trees, indicating absorption through the roots rather than airborne
contamination.
In areas remote from any possibility of airborne contamination, anomalous con-
centrations in trunk wood can usually be related to underlying ore bodies (Walker,
1979; Dunn, 1981). However, in some cases such enrichments may not be entirely due
to natural enrichment of these metals in the ground. If, from mining operations,
there is a constant flux of metalliferous dust, when it is deposited on the ground it
may be partially dissol ved by rainfall and the solution absorbed by the root systems.
Near extensive mine workings or smelters this ‘secondary biogeochemical enrich-
ment’ is a conundrum and a complication that should be considered as a possible
mechanism to create biogeochemical anomalies when interpreting biogeochemical
data.
89
Biogeochemistry in Mineral Exploration
In general, concentrations of many trace elements in dry wood are below detec-
tion levels, even when analyzed by the sensitive ICP-MS instrumentation. However,
since the ash yield of conifer trunk wood is very low, by reducing wood to ash
elements may be concentrated 100- to 400-fold, depending on the tree species. Wood
ash from hardwood species has higher ash yields, higher levels of macronutrients
than conifers and the silica content is frequently lower. Some biogeochemical surveys
using conifer trunk wood ash have been undertaken in areas prior to any mine
developments and have yielded some interesting anomalies, e.g., Au at the Red
Mountain Stockwork, Idaho (Erdman et al., 1985) and U in Saskatchewan at Key
Lake (Walker, 1979) and McClean Lake (Dunn, 1981). In the tropics wood ash was
the medium used in the first recorded biogeochemical survey for Au noted in Chapter
1 (Baromalli and ‘ironwood’, Lungwi tz, 1900).
Treetops
Studies have established that many elements can migrate to the tops of trees – e.g.,
Table 4-VIII. This phenomenon is illustrated further in some data from an undis-
turbed area of the Yukon, in the vicinity of the Brewery Creek Au mine, prior to its
development (Table 4-XVIII ).
TABLE 4-XVII
Gold, As and Ag in the ash of pine tissues (analysis by INAA). Three lodgepole pines located
about 1 km from the former Nickel Plate mine, Hedley, British Columbia
Gold (ppb) in ash
Outer bark Inner bark Trunk wood
Site 1 420 114 128
Site 2 308 28 56
Site 3 238 32 36
Arsenic (ppm) in ash
Outer bark Inner bark Trunk wood
Site 1 220 25 59
Site 2 160 22 41
Site 3 150 20 33
Silver (ppm) in ash
Outer bark Inner bark Trunk wood
Site 1 1.5 o218
Site 2 0.9 o220
Site 3 o2 o220
90 Field Guide 2: Sample Selection and Collection
Sites 1 and 2 were close to known Au mineralization comprising the ‘Canadian
Zone’ prior to its development – it was just a trenched occurrence at the time that
sampling took place. Sites 3 and 4 were from loess-covered northwest-facing slopes
with permafrost and no known minerali zation.
The data in Table 4-XVIII indicate that although the highest levels of some
elements may not always occur in the crowns of trees, concentrations in the crowns
are of significant magnitude that variations in the chemistry of the underlying terrain
can be recognized.
Whereas in some forested areas the tops of scrawny trees can be quite readily
sampled by simply bending them over to collect the upper branches (Fig. 4-6), in
mature forest where trees are well developed this is not an option.
In order to obtain samples from the tops of mature trees a technique has been
developed to hover in a helicopter over the crown and remove the top (or preferably
the uppermost lateral branches) of the tree (Dunn and Scagel, 1989). From subse-
quent experience to achieve maximum utilization of the helicopter time it has been
found that clipping treetops with shears is usually avoided, because the act of main-
taining balance on the helicopter skid (even though a harness is worn) and wielding
clippers at the same time is not a reliably safe procedure. It has proved faster and
safer to simply break off the uppermost lateral branches or top of the chosen sample
tree. Helicopter sampling is a very fast procedure, yielding up to 50 sampl es on a
250 m 250 m grid in an hour, but it is demanding in that the procedure requires
constant and rapid navigation and sampling decisions, while paying close attention
to safety issues. There is very little time for any in-flight sample processing besides
simply bagging the sample. For safety reasons, stand dominants are always the
TABLE 4-XVIII
Concentrations of selected elements in ash of tissues (top, twigs and bark at chest height) from
black spruce (Picea mariana), Brewery Creek Au mine, Yukon. Analysis by INAA (ICP-ES for
Cu and Ni)
Site 1 Site 2 Site 3 Site 4
Top Twig Bark Top Twig Bark Top Twig Top Twig
Au (ppb) 57 85 22 120 335 134 7 12 o510
As (ppm) 70 130 29 150 420 210 3.9 11 2.1 6.3
Ba (ppm) 13,000 24,000 17,000 12,000 14,000 11,000 480 2100 1400 3000
Cr (ppm) 33 37 17 50 49 30 24 24 26 24
Cs (ppm) 11 6.9 3.1 7 4.9 2.9 26 6.6 1.7 1.9
Cu (ppm) 155 144 117 133 117 88 162 209 137 162
Ni (ppm) 132 103 28 91 56 33 5 11 21 16
Sb (ppm) 130 200 47 300 710 290 1.9 5 1.8 3
91Biogeochemistry in Mineral Exploration