Dunn Colin E. Biogeochemistry in Mineral Exploration

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Property Boundary

Mt. Polley, Boundary Zone

Sample # - Cedar Foliage

Zone of relative iodine

enrichment in cedar foliage

pbxh

pm v

jb c

LEG END

jbc

pbxh

pmv

E. Jur. Nicola Gp.

Polymict breccia

L.Trias. Mt. Polley Complex

Altered breccia

L. Trias. Mt. Polley Complex

Monzonite

Fig. 5-9. Trend of iodine anomalies indicated in the proﬁles shown in Fig. 5-8.

0.5

1.5

2.5

3.5

Au ppb in dry tissue.

200 m

Au-bearing quartz vein

Fig. 5-10. Idealized proﬁle over Au-bearing quartz vein. Sample spacing at 10 m intervals

over vein, increasing outward to 25 m, then 50 m and ﬁnally 100 m.

142 Survey Design and Comparisons with Other Sample Media

anomaly. Moving outward from the vein four samples on either side at 25 m intervals

are followed by another four at 50 m intervals and ﬁnally two sites at 100 m intervals.

The slight right skew to the anomaly is indicative of the dip of the vein.

A similar sampling protocol can be employed over geophysical conductors in

order to determine the elemental composition of the conducting body. Whereas the

idealized case shown for Au in Fig. 5-10 is from an artiﬁcial dataset, the proﬁle

shown in Fig. 5-11 is from a real survey designed to differentiate between Ni-bearing

and Ni-d eﬁcient geophysical conductors (massive sulphides) that had not been

drilled. In the example shown, sample spacing was at 200 m, closing to approximately

100 m over the conductor. The results clearly indicate that the massive sulphide that

generated the geophysical anomaly was Ni-bearing.

Another orientation survey design that is particularly appropriate for circular

targets, such as kimberlites, is to collect samples along two lines that cross in the

middle of a prescribed (e.g., geophysical) anomaly, generating a ‘cross-hairs’ pattern.

Figure 5-12 shows an example of sample sites over a kimberlite in the Ekati area of

Canada’s Northwest Territories. The area surveyed was tundra with little relief and a

number of boggy depressions. The dominant plant species was dwarf birch. Over the

circular geophysical target of about 100 m in diameter and for 100 m to either side the

selected sample spacing was 25 m, increasing to 50 m for two sites moving outward

from the anomaly, and ﬁnally three sites at 100 m spacing. This proved to be an

effective pattern for deﬁning the biogeochemical response to the concealed kimberlite

(more details in section on kimberlites – Chapter 11).

COMPARISONS WITH OTHER TYPES OF GEOCHEMICAL SURVEY

A question often asked is ‘How do results from a biogeochemical survey compare

with data obtained from other sample media?’ As a ﬁrst response, the reader is referred

Ni in Black Spruce Tops

200 400 600 800 1,200 1,400 1,600 1,800 2,000 2,200

Ni ppm.

metres

Conductor

1,000

Fig. 5-11. Ni in spruce crowns – concentrations in dry stems (most recent three years of

growth). Analysis by ICP-MS on an aqua regia digestion.

143Biogeochemistry in Mineral Exploration

to the discussions in Chapter 4 where it is noted that the roots from a large tree

commonly penetrate all soil horizons and integrate the geochemical signature of sev-

eral cubic metres of soil. As a result, a similar geochemical response in vegetation and

soils should only be expected when elements are quite evenly distributed throughout a

large volume of soil.

Vegetation versus Soils

In areas where there is either glacial dispersal or down-slope movement of soils,

typically a biogeochemical response is more directly centred over a zone of miner-

alization than is a soil response. At Imperial Metals’ Mount Polley Cu/Au porphyry

open pit mine in central British Columbia, there are several nearby undisturbed zones

of mineralization. The geochemical responses of some, such as the Boundary Zone,

were investigated prior to trenching and drilling. Comparisons of metals in cedar

foliage (Thuja plicata) with concentrations in B-horizon soils demonst rate this spatial

relationship of elements in the two media. Figure 5-13 shows an outline of the

geological contacts, and two plots showing concentrations of Hg in dry foliage from

the cedar and in the –80 mesh fraction of B-horizon soil. Dots are shown as ﬁve

0 100 200

metres

Trail

Lake

Geophysical target

(kimberlite)

Fig. 5-12. ‘Cross-hairs’ sample design over circular geophysical target. Sample survey pattern

over the Bighorn kimberlite, Ekati area, Northwest Territories, Canada (courtesy of Dr. W.B.

Coker, BHP Billiton Ltd.).

144 Survey Design and Comparisons with Other Sample Media

classes of 10 samples (50 samples), with the dots proportional in size to increasing

element concentrations. The sketch shows that the highest concentrations in the soil

are down slope to the south from the mineralized hillcrest, whereas the foliage has

highest concentrations directly over mineralization. This pattern of a biogeochemical

anomaly being located more direct ly over mineralization than a soil anomaly is

characteristic of many sites with undulating relief and/or glaciated terrain. In ﬂat arid

Trail

pbxh

pmv

jbc

LEGEND

jbc

pbxh

pmv

E. Jur. Nicola Gp.

Polymict breccia

L. Trias. Mt. Polley Complex

Altered breccia

L. Trias. Mt. Polley Complex

Monzonite

Mineralization

at surface

0 100 200

metres

Hill

crest

pbxh

pmv

jbc

Mineralization

at surface

rail

Hill

crest

EQUAL NUMBER

Hg ppb

14 to 18

18 to 21

21 to 26

26 to 28

28 to 42

Trail

Mt. Polley,

Boundary Zone

MERCURY (ppb)

Cedar Foliage

0 100 200

metres

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pmv

jbc

Mineralization

at surface

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Hill

crest

EQUAL NUMBER

Hg ppb

16 to 29

29 to 42

42 to 52

52 to 96

96 to 469

Trail

Mt. Polley,

Boundary Zone

MERCURY (ppb

)

B-Horizon Soil

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metres

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Mt. Polley

Boundary Zone

Sample Sites

Fig. 5-13. Mount Polley Cu/Au/Mo porphyry, central British Columbia. Sketch of geological

map with and locations of (1) B-horizon soil and cedar samples and (2) concentrations of Hg in

–80 mesh soil and dry cedar foliage (from Dunn et al., 2006a,b).

145Biogeochemistry in Mineral Exploration

environments upon which residual soils are developed , soil and vegetation anomalies

are commonly more spatially coincident.

Vegetation versus Glacial Tills

Mount Milligan is located on the Nechako Plateau in central British Columbia at

an elevation of 1508 m. Eight kilometres to the northwest are some significant Cu/Au

porphyry deposits that have yet to be developed. The Mou nt Milligan deposits are

centred on Early Jurassic crowded plagioclase-porphyritic monzonite intrusions

known as the MBX and Southern Star stocks. These, and numerous smaller stocks,

intrude Upper Triassic Takla Group augite (+/– plagioclase) porphyry agglomerate,

trachyte breccias and ﬂows, and bedded epiclastic sediments of the Witch Lake

Formation. The last glacial event in the region occurred during the Late Wisconsinan

(Fraser Glaciation) and involved ice movement primarily to the east–northeast. Till

was deposited during the last glacial episode and is commonly hummocky and forms

drumlins. Drift thickness is highly variable, ranging from less than 1 m on rocky

highlands to over 80 m. In the survey area shown in Fig. 5-14, the thinnest till occurs

at the MBX and Southern Star deposits and thickens to the east.

A biogeochemical survey (Dunn et al., 1996b) involved the collection of outer

bark from 134 mature lodgepole pines (Pinus contorta), whereas a simultaneous till

survey involved digging pits at most of these same sites (Sibbick et al., 1996). The

bark samples were reduced to ash by co ntrolled ignition at 475

C, and the tills were

sieved for the –80 mesh fraction to be submitted for analysis. Analyses were by

INAA, and ICP-ES on an aqua regia digestion of each sample medium.

Figure 5-14 shows the results for Au, with lodgepole pine on the left a nd till on

the right. Both media exhibit relative enrichments at both the MBX and Southern

Star Zones where the till cover is thin. The bark anomalies are more tightly centred

over the mineralization than the till for which some dispersion from ice movement is

evident. However, at the Philip Lakes showing, where there is greater thickness of till,

the bark anomaly is centred over the showing, but the till anomaly is displaced in a

down-ice direction by about 2.5 km.

At the Beaver Dam gold project in Nova Scotia, the Mill Shaft deposit comprises

narrow Au-bearing quartz veins in quartzite and argillite of the Goldenville For-

mation (Lower Palaeozoic Meguma Group). Gold is concentrated within and along

the margins of pyrrhotite and chalcopyrite. Samples of B-horizon soil, till and outer

bark of red spruce (Picea rubens) were collected in a grid pattern at intervals of 50 or

100 m (Dunn et al., 1991). Analysis of the soils (o2 mm fraction) and the heavy

mineral concentrates from till indicated disper sal of Au and As in a fan about 100 m

wide, that extended ab out 300 m down-ice from the mineralized zone. The head of

the fan was located 50 m to the south of the Au-bearing quartz veins. Outer bark

from red spruce exhibited a similar trend for Au and As, coincident with a boulder

train of vein quartz on the surface. However, the sites of maximum concentration in

146

Survey Design and Comparisons with Other Sample Media

Mt. Milligan - GOLD in Lodgepole Pine Bark

Au in ash

Max. 185 ppb

metres

0 5000

MBX Zone

Philip Lakes

Showing

Southern Star

Zone

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200

240

280

320

Mt. Milligan - GOLD in Till

Au ppb

Max 732

0 5000

metres

MBX Zone

Philip Lakes

Showing

Southern Star

Zone

Increasing till thickness

Fig. 5-14. Mount Milligan Au/Cu porphyry, central British Columbia. Gold in the ash of

lodgepole pine bark and in the –80 mesh fraction of till. Analysis by INAA.

147Biogeochemistry in Mineral Exploration

the bark were more closely conﬁned to the known mineralization, occurring to both

the north and south of a W/E trending vein (i.e., a ‘rabbit ears’ type of anomaly as

deﬁned by Govett (1976) and Smee (1983)).

Comparison of multi-media reconnaissance-level surveys using plants, soils, and

sediments from lakes and streams

In Nova Scotia comprehensive reconnaissance-level surveys carried out by Rogers

(1988, 1989) and Rogers et al. (1984), as well as National Geochemical Reconnais-

sance surveys conducted by the Geological Survey of Canada in the 1980s, provided

substantial databases that allowed comparison of patterns of biogeochemical data

from later surveys (e.g., Dunn et al., 1989). A comparison was made of the chemistry

of lake sediment and balsam ﬁr twigs (Abies balsamea) collected over a 5000 km

area

of eastern Nova Scotia at a sample density of approximately 1 per 8 km

. The surveys

were conducted 12 years apart. The spatial relationship between areas of metal

enrichment in the two sample media was examined by comparing each kriged da-

taset. Areas where Au and As concentrations were greater than the 70th percentile in

both media succeeded in identifying the main gold camps and identiﬁed other areas

worthy of further investigation (Dunn et al., 1991).

Extensive multi-media reconnaissance-level surveys were conducted in northern

Manitoba during the 1990s by a government-s ponsored programme entitled ‘Oper-

ation Superior’ . Large databases of analyses from these surveys are available in

digital form for statistical interrogation and comparisons (Fedikow et al., 1997a,b,

1998, 1999, 2000, 2002).

In the Cobar area of New South Wales, Australia, Cohen et al. (1998) compared

selective extraction soil geochemistry (lag samples and soils from depths of 10–20 cm)

with the chemistry of needles from cypress-pine (Callitris spp., also known as white

cypress and not a true pine; it is a genus of the coniferous cypress [Cupressaceae]

family). In addition to a good response to gold mineralization by several of the

analytical protocols tested for the various soil components, there was a strong multi-

element response in the cypress needles. It was concluded that the cypress may be

responding to hydromorphic dispersion patterns at depth and that selective extrac-

tions, when integrated with biogeochemistry, offer enhanced potential for detecting

mineralized targets buried by significant thickness of transported cover.

A study to determine vegetation and stream sediment patterns in north-eastern

New South Wales covered approximately 14,000 km

of the Clarence River system

(Cohen et al., 1999). Sampling was designed to conﬁn e each of 924 sub-catchments to

a single lithological group. The composition of the –250 mm fraction of the stream

sediments, and the leaves from over 20 genera (dominated by (Allo-) Casuarina,

Eucalyptus, Acacia, Callistemon and Melaleuca) was determined by INAA. Results

indicated that stream sediment and vegetation geochemistry reﬂected both hydro-

morphic and mechanical dispersion within sub-catchments, with regional patterns

148

Survey Design and Comparisons with Other Sample Media

dominant over local inﬂuences. The vegetation, however, was inﬂuenced to a greater

extent by hydromorphic dispersion, as indicated by differences in the ratio of leaf to

sediment Cr concentrations in sub-catchments draining serpent inites and basalts. A

number of Au targets were detected only on the basis of the biogeochemistry,

whereas others were reﬂected only in the stream sediment geochemistry. As with their

previous regional study in the Cobar region, Cohen et al. (1999) concluded that

differences in the response of the two sampling media suggest their joint use in

exploration would maximise the probability of detecting mineralization.

SUMMARY

Biogeochemical surveys can be carried out over a wide range of scales and sample

designs, and their formats depend upon the objectives, the target (nature of the

mineralization) and various logistical factors. A limitation is that the coverage ob-

tained from a proposed biogeochemical survey can only be as comprehensive as the

distribution of the species selected for collection. Consequently, whenever possible, a

quick reconnaissance assessment of plant coverage should be made. Information

available from local forestry ofﬁces and/or botanists at local universities, government

agencies, botanical gardens and museums can prove invaluable. In some areas

‘biogeoclimatic’ maps are available that provide an overview of the distribution of

plant species to assist in advance planning.

For detailed surveys, sample spacing should be commensurate with the distribu-

tion of the selected plant species. Given the irregular distribution of trees that may

occur in undisturbed forest, the geochemist may need to settle for an uneven grid,

with perhaps some gaps where a recent clear-cut is encountered or where a change of

elevation is sufﬁcient that a different plant assemblage occurs.

As noted earlier, a plant’s root system integrates the geochemical signature of

several cubic metres of the substrate – all soil horizons, interstitial water, and perhaps

penetrates through regolith or glacial deposits into bedrock fractures. As a conse-

quence of the sometimes enormous extent of root systems, a biogeochemical signa-

ture is normally a selective mixture of any or all of these components, and that

signature is unlikely to be identical to the geochemical signature of any of the in-

dividual media penetrated by the roots. Given the complex interactions of physical,

chemical and biological dynamics in the natural environment of a forest or scrub-

land, in most parts of the world it is unrealistic to expect a biogeochemical signature

to be the same as that from an underlying soil.

Slight enrichments of metals in soils and tills are unlikely to be detected in the

vegetation as weak biogeochemical anomalies. This is especially true in the ppb Au

and low ppm ranges of other metals commonl y encountered in surface sediments.

Some metals may not be present in a chemical form that is available for uptake (e.g.,

Cr structurally bound in chromite, and therefore not readily released to root sys-

tems); some may be excluded from uptake at the roots or only partially absorbed

149

Biogeochemistry in Mineral Exploration

(because of plant barrier mechanisms); and some may be taken up but disper sed

among tree tissues to the extent that inter-site variations are so small that they cannot

be detected.

The net result of these factors is that the geochemical information supplied by

vegetation is commonly diff erent from that of soil or till, although there may well be

a more obvious relationship from areas of residual soils. Just as two methods of

geophysical survey will provide totally different information, so will two methods of

geochemical survey. A high correlation between distribution patterns of two geo-

chemical sample media is the exception rather than the rule. In geological environ-

ments, where there is sufﬁcient concentration of metals to form a mineral deposit,

such a ‘critical mass’ of elements may be sufﬁcient to generate biogeochemical

anomalies above (by upward diffusion) or close to (by movement in, for example,

electrochemical cells) the mineral source. Soils may have anomalies displaced down-

slope from mineralization and tills usually have geochemical anomalies displaced

down-ice from the mineralized source. Such factors need to be taken into consid-

eration when interpreting geochemical results. Valuable exploration information can

be obtained from the analysis of most types of surface geochemical sample media.

When this information is coupled with analysis of vegetation samples a powerful

combination is provided for assisting in the exploration for a wide variety of mineral

deposits.

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Survey Design and Comparisons with Other Sample Media

Chapter 6

SAMPLE PREPARATION AND DECOMPOSITION

INTRODUCTION

The selection of appropriate preparation and decomposition procedures is crucial

to the meaningful interpretation and successful outcome of any exploration biogeo-

chemical survey. They require careful consideration prior to introducing vegetation

samples into the analytical instrumentation. For many of the elements of interest to

the exploration biogeochemist, meticulous experimentation by commercial labora-

tories has streamlined this process. However, in order to understand the results that

can be obtained from an analytical package, and to extract relevant and meaningful

information the user should be aware of several procedural options and their re-

spective advantages and limitations.

WASHING

There is on-going debate among those involved in the analysis of plant tissues as

to whether or not samples should be washed prior to analysis. In general, in northern

forests far from dusty roads, for exploration purposes washing may be redundant or

even deleterious to survey results. In arid to semi-arid regions where winds constantly

coat samples with dust, it is commonly better to rinse samples prior to drying and

separating tissues. A judgement call based upon the experience of the biogeochemist

needs to be made in the ﬁeld, that takes into account the degree of dust coating of

vegetation samples and the nature of the dust (i.e., is it from a mining operation or

smelter, and therefore might metal-rich particulates be coating the plants?).

Thorough washing using solvents or dispersants

Wyttenbach et al. (1987) and Wyttenbach and Tobler (1988) advise that washing

should be undertaken with a mixture of toluene and tetrahydrofuran accompanied

by mechanical vibration to remove wax and adhering aerosol particles from the

surface of needles, because for

many elements y quantities on the needle(s) surface are greater that those within the

needle y The treatment used does not remove elements from within the needle.