Dunn Colin E. Biogeochemistry in Mineral Exploration
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TABLE 11-III
Western Amazon: Element concentrations in foliage of ferns and lianas at 366 sample stations
within an area of approximately 100 km
2
Fern foliage Liana foliage Concentration factors
Mean Median Max. Mean Median Max. Mean Median
Fern:Liana Fern:Liana
Ag (ppb) 67 9.5 2748 3.1 2 88 21.6 4.8
Al (%) 0.592 0.19 3.3 0.008 0.005 0.77 74.0 38.0
As (ppm) 0.28 0.2 2.9 0.18 0.1 1 1.6 2.0
Au (ppb) 0.81 0.5 16.8 0.46 0.3 6.8 1.8 1.7
B (ppm) 17.4 17 56 18.5 17 55 0.9 1.0
Ba (ppm) 25 14 208 26 12 404 0.9 1.1
Bi (ppm) 0.011 0.01 0.12 0.01 0.01 0.03 1.1 1.0
Ca (%) 0.302 0.25 1.19 0.983 0.75 3.44 0.3 0.3
Cd (ppm) 0.16 0.07 2.53 0.18 0.06 4.18 0.9 1.2
Co (ppm) 0.21 0.14 1.69 0.08 0.03 1,84 2.8 4.5
Cr (ppm) 2.37 2.1 20.7 1.67 1.63 3.86 1.4 1.3
Cs (ppm) 2.013 0.99 20.07 0.161 0.105 2.807 12.5 9.4
Cu (ppm) 29.3 22 164 4.47 4.2 19.4 6.6 5.2
Fe (%) 0.012 0.008 0.07 0.004 0.003 0.21 3.0 2.7
Ga (ppm) 0.11 0.1 1.7 0.067 0.05 0.3 1.6 2.0
Hg (ppb) 108 76 3943 51 39 725 2.1 1.9
K (%) 1.702 1.48 4.01 0.888 0.82 2.72 1.9 1.8
La (ppm) 15 2 267 0.06 0.02 1.71 255.1 102.3
Mg (%) 0.379 0.335 1.015 0.316 0.284 0.82 1.2 1.2
Mn (ppm) 490 366 2973 1446 1132 8197 0.3 0.3
Mo (ppm) 0.22 0.06 12.3 0.16 0.06 4.23 1.4 1.0
Na (%) 0.003 0.002 0.014 0.001 0.001 0.011 3.0 2.0
Ni (ppm) 2.38 1.3 29.4 0.48 0.2 28.4 4.9 6.5
P (%) 0.171 0.141 0.604 0.062 0.059 0.158 2.8 2.4
Pb (ppm) 2.9 0.8 99.6 0.34 0.26 1.42 8.5 3.0
Rb (ppm) 89 80 244 23 20 272 3.9 4.1
S (%) 0.247 0.22 0.81 0.09 0.08 0.24 2.7 2.8
Sb (ppm) 0.018 0.01 0.21 0.013 0.01 0.05 1.4 1.0
Sc (ppm) 0.0327 0.2 5.2 0.11 0.1 0.6 3.0 2.0
Se (ppm) 0.156 0.1 1 0.096 0.1 0.3 1.6 1.0
Sn (ppm) 0.037 0.02 1.12 0.022 0.01 0.27 1.7 2.0
Sr (ppm) 26 18 146 81 61 484 0.3 0.3
Te (ppm) 0.01 0.01 0.02 0.01 0.01 0.04 1.0 1.0
Th (ppm) 0.007 0.005 0.16 0.005 0.005 0.08 1.4 1.0
Continued
352 Case Histories
concentrations obtained from each sample medium , and the relative enrichment
expressed as the ratio of the average concentration in fern versus liana and also as the
median value for fern versus liana. The only elements more highly concentrated in
the liana leaves were Ca, Mn and Sr. With respect to median values, there was little
or no difference between ferns and lianas for B, Ba, Bi, Cd, Mg, Mo, Sb, Se, Te, Th,
Tl, U, V and W. However, for many of this latter group of elements, concentrations
were mostly below detection.
The two species did not exhibit a linear relationship in element concentrations.
Results indicated that for exploration purposes the fern was the prefer red
sample medium, because it had the de monstrated capability to absorb higher
concentrations of most elements, especially precious metals and their pathfinder
elements, and it was therefore the more sensitive plant with fewer barriers to element
uptake.
Unusually high concentrations of several elements werepresentintheferns.
The following numbers are t he maxima in dry tissue with t he median in paren-
theses: Ag 2748 (10) ppb, Au 16.8 (0.5) ppb, Cu 164 (22) ppm, Hg 3943 (76) ppb,
La 267 (2) ppm, Mo 12.3 (0.06) ppm, Ni 29.4 (1.3) ppm, Pb 100 (0.8) ppm, Rb 244
(80) ppm. From these numbers it is evident that there are unusual enrichments of
Au and base metals within the survey ar ea, with a very high background level of
Cu. The high L a concentrations are characteristic of REE enrichments commonly
found in ferns.
Figure 5-4 (Chapter 5) shows plots of Au and As in the ferns f rom this survey.
All elements were plotted in a similar m anner, resulting in more than 70 element
distribution plots for which t he patterns needed to be summarized. In the lianas
the patterns of a few elements were m ore informative than their corresponding
elements in the ferns – notably Co, Cu, Ga, Mn, Mo, Pb, Sb, Sn and Tl. For
example, Fig. 11-3 shows an area to the north that stands out as enriched in Pb and
Sb. Relative enrichment of Zn, Cd and Se occurred, also, in the eastern part of this
TABLE 11-III Continued
Fern foliage Liana foliage Concentration factors
Mean Median Max. Mean Median Max. Mean Median
Fern:Liana Fern:Liana
Ti (ppm) 4.2 4 22 1.6 1 25 2.6 4.0
Tl (ppm) 0.047 0.01 0.78 0.012 0.01 0.25 3.9 1.0
U (ppm) 0.006 0.005 0.09 0.005 0.005 0.02 1.2 1.0
V (ppm) 1.15 1 4 1.1 1 4 1.1 1.0
W (ppm) 0.05 0.05 0.1 0.05 0.05 0.2 1.0 1.0
Zn (ppm) 43 39 241 17.8 16 79 2.4 2.5
353Biogeochemistry in Mineral Exploration
area. In the ferns, patterns for these elements were less apparent. Consequently,
although the ferns were t he more sensitive to Au and its pathfinder e lements (As,
Hg, Bi and Ag) amongst others, the lianas were more suitable for defining the base
metal signature.
A method to summarize patterns from a large number of plots is to take a per-
centile value (e.g., 95th) for each element and superimpose the limits of these values
using lines with patterns each representing a different element. For example, Au>95th
percentile could be plotted as a polygon with a solid line, As with a dotted line, and Sb
with a dashed line. However, when there are many elements that need to be super-
imposed, it is usually necessary to draw a polygon or an ellipse outlining the approx-
imate limits of the anomalous suite of elements and the elements can be listed alongside
the map in a legend. For the Cordillera del Co
´
ndor survey area the situation is complex
in that there are several known and anticipated styles of mineralization, each with a
different suite of minerals and therefore different element signatures. For this survey,
the dataset from the ferns was examined and areas of anomalous suites of elements
0.15
0.35
0.55
0.75
0.95
Pb ppm
Max. 1.42
LEAD in dry LIANA LEAVES
Cordillera del Condor
Ecuador
Peru
Chinapintza
2 km
0.01
0.02
0.03
0.04
Sb ppm
Max. 0.05
ANTIMONY in dry LIANA LEAVES
Cordillera del Condor
Peru
Ecuador
Chinapintza
2 km
Fig. 11-3. Western Amazon – Pb and Sb in dry liana leaves (‘Churgun’).
354 Case Histories
plotted on a single summary map (Fig. 11-4). A similar map could be produced for
the liana, showing the base metals noted above, and ultimately a single map can be
produced that can rank the anomalies. In the present example only the fern anomalies
are shown, and the two most significant are identified by arrows (Chinapintza and
Conguime). The liana foliage also delineated these zones from most of the same
elements, but yielded lower concentrations.
At this time much of the exploration in this general area is taking place on the
Ecuadorian side of the border, notably northward from Chinapintza, westward from
Conguime, and westward from El Hit o (anoma ly 7S).
ANOMALOUS ZONES - FERNS
Cordillera del Condor
1N
2N
3N
4N
5N
6N
3S
4S
5S
6S
1S
2S
5aS
7S
NORTH
1N. Camino El Carmen
Au, K [Hg, U, Cd, Zn]
2N. Sabintza Baja
Au, [As, Sb]
3N. Las Cuevas
Sb, As,Zn, K [Au, Cu, Pb]
4N. Cobra 1
Pb, Tl, K
5N. Sabintza Alta
Au, As, Sb, K, Th [Mo, Pb]
6N. Sabintza Jog
Au, Bi, K [Sb]
SOUTH
1S. Chinapintza
Au, As, Hg, Bi
[Cs, Sb, Tl]
2S. Conguime
Au, As, Co, V, Al
[Pb, Cs]
3S. Linea 11
Pb, Hg, Zn, As, Sb, Sn
4S. Linea 8
Au, Ag, U, V
[Al, Th, Se]
5S. El Cruce
Pb, Tl
5aS. Skarn
As, Sb, Bi,Cd, V
6S. Linea 22
Au, Cu, Cr, Ni
[Al, Ga]
7S. El Hito
Mo, Cd [Cs]
Ecuador
Peru
2 km
Chinapintza
Conguime
Fig. 11-4. Western Amazon – Zones of relative element enrichments in dry fern foliage
(‘Chonta’).
355Biogeochemistry in Mineral Exploration
Permission from AngloGold Ashanti to reproduce the essence of this study is
gratefully acknowledged.
South America: Bolivia and Argentina
This section has been contributed by Manuel Viladevall
2
and Ignasi Queralt
3
.It
should be noted that these are orientation surveys based upon only a few samples
from each of severa l areas of significant metal enrichment that have been worked, in
some cases, for centuries. As such it can be expected that an historic ‘imprint’ of the
metals has been left in the soils and subsequently transferred into the vegetation.
Therefore, the extraordinarily high concentrations of Au, Sb and Cs may be in part
related to an anthropogenic signature. However, regardless of what that anthro-
pogenic component may be, the plants under discussion indicate a phenomenal
ability to accumulate and tolerate these metals and therefore represent excellent
candidates as biogeochemical sampling media for mineral exploration throughout
vast areas of South America.
Geology and mineralization
1.
Bolivia: Saddle reef-type Sb-Au ore deposits in siliciclast ic materials. Epithermal
Au with Sb – (a) Cevada Mayu and (b) Chilcobija.
2.
Bolivia: Placer Au (Khollpana) derived from conglomerates and epithermal Au
deposits associated with acidic volcani c rocks.
3.
Argentina: Epithermal Au deposits – Aguas Calientes caldera.
Environment
Andean highlands of Bolivia and Argentina (Fig. 11-5). Arid to semi-arid regions
of the Altiplano (‘Puna’).
Biogeochemical surveys
‘Thola’ is the popular name for an intricately branched indigenous bush that
grows at an altitude of between 3350 and 4250 m and covers an area of about
210,000 km
2
in Bolivia and Argentina. There are three genera that are included
in the generic term ‘Thola’: Baccharis spp. (Compositae Family), Parastrephia
lepidophylla (Asteraceae Family) and Fabiana densa (Solanaceae Famil y). Of these,
the first is the more common and includes several species selected for the biogeo-
chemical surveys. Many additional species of Baccharis are found at lower altitudes
throughout the Americas. Baccharis has a network of shallow roots with deep roots
penetrating 5 m or more allowing the plant to live in arid to semi-arid areas deficient
in soil.
2
Department of Geochemistry, Petrology and Geological Prospecting Faculty of Geology, University of
Barcelona. Zona Universitaria de Pedralbes 08071 Barcelona (Spain) mviladevall@ub.edu.
3
Institut Jaume Almera (CSIC). Marti Franques, s.n. 08038 Barcelona (Spain), iqueralt@ija.csic.es.
356 Case Histories
From preli minary studies it was ascertained that leaves absorb higher levels of the
elements of interest than the stems (Fig. 11-6), and that seasonal variations occur.
Subsequently, three orientation surveys for Au deposits using the Thola bush
were carried out in the Andean highlands of Bolivia and Argentina using two species
of Baccharis and the species Fabiana densa.
Samples were rinsed in running water to eliminate adhering dust particles, dried
at 40 1C and leaves separated from stems. Tissues were milled and the dry leaves were
analysed by INAA at Activation Laboratories Ltd. (Ontario, Canada) for Au, As,
Ag, Ba, Br, Ca, Co, Cr, Cs, Fe, Hf, Hg, Ir, K, Mo, Na, Rb, Sb, Sc, Se, Sn, Sr, Ta, Th,
U, W, Zn, La, Ce, Nd, Sm, Tb, Yb and Eu.
Fig. 11-5. Locations of biogeochemical surveys.
357Biogeochemistry in Mineral Exploration
A. Saddle reef Au-Sb deposits. In the southern part of Bolivia, 37 Thola samples
(Baccharis incarum) were collected along traverses in the proximity of the old Cevada
Mayu mine located at Sora Sora (55 km SW of Oruro), and from near the Chilcobija
mine in the Tupiza district (Fig. 11-5). Limited sampling at Cevada Mayu showed
that strong enrichments of Sb and Au in Baccharis incarum are present and that the
highest concentrations appear to be located on the anticlinal limbs of the Ordovician
Llallagua Formation (Fig. 11-7).
In the vicinity of the Chilcobija mine concentrations in dry Thola foliage ranged
from 49.2 to 101 ppb Au and from 330 to 540 ppm Sb. Although samples were rinsed
prior to analysis, it is suspected that these exceptionally high values are partially due
to contamination from the large Sb processing plant nearby. Airborne particulates
can precipitate on the ground, become dissolved by groundwater and then absorbed
through the root systems. Of note is the ability of Tholas to concentrate these
extreme amounts without exhibiting signs of significant stress.
It was found that both species of Thola that were sampled (Baccharis incarum
and B. leptophylla) yielded identical profiles for most elements indicating that their
elemental uptake is such that these species could be mixed without compromising the
integrity of a survey. The Sb content of soils ranged from 3440 to 4400 ppm, yielding
a plant-soil coefficient of approx 0.13 (Fig. 11-8).
B. The Khollpana Formation Placer Au Deposit (Caracollo). The placer Au deposits
associated with the Khollpana Formation are located in the Caracollo and Soledad
areas of western Bolivia (Fig. 11-5). These deposits have been mined since pre-
Colombian times. The Khollpana Formation comprises Tertiary and Quaternary
conglomerates composed of quartzite and sandstone pebbles derived from Ordovician
to Devonian rocks. The matrix is Tertiary acidic volcanic tuffs and sandstone pebbles
cemented by iron oxides.
0.1
1
10
100
1000
Au ppb As Co Cs Mo Rb Sb Zn
StemLeaves
Fig. 11-6. Comparison of element concentrations in dry leaves and stems of the Thola bush
Baccharis incarum at the Sb-Au Antofagasta mine near Oruro, Bolivia (Viladevall, 1994;
Marti, 1996).
358 Case Histories
Of the different Tholas (Baccharis incarum, Baccharis leptophylla, Parastrephia
leptophylla and Fabiana densa) growing on the Kollpana Formation, the most com-
mon is Fabiana densa. In light of the similarities in response shown by two species of
Thola (Fig. 11-8), the orientation sampling was performed by compositing Baccharis
incarum and Fabiana densa at each sample station. Gold in these composite samples
ranged from 5.9 to 90.5 ppb. Although data from this orientation survey look
promising, in ord er to fine tune the responses of the individual Thola species to
mineralization additional work needs to be undertaken to establish the relative
uptake of each species to a wide range of elements.
C. Epithermal gold deposits of Bolivia and northern Argentina. The response of Tholas to
epithermal Au mineralization was tested in areas of acidic volcanic and sub-volcanic
rocks.
In Bolivia, Baccharis incarum grows on Neogene lavas and tuffs in the Korikoya,
Tarikoya and Nueva Esperanza areas in the La Joya Au district (near Caracollo,
approximately 200 km southeast of La Paz). Ten samples contained high levels of
Au (5–54 ppb) and the pathfinder elements As (0.1 –11 ppm) and Sb (0.8–29 ppm).
Other Thola tests were carried out at the ‘Aguas Calientes’ caldera in the San
Antonio de los Cobres district in the Salta district in the Argentinean highlands
(Fig. 11-5). In that area a volcanic caldera contains mesothermal Pb-Ag ore (Poma
Mine) and an epithermal Sb-Au vein ore system exploited as the Incachule mine
Fig. 11-7. Cevada Mayu Sb-Au deposits. Profiles of Au and Sb in dry leaves of Baccharis
incarum.
359Biogeochemistry in Mineral Exploration
(Fig. 11-9). These veins are situated in the chlorite – sericite alteration zone with an
extension of 15 km
2
into surrounding ignimbrites.
The most common Thola bush in this area is Baccharis incarum. Forty-one com-
posite samples were taken from over the epithermal and mesothermal ore deposits.
Samples yielded anomalous Cs–Sb values in the argillic and silicified zones, and anom-
alous Pb–Zn values in the chlorite-sericite zone. Of particular note are Cs concentra-
tions up to 1500 ppm in samples from a silicified zone and up to several hundred ppm
Cs in samples from over chlorite-sericite schists. From discussion with the analyst there
is no possibility that these concentrations could be either analytical artefacts (analysis
by INAA) or from laboratory contamination. Even though the underlying soils had
several hundred ppm Cs thereby accounting for elevated levels in the plant tissues, it is
apparent that Baccharis incarum has a strong propensity to accumulate Cs.
Summary of results from the orientation surveys
Gold and Sb contents are lower in stems than in leaves of Baccharis incarum.
There is a marked increase in the Au content of leaves in spring (October).
In spring, Tholas exhibit an increase in As and Sb whenever concentrations of
these elements are high. Where concentrations of As and Sb are low an increase
occurs in autumn (June).
Fig. 11-8. Chilcobija Mine: plot of Plant:Soil coefficients (PSC). All values in ppm except Au
(ppb).
360 Case Histories
Other elements (e.g., Zn, Mo, Cs and Br) yield higher contents in autumn than in spring.
High Cs, Sb and As anomalies occur in the argillic and silicified zones of epithermal
volcanic areas.
For the different varieties of ‘Thola’, Fabiana densa can accumulate more Au, As,
Ba, Co, Fe, Hf, Sb, Sc, W and U than Baccharis incarum. Conversely, Baccharis
incarum contains more Br, Ca, Mo, Na, Sr and Zn than Fabiana densa.
In all three survey areas Thola leaves proved to be a useful medium for delineating
anomalous zones of Au, Sb, As, Pb and Zn in the Andean highlands.
The authors acknowledge, with thanks, the support of the University of San
Andres (La Paz, Bolivia); the University of Salta (Argentina); and the European
Union International Cooperation for Contracts CT1*-CT90-0784 and CE93-0021.
PLATINUM GROUP METALS/NICKEL/COPPER
Canada: Rottenstone Lake, Northern Saskatchewan
Geology and mineralization
Harzburgite-orthopyroxenite sill with unknown stratigraphic continuity (Hulbert
et al., 1988). Surrounding rocks are prim arily highly metamorphosed (granulite
facies) metasediments and metavolcanics.
Fig. 11-9. ‘Aguas Calientes Caldera’ in San Antonio de los Cobres (Salta, Argentina).
361Biogeochemistry in Mineral Exploration