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Gold in PlantsChristine A. Girling* and Peter J.
PetersonWestfield College, University of London, U. K.*Now with the
Central Electricity Research Laboratories, Leatherhead, U. K.
Many plants have the ability to take up gold from the soil and
to ac-cumulate it in their tissue. Advances have been made in our
understan-ding of these processes to the point where their
exploitation, particularlyin the field of prospecting for gold,
appears practically feasible.
Plants accumulate in their tissue all the macro-
andmicro-elements which are essential to growth andreproduction —
nitrogen, phosphorus, potassium,iron and copper, for example — as
well as elementsfor which no definite role has been established,
suchas chromium, nickel, arsenic and tin (1). Elements ofthis
latter type can occur in plants over wide ranges ofconcentration,
depending on the plant species, theirgrowth rates, and soil and
environmental factors (2).The accumulation of metals in plants has
a number ofinteresting applications. Thus, the analysis of
plantmaterial for lead, zinc and mercury constitutes anelegant
means of monitoring atmospheric releases ofthese pollutants (3).
Similarly, anomalous concentra-tions of cadmium have been found in
numerousspecies growing near metal smelters (4,5). Indeed, ithas
been demonstrated that under some conditions,plants exhibit overt
symptoms of phytotoxicity stem-ming from metal accumulation (6).
Statisticaltreatments involving plant-soil correlations on
naturalplant communities have shown that certain speciescan be used
for the detection of geochemicalenrichments arising from
mineralization in theunderlying bedrock (7).
Accumulation of GoldGold was detected in plant tissues as early
as 1900,
when the method of fire-ashing was used to producegold beads
from hardwood trees (8). Since then, con-centrations of gold have
been reported in a wide varie-ty of plant species collected over
mineralized areas indifferent regions of the world (9 to 12).
Jones (13) has reviewed the literature on the occur-rence of
gold in plants. The gold concentrations areusually in the ppb range
(dry mass) although valuesup to 610 ppm (ash mass) have been
recorded. Thelatter figure was reported for Equisetum
palustre(horsetail), collected from the gold-bearing region
ofOslany, Czechoslovakia (14), but there is now con-siderable doubt
regarding the validity of the methodof analysis. Cannon et al. (15)
studied samples of theEquisetum genus from 22 areas in the U.S.A.
and
found values of only 200 ppb gold (ash mass). Twigsand leaves of
trees growing in gold-enriched areas inBritish Columbia, Canada,
have been reported tocontain up to 20 ppb gold (dry mass) (12).
Schiller et al. (16) found gold in several species
atconcentrations of between 1 and 10 ppb in an en-riched area and
noted seasonal variations in gold ac-cumulation, with the highest
values occurring in thespring. A possible explanation for this
observationwill be derived from the results presented at a
laterpoint in this article. Russian workers have reported arange of
values for gold in plants collected over gold-mineralized areas of
the U.S.S.R. Aripova andTalipov (9), for example, quote enrichment
factors(concentrations of gold in plants grown in such areasdivided
by background sample concentrations) ofbetween 10 and 100.
In order to further our understanding of thesephenomena, a
number of laboratory and field studieson the processes of uptake,
transport and localizationof gold in plants was undertaken, the
results of whichare reported here.
Analysis for GoldNeutron activation analysis is particularly
suited to
the determination of the small quantities of gold inplants and
typically requires samples of 200 mg ofhomogenized, dried material
(17). In the presentstudy small discs of compressed, powdered
materialwere irradiated for 8 hours at a thermal neutron fluxof 1.6
x 10 12 neutrons cm -2s - ' and allowed to `cool'for 3 days; the
resulting gamma-ray spectrum wasdetermined using a
germanium-lithium detector con-nected to a 4 000-channel analyzer.
All samples werecompared against standards having a gold
concentra-tion of similar order. Counting times varied, de-pending
on the activity of the samples, but usuallyextended from one to two
hours to ensure acceptablecounting statistics. Figure 1 shows a
typical gamma-ray spectrum of an irradiated plant sample. The
goldpeak used for analysis occurs at 411.8 keV (0.95 inten-sity)
and that of arsenic at 559.2 keV (0.43 intensity).
MI
-
Fig. I Gam,na-ray spectrumof a dried and pelleted plantsample
irradiated by thermalneutrons in the University ofLondon reactor.
The abscissashows the channel energyand the ordinate the countsper
channel. The gold peakused for analysis occurs at anenergy of 411
keV and that ofarsenic at 559 keV
The validity of this method of analysis was checkedby measuring
the gold and arsenic contents of stan-dard reference materials,
namely Bowen's Kale andNational Bureau of Standards Orchard Leaves,
andfound to agree within 10 per cent with independentlydetermined
standards values for these materials.
Once its reliability and sensitivity had beenestablished,
neutron activation analysis was usedthroughout this study of the
occurrence of gold inplants.
Biogeochemical Prospecting
Biogeochemical prospecting has been of value inthe location of
mineral anomalies (18), but has notbeen extensively used in the
search for gold deposits.The reason for this is the general
unsuitability of theanalytical techniques previously employed.
Atomicabsorption spectrophotometry, fire-ashing, po-larography and
emission spectrography, for exampleare not sufficiently sensitive,
and neutron activationfollowed by radiochemical separation of the
goldspecies is not readily applicable to extended
samplingprogrammes.
A biogeochemical study, employing neutron activa-tion coupled
with multi-channel gamma spectro-scopy, was undertaken in the old
alluvial gold miningregion around the headwaters of Stirrup Creek
insouthern British Columbia, Canada. Plant samplingwas carried out
over areas of partially auriferoussilicified argillite, in which
both rocks and intrusiveswere cast by numerous small veins of
quartz, some ofwhich contained visible gold. Three plant
specieswere sampled across a transect and additional specieswere
collected along a further transect from an adja-
cent area, to examine in more detail the variation ofgold
accumulation between species. The three plantscollected along the
main transect represented a varie-ty of botanical form and habit:
Phacelia sericea (moun-tain phacelia) is a deep-rooted perennial,
Oxytropiscampestris (late yellow locoweed) var. gracilus is
adeep-rooted legume and Sedum lanceolatum(stonecrop) is a
shallow-rooted fleshy succulent.These species were reasonably
widespread over thesampling area.
Approximately 100 g of shoot material was col-lected at 10 m
intervals along both transects. Rootswere not sampled because of
the problems arisingfrom soil contamination. At each sampling
position,leaves and stems of each species were collected from10 to
20 plants, depending on their local frequency ofoccurrence. Dead
material was removed, and thesamples were washed, oven-dried at
80°C for 48hours and powdered in an electric homogenizer.Background
samples from a comparable topographicarea nearby, where earlier
geochemical studies had in-dicated the absence of gold anomalies,
were similarlyprepared.
In addition, several bulk samples of P. sericea werecollected at
a point near to the transects and the golddistributions within the
shoot — that is, the respec-tive concentrations in the leaf, stem
and inflorescence— were determined. Despite a marginally higher
goldcontent in the inflorescence, the selectivity of concen-tration
in the shoot was not considered significant.
Figure 2 illustrates the values obtained for the goldand arsenic
concentrations at each of the samplingpoints along the main
transect. The peak concentra-tion of gold in P. sericea was found
at site No. 5, with
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the two other species also reflecting anomalies at thislocation.
A second, much larger peak was detected in0. campestris and S.
lanceolatum at site No. 9, but un-fortunately P. sericea was not
found growing at thissite. The highest concentration of gold was
found inP. sericea, namely 21.1 ppb, compared with abackground
value of 1.6 ppb.
Seven additional plant species collected adjacent tothe main
transect contained substantial concentra-tions of gold:
Gold, Arsenic,Species ppb, ppm,
dry massCerastium arvense 6.4 18.21Polemonium pulcherrimum 2.5
2.99Arctostaphylos uva-uvsi 2.5 4.96Castilleja miniata 2.3
5.42Dryas octopetala 1.8 0.70Lupinus latifolius 1.7 0.26Pinus
contorta 0.7 0.81
Cerastium arvense was found to contain the highestconcentration
in this area and it is noteworthy thatCerastium species have been
shown to be significantaccumulators of other heavy metals in
severalenvironments (19).
The accumulation of arsenic in plants has beenregarded as a
geochemical indicator of gold (20). Thepresent study, however,
revealed no consistent cor-relation between gold and arsenic
concentrations asshown in Figure 2.
Uptake of GoldThe examination of the gold concentrations in
the
three plant species studied over the transects showsthat P.
sericea has the ability to accumulatesignificantly more gold than
the other two species.Since this is not a function of root depth
(0.campestris is similarly deep-rooted), it would seemnecessary to
investigate this phenomenon further.
At the beginning of the century, Lungwitz (8) sug-gested
possible reasons for the occurrence of gold intree ash. He put
forward the hypothesis that plantsecretions may aid the uptake of
gold from the soiland that cyanide, in particular, might render
gold suf-ficiently soluble to enable plants to accumulate themetal.
At that time, the existence of cyanogenic plantspecies was
unrecognized, but today over one thou-sand species are known which
produce free cyanideby hydrolysis of cyanogenic glycosides within
theirtissues (21).
Macerated tissues of cyanogenic plants have beendemonstrated
(22) to facilitate the uptake of gold inplants. Thus, in the
presence of a cyanogenic extractfrom Prunus laurocerasus (cherry
laurel), gold foilmay be solubilized sufficiently to allow gold
ac-cumulation in Zea mays (maize) to the ppm range ofconcentrations
(23):
J PHACELIA SERICEA
16
GELD
12t1t =— — ARSENIC
I ii —°°
4
1 2 3 4 5 6 7 8 9 tO
UZUi
12 OXYTROP/S CAMPESTRIS.
z d n
r
1 2 3 4 • 5 6 7 8 9 10
0121 SEDUM LANCeOLATUM8
1 2 3 4 5 6 7 8 9 10
SAMPLING LOCATION
Fig. 2 Concentrations of gold and arsenic in driedsamples of
three plant species collected (whenpresent) at 10 locations along a
transect at WatsonBar in the Stirrup Creek region of southern
BritishColumbia, Canada. After (11)
Extract added Gold concentrationto growth medium shoot,
root,
ppm, dry massCyanogenic 1.36 31.10Non-cyanogenic 0.14 6.76
Picric acid-impregnated paper tests, and the morespecific
colorimetric test of Lambert (24), have shownthat fresh root and
shoot material of P. sericea releasescyanide in measurable
quantities. It may therefore beassumed that in natural
environments, cyanide isreleased from the plant as a result of
tissue damageand from leaf litter decomposing on the soil
surface.
Studies relating gold accumulation with the cyanidecontent of
various plant species growing in the field
153
-
Table I
Concentration of Gold in Plants (Dry Mass) Grown in Solutions
Containing Various Gold Compounds atpH 5.5
Compound Phacelia sericea Hordeum vulgareshoot, root, shoot,
root,ppm ppm ppm ppm
Gold potassium cyanide 87.3 3857 1.0 372Gold bromide 68.7 2983
0.7 256Gold chloride 65.3 2937 0.9 253Gold thiosulphate 6.6 458 0.2
50Control solution 0.2 0.7 0.0 0.3
near a gold mine in Wales (23), confirm a
positiveinter-relation, as follows:
Species Gold cone., Cyanide,ppb, dry mass content
Mentha aquatica 49.9 highCirsium palustre 26.5 highRanunculus
aquatilus 8.3 highHedera helix 2.9 lowIlex aquifolium 2.8 lowUlex
gallii 1.3 low
A point of special interest is the observation thatgold is lost
almost completely when dried samples ofP. sericea are ashed at 550
° C:
Treatment Gold concentration,ppb, ash mass
Oven-dried at 80°C 372*Ashed in small muffle
furnace at 550°C 5Ashed in large muffle
furnace at 550°C 2Ashed in `tin' cans
at 550°C 1*based on mean dry mass/ash mass ratio
Since this loss is not observed during the ashing ofmost other
plants, it would appear that gold is ac-cumulated in them in a form
differing from that inP. sericea and, in tune with this, that the
uptake ofgold in natural plant communities can proceedby mechanisms
other than the one applicable toP. sericea. Various mechanisms of
uptake have beensuggested, which depend on chemical factors
withinthe soil (25, 26).
Additional data on the availability of gold to plantswere
obtained by supplying various gold compoundsto Hordeum vulgare
(barley) seedlings growing inhydroponic nutrient solutions, and by
measuring thegold concentrations in the root and shoot
materialafter several days growth. The highest accumulationof gold
was recorded in seedlings grown in thepresence of gold potassium
cyanide and the least inthose grown in the presence of gold
thiosulphate(Table I). Experiments with the cyanogenic P.
sericeaproduced similar results, although this plant ac-
cumulated approximately ten times as much gold asbarley in both
root and shoot. Preferential gold up-take may be considered to stem
from the combined ef-fects of plant species and the selective entry
of goldderivatives produced in the nutrient solution.
The effect of the pH of the growth medium on theuptake of gold
was studied in H. vulgare suppliedwith gold chloride. The uptake in
the root was foundto be markedly dependent on acidity,
increasingthreefold with a lowering of pH from 8 to 3.6. Uptakein
the shoot, however, was not significantly affectedby pH values
(23).
The uptake of gold with time was monitored overseveral hours in
H. vulgare by the use of the radio-active tracer gold-198. Figure 3
illustrates an initialrapid increase in gold accumulation which is
con-sidered to result from ion exchange processes at the
3TIME, HOURS
Fig. 3 Uptake with time of radioactive tracergold•198.by roots
and shoots of Nordsurn vulgaregrown under hydroponic conditions in
nutrienttsolutions of high gold concentration, as measuredby the
activity of samples
154
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root surfaces, while the subsequent much slower risemay be
ascribed to metabolic accumulation.
The lowering of the hydroponic nutrient solutiontemperature to 6
± 2°C resulted in a reduction ingold uptake in the root. Darkness
had a small effecton gold transport from the root to the shoot, but
noeffect on uptake (Table II). The presence of themetabolic
inhibitors sodium azide or dinitrophenol,at concentrations of 10 -4
M in the nutrient solution,caused a large reduction in uptake by
the roots, whilethe addition of an equivalent amount of calcium
hadthe opposite result (Table II). This confirms that golduptake is
at least partially dependent on metabolicprocesses. The
introduction of heavy metal ions,such as those of silver and
arsenic appears to decreasethe uptake of gold in the root, probably
because oftheir toxic action. Ions such as copper and manganesehad
no significant effect on uptake, but iron caused aslight increase
(Table II).
Transport of GoldIncreasing the transpiration rate by blowing a
con-
tinuous stream of air over a group of H. vulgare plantsgrown in
a culture solution containing gold chloride,produced no significant
increase in gold uptake com-pared with that in control specimens.
However,decreasing the transpiration rate by the application
ofvaseline, or vaseline together with the growth in-hibitor
absscisic acid (ABA) at a concentration of10 -`' M, to the leaves
of I-I. vulgare, caused a reductionin the gold content of the
shoot, but did notsignificantly affect uptake in the root. The
quan-titative results were as follows:
Gold contentTreatment shoot, root,
% of controlBreeze 94.9 109.4Vaseline 29.2 96.4Vaseline + ABA
14.4 111.1
It would appear, then, that the transport of gold inplants is
positively affected by the rate of transpira-tion. This, combined
with the knowledge thattranspiration rates are usually at their
highest in thespring, offers a plausible explanation for the
seasonalvariations and peak spring values of gold accumula-tion,
which were observed by Schiller et al. (16).
In a further study, H. vulgare and Phaceliatanacetifolia were
grown in the presence of tracer goldchloride for two days, half of
each set of plants washarvested, assayed for gold, and the
remainder wasdesorbed of gold not contained within the cells,
grownfor a further two days in gold-free nutrient solutionand
similarly analyzed. The results, expressed interms of transport
index (concentration of gold in theshoot as percentage of the
concentration in the wholeplant), were as follows:
Table II
Effect of Various Treatments on Uptake ofGold by Hordeum vulgare
from Nutrient Solu-tions Containing Gold Chloride jExpressed as
Percentage of Control)
RootLow solution
temperature (6°C) 100.0 88.5Darkness 65.8 93.2
Addition of:sodium azide 36.2 16.7dinitrophenol 17.7 0.8Ca?+
84.4 195.9Fe * 115.8P,g+ 25.0 54.0Ass. 106:9 79.9Cuz+ 9715Mn3+
109.1 98.0
not determined
Treatment H.vulgare P. tanacetifoliaGold chloride
(2 days) 2 8Nutrient solution
(further 2 days) 8 34
Thus, the gold was continuously transported tothe shoots after
removal from the gold-containingnutrient solution. This movement of
gold up to theshoot of plants may depend upon its passage in
thetranspiration stream. The ratio of mobilization fromthe roots to
the shoots was the same for both species,but a difference in the
transport index was noted,which reflects the difference in gold
uptake.
The possibility that gold might be washed fromleaves of growing
plants was explored in the followingexperiment: Specimens of H.
vulgare were arrangedto have a number of their attached leaves
immersedin test tubes of water, while the plants were growingin
nutrient solutions containing radioactive goldchloride or gold
cyanide. After 24 hours, the plantswere harvested and the
radioactivity of the shoots andof the leaf wash waters was
determined. The portionsof the gold that had been dissolved,
expressed as apercentage of the total gold content of the
shoots,were as follows:
Solution Sample % SolubleGold chloride 1 negl.
2 0.02Gold cyanide 1 0.18
2 0.38
It will be noted that very small quantities of goldcould be
washed from the leaves of those plantsgrown in gold chloride, but a
much larger proportionfrom those grown in gold cyanide. The
relevance ofthis finding to the environmental cycling of gold
re-quires further examination.
155
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1
•sr
r:
Fig. 4 Autoradiograph of a shoot of Phacelia tanscetijoliagrown
under hydroponic conditions in nutrient solutioncontaining tracer
gold cyanide. Note that a general diffusedistribution of gold is
accompanied by preferentiallocalization in the leaf tips
Localization of GoldIn order to determine the distribution of
gold
within the shoots of various species, plants weregrown in
solutions containing radioactive goldchloride or gold cyanide for
two days and the shootswere subsequently radioautographed for
severalhours. Both H. vulgare and P. tanacetifolia showed
diffuse gold distributions with special accumulationsin the leaf
tips, possibly in metallic form (Figure 4).In the case of plants
treated with gold chloride, thelocalization within the leaves was
less specific.
The concentrations of radioactive gold in water ex-tracts of
various portions of P. tanacetifolia grown insolutions containing
gold as cyanide revealed that themetal is largely in soluble form
in the shoot, but thatin the root an appreciable portion is
insoluble. Goldtaken up from solutions containing the metal as
itschloride was partially soluble in the shoot, but almostentirely
in insoluble form in the root. The addition ofradioactive gold
chloride and gold cyanide to gold-free root and shoot tissue,
followed by water extrac-tion, indicated that gold cyanide is
almost completelysoluble in both root and shoot, but that gold
chlorideis essentially insoluble. Of the insoluble gold asso-ciated
with the cell wall component, only 40 per centcould be removed by
exchange with non-radioactivegold, which tends to indicate a strong
bonding of thegold with the cell wall component of the plant.
Freshly cuts shoots of tracer-free P. tanacetifoliawere placed
(in the manner of cut flowers) into solu-tions of radioactive gold
chloride or gold cyanide,allowed to stand for six hours and were
thenradioautographed. The shoots thus labelled usinggold in cyanide
form showed a distribution of goldsimilar to that occurring in the
shoots of plants grownin the presence of tracer gold cyanide. The
shootslabelled using gold in the form of its chloride,however,
contained only small quantities of gold, ex-cept in the stem region
that had been immersed in thesolution. The shoots of whole plants
grown in amedium containing radioactive gold chloride con-
GOLD CHLORIDE
GOLD CYANIDE
• ROOT ri ROOT
160 EXTRACT 80 EXTRACT
120 60
80 40
pS 40 fl 20ä
1
SHOOT tT60 EXTRACT nfl60 EXTRACT
ö 40 40
C( n7 20 20
^ NUTRIENT NUTRIENT
800 SOLUTION 600 SOLUTION
400 h 400zoo II 200
— 12 8 4 0 4 8 12 16 + — 16 12 EMIGRATION DISTANCE, cm
4 0 4 8 12+
Fig. 5 Separation by high-voltage paper electrophoresisof
various ionic species inaqueous extracts of Phaceliatanacetifolua
grown underhydroponic conditions innutrient solutions
containing.gold•198 as its chloride orcyanide
156
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tamed significantly more gold. This suggests that thesoluble
gold compounds deriving from the root are inthis case not deposited
in the stems in insoluble form.
High-voltage paper electrophoresis on water ex-tracts from P.
tanacetifolia grown in the presence ofradioactive gold chloride
resulted in the separation ofvarious ionic species of gold as shown
in Figure 5.Unidentified soluble gold species were formed in
theplant shoot which were not present in the nutrientsolution,
thereby pointing to their possible produc-tion in the plant root.
However, compounds ofsimilar mobility were identified when tracer
goldchloride was mixed with fresh plant tissue and theaqueous
extracts were analyzed by electrophoresis;this suggests that these
compounds are produced by anon-active metabolic process. Similarly
prepared ex-tracts of plants labelled with gold in cyanide form
alsoindicated that additional soluble gold species hadbeen produced
in P. tanacetifolia.
Relevance to MedicineGold theraphy has been applied to cases
of
rheumatoid arthritis for several decades, although themechanism
of its ameliorative action, recently re-viewed in this journal
(27), is only partiallyunderstood. The technique of cell
fractionation bydifferential centrifugation, when applied to
shoothomogenates of H. vulgare grown in tracer goldchloride or gold
cyanide solutions, can provide someinsight into the possible
biochemical interactions bet-ween gold and cell proteins. Table III
records thecellular distribution of gold between the various
plantorganelles. The distribution was markedly affected bythe
nature of the gold compound supplied. The isola-tion of soluble
protein and ribonucleic acid (RNA)fractions from H. vulgare grown
in tracer gold solu-tions showed variable results, again depending
on thetracer gold compound. Thus, as much as 42 per centof gold
chloride added to serum albumen was foundto bind with this protein,
but much less gold wasassociated with proteins of plants grown in
gold solu-tion. Such studies of the interaction of gold
withmetabolically important cellular constituents mayassist in
elucidating the mechanism of the inhibitionof rheumatism in
man.
This study, in tracing the processes of uptake,transport,
localization and dissolution of gold inplants, suggests that they
play a complex, but signifi-cant, part in the gold cycle. Their
involvement in theformation of ancient gold deposits (28) now
appears adistinct possibility. The study also indicates that
anunderstanding of these effects in plants may con-tribute to the
elucidation of the reactions into whichvarious forms of gold enter
in animal tissue andtherefore to the application of gold in
medicine.
Table IIIDistribution of Gold between Various CellComponents, of
Homogenized Shoots ofHordeum Vulgare Grown in Gold-Containing
Nutrient Solutions
Distribution of gold,Cell per cent of total gold content of
cells
Fractiongold chloride gold cyanide
solution solution
Cell wall 84.7 11.2Chloroplast 4,7 7•5Mitochondria 9.5
17.8Ribosome 1.1 0.8Cell fluids 3.3 59.5
Finally, and perhaps most significantly, it suggestsnew
possibilities in regard to biogeochemical prospec-ting for
gold.
AcknowledgementThis research project was financed from a U.K.
Science
Research Council post-graduate award.
References1 P. J. Peterson, Sci. Prag. Oxford, 1971, 59,
505-5262 P. J. Peterson, International Atomic Energy Agency Rep.
310,
Vienna, 1972, 323-3413 P. Little and M. H. Martin, Environ.
Pollut., 1974, 6, 1-194 G. T. Goodman and T. M. Roberts, Nature
(London), 1971,
231, 287-2925 P. Little and M. H. Martin, Environ. Pollut.,
1972, 3, 241.2546 L. M. Cunningham, F. W. Collins and T. C.
Hutchinson,
`Prot. Int. Conf, Heavy Metals in the Environment',
Toronto,Ontario, edited by T. C. Hutchinson, 1975, Volume 2,pp.
87-120
7 M. H. Timperley, R. R. Brooks and P. J. Peterson, Econ.Geol.,
1972, 67, 669-676
8 E. E. Lungwitz, Min. J. London, 1900, (March 17), 318-3199 Kh.
Aripova and R. H. Talipov, Uzb. Geol. Zh., 1966, 10,
45-5110 C. A. Girling, P. J. Peterson and M. J. Minski, Sci.
Tot. En-
viron., 1978, 10, , 79-8511 C. A. Girling, P. J. Peterson and H.
V. Warren, Econ. Geol.,
1979, 74, 902-90712 H. V. Warren and R. E. Delavault, Bull.
Geol. Sc. Am,, 1950,
61, 123-12813 R. S. Jones, U.S. Geol. Suro. Inf. Circ., 1970,
(625), 1-1514 J. Babicka, Mikrochim. Acta., 1943, 31, 201-25315 H.
L. Cannon, H. T. Shacklette and H. Bastron, U.S. Geol.
Surv. Bull., 1968, (1278-A), 1 -2116 P. Schiller, G. B. Cook, A.
Kitzinger-Skalova and E. Wolff,
Radiochem. Radioanal. Lett., 1973, 13, 283-28617 M. J. Minski,
C. A. Girling and P. J. Peterson, Radiochem.
Radioanal. Lett., 1977, 30, 179-18618 H. L. Cannon, Science,
1960, 132, 591-59819 P. Shewry and P. J. Peterson, 1. Ecol., 1976,
64, 195-21220 H. V. Warren, R. E. Delavault and J. Barakso, Econ.
Geol.,
1966, 59, 1381-138621 E. E. Conn, 7. Agric. Food Chem., 1969,
17, 519-52622 H. T. Shacklette, H. W. Lakin, A. E. Hubert and G. C.
Cur-
tin, U.S. Geol. Surv. Bull., 1970, (1314 -B), 1 -2323 C. A.
Girling and P. J. Peterson, Trace Sub. Environ. Health,
1978, 12, 105-11824 J. L. Lambert, J. Ramasamy and J. V.
Paukstelis, Anal. Chem.,
1975, 47, 916-91725 H. L. Ong and V. Swanson, Colo. Sch. Mines
Q., 1974, 69,
395-42526 A. M MacGregor, Min. Mag., 1953, 88, 281-28227 A.
Lorber and T. M. Simon, Gold Bull., 1979, 12, (4), 149-15828 D. K.
Hallbauer, Miner. Sci. Eng., 1975, 7, 111-131; Gold
Bull., 1978, 11, (1), 18-23
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