Dunn Colin E. Biogeochemistry in Mineral Exploration
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Many other genera occur, but they are likely to be only of local use. In general,
collect all of the above-ground (i.e., aerial) portions of the small shrubs. Separation
of tissues can best be undertaken in the laboratory. Data on these species are pro-
vided in Reading et al. (1987) and Coker et al. (1991).
Arid and semi-arid environments – general comments
The root systems of many plants from arid regions penetrate deep into the subst-
rate. Live roots have been found in mine excavations 60 m beneath the surface. The
volume of ground from which they extract metals is therefore enormous, and
plant composition represents an integration of the geochemical signature of many
cubic metres of soil, rock and groundwater. Consequently, with the exception of the
rare extreme environments such as the Atacama Desert that are too arid to support
even small shrubs, biogeochemical methods have wide application in arid environ-
ments. Many species have tough thorns, so it is advisable to wear strong leather
gloves.
North America – arid and semi-arid
In the arid and semi-arid regions of western North America some of the more
common species that have been used include the following:
Northern basin and range
Sagebrush
Shadscale
Greasewood
Rabbit brush
Hopsage
Horse brush
Mountain mahogany
Artemisia tridentata
Atriplex confertifolia
Sarcobatus vermiculatus
Chrysothamnus nauseosus
Grayia spinosa
Tetradymia glabrata
Cerocarpus betuloides
A long list of additional plants that have been used in U exploration in this
environment is given in Cannon (1952, 1964).
Southern basin and range, and desert of the southwestern USA and Mexico
Mesquite
Creosote
Acacia (‘catclaw’)
Match brush
Palo verde
Prospopis juliflora
Larrea divaricata
Acacia greggii
Gutierrezia lucida
Cercidium microphyllum and C. floridum
52
Field Guide 1: Climatic and Geographic Zones
Central America
In Central America the Mo nterey pine (Pinus radiata) is a useful sample medium
(Honduras). In Costa Rica, high concentrations of Au are recorded in the tree
‘chaparro’ (locally known as ‘raspa’ – Curatella americana)–Siegel and Pagagua
(1991). Chaparro has a large geographic range that includes Cuba, Hispaniola,
southern Mexico, Honduras, Belize, Nicaragua, Costa Rica, Panama, Guatemala, El
Salvador, French Guiana, Suriname, Guyana, Trinidad, Venezuela, Colombia, Peru,
Brazil and Bolivia.
North Africa-arid – southern Morocco into the Sahara Desert
Many of the species common to these areas (pines, figs, palms, oleanders, rock
rose [Cistus spp.]) extend throughout the Balkans and into the Middle East where
acacia becomes one of the most common trees.
The following short lists are for species sampled in Morocco for which biogeo-
chemical data are available (Brooks et al., 1995; Dunn et al., 1996c). They were
found to be the most widespread species in the areas studied, and they are reported
to be present in many countries bordering the Mediterranean and in the Middle
East.
Anvillea garcinii (Arabian oxeye)
Convolvulus trabutianus (morning glory)
Ononis natrix (goat root)
Lavandula spp. (lavender). There are several species, of which the most common is
L. multifida (fer n leaf lavender) and considered a weed because of its ability to
grow in a wide range of environments. This ubiquitous feature is advantageous for
a biogeochemical sample medium
Artemisia herba-alba (sageb rush or wormwood)
Haloxylon (salt tree)
Mediterranean coast
Lavandula spp. (lavender). The L. multifida (described above) and L. stoechas
(French lavender) both occur. L. stoechas is the more common, and grows through-
out the Mediterranean region. It is found, also, in Australia where it has been de-
clared a ‘noxious weed’ in Victoria
Erica arborea (heath tree), found throughout the Mediterranean
Pistacia terebinthus (terebinth or turpentine tree)
Tetraclinis articulata (sandarac [national tree of Malta])
Quercus ilex (holly oak)
Cistus and the similar genus Halimium (both known as rock rose)
53
Biogeochemistry in Mineral Exploration
Middle East – from Turkey and Armenia, through Saudi Arabia to Pakistan,
and Africa east of the Sahara
A short list of common plants that could be used for biogeochemical exploration
includes Acacia tortilis and Acacia negrii (and other Acacia species), Juniperus
excelsa (juniper), Olea europaea (olive tree), Cleome (spider flower), Dodonaea
viscosa (hopbush), Pulicaria crispa (with the appearance of a small sun-flower or
large daisy), Rumex limoniastrum (a type of dock or sorrel; this is a weed with sour
leaves that are unpalatable to most animals, so it is quite common), Astragalus
atropilosus (poison vetch) and Achillea biebersteinii (similar to the common yarrow of
North America).
Armenia has a particularly rich flora that ranges from wet forest to desert steppe
with more than 3500 plant specie s representing more than half of the 6000 contained
within the entire Transcaucasus region. There are 17 vegetation zones ranging from
desert plants to oak, beech and pine forests, wet marshland and sub-tropic plants to
alpine meadows. For the arid regions, Astragalus spp. and Achillea would be good
starting points.
Central, West and South Africa
Some of the earliest geobotanical and biogeochemical studies were conducted in
Central Africa (Rose et al., 1979), and in recent years there has been a resurgence of
interest in the use of vegetation for exploration in this area. M uch of this new
information is currently proprietary in nature, so suffice to say that there have been
some interesting elemental concentrations discovered, and the biogeochemical
method has been achieving considerable success in locating mineralization.
Among the plants for which non-confidential information is available there are
various species of acacia including the following:
Acacia mellifera (Black Thorn), widespread throughout much of sub-Saharan Af-
rica and north-eastward into Egypt. One potential problem as a biogeochemical
sample medium is that goats are very fond of the young leaves!
A. seyal, a useful strongly gregarious tree species that is tolerant of periodically
inundated heavy clays in countries at the southern edge of the Sahara Desert, es-
pecially Mali, Chad and Sudan, and southward into Tanzania. The word seyal
derives from an Arabic word for ‘torrent’ and denotes association with water courses.
A. polyacantha (‘White thorn’, ‘Falcon’s claw’ or ‘catechu tree’) is one of the most
attractive species of acacias in Africa, occurring from Ethiopia southward through
Kenya, Tanzania and Rwanda.
Blepharis (a wildflower, consider ed a weed, with no common name), a common
species throughout tropical and South Africa (Nkoane et al., 2005).
Boswellia (frankincense) in dry areas of Nigeria (Okujeni, 1987).
Aformosia in Nigeria and Namibia.
54
Field Guide 1: Climatic and Geographic Zones
Brachystegia spp. (Prince of Wales feathers) common tree in eastern and southern
Africa. Nitrogen-fixing.
Combretum (large leaved forest bushwillow), a common plant of tropical and sub-
tropical Africa.
Acacia, as in many parts of the world (Fig. 3-6), is widely present throughout
Africa, and many species have been tested for their potential use in biogeochemical
prospecting. Extensive lists of plants are given in Brooks and Malaisse (1985) and
Brooks (1987). The chemistry of common plant species growing over kimberlites in
Central and South Afr ica has been examined, and is worth further con sideration. At
present, details are proprietary.
India
The sub-continent of India has many climatic regimes and a wide diversity of
flora. Details of a number of local biogeochemical studies using a diverse array of
local plants have been reported in the literature. Among the most common that are
encountered across the country are mesquite (Prosopis juliflora) and acacia. In
northern India Berberis (barberry) has been found to accumulate U. Elsewhere,
Impatiens (busy-lizzy) and Lindenbergia are reported to accumulate high concentra-
tions of Zn. The liana Combretum decandrum is reported to accumulate high levels of
Ni. A website by the Botanical Survey of India presents a summary of common
plants of India along with some excellent coloured illustrations: http://envfor.nic.in/
report/0001/chap02.html.
Summaries of most of the more important studies can be found in the Journal of
Geochemical Exploration (abstracts on the CD in the back pocket) and publications
of the Indian Academy of Science.
Fig. 3-6. Worldwide distribution of the genus Acacia.
55Biogeochemistry in Mineral Exploration
China, Mongolia and Japan
The spectru m of climatic regimes across China ranges from desert in the north-
west, through several steppe zones extending from Tibet to Mongolia, and south-
ward into a forest zone. There are few published biogeochemical studies from China.
For those interested in conducting biogeochemical surveys there are several useful
websites for summaries of the flora of China.
http://flora.huh.harvard.edu/china/
http://www.icimod.org/focus/biodiversity/chibio.htm
http://www.fao.org/ag/agl/swlwpnr/reports/y_ea/z_cn/en/text_e/d3.htm
The last gives succinct accounts of what to find in each climatic region and from
these summaries it is evident that there are many species similar to elsewhere in the
world. For more detailed information there is the recent publication of the
1:1,000,000 ‘Vegetation Map of China’ (Xueyu Hou, 2001). It augments the mon-
ograph ‘Vegetation of China’ (1980). This new publication comprises 60 separate
1:1,000,000 sheets; it covers the whole of China and illustrates the geographical dis-
tributions of 11 groups of vegetation types, 796 formations and many smaller cat-
egories. It is a huge publication, weighing 5.8 kg and is in Chinese, but with English
captions to the illustrations. Coupland (1993) provides more focused information on
grasslands, not only in China, but across Europe and the rest of Asia. Other books in
the 30-volume series entitled ‘Ecosystems of the World’ (series editor D.W. Goodall,
published by Elsevier) cover the full spectrum of the world’s ecosystems.
The forests of China can be divided into boreal coniferous, temperate, subtropical
and tropical coniferous forests. Ther e are also deciduous broadleaved forests, ever-
green broadleaved forests, tropical seasonal rain forests, rain fores ts and mangrove
forests. Although there are many species endemic to China, the many genera are
common to other forested regions of the world – e.g., the conifers pine (of which
seven species are endemic), spruce, fir, larch, hemlock; deciduous forest dominated
by birch, beech or oak. Given the general paucity of information available on the
biogeochemistry of Chi na, an orientation survey would be advisable. However, there
are many genera from other locales around the world for which some biogeochemical
information is available, and so information published on similar specie s would
provide some focus for a survey.
The more arid areas of China and Mongolia have plants common to many cold to
temperate arid zones, such as the sagebrush (Artemisia). In the extensive grasslands
of Inner Mon golia there are not many species to cho ose from, and at first sight there
appear to be virtually noth ing other than grasses. However, closer examination does
reveal a number of quite common plants that are not immediately eaten by grazi ng
animals – including sagebrush, caragana, Russian thistle (tumbleweed, Salsola spp.),
and poison vetch (Astragalus spp.). Potaninia mongolica (a member of the rose fam-
ily) has been reported to be effective at con centrating Cu (Kong Ling-shao et al.,
1992). Other bushes and shrubs used in exploration include Anabasis (a succulent
halophyte) and species of the onion genus Allium.
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Field Guide 1: Climatic and Geographic Zones
In Mongolia there are two principal vegetation zones; the Siberian taiga in the
north, dominated by cedar and larch forests, and the Central Asian steppe-desert in
south. The prevailing types of steppe plants are various kinds of Miscanthus (feather
or Japanese silver grass), Artemisia (sagebrush) and Potentilla (cinquefoil). Most
important in Mongolia’s desert steppes are Piptanthus mongolicus (a leguminous
shrub); Convolvulus (‘morning glory’, of which a related species was found to be of
use in southern Morocco (Dunn et al., 1996b)); and Oxytropis (one of the so-called
‘locoweeds’, because when eaten, the poisonous alkaloids that they contain cause
peculiar behaviour in animals, or even death). Carex (sedge) is widespread and could
be of use for, in particular, base metal exploration. Although there are no known
case histories from Mongolia, Carex roots are traditional sample media in Sweden
(Brundin et al., 1987) and Kabata-Pendias (2001) reports that most species seem to
be barrier-free with respect to the translocation of Pb, and exhibit low-barrier char-
acteristics for Ni, Cr and Mo.
The long chain of islands comprising the Japanese archipelago encompasses a
wide range of vegetation zones. In the south of Japan the Japanese beautyberry
(Callicarpa mollis) was found to be an effective indicator of Au mineralization
(Yoshiyuki Kita et al., 1992). Other specie s of use were the fern Arachnioides aristata,
the fig (Ficus erecta) and the artichoke (Gleichenia japonica). The leaves of pepper-
bush (Clethra barbinervis) are known to accumulate certain heavy metals, and bulk
samples of this species have been collected by the Japanese National Institute of
Environmental Sciences for use as a Standard Reference M aterial (SRM-NIES-1).
Elsewhere in Japan, because of the lack of published studies it would be advisable to
conduct an orientation survey before launching into a sampling programme.
Australia
Of the total Australian landmass, 75% falls into four classes of vegetation: Open
Forest (4.6%, mostly eucalypt); Woodlands (13.9%, mostly eucalypt and acacia);
Open W oodlands (25.8%, eucalypt, acacia and cleared grazing land); and Tall
Shrublands (31.1%, mallee, mulga and arid acacias).
There are more than 800 species of eucalypts. Much more detailed research is
required on the chemistry of these species and their sensitivities to underlying min-
eralization. However, it is perhaps not as daunting a problem as it might first appear,
because the vast majority of species are aggregated into groups of related species,
based on the widely held assumption that Eucalyptus began as a more or less single
evolutionary event and that all extant species are descendants. In fact the Australian
National Herbarium divides the 800 or so species of Eucalyptus into just 13 sub-
genera, 2 of which are the ghost gums (subgenus Blakella) and bloodwoods (sub-
genus Corymbia) that constitute the single genus Corymbia. So as far as the true
eucalypts are concerned there are really only eleven subgenera that need careful
evaluation and comparison.
57
Biogeochemistry in Mineral Exploration
Worldwide there are more than 1350 species of acacia. About 1000 acacia species
occur in Australia, and of these, about 955 belong to the subgenus Phyllodineae – the
familiar wattl es. Wattle is an important plant in Australia, such that the national
floral emblem is Acacia pycnantha, the Golden Wattle.
Plant genera in Australia for which biogeochemical exploration data are available
include the following:
Eucalypts (‘gum’)
Acacia
Saltbush
Poverty bush
Paperbark
She-oak
Bottlebrush
Various specie s – e.g., Red River Gum (Eucalyptus cam-
aldulensis)
e.g., Acacia aneura (‘mulga’). Note: a mature tree is usu-
ally >100 years old, which is unusually long-living for
acacias
Atriplex
Eremophila
Melaleuca
Casuarina
Callistemon
A comprehensive website for obtaining detailed informat ion on Australian plants
can be found at http://www.anbg.gov.au.
South America (excluding the Amazon)
The principal widespread plants of potential value for biogeochemical exploration
in the arid regions of South America include the following:
Baccharis spp. (Thola bush of the Andean highlands, or Carqueja of southern
Brazil) is one of the most common plants from the drier areas of South America
(Viladevall and Queralt – see Chapter 11 – case history for Au). In southern Brazil
some local studies have examined a number of species, including Baccharis trimera
(Lima e Cunha et al., 1997). This genus is probably the most studied from a
biogeochemical exploration perspective, and, because of its widespread occurrence
and proven ability to accumulate metals it is a high priority biogeochemical sample
medium.
Larrea spp. is another candidate for biogeochemical exploration from this envi-
ronment. There are five sp ecies of this evergreen shrub, represented in North
America by the Creosote bush Larrea tridentata, and by the ‘Jarillas’ of South
America where species are closely related. It is suffici ently common to be classified
as a weed.
Parkinsonia spp. (formerly Cercidium spp., ‘Palo verde’) occu rs in Argentina,
Mexico, the southern USA, as well as semi-desert areas of Africa. It has been
tested in a few studies and shown to yield high levels of some trace metals, notably
Au, Ba and Br.
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Field Guide 1: Climatic and Geographic Zones
Bulnesia spp. (Palo santo, locally known as ibiocaı
´
) is a tree that occurs on the
Gran Chaco of southern Bolivia and Paraguay. Results have indicated, mostly,
quite low levels of trace metals although modest concentrations of molybdenum.
Humid tropics – Amazon, West Africa, Indonesia
The great diversity of flora in the tropics poses a significant initial problem for
conducting biogeochemical surveys. Interestingly, the earliest report of a biogeo-
chemical study, anywhere in the world, was in tropical forest, using wood from the
Baromalli (Catostemma fragrans) to look for gold in Guyana (Lungwitz, 1900).
In the tropics especially, an effective approach to setting up a biogeochemical
programme in a region of diverse flora is to conduct some preliminary investigations
by visiting botanical gardens, and talking to local botanists at museums and uni-
versities. Also, the use of local guides familiar with the jungle can be of immense
value to a sampling programme – they usually have a fundamental knowledge of the
flora and can point toward species that are commonly encountered. No special
equipment is required – however, the machete commonly used in jungle environ-
ments can be a valuable tool.
In the Tapajos area of the Amazon (Para
´
State) common species that have proved
of value include the following (Dunn and Angelica, 2000).
Imbauba (foliage and bark) Cecropia sp. (there are several species, identified by
leaf shape and fruits)
Vassoura de Bota
˜
o (twigs
and foliage)
Borreria verticillata
Boa macaca (foliage) Parkia ulea
Banana braba (foliage) Ravenala guianensis
The outer bark from the Brazil nut (Castanha-do-para
´
) and bark from other large
trees can be of use.
Tree ferns (Cyathea) from the western Amazon have provided good responses to
Au mineralization and Mo porphyries. Other species, such as the liana Clusia spp.
were less informative. Surveys using bamboo-like plants (Chusquea spp.) from the
same general area have met wi th limited success in that they pro ved to be the least
sensitive of several plants collected at the same sample stations. In a study that
included another bamboo-like plant, Ischnosiphon, collected near the Igarape
´
Bahia
deposit in Para
´
State results were mixed (Machesky et al., 1993).
In the tropics there are many trees that have distinctive shapes or vein patterns to
their leaves, assisting the non-botanist in identifying the frequency of plants in a
given survey area. In Papua New Guinea and throughout Melanesia there is the tree
Astronidium paluense that has a very distinctive leaf and has been of use in locat ing
Au/As mineralization by the biogeochemical method (McInnes et al., 1996).
59
Biogeochemistry in Mineral Exploration
SUMMARY NOTES
Although there are millions of plant species, the world can be divided up into
zones with discrete plant assem blages. Fortunately for the biogeochemical method,
there are commonly only a few common plant species growing within an area of
interest to an exploration programme that are of sufficient biomass for sampling.
Almost invariably, after an hour or two of reconnoitring a survey area while focusing
on the vegetation, the realization dawns that certain species are dotted around at
intervals that render them suitable for conducting a survey.
In the northern forests the task of finding a suitable sample medium is relatively
simple, with only a handful of common trees, most of which are distinct in appear-
ance and easy for the non-botanist to identify after only a short period of instruction.
In arid terrains there are also only a few common species within the limited geo-
graphic area that is usually the extent of a mineral claim, and most are quite easy to
recognize once the key features have been explai ned.
The temperate rain forests have greater diversity and present a greater challenge.
A day or so with a botanist, or a local worker familiar with an area can be invaluable.
Similarly, in the tropics where there is a great diversity of plants, a little advance
planning by talking to a local botanist at a university or a museum before go ing into
the field, can provide considerable focus for what to expect. Simple publications of
museum guides to local flora are an excellent investment, and so is time spent in the
field with local guides who commonly have amazing in-depth knowledge of the local
flora. Not only are they able to provide local names (which can be checked later in a
library to find a botanical name if this is required), but they also have an intimate
knowledge of where certain plants grow and what their uses might be – either as
timber, foodstuffs or for medicinal use. Such information can give the field geologist
clues that can be very useful for a field survey even if they sometimes seem a little
obtuse. As an example, one of the pioneers of exploration biogeochemistry, the late
professor Robert Brooks, quotes in his 1983 book an account from Pagliuchi (1925)
of a bird that the locals described as ‘el minero’ (the miner).
this bird is always found in the vicinity of gold mines or wherever gold is abundant.
Thereafter, as I rode through the country, wherever I hear the cry of ‘el minero’ either I
saw a quartz ledge extending across the trail or was able to find one by looking about a
little. Apparently the natives were correct y Finally a miner told me that if I wanted gold I
must always look for the mora
1
tree y the solution of the puzzle dawned on me. The mora
tree grows and thrives only on siliceous ground, and ‘el minero’ feeds on berries of this tree;
hence, whenever his cry is heard, one is assured of finding a quartz vein in the vicinity. Of
1
The mora tree described by Pagliuchi is probably the large shrub Rubus glaucus – the Andes Berry
(known locally as ‘mora’) and not the tree Mora excelsa that Brooks assumed it to be, because the Andes
Berry fits better with the fact that it has berries that ‘el minero’ feeds on. Mora excelsa has large woody
pods, and no berries.
60 Field Guide 1: Climatic and Geographic Zones
course, the quartz may or may not be auriferous, but when prospecting in a country known
to be auriferous, it must be admitted that the bird is a helpful guide to the prospector.
In this chapter, lists of flora have been provided for most of the key climatic areas
of the world where biogeochemical exploration might be contemplated. The lists of
plants are, intentionally, short and not all plants for which data have be en published
are referenced. Exhaustive lists can be found in the literature, but they tend to be
rather bewildering to the non-botanist. The exploration geologist has enough to
consider with the physical environment and the rocks themselves, and so short lists of
plants are more easily absorbed. The lists are intended to provide a focus – a starting
point, because for many of the plants listed others have gone ahead a nd tested them
for their biogeochemical exploration potential. In any given area other species may
be present that would warrant testing. The science is sufficiently young that new
surprises concerning element enrichments in plants are regularly being made, adding
to the excitement for the field geologist and the exploration industry in general.
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Biogeochemistry in Mineral Exploration