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antibody in a sample (Blake et al., 1998; Johnson, 2003). A number of antibodies for

heavy metal complexes have been developed. Methods to test for Hg, Cd, Co, Pb and

U have been successfully applied in ecotoxicological studies and may be transferable

to exploration purposes (Blake et al., 1998; Johnson, 2003).

Fig. 12-10. The use of DNA microarrays in differential gene expression analysis (adapted

from Albelda and Sheppard, 2000). Comparative hybridization experiment involves isolation

of messenger RNA (mRNA) from two separate samples, e.g. bacteria grown in media without

and amended with different concentrations of heavy metals. The mRNA from each sample is

treated with reverse transcriptase and labelled with a distinct ﬂuorescent tag. The two pools of

labelled RNA are mixed, hybridized to the DNA microarray containing a full set of thousands

or tens of thousands of DNA sequences based on genome or complementary DNA (cDNA)

sequences, and washed. The microarray is scanned using a specialized imager, and the colour

of each spot is determined. In this example, genes expressed dominantly in Sample A would be

red in colour, genes expressed dominantly in Sample B would be green and those genes

expressed equally in both samples would be yellow.

412 Exploration Geomicrobiology – the New Frontier

CONCLUSIONS

This chapter has highlighted the use of emerging molecular biology techniques in

the ﬁeld of exploration geomicrobiology. Exploration geomicrobiology deals with

microbially mediated processes that control the dispersion and re-concentration of

economically important major and trace elements (Fig. 12-1). A number of novel

molecular tools and techniques allow study of complex microbial consortia in soil

materials and biogeochem ical mineral transformations driven by these microbiota,

such as trace metal dissolution or precipitation. The case studies show how these

classical and molecular methods have been used to establish the role of microbiota in

the cycling of Au in the Australian regolith; how the presence of Zn in soils inﬂuences

the composition of the natural bacterial communities; and how biosensors may

provide a quick and reliable tool for mineral exploration.

By identifying biogeochemical processes that lead to the dispersion and accumu-

lation of trace elements in soil, and quantifying the reaction kinetics of these proc-

esses in different materials, this new ﬁeld of research will help mineral explorers to

develop predictive biogeochemical modelling tools (Fig. 12-1). Ultimately, it will be

possible to incorporate appropriate data into numerical geochemical models to pre-

dict dispersion, transport and concentration of trace elements in and around min-

eralized zones. Identiﬁcation of the microbiota and microbial processes that control

the solubilization, transport and precipitation of trace metals in the soil should lead

to the discovery of indicator micro-organisms and the development of microbial

biosensors for detecting concealed mineralization. However, for this to become

reality molecular microbial biosensor methods must ﬁrst demonstrate their full

potential as well as their cost competitiveness and beneﬁt over existing methods.

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Chapter 13

A LOOK TO THE FUTURE

INTRODUCTION

In the mid-20th century, Professor Harry Warren and his associates did much to

raise the credibility of biogeochemistry in mineral exploration from a level of, as

he eloquently put it, ‘general disbelief, through benevolent skepticism to general

acceptance y and that, when used properly, biogeochemistry can be used as a viable

exploration tool’ (Warren and Delavault, 1950; Warren et al., 1968). During the

half century since that pronouncement, science has steadily progressed and a vast

amount of new data has been acquired from thousands of locations around the

world. However, the world of plants is sufﬁciently complex that the current level of

development has been able to look only at the tip of the iceber g. There remains that

immense area of unknown and under-explored processes that present stimulating

challenges to the next generation of researchers in this ﬁeld. This chapter examines

where the next generation of advances might be heading.

HYPERSPECTRAL IMAGERY IN RELATION TO BIOGEOCHEMISTRY

In order to understand the signals and images obtained from hyperspectral imagery

of the forests some ‘ground-truthing’ of the chemistry of treetops is required. This

aspect of trace element biogeochemistry is examined as a future use of plant chemistry

in recognizing signatures that might be related to concealed mineral deposits.

Hyperspectral Imagery (HSI) is a term used to describe the imagery derived from

subdividing the electromagnetic spectrum into very narrow bandwidths. Much of

this imagery is obtained from sensors aboard satellites or aircraft. The narrow

bandwidths, typically in the order of tens of nanometers, may be combined with or

subtracted from each other in various ways to form images useful for detailed terrain

analysis.

The sensors used for HSI obtain vast quantities of data. Some signals can very

clearly be related to obvious terrain features – e.g., forests, bogs, clearings or water

bodies. Other signals are more subtle and from libraries of accumulated signals they

can be tuned to recognize such geological phenomena as alteration zones around

mineral deposits. In forested areas the ground surface signal from alteration zones

may be completely obscured by the forest canopy. It follows, therefore, that if the

subtle signature from the chemistry of the forest canopy can be recognized from HSI,

there becomes the possibility of geochemically mapping the Earth using airborne

sensors. However, in order to do this, various data ‘training sets’ must be established

by providing ground-truthing of the chemistry of the forest canopy. This involves

sample collection (i.e., treetops, or perhaps tissues from the ground that can be

related to the canopy signature) and chemical analysis in order to geochemically

map an area and relate the data to spectral images obtained from remote sensing

instrumentation.

The Greater Victoria Watershed District (GVWD), on Vancouver Island, is one of

several designated test sites across North America that have been established to con-

duct reﬂectance studies using NASA’s AVIRIS (Airborne Visible/Infrared Imaging

Spectrometer). Preliminary biogeochemical work from this area (Dunn, unpublished)

examined the relationship between HSI and the chemistry of Douglas-ﬁr bark from 45

trees within an area of 100 km

in the GVWD. The rationale for this orientation study

was that prior studies, elsewhere, indicated a moderately good relationship between the

chemistry of the bark and that of foliage from the canopy, although absolute con-

centrations of elements are substantially different. Locally, the bark was found to

contain unusually high concentrations of As and Pb, with strong background-

to-anomaly contrast. Investigation of the data involved focusing on the spectral wave-

lengths for As and Pb using ENVI (a remote sensing exploitation software platform).

It was concluded that although there appeared to be a diffuse positive relationship,

results were inconclusive because of the lack of resolution at 10 nm (nanometres).

Goodenough et al. (2003) and McDonald et al. (2003) reported results of a study

to compare the chemistry of foliage from the tops of Douglas-ﬁr, collected from a

helicopter at approximately the same time that a ﬂight took place to collect AVIRIS

data. The AVIRIS system was mounted onboard NASA’s high altitude ER-2

aircraft, and both HSI data and canopy samples were acquired over the GV WD in

the summer of 2001. With 20 m spatial resolution, the HSI sensor has the capability

of capturing 224 contiguous spectral channels at ap proximately 10 nm intervals in the

visible to near-infrared portion (410–2450 nm) of the electromagnetic spectrum

(Green et al., 1998). A total of six AVIRIS scenes were acquired to asses s the canopy

chemistry in the GVWD. A digital 1 m orthophot ograph, based on 1:15,000 scale

aerial orthophotographs, was used to assess the spectral components of each hyper-

spectral pixel (McDonald et al., 2003).

Preliminary results of the data analysis demonstrated that the hyperspectral

reﬂectance data were capable of mapping nitrogen, total chlorophyll and moisture

contents of the foliage (McDonald et al., 2003). However, with the limit of the

spectral resol ution at 10 nm , to date this has proved insufﬁciently precise to reliably

map the elemental content of the foliage. It appears that a spectral resolution of

o1 nm (and preferably 0.1 nm ) would be required to focus on spectral wavelengths

of individual elements, using a wavelength for each element that gives optimum

contrast to the background spectrum.

At this time, it appears that hyperspect ral data have insufﬁcient resolution for

individual element geochemi cal mapping of forested regions. Only major constituents

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A Look to the Future

(e.g., N, moisture and chlorophyll) can be mapped. There is, however, potential

for futur e development of remote geochemical mapping of the forests, once sub-

nanometre resolution is readily available. The integration of the hyperspectral data

will require substantial ground-truthing by systematically collecting suitable plant

tissues for analysis, and ultimately entering the signatures into reference databases of

the type currently available for the reﬂectance characteristics of minerals.

EXPLORATION GEOMICROBIOLOGY

Chapter 12 takes a step back from the conventional concept of biogeochemistry

in mineral exploration (i.e., largely using vascular land plants) to examine the struc-

turally most primitive but the genetically most diverse forms of life: the microorgan-

isms (i.e., bacteria, archaea and fungi). Microorganisms play a vital role in the cycling

of elements in the environment, and many important steps in the nutrient cycle are

catalyzed exclusively by the microbes. They also play a key role in interfacing the

inorganic world, comprising the geological substrate, and the organic world of higher

order plants, by affecting the mobility, speciation and distribution of major and trace

elements in the rhizosphere. The ﬁrst table of in this book (Table 1-I) shows the

extraordinary concentrations of metals that bacteria and fungi can accumulate. There

are estimates that a single gram of soil typically contains 10 billion or more bacterial

cells and that the bacterial population of the world is about 5 10

. Consequently,

this astronomical population plays a fundamental role in biogeochemical processes

and an improved understanding of this part of the biogeochemical cycle can be used to

advantage in reﬁning biogeochemical exploration methods in general. Most bacteria

are 0.5–5 mm in length, which is the common range in the size of heavy metal phases

within plant structures (Chapter 2), and raises the question as to whether this is simply

a coincidence, or is it a physicochemical constraint, or is there a direct link to the

interaction between bacteria and the metal phases found within plant structures?

In order to obtain an improved understanding of biogeochemical processes, the

role of microorganisms needs to be considered. The contribution of Chapter 12 by

Frank Reith and Steve Rogers from Australia provides new ideas on how biogeo-

chemical processes, that were not until recently given consideration by the explo-

ration community, help to understand the cycling of pathﬁnder elements in the

environment, and how the use of microbial signatures (biomarkers) for mineral

exploration is worthy of future consideration.

FORENSIC BIOGEOCHEMISTRY

Although of indirect relevance to miner al exploration, the role of biogeochemistry

in investigating past mining practices has come to the fore in recent years. There have

been situations where mining companies, accused of improper practices by watchdog

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Biogeochemistry in Mineral Exploration

agencies, have required the expertise of the biogeochem ist to resolve issues concern-

ing possible environmental contamination. Although for legal reasons it is inappro-

priate to specify details, through examination of foliar and twig tissues it has been

possible to exonerate companies from certain accusations concerning environmental

contamination. From chemical analysis of tree tissues it has proved pos sible to

ascertain whether tree mortality was consistent with pollution or with death by

drowning of roots that can occur following periods of natural wat er ponding.

As more substantial biogeochemical databases are accumulated, the role of biogeo-

chemistry in forensic investigations is likely to see an increase.

PLANT MINERALOGY

There remains much to be learnt about the mineral phases within plants. Proton

microprobe analysis has been applied successfully to map the distribution of elements

within plant tissues. Using this technique Morrison et al. (1981) were able to show

the locations of Co, Mn and K in leaf tissue of the Co-hyperaccumulating species

Haumaniastrum robertii from West Africa, and recent studies with state-of-the-art

analytical instrumentation have given greater insight into the distribution and

speciation of some elements in plants. Electron microscopy permits the examination

of crystalline phases, and there is scope for modern instruments to add substantially

to the quite limited knowledge of the o2 mm-sized discrete phases within which base

and heavy metals mostly seem to be concentrated. This knowledge will be largely of

academic interest, but it has practical applications by assisting in the understanding

of metals in plants and may help to deﬁne more precisely what and when samples

should be collected and analysed to optimize results from a biogeochemical survey

for minerals.

CHEMICAL ANALYSIS

Over the past 30 years analytical methods have permitted ever-deeper insight

into the composition of plant materials. Of particular importance in generating a vast

amount of multi-element data have been ﬁrst INAA, and more recently commer-

cialization of ICP-MS. Thirty years ago a ‘multi-element package’ typically com-

prised analytical data for about a dozen elements. The ICP-ES broadened this suite,

but most trace elements returned values in dry vegetation that were below detection

limits. A great advance came in the form of INAA, which permitted simultaneous

determination of more than 30 elements at trace levels with excellent accuracy and

precision. In the last 10 years, ICP-MS has broadened this range to more than 60

elements, with even the ‘basic’ packages generating the same high levels of precision

for 35 or more elements, including determinations for some elements (e.g., Au) to

sub-ppb levels. Costs are typically only a few tens of dollars, and for a few extra

418

A Look to the Future

dollars ‘enhanced’ packages provide data for another 20–30 elements with the similar

levels of sensitivity.

High-resolution ICP-MS is now available for determining even lower levels of

more than 60 elements. For some of the REE determinations, routine detection levels

are as low as 0.01 ppb using 1 g of dry vegetation dissolved in acids. For the

exploration biogeochemist, ICP-MS and HR-ICP-MS are the analytical instruments

that currently generate data for almost all practical requirements. However, sample

dissolution requires additional research to establish the optimum methods for

improving data quality at ultratrace levels. Closed-vessel microwave digestion promises

to be a useful technique that is not yet widely available from commercial laboratories

that are generating biogeochemical data for the exploration and environmental

industries. Development of these techniques should improve the precision on deter-

minations for some elements – notably the precious metals.

Partial leaches, such as those described for the halogen elements on the accom-

panying CD, warrant further investigation to determine if they will generate biogeo-

chemical data that better reﬂect the labile phases of elements released from concealed

mineralization.

From a research perspective the synchrot ron promises to provide important new

insight into the location, migration and speciation of trace elements. Armed with

information of this sort, techniques of when and what to collect can be reﬁned. For

example, the data on Se speciation in Astragalus (Chapter 6) clearly demonstrate that

organoselenium is concentrated in young leaf tissue whereas it is preserved as a

selenate in mature leaves, and there is very little Se in stems. This information can

assist the ﬁeld geologist in collecting the most suitable sample medium, and the

chemist in optimizing an appropriate extraction procedure.

Isotopes in plant tissues are being examined at several institutes, but their role in

biogeochemical exploration for minerals is poorly established and remains at the

research stage.

CONCLUDING REMARKS

Collaboration between biogeochemists and geophysicists can assist in establishing

whether certain geophysical signatures are from barren sulphides or from sulphides

rich in valuable base and/or precious metals. Greater liaison with mathematicians

and statisticians should help the biogeochemist to extract subtle structure in datasets

that may prove to be of value in relating patterns to concealed mineralization.

Judicious use of neural-net software might assist in revealing subtle trends in mob ile

pathﬁnder elements and their relationships to poorly- deﬁned signatures of the

valuable, yet less mobile metals.

In his latter years Alexander Kovalevsky was working on methods to predict

from biogeochemical patterns (especially of Mo and Ag), the depth to mineralization

and an estimation of ore grade, and he claimed success in doing so. These bold

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Biogeochemistry in Mineral Exploration

predictions, estimated from sampling different tissues in a ‘non-barrier’ plant, he

deﬁned as ‘biogeochemical logging’. This quantiﬁcation of depth to miner alization is

an exciting concept, but it does not appear to have been tested outside of Siberia.

Clearly, this warrants further investigation.

Biogeochemical methods comprise another tool that explorationists have at their

disposal. Data should be interpreted in conjunction with all other available geologi-

cal, geochemical and geophysical information, because the technique is not a panacea

and in some environments there may be little or no biogeochemical response to

mineralization and so it may not be the best tool to use. However, the ‘case history’

examples demonstrate that a number of mines have been developed long after the

recognition of a biogeochemical signature, attesting to the value of the method.

Increasingly, it appears that biogeochemical data exhibit subtle trends that can

assist in elucidating faults, structural trends, stratigraphic relationships and lithologies,

all of which may combine to indicate a geological setting suitable for the emplacement

of mineral deposits. Such trends, coupled with subtle enrichments of elements typically

associated with speciﬁc styles of mineralization, can provide focus for further explo-

ration activities. Pattern recognition of element distribution patterns and their spatial

relationships is a significant factor in the successful application of biogeochemistry

to mineral exploration. There is now sufﬁcient knowledge of the application and

usefulness of biogeochemical methods for the thoughtful explorationist to consider

using biogeochemistry as an integral part of a comprehensive mineral exploration

program. Frequently, vegetation chemistry can provide information on the substrate

that cannot be obtained by other means.

420

A Look to the Future

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