Dunn Colin E. Biogeochemistry in Mineral Exploration

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presented as posters and talks at a number of internationa l conferences at which

complex maps were displayed showing patterns of Ag distribution. The results were

summarized in several extended abstracts (e.g., Kovalevsky and Kovalevskaya, 1990;

Kovalevskaya and Kovalevsky, 2003). In the latter publication it is stated that

the best case history for NBP (Non-barrier Biogeochemical Prospecting) with ore-grade

prediction is for Ag in the Transbaikal South y Ag-bearing Gilbera Zone of Deep

Faults (GZDF). Exploration of NBP for Ag in GZDF involved analysis for 47–70

elements in 25,000 biogeochemical and 4,000 rock samples. This has resulted in delin-

eation of >250 ‘Supposed Ore Biogeochemical Anomalies’ (SOBA) of Ag. Trenching of

29 SOBA has revealed 27 ‘Veined Silver Ore Bodies’.

The suberized cones and rotted pine and larch stumps outlined

>250 SOBA of Ag up to 8  20–200 m with 70–3,000 ppm Ag in ash [approximately

0.4 to 15 ppm Ag in dry tissue]. These are the highest recorded concentrations of Ag

in plants. Within an area of 10 km

, 11 zones were outlined, ranging from 100 150 m

to 250–400 m. A predictable relationship between Ag in rocks and Ag in plants was

established y sample traverses 40–60 m apart and sample spacing of 1–3 m are

recommended.

Sodium (Na) (Fig. 9-56)

Determinations on V6 and NIST1575a exhibit moderately good precision. Many

plant samples have concentrations quite close to detection at which level precision is

fair to poor. In trace amounts Na is classiﬁed as a beneﬁcial element for many plants,

and only essential for some species. It is usually found more concentrated in foliage

than in twigs. Many samples of foliage from the Eden Project yielded much higher

levels of Na than typically found in natural forest environments, even though the soils

had levels similar to those found in boreal and temperate forests. Concentrations

ranged from 0.004 to 1.6% Na in dry tissue.

Na %

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

0.009

0.002

0.006

0.007

0.0008

0.002

0.004

0.006

0.008

0.01

0.012

0.014

V6 - 30 controls among 600 samples

Concentration

Detection

Fig. 9-56. Sodium – Precision and accuracy on dry tissues – V6 and NIST 1575a (see Fig. 9-1).

302 Biogeochemical Behaviour of the Elements

Plants that are enriched in Na include the halophytes that thrive in salty

environments (e.g., the saltbush Atriplex) and seaweeds. The common rockweed or

wrack (Fucus) has on average 23% Na in ash (Vinogradov, 1953), or about 6% Na

dry weight. Brown, red and green seaweeds all have high levels of Na (Dunn, 1 998a).

Sodium in vegetation is commonly associated with Fe and Fe-related elements.

On occasion Na concentrations can be used to assist in lithogeochemically mapping

Na-rich volcanic rocks, albitization and zones of Na metasomatism to provide focus

to areas favourable for mineralization.

Caution should be exercised in interpreting Na enrichments in vegetation that is

collected in coastal areas (because of salt spray), and in northern climates alongside

roads that may have been salted in the winter months to melt the ice.

Strontium (Sr) (Fig. 9-57)

Concentrations in the control samples V6 and NIST 1575a are all well above the

detection limit, and precision is excellent. Plant tissues commonly have concentra-

tions in the tens to hundreds of ppm Sr, and so data can be plotted with conﬁdence

that they represent true variations among the ﬁeld survey samples.

Strontium is an essential element for some plant species, but its general essen-

tiality has not been conﬁrmed. It performs a function similar to Ca in plants and may

be incorporated into their structural components, but it seems that Sr cann ot replace

Ca in biochemical function (Kabata-Pendias, 2001).

Positive correlations between Sr and Ca occur sometimes in survey samples, but a

strong relationship is not always present. However, in plants growing on carbonat es

there are frequently positive correlations among Ca, Sr and Ba such that, in the

absence of outcrop, this association can give an indication of underlying lithology.

Highest concentrations of Sr are found in the wood of stems and tree-trunks. For

many plants, the concentrations of Sr are similar in leaves and stems. A study in the

western Amazon found that leaves of the vine Clusia yielded higher Sr concentrations

(median of 61 ppm Sr in dry tissue, n ¼ 360) than the fronds of tree-ferns Cyathea

Sr ppm

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

1.6

(6.8)

7.5

0.3

V6 - 30 controls among 600 samples

Concentration

Detection

Fig. 9-57. Strontium– Precision and accuracy on dry tissues – V6 and NIST 1575a (see Fig. 9-1).

303Biogeochemistry in Mineral Exploration

collected at the same sites (median 18 ppm Sr). The foliage of plants from the Eden

Project showed a wide range in composi tion, from a low of 1.4 ppm Sr to over

110 ppm in the ﬁg (Ficus) and Acacia. Acacia at North Mara in northern Tanzania

contained well over 200 ppm Sr in both stems and foliage. Similarly, Acacia from

Western Australia typically has concentrations up to 200 ppm Sr in most components

of its structure, with highest levels occurring in roots and thick stems.

With respect to conifers, in the temperate forests of British Columbia Douglas-ﬁr

have similar concentrations in needles and twigs with values commonly in the range

of 10–30 ppm Sr except higher over carb onates. Black spruce from the boreal forests

has higher concentrations in the twigs. Scales of outer bark from ﬁr, spruce, pine and

larch generally have 20–30 ppm Sr. These are broad generalizations, since concen-

trations up to 100 ppm Sr are not uncommon.

These data indicate that for most species there is some ﬁltering of Sr from roots

through trunk wood into twigs, but that there is little or no barrier mechanism to

inhibit the passage of Sr from twigs into foliage.

Sulphur (S) (Fig. 9-58)

The analytical precision for S at levels below 0.2% is general ly only fair, and most

plant tissues contain concentrations below this. Consequently, the interpretation of

plots of S distributions determined by ICP-MS should be treated with caution, since

data should be considered semi-quantitative.

Sulphur is an element that is essential to all plants, although only in trace

amounts as a component of proteins and enzymes, and as an agent to assist in

resistance to cold conditions. Concentrations in twigs are commonly in the range of

500–1000 ppm S (0.05–0.1% ) and the same to half this amount in foliage. Conifer

outer bark has similar concentrations to twigs. There is usually a good correlation

between S in twigs and foliage, with variations mostly attributable to imprecision in

the analytical data or to inconsistency in sampling procedures. The latter is because

concentrations of S vary with the age of stem/twig growth, and emphasizes the

S %

V17 NIST 1575a

Target

Mean

Std. Dev.

RSD %

0.03

0.01

(0.1)

0.08

0.01

0.02

0.03

0.04

0.05

V17 - 22 samples among 440 samples

Concentration

Detection

Fig. 9-58. Sulphur – Precision and accuracy on dry tissues – V17 and NIST 1575a (see Fig. 9-1).

304 Biogeochemical Behaviour of the Elements

importance of maintaining a consistent number of years of growth to be collected

at each sample station. Figure 9-59 shows this relationship with a histogram of

S concentrations in Douglas-ﬁr top stems.

Sulphur data from the Eden Project samples show concentrations in foliage

from 0.11% in ﬁg (Ficus) up to 1% in several species including sweet garlic. Garlic

in general is known to accumulate both S and Se compounds that account for its

distinctive smell.

Distribution patterns of S concentrations in a population of plants from a

biogeochemical survey can be of relevance to mineral exploration. Whereas the many

published studies on antagonistic relationships between and among elements can

make the thought of arriving at meaningful interpretation quite daunting, patterns

can make geological sense. Many of the antagonisms that have been observed are

with respect to crops or seedli ng plantations. Once a forest or desert scrub vegetation

has become ﬁrmly established, it seems that these antagonistic effects are subdued

and may no longer be relevant. However, the astute geochemist should remain keenly

aware of the possibilities that spatial patterns of element distributions may have

resulted from deﬁciencies or toxic levels of element assemblages. As an example S has

been shown to inhibit the transport of Pb from roots to shoots so that S deﬁciency

can increase the Pb movement into the tops of plants (Jones et al., 1973). From a

practical exploration point of view it is therefore as well to consider if a Pb anomaly

might be related to S deﬁciency. By reviewing the data for both Pb and S a judgement

can be made as to whether this is a possible explanation. The answer is usually that it

is not the reason, and the more data that accumulate the clearer it becomes that by

sampling well-established plants, the biogeochemical method is robust and, when

possible sources of contamination have been taken into account, anomalies are

related to variable concentrations in the substrate – whether it is the soil, groundwater

or underlying bedrock.

Sulphur in Dry Top Stems

0.02

0.04

0.06

0.08

0.1

0.12

S %.

2-3 years of growth 4-5 years of growth

Fig. 9-59. Sulphur in Douglas-ﬁr top stems from 35 sample sites in southern British

Columbia, demonstrating the differences in composition with the amount of twig growth.

305Biogeochemistry in Mineral Exploration

As part of a survey conducted over an epithermal Au/Ag system at the 3Ts property

in central British Columbia, needles from white spruce (Picea glauca) were collected

along traverses over mineralized veins. The Ted vein on this property comprises quartz-

calcite with ﬁnely disseminated sulphide minerals. Pyrite is dominant, but also present

are chalcopyrite, sphalerite, galena, Ag-sulphides, tellurides and sulphosalts. Figure

9-60 shows the S signature in the dry needles, demonstrating a strong positive signature

over the Ted vein and slightly elevated concentrations farther to the east towards some

mineralized boulders. Just west of the Adrian Boulders an undisturbed zone of sulphide

mineralization occurs beneath a cover of glacial till. This exempliﬁes the potential use of

S to assist in delineating the locations of buried sulphides using foliage. In this case

spruce needles were used, but ﬁr and pine should be equally informative. Data listings

for this occurrence are part of the ‘Halogen Project’ for which digital data listings and a

full account are contained on the CD that accompanies this book.

Sulphur data, when used in conjunction with data for commodity metals and

when due consideration is given to the S da ta quality, can provide good focus for

more detailed exploration activities.

Tantalum (Ta) (Fig. 9-61)

At the very low levels of Ta usually present in dry vegetation, analytical precision

by ICP-MS is poor. Samples from the Eden Project were all below the detection limit

of 0.001 ppm Ta, except for three species from the Mediterranean region (maximum

of 0.006 ppm Ta in Arar (Tetraclinus)), and one just above detection from the hot

tropical biome. However, given the poor precision for Ta these data must remain

suspect until they can be veriﬁed.

S %

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Ted

Vein

200 m

Adrian West

(boulders)

Fig. 9-60. Sulphur in dry needles of white spruce (Picea glauca). Ted vein (epithermal Au/Ag

with base metal sulphides), 3Ts property in the Nechako Plateau area of central British

Columbia.

306 Biogeochemical Behaviour of the Elements

Kovalevsky reported values of 10–60 ppm Ta in the ash of a few spe cies using an

emission spectrographic method with a detection limit of 10 ppm Ta. Dete rminations

by both INAA and ICP-MS on samples from the vicinity of the Ta–Li–Cs Bernic

Lake Mine in Manitoba sho wed that twigs and bark of jack pine (Pinus banksiana)

had the highest concentrations with locally more than 1000 ppm Ta in ash. At eight

sites, the average concentrations on a dry-weight basis were, pine bark 34 ppm Ta,

spruce bark 22 ppm Ta, pine and spruce twigs both 13 ppm Ta. Other species and

tissues contained mostly less than 1 ppm Ta. At the time of sample collection in 1990

the Bernic Lake mine was in operation and it was the largest Ta mine in North

America. Whereas the high concentrations in the vegetation could be attributed

partly to airborne contamination, the data indicate that in the exploration for Ta, in

order to provide the best anomaly to background ratios, ba rk of pine or spruce

would be the preferred sample medium, followed by twigs of either species.

Some kimberlites are enriched in Ta, and on occasion Ta anomalies have been

recorded in vegetation growing over or, more usually, marginal to kimberlites. If Ta

enrichment occurs, like many elements associated with kimberlites, anomalies tend to

form an annulus of relative enrichment at the margins of the diatreme. Figure 9-62

shows two parallel proﬁles of Ta in dry foliage from a common South African

thorny shrub known as ‘Hak en Steek’, collected over the Welgevonden kimberlite.

Ta ppm

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

0.002

(0.002)

0.001

0.002

0.004

0.006

0.008

0.01

V6 - 30 controls among 600 samples

Concentration

Detection

Fig. 9-61. Tantalum – Precision and accuracy on dry tissues – V6 and NIST 1575a (see Fig. 9-1).

0.005

0.01

0.015

0.02

0.025

0 100 200 300 400 500 600 700 800 900

Ta ppm

Line 3

NW SE

0.002

0.004

0.006

0.008

0.01

0.012

0 100 200 300 400 500 600 700 800 900

Ta ppm

Kimberlite

NW S

Line 4 (south)

Kimberlite

Fig. 9-62. Tantalum in foliage of South African thorny shrub ‘Hak en Steek’. Two parallel

proﬁles over concealed kimberlite. Concentrations in ppm of dry tissue. Distance in metres.

Data courtesy of Dr. W.B. Coker, BHP Billiton Ltd.

307Biogeochemistry in Mineral Exploration

The translation from the Afrikaans is ‘grab and stab’ that gives a good idea of the

nature of this plant, and signiﬁes that good leather gloves are required for its col-

lection. Background concentrations were close to the detection limit of the ICP-MS

method, but at the margins of the kimberlite values were an order of magnitude higher

(0.01 ppm Ta).

Tellurium (Te) (Fig. 9-63)

Tellurium is a rare element that is barely detectable in most plant tissues. Tellurium

in V6 is at or close to the detection limit of 0.02 ppm Te. Occasional values for V6 of up

to 0.05 ppm have been reported, indicating poor precision at low concentrations. There

are no data published for NIST 1575a and all personal determinations obtained to

date by ICP-MS have returned values below detection. Owing to this lack of control

samples for Te in dry vegetation, the chart to demonstrate precision has used ash

control V7, collected from a PGE-rich site. The data indicate some instrumental drift

with time, and precision is only fair, given an RSD of 23%.

Dried tissue sometimes yields concentrations above detection. As with any

element, single poin t anomalies should be viewed with suspicion since they may be

analytical artefacts. In the event that a series of ﬁeld survey samples show detectable

Te values, repeat analyses would be advisable to conﬁrm their validity, because Te

can be a useful ‘pathﬁnder’ element for precious metal deposits. Situations have

been foun d where a series of values above detection have not been repeated when

re-analysed, and so immediately upon receipt of data it is necessary to scrutinize

them for patterns of element concentrations that may be related to analytical

artefacts such as instrument drift or loss of calibration.

There are reports in older literature of some suspiciously high concentrations of

Te in plants. Schroed er et al. (1967) indicated that concentrations in onion and garlic

can be as high as about 300 ppm Te, and the garlic smell of some plants is from the

volatile compound dimethyl telluride. Analysis of leaves from plants at the Eden

Project showed only one plant (rattan, 0.04 ppm Te) with a concentration ab ove

detection. In Schroeder et al. (1967), it was stated that Te is among the most abundant

V7 (ash) NIST 1575a

Target

Mean

Std. Dev.

RSD %

∼ 1

1.1

0.2

No data

Te(ppm) in Control Ash V7

0.0

0.5

1.0

1.5

2.0

Chronological Sequence: n=45 controls among 900 samples

Fig. 9-63. Tellurium – Precision and accuracy on ashed tissues – V7 (see Fig. 9-1).

308 Biogeochemical Behaviour of the Elements

of trace elements in the human body, which, if true, would indicate an enormous

biomagniﬁcation from rocks through the food chain to humans. It was claimed that the

average human body contains 560 mg Te, which put it fourth in abundance after Fe, Zn

andRb(seealsoCohen, 1984). The evidence seems to be that these workers reported

erroneous data, because it has since been shown that instead of 560 mg Te in humans

there is only 0.7 mg Te in an average body of 70 kg – almost three orders of magnitude

less Te (Emsley, 1998). As a result of this and other ﬁndings, it would appear that the

extraordinarily high levels of Te in biological systems reported by Schroeder et al.

(1967) and Cohen (1984) were incorrect.

Probably a more realistic assessment of typical Te concentrations in plants comes

from a study conducted twenty years later using a more reﬁned analytical technique

(Cowgill, 1988). This was a comprehensive study of Te in vegetation from the vicin ity

of precious metal–mercury–telluride mineralization in the Ely mining district of

Nevada. It involved the collection and analysis of 480 samples of trees and shrubs,

and 505 samples of ﬂowering plants. An additional 105 samples were collected from

various areas of western Colorado. All samples were analysed for Te, Se, Fe, S, Zn,

Cu and Pb. On average, ﬂowers were found to contain significantly more Te than

other plant parts. In trees, the highest Te concentrations were in the foli age and the

lowest in the branches. Seleniferous species of the vetch Astragalus contained larger

amounts of Te than plants in the Te-rich Ely area, whereas non-seleniferous species

of this genus contained much less. This pattern is in accord with the usual sympa-

thetic correlation between Te and Se, because of their geochemi cal afﬁnity in nature.

No plants contained more than 1 ppm Te in dry tissue.

Limited data on dry samples from near PGE/Au mineralization have indicated

that Te in Labrador tea is more concentrated in leaves than twigs, whereas black

spruce twigs have more Te than in needles. To obtain information on the geochemical

‘relief’ of Te datasets, samples need ﬁrst to be reduced to ash by controlled ignition at

475

C because of the very low concentrations of Te in dry tissues. It appears that

little or no Te volatilizes from spruce at this temperature – samples of black spruce

twigs from a PGE deposit have returned concentrations of 0.06 ppm Te in dry tissue

and 0.05 ppm Te when ash of the same sample is normalized to a dry-weight basis.

Preliminary data indicate that more Te volatilizes from foliage of deciduous species

than conifers, but this needs further investigation. Figure 9-64 shows data from the

determination of a set of dry tissue samples. The diamond symbols indicate data

from the direct analysis of 1 g samples of dry tissue, and show that only two samples

yielded values above the detection limit of 0.02 ppm Te (values below detection are

shown at half the detection limit). Fifteen-gram samples of the same material were

reduced to ash and Te determinations were made. The data were then normalized to

a dry-weight basis by adjusting for the loss on ignition. This exercise permitted two

important observations to be made.



The only two samples of dry tissue that yiel ded concentrations above detection

proved to be the samples with the highest (and very similar) concentrations

309

Biogeochemistry in Mineral Exploration

obtained from analysis of the ash. This indicated that Te values that are above

detection in dry tissues may well be meaningful values.



By analysing the ash, considerably more detail of the low-level geochemical

variability of Te could be discerned.

Surveys conducted in Canada over the PGE deposits associated with the Fox

River Sill in Manitoba and the Rottenstone deposit in Saskatchewan both involved

the analysis of ashed twigs of black spruce. In both environments, close to miner-

alization the maximum concentrations were between 1.5 and 2 ppm Te in ash, which

is the equivalent of 0.03–0.04 ppm (30–40 ppb) Te in dry tissue. Over kimberlites in

the Buffalo Head Hills area of Alberta, white spruce top twigs yielded up to 0.12 ppm

Te in ash with values at background sites of o0.02 ppm Te. Over Au deposits in

various parts of the world the maximum values have been 0.5 ppm Te in dry foliage.

At the time of writing, it seems that unless high-resolution ICP-MS is available,

for surveys requiring detection of subtle variations in Te distribution (e.g., many

types of precious metal deposit) it would be advantageous to analyse ashed portions

of the samples.

Terbium (Tb)

See Lanthanum and the Rare Earth Element s (La).

Terbium is one of the rarest rare earth elements. Its concentration in V6 is below

the detection limit of 0.01 ppm Tb and it is rarely above this level in plant tissues. Ash

of V6 contains 0.26 ppm Tb that equates to 0.006 ppm Tb in dry tissue. With the

exception of ferns, most plants contain low er concentrations of REE than V6.

Thallium (Tl) (Fig. 9-65)

V6 has a Tl content of almost exactly the detection limit of 0.02 ppm Tl. V17

has 0.03 ppm Tl and consistently returns this value. Other data also indicate

that precision for Tl by ICP-MS is excellent for values above 0.05 ppm. Figure 9-66

Dry tissue v Ash normalized to dry weight

0.005

0.01

0.015

0.02

0.025

Sequence of Samples

Conc.

Te_dry ppm

Te_Normalized

to dry wt. ppm

Fig. 9-64. Tellurium in dry Acacia tissues. Concentrations in dry material versus ash nor-

malized to dry weight.

310 Biogeochemical Behaviour of the Elements

shows the near perfect laboratory precision obtained for replicate samples of balsam

ﬁr needles. This level of precision is common for Tl and not exceptional for the

dataset shown.

Thallium chemistry is similar to that of alkali salts. In rocks, it closely follows K

and Rb and is, therefore, reﬂected in K metasomatism. Thallium minerals form

during epithermal stages of hydrothermal processes generating a high Tl content to

marcasite, sphalerite and galena. It is especially enriched in polymetallic deposits,

including those with Au along with As, Sb and Ag. It is a highly mobile element that

disperses during oxidation of sulphide ores, so that it can quite readily be made

available to plant roots. Given these properties and the excellent precision obtained

from the analysis of plant tissues, Tl can be a very useful pathﬁnder element for a

number of different types of mineral deposit. Warren and Horsky (1986) noted a near

perfect correl ation between Tl and Au in several tree species from British Columbia

and suggested that the Tl data could be used as a prospecting tool for Au, especially

TI ppm

V17 NIST 1575a

Target

Mean

Std. Dev.

RSD %

0.03

No data

0.02

0.005

0.01

0.015

0.02

0.025

0.03

0.035

V17 - 22 samples among 440 samples

Concentration

Detection

Fig. 9-65. Thallium – Precision and accuracy on dry tissues – V17 and NIST 1575a (see

Fig. 9-1).

THALLIUM in Laboratory Duplicates

BALSAM FIR NEEDLES

y = 0.9581x + 0.0047

= 0.992

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6

Tl(ppm) - First split

Tl (ppm) - Second split

Fig. 9-66. Precision for thallium on replicate samples of balsam ﬁr needles (n ¼ 19).

311Biogeochemistry in Mineral Exploration