Dunn Colin E. Biogeochemistry in Mineral Exploration

Подождите немного. Документ загружается.

recorded unusual enrichment of Ra in plants is by Turner et al. (1958) who reported

that Brazil nuts growing in areas of high natural Ra have orders of magnitude higher

concentration plant-to-soil ratios than those typically found.

Rhenium (Re) (Fig. 9-47)

Rhenium was the last naturally occurring element to be discovered (1925). In

rocks, it occurs primarily with minerals containing Pt, Nb, REE and Mo. Rhenium is

one of the rarest elements, and there was scant information on its biogeochemical

characteristics until the advent of ICP-MS with its high sensitivity to Re (detection of

1 ppb). Large databases now demonstrate that Re is commonly present in dry tissue

at levels of less than 1 ppb. No chart is presented to show accuracy and precision in

dry tissue because there are no known controls with concentrations above the de-

tection limit of 1 ppb Re. Consequently, the histogram illustrating Re precision is

based upon analysis of internal control V7 comprised of plant ash from a bulk

sample collected at a PGE deposit. This ash has returned excellent precision on

concentrations of only 6 ppb Re, with an RSD of 8%. On a dry-weight basis this

corresponds to just 0.1 ppb Re, which is probably a reasonable assessment of back-

ground levels of Re in plants. Tests on the volatility of Re indicate that little or none

is lost during reduction of plant tissue to ash at 475

A comparison of Acacia bark, roots, wood and foliage has revealed that only the

foliage contained Re concentrations above detection, with values up to 5 ppb DW.

This is in general accord with observations for other species. Consequently, for any

surveys requiring Re analyses the foliage would be the preferred sample medium.

Commonly, if Re levels are elevated there is a coincident increase in Mo and/or

PGEs because of their geochemical afﬁnity. Figure 9-48 shows two parallel sample

traverses 100 m apart that both show a strong Re response in needles of sub-alpine ﬁr

near the eastern end of each traverse. These lines were at Mount Polley in central

British Columbia where Cu–Mo–Au porphyry mineralization is present. In this case

Re(ppb) in V7

Re (ppb)in ash

Chronological Sequence

Fig. 9-47. Rhenium – Precision and accuracy on ashed tissues – V7 (see Fig. 9-1).

292 Biogeochemical Behaviour of the Elements

Re ppb

100

592600 592800 593000 593200 593400 593600 593800 594000 594200

NorthernTraverse - Fir needles

West East

Mo ppm

0.1

0.2

0.3

0.4

0.5

0.6

592600 592800 593000 593200 593400 593600 593800 594000 594200

Northern Traverse- Fir needles

West East

Re ppb

592800 593000 593200 593400 593600 593800 594000 594200

West EastSouthern Traverse - Fir Needles

Mo ppm

0.1

0.2

0.3

0.4

0.5

0.6

592800 593000 593200 593400 593600 593800 594000 594200

West EastSouthernTraverse - Fir Needles

Fig. 9-48. Rhenium and Mo in dry sub-alpine ﬁr foliage from undisturbed forest near the Mount Polley Cu–Au–Mo porphyry.

293Biogeochemistry in Mineral Exploration

the Re is clearly in association with Mo. Of note is that the soils from the same

sample stations also yielded Re anomalies, but concentrations were an order of

magnitude lower. The trends of Re and Mo enrichment indicate that the strike of

metal enrichment is towards the northeast.

Kimberlites sometimes yield anomalous levels of Re. The Welgevonden kimber-

lite in South Africa has up to 10 ppb Re in dry foliage of the thorny common plant

‘Hak en Steek’ growing over and marginal to the concealed kimberlite.

A survey involving a pproximately 800 black spruce tree tops from a 120 km

area

centred on the small Rottenstone PGE–Ni–Cu deposit in Saskatchewan yielded a

maximum of 40 ppb Re in dry top twigs. Of note was the spatial relationship of Re

enrichments to the location of known mineralization, since it followed a structural

trend adjacent to the zone of PGE enrichment (Fig. 9-49 ).

Another environment in which elevated levels of Re in plants may occur (invar-

iably associated with Mo) is over black shales, or reduced muds such as ancient lake

sediments which, upon oxidation, can release Re for uptake by plants.

Samples of foliage from the Eden Project have unusually high levels of Re, es-

pecially in samples from the Hot Tropical Biome where concentrations reach 149 ppb

Re. Such concentrations are most unusual and in this environment they are probably

attributable to high levels of organic matter added to the soils.

RHENIUM

in Spruce Tops

5 kms

ROTTENSTONE

PGE-Ni

DEPOSIT

Structural

Boundary

Re ppb

Fig. 9-49. Relationship of Re enrichment in black spruce tops to known PGE-Ni deposit.

Shaded contours represent isolines of concentrations greater than the median of 1.5 ppb Re in

dry tissue. Rottenstone, Saskatchewan. Analysis by ICP-MS on ash (normalized to dry

weight).

294 Biogeochemical Behaviour of the Elements

Rhodium (Rh)

See ‘Platinum and the Platinum Group Elements’.

Rubidium (Rb) (Fig. 9-50)

Rubidium is the sixteenth most abundant element in the earth’s crust. Conse-

quently, the levels present in plants are correspondingly high and well above the

detection limit by ICP-MS of 0.1 ppm Rb. Precision for Rb is consistently excellent,

with controls showing no spurious ‘spikes’ in the data.

Rubidium has no known biological role although there are indications in the

literature that it may be an essential micronutrient. For example, in a study of root

function and overlap Hawkes and Casper (2002) used Rb as a nutrient analogue.

Rubidium has a slight stimulatory effect on plant metabolism, probably because of

its geochemical afﬁnity to the large and essential monovalent cation potassium. The

two elements are found together in minerals, soils and plants although K is much more

abundant than Rb and their paths diverge within plant structures. Consequently,

in plants there is not always a positive correlation between Rb and K, nor does

Rb always exhibit a strong relationship with the other large monovalent cation Cs,

although the association occurs more frequently than with K.

Plants readily absorb Rb and most plant tissues have concentrations considerably

higher than the level of 1.05 ppm Rb that is highly reproducible in V6. The pine

needles comprising NIST 1575a have 17 ppm Rb, such that the precision is even

better with a RSD of 3%. Figure 9-51 shows the near perfect reproducibility of Rb in

needles of balsam ﬁr (Abies balsamea) from two splits of 26 samples (blind duplicates)

interspersed within a series of over 500 samples.

Samples from the Eden Project demonstrate the wide variations in Rb uptake.

Concentrations range from 3 ppm Rb in leaves of avocado and protea to 216 ppm Rb

Rb ppm

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

1.05

0.06

5.4

0.6

0.2

0.4

0.6

0.8

1.2

1.4

V6 - 30 controls among 600 samples

Concentration

Detection

Fig. 9-50. Rubidium – Precision and accuracy on dry tissues – V6 and NIST 1575a (see

Fig. 9-1).

295Biogeochemistry in Mineral Exploration

in torch ginger. Rubidium is usually more concentrated in conifer twigs than needles,

but in Acacia and deciduous species in general it is higher in leaves than twigs. It is

slightly lower in outer bark than in either twigs or foliage.

Several surveys have found Rb to be concentrated, usually with Sr, in vegetation

samples from over and around some kimberlites (Dunn, 1993). Rubidium forms

chemical compounds that can be extremely soluble, especially as carbonates. It is

commonly enriched in phlogopite contained within kimberlite where it is likely to

occur at inter-lattice sites, substituting for K, and held between the lattice layers

by weak van de Waals forces. Carbonatization occurs with many kimberlites, and

during weathering, weak carbonic acids may strip Rb from the phlogopite and trans-

port it in solution until it is absorbed by plants in the acidic environment around their

roots.

Ruthenium (Ru)

See ‘Platinum and the Platinum Group Elements’.

Samarium (Sm)

See Lanthanum and the Rare Earth Element s.

Samarium in V6 exhibits very good precision with a mean of 0.14 ppm Sm.

Samarium is usually present in plants at levels below its detection limit by ICP-MS of

0.02 ppm Sm. In South Africa, foliage of Acacia mellifera (Black thorn) has yielded

y = 1.0238x -0.147

=0.9924

0 5 10 15 20 25 30

Rb (ppm) first split

Rb(ppm) second split

RUBIDIUM in Laboratory Duplicates

BALSAM FIR NEEDLES

Fig. 9-51. Near perfect precision obtained for Rb in 26 analytical pairs of balsam ﬁr

needles.

296 Biogeochemical Behaviour of the Elements

up to 0.56 ppm Sm adjacent to a kimberlite, with stems containing lower concen-

trations. Dwarf birch leaves from the Ekati area of the Northwest Territories of

Canada yielded concentrations below detection. Near the REE-bearing allanite at

Hoidas Lake in northern Saskatchewan alder foliage was the vegetation sample

medium that contained the highest Sm concentration with 0.8 ppm.

Scandium (Sc) (Fig. 9-52)

Precision for Sc is fair to poor largely because Sc concentrations in plants are usually

close to the detection limit. Many types of plant tissue have concentrations below or

close to the detection limit of 0.1 ppm Sc in dry tissue, although concentrations up to

0.5 ppm Sc are not uncommon. The majority of the plants from the Eden Project that

were tested yielded between 0.2 and 0.4 ppm Sc in dry leaf tissue. Ferns seem capable of

accumulating a wide range of elements, and the tree fern Cyathea from the western

Amazon has returned concentrations of up to 5.2 ppm Sc in dry fronds compared to a

background level of 0.2 ppm Sc. A factor analysis of the dataset (n ¼ 354) demon-

strated that Sc is most strongly associated with Al, with positive loadings for Ba, Ga,

REE, S and Se. Leaves of a tropical vine from the same area had lower Sc concen-

trations, but also exhibited an association among Sc, Al and REE. The significance of

this association is not known to be of importance to mineral exploration and probably

simply reﬂects the geochemical afﬁnities of these elements.

In general, Sc concentrations are highest in roots, substantially lower in stems and

least concentrated in leaves (Horovitz, 2000), but Shtangeeva (2005) notes that the

biogeochemistry of Sc is still poorly understood.

In the event that a biogeochemical survey sample shows an anomalous level of Sc,

the Fe and Al data should be checked to see if there is a corresponding increase,

because in most species it appears that Sc is closely associ ated with Fe or Al in plant

tissues. To date Sc distribution patterns have not contributed any obvious assistance

in the exploration for concealed mineralization.

0.05

0.1

0.15

0.2

0.25

0.3

0.35

V6 - 30 controls among 600 samples

Concentration

Detection

Sc ppm

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

0.27

0.2

0.05

0.01

0.18

0.08

Fig. 9-52. Scandium – Precision and accuracy on dry tissues – V6 and NIST 1575a (see Fig. 9-1).

297Biogeochemistry in Mineral Exploration

Selenium (Se) (Fig. 9-53)

Control V6 has close to the de tection limit of 0.1 ppm Se, and exhibits precision at

the usual +/–100% at this level. Values obtained for NIST 1575a indicate poor

accuracy compared to published values. Selenium is an element for which some

laboratories declare that low levels cannot be accurately measured by ICP-MS,

whereas others claim to have circumvented this problem and provide consistently

precise data. Interpretations of Se data by ICP-M S should, therefore, take these

uncertainties into account.

From a biogeochemical exploration perspective Se is an element that has received

considerable attention for more than ﬁfty years. ‘Selenium ﬂoras’ have been iden-

tiﬁed in the western United States, Canada, Columbia and Queensland (Brooks,

1983). The presence of Se ﬂoras indicates the presence of Se enrichm ent in the

substrate, either because they can tolerate high concentrations or they have a speciﬁc

requirement for Se. Perhaps best known of these plants are the poison (or milk) vetch

(Astragalus) and the locoweed (Oxytropis). Some species are geobotanical indicators

(i.e., they are present only when Se is enriched in the ground), whereas others are

Se accumulators that have been found to take up more than 1% Se dry weight.

Concentrations can be sufﬁciently strong to detect in the ﬁeld the garlic and

horseradish-like odour emitted from these plants that is characteristic of volatile

Se compounds. On the Colorado Plateau, Cannon (1957) used various species of

Astragalus for indirect prospecting for U, because carnotite (potassium uranium

vanadate) is commonly associated with zones of Se enrichment in sedimentary U roll-

front deposits. As Brooks (1983) noted

Her classic work represents one of the most successful known applications of the

geobotanical method.

Recent work by Pickering et al. (2000, 2003) on Se in Astragalus using the syn-

chrotron has added significantly to an understanding of the location and speciation

of Se during various growth stages (Fig. 6-6). Among the revelations of this study is

Se ppm

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

0.2

0.18

0.06

0.1

0.226

0.07

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Concentration

Detection

V6 - 30 controls among 600 samples

Fig. 9-53. Selenium – Precision and accuracy on dry tissues – V6 and NIST 1575a (see Fig. 9-1).

298 Biogeochemical Behaviour of the Elements

that selenate is concentrated in leaf tissue, with very little in twigs. This ﬁnding is in

accord with observations on the relative con centrations of Se in conife rous species of

North America and in stems and foliage of a number of deciduous species. In the

widespread genus Combretum (bushwillow) found throughout much of sub-Sahara

Africa and tropical regions of South America, concentrations in foliage and stems are

similar. In general, for surveys requiring measurements of Se the foliage would be the

preferred tissue to collect.

There are no notable concentrations of Se in the foliage samples analysed from

the Eden Project, although it is noticeable that slightly elevated concentrations occur

in members of the Rubiaceae family (bedstraw or coffee family).

Geochemically, Se follows S and relatively high concentrations in plants can be

a good indication of sulphide-bearing minerals. Selenium can also be enriched in

vegetation from over PGE -bearing mineraliz ation, and S/Se ratios can be used to

assist in focusing in on drill targets. However, it should be borne in mind when

evaluating dist ribution patterns that the precision of the data is at best only fair for

both Se and S. Also, it should be noted that reduction of plant tissues to ash results in

partial volatilization of both elements, and so unless these losses can be ﬁrmly

quantiﬁed caution should be exercised when attempting to interpret S/Se ratio

distribution patterns from the analysis of ashed tissues.

In a survey that encompassed the Fox River Sill in eastern Manitoba, black

spruce treetop twig samples were reduced to ash and analysed by ICP-MS after

digestion in a nitric acid/hydrogen peroxide mixture. This method provides more

precise, accurate and stable data than digestion by aqua regia. The distribution

pattern of subtle Se enrichments formed a striking linearity that closely followed

stratigraphic trends. Highest concentrations were located close to but spatially

displaced from sites on the lower part of the Fox River Sill that yielded elevated

levels of Pt and Pd in the treetops. About 7 km to the west, a linear trend of similar

Se intensity lay parallel to a zone of elevated Pt and Pd, and was coincident with the

projected extent of a volcanic unit covered by glacial deposits.

Silver (Ag) (Fig. 9-54)

In dry plant tissue Ag determined by ICP-MS is rarely below the detection limit of

1 or 2 ppb Ag obtained by commercial laborat ories (Fig. 9-54). Values are commonly

in the low to tens of ppb at which levels precision is usually very good. At higher

levels the precision is excellent. V17 has a higher concentration of Ag and has shown

remarkable precision from 336 repeat analyses inserted as blind controls within

a sequence of more than 7000 ﬁeld samples over a two-year period (Fig. 9-55). The

mean value for this control is 74 ppb Ag with a standard deviation of 2.1 and an RSD

of 2.9%.

On occasion within a sequence of ﬁeld survey samples, there are very rare spikes in

the Ag data. They are rarely repeatable and after extensive checks for contamination

299

Biogeochemistry in Mineral Exploration

their source has not been identiﬁed. Consequently, it is as well to be suspicious of single

point Ag ‘spikes’ in the ﬁeld data. Unless they make geological sense they should either

be ignored or, preferably, the analysis should be repeated.

Data on foliage from the Eden Project demonstrate the wide range in uptake of

Ag among the samples tested, but data should be view ed in context of the Ag content

of the soils, which is substantially different in the two biomes. In the warm temperate

biome (WTB) the soils average 162 ppb Ag, whereas in the hot tropical biome (HTB)

concentrations average two orders of magnitude higher than this with almost

17,000 ppb Ag, presumably because of the higher levels of organic matter mixed in

with the HTB soils. Conse quently, the WTB samples (listings shown in black font in

the table of foliage analyses on the CD) should be viewed as a separate population

from the HTB samples (listings shown in red font). Concentrations in the WTB

samples range from 14 to 185 ppb Ag, except for the African Boxwood (Myrsine

africana) that yielded 754 ppb Ag; whereas in the HTB concentrations ranged from

38 ppb to 9670 ppb Ag in rattan (Calamus spp.). Although there is considerable

variability in the Ag content of soils in different en vironments from around the

V6 -30 controls among 600 samples

Concentration

Detection

Ag ppb

V6 NIST 1575a

Target

Mean

Std. Dev.

RSD %

17.6

1.7

(12)

1.5

Fig. 9-54. Silver – Precision and accuracy on dry tissues – V6 and NIST 1575a (see Fig. 9-1).

Ag in Control V17: n = 336

Chronological order

Ag ppb in dry tissue

y =0.0001x + 73.739

=0.00003

Fig. 9-55. Silver in control V17 inserted as a blind control among 7000 samples over a two-

year period.

300 Biogeochemical Behaviour of the Elements

world, soils in general contain Ag concentrations much closer to those in the WTB

than those in the HTB, and so plant tissues usually have a range of Ag concen-

trations similar to those encountered in the W TB.

Conifer trunk wood has a propensity to accumulate Ag. Concentrations in ashed

wood are commonly 10–15 ppm Ag which at ﬁrst sight appear to be extraordinarily

high until data are normalized to a dry-weight basis. These values correspond to

50–75 ppb Ag in dry tissue, because the ash yield of conifer wood is about 0.5% – i.e.,

a 200-fold concentration.

Several tissues were analysed from a single lodgepole pine rooted in tourmali-

nite with massive sulphides near the Sullivan Pb/Zn/Ag mine in southern British

Columbia. The dry roots and outer bark scales each contained 270 ppb Ag; twigs had

50 ppb Ag and at the top of the tree the stem had 15 ppb Ag.

There is an inconsistency among species as to whether foliage or twigs have the

higher concentrations of Ag. As a broad rule of thumb it appears that conifers

have more Ag in twigs than needles, and deciduous plants have the opposite. Some

examples show the patterns obtained from various locations.

Conifers



Mountain hemlock from Mt. Washington on Vancouver Island, where there is unde-

veloped Au/Ag mineralization, has ﬁve times more Ag in twigs than in needles.



Black spruce twigs from near the Rottenstone PGE/Au/Ni/Cu mineralization in

Saskatchewan have twice the Ag concentrations of needles.



Douglas-ﬁr tops from southern British Columbia have slightly higher Ag concen-

trations in the twigs.

Deciduous species tend to have higher Ag concentrations in the foliage.



At North Mara in Tanzania Acacia has considerably more Ag in foliage than

stems. Elsewhere in Central Africa in other species the foliage has similar con-

centrations to the stems.



In northern Saskatchewan Labrador Tea has similar concentrations in foliage and

stems, yet alder foliage has double the Ag content of the stems.

In the design of a biogeochemical survey that focuses on the exploration for

Ag deposits, these patterns of relative enrichments among plants serve as a guide

for selecting a suitable sample medium. However, the low detection limits and

high precision of the data obtained for Ag by ICP-MS mean that either foliage or

stems would be equally informative, because there is a good correlation between the

patterns obtained from each tissue type.

Over a number of years in the 1980s and 1990s, Dr. Alexander Kovalevsky and

his wife Olyesa Kovalevskaya conducted some extensive studies relating the Ag

content of rotted cones and stumps of pine (Pinus sylvestris) and stumps of larch

(Larix dahurica) to concealed Ag mineralization in eastern Siberia. This work was

301

Biogeochemistry in Mineral Exploration