Dunn Colin E. Biogeochemistry in Mineral Exploration

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Chapter 8

THE EDEN PROJECT – SOURCE OF A BIOGEOCHEMICAL DATABASE

INTRODUCTION

This chapter describes a valuable source of biogeochemical data obtained from

plants growing in immense greenhouses comprising the Eden Project. The complex is

located in disused kaolinite quarries cut into Hercynian granites near St. Austell in

south-western England. Conceived and built in the 1990s by Tim Smit, the project was

designed primarily as an educational and research facility ‘to promote the understand-

ing and the responsible management of the vital relationship between plants, people and

resources’. In addition, it serves as a unique natural laboratory for comparing the

relative uptake of elements by a wide range of plants growing under controlled con-

ditions.

The structures comprise two sets of four giant, translucent, geodesic domes, each

emulating a natural biome (an area on the earth with similar climate, plants, and

animals), that house plant species from around the world (Fig. 8-1). Collectively they

comprise the largest greenhouse structures in the world. The larger biome, covering

15,590 square metres (3.8. acres) susta ins the temperature and humidity conditions of

a tropical environment; the smaller biome covers 6,540 square meters (1.6 acres) and

is regulated to provide a warm temperate, Mediterranean-type environment.

SAMPLES IN THE COLLECTION

Plants from selected areas of the world were collected and planted in these biomes

under the temperature and humidity conditions that mimic those from the hot trop-

ical regions (HTB – including the Amazon, West Africa, Malaysia and the Seychelles

Islands), and warm temperate regions (WTB – notably the Mediterranean, South

Africa and California). To date there are few typical plants from Australasia. A third

dome to house plants from desert environments is planned for some future date.

There is a strong educational theme to the e ntire Eden Project; hence most of the

plants were selected because of their importance to human needs, including medicine,

food, dyes, textiles, fuel, construction materials and the environment in general.

Consequently, they are mostly common plants and represent, theref ore, good po-

tential candidates to select for minera l exploration.

Among the unique aspects of the Eden Project is the fact that many species of

trees, shrubs and smaller plants are growing in controlled conditions. Each biome has

its own controlled temperature, humidity and soil type. As such, the uniform con-

ditions under which the plants are growing provide an opportunity to evaluate the

relative uptake of a wide range of elements by common plants. The chemical analysis

of tis sues from plants growing under the same conditions provides a guide for se-

lecting optimum species to collect in the biogeochemical exploration for a particular

commodity and/or for environmental monitoring. Furthermore, such studies can

reveal if any species have the ability to hyper-accumulate metals: such information

would be of relevance to any efforts directed at phyto-mining or phyto-remediation.

The Eden Project, therefore, provides a unique opportunity to compare and contrast

the relative uptake of metals and other elements from diverse areas of the world

without the requirement of undertaking lengthy and expensive expeditions to obtain

comparable materials, which even then would almost certainly have different subst-

rate compositions making inter -species comparisons uncertain.

SOIL IN THE BIOMES

The soil in the biomes is 1 m deep. The air is kept at temperatures varying between

35 1C and 18 1C, providing a variety of environmental niches in different areas of the

biomes. A misting system and a large waterfall provide the high humidity that the

species in the HTB require. Conversely, in the WTB the dry, warm and low humidity

conditions of Mediterranean climates are maintained. The University of Reading

helped devise speciﬁc soil recipes for the two biomes and 85,000 tonnes of soil was

manufactured from a combination of mine waste (mainly silica sand and lignitic clay)

and composted green waste. Inside the HTB, recycled woodchip is used as mulch.

The Eden Project also uses other recycled materials on a day-to-day basis, including

soil improvers such as composted green waste, bark and forestry wastes.

In both biomes the organic-rich soils have been spread over a base of sand-sized

particles obtained from the immediate vicinity of the kaolinite quarries. They are,

therefore, granitic in composition. Locally, fertilizers have been applie d to provide

Fig. 8-1. The Eden Project, Cornwall, England. Left: the two biomes. Right: inside the Hot

Tropical Biome (HTB).

214 The Eden Project – Source of a Biogeochemical Database

the essential nutrients for maintaining the health of certain species – e.g., elevated

levels of K and Mg for banana plants (Musa spp.). Although there are these local

modiﬁcations, soils throughout the HTB and WTB are generally similar in overall

elemental composition, although the HTB soils contain double the amount of or-

ganic matter of that in the WTB (60% compared to 30%). There are some differences

between the soils of the two biomes, but the only element that is much more highly

concentrated in one biome (the HTB) is Ag. Table 8-I presents average concentra-

tions of 53 elements obtained from analysis of the 80 mesh (ASTM) fraction of

soils collected from six widely spaced sites in each biome. To put these data in

context, they are compared with 104 soils from a natural environm ent in central

British Col umbia – Mount Polley, where Cu–Au porphyry mineralization occurs. All

soils were prepared in the same manner by sieving, and were then analyzed at the

same laboratory by digesting in aqua regia with an ICP-MS ﬁnish.

FOLIAGE COLLECTION FROM THE BIOMES

In order to assess the relative uptake of elements by various species contained

within each Eden Project biome, as a ﬁrst step, representative samples of leaf tissues

were collected from a selection of the more common plants. Within a four-hour

period, a few leaves from each of 25 species from the WTB and each of 46 species

from the HTB were collected, generating a total of 71 samples. These were oven-

dried, hermetically sealed, and sent to Canada for milling prior to analysis. One-gram

portions of each powdered sample were digested in nitric acid, then aqua regia, with

an ICP-MS ﬁnish for 51 elements. Tables 8-II and 8-III list the species by biome and

geographic area, and provide botanical and common names as well as some noted

features.

There are many ways to view the data obtained, hence the complete analysis of

each sample is provided on the CD inside the back cover of this book. One per-

spective is to sort the data with respect to the maximum concentrations of each

element and the relative concentration with respect to the mean of the dataset

(‘anomaly contrast ratio’). Results of this exercise are shown in Table 8-IV. From

this table it can be seen that the species concentrating germanium to the greatest

degree is bamboo, with a concentration factor (maximum divided by the mean) of

43.6. The ‘star cluster’ (Pentas) has strong relative enrichment of several elements –

Co (x40), W (x25), Al (x6.5), Th (x6). Fig (Ficus) is enriched with REE (La x34 and

Ce x26) and Tl (x14.7). Gold enrichment is the greatest in kapok bush (Eriocephalus).

Noted above is the fact that Ag is significantly more enriched in soils of the HTB

(average of 16,954 ppb Ag from 6 samples) compared to those in the WTB (average

of 162 ppb Ag from 6 samples) It might be expected, therefore, that plants in the

HTB would have appreciably higher Ag concentrations than those in the WTB and,

in fact, the majority of plants containing the highest levels of Ag occur in the HTB.

However, this pattern is not universal. For example, Acacia nigrescens (HTB) yielded

215

Biogeochemistry in Mineral Exploration

TABLE 8-I

Element concentrations in 80 mesh (ASTM) fraction of soils from the two Eden Project

biomes, and from over a mineralized zone (Cu–Au porphyry) at Mount Polley in British

Columbia. Full analyses of the Eden Project soils are given as Table 8-I D on the CD (back

pocket)

Average

HTB (n ¼ 6) WTB (n ¼ 6) Mt.Polley

(n ¼ 104)

Ag (ppb) 16953 162 274

Al (%) 0.442 0.496 2.038

As (ppm) 9.3 9.9 6.2

Au (ppb) 2.4 4.0 11.0

B (ppm) 16 12 3

Ba (ppm) 64 54 135

Be (ppm) 1.0 0.9 0.5

Bi (ppm) 1.75 2.53 0.16

Ca (%) 0.868 0.527 0.454

Cd (ppm) 0.35 0.22 0.25

Ce (ppm) 12 16 17

Co (ppm) 2.2 2.9 11.5

Cr (ppm) 9.3 8.0 39.7

Cs (ppm) 15.0 18.9 2.6

Cu (ppm) 30 24 92

Fe (%) 0.492 0.543 3.384

Ga (ppm) 3.0 3.0 6.7

Ge (ppm) –0.1 –0.1 –0.1

Hf (ppm) 0.05 0.07 0.06

Hg (ppb) 129 92 57

In (ppm) 0.06 0.07 0.02

K (%) 0.257 0.207 0.094

La (ppm) 5.8 7.5 9.6

Li (ppm) 61 83 21

Mg (%) 0.093 0.104 0.554

Mn (ppm) 235 253 642

Mo (ppm) 1.48 0.81 1.44

Na (%) 0.031 0.019 0.010

Nb (ppm) 1.36 1.80 0.82

Ni (ppm) 4.9 5.2 22.6

P (%) 0.083 0.053 0.112

Pb (ppm) 37 33 8

Pd (ppb) –10 –10 –10

Pt (ppb) –2 –2 –2

Rb (ppm) 51 71 12

Re (ppb) –1 –1 –1

Continued

216 The Eden Project – Source of a Biogeochemical Database

only 85 ppb Ag, whereas the underlying soil contained 74,600 ppb Ag – a plant to soil

ratio of 1:878. Conversely, elsewhere in the HTB, rattan (Calamus) contained 9,670

ppb Ag (the highest of all the species tested), yet the underlying soil had only 3,800

ppb Ag – a plant to soil ratio of 1:0.39. A similar ratio to the latter occurred in the

WTB for the Cape myrtle (Myrsine africana) that had 754 ppb Ag, whereas the

underlying soil had 447 ppb Ag (a plant to soil ratio of 1:0.59).

Where available, because of its ability to concentrate Ag, the rattan would be a

better choice of sample medium than the Acacia. Similarly, in warm temperate re-

gimes, the Cape myrtle would be a good choice for optimizing the Ag signature of the

underlying substrate, and therefore potentially a useful medium in biogeochemical

exploration for Ag-bearing minerals. These examples of Ag serve to demonstrate

further the enormous ranges in the capacities of different species to either exclude or

absorb and tolerate metals. In the case of Acacia leaves, they would not be the

optimum sample medium for Ag, but they should not be totally disregarded because

over mineralization there may be adequate anomaly to background contrast, such

that the patterns of relative enrichment can still be mapped. This reafﬁrms another

point – pattern recognition is of paramount importance in geochemical interpreta-

tions of data.

TABLE 8-I Continued

Average

HTB (n ¼ 6) WTB (n ¼ 6) Mt.Polley

(n ¼ 104)

S (%) 0.208 0.094 0.007

Sb (ppm) 1.40 1.15 0.28

Sc (ppm) 1.1 1.2 3.8

Se (ppm) 1.0 0.7 0.3

Sn (ppm) 3.6 3.5 0.5

Sr (ppm) 36 24 57

Ta (ppm) –0.05 –0.05 –0.05

Te (ppm) –0.02 –0.02 –0.02

Th (ppm) 2.0 3.5 2.0

Ti (%) 0.005 0.006 0.103

Tl (ppm) 0.317 0.397 0.066

U (ppm) 3.4 3.1 0.5

V (ppm) 10 8 97

W (ppm) 2.5 2.3 0.1

Y (ppm) 5.6 6.6 4.0

Zn (ppm) 80 63 98

Zr (ppm) 1.5 2.4 2.8

B-horizon soils from over Cu porphyry mineralization at Mount Polley, British Columbia

(from Dunn et al., 2006a,b).

217Biogeochemistry in Mineral Exploration

TABLE 8-II

List of samples from the Warm Temperate Biome from which leaves were collected for analysis. Samples from each source area sorted

alphabetically by botanical name. Detailed chemical analyses provided on CD in back pocket (Table 8-II D)

Sample # Botanical name Family Common name Notes on some characteristics

WTB Mediterranean

MED 7 Asphodelus ﬁstulosus Liliaceae Onion asphodel;

onionweed

Suspected cause of dermatitis in cattle

MED 3 Ballota acetabulosa Labiatae False dittany Member of the mint family. Stems once

used as wicks for oil lamps

MED 10 Bougainvillea spp. Nyctaginaceae Bougainvillea

MED 8 Chamaerops humilis Palmae Dwarf fan palm

MED 2 Cistus laurifolius Cistaceae Rock rose

MED 1 Ficus carica Moraceae Fig The sap and the half-ripe fruits are said

to be poisonous. The sap can be a serious

eye irritant

MED 4 Nerium oleander Apocynaceae Oleander; Rose bay Poisonous sap – glycosides. A single leaf,

intensively chewed, has been reported to

be lethal

MED 12 Olea europaea Oleaceae Olive Leaf properties include antibacterial,

antifungal, antiseptic, antiviral,

astringent, febrifuge, and tranquilizer

MED 6 Pinus halepensis Pinaceae Aleppo pine Resin provides ﬂavour for Retsina wine

MED 9 Pistacia lentiscus Anarcardiaceae Mastic Chewed to strengthen the gums, and as a

breath sweetener; used as a ﬂavouring in

sweets (e.g., ‘Turkish delight’)

MED 11 Rhododendron (vireya) Ericaceae Rhododendron

MED 5 Tetraclinis articulata Cupressus Arar Endangered species in Malta and S.

Spain

218 The Eden Project – Source of a Biogeochemical Database

WTB South Africa

SA 3 Aloe arborescens Asphodelaceae Krantz aloe Used in cosmetics and medicinal

compounds. Ingesting the plant latex can

cause a cathartic action and nephritis

SA 12 Blechnum tabulare Blechnaceae Table Mountain fern

SA 2 Cunonia capensis Cunoniaceae Butterspoon tree; red

alder

Member of the lily family

SA 9 Erica caffra Ericaceae Heather

SA1 Eriocephalus africanus Compositae Kapok bush Linalyl acetate, cymene, 1,8-cineole,

many sesquiterpinoids

SA 10 Leucadendron argenteum Proteaceae Silver tree

SA 13 Myrsine africana Myrsinaceae Cape myrtle; African

boxwood

Leaves have benzoquinone derivatives,

triterpenoids and steroids. Acylated

cyanidin glycosides

SA 14 Nuxia ﬂoribunda Loganiaceae;

(Buddlejaceae)

Kite tree; forest elder

SA 11 Protea cynaroides Proteaceae King protea

SA 6 Rhumora spp. Polypodiaceae Fern Probable species is ‘adiantiformis’

SA 5 Salvia chamelaeagnea Labiaceae Rough blue sage Leaves produce rosmarinic acid.

Medicinal use as a tea for coughs and

colds

SA 4 Sparrmannia africana Tiliaceae African hemp Proanthocyanidins present; cyanidin.

Not cyanogenic

SA 8 Stoebe plumosa Asteraceae Slangbos

SA 7 Tulbaghia fragrans Alliaceae Sweet garlic

219Biogeochemistry in Mineral Exploration

TABLE 8-III

List of samples from the Hot Tropical Biome from which leaves were collected for analysis. Samples from each source area sorted

alphabetically by botanical name. Detailed chemical analyses provided on CD in back pocket (Table 8-III D)

Sample # Botanical name Family Common name Notes on some characteristics

HTB Islands (Seychelles)

Is 1 Breynia nivosa Euphorbiaceae Snowbush

Is 7 Casuarina

cunninghamiana

Casuarinaceae River oak: She-oak Nitrogen-ﬁxing

Is 3 Chrysobalanus icaco Chrysobalanaceae Cocoplum Various parts of the plant have been

used in folk medicine. It has

hypoglycemic effects

Is 5 Coccoloba uvifera Polygonaceae Sea grape Salt tolerant

Is 6 Cocos nucifera Palmae Coconut palm

Is 10 Cryptostegia grandiﬂora Asclepiadaceae Rubber vine Contains cardiac glycosides

Is 12 Dendrocalamus asper Poaceae Bamboo

Is 4 Hyophorbe lagenicaulis Palmae Bottle palm

Is 11 Impatiens spp Balsaminaceae Busy-lizzie

Is 2 Mangroves Many Mangrove About 80 species, 9 in Seychelles. All

salt-tolerant and high in tannins.

Contain salts, organic acids,

carbohydrates, hydrocarbons,

benzoquinones, naphthofurans,

sesquiterpenes, triterpenes, alkaloids,

ﬂavonoids, polymers, and sulphur

derivatives

Is 9 Rothmannia annae Rubiaceae Wright’s gardenia

Is 8 Terminalia catappa Combretaceae India’s almond Leaves contain agents for chemo-

prevention of cancer and probably have

anticarcinogenic potential. Antioxidant

220 The Eden Project – Source of a Biogeochemical Database

HTB Malaysia

MAL 8 Calamus spp. Arecaceae Rattan

MAL 6 Cananga odorata Annonaceae Ylang-ylang

MAL 2 Etlingera elatior Zingiberaceae Torch ginger

MAL 1 Ficus auriculata Moraceae Fig

MAL 5 Ficus religiosa Moraceae Bo tree

MAL 3 Musa spp Musaceae Banana

MAL 4 Pogostemon cablin Lamiaceae Patchouli Oil used to treat colds, headaches,

nausea, diarrhea and abdominal pain.

Popular aphrodisiac

MAL 7 Syzygium cumini Myrtaceae Java plum Antibacterial properties

TP2 Tectona grandis Verbenaceae Teak Malaysia

HTB West Africa

WA 4 Elaeis guineensis Palmae African oil palm

TP 1 Musa spp. Musaceae Banana

WA 5 Mussaenda

erythrophylla

Rubiaceae Flame of the forest

WA 3 Nauclea diderrichii Rubiaceae Box wood

WA 6 Pentas lanceolata Rubiaceae Star cluster

WA 1 Tabernanthe iboga Apocynaceae Leaf of God Aphrodisiac, CNS-stimulant,

hallucinogenic, stimulant, and tonic

WA 2 Thaumatococcus

daniellii

Marantaceae African serendipity

berry

Natural sweetener ‘thaumatin’

WA7 Thunbergia erecta Acanthaceae King’s mantle

HTB South America

Sam 9 Acacia nigrescens Fabaceae Knob thorn

Sam 15 Allamanda cathartica Apocynaceae Golden trumpet vine Poisonous. Fever, swollen lips, thirst,

nausea, diarrhea. Skin irritation upon

contact with cell sap

Continued

221Biogeochemistry in Mineral Exploration